Greasing the QUIC Bit
A QUIC extension that lets endpoints scramble one fixed header bit so networks cannot come to depend on it.
Thomson, M., "Greasing the QUIC Bit", RFC 9287, DOI 10.17487/RFC9287, August 2022, <https://www.rfc-editor.org/info/rfc9287>.
@misc{rfc9287,
series = {Request for Comments},
number = 9287,
howpublished = {RFC 9287},
publisher = {RFC Editor},
doi = {10.17487/RFC9287},
url = {https://www.rfc-editor.org/info/rfc9287},
author = {M. Thomson},
title = {{Greasing the QUIC Bit}},
pagetotal = 6,
year = 2022,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Thomson
Request for Comments: 9287 Mozilla
Category: Standards Track August 2022
ISSN: 2070-1721
Greasing the QUIC Bit
Abstract
This document describes a method for negotiating the ability to send
an arbitrary value for the second-most significant bit in QUIC
packets.
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/rfc9287.
Copyright Notice
Copyright (c) 2022 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
2. Conventions and Definitions
3. The Grease QUIC Bit Transport Parameter
3.1. Clearing the QUIC Bit
3.2. Using the QUIC Bit
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Author's Address
1. Introduction
The version-independent definition of QUIC [QUIC-INVARIANTS]
intentionally describes a very narrow set of fields that are visible
to entities other than endpoints. Beyond those characteristics that
are invariant, very little about the "wire image" [RFC8546] of QUIC
is visible.
The second-most significant bit of the first byte in every QUIC
packet is defined as having a fixed value in QUIC version 1 [QUIC].
The purpose of having a fixed value is to allow endpoints to
efficiently distinguish QUIC from other protocols; see [DEMUX] for a
description of a system that might use this property. As this bit
can identify a packet as QUIC, it is sometimes referred to as the
"QUIC Bit".
Where endpoints and the intermediaries that support them do not
depend on the QUIC Bit having a fixed value, sending the same value
in every packet is more of a liability than an asset. If systems
come to depend on a fixed value, then it might become infeasible to
define a version of QUIC that attributes semantics to this bit.
In order to safeguard future use of this bit, this document defines a
QUIC transport parameter that indicates that an endpoint is willing
to receive QUIC packets containing any value for this bit. By
sending different values for this bit, the hope is that the value
will remain available for future use [USE-IT].
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].
3. The Grease QUIC Bit Transport Parameter
The grease_quic_bit transport parameter (0x2ab2) is defined for QUIC
version 1 [QUIC]. This transport parameter can be sent by both
client and server. The transport parameter is sent with an empty
value; an endpoint that understands this transport parameter MUST
treat receipt of a non-empty value of the transport parameter as a
connection error of type TRANSPORT_PARAMETER_ERROR.
An endpoint that advertises the grease_quic_bit transport parameter
MUST accept packets with the QUIC Bit set to a value of 0. The QUIC
Bit is defined as the second-most significant bit of the first byte
of QUIC packets (that is, the value 0x40).
3.1. Clearing the QUIC Bit
Endpoints that receive the grease_quic_bit transport parameter from a
peer SHOULD set the QUIC Bit to an unpredictable value unless another
extension assigns specific meaning to the value of the bit.
Endpoints can set the QUIC Bit to 0 on all packets that are sent
after receiving and processing transport parameters. This could
include Initial, Handshake, and Retry packets.
A client MAY also set the QUIC Bit to 0 in Initial, Handshake, or
0-RTT packets that are sent prior to receiving transport parameters
from the server. However, a client MUST NOT set the QUIC Bit to 0
unless the Initial packets it sends include a token provided by the
server in a NEW_TOKEN frame (Section 19.7 of [QUIC]), received less
than 604800 seconds (7 days) prior on a connection where the server
also included the grease_quic_bit transport parameter.
| This 7-day limit allows for changes in server configuration.
| If server configuration changes and a client does not set the
| QUIC Bit, then it is possible that a server will drop packets,
| resulting in connection failures.
A server MUST set the QUIC Bit to 0 only after processing transport
parameters from a client. A server MUST NOT remember that a client
negotiated the extension in a previous connection and set the QUIC
Bit to 0 based on that information.
