Applicability of the QUIC Transport Protocol
draft-ietf-quic-applicability-06

Network Working Group                                      M. Kuehlewind
Internet-Draft                                                  Ericsson
Intended status: Informational                               B. Trammell
Expires: 9 July 2020                                              Google
                                                          6 January 2020

              Applicability of the QUIC Transport Protocol
                    draft-ietf-quic-applicability-06

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 9 July 2020.

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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 . . . . . . . . . . . . .   7
     4.2.  Prioritization  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Flow Control Deadlocks  . . . . . . . . . . . . . . . . .   7
   5.  Packetization and Latency . . . . . . . . . . . . . . . . . .   9
   6.  Port Selection  . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Connection Migration  . . . . . . . . . . . . . . . . . . . .  10
   8.  Connection closure  . . . . . . . . . . . . . . . . . . . . .  10
   9.  Information exposure and the Connection ID  . . . . . . . . .  11
     9.1.  Server-Generated Connection ID  . . . . . . . . . . . . .  11
     9.2.  Mitigating Timing Linkability with Connection ID
           Migration . . . . . . . . . . . . . . . . . . . . . . . .  12
     9.3.  Using Server Retry for Redirection  . . . . . . . . . . .  12
   10. Use of Versions and Cryptographic Handshake . . . . . . . . .  12
   11. Enabling New Versions . . . . . . . . . . . . . . . . . . . .  13
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   14. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  14
   15. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     16.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   QUIC [QUIC] is a new transport protocol currently under development
   in the IETF quic working group, focusing on support of semantics as
   needed for HTTP/2 [QUIC-HTTP] such as stream-multiplexing to avoid
   head-of-line blocking.  Based on current deployment practices, 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.

   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 application use of HTTP/2 over QUIC as
   well as the use of other application layer protocols over QUIC.  For

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   specific guidance on how to integrate HTTP/2 with QUIC, see
   [QUIC-HTTP].

   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 without the TCP Fast Open
   extension does not support 0-RTT session resumption.  TCP Fast Open
   can be requested by the connection initiator but might no be
   supported by the far end or could be blocked on the network path.
   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.

   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

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   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 receiverApplications that cannot treat data
   that may appear in a 0-RTT connection establishment as idempotent
   MUST NOT use 0-RTT establishment.  For this reason the QUIC transport
   SHOULD provide an interface for the application to indicate if 0-RTT
   support is in general desired or a way to indicate whether data is
   idempotent, whether PFS is a hard requirement for the application,
   and/or whether rejected 0-RTT dgitata should be retransmitted or
   withdrawn.

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3.3.  Session resumption versus Keep-alive

   Because QUIC is encapsulated in UDP, applications using QUIC must
   deal with short 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.  According
   to a 2010 study ([Hatonen10]), UDP applications can assume that any
   NAT binding or other state entry will be expired after just thirty
   seconds of inactivity.

   A QUIC application has three strategies to deal with this issue:

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

   *  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.  Note that sending
   PING frames more frequently than every 30 seconds over long idle
   periods may result in a too much unproductive traffic and power usage
   for some situations.

   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

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   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 (UDP)
   packet on the wire can carry one of multiple stream frames.

   Stream can be independently open and closed, gracefully or by error.
   If a critical stream for the application is closed, the application
   can generate respective error messages on the application layer to
   inform the other end 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 to
   exceptional handling of any stream.  There are special streams in
   QUIC that are used for control on the QUIC connection, however, these
   streams are not exposed to the application.

   Mapping of application data to streams is application-specific and
   described for HTTP/s 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 different streams.  If the application requires certain data to
   be received in order, the same stream should be used for that data.
   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.

   Further, streams have a maximum number of bytes that can be sent on
   one stream.  This number is high enough (2^64) that this will usually
   not be reached with current applications.  Applications that send
   chunks of data over a very long period of time (such as days, months,
   or years), should rather utilize the 0-RTT session resumption ability
   provided by QUIC, than trying to maintain one connection open.

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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 8 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

   As QUIC is a general purpose transport protocol, there are no
   requirements that servers use a particular UDP port for QUIC in
   general.  Instead, the same port number is used as would be used for
   the same application over TCP.  In the case of HTTP the expectation
   is that port 443 is used, which has already been registered for "http
   protocol over TLS/SSL".  However, [QUIC-HTTP] also specifies the use
   of Alt-Svc for HTTP/QUIC discovery which allows the server to use and
   announce a different port number.

   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

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   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.

   However, whenever a non-standard port is used which does not enable
   easy mapping to a registered service name, this can lead to blocking
   by network elements such as firewalls that rely on the port number as
   a first order of filtering.

7.  Connection Migration

   QUIC supports connection migration.  Even if lower-layer addresses
   (usually the 4-tuple of IP addresses and ports) changes, QUIC packets
   can still be associated with an existing connection based on the
   Connection ID (see also section Section 9) in the QUIC header, if
   present.  This supports cases where address information changes due
   to e.g.  NAT rebinding or change of the local interface.  Currently
   QUIC only supports failover cases.  Only one "path" can be used at a
   time, and as soon as the new path is validated all traffic will be
   switched over to the next path.  Of course if an endpoint decided to
   not use the Connection ID in short packets (Zero-length Conn ID) for
   a certain connection, migration is not supported for that direction
   of the connection.

8.  Connection closure

   QUIC connections are closed either by expiration of an idle timeout
   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.

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   Alternatively, a QUIC connection will be silently closed by each
   endpoint separately after an idle timeout.  The idle timeout is
   announce for each endpoint during connection established and should
   be accessible by the application.  If an application desires to keep
   the connection open for longer than the announced timeout, it can
   send keep-alives messages.  See {#resumption-v-keepalive} for further
   guidance.

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 will use the same
   local address for the lifetime 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.

   Server-generated connection IDs should seek to obscure any encoding,
   of routing identities or any other information.  Exposing the server
   mapping would allow linkage of multiple IP addresses to the same host
   if the server also supports migration.  Furthermore, this opens an
   attack vector on specific servers or pools.

   The best way to obscure an encoding is to appear random to observers,
   which is most rigorously achieved with encryption.

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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.

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.

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

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   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.

   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.

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   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.

12.  IANA Considerations

   This document has no actions for IANA.

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-24, 3 November 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-quic-
              transport-24.txt>.

   [QUIC-INVARIANTS]
              Thomson, M., "Version-Independent Properties of QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-

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              invariants-07, 11 September 2019, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-invariants-07.txt>.

   [QUIC-TLS] Thomson, M. and S. Turner, "Using TLS to Secure QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-tls-24,
              3 November 2019, <http://www.ietf.org/internet-drafts/
              draft-ietf-quic-tls-24.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-24,
              3 November 2019, <http://www.ietf.org/internet-drafts/
              draft-ietf-quic-tls-24.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-

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              01, 1 February 2017, <http://www.ietf.org/internet-drafts/
              draft-nottingham-httpbis-retry-01.txt>.

   [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-24, 4 November 2019, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-http-24.txt>.

   [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>.

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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