wdiff rfc9308v1.txt rfc9308.txt

Internet Engineering Task Force (IETF) M. Kühlewind
Request for Comments: 9308 Ericsson
Category: Informational B. Trammell
ISSN: 2070-1721 Google
August Switzerland GmbH
September 2022

Applicability of the QUIC Transport Protocol

Abstract

This document discusses the applicability of the QUIC transport
protocol, focusing on caveats impacting application protocol
development and deployment over QUIC. Its intended audience is
designers of application protocol mappings to QUIC and implementors
of these application protocols.

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9308.

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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
2. The Necessity of Fallback
3. 0-RTT
3.1. Replay Attacks
3.2. Session Resumption versus Keep-Alive
4. Use of Streams
4.1. Stream versus Flow Multiplexing
4.2. Prioritization
4.3. Ordered and Reliable Delivery
4.4. Flow Control Deadlocks
4.5. Stream Limit Commitments
5. Packetization and Latency
6. Error Handling
7. Acknowledgment Efficiency
8. Port Selection and Application Endpoint Discovery
8.1. Source Port Selection
9. Connection Migration
10. Connection Termination
11. Information Exposure and the Connection ID
11.1. Server-Generated Connection ID
11.2. Mitigating Timing Linkability with Connection ID Migration
11.3. Using Server Retry for Redirection
12. Quality of Service (QoS) and Diffserv Code Point (DSCP)
13. Use of Versions and Cryptographic Handshake
14. Enabling Deployment of New Versions
15. Unreliable Datagram Service over QUIC
16. IANA Considerations
17. Security Considerations
18. References
18.1. Normative References
18.2. Informative References
Contributors
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

QUIC [QUIC] is a new transport protocol providing a number of
advanced features. While initially designed for the HTTP use case,
it provides capabilities that can be used with a much wider variety
of applications. QUIC is encapsulated in UDP. QUIC version 1
integrates TLS 1.3 [TLS13] to encrypt all payload data and most
control information. The version of HTTP that uses QUIC is known as
HTTP/3 [QUIC-HTTP].

This document provides guidance for application developers who want
to use the QUIC protocol without implementing it on their own. This
includes general guidance for applications operating over HTTP/3 or
directly over QUIC.

In the following sections, we discuss specific caveats to QUIC's
applicability and issues that application developers must consider
when using QUIC as a transport for their applications.

2. The Necessity of Fallback

QUIC uses UDP as a substrate. This enables userspace implementation
and permits traversal of network middleboxes (including NAT) without
requiring updates to existing network infrastructure.

Measurement studies have shown between 3% [Trammell16] and 5%
[Swett16] of networks block all UDP traffic, though there is little
evidence of other forms of systematic disadvantage to UDP traffic
compared to TCP [Edeline16]. This blocking implies that all
applications running on top of QUIC must either be prepared to accept
connectivity failure on such networks or be engineered to fall back
to some other transport protocol. In the case of HTTP, this fallback
is TLS over TCP.

The IETF Transport Services (TAPS) specifications [TAPS-ARCH]
describe a system with a common API for multiple protocols. This is
particularly relevant for QUIC as it addresses the implications of
fallback among multiple protocols.

Specifically, fallback to insecure protocols or to weaker versions of
secure protocols needs to be avoided. In general, an application
that implements fallback needs to consider the security consequences.
A fallback to TCP and TLS exposes control information to modification
and manipulation in the network. Additionally, downgrades to TLS
versions older than 1.3, which is used in QUIC version 1, might
result in significantly weaker cryptographic protection. For
example, the results of protocol negotiation [RFC7301] only have
confidentiality protection if TLS 1.3 is used.

These applications must operate, perhaps with impaired functionality,
in the absence of features provided by QUIC not present in the
fallback protocol. For fallback to TLS over TCP, the most obvious
difference is that TCP does not provide stream multiplexing, and
therefore stream multiplexing would need to be implemented in the
application layer if needed. Further, TCP implementations and
network paths often do not support the TCP Fast Open (TFO) option
[RFC7413], which enables sending of payload data together with the
first control packet of a new connection as also provided by 0-RTT
session resumption in QUIC. Note that there is some evidence of
middleboxes blocking SYN data even if TFO was successfully negotiated
(see [PaaschNanog]). And even if Fast Open successfully operates end
to end, it is limited to a single packet of TLS handshake and
application data, unlike QUIC 0-RTT.

Moreover, while encryption (in this case TLS) is inseparably
integrated with QUIC, TLS negotiation over TCP can be blocked. If
TLS over TCP cannot be supported, the connection should be aborted,
and the application then ought to present a suitable prompt to the
user that secure communication is unavailable.

In summary, any fallback mechanism is likely to impose a degradation
of performance and can degrade security; however, fallback must not
silently violate the application's expectation of confidentiality or
integrity of its payload data.

3. 0-RTT

QUIC provides for 0-RTT connection establishment. Though the same
facility exists in TLS 1.3 with TCP, 0-RTT presents opportunities and
challenges for applications using QUIC.

A transport protocol that provides 0-RTT connection establishment is
qualitatively different from one that does not provide 0-RTT from the
point of view of the application using it. Relative trade-offs
between the cost of closing and reopening a connection and trying to
keep it open are different; see Section 3.2.

An application needs to deliberately choose to use 0-RTT, as 0-RTT
carries a risk of replay attack. Application protocols that use
0-RTT require a profile that describes the types of information that
can be safely sent. For HTTP, this profile is described in
[HTTP-REPLAY].

