Internet Engineering Task Force (IETF) M. Petit-Huguenin
Request for Comments: 7350 Jive Communications
Updates: 5389, 5928 G. Salgueiro
Category: Standards Track Cisco Systems
ISSN: 2070-1721 August 2014
Datagram Transport Layer Security (DTLS) as Transport
for Session Traversal Utilities for NAT (STUN)
Abstract
This document specifies the usage of Datagram Transport Layer
Security (DTLS) as a transport protocol for Session Traversal
Utilities for NAT (STUN). It provides guidance on when and how to
use DTLS with the currently standardized STUN usages. It also
specifies modifications to the STUN and Traversal Using Relay NAT
(TURN) URIs and to the TURN resolution mechanism to facilitate the
resolution of STUN and TURN URIs into the IP address and port of STUN
and TURN servers supporting DTLS as a transport protocol. This
document updates RFCs 5389 and 5928.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7350.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DTLS as Transport for STUN . . . . . . . . . . . . . . . . . 3
4. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. NAT Discovery Usage . . . . . . . . . . . . . . . . . . . 4
4.1.1. DTLS Support in STUN URIs . . . . . . . . . . . . . . 4 5
4.2. Connectivity Check Usage . . . . . . . . . . . . . . . . 5
4.3. Media Keep-Alive Usage . . . . . . . . . . . . . . . . . 5
4.4. SIP Keep-Alive Usage . . . . . . . . . . . . . . . . . . 5 6
4.5. NAT Behavior Discovery Usage . . . . . . . . . . . . . . 5 6
4.6. TURN Usage . . . . . . . . . . . . . . . . . . . . . . . 6
4.6.1. DTLS Support in TURN URIs . . . . . . . . . . . . . . 6 7
4.6.2. Resolution Mechanism for TURN over DTLS . . . . . . . 6 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9
6.1. S-NAPTR Application Protocol Tag . . . . . . . . . . . . 8 9
6.2. Service Name and Transport Protocol Port Number . . . . . 8 9
6.2.1. The "stuns" Service Name . . . . . . . . . . . . . . 9 10
6.2.2. The "turns" Service Name . . . . . . . . . . . . . . 9 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 10 12
8.2. Informative References . . . . . . . . . . . . . . . . . 11 13
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 12 14
1. Introduction
STUN [RFC5389] defines Transport Layer Security (TLS) over TCP
(simply referred to as TLS [RFC5246]) as the transport for STUN due
to additional security advantages it offers over plain UDP or TCP
transport. But, TCP (and thus TLS-over-TCP) is not an optimal
transport when STUN is used for its originally intended purpose,
which is to support multimedia sessions. This is a well documented
and understood transport limitation for real-time communications.
DTLS-over-UDP (referred to in this document as simply DTLS [RFC6347])
offers the same security advantages as TLS-over-TCP, but without the
undesirable concerns.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in [RFC2119] when
they appear in ALL CAPS. When these words are not in ALL CAPS (such
as "must" or "Must"), they have their usual English meanings, and are
not to be interpreted as RFC 2119 key words.
3. DTLS as Transport for STUN
STUN [RFC5389] defines three transports: UDP, TCP, and TLS. This
document adds DTLS as a valid transport for STUN.
STUN over DTLS MUST use the same retransmission rules as STUN over
UDP (as described in Section 7.2.1 of [RFC5389]). It MUST also use
the same rules that are described in Section 7.2.2 of [RFC5389] to
verify the server identity. Instead of TLS_RSA_WITH_AES_128_CBC_SHA,
which is the default cipher suite for STUN over TLS, implementations
of STUN over DTLS, and deployed clients and servers, MUST support
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256, and MAY support other cipher
suites. Perfect Forward Secrecy (PFS) cipher suites MUST be
preferred over non-PFS cipher suites. Cipher suites with known
weaknesses, such as those based on (single) DES and RC4, MUST NOT be
used. Implementations MUST disable TLS-level compression. The same
rules established in Section 7.2.2 of [RFC5389] for keeping open and
closing TCP/TLS connections MUST be used as well for DTLS
associations.
