Applicability of the QUIC Transport Protocol
draft-ietf-quic-applicability-09

Network Working Group                                      M. Kuehlewind
Internet-Draft                                                  Ericsson
Intended status: Informational                               B. Trammell
Expires: 26 July 2021                                             Google
                                                         22 January 2021

              Applicability of the QUIC Transport Protocol
                    draft-ietf-quic-applicability-09

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.

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   This Internet-Draft will expire on 26 July 2021.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   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  . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Stream versus Flow Multiplexing . . . . . . . . . . . . .   8
     4.2.  Prioritization  . . . . . . . . . . . . . . . . . . . . .   8
     4.3.  Flow Control Deadlocks  . . . . . . . . . . . . . . . . .   9
   5.  Packetization and Latency . . . . . . . . . . . . . . . . . .  10
   6.  Port Selection and Application Endpoint Discovery . . . . . .  11
   7.  Connection Migration  . . . . . . . . . . . . . . . . . . . .  12
   8.  Connection Closure  . . . . . . . . . . . . . . . . . . . . .  13
   9.  Information Exposure and the Connection ID  . . . . . . . . .  14
     9.1.  Server-Generated Connection ID  . . . . . . . . . . . . .  14
     9.2.  Mitigating Timing Linkability with Connection ID
           Migration . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.3.  Using Server Retry for Redirection  . . . . . . . . . . .  15
   10. Quality of Service (QoS) and DSCP . . . . . . . . . . . . . .  16
   11. Use of Versions and Cryptographic Handshake . . . . . . . . .  16
   12. Enabling New Versions . . . . . . . . . . . . . . . . . . . .  16
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   15. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   16. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  18
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     17.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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
   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.

2.  The Necessity of Fallback

   QUIC uses UDP as a substrate.  This enables both userspace
   implementation traversal of middleboxes and NAT without requiring
   updates.

   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.  In the case of HTTP, this fallback is TLS 1.3 over TCP.

   An application that implements fallback needs to consider the
   security consequences.  A fallback to TCP and TLS 1.3 exposes control
   information to modification and manipulation in the network.  Further
   downgrades to older TLS versions might result in significantly weaker
   cryptographic protection.  For example, the results of protocol
   negotiation [ALPN] 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 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 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.  Using 0-RTT requires an understanding of the
   implication of sending application data that might be replayed by an
   attacker.

   Application protocols that use 0-RTT require a profile that describes
   the types of information that can be safely sent.  For HTTP, this
   profile is described in [HTTP-REPLAY].

3.2.  Here There Be Dragons

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

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

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   In some cases, it might be sufficient to limit application data sent
   in 0-RTT to that which only causes actions at a server that are known
   to be free of lasting effect.  Initiating data retrieval or
   establishing configuration are examples of actions that could be
   safe.  Idempotent operations - those for which repetition has the
   same net effect as a single operation - might be safe.  However, it
   is also possible to combine individually idempotent operations into a
   non-idempotent sequence of operations.

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

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

3.3.  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, 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 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 could 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.

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   A QUIC application can adjust idle periods to manage the risk of
   timeout (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]), but then there
   are three options:

   *  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.

   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 transient 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.  Using resumption in this way also assumes that the

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   protocol does not accumulate any non-persistent state in association
   with a connection.  State bound to a connection cannot reliably be
   transferred to a resumed connection.

   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.

   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 encoding limitations on stream offsets and connection flow
   control limits, both streams and connections 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, a new connection would need
   to be established.

   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 the egress direction or request that its peer abruptly close
   the ingress direction; these actions are fully independent of each
   other.

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

   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

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   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, for example,
   [QUIC-HTTP], Section 6) either at the sender or receiver end, so
   applications should not assume access to these identifiers.

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.

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   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.

   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.

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   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.

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, many QUIC implementations 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, an
   implementation might 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 application has usually no control about 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.

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   Padding is used by QUIC to ensure that the path is capable of
   transferring datagrams of at least a certain size, both during the
   handshake and for connection migration.  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.  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].  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, and
   NATs.

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

   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 interference by network elements such as
   firewalls that rely on the port number for application
   identification.

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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 using the Destination connection ID field (see
   also Section 9) in the QUIC header, unless a zero-length value is
   used.  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.

   Use of a non-empty connection ID for the server is strongly
   recommended if any clients are behind a NAT or could be.  A non-empty
   connection ID 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.

   Probing packets, which cannot carry application data, can be sent on
   multiple paths at once.  Probing packets can be used to perform
   address validation, measure path characteristics as input for the
   switching decision, or prime the congestion controller in preparation
   for switching to the new path.

   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.  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 also Section 9.6 of [QUIC].

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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 immediate 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 (see, for example, [QUIC-HTTP], Section 8.1).

   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 also include an
   error code, but no reason string.  Application error codes are
   expected to be defined from a single space that applies to all three
   frame types.

   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 values
   advertised by client and server.  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-alive messages, or a QUIC
   implementation may provide an option to defer the time-out to avoid
   unnecessary load, as specified in Section 10.1.2 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.

   Aside from the destination connection ID field of the first packets
   sent by clients, the connection ID can be zero length.  This is a
   choice that is made by each endpoint individually.

   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 its IP address and port
   number to identify which connection is referred to.  An endpoint can
   choose to use the source IP address and port on datagrams, but this
   could mean that the endpoint is unable to match datagrams to
   connections successfully if these values change, making migration
   effectively impossible.

9.1.  Server-Generated Connection ID

   QUIC supports a server-generated connection ID, 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 correct server instance.  This might require
   coordination between servers and infrastructure.  One method of
   achieving this involves encoding routing information into the
   connection ID.  This ensures that there is no need to for servers and
   infrastructure to coordinate routing information for each connection.
   See further [QUIC-LB].

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9.2.  Mitigating Timing Linkability with Connection ID Migration

   QUIC requires that endpoints generate fresh connection IDs for use on
   new network paths.  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 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.

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.

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10.  Quality of Service (QoS) and DSCP

   QUIC 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} will receive similar network treatment as feedback about loss
   or delay of each packet is used as input to the congestion
   controller.  Therefore it is not recommended to use different
   DiffServ Code Points (DSCPs) [RFC2475] for packets belonging to the
   same connection.  If differential network treatment, e.g. by the use
   of different DSCPs, is desired, multiple QUIC connections to the same
   server may be used.  However, in general it is recommended to
   minimize the number of QUIC connections to the same server, to avoid
   increased overheads and, more importantly, competing congestion
   control.

11.  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.

12.  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.

   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.

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   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 authentication of
   the negotiated version with the assumption that the new version is
   fully available.

   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.

13.  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.

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14.  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.  Considerations on
   linkability, replay attacks, and randomness discussed in [QUIC-TLS]
   should be taken into account when deploying and using QUIC.

   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.

15.  Contributors

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

16.  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.

17.  References

17.1.  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-34, 14 January 2021,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-34.txt>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              invariants-13, 14 January 2021, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-invariants-13.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-34,
              14 January 2021, <http://www.ietf.org/internet-drafts/
              draft-ietf-quic-tls-34.txt>.

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

17.2.  Informative References

   [ALPN]     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>.

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

   [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-33, 15 December 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-http-33.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>.

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

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

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

   [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>.

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   [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>.

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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