QUIC Invariants: Version-Independent Properties of QUIC
The short list of QUIC properties that every version must keep, so new versions can always be deployed.
Thomson, M., "Version-Independent Properties of QUIC", RFC 8999, DOI 10.17487/RFC8999, May 2021, <https://www.rfc-editor.org/info/rfc8999>.
@misc{rfc8999,
series = {Request for Comments},
number = 8999,
howpublished = {RFC 8999},
publisher = {RFC Editor},
doi = {10.17487/RFC8999},
url = {https://www.rfc-editor.org/info/rfc8999},
author = {M. Thomson},
title = {{Version-Independent Properties of QUIC}},
pagetotal = 9,
year = 2021,
month = may,
} RFC text
Contents
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
Abstract
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
https://www.rfc-editor.org/info/rfc8999.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
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",
"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].
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
cleared.
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
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.
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,
<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
[QUIC-TLS] Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
<https://www.rfc-editor.org/info/rfc9001>.
[QUIC-TRANSPORT]
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.
[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 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.
Author's Address
Martin Thomson
Mozilla
Email: mt@lowentropy.net
AI ELI5
On this page
§1An Extremely Abstract Description of QUIC
QUIC is a connection-oriented protocol between two endpoints. Those endpoints exchange UDP datagrams, and the datagrams contain QUIC packets. Endpoints use QUIC packets to establish a QUIC connection, which is shared protocol state between them.
That is the whole starting picture. Everything else in this RFC is about which details of that picture a later version is not allowed to change.
Two computers pass notes in envelopes (UDP datagrams). Inside each envelope are smaller notes (QUIC packets) that let them keep up a shared conversation.
§2What Stays Fixed Across Versions
QUIC can negotiate a version, which lets the protocol change as requirements change. This RFC describes the subset that is intended to stay stable as new versions are developed and deployed. All of these invariants are independent of the IP version.
The primary goal is to make sure new versions of QUIC can be deployed. By documenting what cannot change, the RFC preserves the ability of endpoints to negotiate changes to everything else. A side effect is that it also fixes a minimal amount of information that is visible to anyone other than the endpoints.
Unless this document specifically prohibits a change, any aspect of the protocol can change between versions. The RFC also says it places requirements on future QUIC versions even where it uses no normative language.
It is like agreeing that every edition of a board game keeps the same box shape. Inside the box, anything can change.
§5Long and Short Headers
A UDP datagram holds one or more QUIC packets, but the invariants only describe the first packet in a datagram. There are two header types, told apart by the most significant bit of the first byte: set means a long header, cleared means a short header.
A long header has the form bit set to 1, 7 version-specific bits, a 32-bit Version, then a Destination Connection ID Length (8 bits) and Destination Connection ID, then a Source Connection ID Length (8 bits) and Source Connection ID. Each connection ID is between 0 and 255 bytes long. The rest of the packet is version-specific.
A short header has the form bit cleared to 0, 7 version-specific bits, and a Destination Connection ID that follows the first byte immediately. It has no version, no length fields and no source connection ID. The length of its destination connection ID is not encoded and is not constrained by the RFC.
Apart from these values, the payload is version-specific and of arbitrary length. QUIC packets might be integrity protected, including the header, but Version Negotiation packets are not.
The first thing on every note tells you whether it is a long form with a full address block or a short form with just the recipient's name tag.
| Field | Long header | Short header |
|---|---|---|
| Header Form (first bit) | 1 | 0 |
| Version-Specific Bits | 7 bits | 7 bits |
| Version | 32 bits | Absent |
| Destination Connection ID | 0 to 255 bytes, with 8-bit length | Follows the first byte, length not encoded |
| Source Connection ID | 0 to 255 bytes, with 8-bit length | Absent |
§5.3Connection IDs and Versions
A connection ID is an opaque field of arbitrary length. Its main job is to make sure that changes in addressing at lower layers (UDP, IP and below) do not cause packets for a connection to go to the wrong QUIC endpoint. Endpoints and the intermediaries that support them use it to deliver each packet to the correct instance of an endpoint.
Each endpoint picks its connection IDs using version-specific methods, and packets for the same connection might use different connection ID values.
The Version field is a 4-byte identifier that endpoints can use to identify a QUIC version. The value 0x00000000 is reserved for version negotiation, and all other values are potentially valid. A protocol that does not conform to these properties is not QUIC.
A connection ID is like a name tag on a lunch box. Even if the lunch box moves to a different table, the tag still tells people whose it is.
§6Version Negotiation
An endpoint that receives a long header packet with a version it does not understand or support might send a Version Negotiation packet. Short header packets never trigger it. The packet is a long header packet whose Version field is 0x00000000.
After the connection ID fields it carries a list of Supported Version fields, each naming a version the sender supports, and no other fields. An endpoint MUST ignore a packet with no Supported Version fields or with a truncated one. The 7 bits after the form bit are Unused: they can be anything when sent and MUST be ignored on receipt.
The sender MUST echo the connection IDs back, swapped: the Destination Connection ID is taken from the received Source Connection ID, and the Source Connection ID from the received Destination Connection ID, which a client initially picks at random. This gives the client some assurance that the server received its packet and that an attacker who cannot see packets did not forge the reply.
The packet has no integrity or confidentiality protection. A receiving endpoint might change the version it uses afterwards, depending on the version it chooses. RFC 9000 describes how version 1 generates and consumes these packets.
If you speak to someone in a language they do not know, they can hand back a card listing the languages they do speak. They copy your name tag onto it so you know it is an answer to you.
- Long header, unsupported VersionThe Destination Connection ID is initially randomly selected by the client.
- Version Negotiation, Version = 0Connection IDs are echoed; lists Supported Versions.
§7Security and Privacy
Middleboxes could watch the traits of one QUIC version and assume that other versions showing similar traits mean the same thing. Some effort went into removing or hiding observable traits in version 1, but many remain, and other versions may behave differently.
The version number is not in every packet, so a middlebox that wants to use version-specific traits reliably has to keep state for every connection ID it sees.
Because Version Negotiation packets are not integrity protected, they only have modest protection against insertion by attackers. An endpoint MUST authenticate the semantic content of a Version Negotiation packet if it tries a different QUIC version as a result.
Do not assume you know what a note means just because it looks like an old one. And if a stranger hands you a card that says to switch languages, check that it is real first.
§8Assumptions That Are Not Safe
Appendix A lists a sampling of traits of QUIC version 1 that anyone can observe but that are still free to change in a new version. Any of them can be false for a given version, and some are not even true for version 1. The list is illustrative, not exhaustive.
- QUIC uses TLS, and some TLS messages are visible on the wire.
- Long headers are only used while establishing a connection, and the first packets on a flow use a long header.
- Packet numbers are encrypted, and they increase by one for every packet.
- There is a minimum size for the client's first handshake packet, and the client speaks first.
- The second bit of the first byte (
0x40) is always set. - Only a server sends Version Negotiation, and receiving one makes an endpoint change version.
- A connection ID changes infrequently.
- The Version field is the same in both directions, and a given value means that version is in use.
- Only one connection at a time exists between any pair of endpoints.
Just because every edition so far had a certain feature does not mean the next one will. The list warns people not to count on habits that are not real rules.