In addition to the path MTU rules described in Section 7.1 of
[RFC5389], if the path MTU is unknown, the actual STUN message needs
to be adjusted to take into account the size of the (13-byte) DTLS
Record header, the MAC size, and the padding size.
By default, STUN over DTLS MUST use port 5349, the same port number
as STUN over TLS. However, the Service Record (SRV) procedures can
be implemented to use a different port (as described in Section 9 of
[RFC5389]). When using SRV records, the service name MUST be set to
"stuns" and the protocol name to "udp".
Classic STUN [RFC3489] (which was obsoleted by [RFC5389]) defines
only UDP as a transport, and DTLS MUST NOT be used. Any STUN request
or indication without the magic cookie (see Section 6 of [RFC5389])
over DTLS MUST always result in an error.
4. STUN Usages
Section 7.2 of [RFC5389] states that STUN usages must specify which
transport protocol is used. The following sections discuss if and
how the existing STUN usages are used with DTLS as the transport.
Future STUN usages MUST take into account DTLS as a transport and
discuss its applicability. In all cases, new STUN usages MUST
explicitly state if implementing the denial-of-service countermeasure
described in Section 4.2.1 of [RFC6347] is mandatory.
4.1. NAT Discovery Usage
As stated by Section 13 of [RFC5389], "...TLS provides minimal
security benefits..." for this particular STUN usage. DTLS will also
similarly offer only limited benefit. This is because the only
mandatory attribute that is TLS/DTLS protected is the
XOR-MAPPED-ADDRESS, which is already known by an on-path attacker,
since it is the same as the source address and port of the STUN
request. On the other hand, using TLS/DTLS will prevent an active
attacker to inject XOR-MAPPED-ADDRESS in responses. The TLS/DTLS
transport will also protect the SOFTWARE attribute, which can be used
to find vulnerabilities in STUN implementations.
Regardless, this usage is rarely used by itself, since using TURN
[RFC5766] with Interactive Connectivity Establishment (ICE) [RFC5245]
is generally indispensable, and TURN provides the same NAT Discovery
feature as part of an allocation creation. In fact, with ICE, the
NAT Discovery usage is only used when there is no longer any resource
available for new allocations in the TURN server.
A STUN server implementing the NAT Discovery usage and using DTLS
MUST implement the denial-of-service countermeasure described in
Section 4.2.1 of [RFC6347].
4.1.1. DTLS Support in STUN URIs
This document does not make any changes to the syntax of a STUN URI
[RFC7064]. As indicated in Section 3.2 of [RFC7064], secure
transports like STUN over TLS, and now STUN over DTLS, MUST use the
"stuns" URI scheme.
The <host> value MUST be used when using the rules in Section 7.2.2
of [RFC5389] to verify the server identity. A STUN URI containing an
IP address MUST be rejected, unless the domain name is provided by
the same mechanism that provided the STUN URI, and that domain name
can be passed to the verification code.
4.2. Connectivity Check Usage
Using DTLS would hide the USERNAME, PRIORITY, USE-CANDIDATE,
ICE-CONTROLLED, and ICE-CONTROLLING attributes. But, because
MESSAGE-INTEGRITY protects the entire STUN response using a password
that is known only by looking at the Session Description Protocol
(SDP) exchanged, it is not possible for an attacker that does not
have access to this SDP to inject an incorrect XOR-MAPPED-ADDRESS,
which would subsequently be used as a peer reflexive candidate.
Adding DTLS on top of the connectivity check would delay, and
consequently impair, the ICE process. Adding additional round trips
to ICE is undesirable, so much that there is a proposal ([ICE-DTLS])
to use the DTLS handshake used by the WebRTC Secure Real-time
Transport Protocol (SRTP) streams as a replacement for the
connectivity checks.
STUN URIs are not used with this usage.
