wdiff rfc8656.original rfc8656.txt

TRAM WG

Internet Engineering Task Force (IETF) T. Reddy, Ed.
Internet-Draft
Request for Comments: 8656 McAfee
Obsoletes: 5766, 6156 (if approved) A. Johnston, Ed.
Intended status:
Category: Standards Track Villanova University
Expires: January 28, 2020
ISSN: 2070-1721 P. Matthews
Alcatel-Lucent
J. Rosenberg
jdrosen.net
July 27, 2019
February 2020

Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-tram-turnbis-29

Abstract

If a host is located behind a NAT, then in certain situations it can be impossible for that host
to communicate directly with other hosts
(peers). (peers) in certain
situations. In these situations, it is necessary for the host to use
the services of an intermediate node that acts as a communication
relay. This specification defines a protocol, called TURN (Traversal "Traversal
Using Relays around NAT), NAT" (TURN), that allows the host to control the
operation of the relay and to exchange packets with its peers using
the relay. TURN differs from other relay control protocols in that
it allows a client to communicate with multiple peers using a single
relay address.

The TURN protocol was designed to be used as part of the ICE
(Interactive Interactive
Connectivity Establishment) Establishment (ICE) approach to NAT traversal, though it also
can also be used without ICE.

This document obsoletes RFC RFCs 5766 and RFC 6156.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of six months this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 28, 2020.
https://www.rfc-editor.org/info/rfc8656.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 8
3.1. Transports . . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Allocations . . . . . . . . . . . . . . . . . . . . . . . 12
3.3. Permissions . . . . . . . . . . . . . . . . . . . . . . . 14
3.4. Send Mechanism . . . . . . . . . . . . . . . . . . . . . 15
3.5. Channels . . . . . . . . . . . . . . . . . . . . . . . . 17
3.6. Unprivileged TURN Servers . . . . . . . . . . . . . . . . 19
3.7. Avoiding IP Fragmentation . . . . . . . . . . . . . . . . 19
3.8. RTP Support . . . . . . . . . . . . . . . . . . . . . . . 21
3.9. Happy Eyeballs for TURN . . . . . . . . . . . . . . . . . 21
4. Discovery of TURN server . . . . . . . . . . . . . . . . . . 22 Server
4.1. TURN URI Scheme Semantics . . . . . . . . . . . . . . . . 22
5. General Behavior . . . . . . . . . . . . . . . . . . . . . . 23
6. Allocations . . . . . . . . . . . . . . . . . . . . . . . . . 25
7. Creating an Allocation . . . . . . . . . . . . . . . . . . . 26
7.1. Sending an Allocate Request . . . . . . . . . . . . . . . 26
7.2. Receiving an Allocate Request . . . . . . . . . . . . . . 28
7.3. Receiving an Allocate Success Response . . . . . . . . . 33
7.4. Receiving an Allocate Error Response . . . . . . . . . . 34
8. Refreshing an Allocation . . . . . . . . . . . . . . . . . . 36
8.1. Sending a Refresh Request . . . . . . . . . . . . . . . . 37
8.2. Receiving a Refresh Request . . . . . . . . . . . . . . . 37
8.3. Receiving a Refresh Response . . . . . . . . . . . . . . 38
9. Permissions . . . . . . . . . . . . . . . . . . . . . . . . . 38
10. CreatePermission . . . . . . . . . . . . . . . . . . . . . . 40
10.1. Forming a CreatePermission Request . . . . . . . . . . . 40
10.2. Receiving a CreatePermission Request . . . . . . . . . . 40
10.3. Receiving a CreatePermission Response . . . . . . . . . 41
11. Send and Data Methods . . . . . . . . . . . . . . . . . . . . 41
11.1. Forming a Send Indication . . . . . . . . . . . . . . . 41
11.2. Receiving a Send Indication . . . . . . . . . . . . . . 42
11.3. Receiving a UDP Datagram . . . . . . . . . . . . . . . . 42
11.4. Receiving a Data Indication . . . . . . . . . . . . . . 43
11.5. Receiving an ICMP Packet . . . . . . . . . . . . . . . . 43
11.6. Receiving a Data Indication with an ICMP attribute . . . 44 Attribute
12. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . 45
12.1. Sending a ChannelBind Request . . . . . . . . . . . . . 47
12.2. Receiving a ChannelBind Request . . . . . . . . . . . . 48
12.3. Receiving a ChannelBind Response . . . . . . . . . . . . 49
12.4. The ChannelData Message . . . . . . . . . . . . . . . . 49
12.5. Sending a ChannelData Message . . . . . . . . . . . . . 50
12.6. Receiving a ChannelData Message . . . . . . . . . . . . 50
12.7. Relaying Data from the Peer . . . . . . . . . . . . . . 51
13. Packet Translations . . . . . . . . . . . . . . . . . . . . . 51
13.1. IPv4-to-IPv6 Translations . . . . . . . . . . . . . . . 52
13.2. IPv6-to-IPv6 Translations . . . . . . . . . . . . . . . 53
13.3. IPv6-to-IPv4 Translations . . . . . . . . . . . . . . . 55
14. UDP-to-UDP relay . . . . . . . . . . . . . . . . . . . . . . 55 Relay
15. TCP-to-UDP relay . . . . . . . . . . . . . . . . . . . . . . 57 Relay
16. UDP-to-TCP relay . . . . . . . . . . . . . . . . . . . . . . 59 Relay
17. STUN Methods . . . . . . . . . . . . . . . . . . . . . . . . 60
18. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 60
18.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . 61
18.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . 61
18.3. XOR-PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . 62
18.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . 62
18.5. XOR-RELAYED-ADDRESS . . . . . . . . . . . . . . . . . . 62
18.6. REQUESTED-ADDRESS-FAMILY . . . . . . . . . . . . . . . . 62
18.7. EVEN-PORT . . . . . . . . . . . . . . . . . . . . . . . 63
18.8. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . 63
18.9. DONT-FRAGMENT . . . . . . . . . . . . . . . . . . . . . 64
18.10. RESERVATION-TOKEN . . . . . . . . . . . . . . . . . . . 64
18.11. ADDITIONAL-ADDRESS-FAMILY . . . . . . . . . . . . . . . 64
18.12. ADDRESS-ERROR-CODE . . . . . . . . . . . . . . . . . . . 64
18.13. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . 65
19. STUN Error Response Codes . . . . . . . . . . . . . . . . . . 66
20. Detailed Example . . . . . . . . . . . . . . . . . . . . . . 66
21. Security Considerations . . . . . . . . . . . . . . . . . . . 75
21.1. Outsider Attacks . . . . . . . . . . . . . . . . . . . . 75
21.1.1. Obtaining Unauthorized Allocations . . . . . . . . . 75
21.1.2. Offline Dictionary Attacks . . . . . . . . . . . . . 75
21.1.3. Faked Refreshes and Permissions . . . . . . . . . . 76
21.1.4. Fake Data . . . . . . . . . . . . . . . . . . . . . 76
21.1.5. Impersonating a Server . . . . . . . . . . . . . . . 77
21.1.6. Eavesdropping Traffic . . . . . . . . . . . . . . . 77
21.1.7. TURN Loop Attack . . . . . . . . . . . . . . . . . . 78
21.2. Firewall Considerations . . . . . . . . . . . . . . . . 79
21.2.1. Faked Permissions . . . . . . . . . . . . . . . . . 80
21.2.2. Blacklisted IP Addresses . . . . . . . . . . . . . . 80
21.2.3. Running Servers on Well-Known Ports . . . . . . . . 80
21.3. Insider Attacks . . . . . . . . . . . . . . . . . . . . 80
21.3.1. DoS against TURN Server . . . . . . . . . . . . . . 81
21.3.2. Anonymous Relaying of Malicious Traffic . . . . . . 81
21.3.3. Manipulating Other Allocations . . . . . . . . . . . 81
21.4. Tunnel Amplification Attack . . . . . . . . . . . . . . 81
21.5. Other Considerations . . . . . . . . . . . . . . . . . . 83
22. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 83
23. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 84
24. Changes since RFC 5766 . . . . . . . . . . . . . . . . . . . 85
25. Updates to RFC 6156 . . . . . . . . . . . . . . . . . . . . . 86
26. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 86
27. References . . . . . . . . . . . . . . . . . . . . . . . . . 86
27.1.
26.1. Normative References . . . . . . . . . . . . . . . . . . 86
27.2.
26.2. Informative References . . . . . . . . . . . . . . . . . 88
Acknowledgements
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 93

1. Introduction

A host behind a NAT may wish to exchange packets with other hosts,
some of which may also be behind NATs. To do this, the hosts
involved can use "hole punching" techniques (see [RFC5128]) in an
attempt to discover a direct communication path; that is, a
communication path that goes from one host to another through
intervening NATs and routers, routers but does not traverse any relays.

As described in [RFC5128] and [RFC4787], hole punching techniques
will fail if both hosts are behind NATs that are not well behaved.
For example, if both hosts are behind NATs that have a mapping
behavior of "address-dependent mapping" or "address- and port-
dependent mapping" (Section (see Section 4.1 in of [RFC4787]), then hole punching
techniques generally fail.

When a direct communication path cannot be found, it is necessary to
use the services of an intermediate host that acts as a relay for the
packets. This relay typically sits in the public Internet and relays
packets between two hosts that both sit behind NATs.

In many enterprise networks, direct UDP transmissions are not
permitted between clients on the internal networks and external IP
addresses. To permit media sessions in such a situation to use UDP
and avoid forcing them through TCP, an Enterprise Firewall can be
configured to allow UDP traffic relayed through an Enterprise relay
server. WebRTC requires support for this scenario (Section (see
Section 2.3.5.1
in of [RFC7478]). Some users of SIP or WebRTC want IP
location privacy from the remote peer. In this scenario, the client
can select a relay server offering IP location privacy and only
convey the relayed candidates to the peer for ICE connectivity checks
(see Section 4.2.4
in [I-D.ietf-rtcweb-security]). of [SEC-WEBRTC]).

This specification defines a protocol, called TURN, "TURN", that allows a
host behind a NAT (called the TURN client) "TURN client") to request that another
host (called the TURN server) "TURN server") act as a relay. The client can
arrange for the server to relay packets to and from certain other
hosts (called peers) "peers"), and the client can control aspects of how the
relaying is done. The client does this by obtaining an IP address
and port on the server, called the relayed "relayed transport address. address". When
a peer sends a packet to the relayed transport address, the server
relays the transport protocol data from the packet to the client.
The data encapsulated within a message header that allows the client
to know the peer from which the transport protocol data was relayed
by the server. If the server receives an ICMP error packet, the
server also relays certain layer 3/4 Layer 3 and 4 header fields from the ICMP
header to the client. When the client sends a message to the server,
the server identifies the remote peer from the message header and
relays the message data to the intended peer.

A client using TURN must have some way to communicate the relayed
transport address to its peers, peers and to learn each peer's IP address
and port (more precisely, each peer's server-reflexive transport
address,
address; see Section 3). How this is done is out of the scope of the
TURN protocol. One way this might be done is for the client and
peers to exchange email messages. Another way is for the client and
its peers to use a special-purpose "introduction" or "rendezvous"
protocol (see [RFC5128] for more details).

If TURN is used with ICE [RFC8445], then the relayed transport
address and the IP addresses and ports of the peers are included in
the ICE candidate information that the rendezvous protocol must
carry. For example, if TURN and ICE are used as part of a multimedia
solution using SIP [RFC3261], then SIP serves the role of the
rendezvous protocol, carrying the ICE candidate information inside
the body of SIP messages [I-D.ietf-mmusic-ice-sip-sdp]. [SDP-ICE]. If TURN and ICE are used with
some other rendezvous protocol, then ICE provides guidance on the
services the rendezvous protocol must perform.

Though the use of a TURN server to enable communication between two
hosts behind NATs is very likely to work, it comes at a high cost to
the provider of the TURN server, server since the server typically needs a
high-bandwidth connection to the Internet. As a consequence, it is
best to use a TURN server only when a direct communication path
cannot be found. When the client and a peer use ICE to determine the
communication path, ICE will use hole punching techniques to search
for a direct path first and only use a TURN server when a direct path
cannot be found.

TURN was originally invented to support multimedia sessions signaled
using SIP. Since SIP supports forking, TURN supports multiple peers
per relayed transport address; a feature not supported by other
approaches (e.g., SOCKS [RFC1928]). However, care has been taken to
make sure that TURN is suitable for other types of applications.

TURN was designed as one piece in the larger ICE approach to NAT
traversal. Implementors of TURN are urged to investigate ICE and
seriously consider using it for their application. However, it is
possible to use TURN without ICE.

TURN is an extension to the STUN (Session Session Traversal Utilities for
NAT) NAT
(STUN) protocol [I-D.ietf-tram-stunbis]. [RFC8489]. Most, though not all, TURN messages are
STUN-formatted messages. A reader of this document should be
familiar with STUN.

The TURN specification was originally published as [RFC5766], which
was updated by [RFC6156] to add IPv6 support. This document
supersedes and obsoletes both [RFC5766] and [RFC6156].

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119][RFC8174] [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

Readers are expected to be familiar with [I-D.ietf-tram-stunbis] [RFC8489] and the terms
defined there.

The following terms are used in this document:

TURN:
The protocol spoken between a TURN client and a TURN server. It
is an extension to the STUN protocol [I-D.ietf-tram-stunbis]. [RFC8489]. The protocol
allows a client to allocate and use a relayed transport address.

TURN client:
A STUN client that implements this specification.

TURN server:
A STUN server that implements this specification. It relays data
between a TURN client and its peer(s).

Peer:
A host with which the TURN client wishes to communicate. The TURN
server relays traffic between the TURN client and its peer(s).
The peer does not interact with the TURN server using the protocol
defined in this document; rather, the peer receives data sent by
the TURN server server, and the peer sends data towards the TURN server.

Transport Address:
The combination of an IP address and a port.

Host Transport Address:
A transport address on a client or a peer.

Server-Reflexive Transport Address:
A transport address on the "external side" of a NAT. This address
is allocated by the NAT to correspond to a specific host transport
address.

Relayed Transport Address:
A transport address on the TURN server that is used for relaying
packets between the client and a peer. A peer sends to this
address on the TURN server, and the packet is then relayed to the
client.

TURN Server Transport Address:
A transport address on the TURN server that is used for sending
TURN messages to the server. This is the transport address that
the client uses to communicate with the server.

Peer Transport Address:
The transport address of the peer as seen by the server. When the
peer is behind a NAT, this is the peer's server-reflexive
transport address.

Allocation:
The relayed transport address granted to a client through an
Allocate request, along with related state, such as permissions
and expiration timers.

5-tuple:
The combination (client IP address and port, server IP address and
port, and transport protocol (currently one of UDP, TCP, DTLS/UDP DTLS/UDP,
or TLS/TCP) TLS/TCP)) used to communicate between the client and the
server. The 5-tuple uniquely identifies this communication
stream. The 5-tuple also uniquely identifies the Allocation on
the server.

Transport Protocol:
The protocol above IP that carries TURN Requests, Responses, and
Indications as well as providing identifiable flows using a
5-tuple. In this specification, UDP and TCP are defined as
transport protocols, as well as their protocols; this document also describes the use of UDP
and TCP in combination with a security layer using DTLS and TLS TLS,
respectively.

Channel:
A channel number and associated peer transport address. Once a
channel number is bound to a peer's transport address, the client
and server can use the more bandwidth-efficient ChannelData
message to exchange data.

Permission:
The IP address and transport protocol (but not the port) of a peer
that is permitted to send traffic to the TURN server and have that
traffic relayed to the TURN client. The TURN server will only
forward traffic to its client from peers that match an existing
permission.

Realm:
A string used to describe the server or a context within the
server. The realm tells the client which username and password
combination to use to authenticate requests.

Nonce:
A string chosen at random by the server and included in the server
response. To prevent replay attacks, the server should change the
nonce regularly.

(D)TLS:
This term is used for statements that apply to both Transport
Layer Security [RFC8446] and Datagram Transport Layer Security
[RFC6347].

3. Overview of Operation

This section gives an overview of the operation of TURN. It is non-
normative.

In a typical configuration, a TURN client is connected to a private
network [RFC1918] and and, through one or more NATs NATs, to the public
Internet. On the public Internet is a TURN server. Elsewhere in the
Internet are one or more peers with which the TURN client wishes to
communicate. These peers may or may not be behind one or more NATs.
The client uses the server as a relay to send packets to these peers
and to receive packets from these peers.

Peer A
Server-Reflexive +---------+
Transport Address | |
192.0.2.150:32102 | |
| /| |
TURN | / ^| Peer A |
Client's Server | / || |
Host Transport Transport | // || |
Address Address | // |+---------+
198.51.100.2:49721 192.0.2.15:3478 |+-+ // Peer A
| | ||N| / Host Transport
| +-+ | ||A|/ Address
| | | | v|T| 203.0.113.2:49582
| | | | /+-+
+---------+| | | |+---------+ / +---------+
| || |N| || | // | |
| TURN |v | | v| TURN |/ | |
| Client |----|A|----------| |----|A|-------| Server |------------------| Peer B |
| | | |^ | |^ ^| |
| | |T|| | || || |
+---------+ | || +---------+| |+---------+
| || | |
| || | |
+-+| | |
| | |
| | |
Client's | Peer B
Server-Reflexive Relayed Transport
Transport Address Transport Address Address
192.0.2.1:7000 192.0.2.15:50000 192.0.2.210:49191

Figure 1

Figure 1 shows a typical deployment. In this figure, the TURN client
and the TURN server are separated by a NAT, with the client on the
private side and the server on the public side of the NAT. This NAT
is assumed to be a "bad" NAT; for example, it might have a mapping
property of "address-and-port-dependent mapping" (see [RFC4787]).

The client talks to the server from a (IP address, port) combination
called the client's HOST TRANSPORT ADDRESS. "host transport address". (The combination of an
IP address and port is called a TRANSPORT ADDRESS.) "transport address".)

The client sends TURN messages from its host transport address to a
transport address on the TURN server that is known as the TURN SERVER
TRANSPORT ADDRESS. "TURN
server transport address". The client learns the TURN server
transport address through some unspecified means (e.g.,
configuration), and this address is typically used by many clients
simultaneously.

Since the client is behind a NAT, the server sees packets from the
client as coming from a transport address on the NAT itself. This
address is known as the client's SERVER-REFLEXIVE "server-reflexive transport address;
address"; packets sent by the server to the client's server-reflexive
transport address will be forwarded by the NAT to the client's host
transport address.

The client uses TURN commands to create and manipulate an ALLOCATION
on the server. An allocation is a data structure on the server.
This data structure contains, amongst other things, the RELAYED
TRANSPORT ADDRESS relayed
transport address for the allocation. The relayed transport address
is the transport address on the server that peers can use to have the
server relay data to the client. An allocation is uniquely
identified by its relayed transport address.

Once an allocation is created, the client can send application data
to the server along with an indication of to which peer the data is
to be sent, and the server will relay this data to the intended peer.
The client sends the application data to the server inside a TURN
message; at the server, the data is extracted from the TURN message
and sent to the peer in a UDP datagram. In the reverse direction, a
peer can send application data in a UDP datagram to the relayed
transport address for the allocation; the server will then
encapsulate this data inside a TURN message and send it to the client
along with an indication of which peer sent the data. Since the TURN
message always contains an indication of which peer the client is
communicating with, the client can use a single allocation to
communicate with multiple peers.

When the peer is behind a NAT, then the client must identify the peer
using its server-reflexive transport address rather than its host
transport address. For example, to send application data to Peer A
in the example above, the client must specify 192.0.2.150:32102 (Peer
A's server-reflexive transport address) rather than 203.0.113.2:49582
(Peer A's host transport address).

Each allocation on the server belongs to a single client and has
either one or two relayed transport addresses that are used only by
that allocation. Thus, when a packet arrives at a relayed transport
address on the server, the server knows for which client the data is
intended.

The client may have multiple allocations on a server at the same
time.

3.1. Transports

TURN, as defined in this specification, always uses UDP between the
server and the peer. However, this specification allows the use of
any one of UDP, TCP, Transport Layer Security (TLS) over TCP TCP, or
Datagram Transport Layer Security (DTLS) over UDP to carry the TURN
messages between the client and the server.

+----------------------------+---------------------+
| TURN client to TURN server | TURN server to peer |
+----------------------------+---------------------+
+============================+=====================+
| UDP | UDP |
+----------------------------+---------------------+
| TCP | UDP |
+----------------------------+---------------------+
| TLS-over-TCP | UDP |
+----------------------------+---------------------+
| DTLS-over-UDP | UDP |
+----------------------------+---------------------+

Table 1

If TCP or TLS-over-TCP is used between the client and the server,
then the server will convert between these transports and UDP
transport when relaying data to/from the peer.

Since this version of TURN only supports UDP between the server and
the peer, it is expected that most clients will prefer to use UDP
between the client and the server as well. That being the case, some
readers may wonder: Why also support TCP and TLS-over-TCP?