3.1. Replay Attacks

Retransmission or malicious replay of data contained in 0-RTT packets
could cause the server side to receive multiple copies of the same
data.

Application data sent by the client in 0-RTT packets could be
processed more than once if it is replayed. Applications need to be
aware of what is safe to send in 0-RTT. Application protocols that
seek to enable the use of 0-RTT need a careful analysis and a
description of what can be sent in 0-RTT; see Section 5.6 of
[QUIC-TLS].

In some cases, it might be sufficient to limit application data sent
in 0-RTT to data that which only causes does not cause actions with lasting effects at
a server that are known
to be free of lasting effect. server. Initiating data retrieval or establishing configuration
are examples of actions that could be safe. Idempotent operations --
those for which repetition has the same net effect as a single
operation -- might be safe. However, it is also possible to combine
individually idempotent operations into a non-idempotent sequence of
operations.

Once a server accepts 0-RTT data, there is no means of selectively
discarding data that is received. However, protocols can define ways
to reject individual actions that might be unsafe if replayed.

Some TLS implementations and deployments might be able to provide
partial or even complete replay protection, which could be used to
manage replay risk.

3.2. Session Resumption versus Keep-Alive

Because QUIC is encapsulated in UDP, applications using QUIC must
deal with short network idle timeouts. Deployed stateful middleboxes
will generally establish state for UDP flows on the first packet sent
and keep state for much shorter idle periods than for TCP. [RFC5382]
suggests a TCP idle period of at least 124 minutes, though there is
no evidence of widespread implementation of this guideline in the
literature. However, short network timeout for UDP is well-
documented. According to a 2010 study ([Hatonen10]), UDP
applications can assume that any NAT binding or other state entry can
expire after just thirty seconds of inactivity. Section 3.5 of
[RFC8085] further discusses keep-alive intervals for UDP: it requires
that there is a minimum value of 15 seconds, but recommends larger
values, or that keep-alive is omitted entirely.

By using a connection ID, QUIC is designed to be robust to NAT
address
rebinding after a timeout. However, this only helps if one endpoint
maintains availability at the address its peer uses and the peer is
the one to send after the timeout occurs.

Some QUIC connections might not be robust to NAT rebinding because
the routing infrastructure (in particular, load balancers) uses the
address/port 4-tuple to direct traffic. Furthermore, middleboxes
with functions other than address translation could still affect the
path. In particular, some firewalls do not admit server traffic for
which the firewall has no recent state for a corresponding packet
sent from the client.

QUIC applications can adjust idle periods to manage the risk of
timeout. Idle periods and the network idle timeout are distinct from
the connection idle timeout, which is defined as the minimum of
either endpoint's idle timeout parameter; see Section 10.1 of
[QUIC]). [QUIC].
There are three options:

* Ignore the issue if the application-layer protocol consists only
of interactions with no or very short idle periods or if the
protocol's resistance to NAT rebinding is sufficient.

* Ensure there are no long idle periods.

* Resume the session after a long idle period, using 0-RTT
resumption when appropriate.

The first strategy is the easiest, but it only applies to certain
applications.

Either the server or the client in a QUIC application can send PING
frames as keep-alives to prevent the connection and any on-path state
from timing out. Recommendations for the use of keep-alives are
application specific, mainly depending on the latency requirements
and message frequency of the application. In this case, the
application mapping must specify whether the client or server is
responsible for keeping the application alive. While [Hatonen10]
suggests that 30 seconds might be a suitable value for the public
Internet when a NAT is on path, larger values are preferable if the
deployment can consistently survive NAT rebinding or is known to be
in a controlled environment (e.g., data centers) in order to lower
network and computational load.

Sending PING frames more frequently than every 30 seconds over long
idle periods may result in excessive unproductive traffic in some
situations and unacceptable power usage for power-constrained
(mobile) devices. Additionally, timeouts shorter than 30 seconds can
make it harder to handle transient network interruptions, such as
Virtual Machine (VM) migration or coverage loss during mobility. See
[RFC8085], especially Section 3.5.

Alternatively, the client (but not the server) can use session
resumption instead of sending keep-alive traffic. In this case, a
client that wants to send data to a server over a connection that has
been idle longer than the server's idle timeout (available from the
idle_timeout transport parameter) can simply reconnect. When
possible, this reconnection can use 0-RTT session resumption,
reducing the latency involved with restarting the connection. Of
course, this approach is only valid in cases in which it is safe to
use 0-RTT and when the client is the restarting peer.

The trade-offs between resumption and keep-alives need to be
evaluated on a per-application basis. In general, applications
should use keep-alives only in circumstances where continued
communication is highly likely; [QUIC-HTTP], for instance, recommends
using keep-alives only when a request is outstanding.

4. Use of Streams

QUIC's stream multiplexing feature allows applications to run
multiple streams over a single connection without head-of-line
blocking between streams. Stream data is carried within frames where
one QUIC packet on the wire can carry one or multiple stream frames.

Streams can be unidirectional or bidirectional, and a stream may be
initiated either by client or server. Only the initiator of a
unidirectional stream can send data on it.

Streams and connections can each carry a maximum of 2^62-1 bytes in
each direction due to encoding limitations on stream offsets and
connection flow control limits. In the presently unlikely event that
this limit is reached by an application, a new connection would need
to be established.

Streams can be independently opened and closed, gracefully or
abruptly. An application can gracefully close the egress direction
of a stream by instructing QUIC to send a FIN bit in a STREAM frame.
It cannot gracefully close the ingress direction without a peer-
generated FIN, much like in TCP. However, an endpoint can abruptly
close the egress direction or request that its peer abruptly close
the ingress direction; these actions are fully independent of each
other.

