Version-Independent Properties of QUIC
draft-ietf-quic-invariants-08

QUIC                                                          M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                             20 May 2020
Expires: 21 November 2020

                 Version-Independent Properties of QUIC
                     draft-ietf-quic-invariants-08

Abstract

   This document defines the properties of the QUIC transport protocol
   that are expected to remain unchanged over time as new versions of
   the protocol are developed.

Note to Readers

   Discussion of this draft takes place on the QUIC working group
   mailing list (quic@ietf.org (mailto:quic@ietf.org)), which is
   archived at https://mailarchive.ietf.org/arch/
   search/?email_list=quic.

   Working Group information can be found at https://github.com/quicwg;
   source code and issues list for this draft can be found at
   https://github.com/quicwg/base-drafts/labels/-invariants.

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 21 November 2020.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  An Extremely Abstract Description of QUIC . . . . . . . . . .   3
   4.  QUIC Packet Headers . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Long Header . . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Short Header  . . . . . . . . . . . . . . . . . . . . . .   4
     4.3.  Connection ID . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  Version . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Version Negotiation . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security and Privacy Considerations . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Incorrect Assumptions  . . . . . . . . . . . . . . .   8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   In addition to providing secure, multiplexed transport, QUIC
   [QUIC-TRANSPORT] includes the ability to negotiate a version.  This
   allows the protocol to change over time in response to new
   requirements.  Many characteristics of the protocol will change
   between versions.

   This document describes the subset of QUIC that is intended to remain
   stable as new versions are developed and deployed.  All of these
   invariants are IP-version-independent.

   The primary goal of this document is to ensure that it is possible to
   deploy new versions of QUIC.  By documenting the properties that
   can't change, this document aims to preserve the ability to change
   any other aspect of the protocol.  Thus, unless specifically
   described in this document, any aspect of the protocol can change
   between different versions.

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   Appendix A is a non-exhaustive list of some incorrect assumptions
   that might be made based on knowledge of QUIC version 1; these do not
   apply to every version of QUIC.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document uses terms and notational conventions from
   [QUIC-TRANSPORT].

3.  An Extremely Abstract Description of QUIC

   QUIC is a connection-oriented protocol between two endpoints.  Those
   endpoints exchange UDP datagrams.  These UDP datagrams contain QUIC
   packets.  QUIC endpoints use QUIC packets to establish a QUIC
   connection, which is shared protocol state between those endpoints.

4.  QUIC Packet Headers

   A QUIC packet is the content of the UDP datagrams exchanged by QUIC
   endpoints.  This document describes the contents of those datagrams.

   QUIC defines two types of packet header: long and short.  Packets
   with long headers are identified by the most significant bit of the
   first byte being set; packets with a short header have that bit
   cleared.

   Aside from the values described here, the payload of QUIC packets is
   version-specific and of arbitrary length.

4.1.  Long Header

   Long headers take the form described in Figure 1.

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   Long Header Packet {
     Header Form (1) = 1,
     Version-Specific Bits (7),
     Version (32),
     DCID Len (8),
     Destination Connection ID (0..2040),
     SCID Len (8),
     Source Connection ID (0..2040),
     Version-Specific Data (..),
   }

                         Figure 1: QUIC Long Header

   A QUIC packet with a long header has the high bit of the first byte
   set to 1.  All other bits in that byte are version specific.

   The next four bytes include a 32-bit Version field (see Section 4.4).

   The next byte contains the length in bytes of the Destination
   Connection ID (see Section 4.3) field that follows it.  This length
   is encoded as an 8-bit unsigned integer.  The Destination Connection
   ID field follows the DCID Len field and is between 0 and 255 bytes in
   length.

   The next byte contains the length in bytes of the Source Connection
   ID field that follows it.  This length is encoded as a 8-bit unsigned
   integer.  The Source Connection ID field follows the SCID Len field
   and is between 0 and 255 bytes in length.

   The remainder of the packet contains version-specific content.

4.2.  Short Header

   Short headers take the form described in Figure 2.

   Short Header Packet {
     Header Form (1) = 0,
     Version-Specific Bits (7),
     Destination Connection ID (..),
     Version-Specific Data (..),
   }

                        Figure 2: QUIC Short Header

   A QUIC packet with a short header has the high bit of the first byte
   set to 0.

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   A QUIC packet with a short header includes a Destination Connection
   ID immediately following the first byte.  The short header does not
   include the Connection ID Lengths, Source Connection ID, or Version
   fields.  The length of the Destination Connection ID is not specified
   in packets with a short header and is not constrained by this
   specification.

   The remainder of the packet has version-specific semantics.

4.3.  Connection ID

   A connection ID is an opaque field of arbitrary length.

   The primary function of a connection ID is to ensure that changes in
   addressing at lower protocol layers (UDP, IP, and below) don't cause
   packets for a QUIC connection to be delivered to the wrong endpoint.
   The connection ID is used by endpoints and the intermediaries that
   support them to ensure that each QUIC packet can be delivered to the
   correct instance of an endpoint.  At the endpoint, the connection ID
   is used to identify which QUIC connection the packet is intended for.

   The connection ID is chosen by each endpoint using version-specific
   methods.  Packets for the same QUIC connection might use different
   connection ID values.

4.4.  Version

   QUIC versions are identified with a 32-bit integer, encoded in
   network byte order.  Version 0 is reserved for version negotiation
   (see Section 5).  All other version numbers are potentially valid.