TURN supports TCP transport between the client and the server because
some firewalls are configured to block UDP entirely. These firewalls
block UDP but not TCP, in part because TCP has properties that make
the intention of the nodes being protected by the firewall more
obvious to the firewall. For example, TCP has a three-way handshake
that makes in it clearer that the protected node really wishes to have
that particular connection established, while for UDP UDP, the best the
firewall can do is guess which flows are desired by using filtering
rules. Also, TCP has explicit connection teardown; while for UDP,
the firewall has to use timers to guess when the flow is finished.

TURN supports TLS-over-TCP transport and DTLS-over-UDP transport
between the client and the server because (D)TLS provides additional
security properties not provided by TURN's default digest
authentication;
authentication, properties that some clients may wish to take
advantage of. In particular, (D)TLS provides a way for the client to
ascertain that it is talking to the correct server, server and provides for
confidentiality of TURN control messages. If (D)TLS transport is
used between the TURN client and the TURN server, the cipher suites,
server certificate validation and authentication of TURN server are
discussed in refer to
Section 6.2.3 of [I-D.ietf-tram-stunbis]. [RFC8489] for more information about cipher suites,
server certificate validation, and authentication of TURN servers.
The guidance given in [RFC7525] MUST be followed to avoid attacks on
(D)TLS. TURN does not require (D)TLS because the overhead of using
(D)TLS is higher than that of digest authentication; for example,
using (D)TLS likely means that most application data will be doubly
encrypted (once by (D)TLS and once to ensure it is still encrypted in
the UDP datagram).

There is an extension to TURN for TCP transport between the server
and the peers [RFC6062]. For this reason, allocations that use UDP
between the server and the peers are known as UDP allocations, "UDP allocations",
while allocations that use TCP between the server and the peers are
known as TCP allocations. "TCP allocations". This specification describes only UDP
allocations.

In some applications for TURN, the client may send and receive
packets other than TURN packets on the host transport address it uses
to communicate with the server. This can happen, for example, when
using TURN with ICE. In these cases, the client can distinguish TURN
packets from other packets by examining the source address of the
arriving packet: packet; those arriving from the TURN server will be TURN
packets. The algorithm of demultiplexing packets received from
multiple protocols on the host transport address is discussed in
[RFC7983].

3.2. Allocations

To create an allocation on the server, the client uses an Allocate
transaction. The client sends an Allocate request to the server, and
the server replies with an Allocate success response containing the
allocated relayed transport address. The client can include
attributes in the Allocate request that describe the type of
allocation it desires (e.g., the lifetime of the allocation). Since
relaying data has security implications, the server requires that the
client authenticate itself, typically using STUN's long-term
credential mechanism or the STUN Extension for Third-Party
Authorization [RFC7635], to show that it is authorized to use the
server.

Once a relayed transport address is allocated, a client must keep the
allocation alive. To do this, the client periodically sends a
Refresh request to the server. TURN deliberately uses a different
method (Refresh rather than Allocate) for refreshes to ensure that
the client is informed if the allocation vanishes for some reason.

The frequency of the Refresh transaction is determined by the
lifetime of the allocation. The default lifetime of an allocation is
10 minutes -- minutes; this value was chosen to be long enough so that
refreshing is not typically a burden on the client, client while expiring
allocations where the client has unexpectedly quit in a timely
manner. However, the client can request a longer lifetime in the
Allocate request and may modify its request in a Refresh request, and
the server always indicates the actual lifetime in the response. The
client must issue a new Refresh transaction within "lifetime" seconds
of the previous Allocate or Refresh transaction. Once a client no
longer wishes to use an allocation, it should delete the allocation
using a Refresh request with a requested lifetime of 0. zero.

Both the server and client keep track of a value known as the
5-TUPLE.
"5-tuple". At the client, the 5-tuple consists of the client's host
transport address, the server transport address, and the transport
protocol used by the client to communicate with the server. At the
server, the 5-tuple value is the same except that the client's host
transport address is replaced by the client's server-reflexive
address,
address since that is the client's address as seen by the server.

Both the client and the server remember the 5-tuple used in the
Allocate request. Subsequent messages between the client and the
server use the same 5-tuple. In this way, the client and server know
which allocation is being referred to. If the client wishes to
allocate a second relayed transport address, it must create a second
allocation using a different 5-tuple (e.g., by using a different
client host address or port).

| NOTE: While the terminology used in this document refers to
| 5-tuples, the TURN server can store whatever identifier it
| likes that yields identical results. Specifically, an
| implementation may use a file-descriptor file descriptor in place of a 5-tuple
| to represent a TCP connection.

Figure 2

In Figure 2, the client sends an Allocate request to the server with
invalid or missing credentials. Since the server requires that all
requests be authenticated using STUN's long-term credential
mechanism, the server rejects the request with a 401 (Unauthorized)
error code. The client then tries again, this time including
credentials. This time, the server accepts the Allocate request and
returns an Allocate success response containing (amongst other
things) the relayed transport address assigned to the allocation.
Sometime later, the client decides to refresh the allocation and thus allocation; thus,
it sends a Refresh request to the server. The refresh is accepted
and the server replies with a Refresh success response.

3.3. Permissions

To ease concerns amongst enterprise IT administrators that TURN could
be used to bypass corporate firewall security, TURN includes the
notion of permissions. TURN permissions mimic the address-restricted
filtering mechanism of NATs that comply with [RFC4787].

An allocation can have zero or more permissions. Each permission
consists of an IP address and a lifetime. When the server receives a
UDP datagram on the allocation's relayed transport address, it first
checks the list of permissions. If the source IP address of the
datagram matches a permission, the application data is relayed to the
client, otherwise
client; otherwise, the UDP datagram is silently discarded.

A permission expires after 5 minutes if it is not refreshed, and
there is no way to explicitly delete a permission. This behavior was
selected to match the behavior of a NAT that complies with [RFC4787].

The client can install or refresh a permission using either a
CreatePermission request or a ChannelBind request. Using the
CreatePermission request, multiple permissions can be installed or
refreshed with a single request -- request; this is important for applications
that use ICE. For security reasons, permissions can only be
installed or refreshed by transactions that can be authenticated;
thus, Send indications and ChannelData messages (which are used to
send data to peers) do not install or refresh any permissions.

Note that permissions are within the context of an allocation, so
adding or expiring a permission in one allocation does not affect
other allocations.

3.4. Send Mechanism

There are two mechanisms for the client and peers to exchange
application data using the TURN server. The first mechanism uses the
Send and Data methods, the second mechanism uses channels. Common to
both mechanisms is the ability of the client to communicate with
multiple peers using a single allocated relayed transport address;
thus, both mechanisms include a means for the client to indicate to
the server which peer should receive the data, data and for the server to
indicate to the client which peer sent the data.

The Send mechanism uses Send and Data indications. Send indications
are used to send application data from the client to the server,
while Data indications are used to send application data from the
server to the client.

When using the Send mechanism, the client sends a Send indication to
the TURN server containing (a) an XOR-PEER-ADDRESS attribute
specifying the (server-reflexive) transport address of the peer and
(b) a DATA attribute holding the application data. When the TURN
server receives the Send indication, it extracts the application data
from the DATA attribute and sends it in a UDP datagram to the peer,
using the allocated relay address as the source address. Note that
there is no need to specify the relayed transport address, address since it is
implied by the 5-tuple used for the Send indication.

In the reverse direction, UDP datagrams arriving at the relayed
transport address on the TURN server are converted into Data
indications and sent to the client, with the server-reflexive
transport address of the peer included in an XOR-PEER-ADDRESS
attribute and the data itself in a DATA attribute. Since the relayed
transport address uniquely identified the allocation, the server
knows which client should receive the data.

Some ICMP (Internet Control Message Protocol) packets arriving at the
relayed transport address on the TURN server may be converted into
Data indications and sent to the client, with the transport address
of the peer included in an XOR-PEER-ADDRESS attribute and the ICMP
type and code in a an ICMP attribute. ICMP attribute forwarding always
uses Data indications containing the XOR-PEER-ADDRESS and ICMP
attributes, even when using the channel mechanism to forward UDP
data.

Send and Data indications cannot be authenticated, authenticated since the long-
term long-term
credential mechanism of STUN does not support authenticating
indications. This is not as big an issue as it might first appear, appear
since the client-to-server leg is only half of the total path to the
peer. Applications that want end-to-end security should encrypt the
data sent between the client and a peer.

Because Send indications are not authenticated, it is possible for an
attacker to send bogus Send indications to the server, which will
then relay these to a peer. To partly mitigate this attack, TURN
requires that the client install a permission towards a peer before
sending data to it using a Send indication. The technique to fully
mitigate the attack is discussed in Section 21.1.4.

Figure 3

In Figure 3, the client has already created an allocation and now
wishes to send data to its peers. The client first creates a
permission by sending the server a CreatePermission request
specifying Peer A's (server-reflexive) IP address in the XOR-PEER-
ADDRESS attribute; if this was not done, the server would not relay
data between the client and the server. The client then sends data
to Peer A using a Send indication; at the server, the application
data is extracted and forwarded in a UDP datagram to Peer A, using
the relayed transport address as the source transport address. When
a UDP datagram from Peer A is received at the relayed transport
address, the contents are placed into a Data indication and forwarded
to the client. Later, the client attempts to exchange data with Peer
B; however, no permission has been installed for Peer B, so the Send
indication from the client and the UDP datagram from the peer are
both dropped by the server.

3.5. Channels

For some applications (e.g., Voice over IP), IP (VoIP)), the 36 bytes of
overhead that a Send indication or Data indication adds to the
application data can substantially increase the bandwidth required
between the client and the server. To remedy this, TURN offers a
second way for the client and server to associate data with a
specific peer.

This second way uses an alternate packet format known as the
ChannelData message.
"ChannelData message". The ChannelData message does not use the STUN
header used by other TURN messages, but instead has a 4-byte header
that includes a number known as a channel number. "channel number". Each channel
number in use is bound to a specific peer and thus peer; thus, it serves as a
shorthand for the peer's host transport address.

To bind a channel to a peer, the client sends a ChannelBind request
to the server, server and includes an unbound channel number and the
transport address of the peer. Once the channel is bound, the client
can use a ChannelData message to send the server data destined for
the peer. Similarly, the server can relay data from that peer
towards the client using a ChannelData message.

Channel bindings last for 10 minutes unless refreshed -- refreshed; this lifetime
was chosen to be longer than the permission lifetime. Channel
bindings are refreshed by sending another ChannelBind request
rebinding the channel to the peer. Like permissions (but unlike
allocations), there is no way to explicitly delete a channel binding;
the client must simply wait for it to time out.

TURN TURN Peer Peer
client server A B
| | | |
|-- ChannelBind req ---------------->| --------------->| | |
| (Peer A to 0x4001) | | |
| | | |
|<---------- ChannelBind succ resp --| -| | |
| | | |
|-- (0x4001) data ------------------>| ----------------->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<------------------ (0x4001) data --| -| | |
| | | |
|--- Send ind (Peer A)-------------->| A)------------->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<------------------ (0x4001) data --| -| | |
| | | |

Figure 4

Figure 4 shows the channel mechanism in use. The client has already
created an allocation and now wishes to bind a channel to Peer A. To
do this, the client sends a ChannelBind request to the server,
specifying the transport address of Peer A and a channel number
(0x4001). After that, the client can send application data
encapsulated inside ChannelData messages to Peer A: this is shown as
"(0x4001) data" where 0x4001 is the channel number. When the
ChannelData message arrives at the server, the server transfers the
data to a UDP datagram and sends it to Peer A (which is the peer
bound to channel number 0x4001).

In the reverse direction, when Peer A sends a UDP datagram to the
relayed transport address, this UDP datagram arrives at the server on
the relayed transport address assigned to the allocation. Since the
UDP datagram was received from Peer A, which has a channel number
assigned to it, the server encapsulates the data into a ChannelData
message when sending the data to the client.

Once a channel has been bound, the client is free to intermix
ChannelData messages and Send indications. In the figure, the client
later decides to use a Send indication rather than a ChannelData
message to send additional data to Peer A. The client might decide
to do this, for example, so it can use the DONT-FRAGMENT attribute
(see the next section). However, once a channel is bound, the server
will always use a ChannelData message, as shown in the call flow.

Note that ChannelData messages can only be used for peers to which
the client has bound a channel. In the example above, Peer A has
been bound to a channel, but Peer B has not, so application data to
and from Peer B would use the Send mechanism.

3.6. Unprivileged TURN Servers

This version of TURN is designed so that the server can be
implemented as an application that runs in user space under commonly
available operating systems without requiring special privileges.
This design decision was made to make it easy to deploy a TURN
server: for example, to allow a TURN server to be integrated into a
peer-to-peer application so that one peer can offer NAT traversal
services to another peer and to use (D)TLS to secure the TURN
connection.

This design decision has the following implications for data relayed
by a TURN server:

* The value of the Diffserv field may not be preserved across the
server;

* The Time to Live (TTL) field may be reset, rather than
decremented, across the server;

* The Explicit Congestion Notification (ECN) field may be reset by
the server;

* There is no end-to-end fragmentation, fragmentation since the packet is re-
assembled
reassembled at the server.

Future work may specify alternate TURN semantics that address these
limitations.

3.7. Avoiding IP Fragmentation

For reasons described in [Frag-Harmful], [FRAG-HARMFUL], applications, especially
those sending large volumes of data, should avoid having their
packets fragmented. [I-D.ietf-intarea-frag-fragile] [FRAG-FRAGILE] discusses issues associated with
IP fragmentation and proposes alternatives to IP fragmentation.
Applications using TCP can can, more or less less, ignore this issue because
fragmentation avoidance is now a standard part of TCP, but
applications using UDP (and thus (and, thus, any application using this version
of TURN) need to avoid IP fragmentation by sending sufficiently small
messages or use by using UDP fragmentation
[I-D.ietf-tsvwg-udp-options]. [UDP-OPT]. Note that the UDP
fragmentation option needs to be supported by both endpoints, and at
the time of writing of this document, UDP fragmentation support is
under discussion and is not deployed.

The application running on the client and the peer can take one of
two approaches to avoid IP fragmentation until UDP fragmentation
support is available. The first uses messages that are limited to a
predetermined fixed maximum maximum, and the second relies on network
feedback to adapt that maximum.

The first approach is to avoid sending large amounts of application
data in the TURN messages/UDP datagrams exchanged between the client
and the peer. This is the approach taken by most VoIP (Voice-over-
IP) applications.
In this approach, the application MUST assume a
PMTU Path MTU (PMTU) of
1280 bytes, as bytes because IPv6 requires that every link in the Internet
have has
an MTU of 1280 octets or greater as specified in [RFC8200]. If IPv4
support on legacy or otherwise unusual networks is a consideration,
the application MAY assume an effective MTU of 576 bytes for IPv4
datagrams, as every IPv4 host must be capable of receiving a packet whose
with a length is equal to 576 bytes as discussed in [RFC0791] and
[RFC1122].

The exact amount of application data that can be included while
avoiding fragmentation depends on the details of the TURN session
between the client and the server: whether UDP, TCP, or (D)TLS
transport is used, used; whether ChannelData messages or Send/Data
indications are used, used; and whether any additional attributes (such as
the DONT-FRAGMENT attribute) are included. Another factor, which is
hard to determine, is whether the MTU is reduced somewhere along the
path for other reasons, such as the use of IP-in-IP tunneling.

As a guideline, sending a maximum of 500 bytes of application data in
a single TURN message (by the client on the client-to-server leg) or
a UDP datagram (by the peer on the peer-to-server leg) will generally
avoid IP fragmentation. To further reduce the chance of
fragmentation, it is recommended that the client use ChannelData
messages when transferring significant volumes of data, data since the
overhead of the ChannelData message is less than Send and Data
indications.

The second approach the client and peer can take to avoid
fragmentation is to use a path MTU discovery algorithm to determine
the maximum amount of application data that can be sent without
fragmentation. The classic path MTU discovery algorithm defined in
[RFC1191] may not be able to discover the MTU of the transmission
path between the client and the peer since:

- a

* A probe packet with DF a Don't Fragment (DF) bit in the IPv4 header
set to test a path for a larger MTU can be dropped by routers, or
-

* ICMP error messages can be dropped by middle boxes. middleboxes.

As a result, the client and server need to use a path MTU discovery
algorithm that does not require ICMP messages. The Packetized Path
MTU Discovery algorithm defined in [RFC4821] is one such algorithm.

[I-D.ietf-tram-stun-pmtud] algorithm,
and a set of algorithms is defined in [MTU-DATAGRAM].

[MTU-STUN] is an implementation of [RFC4821] that uses STUN to
discover the path MTU, and MTU; so it might be a suitable approach to be used
in conjunction with a TURN server that supports the DONT-FRAGMENT
attribute. When the client includes the DONT-
FRAGMENT DONT-FRAGMENT attribute in a
Send indication, this tells the server to set the DF bit in the
resulting UDP datagram that it sends to the peer. Since some servers
may be unable to set the DF bit, the client should also include this
attribute in the Allocate request -- request; any server that does not support
the DONT-FRAGMENT attribute will indicate this by rejecting the
Allocate request. If the TURN server carrying out packet translation
from IPv4-to-IPv6 is unable to access the state of the Don't Fragment
(DF) bit in the IPv4 header, it MUST reject the Allocate request with
the DONT-FRAGMENT attribute.

3.8. RTP Support

One of the envisioned uses of TURN is as a relay for clients and
peers wishing to exchange real-time data (e.g., voice or video) using
RTP. To facilitate the use of TURN for this purpose, TURN includes
some special support for older versions of RTP.

Old versions of RTP [RFC3550] required that the RTP stream be on an
even port number and the associated RTP Control Protocol (RTCP)
stream, if present, be on the next highest port. To allow clients to
work with peers that still require this, TURN allows the client to
request that the server allocate a relayed transport address with an
even port number, number and to optionally request the server reserve the
next-highest next-
highest port number for a subsequent allocation.

3.9. Happy Eyeballs for TURN

If an IPv4 path to reach a TURN server is found, but the TURN
server's IPv6 path is not working, a dual-stack TURN client can
experience a significant connection delay compared to an IPv4-only
TURN client. To overcome these connection setup problems, the TURN
client needs to query both A and AAAA records for the TURN server
specified using a domain name and try connecting to the TURN server
using both IPv6 and IPv4 addresses in a fashion similar to the Happy
Eyeballs mechanism defined in [RFC8305]. The TURN client performs
the following steps based on the transport protocol being used to
connect to the TURN server.

* For TCP or TLS-over-TCP, the results of the Happy Eyeballs
procedure [RFC8305] are used by the TURN client for sending its
TURN messages to the server.

* For clear text UDP, send TURN Allocate requests to both IP address
families as discussed in [RFC8305], [RFC8305] without authentication
information. If the TURN server requires authentication, it will
send back a 401 unauthenticated response and response; the TURN client uses will use
the first UDP connection on which a 401 error response is
received. If a 401 error response is received from both IP
address families families, then the TURN client can silently abandon the
UDP connection on the IP address family with lower precedence. If
the TURN server does not require authentication (as described in
Section 9 of [RFC8155]), it is possible for both Allocate requests
to succeed. In this case, the TURN client sends a Refresh with a
LIFETIME value of 0 zero on the allocation using the IP address
family with lower precedence to delete the allocation.

* For DTLS over UDP, initiate a DTLS handshake to both IP address
families as discussed in [RFC8305] [RFC8305], and use the first DTLS session
that is established. If the DTLS session is established on both
IP address families families, then the client sends a DTLS close_notify
alert to terminate the DTLS session using the IP address family
with lower precedence. If the TURN over DTLS server has been
configured to require a cookie exchange (Section 4.2 in of [RFC6347])
and a HelloVerifyRequest is received from the TURN servers on both
IP address families families, then the client can silently abandon the
connection on the IP address family with lower precedence.

4. Discovery of TURN server Server

Methods of TURN server discovery, including using anycast, are
described in [RFC8155]. If a host with multiple interfaces discovers
a TURN server in each interface, the mechanism described in [RFC7982]
can be used by the TURN client to influence the TURN server
selection. The syntax of the "turn" and "turns" URIs are defined in
Section 3.1 of [RFC7065]. DTLS as a transport protocol for TURN is
defined in [RFC7350].

4.1. TURN URI Scheme Semantics

The "turn" and "turns" URI schemes are used to designate a TURN
server (also known as a relay) "relay") on Internet hosts accessible using
the TURN protocol. The TURN protocol supports sending messages over
UDP, TCP, TLS-over-TCP TLS-over-TCP, or DTLS-over-UDP. The "turns" URI scheme
MUST be used when TURN is run over TLS-over-TCP or in DTLS-over-UDP,
and the "turn" scheme MUST be used otherwise. The required <host>
part of the "turn" URI denotes the TURN server host. The <port>
part, if present, denotes the port on which the TURN server is
awaiting connection requests. If it is absent, the default port is
3478 for both UDP and TCP. The default port for TURN over TLS and
TURN over DTLS is 5349.

5. General Behavior

This section contains general TURN processing rules that apply to all
TURN messages.

TURN is an extension to STUN. All TURN messages, with the exception
of the ChannelData message, are STUN-formatted messages. All the
base processing rules described in [I-D.ietf-tram-stunbis] [RFC8489] apply to STUN-formatted
messages. This means that all the message-forming and
message-processing message-
processing descriptions in this document are implicitly prefixed with
the rules of [I-D.ietf-tram-stunbis].

