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Greasing the QUIC Bit

Document Type Active Internet-Draft (quic WG)
Author Martin Thomson
Last updated 2022-04-10 (Latest revision 2021-11-10)
Replaces draft-thomson-quic-bit-grease
Stream Internet Engineering Task Force (IETF)
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Associated WG milestone
Greasing the QUIC bit to IESG
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Shepherd write-up Show Last changed 2022-04-08
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quic                                                          M. Thomson
Internet-Draft                                                   Mozilla
Intended status: Standards Track                        10 November 2021
Expires: 14 May 2022

                         Greasing the QUIC Bit


   This document describes a method for negotiating the ability to send
   an arbitrary value for the second-to-most significant bit in QUIC

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the QUIC Working Group
   mailing list (, which is archived at

   Source for this draft and an issue tracker can be found at (

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 14 May 2022.

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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
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The Grease QUIC Bit Transport Parameter . . . . . . . . . . .   3
     3.1.  Clearing the QUIC Bit . . . . . . . . . . . . . . . . . .   3
     3.2.  Using the QUIC Bit  . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   QUIC [QUIC] intentionally describes a very narrow set of fields that
   are visible to entities other than endpoints.  Beyond those
   characteristics that are defined as invariant [QUIC-INVARIANTS], very
   little about the "wire image" [RFC8546] of QUIC is visible.

   The second-to-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 intermediaries and
   endpoints to efficiently distinguish between QUIC and other
   protocols; see [DEMUX] for a description of a scheme that QUIC can
   integrate with as a result.  As this bit effectively identifies 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 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.

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   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",
   "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) 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 as a connection error of type

   Advertising the grease_quic_bit transport parameter indicates that
   packets sent to this endpoint MAY set a value of 0 for the QUIC Bit.
   The QUIC Bit is defined as the second-to-most significant bit of the
   first byte of QUIC packets (that is, the value 0x40).

   A server MUST respect the value it previously provided for the
   grease_quic_bit transport parameter if it accepts 0-RTT.  A client
   MAY forget the value.  In all other cases, only the presence or
   absence of the transport parameter in the current handshake is used
   to determine what values can be sent in the QUIC Bit.

3.1.  Clearing the QUIC Bit

   Endpoints that receive the grease_quic_bit transport parameter from a
   peer MAY set the QUIC Bit to any value in packets they send to that
   peer.  Endpoints SHOULD set the QUIC Bit to an unpredictable value
   unless another extension assigns specific meaning to the value of the
   bit.  All packets sent after receiving and processing transport
   parameters are affected, including Retry, Initial, and Handshake

   A client MAY also clear the QUIC Bit in Initial packets that are sent
   prior to receiving transport parameters from the server.  A client
   can only clear the QUIC Bit if such packets include a token provided

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   by the server in a NEW_TOKEN frame on a connection where the server
   also included the grease_quic_bit transport parameter.  To allow for
   changes in server configuration, clients SHOULD set the QUIC Bit if
   the token was provided more than 7 days prior.

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.  Endpoints MAY send a signal prior
   to this negotiation completing, but any value carried by the bit
   cannot be used until it is clear that the peer is using the

   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.  Including both extensions
   allows for the QUIC Bit to be greased even if the alternative use is
   not supported.

   Receiving 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.

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.

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5.  IANA Considerations

   This document registers the grease_quic_bit transport parameter in
   the "QUIC Transport Parameters" registry established in Section 22.2
   of [QUIC].  The following fields are registered:

   Value:  0x2ab2

   Parameter Name:  grease_quic_bit

   Status:  Permanent

   Specification:  This document.

   Date:  Date of registration.

   Contact:  QUIC Working Group (

   Change Controller:  IETF (

   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,

   [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, <>.

6.2.  Informative References

   [DEMUX]    Petit-Huguenin, M. and G. Salgueiro, "Multiplexing Scheme
              Updates for Secure Real-time Transport Protocol (SRTP)
              Extension for Datagram Transport Layer Security (DTLS)",
              RFC 7983, DOI 10.17487/RFC7983, September 2016,

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              Thomson, M., "Version-Independent Properties of QUIC",
              RFC 8999, DOI 10.17487/RFC8999, May 2021,

   [RFC8546]  Trammell, B. and M. Kuehlewind, "The Wire Image of a
              Network Protocol", RFC 8546, DOI 10.17487/RFC8546, April
              2019, <>.

   [USE-IT]   Thomson, M. and T. Pauly, "Long-term Viability of Protocol
              Extension Mechanisms", Work in Progress, Internet-Draft,
              draft-iab-use-it-or-lose-it-04, 12 October 2021,

Author's Address

   Martin Thomson


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