QUIC Working Group                                            J. Holland
Internet-Draft                                 Akamai Technologies, Inc.
Intended status: Experimental                                  L. Pardue
Expires: 26 December 2022
                                                               M. Franke
                                                               TU Berlin
                                                            24 June 2022


                      Multicast Extension for QUIC
                    draft-jholland-quic-multicast-01

Abstract

   This document defines a multicast extension to QUIC to enable the
   efficient use of mullticast-capable networks to send identical data
   streams to many clients at once, coordinated through individual
   unicast QUIC connections.

About This Document

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

   The latest revision of this draft can be found at
   https://GrumpyOldTroll.github.io/draft-jholland-quic-multicast/draft-
   jholland-quic-multicast.html.  Status information for this document
   may be found at https://datatracker.ietf.org/doc/draft-jholland-quic-
   multicast/.

   Discussion of this document takes place on the QUIC Individual Draft
   mailing list (mailto:quic@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/quic/.

   Source for this draft and an issue tracker can be found at
   https://github.com/GrumpyOldTroll/draft-jholland-quic-multicast.

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 https://datatracker.ietf.org/drafts/current/.






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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 26 December 2022.

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions and Definitions . . . . . . . . . . . . . . .   4
   2.  Multicast Channel . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Transport Parameter . . . . . . . . . . . . . . . . . . . . .   6
   4.  Extension Overview  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Channel Management  . . . . . . . . . . . . . . . . . . .   7
     4.2.  Client Response . . . . . . . . . . . . . . . . . . . . .  11
     4.3.  Data Carried in Channels  . . . . . . . . . . . . . . . .  11
     4.4.  Stream Processing . . . . . . . . . . . . . . . . . . . .  11
   5.  Flow Control  . . . . . . . . . . . . . . . . . . . . . . . .  12
   6.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  13
   7.  Data Integrity  . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Packet Hashes . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Recovery  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   9.  Connection Termination  . . . . . . . . . . . . . . . . . . .  15
     9.1.  Stateless Reset . . . . . . . . . . . . . . . . . . . . .  15
   10. New Frames  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  MC_ANNOUNCE  . . . . . . . . . . . . . . . . . . . . . .  16
     10.2.  MC_KEY . . . . . . . . . . . . . . . . . . . . . . . . .  18
     10.3.  MC_JOIN  . . . . . . . . . . . . . . . . . . . . . . . .  20
     10.4.  MC_LEAVE . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.5.  MC_INTEGRITY . . . . . . . . . . . . . . . . . . . . . .  22
     10.6.  MC_ACK . . . . . . . . . . . . . . . . . . . . . . . . .  23
     10.7.  MC_LIMITS  . . . . . . . . . . . . . . . . . . . . . . .  23
     10.8.  MC_RETIRE  . . . . . . . . . . . . . . . . . . . . . . .  24



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     10.9.  MC_STATE . . . . . . . . . . . . . . . . . . . . . . . .  25
   11. Frames Carried in Channel Packets . . . . . . . . . . . . . .  26
   12. Implementation and Operational Considerations . . . . . . . .  28
     12.1.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . .  28
       12.1.1.  HTTP/3 Server Push . . . . . . . . . . . . . . . . .  28
       12.1.2.  HTTP/3 WebTransport  . . . . . . . . . . . . . . . .  29
       12.1.3.  Non-web Applications . . . . . . . . . . . . . . . .  30
     12.2.  Graceful Degradation . . . . . . . . . . . . . . . . . .  31
       12.2.1.  Circuit Breakers . . . . . . . . . . . . . . . . . .  32
     12.3.  Server Scalability . . . . . . . . . . . . . . . . . . .  32
     12.4.  Address Collisions . . . . . . . . . . . . . . . . . . .  33
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  34
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     15.2.  Informative References . . . . . . . . . . . . . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  36
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

   This document specifies an extension to QUIC version 1 [RFC9000] to
   enable the use of multicast IP transport of identical packets for use
   in many individual QUIC connections.

   The multicast data can only be consumed in conjunction with a unicast
   QUIC connection.  When the client has support for multicast as
   described in Section 3, the server can tell the client about
   multicast channels and ask the client to join and leave them as
   described in Section 4.1.

   The client reports its joins and leaves to the server and acknowleges
   the packets received via multicast after verifying their integrity.

   The purpose of this multicast extension is to realize the large
   scalability benefits for popular traffic over multicast-capable
   networks without compromising on security, network safety, or
   implementation reliabiliity.  Thus, this specification has several
   design goals:

   *  Re-use as much as possible the mechanisms and packet formats of
      QUIC version 1

   *  Provide flow control and congestion control mechanisms that work
      with multicast traffic






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   *  Maintain the confidentiality, integrity, and authentication
      guarantees of QUIC as appropriate for multicast traffic, fully
      meeting the security goals described in
      [I-D.draft-krose-multicast-security]

   *  Leverage the scalability of multicast IP for data that is
      transmitted identically to many clients

   This document does not define any multicast transport except server
   to client and only includes semantics for source-specific multicast.

1.1.  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.

   Commonly used terms in this document are described below.

          +=======+=============================================+
          | Term  | Definition                                  |
          +=======+=============================================+
          | SSM   | Source-specific multicast, as described in  |
          |       | [RFC4607]                                   |
          +-------+---------------------------------------------+
          | ASM   | Any-source multicast, as distinguished from |
          |       | SSM in [RFC4607]                            |
          +-------+---------------------------------------------+
          | (S,G) | A tuple of IP addresses (Source IP, Group   |
          |       | IP) identifying a source-specific multicast |
          |       | channel as described in [RFC4607]           |
          +-------+---------------------------------------------+

                                  Table 1

2.  Multicast Channel

   A QUIC multicast channel (or just channel) is a one-way network path
   that a server can use as an alternate path to send QUIC connection
   data to a client.

   Multicast channels are designed to leverage multicast IP and to be
   shared by many different connections simultaneously for
   unidirectional server-initiated data.





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   One or more servers can use the same QUIC multicast channel to send
   the same data to many clients, as a supplement to the individual QUIC
   connections between those servers and clients.  (Note that QUIC
   connections are defined in Section 5 of [RFC9000] and are not changed
   in this document; each connection is a shared state between a client
   and a server.)

   Each QUIC multicast channel has exactly one associated (S,G) that is
   used for the delivery of the multicast packets on the IP layer.
   Channels do not support any-source multicast (ASM) semantics.

   Channels carry only 1-RTT packets.  Packets associated with a channel
   contain a Channel ID in place of a Destination Connection ID.  (A
   Channel ID cannot be zero length.)  This adds a layer of indirection
   to the process described in Section 5.2 of [RFC9000] for matching
   packets to connections upon receipt.  Incoming packets received on
   the network path associated with a channel use the Channel ID to
   associate the packet with a joined channel.

   A client with a matching joined channel always has at least one
   connection associated with the channel.  If a client has no matching
   joined channel, the packet is discarded.

   Each channel has an independent packet number space.  Since the
   network path for a channel is unidirectional and uses a different
   packet number space than the unicast part of the connection, packets
   associated with a channel are acknowledged with MC_ACK frames
   Section 10.6 instead of ACK frames.