[I-D.ietf-tram-stunbis] [RFC8489].

[RFC8489] specifies an authentication mechanism called the long-term "long-term
credential mechanism. mechanism". TURN servers and clients MUST implement this
mechanism, and the authentication options are discussed in
Section 7.2.

Note that the long-term credential mechanism applies only to requests
and cannot be used to authenticate indications; thus, indications in
TURN are never authenticated. If the server requires requests to be
authenticated, then the server's administrator MUST choose a realm
value that will uniquely identify the username and password
combination that the client must use, even if the client uses
multiple servers under different administrations. The server's
administrator MAY choose to allocate a unique username to each
client, or it MAY choose to allocate the same username to more than
one client (for example, to all clients from the same department or
company). For each Allocate request, the server SHOULD generate a
new random nonce when the allocation is first attempted following the
randomness recommendations in [RFC4086] and SHOULD expire the nonce
at least once every hour during the lifetime of the allocation. The
server uses the mechanism described in section Section 9.2 of
[I-D.ietf-tram-stunbis] [RFC8489] to
indicate that it supports
[I-D.ietf-tram-stunbis]. [RFC8489].

All requests after the initial Allocate must use the same username as
that used to create the allocation, allocation to prevent attackers from
hijacking the client's allocation.

Specifically, if if:

* the server requires the use of the long-term credential mechanism, and if
and;

* a non-
Allocate non-Allocate request passes authentication under this mechanism, and if
and;

* the 5-tuple identifies an existing allocation, but but;

* the request does not use the same username as used to create the
allocation,

then the request MUST be rejected with a 441 (Wrong Credentials)
error.

When a TURN message arrives at the server from the client, the server
uses the 5-tuple in the message to identify the associated
allocation. For all TURN messages (including ChannelData) EXCEPT an
Allocate request, if the 5-tuple does not identify an existing
allocation, then the message MUST either be rejected with a 437
Allocation Mismatch error (if it is a request) or be silently ignored
(if it is an indication or a ChannelData message). A client
receiving a 437 error response to a request other than Allocate MUST
assume the allocation no longer exists.

[I-D.ietf-tram-stunbis]

[RFC8489] defines a number of attributes, including the SOFTWARE and
FINGERPRINT attributes. The client SHOULD include the SOFTWARE
attribute in all Allocate and Refresh requests and MAY include it in
any other requests or indications. The server SHOULD include the
SOFTWARE attribute in all Allocate and Refresh responses (either
success or failure) and MAY include it in other responses or
indications. The client and the server MAY include the FINGERPRINT
attribute in any STUN-formatted messages defined in this document.

TURN does not use the backwards-compatibility mechanism described in
[I-D.ietf-tram-stunbis].
[RFC8489].

TURN, as defined in this specification, supports both IPv4 and IPv6.
IPv6 support in TURN includes IPv4-to-IPv6, IPv6-to-IPv6, and IPv6-
to-IPv4 relaying. When only a single address type is desired, the
REQUESTED-ADDRESS-FAMILY attribute is used to explicitly request the
address type the TURN server will allocate (e.g., an IPv4-only node
may request the TURN server to allocate an IPv6 address). If both
IPv4 and IPv6 are desired, the single ADDITIONAL-ADDRESS-FAMILY
attribute indicates a request to the server to allocate one IPv4 and
one IPv6 relay address in a single Allocate request. This saves
local ports on the client and reduces the number of messages sent
between the client and the TURN server.

By default, TURN runs on the same ports as STUN: 3478 for TURN over
UDP and TCP, and 5349 for TURN over (D)TLS. However, TURN has its
own set of Service Record (SRV) names: "turn" for UDP and TCP, and
"turns" for (D)TLS. Either the DNS resolution procedures or the
ALTERNATE-SERVER procedures, both described in Section 7, can be used
to run TURN on a different port.

To ensure interoperability, a TURN server MUST support the use of UDP
transport between the client and the server, and it SHOULD support
the use of TCP, TLS-over-TCP TLS-over-TCP, and DTLS-over-UDP transports.

When UDP or DTLS-over-UDP transport is used between the client and
the server, the client will retransmit a request if it does not
receive a response within a certain timeout period. Because of this,
the server may receive two (or more) requests with the same 5-tuple
and same transaction id. STUN requires that the server recognize
this case and treat the request as idempotent (see
[I-D.ietf-tram-stunbis]). [RFC8489]). Some
implementations may choose to meet this requirement by remembering
all received requests and the corresponding responses for 40 seconds
(Section 6.3.1 in
[I-D.ietf-tram-stunbis]). of [RFC8489]). Other implementations may choose to
reprocess the request and arrange that such reprocessing returns
essentially the same response. To aid implementors who choose the
latter approach (the so-called "stateless stack approach"), this
specification includes some implementation notes on how this might be
done. Implementations are free to choose either approach or choose some
other approach that gives the same results.

To mitigate either intentional or unintentional denial-of-service
attacks against the server by clients with valid usernames and
passwords, it is RECOMMENDED that the server impose limits on both
the number of allocations active at one time for a given username and
on the amount of bandwidth those allocations can use. The server
should reject new allocations that would exceed the limit on the
allowed number of allocations active at one time with a 486
(Allocation Quota Exceeded) (see Section 7.2), and since UDP does not
include a congestion control mechanism, it should discard application
data traffic that exceeds the bandwidth quota.

6. Allocations

All TURN operations revolve around allocations, and all TURN messages
are associated with either a single or dual allocation. An
allocation conceptually consists of the following state data:

* the relayed transport address or addresses;

* the 5-tuple: (client's IP address, client's port, server IP
address, server port, and transport protocol);

* the authentication information;

* the time-to-expiry for each relayed transport address;

* a list of permissions for each relayed transport address;

* a list of channel to peer channel-to-peer bindings for each relayed transport
address.

The relayed transport address is the transport address allocated by
the server for communicating with peers, while the 5-tuple describes
the communication path between the client and the server. On the
client, the 5-tuple uses the client's host transport address; on the
server, the 5-tuple uses the client's server-reflexive transport
address. The relayed transport address MUST be unique across all
allocations so it can be used to uniquely identify the allocation,
and an allocation in this context can be either a single or dual
allocation.

The authentication information (e.g., username, password, realm, and
nonce) is used to both verify subsequent requests and to compute the
message integrity of responses. The username, realm, and nonce
values are initially those used in the authenticated Allocate request
that creates the allocation, though the server can change the nonce
value during the lifetime of the allocation using a 438 (Stale Nonce)
reply. For security reasons, the server MUST NOT store the password
explicitly and MUST store the key value, which is a cryptographic
hash over the username, realm, and password (see Section 16.1.3 in
[I-D.ietf-tram-stunbis]). of
[RFC8489]).

Note that if the response contains a PASSWORD-ALGORITHMS attribute
and this attribute contains both MD5 and SHA-256 algorithms, and the
client also supports both the algorithms, the request MUST contain a
PASSWORD-ALGORITHM attribute with the SHA-256 algorithm.

The time-to-expiry is the time in seconds left until the allocation
expires. Each Allocate or Refresh transaction sets this timer, which
then ticks down towards 0. zero. By default, each Allocate or Refresh
transaction resets this timer to the default lifetime value of 600
seconds (10 minutes), but the client can request a different value in
the Allocate and Refresh request. Allocations can only be refreshed
using the Refresh request; sending data to a peer does not refresh an
allocation. When an allocation expires, the state data associated
with the allocation can be freed.

The list of permissions is described in Section 9 and the list of
channels is described in Section 12.

7. Creating an Allocation

An allocation on the server is created using an Allocate transaction.

7.1. Sending an Allocate Request

The client forms an Allocate request as follows.

The client first picks a host transport address. It is RECOMMENDED
that the client picks pick a currently unused transport address, typically
by allowing the underlying OS to pick a currently unused port.

The client then picks a transport protocol that the client supports
to use between the client and the server based on the transport
protocols supported by the server. Since this specification only
allows UDP between the server and the peers, it is RECOMMENDED that
the client pick UDP unless it has a reason to use a different
transport. One reason to pick a different transport would be that
the client believes, either through configuration or discovery or by
experiment, that it is unable to contact any TURN server using UDP.
See Section 3.1 for more discussion.

The client also picks a server transport address, which SHOULD be
done as follows. The client uses one or more procedures described in
[RFC8155] to discover a TURN server and uses the TURN server
resolution mechanism defined in [RFC5928] and [RFC7350] to get a list
of server transport addresses that can be tried to create a TURN
allocation.

The client MUST include a REQUESTED-TRANSPORT attribute in the
request. This attribute specifies the transport protocol between the
server and the peers (note that this is NOT *not* the transport protocol
that appears in the 5-tuple). In this specification, the REQUESTED-
TRANSPORT type is always UDP. This attribute is included to allow
future extensions to specify other protocols.

If the client wishes to obtain a relayed transport address of a
specific address type type, then it includes a REQUESTED-ADDRESS-FAMILY
attribute in the request. This attribute indicates the specific
address type the client wishes the TURN server to allocate. Clients
MUST NOT include more than one REQUESTED-ADDRESS-FAMILY attribute in
an Allocate request. Clients MUST NOT include a REQUESTED-ADDRESS-
FAMILY attribute in an Allocate request that contains a RESERVATION-
TOKEN attribute, for the reason that the server uses the previously
reserved transport address corresponding to the included token and
the client cannot obtain a relayed transport address of a specific
address type.

If the client wishes to obtain one IPv6 and one IPv4 relayed
transport address address, then it includes an ADDITIONAL-ADDRESS-FAMILY
attribute in the request. This attribute specifies that the server
must allocate both address types. The attribute value in the
ADDITIONAL-ADDRESS-FAMILY MUST be set to 0x02 (IPv6 address family).
Clients MUST NOT include REQUESTED-ADDRESS-FAMILY and ADDITIONAL-
ADDRESS-FAMILY attributes in the same request. Clients MUST NOT
include the ADDITIONAL-ADDRESS-FAMILY attribute in a an Allocate
request that contains a RESERVATION-TOKEN attribute. Clients MUST
NOT include the ADDITIONAL-ADDRESS-FAMILY attribute in a an Allocate
request that contains an EVEN-PORT attribute with the R (Reserved)
bit set to 1. The reason behind the restriction is that if EVEN-PORT the EVEN-
PORT attribute with the R bit set to 1 is allowed with the
ADDITIONAL-ADDRESS-FAMILY attribute, two tokens will have to be
returned in the success response and requires changes will be required to the
way the RESERVATION-TOKEN attribute is handled.

If the client wishes the server to initialize the time-to-expiry
field of the allocation to some value other than the default
lifetime, then it MAY include a LIFETIME attribute specifying its
desired value. This is just a hint, and the server may elect to use
a different value. Note that the server will ignore requests to
initialize the field to less than the default value.

If the client wishes to later use the DONT-FRAGMENT attribute in one
or more Send indications on this allocation, then the client SHOULD
include the DONT-FRAGMENT attribute in the Allocate request. This
allows the client to test whether this attribute is supported by the
server.

If the client requires the port number of the relayed transport
address to be even, the client includes the EVEN-PORT attribute. If
this attribute is not included, then the port can be even or odd. By
setting the R bit in the EVEN-PORT attribute to 1, the client can
request that the server reserve the next highest port number (on the
same IP address) for a subsequent allocation. If the R bit is 0, no
such request is made.

The client MAY also include a RESERVATION-TOKEN attribute in the
request to ask the server to use a previously reserved port for the
allocation. If the RESERVATION-TOKEN attribute is included, then the
client MUST omit the EVEN-PORT attribute.

Once constructed, the client sends the Allocate request on the
5-tuple.

7.2. Receiving an Allocate Request

When the server receives an Allocate request, it performs the
following checks:

1. The TURN server provided by the local or access network MAY
allow an unauthenticated request in order to accept Allocation
requests from new and/or guest users in the network who do not
necessarily possess long term long-term credentials for STUN
authentication. Making The security implications of STUN and making
STUN authentication optional and its
security implications are discussed in [RFC8155].
Otherwise, the server MUST require that the request be
authenticated. If the request is authenticated, the
authentication MUST be done either using the long-term
credential mechanism of
[I-D.ietf-tram-stunbis] [RFC8489] or using the STUN Extension
for Third-Party Authorization [RFC7635] unless the client and
server agree to use another mechanism through some procedure
outside the scope of this document.

2. The server checks if the 5-tuple is currently in use by an
existing allocation. If yes, the server rejects the request
with a 437 (Allocation Mismatch) error.

3. The server checks if the request contains a REQUESTED-TRANSPORT
attribute. If the REQUESTED-TRANSPORT attribute is not included
or is malformed, the server rejects the request with a 400 (Bad
Request) error. Otherwise, if the attribute is included but
specifies a protocol that is not supported by the server, the
server rejects the request with a 442 (Unsupported Transport
Protocol) error.

4. The request may contain a DONT-FRAGMENT attribute. If it does,
but the server does not support sending UDP datagrams with the
DF bit set to 1 (see Section Sections 14 and Section 15), then the server treats
the DONT-FRAGMENT attribute in the Allocate request as an
unknown comprehension-required attribute.

5. The server checks if the request contains a RESERVATION-TOKEN
attribute. If yes, and the request also contains an EVEN-PORT
or REQUESTED-ADDRESS-FAMILY or ADDITIONAL-ADDRESS-FAMILY
attribute, the server rejects the request with a 400 (Bad
Request) error. Otherwise, it checks to see if the token is
valid (i.e., the token is in range and has not expired expired, and the
corresponding relayed transport address is still available). If
the token is not valid for some reason, the server rejects the
request with a 508 (Insufficient Capacity) error.

6. The server checks if the request contains both REQUESTED-
ADDRESS-FAMILY and ADDITIONAL-ADDRESS-FAMILY attributes. If
yes, then the server rejects the request with a 400 (Bad
Request) error.

7. If the server does not support the address family requested by
the client in REQUESTED-ADDRESS-FAMILY REQUESTED-ADDRESS-FAMILY, or if the allocation of
the requested address family is disabled by local policy, it
MUST generate an Allocate error response, and it MUST include an
ERROR-CODE attribute with the 440 (Address Family not Supported)
response code. If the REQUESTED-ADDRESS-FAMILY attribute is
absent and the server does not support the IPv4 address family,
the server MUST include an ERROR-CODE attribute with the 440
(Address Family not Supported) response code. If the REQUESTED-
ADDRESS-FAMILY attribute is absent and the server supports the
IPv4 address family, the server MUST allocate an IPv4 relayed
transport address for the TURN client.

8. The server checks if the request contains an EVEN-PORT attribute
with the R bit set to 1. If yes, and the request also contains
an ADDITIONAL-ADDRESS-FAMILY attribute, the server rejects the
request with a 400 (Bad Request) error. Otherwise, the server
checks if it can satisfy the request (i.e., can allocate a
relayed transport address as described below). If the server
cannot satisfy the request, then the server rejects the request
with a 508 (Insufficient Capacity) error.

9. The server checks if the request contains an ADDITIONAL-ADDRESS-
FAMILY attribute. If yes, and the attribute value is 0x01 (IPv4
address family), then the server rejects the request with a 400
(Bad Request) error. Otherwise, the server checks if it can
allocate relayed transport addresses of both address types. If
the server cannot satisfy the request, then the server rejects
the request with a 508 (Insufficient Capacity) error. If the
server can partially meet the request, i.e. i.e., if it can only
allocate one relayed transport address of a specific address
type, then it includes ADDRESS-ERROR-CODE attribute in the
success response to inform the client the reason for partial
failure of the request. The error code value signaled in the
ADDRESS-ERROR-CODE attribute could be 440 (Address Family not
Supported) or 508 (Insufficient Capacity). If the server can
fully meet the request, then the server allocates one IPv4 and
one IPv6 relay address, address and returns an Allocate success response
containing the relayed transport addresses assigned to the dual
allocation in two XOR-RELAYED-ADDRESS attributes.

10. At any point, the server MAY choose to reject the request with a
486 (Allocation Quota Reached) error if it feels the client is
trying to exceed some locally defined allocation quota. The
server is free to define this allocation quota any way it
wishes, but it SHOULD define it based on the username used to
authenticate the request, request and not on the client's transport
address.

11. Also Also, at any point, the server MAY choose to reject the request
with a 300 (Try Alternate) error if it wishes to redirect the
client to a different server. The use of this error code and
attribute follow follows the specification in [I-D.ietf-tram-stunbis]. [RFC8489].

If all the checks pass, the server creates the allocation. The
5-tuple is set to the 5-tuple from the Allocate request, while the
list of permissions and the list of channels are initially empty.

The server chooses a relayed transport address for the allocation as
follows:

* If the request contains a RESERVATION-TOKEN attribute, the server
uses the previously reserved transport address corresponding to
the included token (if it is still available). Note that the
reservation is a server-wide reservation and is not specific to a
particular allocation, allocation since the Allocate request containing the
RESERVATION-TOKEN uses a different 5-tuple than the Allocate
request that made the reservation. The 5-tuple for the Allocate
request containing the RESERVATION-TOKEN attribute can be any
allowed 5-tuple; it can use a different client IP address and
port, a different transport protocol, and even a different server
IP address and port (provided, of course, that the server IP
address and port are ones on which the server is listening for
TURN requests).

* If the request contains an EVEN-PORT attribute with the R bit set
to 0, then the server allocates a relayed transport address with
an even port number.

* If the request contains an EVEN-PORT attribute with the R bit set
to 1, then the server looks for a pair of port numbers N and N+1
on the same IP address, where N is even. Port N is used in the
current allocation, while the relayed transport address with port
N+1 is assigned a token and reserved for a future allocation. The
server MUST hold this reservation for at least 30 seconds, seconds and MAY
choose to hold longer (e.g., until the allocation with port N
expires). The server then includes the token in a RESERVATION-
TOKEN attribute in the success response.

* Otherwise, the server allocates any available relayed transport
address.

In all cases, the server SHOULD only allocate ports from the range
49152 - 65535 (the Dynamic and/or Private Port range [Port-Numbers]), [PORT-NUMBERS]),
unless the TURN server application knows, through some means not
specified here, that other applications running on the same host as
the TURN server application will not be impacted by allocating ports
outside this range. This condition can often be satisfied by running
the TURN server application on a dedicated machine and/or by
arranging that any other applications on the machine allocate ports
before the TURN server application starts. In any case, the TURN
server SHOULD NOT allocate ports in the range 0 - 1023 (the Well-
Known Port range) to discourage clients from using TURN to run
standard services.

| NOTE: The use of randomized port assignments to avoid certain
| types of attacks is described in [RFC6056]. It is RECOMMENDED
| that a TURN server implement a randomized port assignment
| algorithm from [RFC6056]. This is especially applicable to
| servers that choose to pre-allocate a number of ports from the
| underlying OS and then later assign them to allocations; for
| example, a server may choose this technique to implement the EVEN-
PORT
| EVEN-PORT attribute.

The server determines the initial value of the time-to-expiry field
as follows. If the request contains a LIFETIME attribute, then the
server computes the minimum of the client's proposed lifetime and the
server's maximum allowed lifetime. If this computed value is greater
than the default lifetime, then the server uses the computed lifetime
as the initial value of the time-to-expiry field. Otherwise, the
server uses the default lifetime. It is RECOMMENDED that the server
use a maximum allowed lifetime value of no more than 3600 seconds (1
hour). Servers that implement allocation quotas or charge users for
allocations in some way may wish to use a smaller maximum allowed
lifetime (perhaps as small as the default lifetime) to more quickly
remove orphaned allocations (that is, allocations where the
corresponding client has crashed or terminated terminated, or the client
connection has been lost for some reason). Also, note that the time-
to-expiry is recomputed with each successful Refresh request, and
thus
thus, the value computed here applies only until the first refresh.

Once the allocation is created, the server replies with a success
response. The success response contains:

* An XOR-RELAYED-ADDRESS attribute containing the relayed transport
address or two XOR-RELAYED-ADDRESS attributes containing the
relayed transport addresses.

* A LIFETIME attribute containing the current value of the time-to-
expiry timer.

* A RESERVATION-TOKEN attribute (if a second relayed transport
address was reserved).

* An XOR-MAPPED-ADDRESS attribute containing the client's IP address
and port (from the 5-tuple).

| NOTE: The XOR-MAPPED-ADDRESS attribute is included in the
| response as a convenience to the client. TURN itself does not
| make use of this value, but clients running ICE can often need
| this value and can thus avoid having to do an extra Binding
| transaction with some STUN server to learn it.