QUIC does not provide an interface for exceptional handling of any
stream. If a stream that is critical for an application is closed,
the application can generate error messages on the application layer
to inform the other end and/or the higher layer, which can eventually
terminate the QUIC connection.

Mapping of application data to streams is application specific and
described for HTTP/3 in [QUIC-HTTP]. There are a few general
principles to apply when designing an application's use of streams:

* A single stream provides ordering. If the application requires
certain data to be received in order, that data should be sent on
the same stream. There is no guarantee of transmission,
reception, or delivery order across streams.

* Multiple streams provide concurrency. Data that can be processed
independently, and therefore would suffer from head-of-line
blocking if forced to be received in order, should be transmitted
over separate streams.

* Streams can provide message orientation and allow messages to be
canceled. If one message is mapped to a single stream, resetting
the stream to expire an unacknowledged message can be used to
emulate partial reliability for that message.

If a QUIC receiver has opened the maximum allowed concurrent streams,
and the sender indicates that more streams are needed, it does not
automatically lead to an increase of the maximum number of streams by
the receiver. Therefore, an application should consider the maximum
number of allowed, currently open, and currently used streams when
determining how to map data to streams.

QUIC assigns a numerical identifier, called the stream ID, to each
stream. While the relationship between these identifiers and stream
types is clearly defined in version 1 of QUIC, future versions might
change this relationship for various reasons. QUIC implementations
should expose the properties of each stream (which endpoint initiated
the stream, whether the stream is unidirectional or bidirectional,
the stream ID used for the stream); applications should query for
these properties rather than attempting to infer them from the stream
ID.

The method of allocating stream identifiers to streams opened by the
application might vary between transport implementations. Therefore,
an application should not assume a particular stream ID will be
assigned to a stream that has not yet been allocated. For example,
HTTP/3 uses stream IDs to refer to streams that have already been
opened but makes no assumptions about future stream IDs or the way in
which they are assigned (see Section 6 of [QUIC-HTTP]).

4.1. Stream versus Flow Multiplexing

Streams are meaningful only to the application; since stream
information is carried inside QUIC's encryption boundary, a given
packet exposes no information about which stream(s) are carried
within the packet. Therefore, stream multiplexing is not intended to
be used for differentiating streams in terms of network treatment.
Application traffic requiring different network treatment should
therefore be carried over different 5-tuples (i.e., multiple QUIC
connections). Given QUIC's ability to send application data in the
first RTT of a connection (if a previous connection to the same host
has been successfully established to provide the necessary
credentials), the cost of establishing another connection is
extremely low.

4.2. Prioritization

Stream prioritization is not exposed to either the network or the
receiver. Prioritization is managed by the sender, and the QUIC
transport should provide an interface for applications to prioritize
streams [QUIC]. Applications can implement their own prioritization
scheme on top of QUIC: an application protocol that runs on top of
QUIC can define explicit messages for signaling priority, such as
those defined in [RFC9218] for HTTP. An application protocol can
define rules that allow an endpoint to determine priority based on
context or can provide a higher-level interface and leave the
determination to the application on top.

Priority handling of retransmissions can be implemented by the sender
in the transport layer. [QUIC] recommends retransmitting lost data
before new data, unless indicated differently by the application.
When a QUIC endpoint uses fully reliable streams for transmission,
prioritization of retransmissions will be beneficial in most cases,
filling in gaps and freeing up the flow control window. For
partially reliable or unreliable streams, priority scheduling of
retransmissions over data of higher-priority streams might not be
desirable. For such streams, QUIC could either provide an explicit
interface to control prioritization or derive the prioritization
decision from the reliability level of the stream.

4.3. Ordered and Reliable Delivery

QUIC streams enable ordered and reliable delivery. Though it is
possible for an implementation to provide options that use streams
for partial reliability or out-of-order delivery, most
implementations will assume that data is reliably delivered in order.

Under this assumption, an endpoint that receives stream data might
not make forward progress until data that is contiguous with the
start of a stream is available. In particular, a receiver might
withhold flow control credit until contiguous data is delivered to
the application; see Section 2.2 of [QUIC]. To support this receive
logic, an endpoint will send stream data until it is acknowledged,
ensuring that data at the start of the stream is sent and
acknowledged first.

An endpoint that uses a different sending behavior and does not
negotiate that change with its peer might encounter performance
issues or deadlocks.

4.4. Flow Control Deadlocks

QUIC flow control (Section 4 of [QUIC]) provides a means of managing
access to the limited buffers that endpoints have for incoming data.
This mechanism limits the amount of data that can be in buffers in
endpoints or in transit on the network. However, there are several
ways in which limits can produce conditions that can cause a
connection to either perform suboptimally or become deadlocked.

Deadlocks in flow control are possible for any protocol that uses
QUIC, though whether they become a problem depends on how
implementations consume data and provide flow control credit.
Understanding what causes deadlocking might help implementations
avoid deadlocks.

The size and rate of transport updates to flow control credit updates can affect
performance. Applications that use QUIC often have a data consumer
that reads data from transport buffers. Some implementations might
have independent transport-layer and application-layer receive
buffers. buffers at the transport layer and
application layer. Consuming data does not always imply it is
immediately processed. However, a common flow control implementation technique is
to extend flow control credit to the sender by emitting MAX_DATA and/or and/
or MAX_STREAM_DATA frames as data is consumed. Delivery of these
frames is affected by the latency of the back channel from the
receiver to the data sender. If credit is not extended in a timely
manner, the sending application can be blocked, effectively
throttling the sender.