   The use of any particular channel is OPTIONAL for both the server and
   the client.  It is recommended that applications designed to leverage
   the multicast capabilities of this extension also provide graceful
   degradation for endpoints that do not or cannot make use of the
   multicast functionality (see Section 12.2).

   The server has access to all data transmitted on any multicast
   channel it uses, and could optionally send this data with unicast
   instead.

   No special handling of the data is required in a client application
   that has enabled multicast.  A datagram or any particular bytes from
   a server-initiated unidirectional stream can be delivered over the
   unicast connection or a multicast channel transparently to a client
   application consuming the stream or datagram.







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   Client applications should have a mechanism that disables the use of
   multicast on connections with enhanced privacy requirements for the
   privacy-related reasons covered in
   [I-D.draft-krose-multicast-security].

3.  Transport Parameter

   Support for multicast extensions in a client is advertised by means
   of a QUIC transport parameter:

   *  name: multicast_client_params (TBD - experiments use 0xff3e800)

   If a multicast_client_params transport parameter is not included,
   servers MUST NOT send any frames defined in this document.  (Given
   that a server never sends any MC_JOIN frames, the clients also will
   never send any frames in this document so only the client-to-server
   advertisement is necessary.)

   The multicast_client_params parameter has the structure shown below
   in Figure 1.

   multicast_client_params {
     Capabilities Field (i),
     Max Aggregate Rate (i),
     Max Channel IDs (i),
     Hash Algorithms Supported (i),
     AEAD Algorithms Supported (i),
     Hash Algorithms List (16 * Hash Algorithms Supported),
     AEAD Algorithms List (16 * AEAD Algorithms Supported)
   }

                  Figure 1: multicast_client_params Format

   Capabilities Flags is a bit field structured as follows:

   *  0x1 is set if IPv4 channels are permitted

   *  0x2 is set if IPv6 channels are permitted

   A server MUST NOT send MC_ANNOUNCE (Section 10.1) frames with
   addresses using an IP Family that is not supported according to the
   Capabilities in the multicast_client_params, unless and until a later
   MC_LIMITS (Section 10.7) frame adds permission for a different
   address family.

   The Capabilities Field, Max Aggregate Rate, and Max Channel IDs are
   the same as in MC_LIMITS frames (Section 10.7) and provide the
   initial client values.



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   The AEAD Algorithms List field is in order of preference (most
   preferred occuring first) using values from the TLS Cipher Suite
   registry (https://www.iana.org/assignments/tls-parameters/tls-
   parameters.xhtml#tls-parameters-4 (https://www.iana.org/assignments/
   tls-parameters/tls-parameters.xhtml#tls-parameters-4)).  It lists the
   algorithms the client is willing to use to decrypt data in multicast
   channels, and the server MUST NOT send an MC_ANNOUNCE to this client
   for any channels using unsupported algorithms.  If the server does
   send an MC_ANNOUNCE with an unsupported cipher suite, the client
   SHOULD treat it as a connection error of type PROTOCOL_VIOLATION.

   The Hash Algorithms List field is in order of preference (most
   preferred occurring first) using values from the registry below.  It
   lists the algorithms the client is willing to use to check integrity
   of data in multicast channels, and the server MUST NOT send an
   MC_ANNOUNCE to this client for any channels using unsupported
   algorithms, or the client SHOULD treat it as a connection error of
   type PROTOCOL_VIOLATION:

   *  https://www.iana.org/assignments/named-information/named-
      information.xhtml#hash-alg (https://www.iana.org/assignments/
      named-information/named-information.xhtml#hash-alg)

4.  Extension Overview

   A client has the option of refusal and the power to impose upper
   bound maxima on several resources (see Section 5), but otherwise its
   join status for all multicast channels is entirely managed by the
   server.

   *  A client MUST NOT join a channel without receiving instructions
      from a server to do so.

   *  A client MUST leave joined channels when instructed by the server
      to do so.

   *  A client MAY leave channels or refuse to join channels, regardless
      of instructions from the server.

4.1.  Channel Management

   The client tells its server about some restrictions on resources that
   it is capable of processing with the initial values in the
   multicast_client_params transport parameter (Section 3) and later can
   update these limits with MC_LIMITS Section 10.7 frames.  Servers
   ensure the set of channels the client is currently requested to join
   remains within these advertised client limits as covered in
   Section 5.



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   The server asks the client to join channels with MC_JOIN
   (Section 10.3) frames and to leave channels with MC_LEAVE
   (Section 10.4) frames.

   The server uses the MC_ANNOUNCE (Section 10.1) frame before any join
   or leave frames for the channel to describe the channel properties to
   the client, including values the client can use to ensure the
   server's requests remain within the limits it has sent to the server,
   as well as the secrets necessary to decode the headers of packets in
   the channel.  MC_KEY frames provide the secrets necessary to decode
   the payload of packets in the channel.  Figure 2 shows the states a
   channel has from the clients point of view.

   Joining a channel after receiving an MC_JOIN frame is OPTIONAL for
   clients.  If a client decides not to join after being asked to do so,
   it can indicate this decision by sending an MC_STATE (Section 10.9)
   frame with state Declined Join and an appropriate reason.

   The server ensures that in aggregate, all channels that the client
   has currently been asked to join and that the client has not left or
   declined to join fit within the limits indicated by the initial
   values in the transport parameter or last MC_LIMITS (Section 10.7)
   frame the server received.




























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                               o
                               |
   ----------------------->|   | Receive MC_ANNOUNCE and/or MC_KEY
   ^                       |   |
   |                       |   |
   |  Receive MC_JOIN (and v   v
   |     unable to join) +----------+
   |<--------------------*          |
                         | unjoined | Receive MC_RETIRE
   --------------------->|          *------------------------>|
   ^                     +----*-----+                         |
   |                          | Receive MC_JOIN               |
   |                          |   (and able to join)          |
   |                          |                               |
   |                          v                               v
   |                     +----------+                    +---------+
   |    Receive MC_LEAVE |          |                    |         |
   |     (or error case) |  joined  | Receive MC_RETIRE  | retired |
   |<--------------------*          *------------------->|         |
                         +----------+                    +---------+

   *: Each transition except the initial receiving of MC_ANNOUNCE
      and MC_KEY frames causes the client to send an MC_STATE frame
      describing the state transition (for Left or Declined Join, this
      includes a reason for the transition).

   "able to join" means:
   - Both MC_KEY and MC_ANNOUNCE have been received
   - Result will be within latest advertised client limits
   - Nothing preventing a join is active (e.g. a hold-down timer,
     administrative blocking, etc.)

         Figure 2: States a channel from the clients point of view.

   When the server has asked the client to join a channel and has not
   received any MC_STATE frames Section 10.9 with state Declined Join or
   Left, it also sends MC_INTEGRITY frames (Section 10.5) to enable the
   client to verify packet integrity before processing the packet.  A
   client MUST NOT decode packets for a channel for which it has not
   received an applicable MC_ANNOUNCE (Section 10.1), or for which it
   has not received a matching packet hash in an MC_INTEGRITY
   (Section 10.5) frame, or for which it has not received an applicable
   MC_KEY frame Section 10.2.