4.3. Media Keep-Alive Usage
When STUN Binding Indications are being used for media keep-alive
(described in Section 10 of [RFC5245]), it runs alongside an RTP or
RTP Control Protocol (RTCP) session. It is possible to send these
media keep-alive packets inside a separately negotiated non-SRTP DTLS
session if DTLS-SRTP [RFC5764] is used, but that would add overhead,
with minimal security benefit.
STUN URIs are not used with this usage.
4.4. SIP Keep-Alive Usage
The SIP keep-alive (described in [RFC5626]) runs inside a SIP flow.
This flow would be protected if a SIP over DTLS transport mechanism
is implemented (such as described in [SIP-DTLS]).
STUN URIs are not used with this usage.
4.5. NAT Behavior Discovery Usage
The NAT Behavior Discovery usage is Experimental and to date has
never been effectively deployed. Despite this, using DTLS would add
the same security properties as for the NAT Discovery usage
(Section 4.1).
The STUN URI can be used to access the NAT Discovery feature of a NAT
Behavior Discovery server, but accessing the full features would
require definition of a "stun-behaviors:" URI, which is out of scope
for this document.
A STUN server implementing the NAT Behavior Discovery usage and using
DTLS MUST implement the denial-of-service countermeasure described in
Section 4.2.1 of [RFC6347].
4.6. TURN Usage
TURN [RFC5766] defines three combinations of transports/allocations:
UDP/UDP, TCP/UDP, and TLS/UDP. This document adds DTLS/UDP as a
valid combination. A TURN server using DTLS MUST implement the
denial-of-service countermeasure described in Section 4.2.1 of
[RFC6347].
[RFC6062] states that TCP allocations cannot be obtained using a UDP
association between client and server. The fact that DTLS uses UDP
implies that TCP allocations MUST NOT be obtained using a DTLS
association between client and server.
By default, TURN over DTLS uses port 5349, the same port number as
TURN over TLS. However, the SRV procedures can be implemented to use
a different port (as described in Section 6 of [RFC5766]). When
using SRV records, the service name MUST be set to "turns" and the
protocol name to "udp".
4.6.1. DTLS Support in TURN URIs
This document does not make any changes to the syntax of a TURN URI
[RFC7065]. As indicated in Section 3 of [RFC7065], secure transports
like TURN over TLS, and now TURN over DTLS, MUST use the "turns" URI
scheme. When using the "turns" URI scheme to designate TURN over
DTLS, the transport value of the TURN URI, if set, MUST be "udp".
The <host> value MUST be used when using the rules in Section 7.2.2
of [RFC5389] to verify the server identity. A TURN URI containing an
IP address MUST be rejected, unless the domain is provided by the
same mechanism that provided the TURN URI, and that domain name can
be passed to the verification code.
4.6.2. Resolution Mechanism for TURN over DTLS
This document defines a new Straightforward-Naming Authority Pointer
(S-NAPTR) application protocol tag: "turn.dtls".
The <transport> component, as provisioned or resulting from the
parsing of a TURN URI, is passed without modification to the TURN
resolution mechanism defined in Section 3 of [RFC5928], but with the
following alterations to that algorithm:
o The acceptable values for the transport name are extended with the
addition of "dtls".
o The acceptable values in the ordered list of supported TURN
transports is extended with the addition of "Datagram Transport
Layer Security (DTLS)".
o The resolution algorithm check rules list is extended with the
addition of the following step:
If <secure> is true and <transport> is defined as "udp" but the
list of TURN transports supported by the application does not
contain DTLS, then the resolution MUST stop with an error.
o The 5th rule of the resolution algorithm check rules list is
modified to read like this:
If <secure> is true and <transport> is not defined but the list
of TURN transports supported by the application does not
contain TLS or DTLS, then the resolution MUST stop with an
error.
o Table 1 is modified to add the following line:
+----------+-------------+----------------+
| <secure> | <transport> | TURN Transport |
+----------+-------------+----------------+
| true | "udp" | DTLS |
+----------+-------------+----------------+
o In step 1 of the resolution algorithm, the default port for DTLS
is 5349.
o In step 4 of the resolution algorithm, the following is added to
the list of conversions between the filtered list of TURN
transports supported by the application and application protocol
tags:
"turn.dtls" is used if the TURN transport is DTLS.