Large application messages can produce deadlocking if the recipient
does not read data from the transport incrementally. If the message
is larger than the flow control credit available and the recipient
does not release additional flow control credit until the entire
message is received and delivered, a deadlock can occur. This is
possible even where stream flow control limits are not reached
because connection flow control limits can be consumed by other
streams.

A length-prefixed message format makes it easier for a data consumer
to leave data unread in the transport buffer and thereby withhold
flow control credit. If flow control limits prevent the remainder of
a message from being sent, a deadlock will result. A length prefix
might also enable the detection of this sort of deadlock. Where
application protocols have messages that might be processed as a
single unit, reserving flow control credit for the entire message
atomically makes this style of deadlock less likely.

A data consumer can eagerly read all data as it becomes available in
order to make the receiver extend flow control credit and reduce the
chances of a deadlock. However, such a data consumer might need
other means for holding a peer accountable for the additional state
it keeps for partially processed messages.

Deadlocking can also occur if data on different streams is
interdependent. Suppose that data on one stream arrives before the
data on a second stream on which it depends. A deadlock can occur if
the first stream is left unread, preventing the receiver from
extending flow control credit for the second stream. To reduce the
likelihood of deadlock for interdependent data, the sender should
ensure that dependent data is not sent until the data it depends on
has been accounted for in both stream- and connection-level flow
control credit.

Some deadlocking scenarios might be resolved by canceling affected
streams with STOP_SENDING or RESET_STREAM. Canceling some streams
results in the connection being terminated in some protocols.

4.5. Stream Limit Commitments

QUIC endpoints are responsible for communicating the cumulative limit
of streams they would allow to be opened by their peer. Initial
limits are advertised using the initial_max_streams_bidi and
initial_max_streams_uni transport parameters. As streams are opened
and closed, they are consumed, and the cumulative total is
incremented. Limits can be increased using the MAX_STREAMS frame,
but there is no mechanism to reduce limits. Once stream limits are
reached, no more streams can be opened, which prevents applications
using QUIC from making further progress. At this stage, connections
can be terminated via idle timeout or explicit close; see Section 10.

An application that uses QUIC and communicates a cumulative stream
limit might require the connection to be closed before the limit is
reached, e.g., to stop the server in order to perform scheduled
maintenance. Immediate connection close causes abrupt closure of
actively used streams. Depending on how an application uses QUIC
streams, this could be undesirable or detrimental to behavior or
performance.

A more graceful closure technique is to stop sending increases to
stream limits and allow the connection to naturally terminate once
remaining streams are consumed. However, the period of time it takes
to do so is dependent on the peer, and an unpredictable closing
period might not fit application or operational needs. Applications
using QUIC can be conservative with open stream limits in order to
reduce the commitment and indeterminism. However, being overly
conservative with stream limits affects stream concurrency.
Balancing these aspects can be specific to applications and their
deployments.

Instead of relying on stream limits to avoid abrupt closure, an
application layer's graceful close mechanism can be used to
communicate the intention to explicitly close the connection at some
future point. HTTP/3 provides such a mechanism using the GOAWAY
frame. In HTTP/3, when the GOAWAY frame is received by a client, it
stops opening new streams even if the cumulative stream limit would
allow. Instead, the client would create a new connection on which to
open further streams. Once all streams are closed on the old
connection, it can be terminated safely by a connection close or
after expiration of the idle timeout (see Section 10).

5. Packetization and Latency

QUIC exposes an interface that provides multiple streams to the
application; however, the application usually cannot control how data
transmitted over those streams is mapped into frames or how those
frames are bundled into packets.

By default, many implementations will try to maximally pack QUIC
packets DATA STREAM frames from
one or more streams into each QUIC packet, in order to minimize
bandwidth consumption and computational costs (see Section 13 of
[QUIC]). If there is not enough data available to fill a packet, an
implementation might wait for a short time to optimize bandwidth
efficiency instead of latency. This delay can either be
preconfigured or dynamically adjusted based on the observed sending
pattern of the application.

If the application requires low latency, with only small chunks of
data to send, it may be valuable to indicate to QUIC that all data
should be sent out immediately. Alternatively, if the application
expects to use a specific sending pattern, it can also provide a
suggested delay to QUIC for how long to wait before bundling frames
into a packet.

Similarly, an application usually has no control over the length of a
QUIC packet on the wire. QUIC provides the ability to add a PADDING
frame to arbitrarily increase the size of packets. Padding is used
by QUIC to ensure that the path is capable of transferring datagrams
of at least a certain size during the handshake (see Sections 8.1 and
14.1 of [QUIC]) and for path validation after connection migration
(see Section 8.2 of [QUIC]) as well as for Datagram Packetization
Layer PMTU Discovery (DPLPMTUD) (see Section 14.3 of [QUIC]).

Padding can also be used by an application to reduce leakage of
information about the data that is sent. A QUIC implementation can
expose an interface that allows an application layer to specify how
to apply padding.

6. Error Handling

QUIC recommends that endpoints signal any detected errors to the
peer. Errors can occur at the transport layer and the application
layer. Transport errors, such as a protocol violation, affect the
entire connection. Applications that use QUIC can define their own
error detection and signaling (see, for example, Section 8 of
[QUIC-HTTP]). Application errors can affect an entire connection or
a single stream.

