Applicability of the QUIC Transport Protocol
draft-ietf-quic-applicability-08

Network Working Group                                      M. Kuehlewind
Internet-Draft                                                  Ericsson
Intended status: Informational                               B. Trammell
Expires: 6 May 2021                                               Google
                                                         2 November 2020

              Applicability of the QUIC Transport Protocol
                    draft-ietf-quic-applicability-08

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 6 May 2021.

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   document authors.  All rights reserved.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  The Necessity of Fallback . . . . . . . . . . . . . . . . . .   3
   3.  Zero RTT  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Thinking in Zero RTT  . . . . . . . . . . . . . . . . . .   4
     3.2.  Here There Be Dragons . . . . . . . . . . . . . . . . . .   4
     3.3.  Session resumption versus Keep-alive  . . . . . . . . . .   5
   4.  Use of Streams  . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Stream versus Flow Multiplexing . . . . . . . . . . . . .   8
     4.2.  Prioritization  . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Flow Control Deadlocks  . . . . . . . . . . . . . . . . .   8
   5.  Packetization and Latency . . . . . . . . . . . . . . . . . .  10
   6.  Port Selection and Application Endpoint Discovery . . . . . .  10
   7.  Connection Migration  . . . . . . . . . . . . . . . . . . . .  11
   8.  Connection closure  . . . . . . . . . . . . . . . . . . . . .  12
   9.  Information exposure and the Connection ID  . . . . . . . . .  13
     9.1.  Server-Generated Connection ID  . . . . . . . . . . . . .  13
     9.2.  Mitigating Timing Linkability with Connection ID
           Migration . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.3.  Using Server Retry for Redirection  . . . . . . . . . . .  14
   10. Use of Versions and Cryptographic Handshake . . . . . . . . .  14
   11. Enabling New Versions . . . . . . . . . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  16
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  16
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     16.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   QUIC [QUIC] is a new transport protocol providing a number of
   advanced features.  While initially designed for the HTTP use case,
   like most transports it is intended for use with a much wider variety
   of applications.  QUIC is encapsulated in UDP.  The version of QUIC
   that is currently under development will integrate TLS 1.3 [TLS13] to
   encrypt all payload data and most control information.  HTTP
   operating over QUIC is known as HTTP/3.

   This document provides guidance for application developers that 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.  For specific guidance on how to integrate HTTP/3
   with QUIC, see [QUIC-HTTP].

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   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 application.

1.1.  Notational Conventions

   The words "MUST", "MUST NOT", "SHOULD", and "MAY" are used in this
   document.  It's not shouting; when these words are capitalized, they
   have a special meaning as defined in [RFC2119].

2.  The Necessity of Fallback

   QUIC uses UDP as a substrate for userspace implementation and port
   numbers for NAT and middlebox traversal.  While there is no evidence
   of widespread, systematic disadvantage of UDP traffic compared to TCP
   in the Internet [Edeline16], somewhere between three [Trammell16] and
   five [Swett16] percent of networks simply block UDP traffic.  All
   applications running on top of QUIC must therefore either be prepared
   to accept connectivity failure on such networks, or be engineered to
   fall back to some other transport protocol.  This fallback SHOULD
   provide TLS 1.3 or equivalent cryptographic protection, if available,
   in order to keep fallback from being exploited as a downgrade attack.
   In the case of HTTP, this fallback is TLS 1.3 over TCP.

   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 Fast Open option, which is analogous to 0-RTT session resumption.
   Even if Fast Open successfully operates end-to-end, it is limited to
   a single packet of payload, unlike QUIC 0-RTT.

   Note that there is some evidence of middleboxes blocking SYN data
   even if TFO was successfully negotiated (see [PaaschNanog]).

   Any fallback mechanism is likely to impose a degradation of
   performance; however, fallback MUST not silently violate the
   application's expectation of confidentiality or integrity of its
   payload data.

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   Moreover, while encryption (in this case TLS) is inseparably
   integrated with QUIC, TLS negotiation over TCP can be blocked.  In
   case it is RECOMMENDED to abort the connection, allowing the
   application to present a suitable prompt to the user that secure
   communication is unavailable.

3.  Zero RTT

   QUIC provides for 0-RTT connection establishment.  This presents
   opportunities and challenges for applications using QUIC.

3.1.  Thinking in Zero RTT

   A transport protocol that provides 0-RTT connection establishment to
   recently contacted servers is qualitatively different than one that
   does not 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 3.3.