   Figure 3 shows the frames that are being exchanged about and over a
   channel during the lifetime of an example channel.





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   Client                                        Server

   MC_LIMITS/initial_limits  --->

                                                 MC_ANNOUNCE
                                                 MC_KEY
                                          <----  MC_JOIN

   MC_STATE(Joined)  --->

                                                 MC_INTEGRITY
                                          <----  [STREAM(...)]
   MC_ACK  --->                                  ...
   ...                                    <----  MC_KEY
   ...
   MC_LIMITS  --->

                                          <----  MC_LEAVE

   MC_STATE(Left)  --->

                                          <----  MC_JOIN

   MC_STATE(Joined)  --->

                                                 MC_INTEGRITY
                                          <----  [STREAM(...)]
   MC_ACK  --->                                  ...
   ...

                                          <----  MC_LEAVE

   MC_STATE(Left)  --->

                                          <----  MC_RETIRE

   MC_STATE(Retired)  --->

     Figure 3: Example flow of frames for a channel.  Frames in square
                     brackets are sent over multicast.

   TODO: incorporate server-side state diagram and explanation, latest
   proposed sketch at https://github.com/GrumpyOldTroll/draft-jholland-
   quic-multicast/issues/62 (https://github.com/GrumpyOldTroll/draft-
   jholland-quic-multicast/issues/62)






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4.2.  Client Response

   The client sends back information about how it has responded to the
   server's requests to join and leave channels in MC_STATE
   (Section 10.9) frames.  MC_STATE frames are only sent for channels
   after the server has requested the client to join the channel, and
   are thereafter sent any time the state changes.

   Clients that receive and decode data on a multicast channel send
   acknowledgements for the data on the unicast connection using MC_ACK
   (Section 10.6) frames.

   A server can determine if a client receives packets for a multicast
   channel if it receives MC_ACK frames associated with that channel.
   As such, it is in general up to the server to decide on the time
   after which it deems a client to be unable to receive packets on a
   given channel and take appropriate steps, e.g. sending an MC_LEAVE
   frame to the client.  Note that clients willing to join a channel
   SHOULD remain joined to the channel even if they receive no channel
   data for an extended period, to enable multicast-capable networks to
   perform popularity-based admission control for multicast channels.

4.3.  Data Carried in Channels

   Data transmitted in a multicast channel is encrypted with symmetric
   keys so that on-path observers without access to these keys cannot
   decode the data.  However, since potentially many receivers receive
   identical packets and identical keys for the multicast channel and
   some receivers might be malicious, the packets are also protected by
   MC_INTEGRITY (Section 10.5) frames transmitted over a separate
   integrity-protected path.

   A client MUST NOT decode packets on a multicast channel for which it
   has not received a matching hash in an MC_INTEGRITY frame over a
   different integrity-protected communication path.  The different path
   can be either the unicast connection or another multicast channel
   with packets that were verified with an earlier MC_INTEGRITY frame.

   See Section 7 for a more complete overview of the security issues
   involved here.

4.4.  Stream Processing

   Stream IDs in channels are restricted to unidirectional server
   initiated streams, or those with the least significant 2 bits of the
   stream ID equal to 3 (see [RFC9000] Section 2.1).





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   When a channel contains streams with ids above the client's
   unidirectional MAX_STREAMS, the server MUST NOT instruct the client
   to join that channel and SHOULD send a STREAMS_BLOCKED frame, as
   described in Sections 4.6 and 19.14 of [RFC9000].

   If the client is already joined to a channel that carries streams
   that exceed or will soon exceed the client's unidirectional
   MAX_STREAMS, the server SHOULD send an MC_LEAVE frame.

   If a client receives a STREAM frame with an ID above its MAX_STREAMS
   on a channel, the client MAY increase its unidirectional MAX_STREAMS
   to a value greater than the new ID and send an update to the server,
   otherwise it MUST drop the packet and leave the channel with reason
   Max Streams Exceeded.

   Since clients can join later than a channel began, it is RECOMMENDED
   that clients supporting the multicast extensions to QUIC be prepared
   to handle stream IDs that do not begin at early values, since by the
   time a client joins a channel in progress the stream id count might
   have been increasing for a long time.  Clients should therefore begin
   with a high initial_max_streams_uni or send an early MAX_STREAMS type
   0x13 value (see Section 19.11 of [RFC9000]) with a high limit.
   Clients MAY use the maximum 2^60 for this high initial limit, but the
   specific choice is implementation-dependent.

   The same stream ID may be used in both one or more multicast channels
   and the unicast connection.  As described in Section 2.2 of
   [RFC9000], stream data received multiple times for the same offset
   MUST be identical, even across different network paths; if it's not
   identical it MAY be treated as a connection error of type
   PROTOCOL_VIOLATION.

5.  Flow Control

   The values used for unicast flow control cannot be used to limit the
   transmission rate of a multicast channel because a single client with
   a low MAX_STREAM_DATA or MAX_DATA value that did not acknowledge
   receipt could block many other receivers if the servers had to ensure
   that channels responded to each client's limits.

   Instead, clients advertise resource limits via MC_LIMITS
   (Section 10.7) frames and their initial values from the transport
   parameter (Section 3).  The server is responsible for keeping the
   client within its advertised limits, by ensuring via MC_JOIN and
   MC_LEAVE frames that the set of channels the client is asked to be
   joined to will not, in aggregate, exceed the client's advertised
   limits.  The server also advertises the expected maxima of the values
   that can contribute toward client resource limits within a channel in



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   an MC_ANNOUNCE (Section 10.1) frame, and the client also ensures that
   the set of channels it's joined to does not exceed its limits,
   according to the advertised values.  The client also monitors the
   packets received to ensure that channels don't exceed their
   advertised values, and leaves channels that do.

   If the server asks the client to join a channel that would exceed the
   client's limits with an up-to-date Client Limit Sequence Number, the
   client should send back an MC_STATE frame (Section 10.9) with
   "Declined Join" and reason "Property Violation".  If the server asks
   the client to join a channel that would exceed the client's limits
   with an out-of-date Client Limit Sequence Number or a Channel Key
   Sequence Number that the client has not yet seen, the client should
   instead send back a "Declined Join" with "Desynchronized Limit
   Violation".  If the actual contents sent in the channel exceed the
   advertised limits from the MC_ANNOUNCE, clients SHOULD leave the
   stream and send an MC_STATE(Left) frame, using the Limit Violated
   reason.

6.  Congestion Control

   Both the server and the client perform congestion control operations,
   so that according to the guidelines in Section 4.1 of [RFC8085],
   mechanisms for both feedback-based and receiver-driven styles of
   congestion control are present and operational.

   The server maintains a full view of the traffic received by the
   client via the MC_ACK (Section 10.6) frames and ACK frames it
   receives, and can detect loss experienced by the client.  Under
   sustained persistent loss that exceeds server-configured thresholds,
   the server SHOULD instruct the client to leave channels as
   appropriate to avoid having the client continue to see sustained
   persistent loss.