The response (either success or error) is sent back to the client on
the 5-tuple.

| NOTE: When the Allocate request is sent over UDP,
[I-D.ietf-tram-stunbis] [RFC8489]
| requires that the server handle the possible retransmissions of
| the request so that retransmissions do not cause multiple
| allocations to be created. Implementations may achieve this
| using the so-called "stateless stack approach" as follows. To
| detect retransmissions when the original request was successful
| in creating an allocation, the server can store the transaction
| id that created the request with the allocation data and
| compare it with incoming Allocate requests on the same 5-tuple.
| Once such a request is detected, the server can stop parsing
| the request and immediately generate a success response. When
| building this response, the value of the LIFETIME attribute can
| be taken from the time-to-expiry field in the allocate state
| data, even though this value may differ slightly from the
| LIFETIME value originally returned. In addition, the server
| may need to store an indication of any reservation token
| returned in the original response, response so that this may be returned
| in any retransmitted responses.
|
| For the case where the original request was unsuccessful in
| creating an allocation, the server may choose to do nothing
| special. Note, however, that there is a rare case where the
| server rejects the original request but accepts the
| retransmitted request (because conditions have changed in the
| brief intervening time period). If the client receives the
| first failure response, it will ignore the second (success)
| response and believe that an allocation was not created. An
| allocation created in this matter manner will eventually timeout, time out
| since the client will not refresh it. Furthermore, if the
| client later retries with the same 5-tuple but a different
| transaction id, it will receive a 437 (Allocation
Mismatch), Mismatch)
| error response, which will cause it to retry with a different
| 5-tuple. The server may use a smaller maximum lifetime value
| to minimize the lifetime of allocations "orphaned" in this
| manner.

7.3. Receiving an Allocate Success Response

If the client receives an Allocate success response, then it MUST
check that the mapped address and the relayed transport address or
addresses are part of an address family or families that the client
understands and is prepared to handle. If these addresses are not
part of an address family or families which that the client is prepared to
handle, then the client MUST delete the allocation (Section 8) and
MUST NOT attempt to create another allocation on that server until it
believes the mismatch has been fixed.

Otherwise, the client creates its own copy of the allocation data
structure to track what is happening on the server. In particular,
the client needs to remember the actual lifetime received back from
the server, rather than the value sent to the server in the request.
The client must also remember the 5-tuple used for the request and
the username and password it used to authenticate the request to
ensure that it reuses them for subsequent messages. The client also
needs to track the channels and permissions it establishes on the
server.

If the client receives an Allocate success response but with ADDRESS-
ERROR-CODE an
ADDRESS-ERROR-CODE attribute in the response and the error code value
signaled in the ADDRESS-ERROR-CODE attribute is 440 (Address Family
not Supported), the client MUST NOT retry its request for the
rejected address type. If the client receives an ADDRESS-ERROR-CODE
attribute in the response and the error code value signaled in the
ADDRESS-ERROR-CODE attribute is 508 (Insufficient Capacity), the
client SHOULD wait at least 1 minute before trying to request any
more allocations on this server for the rejected address type.

The client will probably wish to send the relayed transport address
to peers (using some method not specified here) so the peers can
communicate with it. The client may also wish to use the server-
reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in
its ICE processing.

7.4. Receiving an Allocate Error Response

If the client receives an Allocate error response, then the
processing depends on the actual error code returned:

408 (Request timed out):
There is either a problem with the server, server or a problem reaching
the server with the chosen transport. The client considers the
current transaction as having failed but MAY choose to retry the
Allocate request using a different transport (e.g., TCP instead of
UDP).

300 (Try Alternate):
The server would like the client to use the server specified in
the ALTERNATE-SERVER attribute instead. The client considers the
current transaction as having failed, but it SHOULD try the
Allocate request with the alternate server before trying any other
servers (e.g., other servers discovered using the DNS resolution
procedures). When trying the Allocate request with the alternate
server, the client follows the ALTERNATE-SERVER procedures
specified in [I-D.ietf-tram-stunbis].

o [RFC8489].

400 (Bad Request):
The server believes the client's request is malformed for some
reason. The client considers the current transaction as having
failed. The client MAY notify the user or operator and SHOULD NOT
retry the request with this server until it believes the problem
has been fixed.

401 (Unauthorized):
If the client has followed the procedures of the long-term
credential mechanism and still gets this error, then the server is
not accepting the client's credentials. In this case, the client
considers the current transaction as having failed and SHOULD
notify the user or operator. The client SHOULD NOT send any
further requests to this server until it believes the problem has
been fixed.

403 (Forbidden):
The request is valid, but the server is refusing to perform it,
likely due to administrative restrictions. The client considers
the current transaction as having failed. The client MAY notify
the user or operator and SHOULD NOT retry the same request with
this server until it believes the problem has been fixed.

420 (Unknown Attribute):
If the client included a DONT-FRAGMENT attribute in the request
and the server rejected the request with a 420 error code and
listed the DONT-FRAGMENT attribute in the UNKNOWN-ATTRIBUTES
attribute in the error response, then the client now knows that
the server does not support the DONT-
FRAGMENT DONT-FRAGMENT attribute. The
client considers the current transaction as having failed but MAY
choose to retry the Allocate request without the DONT-FRAGMENT
attribute.

437 (Allocation Mismatch):
This indicates that the client has picked a 5-tuple that the
server sees as already in use. One way this could happen is if an
intervening NAT assigned a mapped transport address that was used
by another client that recently crashed. The client considers the
current transaction as having failed. The client SHOULD pick
another client transport address and retry the Allocate request
(using a different transaction id). The client SHOULD try three
different client transport addresses before giving up on this
server. Once the client gives up on the server, it SHOULD NOT try
to create another allocation on the server for 2 minutes.

438 (Stale Nonce):
See the procedures for the long-term credential mechanism [I-D.ietf-tram-stunbis].

o
[RFC8489].

440 (Address Family not Supported):
The server does not support the address family requested by the
client. If the client receives an Allocate error response with
the 440 (Unsupported
Address Family) (Address Family not Supported) error code, the client MUST
NOT retry the request.

441 (Wrong Credentials):
The client should not receive this error in response to a an
Allocate request. The client MAY notify the user or operator and
SHOULD NOT retry the same request with this server until it
believes the problem has been fixed.

442 (Unsupported Transport Address):
The client should not receive this error in response to a request
for a UDP allocation. The client MAY notify the user or operator
and SHOULD NOT reattempt the request with this server until it
believes the problem has been fixed.

486 (Allocation Quota Reached):
The server is currently unable to create any more allocations with
this username. The client considers the current transaction as
having failed. The client SHOULD wait at least 1 minute before
trying to create any more allocations on the server.

508 (Insufficient Capacity):
The server has no more relayed transport addresses available, available or
has none with the requested properties, or the one that was
reserved is no longer available. The client considers the current
operation as having failed. If the client is using either the
EVEN-PORT or the RESERVATION-TOKEN attribute, then the client MAY
choose to remove or modify this attribute and try again
immediately. Otherwise, the client SHOULD wait at least 1 minute
before trying to create any more allocations on this server.

Note that the error code values 486 and 508 indicate to a
eavesdropper that several other users are using the server at this
time, similar to that of the HTTP error response code 503, but it
does not reveal any information about the users using the TURN
server.

An unknown error response MUST be handled as described in
[I-D.ietf-tram-stunbis]. [RFC8489].

8. Refreshing an Allocation

A Refresh transaction can be used to either (a) refresh an existing
allocation and update its time-to-expiry or (b) delete an existing
allocation.

If a client wishes to continue using an allocation, then the client
MUST refresh it before it expires. It is suggested that the client
refresh the allocation roughly 1 minute before it expires. If a
client no longer wishes to use an allocation, then it SHOULD
explicitly delete the allocation. A client MAY refresh an allocation
at any time for other reasons.

8.1. Sending a Refresh Request

If the client wishes to immediately delete an existing allocation, it
includes a LIFETIME attribute with a value of 0. zero. All other forms
of the request refresh the allocation.

When refreshing a dual allocation, the client includes a REQUESTED-
ADDRESS-FAMILY attribute indicating the address family type that
should be refreshed. If no REQUESTED-ADDRESS-FAMILY attribute is included
included, then the request should be treated as applying to all
current allocations. The client MUST only include a family type it
previously allocated and has not yet deleted. This process can also
be used to delete an allocation of a specific address type, type by setting
the lifetime of that refresh Refresh request to 0. zero. Deleting a single
allocation destroys any permissions or channels associated with that
particular allocation; it MUST NOT affect any permissions or channels
associated with allocations for the other address family.

The Refresh transaction updates the time-to-expiry timer of an
allocation. If the client wishes the server to set the time-to-
expiry timer to something other than the default lifetime, it
includes a LIFETIME attribute with the requested value. The server
then computes a new time-to-expiry value in the same way as it does
for an Allocate transaction, with the exception that a requested
lifetime of 0 zero causes the server to immediately delete the
allocation.

8.2. Receiving a Refresh Request

When the server receives a Refresh request, it processes the request
as per Section 5 plus the specific rules mentioned here.

If the server receives a Refresh Request with a REQUESTED-ADDRESS-
FAMILY attribute and the attribute value does not match the address
family of the allocation, the server MUST reply with a 443 (Peer
Address Family Mismatch) Refresh error response.

The server computes a value called the "desired lifetime" as follows:
if the request contains a LIFETIME attribute and the attribute value
is 0, zero, then the "desired lifetime" is 0. zero. Otherwise, if the
request contains a LIFETIME attribute, then the server computes the
minimum of the client's requested lifetime and the server's maximum
allowed lifetime. If this computed value is greater than the default
lifetime, then the "desired lifetime" is the computed value.
Otherwise, the "desired lifetime" is the default lifetime.

Subsequent processing depends on the "desired lifetime" value:

* If the "desired lifetime" is 0, zero, then the request succeeds and
the allocation is deleted.

* If the "desired lifetime" is non-zero, then the request succeeds
and the allocation's time-to-expiry is set to the "desired
lifetime".

If the request succeeds, then the server sends a success response
containing:

* A LIFETIME attribute containing the current value of the time-to-
expiry timer.

| NOTE: A server need not do anything special to implement
| idempotency of Refresh requests over UDP using the "stateless
| stack approach". Retransmitted Refresh requests with a non-zero non-
| zero "desired lifetime" will simply refresh the allocation. A
| retransmitted Refresh request with a zero "desired lifetime"
| will cause a 437 (Allocation Mismatch) response if the
| allocation has already been deleted, but the client will treat
| this as equivalent to a success response (see below).

8.3. Receiving a Refresh Response

If the client receives a success response to its Refresh request with
a non-zero lifetime, it updates its copy of the allocation data
structure with the time-to-expiry value contained in the response.
If the client receives a 437 (Allocation Mismatch) error response to
its request to refresh the allocation, it should consider the
allocation no longer exists. If the client receives a 438 (Stale
Nonce) error to its request to refresh the allocation, it should
reattempt the request with the new nonce value.

If the client receives a 437 (Allocation Mismatch) error response to
a request to delete the allocation, then the allocation no longer
exists and it should consider its request as having effectively
succeeded.

9. Permissions

For each allocation, the server keeps a list of zero or more
permissions. Each permission consists of an IP address and an
associated time-to-expiry. While a permission exists, all peers
using the IP address in the permission are allowed to send data to
the client. The time-to-expiry is the number of seconds until the
permission expires. Within the context of an allocation, a
permission is uniquely identified by its associated IP address.

By sending either CreatePermission requests or ChannelBind requests,
the client can cause the server to install or refresh a permission
for a given IP address. This causes one of two things to happen:

* If no permission for that IP address exists, then a permission is
created with the given IP address and a time-to-expiry equal to
Permission Lifetime.

* If a permission for that IP address already exists, then the time-
to-expiry for that permission is reset to Permission Lifetime.

The Permission Lifetime MUST be 300 seconds (= 5 minutes).

Each permission's time-to-expiry decreases down once per second until
it reaches 0; zero, at which point, the permission expires and is
deleted.

CreatePermission and ChannelBind requests may be freely intermixed on
a permission. A given permission may be initially installed and/or
refreshed with a CreatePermission request, request and then later refreshed
with a ChannelBind request, or vice versa.

When a UDP datagram arrives at the relayed transport address for the
allocation, the server extracts the source IP address from the IP
header. The server then compares this address with the IP address
associated with each permission in the list of permissions for the
allocation. Note that only addresses are compared and port numbers
are not considered. If no match is found, relaying is not permitted, permitted
and the server silently discards the UDP datagram. If an exact match
is found, the permission check is considered to have succeeded and
the server continues to process the UDP datagram as specified
elsewhere (Section 11.3).

The permissions for one allocation are totally unrelated to the
permissions for a different allocation. If an allocation expires,
all its permissions expire with it.

| NOTE: Though TURN permissions expire after 5 minutes, many NATs
| deployed at the time of publication expire their UDP bindings
| considerably faster. Thus, an application using TURN will
| probably wish to send some sort of keep-alive traffic at a much
| faster rate. Applications using ICE should follow the keep-alive keep-
| alive guidelines of ICE [RFC8445], and applications not using
| ICE are advised to do something similar.

10. CreatePermission

TURN supports two ways for the client to install or refresh
permissions on the server. This section describes one way: the
CreatePermission request.

A CreatePermission request may be used in conjunction with either the
Send mechanism in Section 11 or the Channel mechanism in Section 12.

10.1. Forming a CreatePermission Request

The client who wishes to install or refresh one or more permissions
can send a CreatePermission request to the server.

When forming a CreatePermission request, the client MUST include at
least one XOR-PEER-ADDRESS attribute, attribute and MAY include more than one
such attribute. The IP address portion of each XOR-PEER-ADDRESS
attribute contains the IP address for which a permission should be
installed or refreshed. The port portion of each XOR-PEER-ADDRESS
attribute will be ignored and can be any arbitrary value. The
various XOR-PEER-ADDRESS attributes MAY appear in any order. The
client MUST only include XOR-PEER-ADDRESS attributes with addresses
of the same address family as that of the relayed transport address
for the allocation. For dual allocations obtained using the
ADDITIONAL-ADDRESS-FAMILY attribute, the client MAY include XOR-PEER-
ADDRESS attributes with addresses of IPv4 and IPv6 address families.

10.2. Receiving a CreatePermission Request

When the server receives the CreatePermission request, it processes
as per Section 5 plus the specific rules mentioned here.

The message is checked for validity. The CreatePermission request
MUST contain at least one XOR-PEER-ADDRESS attribute and MAY contain
multiple such attributes. If no such attribute exists, or if any of
these attributes are invalid, then a 400 (Bad Request) error is
returned. If the request is valid, but the server is unable to
satisfy the request due to some capacity limit or similar, then a 508
(Insufficient Capacity) error is returned.

If an XOR-PEER-ADDRESS attribute contains an address of an address
family that is not the same as that of a relayed transport address
for the allocation, the server MUST generate an error response with
the 443 (Peer Address Family Mismatch) response code.

The server MAY impose restrictions on the IP address allowed in the
XOR-PEER-ADDRESS attribute -- attribute; if a value is not allowed, the server
rejects the request with a 403 (Forbidden) error.

If the message is valid and the server is capable of carrying out the
request, then the server installs or refreshes a permission for the
IP address contained in each XOR-PEER-ADDRESS attribute as described
in Section 9. The port portion of each attribute is ignored and may
be any arbitrary value.

The server then responds with a CreatePermission success response.
There are no mandatory attributes in the success response.

| NOTE: A server need not do anything special to implement
| idempotency of CreatePermission requests over UDP using the
| "stateless stack approach". Retransmitted CreatePermission
| requests will simply refresh the permissions.

10.3. Receiving a CreatePermission Response

If the client receives a valid CreatePermission success response,
then the client updates its data structures to indicate that the
permissions have been installed or refreshed.

11. Send and Data Methods

TURN supports two mechanisms for sending and receiving data from
peers. This section describes the use of the Send and Data
mechanisms, while Section 12 describes the use of the Channel
mechanism.

11.1. Forming a Send Indication

The client can use a Send indication to pass data to the server for
relaying to a peer. A client may use a Send indication even if a
channel is bound to that peer. However, the client MUST ensure that
there is a permission installed for the IP address of the peer to
which the Send indication is being sent; this prevents a third party
from using a TURN server to send data to arbitrary destinations.

When forming a Send indication, the client MUST include an XOR-PEER-
ADDRESS attribute and a DATA attribute. The XOR-PEER-ADDRESS
attribute contains the transport address of the peer to which the
data is to be sent, and the DATA attribute contains the actual
application data to be sent to the peer.

The client MAY include a DONT-FRAGMENT attribute in the Send
indication if it wishes the server to set the DF bit on the UDP
datagram sent to the peer.

11.2. Receiving a Send Indication

When the server receives a Send indication, it processes as per
Section 5 plus the specific rules mentioned here.

The message is first checked for validity. The Send indication MUST
contain both an XOR-PEER-ADDRESS attribute and a DATA attribute. If
one of these attributes is missing or invalid, then the message is
discarded. Note that the DATA attribute is allowed to contain zero
bytes of data.

The Send indication may also contain the DONT-FRAGMENT attribute. If
the server is unable to set the DF bit on outgoing UDP datagrams when
this attribute is present, then the server acts as if the DONT-
FRAGMENT attribute is an unknown comprehension-required attribute
(and thus the Send indication is discarded).

The server also checks that there is a permission installed for the
IP address contained in the XOR-PEER-ADDRESS attribute. If no such
permission exists, the message is discarded. Note that a Send
indication never causes the server to refresh the permission.

The server MAY impose restrictions on the IP address and port values
allowed in the XOR-PEER-ADDRESS attribute -- attribute; if a value is not allowed,
the server silently discards the Send indication.

If everything is OK, then the server forms a UDP datagram as follows:

* the source transport address is the relayed transport address of
the allocation, where the allocation is determined by the 5-tuple
on which the Send indication arrived;

* the destination transport address is taken from the XOR-PEER-
ADDRESS attribute;

* the data following the UDP header is the contents of the value
field of the DATA attribute.

The handling of the DONT-FRAGMENT attribute (if present), is
described in Section Sections 14 and Section 15.

The resulting UDP datagram is then sent to the peer.

11.3. Receiving a UDP Datagram

When the server receives a UDP datagram at a currently allocated
relayed transport address, the server looks up the allocation
associated with the relayed transport address. The server then
checks to see whether the set of permissions for the allocation allow
the relaying of the UDP datagram as described in Section 9.

If relaying is permitted, then the server checks if there is a
channel bound to the peer that sent the UDP datagram (see
Section 12). If a channel is bound, then processing proceeds as
described in Section 12.7.

If relaying is permitted but no channel is bound to the peer, then
the server forms and sends a Data indication. The Data indication
MUST contain both an XOR-PEER-ADDRESS and a DATA attribute. The DATA
attribute is set to the value of the 'data octets' "data octets" field from the
datagram, and the XOR-PEER-ADDRESS attribute is set to the source
transport address of the received UDP datagram. The Data indication
is then sent on the 5-tuple associated with the allocation.

11.4. Receiving a Data Indication

When the client receives a Data indication, it checks that the Data
indication contains an XOR-PEER-ADDRESS attribute, attribute and discards the
indication if it does not. The client SHOULD also check that the
XOR-PEER-ADDRESS attribute value contains an IP address with which
the client believes there is an active permission, permission and discard the
Data indication otherwise.

| NOTE: The latter check protects the client against an attacker
| who somehow manages to trick the server into installing
| permissions not desired by the client.

If the XOR-PEER-ADDRESS is present and valid, the client checks that
the Data indication contains either a DATA attribute or an ICMP
attribute and discards the indication if it does not. Note that a
DATA attribute is allowed to contain zero bytes of data. Processing
of Data indications with an ICMP attribute is described in
Section 11.6.

If the Data indication passes the above checks, the client delivers
the data octets inside the DATA attribute to the application, along
with an indication that they were received from the peer whose
transport address is given by the XOR-PEER-ADDRESS attribute.

11.5. Receiving an ICMP Packet

When the server receives an ICMP packet, the server verifies that the
type is either 3 or 11 for an ICMPv4 [RFC0792] packet or either 1, 2,
or 3 for an ICMPv6 [RFC4443] packet. It also verifies that the IP
packet in the ICMP packet payload contains a UDP header. If either
of these conditions fail, then the ICMP packet is silently dropped.
If a UDP header is present, the server extracts the source and
destination IP address and UDP port information.

The server looks up the allocation whose relayed transport address
corresponds to the encapsulated packet's source IP address and UDP
port. If no such allocation exists, the packet is silently dropped.
The server then checks to see whether the set of permissions for the
allocation allows the relaying of the ICMP packet. For ICMP packets,
the source IP address MUST NOT be checked against the permissions
list as it would be for UDP packets. Instead, the server extracts
the destination IP address from the encapsulated IP header. The
server then compares this address with the IP address associated with
each permission in the list of permissions for the allocation. If no
match is found, relaying is not permitted, permitted and the server silently
discards the ICMP packet. Note that only addresses are compared and
port numbers are not considered.

If relaying is permitted permitted, then the server forms and sends a Data
indication. The Data indication MUST contain both an XOR-PEER-
ADDRESS and an ICMP attribute. The ICMP attribute is set to the
value of the type and code fields from the ICMP packet. The IP
address portion of XOR-PEER-ADDRESS attribute is set to the
destination IP address in the encapsulated IP header. At the time of
writing of this specification, Socket APIs on some operating systems
do not deliver the destination port in the encapsulated UDP header to
applications without superuser privileges. If destination port in
the encapsulated UDP header is available to the server server, then the port
portion of the XOR-PEER-ADDRESS attribute is set to the destination port
otherwise
port; otherwise, the port portion is set to 0. zero. The Data
indication is then sent on the 5-tuple associated with the
allocation.

| Implementation Note: New ICMP types or codes can be defined in
| future specifications. If the server receives an ICMP error
| packet, and the new type or code field can help the client to
| make use of the ICMP error notification and generate feedback
| to the application layer, the server sends the Data indication
| with an ICMP attribute conveying the new ICMP type or code.