Note that using the resolution mechanism in [RFC5928] does not imply
that additional round trips to the DNS server will be needed (e.g.,
the TURN client will start immediately if the TURN URI contains an IP
address).
5. Security Considerations
STUN over DTLS as a STUN transport does not introduce any specific
security considerations beyond those for STUN over TLS detailed in
[RFC5389].
The usage of "udp" as a transport parameter with the "stuns" URI
scheme does not introduce any specific security issues beyond those
discussed in [RFC7064].
TURN over DTLS as a TURN transport does not introduce any specific
security considerations beyond those for TURN over TLS detailed in
[RFC5766].
The usage of "udp" as a transport parameter with the "turns" URI
scheme does not introduce any specific security issues beyond those
discussed in [RFC7065].
The new S-NAPTR application protocol tag defined in this document as
well as the modifications this document makes to the TURN resolution
mechanism described in [RFC5928] do not introduce any additional
security considerations beyond those outlined in [RFC5928].
6. IANA Considerations
6.1. S-NAPTR Application Protocol Tag
This specification contains the registration information for one
S-NAPTR application protocol tag in the "Straightforward-NAPTR
(S-NAPTR) Parameters" registry under "S-NAPTR Application Protocol
Tags" (in accordance with [RFC3958]).
Application Protocol Tag: turn.dtls
Intended Usage: See Section 4.6.2
Interoperability considerations: N/A
Security considerations: See Section 5
Relevant publications: This document
Contact information: Marc Petit-Huguenin <petithug@acm.org>
Author/Change controller: The IESG
6.2. Service Name and Transport Protocol Port Number
This specification contains the registration information for two
Service Name and Transport Protocol Port Numbers in the "Service
Names and Transport Protocol Port Numbers/Service Name and Transport
Protocol Port Number" registry (in accordance with [RFC6335]).
6.2.1. The "stuns" Service Name
IANA has modified the following entry in the registry "Service Names
and Transport Protocol Port Numbers/Service Name and Transport
Protocol Port Number":
Service Name: stuns
PortNumber: 5349
Transport Protocol(s): UDP
Assignee:
Contact: Protocol: udp
Description: Reserved for a future enhancement of STUN
Assignee:
Contact:
Reference: RFC 5389
Port Number: 5349
So that it contains the following:
Service Name: stuns
Port Number: 5349
Transport Protocol(s): UDP Protocol: udp
Description: STUN over DTLS
Assignee: IESG
Contact: IETF Chair <chair@ietf.org>
Description: STUN over DTLS
Reference: RFC 7350
Port Number: 5349
Assignment Notes: This service name was initially created by
RFC 5389.
6.2.2. The "turns" Service Name
IANA has modified the following entry in the registry "Service Names
and Transport Protocol Port Numbers/Service Name and Transport
Protocol Port Number":
Service Name: turns
Port Number: 5349
Transport Protocol(s): UDP
Assignee:
Contact: Protocol: udp
Description: Reserved for a future enhancement of TURN
Assignee:
Contact:
Reference: RFC 5766
Port Number: 5349
So that it contains the following:
Service Name: turns
Port Number: 5349
Transport Protocol(s): UDP Protocol: udp
Description: TURN over DTLS
Assignee: IESG
Contact: IETF Chair <chair@ietf.org>
Description: TURN over DTLS
Reference: RFC 7350
Port Number: 5349
Assignment Notes: This service name was initially created by
RFC 5766.
