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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the Trust Legal Provisions and are provided without warranty as
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
ISO Plain Language
On this page
§1An Abstract Description of QUIC
QUIC is a connection-oriented protocol between two endpoints. Those endpoints exchange UDP datagrams, and those datagrams contain QUIC packets. QUIC endpoints use QUIC packets to establish a QUIC connection, which is shared protocol state between those endpoints.
That is the whole description. Everything else in QUIC, from the encryption to the way streams are multiplexed, is a matter for the transport specification itself.
QUIC is a connection-oriented protocol carried inside UDP datagrams. Two ends use it to build a connection, which is the state they share while the conversation lasts.
§2The Fixed Properties of Every Version
In addition to providing secure, multiplexed transport, QUIC allows for the option to negotiate a version. That is what lets the protocol change over time in response to new requirements, and 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 is to ensure that it is possible to deploy new versions of QUIC: by documenting the properties that cannot change, the document aims to preserve the ability for QUIC endpoints to negotiate changes to any other aspect of the protocol. As a consequence it also guarantees a minimal amount of information that is made available to entities other than endpoints. Unless specifically prohibited, any aspect of the protocol can change between different versions.
The document defines requirements on future QUIC versions even where normative language is not used, and it uses terms and notational conventions from the QUIC transport specification. The packet formats are described with a notation in which complex fields are named and then followed by a list of fields, individual fields carry length information and indications about fixed value, optionality or repetition, and lengths are given in bits. The document uses network byte order, that is big endian values, and fields are placed starting from the high-order bits of each byte.
Only a few things are frozen. Everything else, including the encryption, may be different in the next version, as long as the two ends agree to use it.
- Two header formsLong and short, told apart by the top bit of the first byte.
- Connection IDAn opaque field of arbitrary length, chosen by each endpoint.
- Version fieldA 4-byte identifier; zero is reserved for version negotiation.
- Integrity protectionEvery QUIC packet is protected, except Version Negotiation packets.
§5.1The Long Header
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.
The long header carries a fixed set of fields. The first bit is the header form, fixed at 1, and the remaining seven bits of that byte are version specific. The next four bytes hold a 32-bit Version field. The next byte holds the length in bytes of the Destination Connection ID field that follows it, encoded as an 8-bit unsigned integer, and the Destination Connection ID field itself is between 0 and 255 bytes in length. The next byte likewise holds the length in bytes of the Source Connection ID field, encoded as an 8-bit unsigned integer, and that field is also between 0 and 255 bytes long. The remainder of the packet contains version-specific content.
Aside from the values described here, the payload of QUIC packets is version specific and of arbitrary length. 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.
The long header is used when the two ends need to agree on things, such as which version they are speaking. It carries the version number and the two identifiers that name the connection.
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 (..),
}§5.2The Short Header
A QUIC packet with a short header has the high bit of the first byte set to 0. It includes a Destination Connection ID immediately following the first byte, and its remaining seven bits are version specific. The short header does not include the Destination Connection ID Length, the Source Connection ID Length, the Source Connection ID, or the 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.
QUIC packets might be integrity protected, including the header. Version Negotiation packets are the exception, and are not integrity protected.
The short header is the compact one used once a connection is running. It carries only the identifier of the connection, so there is very little for the network to look at.
| Aspect | Long header | Short header |
|---|---|---|
| Header form bit THIS RFC | 1 | 0 |
| Version field | Present, 4 bytes | Absent |
| Source connection ID | Present | Absent |
| Destination connection ID length | Present, 1 byte | Absent |
| Destination connection ID | 0 to 255 bytes | Length not constrained |
§5.3The Connection ID
A connection ID is an opaque field of arbitrary length. Its primary function is to ensure that changes in addressing at lower protocol layers, in UDP and 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, it identifies the QUIC connection for which the packet is intended.
The connection ID is chosen by each endpoint using version-specific methods, and packets for the same QUIC connection might use different connection ID values.
The Version field contains a 4-byte identifier that can be used by endpoints to identify a QUIC version. A Version field with a value of 0x00000000 is reserved for version negotiation, and all other values are potentially valid. The properties described in the document apply to all versions of QUIC: a protocol that does not conform to them is not QUIC. Future documents might describe additional properties that apply to a specific version or to a range of versions.
The connection ID is a label the two ends agree on so that packets find their way even when the network rewrites addresses. The version number is a four-byte tag, and one value of it is reserved for saying "let us agree on a version".
§6Version 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 conforms to the format of a long header packet. It is identifiable as such by its Version field, which is set to 0x00000000. Only the most significant bit of the first byte has any defined value; the remaining seven bits, labelled "Unused", can be set to any value when sending and MUST be ignored on receipt. After the Source Connection ID field, the packet contains a list of Supported Version fields, each identifying a version the sending endpoint supports, and no other fields. An endpoint MUST ignore a packet that contains no Supported Version fields, or that 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, and 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 who is unable to observe packets. An endpoint that receives a Version Negotiation packet might change the version it decides to use for subsequent packets, and the conditions under which it does so depend on the version of QUIC it chooses.
When a server hears a version it does not know, it can send back a list of versions it does know. The reply is not protected in any way, so the client only trusts its contents after checking them itself.
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) ...,
}§7Security and Privacy Considerations
It is possible for middleboxes to observe traits of a specific version of QUIC and assume that when other versions exhibit similar traits the same underlying meaning is being expressed. There are potentially many such traits, and some effort has been made to either eliminate or obscure some observable traits in QUIC version 1, but many 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 is not integrity protected and has only 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.
Equipment in the middle of the network can see the shape of what passes by it, and it may draw the wrong conclusion from what it sees in a later version. The version negotiation reply in particular can be forged, so a client must check it before acting on it.
§8Assumptions That Are Not Safe
Appendix A 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, and the list is not exhaustive. The document states that any and all of the following statements can be false for a given QUIC version.
Among them are that QUIC uses TLS and some TLS messages are visible on the wire; that QUIC long headers are only exchanged during connection establishment; that every flow on a given five-tuple will include a connection establishment phase; that the first packets exchanged on a flow use the long header; that the last packet before a long period of quiescence might be assumed to contain only an acknowledgment; that QUIC uses an AEAD_AES_128_GCM function to protect the packets it exchanges during connection establishment; that QUIC packet numbers are encrypted and appear as the first encrypted bytes; that QUIC packet numbers increase by one for every packet sent; that QUIC has a minimum size for the first handshake packet sent by a client; that QUIC stipulates that a client speak first; that QUIC packets always have the second bit of the first byte, 0x40, set; that a QUIC Version Negotiation packet is only sent by a server; that a QUIC connection ID changes infrequently; that QUIC endpoints change the version they speak if they are sent a Version Negotiation packet; that the Version field in a QUIC long header is the same in both directions; that a QUIC packet with a particular value in the Version field means that the corresponding version of QUIC is in use; and that only one connection at a time is established between any pair of QUIC endpoints.
The RFC ends with a list of things people often assume about QUIC because of version 1, and warns that none of them is guaranteed. A future version may look quite different on the wire.