11.6. Receiving a Data Indication with an ICMP attribute Attribute

When the client receives a Data indication with an ICMP attribute, it
checks that the Data indication contains an XOR-PEER-ADDRESS
attribute,
attribute and discards the indication if it does not. The client
SHOULD also check that the XOR-PEER-ADDRESS attribute value contains
an IP address with an active permission, permission and discard the Data
indication otherwise.

If the Data indication passes the above checks, the client signals
the application of the error condition, condition along with an indication that
it was received from the peer whose transport address is given by the
XOR-PEER-ADDRESS attribute. The application can make sense of the
meaning of the type and code values in the ICMP attribute by using
the family field in the XOR-PEER-ADDRESS attribute.

12. Channels

Channels provide a way for the client and server to send application
data using ChannelData messages, which have less overhead than Send
and Data indications.

The ChannelData message (see Section 12.4) starts with a two-byte
field that carries the channel number. The values of this field are
allocated as follows:

0x5000-0xFFFF: |
+------------------------+--------------------------------------+
| 0x5000 through 0xFFFF: | Reserved (For DTLS-SRTP multiplexing |
| | collision avoidance, see [RFC7983]. [RFC7983]). |
+------------------------+--------------------------------------+

Table 2

Note that the channel number range is not backwards compatible with
[RFC5766], which could prevent an RFC5766-compliant a client compliant with RFC 5766 from
establishing channel bindings with a TURN server that complies with
this specification. .

According to [RFC7983], ChannelData messages can be distinguished
from other multiplexed protocols by examining the first byte of the
message:

+------------+------------------------------+

+------------+------------------------------------------------------+
| [0..3] | STUN |
| | |
+-------------------------------------------+
+------------+------------------------------------------------------+
| [16..19] | ZRTP |
| | |
+-------------------------------------------+
+------------+------------------------------------------------------+
| [20..63] | DTLS |
| | |
+-------------------------------------------+
+------------+------------------------------------------------------+
| [64..79] | TURN Channel |
| | |
+-------------------------------------------+
+------------+------------------------------------------------------+
| [128..191] | RTP/RTCP |
| | |
+-------------------------------------------+
+------------+------------------------------------------------------+
| Others | Reserved, Reserved; MUST be dropped |
| | and an alert MAY be logged |
+-------------------------------------------+

Figure 5
+------------+------------------------------------------------------+

Table 3

Reserved values may be used in the future by other protocols. When
the client uses channel binding, it MUST comply with the
demultiplexing scheme discussed above.

Channel bindings are always initiated by the client. The client can
bind a channel to a peer at any time during the lifetime of the
allocation. The client may bind a channel to a peer before
exchanging data with it, it or after exchanging data with it (using Send
and Data indications) for some time, or may choose never to bind a
channel to it. The client can also bind channels to some peers while
not binding channels to other peers.

Channel bindings are specific to an allocation, allocation so that the use of a
channel number or peer transport address in a channel binding in one
allocation has no impact on their use in a different allocation. If
an allocation expires, all its channel bindings expire with it.

A channel binding consists of:

* a channel number;

* a transport address (of the peer); and

* A time-to-expiry timer.

Within the context of an allocation, a channel binding is uniquely
identified either by the channel number or by the peer's transport
address. Thus, the same channel cannot be bound to two different
transport addresses, nor can the same transport address be bound to
two different channels.

A channel binding lasts for 10 minutes unless refreshed. Refreshing
the binding (by the server receiving a ChannelBind request rebinding
the channel to the same peer) resets the time-to-expiry timer back to
10 minutes.

When the channel binding expires, the channel becomes unbound. Once
unbound, the channel number can be bound to a different transport
address, and the transport address can be bound to a different
channel number. To prevent race conditions, the client MUST wait 5
minutes after the channel binding expires before attempting to bind
the channel number to a different transport address or the transport
address to a different channel number.

When binding a channel to a peer, the client SHOULD be prepared to
receive ChannelData messages on the channel from the server as soon
as it has sent the ChannelBind request. Over UDP, it is possible for
the client to receive ChannelData messages from the server before it
receives a ChannelBind success response.

In the other direction, the client MAY elect to send ChannelData
messages before receiving the ChannelBind success response. Doing
so, however, runs the risk of having the ChannelData messages dropped
by the server if the ChannelBind request does not succeed for some
reason (e.g., packet lost if the request is sent over UDP, UDP or the
server being unable to fulfill the request). A client that wishes to
be safe should either queue the data or use Send indications until
the channel binding is confirmed.

12.1. Sending a ChannelBind Request

A channel binding is created or refreshed using a ChannelBind
transaction. A ChannelBind transaction also creates or refreshes a
permission towards the peer (see Section 9).

To initiate the ChannelBind transaction, the client forms a
ChannelBind request. The channel to be bound is specified in a
CHANNEL-NUMBER attribute, and the peer's transport address is
specified in an XOR-PEER-ADDRESS attribute. Section 12.2 describes
the restrictions on these attributes. The client MUST only include
an XOR-PEER-ADDRESS attribute with an address of the same address
family as that of a relayed transport address for the allocation.

Rebinding a channel to the same transport address that it is already
bound to provides a way to refresh a channel binding and the
corresponding permission without sending data to the peer. Note Note,
however, that permissions need to be refreshed more frequently than
channels.

12.2. Receiving a ChannelBind Request

When the server receives a ChannelBind request, it processes as per
Section 5 plus the specific rules mentioned here.

The server checks the following:

* The request contains both a CHANNEL-NUMBER and an XOR-PEER-ADDRESS
attribute;

* The channel number is in the range 0x4000 through 0x4FFF
(inclusive);

* The channel number is not currently bound to a different transport
address (same transport address is OK);

* The transport address is not currently bound to a different
channel number.

If any of these tests fail, the server replies with a 400 (Bad
Request) error. If the XOR-PEER-ADDRESS attribute contains an
address of an address family that is not the same as that of a
relayed transport address for the allocation, the server MUST
generate an error response with the 443 (Peer Address Family
Mismatch) response code.

The server MAY impose restrictions on the IP address and port values
allowed in the XOR-PEER-ADDRESS attribute -- attribute; if a value is not allowed,
the server rejects the request with a 403 (Forbidden) error.

If the request is valid, but the server is unable to fulfill the
request due to some capacity limit or similar, the server replies
with a 508 (Insufficient Capacity) error.

Otherwise, the server replies with a ChannelBind success response.
There are no required attributes in a successful ChannelBind
response.

If the server can satisfy the request, then the server creates or
refreshes the channel binding using the channel number in the
CHANNEL-NUMBER attribute and the transport address in the XOR-PEER-
ADDRESS attribute. The server also installs or refreshes a
permission for the IP address in the XOR-PEER-ADDRESS attribute as
described in Section 9.

| NOTE: A server need not do anything special to implement
| idempotency of ChannelBind requests over UDP using the
| "stateless stack approach". Retransmitted ChannelBind requests
| will simply refresh the channel binding and the corresponding
| permission. Furthermore, the client must wait 5 minutes before
| binding a previously bound channel number or peer address to a
| different channel, eliminating the possibility that the
| transaction would initially fail but succeed on a
| retransmission.

12.3. Receiving a ChannelBind Response

When the client receives a ChannelBind success response, it updates
its data structures to record that the channel binding is now active.
It also updates its data structures to record that the corresponding
permission has been installed or refreshed.

If the client receives a ChannelBind failure response that indicates
that the channel information is out-of-sync out of sync between the client and
the server (e.g., an unexpected 400 "Bad Request" response), then it
is RECOMMENDED that the client immediately delete the allocation and
start afresh with a new allocation.

12.4. The ChannelData Message

The ChannelData message is used to carry application data between the
client and the server. It has the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Application Data /
/ /
| |
| +-------------------------------+
| |
+-------------------------------+

Figure 5

The Channel Number field specifies the number of the channel on which
the data is traveling, and thus thus, the address of the peer that is
sending or is to receive the data.

The Length field specifies the length in bytes of the application
data field (i.e., it does not include the size of the ChannelData
header). Note that 0 is a valid length.

The Application Data field carries the data the client is trying to
send to the peer, or that the peer is sending to the client.

12.5. Sending a ChannelData Message

Once a client has bound a channel to a peer, then when the client has
data to send to that peer peer, it may use either a ChannelData message or
a Send indication; that is, the client is not obligated to use the
channel when it exists and may freely intermix the two message types
when sending data to the peer. The server, on the other hand, MUST
use the ChannelData message if a channel has been bound to the peer.
The server uses a Data indication to signal the XOR-PEER-ADDRESS and
ICMP attributes to the client even if a channel has been bound to the
peer.

The fields of the ChannelData message are filled in as described in
Section 12.4.

Over TCP and TLS-over-TCP, the ChannelData message MUST be padded to
a multiple of four bytes in order to ensure the alignment of
subsequent messages. The padding is not reflected in the length
field of the ChannelData message, so the actual size of a ChannelData
message (including padding) is (4 + Length) rounded up to the nearest
multiple of 4 (See section (see Section 14 in [I-D.ietf-tram-stunbis]). of [RFC8489]). Over UDP, the padding
is not required but MAY be included.

The ChannelData message is then sent on the 5-tuple associated with
the allocation.

12.6. Receiving a ChannelData Message

The receiver of the ChannelData message uses the first byte to
distinguish it from other multiplexed protocols, protocols as described in
Figure 5.
Table 3. If the message uses a value in the reserved range (0x5000
through 0xFFFF), then the message is silently discarded.

If the ChannelData message is received in a UDP datagram, and if the
UDP datagram is too short to contain the claimed length of the
ChannelData message (i.e., the UDP header length field value is less
than the ChannelData header length field value + 4 + 8), then the
message is silently discarded.

If the ChannelData message is received over TCP or over TLS-over-TCP,
then the actual length of the ChannelData message is as described in
Section 12.5.

If the ChannelData message is received on a channel that is not bound
to any peer, then the message is silently discarded.

On the client, it is RECOMMENDED that the client discard the
ChannelData message if the client believes there is no active
permission towards the peer. On the server, the receipt of a
ChannelData message MUST NOT refresh either the channel binding or
the permission towards the peer.

On the server, if no errors are detected, the server relays the
application data to the peer by forming a UDP datagram as follows:

* the source transport address is the relayed transport address of
the allocation, where the allocation is determined by the 5-tuple
on which the ChannelData message arrived;

* the destination transport address is the transport address to
which the channel is bound;

* the data following the UDP header is the contents of the data
field of the ChannelData message.

The resulting UDP datagram is then sent to the peer. Note that if
the Length field in the ChannelData message is 0, then there will be
no data in the UDP datagram, but the UDP datagram is still formed and
sent (Section 4.1 in of [RFC6263]).

12.7. Relaying Data from the Peer

When the server receives a UDP datagram on the relayed transport
address associated with an allocation, the server processes it as
described in Section 11.3. If that section indicates that a
ChannelData message should be sent (because there is a channel bound
to the peer that sent to the UDP datagram), then the server forms and
sends a ChannelData message as described in Section 12.5.

When the server receives an ICMP packet, the server processes it as
described in Section 11.5.

13. Packet Translations

This section addresses IPv4-to-IPv6, IPv6-to-IPv4, and IPv6-to-IPv6
translations. Requirements for translation of the IP addresses and
port numbers of the packets are described above. The following
sections specify how to translate other header fields.

As discussed in Section 3.6, translations in TURN are designed so
that a TURN server can be implemented as an application that runs in
user space under commonly available operating systems and that does
not require special privileges. The translations specified in the
following sections follow this principle.

The descriptions below have two parts: a preferred behavior and an
alternate behavior. The server SHOULD implement the preferred
behavior, but if that is not possible for a particular field, the
server MUST implement the alternate behavior and MUST NOT do anything
else for the reasons detailed in [RFC7915]. The TURN server solely
relies on the DF bit in the IPv4 header and Fragmentation the Fragment header in
the IPv6 header to handle fragmentation using the approach described
in [RFC7915] and does not rely on the DONT-FRAGMENT attribute, attribute;
ignoring the DONT-FRAGMENT attribute is only applicable for UDP-to-UDP relay, UDP-to-
UDP relay and not for TCP-to-UDP relay.

13.1. IPv4-to-IPv6 Translations

Time to Live (TTL) field

Preferred Behavior: As specified in Section 4 of [RFC7915].

Alternate Behavior: Set the outgoing value to the default for
outgoing packets.

Traffic Class

Preferred behavior: As specified in Section 4 of [RFC7915].

Alternate behavior: The TURN server sets the Traffic Class to the
default value for outgoing packets.

Flow Label

Preferred behavior: The TURN server can use the 5-tuple of relayed
transport address, peer transport address address, and UDP protocol number
to identify each flow, flow and to generate and set the flow label value
in the IPv6 packet as discussed in Section 3 of [RFC6437]. If the
TURN server is incapable of generating the flow label value from
the IPv6 packet's 5-tuple, it sets the Flow label to 0. zero.

Alternate behavior: The alternate behavior is the same as the
preferred behavior for a TURN server that does not support flow
labels.

Hop Limit

Preferred behavior: As specified in Section 4 of [RFC7915].

Alternate behavior: The TURN server sets the Hop Limit to the
default value for outgoing packets.

Fragmentation

Preferred behavior: As specified in Section 4 of [RFC7915].

Alternate behavior: The TURN server assembles incoming fragments.
The TURN server follows its default behavior to send outgoing
packets.

For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.

Extension Headers

Preferred behavior: The outgoing packet uses the system defaults
for IPv6 extension headers, with the exception of the
Fragmentation Fragment
header as described above.

Alternate behavior: Same as preferred.

13.2. IPv6-to-IPv6 Translations

Flow Label

NOTE: The TURN server should consider that it is handling two
different IPv6 flows. Therefore, the Flow label [RFC6437] SHOULD NOT
be copied as part of the translation.

Alternate behavior: The alternate behavior is the same as the
preferred behavior for a TURN server that does not support flow
labels.

Hop Limit

Preferred behavior: The TURN server acts as a regular router with
respect to decrementing the Hop Limit and generating an ICMPv6
error if it reaches zero.

Alternate behavior: The TURN server sets the Hop Limit to the
default value for outgoing packets.

Fragmentation

Preferred behavior: If the incoming packet did not include a
Fragment header and the outgoing packet size does not exceed the
outgoing link's MTU, the TURN server sends the outgoing packet
without a Fragment header.

If the incoming packet did not include a Fragment header and the
outgoing packet size exceeds the outgoing link's MTU, the TURN
server drops the outgoing packet and send sends an ICMP message of type
2 code 0 ("Packet too big") to the sender of the incoming packet.
If the ICMPv6 packet ("Packet too big") is being sent to the peer,
the TURN server SHOULD reduce the MTU reported in the ICMP message
by 48 bytes to allow room for the overhead of a Data indication.

If the incoming packet included a Fragment header and the outgoing
packet size (with a Fragment header included) does not exceed the
outgoing link's MTU, the TURN server sends the outgoing packet
with a Fragment header. The TURN server sets the fields of the
Fragment header as appropriate for a packet originating from the
server.

If the incoming packet included a Fragment header and the outgoing
packet size exceeds the outgoing link's MTU, the TURN server MUST
fragment the outgoing packet into fragments of no more than 1280
bytes. The TURN server sets the fields of the Fragment header as
appropriate for a packet originating from the server.

Alternate behavior: The TURN server assembles incoming fragments.
The TURN server follows its default behavior to send outgoing
packets.

For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.

Extension Headers

Preferred behavior: The outgoing packet uses the system defaults
for IPv6 extension headers, with the exception of the
Fragmentation Fragment
header as described above.

Alternate behavior: Same as preferred.

13.3. IPv6-to-IPv4 Translations

Type of Service and Precedence

Preferred behavior: As specified in Section 5 of [RFC7915].

Alternate behavior: The TURN server sets the Type of Service and
Precedence to the default value for outgoing packets.

Time to Live

Preferred behavior: As specified in Section 5 of [RFC7915].

Alternate behavior: The TURN server sets the Time to Live to the
default value for outgoing packets.

Fragmentation

Preferred behavior: As specified in Section 5 of [RFC7915].
Additionally, when the outgoing packet's size exceeds the outgoing
link's MTU, the TURN server needs to generate an ICMP error
(ICMPv6 Packet Too Big) "Packet too big") reporting the MTU size. If the ICMPv4
packet (Destination Unreachable (Type 3) with Code 4) is being
sent to the peer, the TURN server SHOULD reduce the MTU reported
in the ICMP message by 48 bytes to allow room for the overhead of
a Data indication.

Alternate behavior: The TURN server assembles incoming fragments.
The TURN server follows its default behavior to send outgoing
packets.

For both preferred and alternate behavior, the DONT-FRAGMENT
attribute MUST be ignored by the server.

14. UDP-to-UDP relay Relay

This section describes how the server sets various fields in the IP
header for UDP-to-UDP relay from the client to the peer or vice
versa. The descriptions in this section apply: apply (a) when the server
sends a UDP datagram to the peer, peer or (b) when the server sends a Data
indication or ChannelData message to the client over UDP transport.
The descriptions in this section do not apply to TURN messages sent
over TCP or TLS transport from the server to the client.

Differentiated Services Code Point (DSCP) field [RFC2474]

Preferred Behavior: Set the outgoing value to the incoming value, value
unless the server includes a differentiated services classifier
and marker [RFC2474].

Alternate Behavior: Set the outgoing value to a fixed value, which
by default is Best Effort unless configured otherwise.

In both cases, if the server is immediately adjacent to a
differentiated services classifier and marker, then DSCP MAY be
set to any arbitrary value in the direction towards the
classifier.

Explicit Congestion Notification (ECN) field [RFC3168]

Preferred Behavior: Set the outgoing value to the incoming value.
The server may perform Active Queue Management, in which case it
SHOULD behave as a ECN aware an ECN-aware router [RFC3168] and can mark
traffic with Congestion Experienced (CE) instead of dropping the
packet. The use of ECT(1) is subject to experimental usage
[RFC8311].

Alternate Behavior: Set the outgoing value to Not-ECT (=0b00).

IPv4 Fragmentation fields (applicable only for IPv4-to-IPv4 relay)

Preferred Behavior: When the server sends a packet to a peer in
response to a Send indication containing the DONT-FRAGMENT
attribute, then set the outgoing UDP packet to not fragment. In
all other cases cases, when sending an outgoing packet containing
application data (e.g., Data indication, a ChannelData message, or
the DONT-FRAGMENT attribute not included in the Send indication),
copy the DF bit from the DF bit of the incoming packet that
contained the application data.

Set the other fragmentation fields (Identification, More
Fragments, Fragment Offset) as appropriate for a packet
originating from the server.

Alternate Behavior: As described in the Preferred Behavior, except
always assume the incoming DF bit is 0.

In both the Preferred and Alternate Behaviors, the resulting
packet may be too large for the outgoing link. If this is the
case, then the normal fragmentation rules apply [RFC1122].

IPv4 Options

Preferred Behavior: The outgoing packet uses the system defaults
for IPv4 options.

Alternate Behavior: Same as preferred.

15. TCP-to-UDP relay Relay

This section describes how the server sets various fields in the IP
header for TCP-to-UDP relay from the client to the peer. The
descriptions in this section apply when the server sends a UDP
datagram to the peer. Note that the server does not perform per-
packet translation for TCP-to-UDP relaying.

Multipath TCP [I-D.ietf-mptcp-rfc6824bis] [TCP-EXT] is not supported by this version of TURN
because TCP multi-path multipath is not used by either SIP or WebRTC protocols
[RFC7478] for media and non-media data. TCP connection between the
TURN client and server can use TCP-AO
[RFC5925] the TCP Authentication Option (TCP-AO)
[RFC5925], but UDP does not provide a similar type of authentication authentication,
though it might be added in the future [I-D.ietf-tsvwg-udp-options]. [UDP-OPT]. Even if both TCP-AO TCP-
AO and UDP authentication would be used between TURN client and
server, it would not change the end-to-end security properties of the
application payload being relayed. Therefore Therefore, applications using
TURN will need to secure their application data
end-to-end end to end
appropriately, e.g., SRTP Secure Real-time Transport Protocol (SRTP) for
RTP applications. Note that the TCP-AO option obsoletes the TCP MD5
option.

Unlike UDP, TCP without the TCP Fast Open extension [RFC7413] does
not support 0-RTT session resumption. The TCP user timeout [RFC5482]
equivalent for application data relayed by the TURN is the use of RTP
control protocol (RTCP). As a reminder, RTCP is a fundamental and
integral part of RTP.