   Applications must be slightly rethought in order to make best use of
   0-RTT resumption.  Most importantly, application operations must be
   divided into idempotent and non-idempotent operations, as only
   idempotent operations may appear in 0-RTT packets.  This implies that
   the interface between the application and transport layer exposes
   idempotence either explicitly or implicitly.

3.2.  Here There Be Dragons

   Retransmission or (malicious) replay of data contained in 0-RTT
   resumption packets could cause the server side to receive two copies
   of the same data.  This is further described in [HTTP-RETRY].  Data
   sent during 0-RTT resumption also cannot benefit from perfect forward
   secrecy (PFS).

   Data in the first flight sent by the client in a connection
   established with 0-RTT MUST be idempotent (as specified in section
   2.1 in [QUIC-TLS]).  Applications MUST be designed, and their data
   MUST be framed, such that multiple reception of idempotent data is
   recognized as such by the receiver.  Applications that cannot treat
   data that may appear in a 0-RTT connection establishment as
   idempotent MUST NOT use 0-RTT establishment.  For these reason the
   QUIC transport SHOULD provide some or all of the following interfaces
   to applications:

   *  indicate if 0-RTT support is in general desired, which implies
      that lack of PFS is acceptable for some data;

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   *  an indication when 0RTT data for both egress and ingress, so that
      both sender and receiver understand the properties of the
      communication channel when the data is sent; and/or

   *  whether rejected 0-RTT data should be retransmitted or withdrawn.

   Some TLS implementations may offer replay protection, which may
   mitigate some of these issues.

3.3.  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
   state, 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 not evidence of widespread implementation of this guideline
   in the literature.  Short network timeout for UDP, however, 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
   a minimum value of 15 seconds, but recommends larger values, or
   omitting keepalive entirely.

   By using a Connection ID, QUIC is designed to be robust to NAT
   address rebinding after a timeout.  However, some QUIC connections
   may not be robust to rebinding because the routing infrastructure (in
   particular, load balancers) uses the address/port four-tuple to
   direct traffic.  Furthermore, middleboxes with functions other than
   address translation may still affect the path.  In particular,
   firewalls will often not admit server traffic for which it has not
   kept state for corresponding packets from the client.

   A QUIC application has three strategies to deal with this issue by
   adjusting idle periods (noting that idle periods and the network idle
   timeout is distinct from the connection idle timeout, defined as the
   minimum of the idle timeout parameter in Section 10.1 of [QUIC]):

   *  Ignore it, if the application-layer protocol consists only of
      interactions with no or very short idle periods, or 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.

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   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 environments like e.g. data centres 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 to unacceptable power
   usage for power-constrained (mobile) devices.  Additionally, time-
   outs shorter than 30 seconds can make it harder to handle trasient
   network interruptions, such as VM migration or coverage loss during
   mobilty.

   Alternatively, the client (but not the server) can use session
   resumption instead of sending keepalive traffic.  In this case, a
   client that wants to send data to a server over a connection 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.  This
   of course only applies in cases in which 0-RTT data is safe, when the
   client is the restarting peer, and when the data to be sent is
   idempotent.

   The tradeoffs between resumption and keepalive need to be evaluated
   on a per-application basis.  However, in general applications should
   use keepalives only in circumstances where continued communication is
   highly likely; [QUIC-HTTP], for instance, recommends using PING
   frames for keepalive 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, associated at a point in time with a single
   five-tuple.  Stream data is carried within Frames, where one QUIC
   packet on the wire can carry one or multiple stream frames.

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   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.  Due to offset encoding
   limitations, a stream can carry a maximum of 2^62-1 bytes in each
   direction.  In the presently unlikely event that this limit is
   reached by an application, the stream can simply be closed and
   replaced with a new one.

   Streams can be independently opened and closed, gracefully or by
   error.  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 either the ingress or egress direction; these actions are fully
   independent of each other.

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

   Mapping of application data to streams is application-specific and
   described for HTTP/3 in [QUIC-HTTP].  In general, 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.  If the application requires certain data to
   be received in order, that data should be sent on the same stream.
   If there is a logical grouping of data chunks or messages, streams
   can be reused, or a new stream can be opened for each chunk/message.
   If one message is mapped to a single stream, resetting the stream to
   expire an unacknowledged message can be used to emulate partial
   reliability on a message basis.  If a QUIC receiver has maximum
   allowed concurrent streams open and the sender on the other end
   indicates that more streams are needed, it doesn't automatically lead
   to an increase of the maximum number of streams by the receiver.
   Therefore it can be valuable to expose maximum number of allowed,
   currently open and currently used streams to the application to make
   the mapping of data to streams dependent on this information.

