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Applicability of the QUIC Transport Protocol
RFC 9308

Document Type RFC - Informational (September 2022)
Authors Mirja Kühlewind , Brian Trammell
Last updated 2022-09-27
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Zaheduzzaman Sarker
Send notices to (None)
RFC 9308

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

              Applicability of the QUIC Transport Protocol


   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

Copyright Notice

   Copyright (c) 2022 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
   Provisions Relating to IETF Documents
   ( 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
   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

   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

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

   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

   In some cases, it might be sufficient to limit application data sent
   in 0-RTT to data that does not cause actions with lasting effects at
   a 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

   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
   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].
   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

   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

   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

   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 updates to flow control credit can affect
   performance.  Applications that use QUIC often have a data consumer
   that reads data from transport buffers.  Some implementations might
   have independent receive buffers at the transport layer and
   application layer.  Consuming data does not always imply it is
   immediately processed.  However, a common implementation technique is
   to extend flow control credit to the sender by emitting MAX_DATA 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

   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

   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

   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 pack 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

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

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 an
   application with a fallback to TCP that does not already have an
   alternate mapping to UDP, it is usually appropriate to register (if
   necessary) and use the UDP port number corresponding to the TCP port
   already registered for the 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 - 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

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

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, where every server handles multiple simultaneous
   migrations, even an exposed server mapping may be insufficient

   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

   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] for constructing TLS
   resumption tickets provides an example that can be also applied to
   validation 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

16.  IANA Considerations

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

              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,

   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,

18.2.  Informative References

              Edeline, K., Kühlewind, M., Trammell, B., Aben, E., and B.
              Donnet, "Using UDP for Internet Transport Evolution",
              DOI 10.48550/arXiv.1612.07816, 22 December 2016,

              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, <

              Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <>.

              Paasch, C., "Network support for TCP Fast Open", NANOG 67
              Presentation, 13 June 2016,

              Iyengar, J. and I. Swett, "QUIC Acknowledgement
              Frequency", Work in Progress, Internet-Draft, draft-ietf-
              quic-ack-frequency-02, 11 July 2022,

              Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <>.

   [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,

              Kühlewind, M. and B. Trammell, "Manageability of the QUIC
              Transport Protocol", RFC 9312, DOI 10.17487/RFC9312,
              September 2022, <>.

              Duke, M. and N. Banks, "QUIC Retry Offload", Work in
              Progress, Internet-Draft, draft-ietf-quic-retry-offload-
              00, 25 May 2022, <

              Schinazi, D. and E. Rescorla, "Compatible Version
              Negotiation for QUIC", Work in Progress, Internet-Draft,
              draft-ietf-quic-version-negotiation-10, 27 September 2022,

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [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,

   [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, <>.

   [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,

   [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,

   [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,

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,

   [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, <>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7657]  Black, D., Ed. and P. Jones, "Differentiated Services
              (Diffserv) and Real-Time Communication", RFC 7657,
              DOI 10.17487/RFC7657, November 2015,

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <>.

   [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,

   [RFC9218]  Oku, K. and L. Pardue, "Extensible Prioritization Scheme
              for HTTP", RFC 9218, DOI 10.17487/RFC9218, June 2022,

   [RFC9221]  Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", RFC 9221,
              DOI 10.17487/RFC9221, March 2022,

   [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,

   [Swett16]  Swett, I., "QUIC Deployment Experience @Google", IETF96
              QUIC BoF Presentation, 20 July 2016,

              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-14, 27
              September 2022, <

   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

              Trammell, B. and M. Kühlewind, "Internet Path Transparency
              Measurements using RIPE Atlas", RIPE 72 MAT Presentation,
              25 May 2016, <


   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.


   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

   Brian Trammell
   Google Switzerland GmbH
   Gustav-Gull-Platz 1
   CH-8004 Zurich