   The properties described in this document apply to all versions of
   QUIC.  A protocol that does not conform to the properties described
   in this document is not QUIC.  Future documents might describe
   additional properties which apply to a specific QUIC version, or to a
   range of QUIC versions.

5.  Version Negotiation

   A QUIC endpoint that receives a packet with a long header and a
   version it either does not understand or does not support might send
   a Version Negotiation packet in response.  Packets with a short
   header do not trigger version negotiation.

   A Version Negotiation packet sets the high bit of the first byte, and
   thus it conforms with the format of a packet with a long header as
   defined in Section 4.1.  A Version Negotiation packet is identifiable
   as such by the Version field, which is set to 0x00000000.

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   Version Negotiation Packet {
     Header Form (1) = 1,
     Unused (7),
     Version (32) = 0,
     DCID Len (8),
     Destination Connection ID (0..2040),
     SCID Len (8),
     Source Connection ID (0..2040),
     Supported Version (32) ...,
   }

                    Figure 3: Version Negotiation Packet

   The Version Negotiation packet contains a list of Supported Version
   fields, each identifying a version that the endpoint sending the
   packet supports.  The Supported Version fields follow the Version
   field.  A Version Negotiation packet contains no other fields.  An
   endpoint MUST ignore a packet that contains no Supported Version
   fields, or a truncated Supported Version.

   Version Negotiation packets do not use integrity or confidentiality
   protection.  A specific QUIC version might authenticate the packet as
   part of its connection establishment process.

   An endpoint MUST include the value from the Source Connection ID
   field of the packet it receives in the Destination Connection ID
   field.  The value for Source Connection ID MUST be copied from the
   Destination Connection ID of the received packet, which is initially
   randomly selected by a client.  Echoing both connection IDs gives
   clients some assurance that the server received the packet and that
   the Version Negotiation packet was not generated by an off-path
   attacker.

   An endpoint that receives a Version Negotiation packet might change
   the version that it decides to use for subsequent packets.  The
   conditions under which an endpoint changes QUIC version will depend
   on the version of QUIC that it chooses.

   See [QUIC-TRANSPORT] for a more thorough description of how an
   endpoint that supports QUIC version 1 generates and consumes a
   Version Negotiation packet.

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6.  Security and Privacy Considerations

   It is possible that middleboxes could use traits of a specific
   version of QUIC and assume that when other versions of QUIC exhibit
   similar traits the same underlying semantic is being expressed.
   There are potentially many such traits (see Appendix A).  Some effort
   has been made to either eliminate or obscure some observable traits
   in QUIC version 1, but many of these remain.  Other QUIC versions
   might make different design decisions and so exhibit different
   traits.

   The QUIC version number does not appear in all QUIC packets, which
   means that reliably extracting information from a flow based on
   version-specific traits requires that middleboxes retain state for
   every connection ID they see.

   The Version Negotiation packet described in this document is not
   integrity-protected; it only has modest protection against insertion
   by off-path attackers.  QUIC versions MUST define a mechanism that
   authenticates the values it contains.

7.  IANA Considerations

   This document makes no request of IANA.

8.  References

8.1.  Normative References

   [QUIC-TRANSPORT]
              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", Work in Progress,
              Internet-Draft, draft-ietf-quic-transport-28, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-transport-
              28>.

   [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>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

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   [QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using Transport
              Layer Security (TLS) to Secure QUIC", Work in Progress,
              Internet-Draft, draft-ietf-quic-tls-28, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-quic-tls-28>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

Appendix A.  Incorrect Assumptions

   There are several traits of QUIC version 1 [QUIC-TRANSPORT] that are
   not protected from observation, but are nonetheless considered to be
   changeable when a new version is deployed.

   This section lists a sampling of incorrect assumptions that might be
   made based on knowledge of QUIC version 1.  Some of these statements
   are not even true for QUIC version 1.  This is not an exhaustive
   list, it is intended to be illustrative only.

   The following statements are NOT guaranteed to be true for every QUIC
   version:

   *  QUIC uses TLS [QUIC-TLS] and some TLS messages are visible on the
      wire

   *  QUIC long headers are only exchanged during connection
      establishment

   *  Every flow on a given 5-tuple will include a connection
      establishment phase

   *  The first packets exchanged on a flow use the long header

   *  QUIC forbids acknowledgments of packets that only contain ACK
      frames, therefore the last packet before a long period of
      quiescence might be assumed to contain an acknowledgment

   *  QUIC uses an AEAD (AEAD_AES_128_GCM [RFC5116]) to protect the
      packets it exchanges during connection establishment

   *  QUIC packet numbers are encrypted and appear as the first
      encrypted bytes

   *  QUIC packet numbers increase by one for every packet sent

   *  QUIC has a minimum size for the first handshake packet sent by a
      client

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   *  QUIC stipulates that a client speaks first

   *  QUIC packets always have the second bit of the first byte (0x40)
      set

   *  A QUIC Version Negotiation packet is only sent by a server

   *  A QUIC connection ID changes infrequently

   *  QUIC endpoints change the version they speak if they are sent a
      Version Negotiation packet

   *  The version field in a QUIC long header is the same in both
      directions

   *  Only one connection at a time is established between any pair of
      QUIC endpoints

Author's Address

   Martin Thomson
   Mozilla

   Email: mt@lowentropy.net

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