   While a QUIC implementation must necessarily provide a way for an
   application to send data on separate streams, it does not necessarily
   expose stream identifiers to the application (see e.g.  [QUIC-HTTP]
   section 6) either at the sender or receiver end, so applications
   should not assume access to these identifiers.

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4.1.  Stream versus Flow Multiplexing

   Streams are meaningful only to the application; since stream
   information is carried inside QUIC's encryption boundary, no
   information about the stream(s) whose frames are carried by a given
   packet is visible to the network.  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 five-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 respective 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].  Further 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 for HTTP/2; it can define rules that
   allow an endpoint to determine priority based on context; or it 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 to retransmit lost data
   before new data, unless indicated differently by the application.
   Currently, QUIC only provides fully reliable stream transmission,
   which means that prioritization of retransmissions will be beneficial
   in most cases, by 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.  Flow Control Deadlocks

   Flow control provides a means of managing access to the limited
   buffers 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 deadlock.

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   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.

   Large messages can produce deadlocking if the recipient does not
   process the message incrementally.  If the message is larger than
   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 common flow control implementation technique is for a receiver to
   extend credit to the sender as a the data consumer reads data.  In
   this setting, a length-prefixed message format makes it easier for
   the data consumer to leave data unread in the receiver's buffers 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 protocols have messages that might be processed as a
   single unit, reserving flow control credit for the entire message
   atomically ensures that this style of deadlock is less likely.

   A data consumer can read all data as it becomes available to cause
   the receiver to extend flow control credit to the sender and reduce
   the chances of a deadlock.  However, releasing flow control credit
   might mean that the data consumer might need other means for holding
   a peer accountable for the 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 cancelling affected
   streams with STOP_SENDING or RST_STREAM.  Cancelling some streams
   results in the connection being terminated in some protocols.

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5.  Packetization and Latency

   QUIC provides an interface that provides multiple streams to the
   application; however, the application usually cannot control how data
   transmitted over one stream is mapped into frames or how those frames
   are bundled into packets.  By default, QUIC will try to maximally
   pack packets with one or more stream data frames to minimize
   bandwidth consumption and computational costs (see section 13 of
   [QUIC]).  If there is not enough data available to fill a packet,
   QUIC may even wait for a short time, to optimize bandwidth efficiency
   instead of latency.  This delay can either be pre-configured 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 send 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 bundle
   frames into a packet.

   Similarly, an appliaction has usually no control about the length of
   a QUIC packet on the wire.  However, QUIC provides the ability to add
   a padding frame to impact the packet size.  This is mainly used by
   QUIC itself in the first packet in order to ensure that the path is
   capable of transferring packets of at least a certain size.
   Additionally, a QUIC implementation can expose an application layer
   interface to specify a certain packet size.  This can either be used
   by the application to force certian packet sizes in specific use
   cases/networks, or ensure that all packets are equally sized to
   conceal potential leakage of application layer information when the
   data sent by the application are not greedy.  Note the initial packet
   must have a minimum size of 1200 bytes according to the QUIC
   specification.  A receiver of a smaller initial packet may reject
   this packet in order to avoid amplification attacks.

6.  Port Selection and Application Endpoint Discovery

   In general, port numbers serves 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" [RFC6335].  Note that the assumption that an application can
   be identified in the network based on the port number is less true
   today, due to encapsulation, mechanisms for dynamic port assignments
   as well as NATs.

   As QUIC is a general purpose transport protocol, there are no
   requirements that servers use a particular UDP port for QUIC in
   general.  For applications with a fallback to TCP which do not

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   already have an alternate mapping to UDP, the registration (if
   necessary) and use of the UDP port number corresponding to the TCP
   port already registered for the application is RECOMMENDED.  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.

   Applications SHOULD define an alternate endpoint discovery mechanism
   to allow the usage of ports other than the default.  For example,
   HTTP/3 ([QUIC-HTTP] sections 3.2 and 3.3) specifies the use of ALPN
   [RFC7301] for service discovery which allows the server to use and
   announce a different port number.  Note that HTTP/3's ALPN token
   ("h3") identifies not only the version of the application protocol,
   but also the binding to QUIC as well as the version of QUIC itself;
   this approach allows unambiguous agreement between the endpoints on
   the protocol stack in use.

   Note that 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 may lead to blocking or other interference by
   network elements such as firewalls that rely on the port number for
   application identification.