7. Acknowledgements
Thanks to Alan Johnston, Oleg Moskalenko, Simon Perreault, Thomas
Stach, Simon Josefsson, Roni Even, Kathleen Moriarty, Benoit Claise,
Martin Stiemerling, Jari Arkko, and Stephen Farrell for the comments,
suggestions, and questions that helped improve this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[RFC3958] Daigle, L. and A. Newton, "Domain-Based Application
Service Location Using SRV RRs and the Dynamic Delegation
Discovery Service (DDDS)", RFC 3958, January 2005.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245, April
2010.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
[RFC5764] McGrew, D. and E. Rescorla, "Datagram Transport Layer
Security (DTLS) Extension to Establish Keys for the Secure
Real-time Transport Protocol (SRTP)", RFC 5764, May 2010.
[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, April 2010.
[RFC5928] Petit-Huguenin, M., "Traversal Using Relays around NAT
(TURN) Resolution Mechanism", RFC 5928, August 2010.
[RFC6062] Perreault, S. and J. Rosenberg, "Traversal Using Relays
around NAT (TURN) Extensions for TCP Allocations", RFC
6062, November 2010.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, RFC
6335, August 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC7064] Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
Huguenin, "URI Scheme for the Session Traversal Utilities
for NAT (STUN) Protocol", RFC 7064, November 2013.
[RFC7065] Petit-Huguenin, M., Nandakumar, S., Salgueiro, G., and P.
Jones, "Traversal Using Relays around NAT (TURN) Uniform
Resource Identifiers", RFC 7065, November 2013.
8.2. Informative References
[ICE-DTLS] Thomson, M., "Using Datagram Transport Layer Security
(DTLS) For Interactivity Connectivity Establishment (ICE)
Connectivity Checking: ICE-DTLS", Work in Progress, March
2012.
[SIP-DTLS] Jennings, C. and N. Modadugu, "Session Initiation Protocol
(SIP) over Datagram Transport Layer Security (DTLS)", Work
in Progress, October 2007.
Appendix A. Examples
Table 1 shows how the <secure>, <port>, and <transport> components
are populated for a TURN URI that uses DTLS as its transport. For
all these examples, the <host> component is populated with
"example.net".
+---------------------------------+----------+--------+-------------+
| URI | <secure> | <port> | <transport> |
+---------------------------------+----------+--------+-------------+
| turns:example.net?transport=udp | true | | DTLS |
+---------------------------------+----------+--------+-------------+
Table 1
With the DNS Resource Records (RRs) in Figure 1 and an ordered TURN
transport list of {DTLS, TLS, TCP, UDP}, the resolution algorithm
will convert the TURN URI "turns:example.net" to the ordered list of
IP address, port, and protocol tuples in Table 2.
example.net.
IN NAPTR 100 10 "" RELAY:turn.udp:turn.dtls "" datagram.example.net.
IN NAPTR 200 10 "" RELAY:turn.tcp:turn.tls "" stream.example.net.
datagram.example.net.
IN NAPTR 100 10 S RELAY:turn.udp "" _turn._udp.example.net.
IN NAPTR 200 10 S RELAY:turn.dtls "" _turns._udp.example.net.
stream.example.net.
IN NAPTR 100 10 S RELAY:turn.tcp "" _turn._tcp.example.net.
IN NAPTR 200 10 A RELAY:turn.tls "" a.example.net.
_turn._udp.example.net.
IN SRV 0 0 3478 a.example.net.
_turn._tcp.example.net.
IN SRV 0 0 5000 a.example.net.
_turns._udp.example.net.
IN SRV 0 0 5349 a.example.net.
a.example.net.
IN A 192.0.2.1
Figure 1
+-------+----------+------------+------+
| Order | Protocol | IP address | Port |
+-------+----------+------------+------+
| 1 | DTLS | 192.0.2.1 | 5349 |
| 2 | TLS | 192.0.2.1 | 5349 |
+-------+----------+------------+------+
Table 2
Authors' Addresses
Marc Petit-Huguenin
Jive Communications
1275 West 1600 North, Suite 100
Orem, UT 84057
USA
EMail: marcph@getjive.com
Gonzalo Salgueiro
Cisco Systems
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
USA
EMail: gsalguei@cisco.com