   Under sustained persistent loss that exceeds client-configured
   thresholds, the client SHOULD reduce its Max Rate and tell the server
   via MC_LIMITS frames, which also will result in the server
   instructing the client to leave channels until the clients aggregate
   rate is below its advertised Max Rate.  Under a higher threshold of
   sustained persistent loss, the client also SHOULD leave channels,
   using an MC_STATE(Left) frame with the High Loss reason, as well as
   reducing the Max Rate in MC_LIMITS.

   The unicast connection's congestion control is unaffected.  However a
   few potential interactions with the unicast connection are worth
   highlighting:





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   *  if the client notices high loss on the unicast connection while
      multicast channel packets are arriving, the client MAY leave
      channels with reason High Loss.

   *  if the client notices congestion from unicast this MAY also drive
      reductions in the client's Max Rate, and a lack of unicast
      congestion under unicast load MAY also drive increases to the
      client's Max Rate (along with an updated MC_LIMITS frame).

   Hybrid multicast-unicast congestion control is still an experimental
   research topic.  Implementations SHOULD follow the guidelines given
   in Section 4.1.1 of [RFC8085] under the assumption that applications
   using QUIC multicast will operate as Bulk-Transfer applications.

7.  Data Integrity

   TODO: import the [I-D.draft-krose-multicast-security] explanation for
   why extra integrity protection is necessary (many client have the
   shared key, so AEAD doesn't provide authentication against other
   valid clients on its own, since the same key is given to multiple
   clients and as the client count grows so does the chance that at
   least one client is controlled by an attacker.)

7.1.  Packet Hashes

   TODO: explanation and example for how to calculate the packet hash.
   Note that the hash is on the encrypted packet to avoid leaking data
   about the encrypted contents to those who can see a hash but not the
   key.  (This approach also may help make better use of
   [I-D.draft-ietf-mboned-ambi] by making it possible to generate the
   same hashes for use in both AMBI and QUIC MC_INTEGRITY frames.)

8.  Recovery

   TODO: Articulate key differences with [RFC9002].  The main known
   difference is that servers might not be running on the same devices
   that are sending the channel packets, therefore the RTT for channel
   packets might use an estimated send time that can vary according to
   the clock synchronization among servers and the deployment and
   implementation details of how the servers find out the sending
   timestamps of channel packets.  Experience-based guidance on the
   recovery timing estimates is one anticipated outcome of experimenting
   with deployments of this experimental extension.

   All the new frames defined in this document except MC_ACK are ack-
   eliciting and are retransmitted until acknowledged to provide
   reliable, though possibly out of order, delivery.




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   Note that recovery MAY be achieved either by retransmitting frame
   data that was lost and needs reliable transport either by sending the
   frame data on the unicast connection or by coordinating to cause an
   aggregated retransmission of widely dropped data on a multicast
   channel, at the server's discretion.  However, the server in each
   connection is responsible for ensuring that any necessary server-to-
   client frame data lost by a multicast channel packet loss ultimately
   arrives at the client.

9.  Connection Termination

   Termination of the unicast connection behaves as described in
   Section 10 of [RFC9000], with the following notable differences:

   *  On the client side, termination of the unicast connection means
      that it MUST leave all multicast channels and discard any state
      associated with them.  Servers MAY stop sending to multicast
      channels if there are no unicast connections left that are
      associated with them.

   *  For determining the liveness of a connection, the client MUST only
      consider packets received on the unicast connection.  Any packets
      received on a multicast channel MUST NOT be used to reset a timer
      checking if a potentially specified max_idle_timeout has been
      reached.  If the unicast connection becomes idle and the server
      does not respond to a liveness test, the client MUST terminate the
      connection as described above.

9.1.  Stateless Reset

   As clients can unilaterally stop the delivery of multicast packets by
   leaving the relevant (S,G), channels do not need stateless reset
   tokens.  Clients therefore do not share the stateless reset tokens of
   channels with the server.  Instead, if an endpoint receives packets
   addressed to an (S,G) that it can not associate with any existing
   channel, it MAY take the necessary steps to prevent the reception of
   further such packets, without the need to signal to the server that
   it should stop sending.

   If a server or client detect a stateless reset for a channel, they
   MUST ignore it.

10.  New Frames








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10.1.  MC_ANNOUNCE

   Once a server learns that a client supports multicast through its
   transport parameters, it can send one or multiple MC_ANNOUNCE frames
   (type=TBD-11..TBD-12) to share information about available channels
   with the client.  The MC_ANNOUNCE frame contains the properties of a
   channel that do not change during its lifetime.

   MC_ANNOUNCE frames are formatted as shown in Figure 4.

   MC_ANNOUNCE Frame {
     Type (i) = TBD-11..TBD-12 (experiments use 0xff3e811/0xff3e812),
     ID Length (8),
     Channel ID (8..160),
     Source IP (32..128),
     Group IP (32..128),
     UDP Port (16),
     Header AEAD Algorithm (16),
     Header Secret Length (i),
     Header Secret (..),
     AEAD Algorithm (16),
     Integrity Hash Algorithm (16),
     Max Rate (i),
     Max ACK Delay (i)
   }

                     Figure 4: MC_ANNOUNCE Frame Format

   Frames of type TBD-11 are used for IPv4 and both Source and Group
   address are 32 bits long.  Frames of type TBD-12 are used for IPv6
   and both Source and Group address are 128 bits long.

   MC_ANNOUNCE frames contain the following fields:

   *  ID Length: The length in bytes of the Channel ID field.

   *  Channel ID: The channel ID of the channel that is getting
      announced.

   *  Source IP: The IP Address of the source of the (S,G) for the
      channel.  Either a 32-bit IPv4 address or a 128-bit IPv6 address,
      as indicated by the frame type (TBD-11 indicates IPv4, TBD-12
      indicates IPv6).

   *  Group IP: The IP Address of the group of the (S,G) for the
      channel.  Either a 32-bit IPv4 address or a 128-bit IPv6 address,
      as indicated by the frame type (TBD-11 indicates IPv4, TBD-12
      indicates IPv6).



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   *  UDP Port: The 16-bit UDP Port of traffic for the channel.

   *  Header AEAD Algorithm: A value from the TLS Cipher Suite registry
      (https://www.iana.org/assignments/tls-parameters/tls-
      parameters.xhtml#tls-parameters-4
      (https://www.iana.org/assignments/tls-parameters/tls-
      parameters.xhtml#tls-parameters-4)), used to protect the header
      fields in the channel packets.  The value MUST match a value
      provided in the "AEAD Algorithms List" of the transport parameter
      (see Section 3).

   *  Header Secret Length: Provides the length of the Secret field.

   *  Header Secret: A secret for use with the Header AEAD Algorithm for
      protecting the header fields of 1-RTT packets in the channel as
      described in [RFC9001].  The Key and Initial Vector for the
      application data carried in the 1-RTT packet header fields are
      derived from this secret as described in Section 7.3 of [RFC8446].

   *  AEAD Algorithm: A value from the TLS Cipher Suite registry
      (https://www.iana.org/assignments/tls-parameters/tls-
      parameters.xhtml#tls-parameters-4
      (https://www.iana.org/assignments/tls-parameters/tls-
      parameters.xhtml#tls-parameters-4)), used to protect the payloads
      in the channel packets.  The value MUST match a value provided in
      the "AEAD Algorithms List" of the transport parameter (see
      Section 3).