7.  Connection Migration

   QUIC supports connection migration by the client.  If a lower-layer
   address changes, a QUIC endpoint can still associate packets with an
   existing connection based on the Connection ID (see also Section 9)
   in the QUIC header, if present.  This supports cases where address
   information changes, such as NAT rebinding, intentional change of the
   local interface, or based on an indication in the handshake of the
   server for a preferred address to be used.  As such if the client is
   known or likely to sit behind a NAT, use of a connection ID for the
   server is strongly recommended.  A non-empty connection ID for the
   server is also strongly recommended when migration is supported.

   Currently QUIC only supports failover cases.  Only one "path" can be
   used at a time, and only when the new path is validated all traffic
   can be switched over to that new path.  Path validation means that
   the other endpoint in required to validate the new path before use in
   order to avoid address spoofing attacks.  Path validation takes at
   least one RTT and congestion control will also be reset on path
   migration.  Therefore migration usually has a performance impact.

   As long as the new path is not validated only probing packets can be
   sent.  However, the probing packets can be used measure path
   characteristics as input for the switching decision or the congestion
   controller on the new path.

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   Only the client can actively migrate.  However, servers can indicate
   during the handshake that they prefer to transfer the connection to a
   different address after the handshake, e.g. 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 also
   Section 9.6 of [QUIC].

8.  Connection closure

   QUIC connections are closed either by expiration of an idle timeout,
   as determined by transport parameters, or by an explicit indication
   of the application that a connection should be closed (immediate
   close).  While data could still be received after the immediate close
   has been initiated by one endpoint (for a limited time period), the
   expectation is that an immediate close was negotiated at the
   application layer and therefore no additional data is expected from
   both sides.

   An immidate close will emit an CONNECTION_CLOSE frame.  This frames
   has two sets of types: one for QUIC internal problems that might lead
   to connection closure, and one for closures initiated by the
   application.  An application using QUIC can define application-
   specific error codes, e.g. see [QUIC-HTTP] section 8.1.  In the case
   of a grateful shut-down initiated by the application after
   application layer negotiation, a NO_ERROR code is expected.  Further,
   the CONNECTION_CLOSE frame provides an optional reason field, that
   can be used to append human-readable information to an error code.
   Note that QUIC RESET_STREAM and STOP_SENDING frames provide similar
   capablities.  Usually application error codes are defined to be
   applicabile to all three frames.

   Alternatively, a QUIC connection can be silently closed by each
   endpoint separately after an idle timeout.  If enabled as indicated
   by a transport parameter in the handshake, the idle timeout is
   announced for each endpoint during connection establishment and the
   effective value for this connection is the minimum of the two
   advertised values.  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 based on the number of open or
   expected connections as shorter timeout values may free-up memory
   more quickly.  If an application desires to keep the connection open
   for longer than the announced timeout, it can send keep-alives
   messages, or a QUIC implementation may provide an option to defer the
   time-out to avoid unnecessary load, as specified in Section 10.2.2 of
   [QUIC].  See Section 3.3 for further guidance on keep-alives.

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9.  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,
   because the information is intended to be used by the network.  QUIC
   has a long header that is used during connection establishment and
   for other control processes, and a short header that may be used for
   data transmission in an established connection.  While the long
   header always exposes some information (such as the version and
   Connection IDs), the short header exposes at most only a single
   Connection ID.

   Note that the Connection ID in the short header may be omitted.  This
   is a per-connection configuration option; if the Connection ID is not
   present, then the peer omitting the connection ID needs to use the
   same local address for the lifetime of the connection and connection
   migration is not supported for that direction of the connection.

9.1.  Server-Generated Connection ID

   QUIC supports a server-generated Connection ID, transmitted to the
   client during connection establishment (see Section 6.1 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.  Once the server generates a Connection ID
   that encodes its identity, every CDN load balancer would be able to
   forward the packets to that server without retaining connection
   state.

9.2.  Mitigating Timing Linkability with Connection ID Migration

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

   In the limit 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 mitigation for these attacks is operational,
   either by using a load balancing architecture that loads more flows
   onto a single server-side address, by coordinating the timing of
   migrations to attempt to increase the number of simultaneous
   migrations at a given time, or through other means.

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9.3.  Using Server Retry for Redirection

   QUIC provides a Server 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 Server Retry packet such
   that the load balancer will redirect the next Client Initial packet
   to a different server in that pool.  Alternatively the load balancer
   can directly offer a Retry services as further described in
   [QUIC-LB].

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

10.  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
   light-weight versioning with different cryptographic handshakes.