QUIC defines an error code space that is used for error handling at
the transport layer. QUIC encourages endpoints to use the most
specific code, although any applicable code is permitted, including
generic ones.

Applications using QUIC define an error code space that is
independent of QUIC or other applications (see, for example,
Section 8.1 of [QUIC-HTTP]). The values in an application error code
space can be reused across connection-level and stream-level errors.

Connection errors lead to connection termination. They are signaled
using a CONNECTION_CLOSE frame, which contains an error code and a
reason field that can be zero length. Different types of
CONNECTION_CLOSE frames are used to signal transport and application
errors.

Stream errors lead to stream termination. These are signaled using
STOP_SENDING or RESET_STREAM frames, which contain only an error
code.

7. Acknowledgment Efficiency

QUIC version 1 without extensions uses an acknowledgment strategy
adopted from TCP (see Section 13.2 of [QUIC]). That is, it
recommends that every other packet is acknowledged. However,
generating and processing QUIC acknowledgments consumes resources at
a sender and receiver. Acknowledgments also incur forwarding costs
and contribute to link utilization, which can impact performance over
some types of network. Applications might be able to improve overall
performance by using alternative strategies that reduce the rate of
acknowledgments. [QUIC-ACK-FREQUENCY] describes an extension to
signal the desired delay of acknowledgments and discusses use cases
as well as implications for congestion control and recovery.

8. Port Selection and Application Endpoint Discovery

In general, port numbers serve two purposes: "first, they provide a
demultiplexing identifier to differentiate transport sessions between
the same pair of endpoints, and second, they may also identify the
application protocol and associated service to which processes
connect" (Section 3 of [RFC6335]). The assumption that an
application can be identified in the network based on the port number
is less true today due to encapsulation and mechanisms for dynamic
port assignments, as noted in [RFC6335].

As QUIC is a general-purpose transport protocol, there are no
requirements that servers use a particular UDP port for QUIC. For
applications an
application with a fallback to TCP that do does not already have an
alternate mapping to UDP, it is usually the registration appropriate to register (if
necessary) and use of the UDP port number corresponding to the TCP port
already registered for the application is appropriate. application. For example, the default
port for HTTP/3 [QUIC-HTTP] is UDP port 443, analogous to HTTP/1.1 or
HTTP/2 over TLS over TCP.

Given the prevalence of the assumption in network management practice
that a port number maps unambiguously to an application, the use of
ports that cannot easily be mapped to a registered service name might
lead to blocking or other changes to the forwarding behavior by
network elements such as firewalls that use the port number for
application identification.

Applications could define an alternate endpoint discovery mechanism
to allow the usage of ports other than the default. For example,
HTTP/3 (Sections 3.2 and 3.3 of [QUIC-HTTP]) specifies the use of
HTTP Alternative Services [RFC7838] for an HTTP origin to advertise
the availability of an equivalent HTTP/3 endpoint on a certain UDP
port by using "h3" as the Application-Layer Protocol Negotiation
(ALPN) [RFC7301] token.

ALPN permits the client and server to negotiate which of several
protocols will be used on a given connection. Therefore, multiple
applications might be supported on a single UDP port based on the
ALPN token offered. Applications using QUIC are required to register
an ALPN token for use in the TLS handshake.

As QUIC version 1 deferred defining a complete version negotiation
mechanism, HTTP/3 requires QUIC version 1 and defines the ALPN token
("h3") to only apply to that version. So far, no single approach has
been selected for managing the use of different QUIC versions,
neither in HTTP/3 nor in general. Application protocols that use
QUIC need to consider how the protocol will manage different QUIC
versions. Decisions for those protocols might be informed by choices
made by other protocols, like HTTP/3.

8.1. Source Port Selection

Some UDP protocols are vulnerable to reflection attacks, where an
attacker is able to direct traffic to a third party as a denial of
service. For example, these source ports are associated with
applications known to be vulnerable to reflection attacks, often due
to server misconfiguration:

* port 53 - DNS [RFC1034]

* port 123 - NTP [RFC5905]

* port 1900 - SSDP [SSDP]

* port 5353 - mDNS [RFC6762]

* port 11211 - memcached memcache

Services might block source ports associated with protocols known to
be vulnerable to reflection attacks to avoid the overhead of
processing large numbers of packets. However, this practice has
negative effects on clients -- not only does it require establishment
of a new connection but in some instances might cause the client to
avoid using QUIC for that service for a period of time and downgrade
to a non-UDP protocol (see Section 2).

As a result, client implementations are encouraged to avoid using
source ports associated with protocols known to be vulnerable to
reflection attacks. Note that following the general guidance for
client implementations given in [RFC6335], to use ephemeral ports in
the range 49152-65535, has the effect of avoiding these ports. Note
that other source ports might be reflection vectors as well.

9. Connection Migration

QUIC supports connection migration by the client. If the client's IP
address changes, a QUIC endpoint can still associate packets with an
existing transport connection using the Destination Connection ID
field (see Section 11) in the QUIC header. This supports cases where
the address information changes, such as NAT rebinding, the
intentional change of the local interface, the expiration of a
temporary IPv6 address [RFC8981], or the indication from the server
of a preferred address (Section 9.6 of [QUIC]).

Use of a non-zero-length connection ID for the server is strongly
recommended if any clients are or could be behind a NAT. A non-zero-
length connection ID is also strongly recommended when active
migration is supported. If a connection is intentionally migrated to
a new path, a new connection ID is used to minimize linkability by
network observers. The other QUIC endpoint uses the connection ID to
link different addresses to the same connection and entity if a non-
zero-length connection ID is provided.