An endpoint MUST NOT set the QUIC Bit to 0 without knowing whether
the peer supports the extension. As Stateless Reset packets
(Section 10.3 of [QUIC]) are only used after a loss of connection
state, endpoints are unlikely to be able to set the QUIC Bit to 0 on
Stateless Reset packets.
3.2. Using the QUIC Bit
The purpose of this extension is to allow for the use of the QUIC Bit
by later extensions.
Extensions to QUIC that define semantics for the QUIC Bit can be
negotiated at the same time as the grease_quic_bit transport
parameter. In this case, a recipient needs to be able to distinguish
a randomized value from a value carrying information according to the
extension. Extensions that use the QUIC Bit MUST negotiate their use
prior to acting on any semantic.
For example, an extension might define a transport parameter that is
sent in addition to the grease_quic_bit transport parameter. Though
the value of the QUIC Bit in packets received by a peer might be set
according to rules defined by the extension, they might also be
randomized as specified in this document.
The receipt of a transport parameter for an extension that uses the
QUIC Bit could be used to confirm that a peer supports the semantic
defined in the extension. To avoid acting on a randomized signal,
the extension can require that endpoints set the QUIC Bit according
to the rules of the extension but defer acting on the information
conveyed until the transport parameter for the extension is received.
Extensions that define semantics for the QUIC Bit can be negotiated
without using the grease_quic_bit transport parameter. However,
including both extensions allows for the QUIC Bit to be greased even
if the alternative use is not supported.
4. Security Considerations
This document introduces no new security considerations for endpoints
or entities that can rely on endpoint cooperation. However, this
change makes the task of identifying QUIC more difficult without
cooperation of endpoints. This sometimes works counter to the
security goals of network operators who rely on network
classification to identify threats; see Section 3.1 of
[MANAGEABILITY] for a more comprehensive treatment of this topic.
5. IANA Considerations
This document registers the grease_quic_bit transport parameter in
the "QUIC Transport Parameters" registry established in Section 22.3
of [QUIC]. The following fields are registered:
Value: 0x2ab2
Parameter Name: grease_quic_bit
Status: Permanent
Specification: RFC 9287
Date: 2022-07-13
Change Controller: IETF (iesg@ietf.org)
Contact: QUIC Working Group (quic@ietf.org)
Notes: (none)
6. References
6.1. Normative References
[QUIC] 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>.
[QUIC-INVARIANTS]
Thomson, M., "Version-Independent Properties of QUIC",
RFC 8999, DOI 10.17487/RFC8999, May 2021,
<https://www.rfc-editor.org/info/rfc8999>.
[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>.
6.2. Informative References
[DEMUX] Aboba, B., Salgueiro, G., and C. Perkins, "Multiplexing
Scheme Updates for QUIC", Work in Progress, Internet-
Draft, draft-ietf-avtcore-rfc7983bis-06, 5 August 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-avtcore-
rfc7983bis-06>.
[MANAGEABILITY]
Kuehlewind, M. and B. Trammell, "Manageability of the QUIC
Transport Protocol", Work in Progress, Internet-Draft,
draft-ietf-quic-manageability-18, 15 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
manageability-18>.
[RFC8546] Trammell, B. and M. Kuehlewind, "The Wire Image of a
Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
2019, <https://www.rfc-editor.org/info/rfc8546>.
[USE-IT] Thomson, M. and T. Pauly, "Long-Term Viability of Protocol
Extension Mechanisms", RFC 9170, DOI 10.17487/RFC9170,
December 2021, <https://www.rfc-editor.org/info/rfc9170>.
Author's Address
Martin Thomson
Mozilla
Email: mt@lowentropy.net
AI ELI5
On this page
§1Why the QUIC Bit Needs Protecting
The version-independent definition of QUIC (RFC 8999) deliberately describes only a very narrow set of fields that anyone other than the endpoints can see. Beyond those invariant characteristics, very little of QUIC's "wire image" (RFC 8546) is visible.
In QUIC version 1 (RFC 9000), the second-most significant bit of the first byte of every packet has a fixed value. The point of the fixed value is to let endpoints efficiently tell QUIC apart from other protocols. Because it can identify a packet as QUIC, it is sometimes called the QUIC Bit.