For the UDP datagram sent to the peer based on a Send Indication or
ChannelData message arriving at the TURN server over a TCP Transport,
the server sets various fields in the IP header as follows:

Differentiated Services Code Point (DSCP) field [RFC2474]

Preferred Behavior: The TCP connection can only use a single DSCP
code point DSCP,
so inter flow inter-flow differentiation is not possible, possible; see Section 5.1 of
[RFC7657]. The server sets the outgoing value to the DSCP code point used by
the TCP connection, unless the server includes a differentiated
services classifier and marker [RFC2474].

Alternate Behavior: Set the outgoing value to a fixed value, which
by default is Best Effort unless configured otherwise.

In both cases, if the server is immediately adjacent to a
differentiated services classifier and marker, then DSCP MAY be
set to any arbitrary value in the direction towards the
classifier.

Explicit Congestion Notification (ECN) field [RFC3168]

Preferred Behavior: No mechanism is defined to indicate what ECN
value should be used for the outgoing UDP datagrams of an
allocation, therefore
allocation; therefore, set the outgoing value to Not-ECT (=0b00).

Alternate Behavior: Same as preferred.

IPv4 Fragmentation fields (applicable only for IPv4-to-IPv4 relay)

Preferred Behavior: When the server sends a packet to a peer in
response to a Send indication containing the DONT-FRAGMENT
attribute, then set the outgoing UDP packet to not fragment. In all
other cases cases, when sending an outgoing UDP packet containing
application data (e.g., Data indication, ChannelData message, or
DONT-FRAGMENT attribute not included in the Send indication), set
the DF bit in the outgoing IP header to 0.

Alternate Behavior: Same as preferred.

IPv6 Fragmentation fields

Preferred Behavior: If the TCP traffic arrives over IPv6, the
server relies on the presence of the DONT-FRAGMENT attribute in
the send indication to set the outgoing UDP packet to not
fragment.

Alternate Behavior: Same as preferred.

IPv4 Options

Preferred Behavior: The outgoing packet uses the system defaults
for IPv4 options.

Alternate Behavior: Same as preferred.

16. UDP-to-TCP relay Relay

This section describes how the server sets various fields in the IP
header for UDP-to-TCP relay from the peer to the client. The
descriptions in this section apply when the server sends a Data
indication or ChannelData message to the client over TCP or TLS
transport. Note that the server does not perform per-packet
translation for UDP-to-TCP relaying.

The TURN server sets IP header fields in the TCP packets on a per-
connection basis for the TCP connection as follows:

Differentiated Services Code Point (DSCP) field [RFC2474]

Preferred Behavior: Ignore the incoming DSCP value. When TCP is
used between the client and the server, a single DSCP should be
used for all traffic on that TCP connection. Note, TURN/ICE
occurs before application data is exchanged.

Alternate Behavior: Same as preferred.

Explicit Congestion Notification (ECN) field [RFC3168]

Preferred Behavior: Ignore, Ignore; ECN signals are dropped in the TURN
server for the incoming UDP datagrams from the peer.

Alternate Behavior: Same as preferred.

Fragmentation

Preferred Behavior: Any fragmented packets are reassembled in the
server and then forwarded to the client over the TCP connection.
ICMP messages resulting from the UDP datagrams sent to the peer
are processed by the server as described in Section 11.5 and
forwarded to the client using TURN's mechanism for relevant ICMP
types and codes.

Alternate Behavior: Same as preferred.

Extension Headers

Preferred behavior: The outgoing packet uses the system defaults
for IPv6 extension headers.

Alternate behavior: Same as preferred.

IPv4 Options

Preferred Behavior: The outgoing packet uses the system defaults
for IPv4 options.

Alternate Behavior: Same as preferred.

17. STUN Methods

This section lists the codepoints code points for the STUN methods defined in
this specification. See elsewhere in this document for the semantics
of these methods.

+-------+------------------+------------------------+
| 0x003 : | Allocate | (only request/response |
| | | semantics defined) |
+-------+------------------+------------------------+
| 0x004 : | Refresh | (only request/response |
| | | semantics defined) |
+-------+------------------+------------------------+
| 0x006 : | Send | (only indication |
| | | semantics defined) |
+-------+------------------+------------------------+
| 0x007 : | Data | (only indication |
| | | semantics defined) |
+-------+------------------+------------------------+
| 0x008 : | CreatePermission | (only request/response |
| | | semantics defined defined) |
+-------+------------------+------------------------+
| 0x009 : | ChannelBind | (only request/response |
| | | semantics defined) |
+-------+------------------+------------------------+

Table 4

18. STUN Attributes

This STUN extension defines the following attributes:

0x000C:

+--------+---------------------------+
| 0x000C | CHANNEL-NUMBER
0x000D: |
+--------+---------------------------+
| 0x000D | LIFETIME
0x0010: |
+--------+---------------------------+
| 0x0010 | Reserved (was BANDWIDTH)
0x0012: |
+--------+---------------------------+
| 0x0012 | XOR-PEER-ADDRESS
0x0013: |
+--------+---------------------------+
| 0x0013 | DATA
0x0016: |
+--------+---------------------------+
| 0x0016 | XOR-RELAYED-ADDRESS
0x0017: |
+--------+---------------------------+
| 0x0017 | REQUESTED-ADDRESS-FAMILY
0x0018: |
+--------+---------------------------+
| 0x0018 | EVEN-PORT
0x0019: |
+--------+---------------------------+
| 0x0019 | REQUESTED-TRANSPORT
0x001A: |
+--------+---------------------------+
| 0x001A | DONT-FRAGMENT
0x0021: |
+--------+---------------------------+
| 0x0021 | Reserved (was TIMER-VAL)
0x0022: |
+--------+---------------------------+
| 0x0022 | RESERVATION-TOKEN
TBD-CA: |
+--------+---------------------------+
| 0x8000 | ADDITIONAL-ADDRESS-FAMILY
TBD-CA: |
+--------+---------------------------+
| 0x8001 | ADDRESS-ERROR-CODE
TBD-CA: |
+--------+---------------------------+
| 0x8004 | ICMP |
+--------+---------------------------+

Table 5

Some of these attributes have lengths that are not multiples of 4.
By the rules of STUN, any attribute whose length is not a multiple of
4 bytes MUST be immediately followed by 1 to 3 padding bytes to
ensure the next attribute (if any) would start on a 4-byte boundary
(see [I-D.ietf-tram-stunbis]). [RFC8489]).

18.1. CHANNEL-NUMBER

The CHANNEL-NUMBER attribute contains the number of the channel. The
value portion of this attribute is 4 bytes long and consists of a
16-bit unsigned integer, integer followed by a two-octet RFFU (Reserved For
Future Use) field, which MUST be set to 0 on transmission and MUST be
ignored on reception.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | RFFU = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6

18.2. LIFETIME

The LIFETIME attribute represents the duration for which the server
will maintain an allocation in the absence of a refresh. The TURN
client can include the LIFETIME attribute with the desired lifetime
in Allocate and Refresh requests. The value portion of this
attribute is 4-bytes 4 bytes long and consists of a 32-bit unsigned integral
value representing the number of seconds remaining until expiration.

18.3. XOR-PEER-ADDRESS

The XOR-PEER-ADDRESS attribute specifies the address and port of the
peer as seen from the TURN server. (For example, the peer's server-reflexive server-
reflexive transport address if the peer is behind a NAT.) It is
encoded in the same way as the XOR-MAPPED-ADDRESS [I-D.ietf-tram-stunbis]. attribute
[RFC8489].

18.4. DATA

The DATA attribute is present in all Send indications. If the ICMP
attribute is not present in a Data indication, it contains a DATA
attribute. The value portion of this attribute is variable length
and consists of the application data (that is, the data that would
immediately follow the UDP header if the data was been sent directly
between the client and the peer). The application data is equivalent
to the "UDP user data" and does not include the "surplus area"
defined in Section 4 of [I-D.ietf-tsvwg-udp-options]. [UDP-OPT]. If the length of this attribute
is not a multiple of 4, then padding must be added after this
attribute.

18.5. XOR-RELAYED-ADDRESS

The XOR-RELAYED-ADDRESS attribute is present in Allocate responses.
It specifies the address and port that the server allocated to the
client. It is encoded in the same way as the XOR-MAPPED-ADDRESS
[I-D.ietf-tram-stunbis].
attribute [RFC8489].

18.6. REQUESTED-ADDRESS-FAMILY

This attribute is used in Allocate and Refresh requests to specify
the address type requested by the client. The value of this
attribute is 4 bytes with the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Family | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7

Family: there There are two values defined for this field and specified in
[I-D.ietf-tram-stunbis],
Section 14.1: 14.1 of [RFC8489]: 0x01 for IPv4 addresses and 0x02 for
IPv6 addresses.

Reserved: at At this point, the 24 bits in the Reserved field MUST be
set to zero by the client and MUST be ignored by the server.

18.7. EVEN-PORT

This attribute allows the client to request that the port in the
relayed transport address be even, even and (optionally) that the server
reserve the next-higher port number. The value portion of this
attribute is 1 byte long. Its format is:

0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|R| RFFU |
+-+-+-+-+-+-+-+-+

Figure 8

The value contains a single 1-bit flag:

R: If 1, the server is requested to reserve the next-higher port
number (on the same IP address) for a subsequent allocation. If
0, no such reservation is requested.

RFFU: Reserved For Future Use.

The RFFU field must be set to zero on transmission and ignored on
reception.

Since the length of this attribute is not a multiple of 4, padding
must immediately follow this attribute.

18.8. REQUESTED-TRANSPORT

This attribute is used by the client to request a specific transport
protocol for the allocated transport address. The value of this
attribute is 4 bytes with the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9

The Protocol field specifies the desired protocol. The codepoints code points
used in this field are taken from those allowed in the Protocol field
in the IPv4 header and the NextHeader field in the IPv6 header
[Protocol-Numbers].
[PROTOCOL-NUMBERS]. This specification only allows the use of
codepoint code
point 17 (User Datagram Protocol).

The RFFU field MUST be set to zero on transmission and MUST be
ignored on reception. It is reserved for future uses.

18.9. DONT-FRAGMENT

This attribute is used by the client to request that the server set
the DF (Don't Fragment) bit in the IP header when relaying the
application data onward to the peer, peer and for determining the server
capability in Allocate requests. This attribute has no value part part,
and thus thus, the attribute length field is 0.

18.10. RESERVATION-TOKEN

The RESERVATION-TOKEN attribute contains a token that uniquely
identifies a relayed transport address being held in reserve by the
server. The server includes this attribute in a success response to
tell the client about the token, and the client includes this
attribute in a subsequent Allocate request to request the server use
that relayed transport address for the allocation.

The attribute value is 8 bytes and contains the token value.

18.11. ADDITIONAL-ADDRESS-FAMILY

This attribute is used by clients to request the allocation of a an
IPv4 and IPv6 address type from a server. It is encoded in the same
way as the REQUESTED-ADDRESS-FAMILY attribute; see Section 18.6. The ADDITIONAL-ADDRESS-
FAMILY
ADDITIONAL-ADDRESS-FAMILY attribute MAY be present in the Allocate
request. The attribute value of 0x02 (IPv6 address) is the only
valid value in Allocate request.

18.12. ADDRESS-ERROR-CODE

This attribute is used by servers to signal the reason for not
allocating the requested address family. The value portion of this
attribute is variable length with the following format:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Family | Reserved |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10

Family: there There are two values defined for this field and specified in
[I-D.ietf-tram-stunbis],
Section 14.1: 14.1 of [RFC8489]: 0x01 for IPv4 addresses and 0x02 for
IPv6 addresses.

Reserved: at At this point, the 13 bits in the Reserved field MUST be
set to zero by the server and MUST be ignored by the client.

Class: The Class represents the hundreds digit of the error code and
is defined in section Section 14.8 of [I-D.ietf-tram-stunbis]. [RFC8489].

Number: this This 8-bit field contains the reason the server cannot
allocate one of the requested address types. The error code
values could be either 440 (unsupported address family) (Address Family not Supported) or 508 (insufficient
capacity).
(Insufficient Capacity). The number representation is defined in section
Section 14.8 of [I-D.ietf-tram-stunbis]. [RFC8489].

Reason Phrase: The recommended reason phrases for error codes 440
and 508 are explained in Section 19. The reason phrase MUST be a
UTF-8 [RFC3629] encoded sequence of less than 128 characters
(which can be as long as 509 bytes when encoding them or 763 bytes
when decoding them).

18.13. ICMP

This attribute is used by servers to signal the reason an a UDP packet
was dropped. The following is the format of the ICMP attribute.

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | ICMP Type | ICMP Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 11

Reserved: This field MUST be set to 0 when sent, sent and MUST be ignored
when received.

ICMP Type: The field contains the value in of the ICMP type. Its
interpretation depends on whether the ICMP was received over IPv4
or IPv6.

ICMP Code: The field contains the value in of the ICMP code. Its
interpretation depends on whether the ICMP was received over IPv4
or IPv6.

Error Data: This field size is 4 bytes long. If the ICMPv6 type is
2 (Packet Too Big Message) ("Packet too big" message) or ICMPv4 type is 3 ( Destination (Destination
Unreachable) and Code is 4 (fragmentation needed and DF set), the
Error Data field will be set to the Maximum Transmission Unit of
the next-hop link (Section 3.2 of [RFC4443]) [RFC4443] and Section 4 of
[RFC1191]). For other ICMPv6 types and ICMPv4 types and codes,
the Error Data field MUST be set to zero.

19. STUN Error Response Codes

This document defines the following error response codes:

403 (Forbidden):
The request was valid but cannot be performed due to
administrative or similar restrictions.

437 (Allocation Mismatch):
A request was received by the server that requires an allocation
to be in place, but no allocation exists, or a request was
received that requires no allocation, but an allocation exists.

440 (Address Family not Supported):
The server does not support the address family requested by the
client.

441 (Wrong Credentials):
(Wrong Credentials): The credentials in the (non-Allocate) request
do not match those used to create the allocation.

442 (Unsupported Transport Protocol):
The Allocate request asked the server to use a transport protocol
between the server and the peer that the server does not support.
NOTE: This does NOT refer to the transport protocol used in the
5-tuple.

443 (Peer Address Family Mismatch). Mismatch):
A peer address is part of a different address family than that of
the relayed transport address of the allocation.

486 (Allocation Quota Reached):
No more allocations using this username can be created at the
present time.

508 (Insufficient Capacity):
The server is unable to carry out the request due to some capacity
limit being reached. In an Allocate response, this could be due
to the server having no more relayed transport addresses available
at that time, having none with the requested properties, or the
one that corresponds to the specified reservation token is not
available.

20. Detailed Example

This section gives an example of the use of TURN, showing in detail
the contents of the messages exchanged. The example uses the network
diagram shown in the Overview (Figure 1).

For each message, the attributes included in the message and their
values are shown. For convenience, values are shown in a human-
readable format rather than showing the actual octets; for example,
"XOR-RELAYED-ADDRESS=192.0.2.15:9000" shows that the XOR-RELAYED-
ADDRESS attribute is included with an address of 192.0.2.15 and a
port of 9000, here 9000; here, the address and port are shown before the xor-ing
is done. For attributes with string-like values (e.g.,
SOFTWARE="Example client, version 1.03" and
NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda"), the value of the attribute
is shown in quotes for readability, but these quotes do not appear in
the actual value.

TURN TURN Peer Peer
client server A B
| | | |
|--- Allocate request -------------->| | |
| Transaction-Id=0xA56250D3F17ABE679422DE85 | |
| SOFTWARE="Example client, version 1.03" | |
| LIFETIME=3600 (1 hour) | | |
| REQUESTED-TRANSPORT=17 (UDP) | | |
| DONT-FRAGMENT | | |
| | | |
|<-- Allocate error response --------| | |
| Transaction-Id=0xA56250D3F17ABE679422DE85 | |
| SOFTWARE="Example server, version 1.17" | |
| ERROR-CODE=401 (Unauthorized) | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| | | |
|--- Allocate request -------------->| | |
| Transaction-Id=0xC271E932AD7446A32C234492 | |
| SOFTWARE="Example client 1.03" | | |
| LIFETIME=3600 (1 hour) | | |
| REQUESTED-TRANSPORT=17 (UDP) | | |
| DONT-FRAGMENT | | |
| USERNAME="George" | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| PASSWORD-ALGORITHM=SHA256 | | |
| MESSAGE-INTEGRITY=... | | |
| MESSAGE-INTEGRITY-SHA256=... | | |
| | | |
|<-- Allocate success response ------| | |
| Transaction-Id=0xC271E932AD7446A32C234492 | |
| SOFTWARE="Example server, version 1.17" | |
| LIFETIME=1200 (20 minutes) | | |
| XOR-RELAYED-ADDRESS=192.0.2.15:50000 | |
| XOR-MAPPED-ADDRESS=192.0.2.1:7000 | |
| MESSAGE-INTEGRITY-SHA256=... | | |

Figure 12

The client begins by selecting a host transport address to use for
the TURN session; in this example, the client has selected
198.51.100.2:49721 as shown in Figure 1. The client then sends an
Allocate request to the server at the server transport address. The
client randomly selects a 96-bit transaction id of
0xA56250D3F17ABE679422DE85 for this transaction; this is encoded in
the transaction id field in the fixed header. The client includes a
SOFTWARE attribute that gives information about the client's
software; here here, the value is "Example client, version 1.03" to
indicate that this is version 1.03 of something called the Example
client. "Example
client". The client includes the LIFETIME attribute because it
wishes the allocation to have a longer lifetime than the default of
10 minutes; the value of this attribute is 3600 seconds, which
corresponds to 1 hour. The client must always include a REQUESTED-
TRANSPORT attribute in an Allocate request request, and the only value
allowed by this specification is 17, which indicates UDP transport
between the server and the peers. The client also includes the DONT-FRAGMENT DONT-
FRAGMENT attribute because it wishes to use the DONT-FRAGMENT
attribute later in Send indications; this attribute consists of only
an attribute
header, header; there is no value part. We assume the client
has not recently interacted with the server, thus server; thus, the client does
not include the USERNAME, USERHASH, REALM, NONCE, PASSWORD-ALGORITHMS, PASSWORD-
ALGORITHM, MESSAGE-INTEGRITY
ALGORITHMS, PASSWORD-ALGORITHM, MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 MESSAGE-
INTEGRITY-SHA256 attribute. Finally, note that the order of
attributes in a message is arbitrary (except for the MESSAGE-INTEGRITY, MESSAGE-
INTEGRITY, MESSAGE-INTEGRITY-SHA256 and FINGERPRINT attributes) attributes), and
the client could have used a different order.

Servers require any request to be authenticated. Thus, when the
server receives the initial Allocate request, it rejects the request
because the request does not contain the authentication attributes.
Following the procedures of the long-term credential mechanism of
STUN [I-D.ietf-tram-stunbis], [RFC8489], the server includes an ERROR-CODE attribute with a
value of 401 (Unauthorized), a REALM attribute that specifies the
authentication realm used by the server (in this case, the server's
domain "example.com"), and a nonce value in a NONCE attribute. The
NONCE attribute starts with the "nonce cookie" with the STUN Security
Feature "Password algorithm" bit set to 1. The server includes a
PASSWORD-ALGORITHMS attribute that specifies the list of algorithms
that the server can use to derive the long-term password. If the
server sets the STUN Security Feature "Username anonymity" bit to 1 1,
then the client uses the USERHASH attribute instead of the USERNAME
attribute in the Allocate request to
anonymise anonymize the username. The
server also includes a SOFTWARE attribute that gives information
about the server's software.

The client, upon receipt of the 401 error, re-attempts reattempts the Allocate
request, this time including the authentication attributes. The
client selects a new transaction id, id and then populates the new
Allocate request with the same attributes as before. The client
includes a USERNAME attribute and uses the realm value received from
the server to help it determine which value to use; here here, the client
is configured to use the username "George" for the realm
"example.com". The client includes the PASSWORD-ALGORITHM attribute
indicating the algorithm that the server must use to derive the long-
term password. The client also includes the REALM, PASSWORD-
ALGORITHMS
ALGORITHMS, and NONCE attributes, which are just copied from the 401
error response. Finally, the client includes MESSAGE-INTEGRITY-
SHA256 attribute as the last attributes in the message, message whose value is
Hashed Message Authentication Code - Secure Hash Algorithm 2
(HMAC-SHA2) (HMAC-
SHA2) hash over the contents of the message (shown as just "..."
above); this HMAC-SHA2 computation includes a password value. Thus,
an attacker cannot compute the message integrity value without
somehow knowing the secret password.

The server, upon receipt of the authenticated Allocate request,
checks that everything is OK, then creates an allocation. The server
replies with an Allocate success response. The server includes a
LIFETIME attribute giving the lifetime of the allocation; here, the
server has reduced the client's requested 1-hour lifetime to just 20
minutes,
minutes because this particular server doesn't allow lifetimes longer
than 20 minutes. The server includes an XOR-RELAYED-ADDRESS
attribute whose value is the relayed transport address of the
allocation. The server includes an XOR-MAPPED-ADDRESS attribute
whose value is the server-reflexive address of the client; this value
is not used otherwise in TURN but is returned as a convenience to the
client. The server includes a MESSAGE-INTEGRITY-SHA256 attribute to
authenticate the response and to ensure its integrity; note that the
response does not contain the USERNAME, REALM, and NONCE attributes.
The server also includes a SOFTWARE attribute.