   *  Integrity Hash Algorithm: The hash algorithm used in integrity
      frames.

      -  *Author's Note:* Several candidate IANA registries, not sure
         which one to use?  Some have only text for some possibly useful
         values.  For now we use the first of these:

         o  https://www.iana.org/assignments/named-information/named-
            information.xhtml#hash-alg
            (https://www.iana.org/assignments/named-information/named-
            information.xhtml#hash-alg)

         o  https://www.iana.org/assignments/tls-parameters/tls-
            parameters.xhtml#tls-parameters-18
            (https://www.iana.org/assignments/tls-parameters/tls-
            parameters.xhtml#tls-parameters-18)







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         o  (text-only): https://www.iana.org/assignments/hash-function-
            text-names/hash-function-text-names.xhtml
            (https://www.iana.org/assignments/hash-function-text-names/
            hash-function-text-names.xhtml)

   *  Max Rate: The maximum rate in Kibps of the payload data for this
      channel.  Channel data MUST NOT exceed this rate over any 5s
      window, if it does clients SHOULD leave the channel with reason
      Max Rate Exceeded.

   *  Max ACK Delay: A value used similarly to max_ack_delay
      (Section 18.2 of [RFC9000]}) that applies to traffic in this
      channel.  Clients SHOULD NOT intentionally add delay to MC_ACK
      frames for traffic in this channel beyond this value, in
      milliseconds, and SHOULD NOT add any delay to the first MC_ACK of
      data packets for a channel.  As long as they stay inside these
      limits, clients can improve efficiency and network load for the
      uplink by aggregating MC_ACK frames whenever possible.

   A client MUST NOT use the channel ID included in an MC_ANNOUNCE frame
   as a connection ID for the unicast connection.  If it is already in
   use, the client should retire it as soon as possible.  As the server
   knows which connection IDs are in use by the client, it MUST wait
   with the sending of an MC_JOIN frame until the channel ID associated
   with it has been retired by the client.

   As all the properties in MC_ANNOUNCE frames are immutable during the
   lifetime of a channel, a server SHOULD NOT send an MC_ANNOUNCE frame
   for the same channel more than once to each client except as needed
   for recovery.

   A server SHOULD send an MC_ANNOUNCE frame for a channel before
   sending an MC_KEY and SHOULD send an MC_KEY frame for a channel
   before sending an MC_JOIN frame for it.  Each of these recommended
   orderings MAY occur within the same packet.

10.2.  MC_KEY

   An MC_KEY frame (type=TBD-01) is sent from server to client, either
   with the unicast connection or in an existing joined multicast
   channel.  The MC_KEY frame contains an updated secret that is used to
   generate the keying material for the payload of 1-RTT packets
   received on the multicast channel.

   A server can send a new MC_KEY frame with a sequence number increased
   by one.  A server MUST generate continuous sequence numbers, and MAY
   start at a value higher than 0.  Note that while not joined, a client
   will not receive updates to channel secrets, and thus may see jumps



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   in the Key Sequence Number values between MC_KEY frames.  However,
   while joined the Key Sequence Numbers in the MC_KEY frames MUST
   increment by 1 for each new secret.

   Secrets with even-valued Key Sequence Numbers have a Key Phase of 0
   in the 1-RTT packet, and secrets with odd-valued Key Seqence Numbers
   have a Key Phase of 1 in the 1-RTT packet.  Secrets with a Key Phase
   indicating an unknown key SHOULD be discarded without attempting to
   decrypt them.  (An unknown key might happen after loss of the latest
   MC_KEY frame, so that packets on a channel have an updated Key Phase
   starting at a particular packet number, but the client does not yet
   know about the key change.)

   It is RECOMMENDED that servers send regular secret updates.

   MC_KEY frames are formatted as shown in Figure 5.

   MC_KEY Frame {
     Type (i) = TBD-01 (experiments use 0xff3e801),
     ID Length (8),
     Channel ID (8..160),
     Key Sequence Number (i),
     From Packet Number (i),
     Secret Length (i),
     Secret (..)
   }

                       Figure 5: MC_KEY Frame Format

   MC_KEY frames contain the following fields:

   *  ID Length: The length in bytes of the Channel ID field.

   *  Channel ID: The channel ID for the channel associated with this
      frame.

   *  Key Sequence Number: Increases by 1 each time the secret for the
      channel is changed by the server.  If there is a gap in sequence
      numbers due to reordering or retransmission of packets, on receipt
      of the older MC_KEY frame, the client MUST apply the secret
      contained and the packet numbers on which it applies as if they
      arrived in order.

   *  From Packet Number: The values in this MC_KEY frame apply only to
      packets starting at From Packet Number and continuing until they
      are overwritten by a new MC_KEY frame with a higher From Packet
      Number.  The Packet Number MUST never decrease with an increased
      Key Sequence Number.



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   *  Secret Length: Provides the length of the secret field.

   *  Secret: Used to protect the packet contents of 1-RTT packets for
      the channel as described in [RFC9001].  The Key and Initial Vector
      for the application data carried in the 1-RTT packet payloads are
      derived from the secret as described in Section 7.3 of [RFC8446].
      To maintain forward secrecy and prevent malicious clients from
      decrypting packets long after they have left or were removed from
      the unicast connection, servers SHOULD periodically send key
      updates using only unicast.

   Clients MUST delete old secrets within 10 seconds after receiving a
   new key, and within 3 seconds after receiving a new key and not
   receiving any data traffic decrypted with the old key.

   The From Packet Number is used to indicate the starting packet number
   (Section 17.1 of [RFC9000]) of the 1-RTT packets for which the secret
   contained in an MC_KEY frame is applicable.  This secret is
   applicable to all future packets until it is updated by a new MC_KEY
   frame.

   A server SHOULD NOT send MC_KEY frames for channels except those the
   client has joined or will be imminently asked to join.

10.3.  MC_JOIN

   An MC_JOIN frame (type TBD-02) is sent from server to client and
   requests that a client join the given transport addresses and ports
   and process packets with the given Channel ID according to the
   corresponding MC_ANNOUNCE frame and the latest MC_KEY frame for the
   channel.

   A client cannot join a multicast channel without first receiving an
   MC_ANNOUNCE frame and an MC_KEY frame, which together set all the
   values necessary to process the channel.

   If a client receives an MC_JOIN for a channel for which it has not
   received both an MC_ANNOUNCE frame and an MC_KEY frame, it MUST
   respond with an MC_STATE with State "Declined Join" and reason
   "Missing Properties".  The server MAY send another MC_JOIN after
   receiving an acknowledgement indicating receipt of the MC_ANNOUNCE
   frame and the MC_KEY frame.

   MC_JOIN frames are formatted as shown in Figure 6.







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   MC_JOIN Frame {
     Type (i) = TBD-02 (experiments use 0xff3e802),
     MC_LIMITS Sequence Number (i),
     MC_STATE Sequence Number (i),
     MC_KEY Sequence Number (i),
     ID Length (8),
     Channel ID (8..160)
   }

                       Figure 6: MC_JOIN Frame Format

   The sequence numbers are the most recently processed sequence number
   by the server from the respective frame type.  They are present to
   allow the client to distinguish between a broken server that has
   performed an illegal action and an instruction that's based on
   updates that are out of sync (either one or more missing updates to
   MC_KEY not yet received by the client or one or more missing updates
   to MC_LIMITS or MC_STATE not yet received by the server).

