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A Configurable Retransmission Extension for HTTP/3 Datagrams

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Furong Yang , Yanmei Liu , Yunfei Ma , Qinghua Wu
Last updated 2022-12-12
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MASQUE                                                           F. Yang
Internet-Draft                                                    Y. Liu
Intended status: Experimental                                      Y. Ma
Expires: 16 June 2023                                       Alibaba Inc.
                                                                   Q. Wu
                                                                 ICT CAS
                                                        13 December 2022

      A Configurable Retransmission Extension for HTTP/3 Datagrams


   When using HTTP/3 Datagrams for traffic tunneling, it is desirable to
   retransmit HTTP/3 Datagrams in some scenarios where the
   retransmission is beneficial for the tunneled end-to-end connection.
   This document defines an extension to the HTTP Datagrams and the
   Capsule Protocol, which allows HTTP/3 Datagrams to be retransmitted
   according to the configuration of the HTTP/3 Datagram flow.

Discussion Venues

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

   Discussion of this document takes place on the Multiplexed
   Application Substrate over QUIC Encryption 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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 16 June 2023.

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 (
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   Please review these documents carefully, as they describe your rights
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   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Negotiating The Extension Between Peers . . . . . . . . . . .   4
   4.  Signaling HTTP/3 Datagram Retransmission Limit  . . . . . . .   4
   5.  Updating HTTP/3 Datagram Retransmission Limit . . . . . . . .   5
   6.  Handling Lost HTTP/3 Datagrams  . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   HTTP Datagrams and the Capsule Protocol [HTTP-DATAGRAM] defines how
   HTTP Datagrams can be sent either unreliably using the QUIC DATAGRAM
   extension [QUIC-DATAGRAM] or reliably using the Capsule Protocol that
   encapsulates HTTP Datagrams into HTTP/2 [RFC7540] streams, HTTP/3
   [RFC9114] streams or HTTP/1.x connections.  The two modes, "reliable
   mode" and "unreliable mode", all have their pros and cons.

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   This document takes the scenario where HTTP Datagrams are leveraged
   to tunnel QUIC [QUIC] connections from a QUIC client and a target
   QUIC server via an HTTP UDP proxy [CONNECT-UDP] as a reference.
   However, the problems discussed below are not restricted to the
   reference scenario.  Instead, the problems are general in other
   scenarios using HTTP Datagrams for traffic tunneling, e.g.

   In the reference scenario, the reliable mode is usually worse than
   the unreliable mode in terms of the transport performance of the end-
   to-end QUIC connection (i.e. the connection tunneled by the proxy).
   The culprit is that the stream-based Capsule Protocol can stall the
   end-to-end QUIC connection due to head-of-line blocking, which can
   inflate the RTT estimation of the end-to-end connection, make the
   connection perceive bursty losses, and hinder different streams of
   the connection from independent delivery.  However, the reliable mode
   also has advantages sometimes.  If the network path between the
   client and the UDP proxy is lossy and the end-to-end delay is a few
   times higher than the delay of the tunnel, the reliable mode can
   quickly recover the lost packets in the tunnel, hide the losses from
   the end-to-end connection, and avoid the reduction of the
   connection's congestion window.  Some of the above behaviors were
   observed by a study [MASQUE-EVALUATION].

   This document defines an extension to the Capsule Protocol
   [HTTP-DATAGRAM], which allows HTTP/3 Datagrams to be retransmitted
   according to the configuration of the HTTP/3 Datagram flow.  In
   Section 4, a new Capsule Type is added to configure peers'
   retransmission limit of HTTP/3 Datagrams.  Having such a signaling
   mechanism instead of just locally configuring the retransmission
   capability at endpoints (i.e. the client and the proxy) is necessary
   for enforcing retransmission policies in both upstream and downstream
   directions.  As the proxy does not know the end-to-end connection's
   preference for retransmission, the client needs to inform the proxy
   what is the retransmission preference.  Depending on the
   retransmission limit of HTTP/3 Datagrams, the handling of lost HTTP/3
   Datagrams is discussed in Section 6.

