---
title: "Version-Independent Properties of QUIC"
abbrev: QUIC Invariants
number: 8999
docName: draft-ietf-quic-invariants-13
date: 2021-05
category: std
consensus: true
ipr: trust200902
area: Transport
workgroup: QUIC
keyword:
  - crypto
  - next generation
  - protocol
  - secure
  - transport
  - UDP

stand_alone: yes
pi: [toc, sortrefs, symrefs, docmapping]

author:
  -
    ins: M. Thomson
    name: Martin Thomson
    org: Mozilla
    email: mt@lowentropy.net

informative:

  QUIC-TRANSPORT:
    title: "QUIC: A UDP-Based Multiplexed and Secure Transport"
    date: 2021-05
    seriesinfo:
      RFC: 9000
      DOI: 10.17487/RFC9000
    author:
      -
        ins: J. Iyengar
        name: Jana Iyengar
        org: Google
        role: editor
      -
        ins: M. Thomson
        name: Martin Thomson
        org: Mozilla
        role: editor

  QUIC-TLS:
    title: "Using TLS to Secure QUIC"
    date: 2021-05
    seriesinfo:
      RFC: 9001
      DOI: 10.17487/RFC9001
    author:
      -
        ins: M. Thomson
        name: Martin Thomson
        org: Mozilla
        role: editor
      -
        ins: S. Turner
        name: Sean Turner
        org: sn3rd
        role: editor


--- abstract

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


--- middle

# 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.


# 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.

{{bad-assumptions}} 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.


# 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 defines requirements on future QUIC versions, even where normative
language is not used.

This document uses terms and notational conventions from {{QUIC-TRANSPORT}}.


# 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.

{{fig-ex-format}} 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) ...,
}
~~~
{: #fig-ex-format title="Example Format"}


# 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 cleared.

QUIC packets might be integrity protected, including the header.  However, QUIC
Version Negotiation packets are not integrity protected; see {{vn}}.

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


## Long Header

Long headers take the form described in {{fig-long}}.

~~~
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 (..),
}
~~~
{: #fig-long title="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
{{version}}.

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
{{connection-id}}.

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.


## Short Header

Short headers take the form described in {{fig-short}}.

~~~~~
Short Header Packet {
  Header Form (1) = 0,
  Version-Specific Bits (7),
  Destination Connection ID (..),
  Version-Specific Data (..),
}
~~~~~
{: #fig-short title="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.


## 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.


## 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 {{vn}}.  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.


# Version Negotiation {#vn}

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
{{long-header}}.  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) ...,
}
~~~
{: #version-negotiation-format title="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.


# 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 {{bad-assumptions}}.  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
attackers.  An endpoint MUST authenticate the semantic content of a Version
Negotiation packet if it attempts a different QUIC version as a result.


--- back

# Incorrect Assumptions {#bad-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 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.

* 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 client.

* QUIC stipulates that a client speak 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.

* 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.