The RFC argues that where endpoints and their intermediaries do not need the fixed value, sending the same value in every packet is more of a liability than an asset. If systems come to depend on it, it might become infeasible to define a QUIC version that gives the bit a meaning.
One tiny switch on every QUIC message is always set the same way. If people start counting on that, nobody can ever use the switch for something new.
§3The Grease QUIC Bit Transport Parameter
The extension defines the grease_quic_bit transport parameter, with value 0x2ab2, for QUIC version 1. Both client and server can send it. It is sent with an empty value, and an endpoint that understands it MUST treat a non-empty value as a connection error of type TRANSPORT_PARAMETER_ERROR.
An endpoint that advertises the parameter MUST accept packets with the QUIC Bit set to 0. The QUIC Bit is the second-most significant bit of the first byte, that is, the value 0x40.
In other words, sending the parameter is a promise: "I do not depend on this bit, so you can put anything in it." The hope stated in the RFC is that by sending different values the bit will remain available for future use.
Each side says "I will not be upset if that switch is off." Only after hearing that is the other side allowed to flip it.
§3.1Clearing the QUIC Bit
An endpoint that receives the parameter from its peer SHOULD set the QUIC Bit to an unpredictable value, unless another extension assigns the bit a specific meaning. Once an endpoint has received and processed the peer's transport parameters, it can set the bit to 0 on all packets it sends, which could include Initial, Handshake and Retry packets.
A client MAY also set the bit to 0 in Initial, Handshake or 0-RTT packets sent before it has the server's transport parameters. It MUST NOT do so unless its Initial packets carry a token from the server's NEW_TOKEN frame, received less than 604800 seconds (7 days) earlier on a connection where the server also included the parameter. The RFC says this limit allows for changes in server configuration: if a server stops supporting the extension and the client does not set the bit, the server might drop packets and the connection fails.
A server MUST set the bit to 0 only after processing the client's transport parameters. A server MUST NOT remember from a previous connection that a client negotiated the extension. In general, an endpoint MUST NOT set the bit to 0 without knowing the peer supports the extension. Stateless Reset packets are only used after connection state is lost, so endpoints are unlikely to be able to clear the bit on them.
You may only flip the switch once you are sure the other side said it is fine. Servers must ask again each time and cannot just remember from last time.
| Endpoint | May set the bit to 0 | Must not |
|---|---|---|
| Any endpoint | After processing the peer's transport parameters | Without knowing the peer supports the extension |
| Client | Earlier, in Initial, Handshake or 0-RTT, with a NEW_TOKEN token less than 604800 seconds old | Do so without such a token |
| Server | Only after processing the client's transport parameters | Remember the extension from a previous connection |
§3.2Using the QUIC Bit for Something Else
The purpose of the extension is to let later extensions use the QUIC Bit. An extension that gives the bit a meaning can be negotiated at the same time as grease_quic_bit. The recipient then has to be able to tell a randomized value from one that carries information. Extensions that use the bit MUST negotiate their use before acting on any semantic.
For example, an extension might define its own transport parameter alongside grease_quic_bit. The bit might be set by the extension's rules, or it might be randomized. To avoid acting on a randomized signal, the extension can require endpoints to set the bit by its rules but defer acting on it until the extension's transport parameter has been received.
Such an extension can also be negotiated without grease_quic_bit, but including both lets the bit be greased even when the alternative use is not supported.
If someone invents a real job for the switch, both sides first agree to it. Until then, a flip might just be random, so you do not act on it.
§4Security Considerations
The RFC says it introduces no new security considerations for endpoints or for entities that can rely on endpoint cooperation. However, the change makes it more difficult to identify QUIC without the endpoints' cooperation.
That sometimes works against the security goals of network operators who rely on network classification to identify threats. The RFC points to a draft on the manageability of QUIC for a fuller treatment.
Hiding the always-the-same switch makes it harder for network guards to spot QUIC traffic by looking at it, and some guards do not like that.
§5IANA Considerations
The RFC registers the grease_quic_bit transport parameter in the "QUIC Transport Parameters" registry. The registration has value 0x2ab2, status Permanent, specification RFC 9287 and date 2022-07-13. The change controller is the IETF and the contact is the QUIC Working Group.
The number 0x2ab2 is officially written down in a list so nobody else uses it for something different.