Figure 13

The client then creates a permission towards Peer A in preparation
for sending it some application data. This is done through a
CreatePermission request. The XOR-PEER-ADDRESS attribute contains
the IP address for which a permission is established (the IP address
of peer A); note that the port number in the attribute is ignored
when used in a CreatePermission request, and here it has been set to
0; also, note how the client uses Peer A's server-reflexive IP
address and not its (private) host address. The client uses the same
username, realm, and nonce values as in the previous request on the
allocation. Though it is allowed to do so, the client has chosen not
to include a SOFTWARE attribute in this request.

The server receives the CreatePermission request, creates the
corresponding permission, and then replies with a CreatePermission
success response. Like the client, the server chooses not to include
the SOFTWARE attribute in its reply. Again, note how success
responses contains contain a MESSAGE-INTEGRITY-SHA256 attribute (assuming the
server uses the long-term credential mechanism), mechanism) but no USERNAME,
REALM, and NONCE attributes.

TURN TURN Peer Peer
client server A B
|--- Send indication --------------->| | |
| Transaction-Id=0x1278E9ACA2711637EF7D3328 | |
| XOR-PEER-ADDRESS=192.0.2.150:32102 | |
| DONT-FRAGMENT | | |
| DATA=... | | |
| |-- |- UDP dgm ->| |
| | data=... | |
| | | |
| |<- UDP dgm --| -| |
| | data=... | |
|<-- Data indication ----------------| | |
| Transaction-Id=0x8231AE8F9242DA9FF287FEFF | |
| XOR-PEER-ADDRESS=192.0.2.150:32102 | |
| DATA=... | | |

Figure 14

The client now sends application data to Peer A using a Send
indication. Peer A's server-reflexive transport address is specified
in the XOR-PEER-ADDRESS attribute, and the application data (shown
here as just "...") is specified in the DATA attribute. The client
is doing a form of path MTU discovery at the application layer and
thus and,
thus, specifies (by including the DONT-FRAGMENT attribute) that the
server should set the DF bit in the UDP datagram to send to the peer.
Indications cannot be authenticated using the long-term credential
mechanism of STUN, so no MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute is included in the message. An application wishing
to ensure that its data is not altered or forged must integrity-
protect its data at the application level.

Upon receipt of the Send indication, the server extracts the
application data and sends it in a UDP datagram to Peer A, with the
relayed transport address as the source transport address of the
datagram,
datagram and with the DF bit set as requested. Note that, that had the
client not previously established a permission for Peer A's server-
reflexive IP address, then the server would have silently discarded the
Send indication instead.

Peer A then replies with its own UDP datagram containing application
data. The datagram is sent to the relayed transport address on the
server. When this arrives, the server creates a Data indication
containing the source of the UDP datagram in the XOR-PEER-ADDRESS
attribute, and the data from the UDP datagram in the DATA attribute.
The resulting Data indication is then sent to the client.

TURN TURN Peer Peer
client server A B
|--- ChannelBind request ----------->| | |
| Transaction-Id=0x6490D3BC175AFF3D84513212 | |
| CHANNEL-NUMBER=0x4000 | | |
| XOR-PEER-ADDRESS=192.0.2.210:49191 | |
| USERNAME="George" | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| PASSWORD-ALGORITHM=SHA256 | | |
| MESSAGE-INTEGRITY-SHA256=... | | |
| | | |
|<-- ChannelBind success response ---| | |
| Transaction-Id=0x6490D3BC175AFF3D84513212 | |
| MESSAGE-INTEGRITY-SHA256=... | | |

Figure 15

The client now binds a channel to Peer B, specifying a free channel
number (0x4000) in the CHANNEL-NUMBER attribute, and Peer B's
transport address in the XOR-PEER-ADDRESS attribute. As before, the
client re-uses reuses the username, realm, and nonce from its last request in
the message.

Upon receipt of the request, the server binds the channel number to
the peer, installs a permission for Peer B's IP address, and then
replies with a ChannelBind success response.

TURN TURN Peer Peer
client server A B
|--- ChannelData ------------------->| ------------------>| | |
| Channel-number=0x4000 |--- UDP datagram --------->|
| Data=... | Data=... |
| | | |
| |<-- UDP datagram ----------|
| | Data=... | |
|<-- ChannelData --------------------| -------------------| | |
| Channel-number=0x4000 | | |
| Data=... | | |

Figure 16

The client now sends a ChannelData message to the server with data
destined for Peer B. The ChannelData message is not a STUN message,
and thus message;
thus, it has no transaction id. Instead, it has only three fields: a
channel number, data, and data length; here here, the channel number field
is 0x4000 (the channel the client just bound to Peer B). When the
server receives the ChannelData message, it checks that the channel
is currently bound (which it is) and then sends the data onward to
Peer B in a UDP datagram, using the relayed transport address as the
source transport address address, and 192.0.2.210:49191 (the value of the XOR-
PEER-ADDRESS
XOR-PEER-ADDRESS attribute in the ChannelBind request) as the
destination transport address.

Later, Peer B sends a UDP datagram back to the relayed transport
address. This causes the server to send a ChannelData message to the
client containing the data from the UDP datagram. The server knows
to which client to send the ChannelData message because of the
relayed transport address at which the UDP datagram arrived, and it
knows to use channel 0x4000 because this is the channel bound to
192.0.2.210:49191. Note that if there had not been any channel
number bound to that address, the server would have used a Data
indication instead.

TURN TURN Peer Peer
client server A B
|--- ChannelBind request ----------->| | |
| Transaction-Id=0xE5913A8F46091637EF7D3328 | |
| CHANNEL-NUMBER=0x4000 | | |
| XOR-PEER-ADDRESS=192.0.2.210:49191 | |
| USERNAME="George" | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| PASSWORD-ALGORITHM=SHA256 | | |
| MESSAGE-INTEGRITY-SHA256=... | | |
| | | |
|<-- ChannelBind success response ---| | |
| Transaction-Id=0xE5913A8F46091637EF7D3328 | |
| MESSAGE-INTEGRITY-SHA256=... | | |

Figure 17

The channel binding lasts for 10 minutes unless refreshed. The TURN
client refreshes the binding by sending a ChannelBind request
rebinding the channel to the same peer (Peer B's IP address). The
server processes the ChannelBind request, rebinds the channel to the
same
peer peer, and resets the time-to-expiry timer back to 10 minutes.

TURN TURN Peer Peer
client server A B
|--- Refresh request --------------->| | |
| Transaction-Id=0x0864B3C27ADE9354B4312414 | |
| SOFTWARE="Example client 1.03" | | |
| USERNAME="George" | | |
| REALM="example.com" | | |
| NONCE="oobMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| PASSWORD-ALGORITHM=SHA256 | | |
| MESSAGE-INTEGRITY-SHA256=... | | |
| | | |
|<-- Refresh error response ---------| | |
| Transaction-Id=0x0864B3C27ADE9354B4312414 | |
| SOFTWARE="Example server, version 1.17" | |
| ERROR-CODE=438 (Stale Nonce) | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| | | |
|--- Refresh request --------------->| | |
| Transaction-Id=0x427BD3E625A85FC731DC4191 | |
| SOFTWARE="Example client 1.03" | | |
| USERNAME="George" | | |
| REALM="example.com" | | |
| NONCE="obMatJos2gAAAadl7W7PeDU4hKE72jda" | |
| PASSWORD-ALGORITHMS=MD5 and SHA256 | |
| PASSWORD-ALGORITHM=SHA256 | | |
| MESSAGE-INTEGRITY-SHA256=... | | |
| | | |
|<-- Refresh success response -------| | |
| Transaction-Id=0x427BD3E625A85FC731DC4191 | |
| SOFTWARE="Example server, version 1.17" | |
| LIFETIME=600 (10 minutes) | | |
| MESSAGE-INTEGRITY=... | | |

Figure 18

Sometime before the 20 minute 20-minute lifetime is up, the client refreshes
the allocation. This is done using a Refresh request. As before,
the client includes the latest username, realm, and nonce values in
the request. The client also includes the SOFTWARE attribute,
following the recommended practice of always including this attribute
in Allocate and Refresh messages. When the server receives the
Refresh request, it notices that the nonce value has expired, expired and so
replies with a 438 (Stale Nonce) error given a new nonce value. The
client then reattempts the request, this time with the new nonce
value. This second attempt is accepted, and the server replies with
a success response. Note that the client did not include a LIFETIME
attribute in the request, so the server refreshes the allocation for
the default lifetime of 10 minutes (as can be seen by the LIFETIME
attribute in the success response).

21. Security Considerations

This section considers attacks that are possible in a TURN
deployment, deployment
and discusses how they are mitigated by mechanisms in the protocol or
recommended practices in the implementation.

Most of the attacks on TURN are mitigated by the server requiring
requests be authenticated. Thus, this specification requires the use
of authentication. The mandatory-to-implement mechanism is the long-
term credential mechanism of STUN. Other authentication mechanisms
of equal or stronger security properties may be used. However, it is
important to ensure that they can be invoked in an inter-operable interoperable way.

21.1. Outsider Attacks

Outsider attacks are ones where the attacker has no credentials in
the system, system and is attempting to disrupt the service seen by the
client or the server.

21.1.1. Obtaining Unauthorized Allocations

An attacker might wish to obtain allocations on a TURN server for any
number of nefarious purposes. A TURN server provides a mechanism for
sending and receiving packets while cloaking the actual IP address of
the client. This makes TURN servers an attractive target for
attackers who wish to use it to mask their true identity.

An attacker might also wish to simply utilize the services of a TURN
server without paying for them. Since TURN services require
resources from the provider, it is anticipated that their usage will
come with a cost.

These attacks are prevented using the long-term credential mechanism,
which allows the TURN server to determine the identity of the
requestor and whether the requestor is allowed to obtain the
allocation.

21.1.2. Offline Dictionary Attacks

The long-term credential mechanism used by TURN is subject to offline
dictionary attacks. An attacker that is capable of eavesdropping on
a message exchange between a client and server can determine the
password by trying a number of candidate passwords and seeing if one
of them is correct. This attack works when the passwords are low
entropy,
entropy such as a word from the dictionary. This attack can be
mitigated by using strong passwords with large entropy. In
situations where even stronger mitigation is required, (D)TLS
transport between the client and the server can be used.

21.1.3. Faked Refreshes and Permissions

An attacker might wish to attack an active allocation by sending it a
Refresh request with an immediate expiration, expiration in order to delete it
and disrupt service to the client. This is prevented by
authentication of refreshes. Similarly, an attacker wishing to send
CreatePermission requests to create permissions to undesirable
destinations is prevented from doing so through authentication. The
motivations for such an attack are described in Section 21.2.

21.1.4. Fake Data

An attacker might wish to send data to the client or the peer, peer as if
they came from the peer or client, respectively. To do that, the
attacker can send the client a faked Data Indication indication or ChannelData
message, or send the TURN server a faked Send Indication indication or
ChannelData message.

Since indications and ChannelData messages are not authenticated,
this attack is not prevented by TURN. However, this attack is
generally present in IP-based communications and is not substantially
worsened by TURN. Consider a normal, non-TURN IP session between
hosts A and B. An attacker can send packets to B as if they came
from A by sending packets towards B with a spoofed IP address of A.
This attack requires the attacker to know the IP addresses of A and
B. With TURN, an attacker wishing to send packets towards a client
using a Data indication needs to know its IP address (and port), the
IP address and port of the TURN server, and the IP address and port
of the peer (for inclusion in the XOR-PEER-ADDRESS attribute). To
send a fake ChannelData message to a client, an attacker needs to
know the IP address and port of the client, the IP address and port
of the TURN server, and the channel number. This particular
combination is mildly more guessable than in the non-TURN case.

These attacks are more properly mitigated by application-layer
authentication techniques. In the case of real-time traffic, usage
of SRTP [RFC3711] prevents these attacks.

In some situations, the TURN server may be situated in the network
such that it is able to send to hosts to which the client cannot
directly send. This can happen, for example, if the server is
located behind a firewall that allows packets from outside the
firewall to be delivered to the server, but not to other hosts behind
the firewall. In these situations, an attacker could send the server
a Send indication with an XOR-PEER-ADDRESS attribute containing the
transport address of one of the other hosts behind the firewall. If
the server was to allow relaying of traffic to arbitrary peers, then
this would provide a way for the attacker to attack arbitrary hosts
behind the firewall.

To mitigate this attack, TURN requires that the client establish a
permission to a host before sending it data. Thus, an attacker can
only attack hosts with which the client is already communicating, communicating
unless the attacker is able to create authenticated requests.
Furthermore, the server administrator may configure the server to
restrict the range of IP addresses and ports to which it will relay
data. To provide even greater security, the server administrator can
require that the client use (D)TLS for all communication between the
client and the server.

21.1.5. Impersonating a Server

When a client learns a relayed address from a TURN server, it uses
that relayed address in application protocols to receive traffic.
Therefore, an attacker wishing to intercept or redirect that traffic
might try to impersonate a TURN server and provide the client with a
faked relayed address.

This attack is prevented through the long-term credential mechanism,
which provides message integrity for responses in addition to
verifying that they came from the server. Furthermore, an attacker
cannot replay old server responses as the transaction id in the STUN
header prevents this. Replay attacks are further thwarted through
frequent changes to the nonce value.

21.1.6. Eavesdropping Traffic

If the TURN client and server use the STUN Extension for Third-Party
Authorization [RFC7635] (for example example, it is used in WebRTC), the
username does not reveal the real user's identity, identity; the USERNAME
attribute carries an ephemeral and unique key identifier. If the
TURN client and server use the STUN long-term credential mechanism
and the username reveals the real user's identity, the client MUST
either use the USERHASH attribute instead of the USERNAME attribute
to anonynmize anonymize the username or use (D)TLS transport between the client
and the server.

If the TURN client and server use the STUN long-term credential
mechanism
mechanism, and realm information is privacy sensitive, TURN can be
run over (D)TLS. As a reminder, STUN Extension for Third-Party
Authorization does not use realm.

The SOFTWARE attribute can reveal the specific software version of
the TURN client and server to eavesdropper the eavesdropper, and it might possibly
allow attacks against vulnerable software that is known to contain
security vulnerabilities. If the software version is known to
contain security vulnerabilities, TURN SHOULD be run over (D)TLS to
prevent leaking the SOFTWARE attribute in clear text. If zero-day
vulnerabilities are detected in the software version, the endpoint
policy can be modified to mandate the use of (D)TLS until the patch
is in place to fix the flaw.

TURN concerns itself primarily with authentication and message
integrity. Confidentiality is only a secondary concern, concern as TURN
control messages do not include information that is particularly
sensitive,
sensitive with the exception of USERNAME, REALM REALM, and SOFTWARE. The
primary protocol content of the messages is the IP address of the
peer. If it is important to prevent an eavesdropper on a TURN
connection from learning this, TURN can be run over (D)TLS.

Confidentiality for the application data relayed by TURN is best
provided by the application protocol itself, itself since running TURN over
(D)TLS does not protect application data between the server and the
peer. If confidentiality of application data is important, then the
application should encrypt or otherwise protect its data. For
example, for real-time media, confidentiality can be provided by
using SRTP.

21.1.7. TURN Loop Attack

An attacker might attempt to cause data packets to loop indefinitely
between two TURN servers. The attack goes as follows. First, follows: first, the
attacker sends an Allocate request to server A, A using the source
address of server B. Server A will send its response to server B,
and for the attack to succeed, the attacker must have the ability to
either view or guess the contents of this response, response so that the
attacker can learn the allocated relayed transport address. The
attacker then sends an Allocate request to server B, B using the source
address of server A. Again, the attacker must be able to view or
guess the contents of the response, response so it can send learn the allocated
relayed transport address. Using the same spoofed source address
technique, the attacker then binds a channel number on server A to
the relayed transport address on server B, B and similarly binds the
same channel number on server B to the relayed transport address on
server A. Finally, the attacker sends a ChannelData message to
server A.

The result is a data packet that loops from the relayed transport
address on server A to the relayed transport address on server B,
then from server B's transport address to server A's transport
address, and then around the loop again.

This attack is mitigated as follows. By follows: by requiring all requests to be
authenticated and/or by randomizing the port number allocated for the
relayed transport address, the server forces the attacker to either
intercept or view responses sent to a third party (in this case, the
other server) so that the attacker can authenticate the requests and
learn the relayed transport address. Without one of these two
measures, an attacker can guess the contents of the responses without
needing to see them, which makes the attack much easier to perform.
Furthermore, by requiring authenticated requests, the server forces
the attacker to have credentials acceptable to the server, which
turns this from an outsider attack into an insider attack and allows
the attack to be traced back to the client initiating it.

The attack can be further mitigated by imposing a per-username limit
on the bandwidth used to relay data by allocations owned by that
username,
username to limit the impact of this attack on other allocations.
More mitigation can be achieved by decrementing the TTL when relaying
data packets (if the underlying OS allows this).

21.2. Firewall Considerations

A key security consideration of TURN is that TURN should not weaken
the protections afforded by firewalls deployed between a client and a
TURN server. It is anticipated that TURN servers will often be
present on the public Internet, and clients may often be inside
enterprise networks with corporate firewalls. If TURN servers
provide a 'backdoor' "backdoor" for reaching into the enterprise, TURN will be
blocked by these firewalls.

TURN servers therefore emulate the behavior of NAT devices that
implement address-dependent filtering [RFC4787], a property common in
many firewalls as well. When a NAT or firewall implements this
behavior, packets from an outside IP address are only allowed to be
sent to an internal IP address and port if the internal IP address
and port had recently sent a packet to that outside IP address. TURN
servers introduce the concept of permissions, which provide exactly
this same behavior on the TURN server. An attacker cannot send a
packet to a TURN server and expect it to be relayed towards the
client, unless the client has tried to contact the attacker first.

It is important to note that some firewalls have policies that are
even more restrictive than address-dependent filtering. Firewalls
can also be configured with address- and port-dependent filtering, or
they can be configured to disallow inbound traffic entirely. In
these cases, if a client is allowed to connect the TURN server,
communications to the client will be less restrictive than what the
firewall would normally allow.

21.2.1. Faked Permissions

In firewalls and NAT devices, permissions are granted implicitly
through the traversal of a packet from the inside of the network
towards the outside peer. Thus, a permission cannot, by definition,
be created by any entity except one inside the firewall or NAT. With
TURN, this restriction no longer holds. Since the TURN server sits
outside the firewall, at an attacker outside the firewall can now send a
message to the TURN server and try to create a permission for itself.

This attack is prevented because all messages that create permissions
(i.e., ChannelBind and CreatePermission) are authenticated.

21.2.2. Blacklisted IP Addresses

Many firewalls can be configured with blacklists that prevent a
client behind the firewall from sending packets to, or receiving
packets from, ranges of blacklisted IP addresses. This is
accomplished by inspecting the source and destination addresses of
packets entering and exiting the firewall, respectively.

This feature is also present in TURN, TURN since TURN servers are allowed
to arbitrarily restrict the range of addresses of peers that they
will relay to.

21.2.3. Running Servers on Well-Known Ports

A malicious client behind a firewall might try to connect to a TURN
server and obtain an allocation which that it then uses to run a server.
For example, a client might try to run a DNS server or FTP server.

This is not possible in TURN. A TURN server will never accept
traffic from a peer for which the client has not installed a
permission. Thus, peers cannot just connect to the allocated port in
order to obtain the service.

21.3. Insider Attacks

In insider attacks, a client has legitimate credentials but defies
the trust relationship that goes with those credentials. These
attacks cannot be prevented by cryptographic means but need to be
considered in the design of the protocol.

21.3.1. DoS against TURN Server

A client wishing to disrupt service to other clients might obtain an
allocation and then flood it with traffic, traffic in an attempt to swamp the
server and prevent it from servicing other legitimate clients. This
is mitigated by the recommendation that the server limit the amount
of bandwidth it will relay for a given username. This won't prevent
a client from sending a large amount of traffic, but it allows the
server to immediately discard traffic in excess.

Since each allocation uses a port number on the IP address of the
TURN server, the number of allocations on a server is finite. An
attacker might attempt to consume all of them by requesting a large
number of allocations. This is prevented by the recommendation that
the server impose a limit of on the number of allocations active at a
time for a given username.

21.3.2. Anonymous Relaying of Malicious Traffic

TURN servers provide a degree of anonymization. A client can send
data to peers without revealing its own IP address. TURN servers may
therefore become attractive vehicles for attackers to launch attacks
against targets without fear of detection. Indeed, it is possible
for a client to chain together multiple TURN servers, servers such that any
number of relays can be used before a target receives a packet.

Administrators who are worried about this attack can maintain logs
that capture the actual source IP and port of the client, client and perhaps
even every permission that client installs. This will allow for
forensic tracing to determine the original source, source should it be
discovered that an attack is being relayed through a TURN server.

21.3.3. Manipulating Other Allocations

An attacker might attempt to disrupt service to other users of the
TURN server by sending Refresh requests or CreatePermission requests
that (through source address spoofing) appear to be coming from
another user of the TURN server. TURN prevents this by requiring
that the credentials used in CreatePermission, Refresh, and
ChannelBind messages match those used to create the initial
allocation. Thus, the fake requests from the attacker will be
rejected.