   A client MAY perform the join if it has the sequence number of the
   corresponding channel properties and the client's limits will not be
   exceeded, even if the client sequence numbers are not up-to-date.

   If the client does not join, it MUST send an MC_STATE frame with
   "Declined Join" and a reason.

   If the client does join, it MUST send an MC_STATE frame with
   "Joined".

10.4.  MC_LEAVE

   An MC_LEAVE frame (type=TBD-03) is sent from server to client, and
   requests that a client leave the given channel.

   If the client has already left or declined to join the channel, the
   MC_LEAVE is ignored.

   If an MC_JOIN or an MC_LEAVE with the same Channel ID and a higher
   MC_STATE Sequence number has previously been received, the MC_LEAVE
   is ignored.

   Otherwise, the client MUST leave the channel and send a new MC_STATE
   frame with reason Left as requested by server.

   MC_LEAVE frames are formatted as shown in Figure 7.






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   MC_LEAVE Frame {
     Type (i) = TBD-03 (experiments use 0xff3e803),
     ID Length (8),
     Channel ID (8..160),
     MC_STATE Sequence Number (i),
     After Packet Number (i)
   }

                      Figure 7: MC_LEAVE Frame Format

   If After Packet Number is nonzero, wait until receiving that packet
   or a higher valued number before leaving.

10.5.  MC_INTEGRITY

   MC_INTEGRITY frames are sent from server to client and are used to
   convey packet hashes for validating the integrity of packets received
   over the multicast channel as described in Section 7.1.

   MC_INTEGRITY frames are formatted as shown in Figure 8.

   MC_INTEGRITY Frame {
     Type (i) = TBD-04..TBD-05 (experiments use 0xff3e804/0xff3e805),
     ID Length (8),
     Channel ID (8..160),
     Packet Number Start (i),
     [Length (i)],
     Packet Hashes (..)
   }

                    Figure 8: MC_INTEGRITY Frame Format

   For type TBD-05, Length is present and is a count of packet hashes.
   For TBD-04, Length is not present and the packet hashes extend to the
   end of the packet.

   The first hash in the Packet Hashes list is a hash of a 1-RTT packet
   with the Channel ID equal to the Channel ID in the MC_INTEGRITY frame
   and packet number equal to the Packet Number Start field.  Subsequent
   hashes refer to the packets for the channel with packet numbers
   increasing by 1.

   Packet hashes MUST have length with an integer multiple of the length
   indicated by the Hash Algorithm from the MC_ANNOUNCE frame.

   See Section 7.1 for a description of the packet hash calculation.





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10.6.  MC_ACK

   The MC_ACK frame (types TBD-06 and TBD-07; experiments use
   0xff3e806..0xff3e807) is an extension of the ACK frame defined by
   [RFC9000].  It is used to acknowledge packets that were sent on
   multicast channels.  If the frame type is TBD-07, MC_ACK frames also
   contain the sum of QUIC packets with associated ECN marks received on
   the connection up to this point.

   (TODO: Would there be value in reusing the multiple packet number
   space version of ACK_MP from Section 12.2 of
   [I-D.draft-ietf-quic-multipath], defining channel id as the packet
   number space?  at 2022-05 they're identical except the Channel ID and
   types.)

   MC_ACK frames are formatted as shown in Figure 9.

   MC_ACK Frame {
     Type (i) = TBD-06..TBD-07 (experiments use 0xff3e806, 0xff3e807),
     ID Length (8),
     Channel ID (8..160),
     Largest Acknowledged (i),
     ACK Delay (i),
     ACK Range Count (i),
     First ACK Range (i),
     ACK Range (..) ...,
     [ECN Counts (..)],
   }

                       Figure 9: MC_ACK Frame Format

10.7.  MC_LIMITS

   MC_LIMITS frames are formatted as shown in Figure 10.

   MC_LIMITS Frame {
     Type (i) = TBD-09 (experiments use 0xff3e809),
     Client Limits Sequence Number (i),
     Capabilities Flags(i),
     Max Aggregate Rate (i),
     Max Channel IDs (i),
     Max Joined Count (i),
   }

                     Figure 10: MC_LIMITS Frame Format






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   The sequence number is implicitly 0 before the first MC_LIMITS frame
   from the client, and increases by 1 each new frame that's sent.
   Newer frames override older ones.

   Capabilities Flags is a bit field structured as follows:

   *  0x1 is set if IPv4 channels are permitted

   *  0x2 is set if IPv6 channels are permitted

   For example, a Capabilities Flags value of 3 (0x11) indicates that
   both IPv4 and IPv6 channels are permitted.

   Max Aggregate Rate allowed across all joined channels is in Kibps.

   Max Channel IDs is the count of channel IDs that can be announced to
   this client and have keys.  Retired Channel IDs don't count against
   this value.

   Max Joined Count is the count of channels that are allowed to be
   joined concurrently.

10.8.  MC_RETIRE

   MC_RETIRE frames are formatted as shown in Figure 11.

   MC_RETIRE Frame {
     Type (i) = TBD-0a (experiments use 0xff3e80a),
     ID Length (8),
     Channel ID (8..160),
     After Packet Number (i)
   }

                     Figure 11: MC_RETIRE Frame Format

   Retires a channel by id, discarding any state associated with it.
   (Author comment: We can't use RETIRE_CONNECTION_ID because we don't
   have a coherent sequence number.)  If After Packet Number is nonzero
   and the channel is joined and has received any data, the channel will
   be retired after receiving that packet or a higher valued number,
   otherwise it will be retired immediately.

   After retiring a channel, the client MUST send a new MC_STATE frame
   with reason Retired to the server.







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   If the client is still joined in the channel that is being retired,
   it MUST also leave it.  If a channel is left this way, it does not
   need to send an additional MC_STATE frame with reason Left, as reason
   Retired also implies the channel was left.

10.9.  MC_STATE

   MC_STATE frames are sent from client to server to report changes in
   the client's channel state.  Each time the channel state changes, the
   Client Channel State Sequence number is increased by one.  It is a
   state change to the channel if the server requests that a client join
   a channel and the client declines the join, even though no join
   occurs on the network.

   MC_STATE frames are formatted as shown in Figure 12.

   MC_STATE Frame {
     Type (i) = TBD-0b (experiments use 0xff3e80b),
     Client Channel State Sequence Number (i),
     ID Length (8),
     Channel ID (8..160),
     State (i),
     Reason (0..i)
   }

                      Figure 12: MC_STATE Frame Format

   State has these defined values:

   *  0x1: Left

   *  0x2: Declined Join

   *  0x3: Joined

   *  0x4: Retired

   If State is Joined or Retired, the Reason field is absent.