The base specification of QUIC version 1 only supports the use of a
single network path at a time, which enables failover use cases.
Path validation is required so that endpoints validate paths before
use to avoid address spoofing attacks. Path validation takes at
least one RTT, and congestion control will also be reset after path
migration. Therefore, migration usually has a performance impact.

QUIC probing packets, which can be sent on multiple paths at once,
are used to perform address validation as well as measure path
characteristics. Probing packets cannot carry application data but
likely contain padding frames. Endpoints can use information about
their receipt as input to congestion control for that path.
Applications could use information learned from probing to inform a
decision to switch paths.

Only the client can actively migrate in version 1 of QUIC. However,
servers can indicate during the handshake that they prefer to
transfer the connection to a different address after the handshake.
For instance, this could be used to move from an address that is
shared by multiple servers to an address that is unique to the server
instance. The server can provide an IPv4 and an IPv6 address in a
transport parameter during the TLS handshake, and the client can
select between the two if both are provided. See Section 9.6 of
[QUIC].

10. Connection Termination

QUIC connections are terminated in one of three ways: implicit idle
timeout, explicit immediate close, or explicit stateless reset.

QUIC does not provide any mechanism for graceful connection
termination; applications using QUIC can define their own graceful
termination process (see, for example, Section 5.2 of [QUIC-HTTP]).

QUIC idle timeout is enabled via transport parameters. The client
and server announce a timeout period, and the effective value for the
connection is the minimum of the two values. After the timeout
period elapses, the connection is silently closed. An application
therefore should be able to configure its own maximum value, as well
as have access to the computed minimum value for this connection. An
application may adjust the maximum idle timeout for new connections
based on the number of open or expected connections since shorter
timeout values may free up resources more quickly.

Application data exchanged on streams or in datagrams defers the QUIC
idle timeout. Applications that provide their own keep-alive
mechanisms will therefore keep a QUIC connection alive. Applications
that do not provide their own keep-alive can use transport-layer
mechanisms (see Section 10.1.2 of [QUIC] and Section 3.2). However,
QUIC implementation interfaces for controlling such transport
behavior can vary, affecting the robustness of such approaches.

An immediate close is signaled by a CONNECTION_CLOSE frame (see
Section 6). Immediate close causes all streams to become immediately
closed, which may affect applications; see Section 4.5.

A stateless reset is an option of last resort for an endpoint that
does not have access to connection state. Receiving a stateless
reset is an indication of an unrecoverable error distinct from
connection errors in that there is no application-layer information
provided.

11. Information Exposure and the Connection ID

QUIC exposes some information to the network in the unencrypted part
of the header either before the encryption context is established or
because the information is intended to be used by the network. For
more information on manageability of QUIC, see [QUIC-MANAGEABILITY].
QUIC has a long header that exposes some additional information (the
version and the source connection ID), while the short header exposes
only the destination connection ID. In QUIC version 1, the long
header is used during connection establishment, while the short
header is used for data transmission in an established connection.

The connection ID can be zero length. Zero-length connection IDs can
be chosen on each endpoint individually and on any packet except the
first packets sent by clients during connection establishment.

An endpoint that selects a zero-length connection ID will receive
packets with a zero-length destination connection ID. The endpoint
needs to use other information, such as the source and destination IP
address and port number to identify which connection is referred to.
This could mean that the endpoint is unable to match datagrams to
connections successfully if these values change, making the
connection effectively unable to survive NAT rebinding or migrate to
a new path.

11.1. Server-Generated Connection ID

QUIC supports a server-generated connection ID that is transmitted to
the client during connection establishment (see Section 7.2 of
[QUIC]). Servers behind load balancers may need to change the
connection ID during the handshake, encoding the identity of the
server or information about its load balancing pool, in order to
support stateless load balancing.

Server deployments with load balancers and other routing
infrastructure need to ensure that this infrastructure consistently
routes packets to the server instance that has the connection state,
even if addresses, ports, or connection IDs change. This might
require coordination between servers and infrastructure. One method
of achieving this involves encoding routing information into the
connection ID. For an example of this technique, see [QUIC-LB].

11.2. Mitigating Timing Linkability with Connection ID Migration

If QUIC endpoints do not issue fresh connection IDs, then clients
cannot reduce the linkability of address migration by using them.
Choosing values that are unlinkable to an outside observer ensures
that activity on different paths cannot be trivially correlated using
the connection ID.

While sufficiently robust connection ID generation schemes will
mitigate linkability issues, they do not provide full protection.
Analysis of the lifetimes of 6-tuples (source and destination
addresses as well as the migrated Connection ID) may expose these
links anyway.

In the case where connection migration in a server pool is rare, it
is trivial for an observer to associate two connection IDs.
Conversely, in the opposite limit where every server handles multiple simultaneous
migrations, even an exposed server mapping may be insufficient
information.

The most efficient mitigations for these attacks are through network
design and/or operational practices, by using a load-balancing
architecture that loads more flows onto a single server-side address,
by coordinating the timing of migrations in an attempt to increase
the number of simultaneous migrations at a given time, or by using
other means.

11.3. Using Server Retry for Redirection

QUIC provides a Retry packet that can be sent by a server in response
to the client Initial packet. The server may choose a new connection
ID in that packet, and the client will retry by sending another
client Initial packet with the server-selected connection ID. This
mechanism can be used to redirect a connection to a different server,
e.g., due to performance reasons or when servers in a server pool are
upgraded gradually and therefore may support different versions of
QUIC.