   This extension brings the benefits of the reliable mode to the
   unreliable mode.  It is beneficial for traffic tunneling scenarios
   where the last-mile link could be very lossy (e.g.  Apple's iCloud
   Private Relay scenario [PR] where the last-mile link is usually

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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 the notation from [QUIC] for the format of the new
   capsule definition.  Where fields are encoded using the variable-
   length integer, they need not be encoded on the minimum number of

   In this document, the term "UDP proxy" aligns with the definition in
   [CONNECT-UDP], and the term "intermediary" refers to an HTTP
   intermediary as defined in Section 3.7 of [RFC9110].

   The term "HTTP/3 Datagram flow" describes the HTTP/3 Datagrams
   associated with the same HTTP request, .e.g a Connect-UDP request
   [CONNECT-UDP] or a Connect-IP request [CONNECT-IP].

3.  Negotiating The Extension Between Peers

   Peers indicate support for this extension by including the boolean-
   valued Item Structured Field "DG-Retrans: ?1" in the HTTP Request and
   Response headers (See Section 3.3.6 of [RFC8941] for information
   about the boolean format.).  Peers MUST NOT use any following
   mechanisms described by this extension unless the support is
   explicitly expressed.

4.  Signaling HTTP/3 Datagram Retransmission Limit

   This document defines a new Capsule Type SET_H3_DGRAM_RETX_LIMIT to
   communicate how many times an HTTP/3 Datagram can be retransmitted at
   most between peers.  Note, the retransmission limit takes effect
   within the scope of an HTTP/3 Datagram flow.

   The format of the SET_H3_DGRAM_RETX_LIMIT capsule is shown in
   Figure 1.  It has the following fields:

   Context ID: It is the Context ID defined in [CONNECT-UDP] or
   [CONNECT-IP].  It describes the effect scope of the capsule.  It is
   optional.  If the Capsule Type is 0xbb (tentative), the capsule has
   no Context ID field, and the retransmission limit applies to all

   Retransmission Limit: It is the maximum retransmission number of an
   HTTP/3 Datagram.

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       Capsule Type (i) = 0xba..0xbb,
       Capsule Length (i),
       [Context ID (i)],
       Retransmission Limit (i),

                  Figure 1: SET_H3_DGRAM_RETX_LIMIT Format

   When a peer that recognizes SET_H3_DGRAM_RETX_LIMIT capsules receives
   a SET_H3_DGRAM_RETX_LIMIT capsule, if it is using HTTP/3 Datagrams,
   it MUST start to retransmit lost HTTP/3 Datagrams until they are
   acknowledged or their retransmission limit specified in the capsule
   is reached.  If the peer is an intermediary, it SHOULD NOT forward
   the capsule to the next hop, as the aim of retransmissions is to
   recover the lost packets at the probably lossy last-mile link between
   the client and the first hop proxy.  If an intermediary does not
   recognize SET_H3_DGRAM_RETX_LIMIT capsules, it SHOULD forward the
   capsules without any modification for the future extensibility as
   suggested by [HTTP-DATAGRAM].

   Finding the best way to set the limit of retransmission is out of
   this document's scope.  Nonetheless, a possible way to calculate the
   retransmission limit is as follows.  Considering the reference
   scenario of this document (shown in Figure 2), the client can set its
   local retransmission limit to floor(RTT2 / RTT1) and use the
   SET_H3_DGRAM_RETX_LIMIT capsule to set the proxy's retransmission
   limit to floor(RTT2 / RTT1).  As the loss detection algorithm takes
   at least one RTT to detect a packet loss, this setting intends to
   only allow a lost packet to be retransmitted by the tunnel before it
   is retransmitted by the end-to-end QUIC connection.  Note, the client
   can subtract RTT1 from the RTT of the end-to-end QUIC connection to
   get RTT2.