21.4. Tunnel Amplification Attack

An attacker might attempt to cause data packets to loop numerous
times between a TURN server and a tunnel between IPv4 and IPv6. The
attack goes as follows. follows:

Suppose an attacker knows that a tunnel endpoint will forward
encapsulated packets from a given IPv6 address (this doesn't
necessarily need to be the tunnel endpoint's address). Suppose he
then spoofs two packets from this address:

1. An Allocate request asking for a v4 address, and

2. A ChannelBind request establishing a channel to the IPv4 address
of the tunnel endpoint

Then endpoint.

Then, he has set up an amplification attack:

* The TURN server will re-encapsulate IPv6 UDP data in v4 and send
it to the tunnel endpoint

o endpoint.

* The tunnel endpoint will de-encapsulate packets from the v4
interface and send them to v6

So v6.

So, if the attacker sends a packet of the following form... form:

IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
UDP: <ports>
TURN: <channel id>
IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
UDP: <ports>
TURN: <channel id>
IPv6: src=2001:DB8:1::1 dst=2001:DB8::2
UDP: <ports>
TURN: <channel id>
...

Then

Figure 19

then the TURN server and the tunnel endpoint will send it back and
forth until the last TURN header is consumed, at which point the TURN
server will send an empty packet, which packet that the tunnel endpoint will drop.

The amplification potential here is limited by the MTU, so it's not
huge: IPv6+UDP+TURN takes 334 bytes, so a four-to-one amplification
out of a 1500-byte packet is possible. But But, the attacker could still
increase traffic volume by sending multiple packets or by
establishing multiple channels spoofed from different addresses
behind the same tunnel endpoint.

The attack is mitigated as follows. It is RECOMMENDED that TURN
servers not accept allocation or channel binding channel-binding requests from
addresses known to be tunneled, and that they not forward data to
such addresses. In particular, a TURN server MUST NOT accept Teredo
or 6to4 addresses in these requests.

21.5. Other Considerations

Any relay addresses learned through an Allocate request will not
operate properly with IPsec Authentication Header (AH) [RFC4302] in
transport or tunnel mode. However, tunnel-mode IPsec Encapsulating
Security Payload (ESP) [RFC4303] should still operate.

22. IANA Considerations

[Paragraphs in braces should be removed by the RFC Editor upon
publication]

The codepoints code points for the STUN methods defined in this specification
are listed in Section 17. [IANA is requested to update IANA has updated the reference references from
[RFC5766] to RFC-to-be for this document (for the STUN methods listed in
Section 17.] 17).

The codepoints code points for the STUN attributes defined in this specification
are listed in Section 18. [IANA is requested to update IANA has updated the reference references from
[RFC5766] to RFC-to-be for this document (for the STUN attributes CHANNEL-NUMBER,
LIFETIME, Reserved (was BANDWIDTH), XOR-PEER-ADDRESS, DATA, XOR-
RELAYED-ADDRESS, REQUESTED-ADDRESS-FAMILY, EVEN-PORT, REQUESTED-
TRANSPORT, DONT-FRAGMENT, Reserved (was TIMER-VAL) TIMER-VAL), and RESERVATION-
TOKEN listed in Section 18.]

[The ADDITIONAL-ADDRESS-FAMILY, ADDRESS-ERROR-CODE and ICMP
attributes requires that IANA allocate a value in the "STUN
attributes Registry" from the comprehension-optional range
(0x8000-0xFFFF), to be replaced for TBD-CA throughout this document] 18).

The codepoints code points for the STUN error codes defined in this
specification are listed in Section 19. [IANA is requested to update IANA has updated the reference
references from [RFC5766] and [RFC6156] to RFC-to-be for this document (for the
STUN error codes listed in Section 19.] 19).

IANA has allocated updated the references to [RFC5766] to this document for the
SRV service name of "turn" for TURN over UDP or TCP, TCP and the service
name of "turns" for TURN over (D)TLS.

IANA has created a registry for TURN channel numbers, numbers (the "Traversal
Using Relays around NAT (TURN) Channel Numbers" registry), initially
populated as follows:

o |
+------------------------+------------------------------------------+
| 0x5000 through | Reserved (For DTLS-SRTP multiplexing |
| 0xFFFF: Unassigned. | collision avoidance, see [RFC7983]) |
+------------------------+------------------------------------------+

Table 6

Any change to this registry must be made through an IETF Standards
Action.

23. IAB Considerations

The IAB has studied the problem of "Unilateral Self Address Fixing"
(UNSAF), which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
through a collaborative protocol-reflection mechanism [RFC3424]. The
TURN extension is an example of a protocol that performs this type of
function. The IAB has mandated that any protocols developed for this
purpose document a specific set of considerations. These
considerations and the responses for TURN are documented in this
section.

Consideration 1: Precise definition of a specific, limited-scope
problem that is to be solved with the UNSAF proposal. A short-term
fix should not be generalized to solve other problems. Such
generalizations lead to the prolonged dependence on and usage of the
supposed short-term fix -- fix, meaning that it is no longer accurate to
call it "short-term".

Response: TURN is a protocol for communication between a relay (=
TURN server) and its client. The protocol allows a client that is
behind a NAT to obtain and use a public IP address on the relay. As
a convenience to the client, TURN also allows the client to determine
its server-reflexive transport address.

Consideration 2: Description of an exit strategy/transition plan.
The better short-term fixes are the ones that will naturally see less
and less use as the appropriate technology is deployed.

Response: TURN will no longer be needed once there are no longer any
NATs. Unfortunately, as of the date of publication of this document,
it no longer seems very likely that NATs will go away any time soon.
However, the need for TURN will also decrease as the number of NATs
with the mapping property of Endpoint-Independent Mapping [RFC4787]
increases.

Consideration 3: Discussion of specific issues that may render
systems more "brittle". For example, approaches that involve using
data at multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.

Response: TURN is "brittle" in that it requires the NAT bindings
between the client and the server to be maintained unchanged for the
lifetime of the allocation. This is typically done using keep-
alives. If this is not done, then the client will lose its
allocation and can no longer exchange data with its peers.

Consideration 4: Identify requirements for longer-term, sound
technical solutions; contribute to the process of finding the right
longer-term solution.

Response: The need for TURN will be reduced once NATs implement the
recommendations for NAT UDP behavior documented in [RFC4787].
Applications are also strongly urged to use ICE [RFC8445] to
communicate with peers; though ICE uses TURN, it does so only as a
last resort, and it uses it in a controlled manner.

Consideration 5: Discussion of the impact of the noted practical
issues with existing deployed NATs and experience reports.

Response: Some NATs deployed today exhibit a mapping behavior other
than Endpoint-Independent mapping. These NATs are difficult to work
with, as they make it difficult or impossible for protocols like ICE
to use server-reflexive transport addresses on those NATs. A client
behind such a NAT is often forced to use a relay protocol like TURN
because "UDP hole punching" techniques [RFC5128] do not work.

24. Changes since RFC 5766

This section lists the major changes in the TURN protocol from the
original [RFC5766] specification.

* IPv6 support.

* REQUESTED-ADDRESS-FAMILY attribute.

* Description of the tunnel amplification attack.

* DTLS support.

* Add support for receiving ICMP packets.

* Updates PMTUD.

* Discovery of TURN server.

* TURN URI Scheme Semantics.

* Happy Eyeballs for TURN.

* Align with the changes in STUNbis. STUN [RFC8489].

25. Updates to RFC 6156

This section lists the major updates to [RFC6156] in this
specification.

o ADDITIONAL-ADDRESS-FAMILY, AND ADDRESS-ERR-CODE

* ADDITIONAL-ADDRESS-FAMILY and ADDRESS-ERROR-CODE attributes.

* 440 (Address Family not Supported) and 443 (Peer Address Family
Mismatch) responses.

* More details on packet translation.

* TCP-to-UDP and UDP-to-TCP relaying.

26. Acknowledgements

Most of the text in this note comes from the original TURN
specification, [RFC5766]. The authors would like to thank Rohan Mahy
co-author of original TURN specification and everyone who had
contributed to that document. The authors would also like to
acknowledge that this document inherits material from [RFC6156].

Thanks to Justin Uberti, Pal Martinsen, Oleg Moskalenko, Aijun Wang
and Simon Perreault for their help on the ADDITIONAL-ADDRESS-FAMILY
mechanism. Authors would like to thank Gonzalo Salgueiro, Simon
Perreault, Jonathan Lennox, Brandon Williams, Karl Stahl, Noriyuki
Torii, Nils Ohlmeier, Dan Wing, Vijay Gurbani, Joseph Touch, Justin
Uberti, Christopher Wood, Roman Danyliw, Eric Vyncke, Adam Roach,
Suresh Krishnan, Mirja Kuehlewind, Benjamin Kaduk and Oleg Moskalenko
for comments and review. The authors would like to thank Marc for
his contributions to the text.

Special thanks to Magnus Westerlund for the detailed AD review.

27. References

27.1.

26.1. Normative References

[I-D.ietf-tram-stunbis]
Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and P. Matthews, "Session Traversal
Utilities for NAT (STUN)", draft-ietf-tram-stunbis-21
(work in progress), March 2019.

[Protocol-Numbers]
"IANA Protocol Numbers Registry", 2005,
<http://www.iana.org/assignments/protocol-numbers>.

[PROTOCOL-NUMBERS]
IANA, "Protocol Numbers",
<https://www.iana.org/assignments/protocol-numbers>.

[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, DOI 10.17487/RFC0792, September 1981,
<https://www.rfc-editor.org/info/rfc792>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
DOI 10.17487/RFC2474, December 1998,
<https://www.rfc-editor.org/info/rfc2474>.

[RFC3168] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP",
RFC 3168, DOI 10.17487/RFC3168, September 2001,
<https://www.rfc-editor.org/info/rfc3168>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC6437] Amante, S., Carpenter, B., Jiang, S., and J. Rajahalme,
"IPv6 Flow Label Specification", RFC 6437,
DOI 10.17487/RFC6437, November 2011,
<https://www.rfc-editor.org/info/rfc6437>.

[RFC7065] Petit-Huguenin, M., Nandakumar, S., Salgueiro, G., and P.
Jones, "Traversal Using Relays around NAT (TURN) Uniform
Resource Identifiers", RFC 7065, DOI 10.17487/RFC7065,
November 2013, <https://www.rfc-editor.org/info/rfc7065>.

[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
August 2014, <https://www.rfc-editor.org/info/rfc7350>.

[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.

[RFC7915] Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
"IP/ICMP Translation Algorithm", RFC 7915,
DOI 10.17487/RFC7915, June 2016,
<https://www.rfc-editor.org/info/rfc7915>.

[RFC7982] Martinsen, P., Reddy, T., Wing, D., and V. Singh,
"Measurement of Round-Trip Time and Fractional Loss Using
Session Traversal Utilities for NAT (STUN)", RFC 7982,
DOI 10.17487/RFC7982, September 2016,
<https://www.rfc-editor.org/info/rfc7982>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

27.2. Informative References
[Frag-Harmful]
Kent, C.

[RFC8489] Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
D., Mahy, R., and J. Mogul, ""Fragmentation Considered
Harmful", In Proc. SIGCOMM '87 Workshop on Frontiers in
Computer Communications Technology, P. Matthews, "Session Traversal
Utilities for NAT (STUN)", RFC 8489, DOI
10.1145/55483.55524", August 1987,
<http://www.hpl.hp.com/techreports/Compaq-DEC/
WRL-87-3.pdf>.

[I-D.ietf-intarea-frag-fragile] 10.17487/RFC8489,
February 2020, <https://doi.org/10.17487/RFC8489>.

26.2. Informative References

[FRAG-FRAGILE]
Bonica, R., Baker, F., Huston, G., Hinden, R., Troan, O.,
and F. Gont, "IP Fragmentation Considered Fragile", draft-
ietf-intarea-frag-fragile-15 (work Work
in progress), July
2019.

[I-D.ietf-mmusic-ice-sip-sdp]
Petit-Huguenin, Progress, Internet-Draft, draft-ietf-intarea-frag-
fragile-17, 30 September 2019,
<https://tools.ietf.org/html/draft-ietf-intarea-frag-
fragile-17>.

[FRAG-HARMFUL]
Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
December 1987, <https://www.hpl.hp.com/techreports/Compaq-
DEC/WRL-87-3.pdf>.

[MTU-DATAGRAM]
Fairhurst, G., Jones, T., Tuexen, M., Nandakumar, S., Ruengeler, I., and A. Keranen,
"Session Description Protocol (SDP) Offer/Answer
procedures for Interactive Connectivity Establishment
(ICE)", draft-ietf-mmusic-ice-sip-sdp-36 (work in
progress), June 2019.

[I-D.ietf-mptcp-rfc6824bis]
Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", draft-ietf-mptcp-rfc6824bis-18 (work
in progress), June 2019.

[I-D.ietf-rtcweb-security]
Rescorla, E., "Security Considerations
T. Voelker, "Packetization Layer Path MTU Discovery for WebRTC", draft-
ietf-rtcweb-security-12 (work
Datagram Transports", Work in progress), July 2019.

[I-D.ietf-tram-stun-pmtud] Progress, Internet-Draft,
draft-ietf-tsvwg-datagram-plpmtud-14, 12 February 2020,
<https://tools.ietf.org/html/draft-ietf-tsvwg-datagram-
plpmtud-14>.

[MTU-STUN] Petit-Huguenin, M. and G. M., Salgueiro, "Path G., and F. Garrido,
"Packetization Layer Path MTU Discovery (PLMTUD) For UDP
Transports Using Session Traversal Utilities for NAT
(STUN)", draft-
ietf-tram-stun-pmtud-10 (work Work in progress), September
2018.

[I-D.ietf-tsvwg-udp-options]
Touch, J., "Transport Options for UDP", draft-ietf-tsvwg-
udp-options-07 (work in progress), March 2019.

[Port-Numbers]
"IANA Progress, Internet-Draft, draft-ietf-
tram-stun-pmtud-15, 17 December 2019,
<https://tools.ietf.org/html/draft-ietf-tram-stun-pmtud-
15>.

[PORT-NUMBERS]
IANA, "Service Name and Transport Protocol Port Numbers Number
Registry", 2005,
<http://www.iana.org/assignments/port-numbers>.
<https://www.iana.org/assignments/port-numbers>.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
DOI 10.17487/RFC1191, November 1990,
<https://www.rfc-editor.org/info/rfc1191>.

[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.

[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
DOI 10.17487/RFC1928, March 1996,
<https://www.rfc-editor.org/info/rfc1928>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for
UNilateral Self-Address Fixing (UNSAF) Across Network
Address Translation", RFC 3424, DOI 10.17487/RFC3424,
November 2002, <https://www.rfc-editor.org/info/rfc3424>.

[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
July 2003, <https://www.rfc-editor.org/info/rfc3550>.

[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, DOI 10.17487/RFC3711, March 2004,
<https://www.rfc-editor.org/info/rfc3711>.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<https://www.rfc-editor.org/info/rfc4086>.

[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.

[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.

[RFC4787] Audet, F., Ed. and C. Jennings, "Network Address
Translation (NAT) Behavioral Requirements for Unicast
UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
2007, <https://www.rfc-editor.org/info/rfc4787>.

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<https://www.rfc-editor.org/info/rfc4821>.

[RFC5128] Srisuresh, P., Ford, B., and D. Kegel, "State of Peer-to-
Peer (P2P) Communication across Network Address
Translators (NATs)", RFC 5128, DOI 10.17487/RFC5128, March
2008, <https://www.rfc-editor.org/info/rfc5128>.

[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/info/rfc5482>.

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.

[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.

[RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT
(TURN) Resolution Mechanism", RFC 5928,
DOI 10.17487/RFC5928, August 2010,
<https://www.rfc-editor.org/info/rfc5928>.

[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
DOI 10.17487/RFC6056, January 2011,
<https://www.rfc-editor.org/info/rfc6056>.

[RFC6062] Perreault, S., Ed. and J. Rosenberg, "Traversal Using
Relays around NAT (TURN) Extensions for TCP Allocations",
RFC 6062, DOI 10.17487/RFC6062, November 2010,
<https://www.rfc-editor.org/info/rfc6062>.

[RFC6156] Camarillo, G., Novo, O., and S. Perreault, Ed., "Traversal
Using Relays around NAT (TURN) Extension for IPv6",
RFC 6156, DOI 10.17487/RFC6156, April 2011,
<https://www.rfc-editor.org/info/rfc6156>.

[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263,
DOI 10.17487/RFC6263, June 2011,
<https://www.rfc-editor.org/info/rfc6263>.

[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.

[RFC7478] Holmberg, C., Hakansson, S., and G. Eriksson, "Web Real-
Time Communication Use Cases and Requirements", RFC 7478,
DOI 10.17487/RFC7478, March 2015,
<https://www.rfc-editor.org/info/rfc7478>.

[RFC7635] Reddy, T., Patil, P., Ravindranath, R., and J. Uberti,
"Session Traversal Utilities for NAT (STUN) Extension for
Third-Party Authorization", RFC 7635,
DOI 10.17487/RFC7635, August 2015,
<https://www.rfc-editor.org/info/rfc7635>.

[RFC7657] Black, D., Ed. and P. Jones, "Differentiated Services
(Diffserv) and Real-Time Communication", RFC 7657,
DOI 10.17487/RFC7657, November 2015,
<https://www.rfc-editor.org/info/rfc7657>.

[RFC7983] Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
Updates for Secure Real-time Transport Protocol (SRTP)
Extension for Datagram Transport Layer Security (DTLS)",
RFC 7983, DOI 10.17487/RFC7983, September 2016,
<https://www.rfc-editor.org/info/rfc7983>.

[RFC8155] Patil, P., Reddy, T., and D. Wing, "Traversal Using Relays
around NAT (TURN) Server Auto Discovery", RFC 8155,
DOI 10.17487/RFC8155, April 2017,
<https://www.rfc-editor.org/info/rfc8155>.

[RFC8311] Black, D., "Relaxing Restrictions on Explicit Congestion
Notification (ECN) Experimentation", RFC 8311,
DOI 10.17487/RFC8311, January 2018,
<https://www.rfc-editor.org/info/rfc8311>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[SDP-ICE] Petit-Huguenin, M., Nandakumar, S., Holmberg, C., Keranen,
A., and R. Shpount, "Session Description Protocol (SDP)
Offer/Answer procedures for Interactive Connectivity
Establishment (ICE)", Work in Progress, Internet-Draft,
draft-ietf-mmusic-ice-sip-sdp-39, 13 August 2019,
<https://tools.ietf.org/html/draft-ietf-mmusic-ice-sip-
sdp-39>.

[SEC-WEBRTC]
Rescorla, E., "Security Considerations for WebRTC", Work
in Progress, Internet-Draft, draft-ietf-rtcweb-security-
12, 5 July 2019, <https://tools.ietf.org/html/draft-ietf-
rtcweb-security-12>.

[TCP-EXT] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", Work in Progress, Internet-Draft,
draft-ietf-mptcp-rfc6824bis-18, 8 June 2019,
<https://tools.ietf.org/html/draft-ietf-mptcp-rfc6824bis-
18>.

[UDP-OPT] Touch, J., "Transport Options for UDP", Work in Progress,
September 2019, <https://tools.ietf.org/html/draft-ietf-
tsvwg-udp-options-08>.

Acknowledgements

Most of the text in this note comes from the original TURN
specification, [RFC5766]. The authors would like to thank Rohan
Mahy, coauthor of the original TURN specification, and everyone who
had contributed to that document. The authors would also like to
acknowledge that this document inherits material from [RFC6156].

Thanks to Justin Uberti, Pal Martinsen, Oleg Moskalenko, Aijun Wang,
and Simon Perreault for their help on the ADDITIONAL-ADDRESS-FAMILY
mechanism. The authors would like to thank Gonzalo Salgueiro, Simon
Perreault, Jonathan Lennox, Brandon Williams, Karl Stahl, Noriyuki
Torii, Nils Ohlmeier, Dan Wing, Vijay Gurbani, Joseph Touch, Justin
Uberti, Christopher Wood, Roman Danyliw, Eric Vyncke, Adam Roach,
Suresh Krishnan, Mirja Kuehlewind, Benjamin Kaduk, and Oleg
Moskalenko for comments and review. The authors would like to thank
Marc Petit-Huguenin for his contributions to the text.

Special thanks to Magnus Westerlund for the detailed AD review.

Authors' Addresses

Tirumaleswar Reddy (editor)
McAfee, Inc.
Embassy Golf Link Business Park
Bangalore, Karnataka
Bangalore 560071
Karnataka
India

Email: kondtir@gmail.com

Alan Johnston (editor)
Villanova University
Villanova, PA
USA
United States of America

Email: alan.b.johnston@gmail.com

Philip Matthews
Alcatel-Lucent
600 March Road
Ottawa,
Ottawa Ontario
Canada

Email: philip_matthews@magma.ca

Jonathan Rosenberg
jdrosen.net
Edison, NJ
USA
United States of America

Email: jdrosen@jdrosen.net
URI: http://www.jdrosen.net