In this case, it is assumed that all servers belonging to a certain
pool are served in cooperation with load balancers that forward the
traffic based on the connection ID. A server can choose the
connection ID in the Retry packet such that the load balancer will
redirect the next Initial packet to a different server in that pool.
Alternatively, the load balancer can directly offer a Retry offload
as further described in [QUIC-RETRY].

The approach described in Section 4 of [RFC5077] describes an example approach for constructing TLS
resumption tickets provides an example that can be also applied for to
validation
tokens; however, tokens. However, the use of more modern cryptographic
algorithms is highly recommended.

12. Quality of Service (QoS) and Diffserv Code Point (DSCP)

QUIC, as defined in [QUIC], has a single congestion controller and
recovery handler. This design assumes that all packets of a QUIC
connection, or at least with the same 5-tuple {dest addr, source
addr, protocol, dest port, source port}, that have the same Diffserv
Code Point (DSCP) [RFC2475] will receive similar network treatment
since feedback about loss or delay of each packet is used as input to
the congestion controller. Therefore, packets belonging to the same
connection should use a single DSCP. Section 5.1 of [RFC7657]
provides a discussion of Diffserv interactions with datagram
transport protocols [RFC7657] (in this respect, the interactions with
QUIC resemble those of Stream Control Transmission Protocol (SCTP)).

When multiplexing multiple flows over a single QUIC connection, the
selected DSCP value should be the one associated with the highest
priority requested for all multiplexed flows.

If differential network treatment is desired, e.g., by the use of
different DSCPs, multiple QUIC connections to the same server may be
used. In general, it is recommended to minimize the number of QUIC
connections to the same server to avoid increased overhead and, more
importantly, competing congestion control.

As in other uses of Diffserv, when a packet enters a network segment
that does not support the DSCP value, this could result in the
connection not receiving the network treatment it expects. The DSCP
value in this packet could also be remarked as the packet travels
along the network path, changing the requested treatment.

13. Use of Versions and Cryptographic Handshake

Versioning in QUIC may change the protocol's behavior completely,
except for the meaning of a few header fields that have been declared
to be invariant [QUIC-INVARIANTS]. A version of QUIC with a higher
version number will not necessarily provide a better service but
might simply provide a different feature set. As such, an
application needs to be able to select which versions of QUIC it
wants to use.

A new version could use an encryption scheme other than TLS 1.3 or
higher. [QUIC] specifies requirements for the cryptographic
handshake as currently realized by TLS 1.3 and described in a
separate specification [QUIC-TLS]. This split is performed to enable
lightweight versioning with different cryptographic handshakes.

The "QUIC Versions" registry established in [QUIC] allows for
provisional registrations for experimentation. Registration, also of
experimental versions, is important to avoid collision. Experimental
versions should not be used long-term or registered as permanent to
minimize the risk of fingerprinting based on the version number.

14. Enabling Deployment of New Versions

QUIC version 1 does not specify a version negotiation mechanism in
the base specification, but [QUIC-VERSION-NEGOTIATION] proposes an
extension that provides compatible version negotiation.

This approach uses a three-stage deployment mechanism, enabling
progressive rollout and experimentation with multiple versions across
a large server deployment. In this approach, all servers in the
deployment must accept connections using a new version (stage 1)
before any server advertises it (stage 2), and authentication of the
new version (stage 3) only proceeds after advertising of that version
is completely deployed.

See Section 5 of [QUIC-VERSION-NEGOTIATION] for details.

15. Unreliable Datagram Service over QUIC

[RFC9221] specifies a QUIC extension to enable sending and receiving
unreliable datagrams over QUIC. Unlike operating directly over UDP,
applications that use the QUIC datagram service do not need to
implement their own congestion control, per [RFC8085], as QUIC
datagrams are congestion controlled.

QUIC datagrams are not flow controlled, and as such data chunks may
be dropped if the receiver is overloaded. While the reliable
transmission service of QUIC provides a stream-based interface to
send and receive data in order over multiple QUIC streams, the
datagram service has an unordered message-based interface. If
needed, an application-layer framing can be used on top to allow
separate flows of unreliable datagrams to be multiplexed on one QUIC
connection.

16. IANA Considerations

This document has no actions for IANA; however, note that Section 8
recommends that an application bindings that has already registered a TCP port
but wants to specify QUIC for applications using
TCP as a transport should register a UDP ports port
analogous to their existing TCP registrations. registration.

17. Security Considerations

See the security considerations in [QUIC] and [QUIC-TLS]; the
security considerations for the underlying transport protocol are
relevant for applications using QUIC. Considerations on linkability,
replay attacks, and randomness discussed in [QUIC-TLS] should be
taken into account when deploying and using QUIC.

Further, migration to a new address exposes a linkage between client
addresses to the server and may expose this linkage also to the path
if the connection ID cannot be changed or flows can otherwise be
correlated. When migration is supported, this needs to be considered
with respective to user privacy.

Application developers should note that any fallback they use when
QUIC cannot be used due to network blocking of UDP should guarantee
the same security properties as QUIC. If this is not possible, the
connection should fail to allow the application to explicitly handle
fallback to a less-secure alternative. See Section 2.

Further, [QUIC-HTTP] provides security considerations specific to
HTTP. However, discussions such as on cross-protocol attacks,
traffic analysis and padding, or migration might be relevant for
other applications using QUIC as well.

18. References

18.1. Normative References

[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/info/rfc8999>.

[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.