   If State is Left or Declined Join, the Reason field is set to one of:

   *  0x0: Unspecified Other

   *  0x1: Left as requested by server

   *  0x2: Administrative Block

   *  0x3: Protocol Error



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   *  0x4: Property Violation

   *  0x5: Unsynchronized Properties

   *  0x6: ID Collision

   *  0x10: Held Down

   *  0x12: Max Rate Exceeded

   *  0x13: High Loss

   *  0x14: Excessive Spurious Traffic

   *  0x15: Max Streams Exceeded

   *  0x1000000-0x3fffffff: Application-defined Reason

   A client might receive multicast packets that it can not associate
   with any channel ID, or that cannot be verified as matching hashes
   from MC_INTEGRITY frames, or cannot be decrypted.  This traffic is
   presumed either to have been corrupted in transit or to have been
   sent by someone other than the legitimate sender of traffic for the
   channel, possibly by an attacker or a misconfigured sender.  If these
   packets are addressed to an (S,G) that is used for reception in one
   or more known channels, the client MAY leave these channels with
   reason "Excessive Spurious traffic".

11.  Frames Carried in Channel Packets

   Multicast channels will contain normal QUIC 1-RTT data packets (see
   Section 17.3.1 of [RFC9000]) except using the Channel ID instead of a
   Connection ID.  The packets are protected with the keys derived from
   the secrets in MC_KEY frames for the corresponding channel.

   Data packet hashes will also be sent in MC_INTEGRITY frames, as keys
   cannot be trusted for integrity due to giving them to too many
   receivers, as described in [I-D.draft-krose-multicast-security].

   The 1-RTT packets in multicast channels will have a restricted set of
   frames.  Since the channel is strictly 1-way server to client, the
   general principle is that broadcastable shared server->client data
   frames can be sent, but frames that make sense only for
   individualized connections or that are sent client-to-server cannot.

   Permitted:

   *  PADDING Frames ([RFC9000] Section 19.1)



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   *  PING Frames ([RFC9000] Section 19.2)

   *  RESET_STREAM Frames ([RFC9000] Section 19.4)

   *  STREAM Frames ([RFC9000] Section 19.8)

   *  DATAGRAM Frames (both types) ([RFC9221] Section 4)

   *  MC_KEY

   *  MC_LEAVE (however, join must come over unicast?)

   *  MC_INTEGRITY (not for this channel, only for another)

   *  MC_RETIRE

   Not permitted:

   *  19.3.  ACK Frames

   *  19.6.  CRYPTO Frames (crypto handshake does not happen on mc
      channels)

   *  19.7.  NEW_TOKEN Frames

   *  Flow control is different:

      -  19.5.  STOP_SENDING Frames

      -  19.9.  MAX_DATA Frames (flow control for mc channels is by
         rate)

      -  19.10.  MAX_STREAM_DATA Frames

      -  19.11.  MAX_STREAMS Frames

      -  19.12.  DATA_BLOCKED Frames

      -  19.13.  STREAM_DATA_BLOCKED Frames

      -  19.14.  STREAMS_BLOCKED Frames

   *  Channel ID Migration can't use the "prior to" concept within a
      channel, not 0-starting

      -  19.15.  NEW_CONNECTION_ID Frames

      -  19.16.  RETIRE_CONNECTION_ID Frames



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   *  Channels don't have the same kind of path validation, as there's a
      unicast anchor with acks for the multicast packets:

      -  19.17.  PATH_CHALLENGE Frames

      -  19.18.  PATH_RESPONSE Frames

   *  19.19.  CONNECTION_CLOSE Frames

   *  19.20.  HANDSHAKE_DONE Frames

   *  MC_ANNOUNCE

   *  MC_LIMITS

   *  MC_STATE

   *  MC_ACK

12.  Implementation and Operational Considerations

12.1.  Use Cases

   This section outlines considerations for some known transport
   mechanisms that are worth highlighting as potentially useful with
   multicast QUIC.

12.1.1.  HTTP/3 Server Push

   HTTP/3 Server Push is defined in Section 4.6 of [RFC9114].

   Server push is a good use case for multicast transport because the
   same data can be pushed to many different receivers on a multicast
   channel.  Applications designed to work well with server push can
   leverage multicast QUIC very effectively with only a few extra
   considerations.

   A QUIC connection using HTTP/3 can use multicast channels to deliver
   server-initiated streams that implement HTTP/3 Server Push.

   Applications expecting to use server push with multicast SHOULD use a
   high MAX_PUSH_ID in order to work with channels that have been active
   for a long time already when the connection is first established.
   Servers SHOULD NOT allow clients to remain joined to channels if
   their MAX_PUSH_ID will be exceeded by push streams that are to be
   sent imminently.





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   If a client receives data from a push ID that exceeds its MAX_PUSH_ID
   causing an H3_ID_ERROR on a multicast channel, it SHOULD leave the
   channel with reason 0x1000108 (computed by adding the H3_ID_ERROR
   value 0x0108 to the Application-defined Reason start value
   0x1000000).  This SHOULD NOT cause a close of the whole connection
   but MAY cause a stream error and reset of the stream.

   TODO: flesh out this principle for application-level error code
   assignment in general for known error code values, and specifically
   all HTTP/3 ones?  (And is it useful?)

12.1.2.  HTTP/3 WebTransport

   WebTransport over HTTP/3 is defined in
   [I-D.draft-ietf-webtrans-http3].

   Popular data that can be sent with server-initiated streams or
   server-sent datagrams and carried over WebTransport are good use
   cases for multicast transport because the same server-to-client data
   can be pushed to many different receivers on a multicast channel.

   A QUIC connection using HTTP/3 and WebTransport can use multicast
   channels to deliver WebTransport server-initiated streams.  At the
   time of this writing (version -02 of [I-D.draft-ietf-webtrans-http3])
   this comes with the significant penalty that in order to do so,
   servers would have to run up to 4 multicast channels per shared set
   of data to send, one for each possible size of the client-chosen
   Session ID.

   Servers can achieve this by sending the initial few bytes of the
   server-initiated stream containing the Session ID (currently defined
   as the Stream ID of the QUIC stream containing the original HTTP/3
   request for the WebTransport extended CONNECT request), then sending
   the rest of the stream data over a multicast channel.

   However, since the client-initiated Stream ID used for the Session ID
   is a variable-length integer with 4 possible sizes (1, 2, 4, or 8
   octets), clients will need the shared data in the stream to be at one
   of 4 different possible stream offsets in order to process it.
   Hence, for WebTransport as currently specified servers would need to
   run up to 4 separate channels instead of a single channel in order to
   send the same WebTransport data to many different clients.

   WebTransport Datagrams are delivered over HTTP/3 Datagrams as defined
   in [I-D.draft-ietf-masque-h3-datagram] and in version -04 have the
   same characteristic of relying on the client-chosen Quarter Stream ID
   value.




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   While using 4 multicast channels instead of 1 still represents a
   potentially vast scalability improvement over unicast delivery for
   popular content, it causes other scalability problems, especially in
   networks that have small limits on the number of multicast channels
   that are allowed to be provisioned at the same time.  The total
   channel count limits have a surprisingly low bound in many multicast-
   capable networking devices designed for TV services that were
   expected to support only up to a few tens of very popular channels at
   the same time.