11.  Enabling New Versions

   QUIC provides integrity protection for its version negotiation
   process.  This process assumes that the set of versions that a server
   supports is fixed.  This complicates the process for deploying new
   QUIC versions or disabling old versions when servers operate in
   clusters.

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   A server that rolls out a new version of QUIC can do so in three
   stages.  Each stage is completed across all server instances before
   moving to the next stage.

   In the first stage of deployment, all server instances start
   accepting new connections with the new version.  The new version can
   be enabled progressively across a deployment, which allows for
   selective testing.  This is especially useful when the new version is
   compatible with an old version, because the new version is more
   likely to be used.

   While enabling the new version, servers do not advertise the new
   version in any Version Negotiation packets they send.  This prevents
   clients that receive a Version Negotiation packet from attempting to
   connect to server instances that might not have the new version
   enabled.

   During the initial deployment, some clients will have received
   Version Negotiation packets that indicate that the server does not
   support the new version.  Other clients might have successfully
   connected with the new version and so will believe that the server
   supports the new version.  Therefore, servers need to allow for this
   ambiguity when validating the negotiated version.

   The second stage of deployment commences once all server instances
   are able accept new connections with the new version.  At this point,
   all servers can start sending the new version in Version Negotiation
   packets.

   During the second stage, the server still allows for the possibility
   that some clients believe the new version to be available and some do
   not.  This state will persist only for as long as any Version
   Negotiation packets take to be transmitted and responded to.  So the
   third stage can follow after a relatively short delay.

   The third stage completes the process by enabling validation of the
   negotiation version as though the new version were disabled.

   The process for disabling an old version or rolling back the
   introduction of a new version uses the same process in reverse.
   Servers disable validation of the old version, stop sending the old
   version in Version Negotiation packets, then the old version is no
   longer accepted.

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12.  IANA Considerations

   This document has no actions for IANA; however, note that Section 6
   recommends that application bindings to QUIC for applications using
   TCP register UDP ports analogous to their existing TCP registrations.

13.  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, as well.

   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.

14.  Contributors

   Igor Lubashev contributed text to Section 9 on server-selected
   Connection IDs.

15.  Acknowledgments

   This work is 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.

16.  References

16.1.  Normative References

   [QUIC]     Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", Work in Progress, Internet-Draft,
              draft-ietf-quic-transport-32, 20 October 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-32.txt>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-11, 24 September 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-invariants-11.txt>.

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   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-32,
              20 October 2020, <http://www.ietf.org/internet-drafts/
              draft-ietf-quic-tls-32.txt>.

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

   [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>.

   [TLS13]    Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-32,
              20 October 2020, <http://www.ietf.org/internet-drafts/
              draft-ietf-quic-tls-32.txt>.

16.2.  Informative References

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

   [Hatonen10]
              Hatonen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
              Sarolahti, P., and M. Kojo, "An experimental study of home
              gateway characteristics (Proc. ACM IMC 2010)", October
              2010.

   [HTTP-RETRY]
              Nottingham, M., "Retrying HTTP Requests", Work in
              Progress, Internet-Draft, draft-nottingham-httpbis-retry-
              01, 1 February 2017, <http://www.ietf.org/internet-drafts/
              draft-nottingham-httpbis-retry-01.txt>.

   [I-D.nottingham-httpbis-retry]
              Nottingham, M., "Retrying HTTP Requests", Work in
              Progress, Internet-Draft, draft-nottingham-httpbis-retry-
              01, 1 February 2017, <http://www.ietf.org/internet-drafts/
              draft-nottingham-httpbis-retry-01.txt>.

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   [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-HTTP]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-32, 20 October 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-http-32.txt>.

   [QUIC-LB]  Duke, M. and N. Banks, "QUIC-LB: Generating Routable QUIC
              Connection IDs", Work in Progress, Internet-Draft, draft-
              ietf-quic-load-balancers-05, 30 October 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-load-
              balancers-05.txt>.

   [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>.

   [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>.

   [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>.

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

   [Trammell16]
              Trammell, B. and M. Kuehlewind, "Internet Path
              Transparency Measurements using RIPE Atlas (RIPE72 MAT
              presentation)", 25 May 2016, <https://ripe72.ripe.net/wp-
              content/uploads/presentations/86-atlas-udpdiff.pdf>.

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Authors' Addresses

   Mirja Kuehlewind
   Ericsson

   Email: mirja.kuehlewind@ericsson.com

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

   Email: ietf@trammell.ch

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