18.2. Informative References

[Edeline16]
Edeline, K., Kuehlewind, Kühlewind, M., Trammell, B., Aben, E., and B.
Donnet, "Using UDP for Internet Transport Evolution",
DOI 10.48550/arXiv.1612.07816, 22 December 2016,
<https://arxiv.org/abs/1612.07816>.

[Hatonen10]
Hätönen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., and M. Kojo, "An Experimental Study of Home
Gateway Characteristics", Proc. ACM IMC 2010, November
2010, <https://conferences.sigcomm.org/imc/2010/papers/
p260.pdf>.

[HTTP-REPLAY]
Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.

[PaaschNanog]
Paasch, C., "Network support for TCP Fast Open", NANOG 67
Presentation, 13 June 2016,
<https://www.nanog.org/sites/default/files/
Paasch_Network_Support.pdf>.

[QUIC-ACK-FREQUENCY]
Iyengar, J. and I. Swett, "QUIC Acknowledgement
Frequency", Work in Progress, Internet-Draft, draft-ietf-
quic-ack-frequency-02, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
ack-frequency-02>.

[QUIC-HTTP]
Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/info/rfc9114>.

[QUIC-LB] Duke, M., Banks, N., and C. Huitema, "QUIC-LB: Generating
Routable QUIC Connection IDs", Work in Progress, Internet-
Draft, draft-ietf-quic-load-balancers-14, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
load-balancers-14>.

[QUIC-MANAGEABILITY]
Kuehlewind,
Kühlewind, M. and B. Trammell, "Manageability of the QUIC
Transport Protocol", Work in Progress, Internet-Draft,
draft-ietf-quic-manageability-18, 15 July RFC 9312, DOI 10.17487/RFC9312,
September 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
manageability-18>. <https://www.rfc-editor.org/info/rfc9312>.

[QUIC-RETRY]
Duke, M. and N. Banks, "QUIC Retry Offload", Work in
Progress, Internet-Draft, draft-ietf-quic-retry-offload-
00, 25 May 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-quic-retry-offload-00>.

[QUIC-VERSION-NEGOTIATION]
Schinazi, D. and E. Rescorla, "Compatible Version
Negotiation for QUIC", Work in Progress, Internet-Draft,
draft-ietf-quic-version-negotiation-09, 11 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
version-negotiation-09>.

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
<https://www.rfc-editor.org/info/rfc2475>.

[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.

[RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, DOI 10.17487/RFC5382, October 2008,
<https://www.rfc-editor.org/info/rfc5382>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.

[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, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.

[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.

[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.

[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>.

[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>.

[RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP
Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
April 2016, <https://www.rfc-editor.org/info/rfc7838>.

[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.

[RFC8981] Gont, F., Krishnan, S., Narten, T., and R. Draves,
"Temporary Address Extensions for Stateless Address
Autoconfiguration in IPv6", RFC 8981,
DOI 10.17487/RFC8981, February 2021,
<https://www.rfc-editor.org/info/rfc8981>.

[RFC9218] Oku, K. and L. Pardue, "Extensible Prioritization Scheme
for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,
<https://www.rfc-editor.org/info/rfc9218>.

[RFC9221] Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
Datagram Extension to QUIC", RFC 9221,
DOI 10.17487/RFC9221, March 2022,
<https://www.rfc-editor.org/info/rfc9221>.

[SSDP] Donoho, A., Roe, B., Bodlaender, M., Gildred, J., Messer,
A., Kim, Y., Fairman, B., and J. Tourzan, "UPnP Device
Architecture 2.0", 17 April 2020,
<https://openconnectivity.org/upnp-specs/UPnP-arch-
DeviceArchitecture-v2.0-20200417.pdf>.

[Swett16] Swett, I., "QUIC Deployment Experience @Google", IETF96
QUIC BoF Presentation, 20 July 2016,
<https://www.ietf.org/proceedings/96/slides/slides-96-
quic-3.pdf>.

[TAPS-ARCH]
Pauly, T., Trammell, B., Brunstrom, A., Fairhurst, G., and
C. Perkins, "An Architecture for Transport Services", Work
in Progress, Internet-Draft, draft-ietf-taps-arch-13, 27
June 2022, <https://datatracker.ietf.org/doc/html/draft-
ietf-taps-arch-13>.

[TLS13] 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>.

[Trammell16]
Trammell, B. and M. Kühlewind, "Internet Path Transparency
Measurements using RIPE Atlas", RIPE 72 MAT Presentation,
25 May 2016, <https://ripe72.ripe.net/wp-content/uploads/
presentations/86-atlas-udpdiff.pdf>.

Contributors

The following people have contributed significant text to or feedback
on this document:

* Gorry Fairhurst

* Ian Swett

* Igor Lubashev

* Lucas Pardue

* Mike Bishop

* Mark Nottingham

* Martin Duke

* Martin Thomson

* Sean Turner

* Tommy Pauly

Acknowledgments

Special thanks to Last Call reviewers Chris Lonvick and Ines Robles.

This work was partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI) and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.

Contributors

The following people have contributed significant text to or feedback
on this document:

Gorry Fairhurst

Ian Swett

Igor Lubashev

Lucas Pardue

Mike Bishop

Mark Nottingham

Martin Duke

Martin Thomson

Sean Turner

Tommy Pauly

Authors' Addresses

Mirja Kühlewind
Ericsson
Email: mirja.kuehlewind@ericsson.com

Brian Trammell
Google Switzerland GmbH
Gustav-Gull-Platz 1
CH-8004 Zurich
Switzerland
Email: ietf@trammell.ch