RFC 8999

Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8999                                       Mozilla
Category: Standards Track                                       May 2021
ISSN: 2070-1721

                 Version-Independent Properties of QUIC


   This document defines the properties of the QUIC transport protocol
   that are common to all versions of the protocol.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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Table of Contents

   1.  An Extremely Abstract Description of QUIC
   2.  Fixed Properties of All QUIC Versions
   3.  Conventions and Definitions
   4.  Notational Conventions
   5.  QUIC Packets
     5.1.  Long Header
     5.2.  Short Header
     5.3.  Connection ID
     5.4.  Version
   6.  Version Negotiation
   7.  Security and Privacy Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Incorrect Assumptions

   Author's Address

1.  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.

2.  Fixed Properties of All QUIC Versions

   In addition to providing secure, multiplexed transport, QUIC
   [QUIC-TRANSPORT] allows for the option to negotiate a version.  This
   allows the protocol to change over time in response to new
   requirements.  Many characteristics of the protocol could 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 independent of the IP version.

   The primary goal of this document is to ensure that it is possible to
   deploy new versions of QUIC.  By documenting the properties that
   cannot change, this document aims to preserve the ability for QUIC
   endpoints to negotiate changes to any other aspect of the protocol.
   As a consequence, this also guarantees a minimal amount of
   information that is made available to entities other than endpoints.
   Unless specifically prohibited in this document, any aspect of the
   protocol can change between different versions.

   Appendix A contains 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.

3.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 defines requirements on future QUIC versions, even
   where normative language is not used.

   This document uses terms and notational conventions from

4.  Notational Conventions

   The format of packets is described using the notation defined in this
   section.  This notation is the same as that used in [QUIC-TRANSPORT].

   Complex fields are named and then followed by a list of fields
   surrounded by a pair of matching braces.  Each field in this list is
   separated by commas.

   Individual fields include length information, plus indications about
   fixed value, optionality, or repetitions.  Individual fields use the
   following notational conventions, with all lengths in bits:

   x (A):  Indicates that x is A bits long

   x (A..B):  Indicates that x can be any length from A to B; A can be
      omitted to indicate a minimum of zero bits, and B can be omitted
      to indicate no set upper limit; values in this format always end
      on a byte boundary

   x (L) = C:  Indicates that x has a fixed value of C; the length of x
      is described by L, which can use any of the length forms above

   x (L) ...:  Indicates that x is repeated zero or more times and that
      each instance has a length of L

   This document uses network byte order (that is, big endian) values.
   Fields are placed starting from the high-order bits of each byte.

   Figure 1 shows an example structure:

   Example Structure {
     One-bit Field (1),
     7-bit Field with Fixed Value (7) = 61,
     Arbitrary-Length Field (..),
     Variable-Length Field (8..24),
     Repeated Field (8) ...,

                          Figure 1: Example Format

5.  QUIC Packets

   QUIC endpoints exchange UDP datagrams that contain one or more QUIC
   packets.  This section describes the invariant characteristics of a
   QUIC packet.  A version of QUIC could permit multiple QUIC packets in
   a single UDP datagram, but the invariant properties only describe the
   first packet in a datagram.

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

   QUIC packets might be integrity protected, including the header.
   However, QUIC Version Negotiation packets are not integrity
   protected; see Section 6.

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

5.1.  Long Header

   Long headers take the form described in Figure 2.

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

                         Figure 2: 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.  Versions are
   described in Section 5.4.

   The next byte contains the length in bytes of the Destination
   Connection ID field that follows it.  This length is encoded as an
   8-bit unsigned integer.  The Destination Connection ID field follows
   the Destination Connection ID Length field and is between 0 and 255
   bytes in length.  Connection IDs are described in Section 5.3.

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

   The remainder of the packet contains version-specific content.

5.2.  Short Header

   Short headers take the form described in Figure 3.

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

                        Figure 3: QUIC Short Header

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

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

   The remainder of the packet has version-specific semantics.

5.3.  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) do not cause
   packets for a QUIC connection to be delivered to the wrong QUIC
   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 the QUIC connection
   for which the packet is intended.

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

5.4.  Version

   The Version field contains a 4-byte identifier.  This value can be
   used by endpoints to identify a QUIC version.  A Version field with a
   value of 0x00000000 is reserved for version negotiation; see
   Section 6.  All other values 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 that apply to a specific QUIC version or to a
   range of QUIC versions.

6.  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 5.1.  A Version Negotiation packet is identifiable
   as such by the Version field, which is set to 0x00000000.

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

                    Figure 4: Version Negotiation Packet

   Only the most significant bit of the first byte of a Version
   Negotiation packet has any defined value.  The remaining 7 bits,
   labeled "Unused", can be set to any value when sending and MUST be
   ignored on receipt.

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

   Version Negotiation packets do not use integrity or confidentiality
   protection.  Specific QUIC versions might include protocol elements
   that allow endpoints to detect modification or corruption in the set
   of supported versions.

   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 the Source Connection ID field MUST be copied
   from the Destination Connection ID field 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 attacker that is unable to observe packets.

   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 its 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.

7.  Security and Privacy Considerations

   It is possible that middleboxes could observe 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

   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 attackers.  An endpoint MUST authenticate the semantic content of
   a Version Negotiation packet if it attempts a different QUIC version
   as a result.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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

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

              Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,

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 about QUIC 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.

   *Any and all of the following statements can be false for a given
   QUIC version:*

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

   *  QUIC long headers are only exchanged during connection

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

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

   *  The last packet before a long period of quiescence might be
      assumed to contain only an acknowledgment.

   *  QUIC uses an Authenticated Encryption with Associated Data (AEAD)
      function (AEAD_AES_128_GCM; see [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

   *  QUIC stipulates that a client speak first.

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

   *  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

   *  A QUIC packet with a particular value in the Version field means
      that the corresponding version of QUIC is in use.

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

Author's Address

   Martin Thomson

   Email: mt@lowentropy.net