   ┌───────────────┐              ┌─────────┐            ┌───────────┐
   │  QUIC Client  │    RTT1      │UDP Proxy│    RTT2    │QUIC Server│
   ├───────────────┼──────────────┤         ├────────────┤           │
   │ MASQUE Client │              │         │            │           │
   └───────────────┘              └─────────┘            └───────────┘

                      Figure 2: The reference scenario

5.  Updating HTTP/3 Datagram Retransmission Limit

   A peer can just send a new SET_H3_DGRAM_RETX_LIMIT capsule to update
   the retransmission limit of its peer if necessary.  Note, the new
   limit will overwrite the old limit specified by a previous

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6.  Handling Lost HTTP/3 Datagrams

   HTTP/3 Datagrams are encoded in QUIC DATAGRAM frames.  As described
   in [QUIC-DATAGRAM], QUIC MAY notify the sender upon a QUIC DATAGRAM
   frame is acknowledged or declared lost by the loss detection
   algorithm.  This extension relies on the notifications of the
   acknowledgement and loss of QUIC DATAGRAM frames to handle the
   retransmission of lost HTTP/3 Datagrams.

   A reference way of implementation is as follows.  First, when the
   HTTP/3 Datagram layer calls the unreliable sending API of QUIC to
   send an HTTP/3 Datagram, it gets a connection-level unique ID
   (DATAGRAM_ID) from QUIC that corresponds to the underlying QUIC
   DATAGRAM frame.  Then, if the retransmission limit is larger than
   zero, the HTTP/3 Datagram layer generates a record {id = DATAGRAM_ID,
   retx_times = 0} for the HTTP/3 Datagram.  Afterwards, whether the
   HTTP/3 Datagram is acknowledged or declared lost, the HTTP/3 Datagram
   layer will get a corresponding notification.  For the acknowledgement
   notification, the HTTP/3 Datagram layer just deletes the record.  For
   the loss notification, the HTTP/3 Datagram layer retransmits the
   HTTP/3 Datagram and updates the id and retx_times of the record if
   the retransmission limit permits, otherwise, the record is deleted.
   Note, as QUIC holds the HTTP/3 Datagram as the payload of the QUIC
   DATAGRAM frame, the payload can be returned to the HTTP/3 Datagram
   layer for retransmission, which saves the HTTP/3 Datagram layer from
   buffering HTTP/3 Datagrams for retransmission.

7.  Security Considerations

   This extension adds no additional considerations to those presented

8.  IANA Considerations

   This document adds following entry to the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry":

                 | Header Field | Status | Reference     |
                 | DG-Retrans   | Exp    | This document |

                       Table 1: New HTTP Header Field

   This document adds following entries to the "HTTP Capsule Types"

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         | Capsule Type            | Value      | Specification |
         | SET_H3_DGRAM_RETX_LIMIT | 0xba, 0xbb | This document |

                        Table 2: New Capsule Type

9.  References

9.1.  Normative References

              Pauly, T., Schinazi, D., Chernyakhovsky, A., Kühlewind,
              M., and M. Westerlund, "IP Proxying Support for HTTP",
              Work in Progress, Internet-Draft, draft-ietf-masque-
              connect-ip-03, 27 September 2022,

              Schinazi, D., "Proxying UDP in HTTP", RFC 9298,
              DOI 10.17487/RFC9298, August 2022,

              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", RFC 9297, DOI 10.17487/RFC9297, August
              2022, <>.

   [QUIC]     Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,

              Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", RFC 9221,
              DOI 10.17487/RFC9221, March 2022,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

9.2.  Informative References

              Kühlewind, M., Carlander-Reuterfelt, M., Ihlar, M., and M.
              Westerlund, "Evaluation of QUIC-based MASQUE proxying",
              Proceedings of the 2021 Workshop on Evolution, Performance
              and Interoperability of QUIC, DOI 10.1145/3488660.3493806,
              December 2021, <>.

   [PR]       Apple Inc., "iCloud Private Relay Overview", 2021.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,

   [RFC8941]  Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

   [RFC9114]  Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <>.




   TODO acknowledge.

Authors' Addresses

   Furong Yang
   Alibaba Inc.

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   Yanmei Liu
   Alibaba Inc.

   Yunfei Ma
   Alibaba Inc.

   Qinghua Wu

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