   It is therefore hoped that an extension or revision to WebTransport
   and HTTP/3 Datagrams can be adopted in a future version of their
   specifications that make it possible to use a single channel for all
   the shared data.

   For example, an extension that permits a server-chosen value to be
   used as a Session ID.  Such a value could for instance be sent in an
   HTTP/3 response header, and as long as it is unique within the
   connection and avoided collision with any client-initiated stream ID
   values, could still be used to multiplex data associated with
   different HTTP/3 traffic and different WebTransport sessions carried
   on the same connection.  Then by choosing the same server-chosen
   session ID for all the connections, it would allow the server to use
   the same channel to carry the identical data, including the Session
   ID, to be received by multiple receivers.  Such a change could either
   replace the current client-chosen definition for Session ID, or could
   add new frame types that allow a server-chosen Session ID when the
   client has advertised support for this extended functionality.

   As an alternate example of an extension, a mechanism that allows
   padding at the beginning of the WebTransport stream before the
   Session ID, for example with 0 bytes, would at least allow the server
   to align the client-chosen ID value to end at the same QUIC stream
   offset, which would allow the shared portion of the stream data (all
   or most of the data following the Session ID) to be transmitted via
   the same multicast channel, since it would then be the same at the
   same stream offset.

12.1.3.  Non-web Applications

   There are also several non-web application protocols that could
   benefit from using multicast QUIC.  A few examples include:

   *  Existing multicast-capable applications that are modified to use
      QUIC datagrams instead of UDP payloads can potentially get
      improved encryption and congestion feedback, while keeping its
      existing FEC/error recovery techniques.




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      -  An external tunnel could supply this kind of encapsulation
         without modification to the sender or receiver for some
         applications, while retaining the benefits of multicast
         scalability

      -  This could usefully support existing implementations of file-
         transfer protocols like FLUTE [RFC6726] or FCAST [RFC6968] to
         enable file downloads such as operating system updates or
         popular game downloads with encryption and packet-level
         authentication.

   *  Conferencing systems, especially within an enterprise that can
      deploy multicast network support, often can save significantly on
      server costs by using multicast

   *  The traditional multicast use case of broadcasting of live sports
      with a set-top box would benefit from a system that uses the same
      QUIC receiver code even for customers who installed a non-
      multicast-capable home router.

   *  Smart TVs or other video playing in-home devices could
      interoperate with a standard sender using multicast QUIC, rather
      than requiring proprietary integrations with TV operators.

12.2.  Graceful Degradation

   Clients with multicast QUIC support can stop accepting multicast for
   a variety of reasons.

   Applications like live broadcast-scale video that rely on multicast
   QUIC may benefit from anticipating that clients might stop using
   multicast and providing data feeds with similar content that can
   scale even if many clients stop using multicast, for example by
   ensuring that a lower-bitrate rendition can still be delivered over
   unicast to all or most of the clients simultaneously, and ensuring
   that the server has a way to make the client start using the low-
   bitrate version when it switches to unicast.














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   While some existing Adaptive Bitrate video players might have an easy
   way to provide this, other video players might need specialized logic
   to provide the server a way to control what bitrate individual
   clients consume.  Although under ideal conditions it may often be
   possible using features like server push (Section 12.1.1) to use
   unmodified existing HTTP-based video players with multicast QUIC, in
   practice it may require extra devlopment at the application level to
   make a player that robustly delivers a good user experience under
   variable network conditions, depending on the scalability gains that
   multicast transport is providing and the Adaptive Bitrate algorithms
   the player is using.

12.2.1.  Circuit Breakers

   Operators of multicast QUIC services should consider that some
   networks may implement circuit breakers such as the one described in
   [I-D.draft-ietf-mboned-cbacc], or similar network-level safety
   features that might cut off previously operational multicast
   transport under certain conditions.

   The servers will notice the transport loss from the lack of MC_ACK
   frames from receivers in a network that cut off multicast transport,
   but it may be beneficial when possible in a transport cutoff event
   correlated across many clients to pace the recovery response
   according to aggregations of the affected clients so that a sudden
   unicast storm doesn't overload the network further.

12.3.  Server Scalability

   Use of QUIC multicast channels can provide large scalability gains,
   but there still will be significant scaling requiremnts on server
   operators to support a large client footprint.

   Servers, possibly many of them, still will be required to maintain
   unicast connections with all the clients and provide for handling
   MC_ACK frames from the clients, delivering MC_INTEGRITY frames,
   managing the clients' channel join states, and providing recovery for
   lost packets.

   Further, the use of multicast channels likely requires increased
   coordination between the different servers, relative to services that
   operate completely independently.

   For large deployments, server implementations will often need to
   operate on separate devices from the ones generating the multicast
   channel packets, and will need to be designed accordingly.





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12.4.  Address Collisions

   Multicast channels at the network layer are addressed with a source
   IP, a destination group IP address, and a destination UDP port.

   These offers a number of potential address collision considerations
   that are worth mentioning:

   1.  If properties change for the data being used in a channel (for
       example, new video encoding settings might result in a change to
       the expected max rate for a video feed), a server might reuse the
       same network addresses in a new QUIC multicast channel, and might
       send a join for the new channel and a leave for the old channel
       to clients that can support the new max rate.  If they arrive
       together, this could be handled by the client without making a
       change to the IGMP or MLD membership state, as an optimization
       that can prevent the need for some recovery, or even by reusing
       the same UDP socket.  Doing so does not change any requirements
       for the channel state management at the QUIC layer, and as long
       as the situation is transient, should not result in leaving due
       to Excessive Spurious Traffic even if some packets were reordered
       or may still be in flight.

   2.  As described in Section 6 of [RFC4607], link-layer addresses can
       be linked to the low-order bits of multicast addresses, and may
       be the same for different group destinations.  Collisions in the
       link-layer addressing, even with traffic that comes from other
       sources, can cause congestion or receiver CPU load for colliding
       channels that might be different from that seen with other
       channels that were delivered with apparently the same network
       paths.

   3.  Even though multicast QUIC uses only source-specific multicast,
       older networks with devices that don't have IGMPv3 or MLDv2
       support can propagate the joins as any-source multicast.  If
       there are active senders sending to that destination, this can
       cause network congestion and CPU load due to discarding packets
       from the wrong source, even though at the application layer the
       UDP socket won't receive those packets from the wrong source.

   4.  If different channels use the same (S,G) but different UDP ports,
       they will share the same multicast forwarding tree in an IP
       network.  This is often useful when the data in the channels are
       linked, for example if MC_INTEGRITY frames are carried on one
       channel for packets carried on another channel, because it
       provides some fate-sharing for the linked data.  However, for
       data that is not so linked, it would generally be a disadvantage
       to share the (S,G) because the network link of any receiver



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       joined to one of those channels but not the other would receive
       both packets and throw away the data for the un-joined port,
       causing extra congestion and CPU load for the receiving device.

13.  Security Considerations

   (Authors comment: Mostly incorporate
   [I-D.draft-krose-multicast-security].  Anything else?

   e.g. if a different legitimate quic connection says someone else's
   quic multicast stream is theirs, that's maybe a problem worth
   protecting against.  Maybe we need a periodic asymmetric challenge?
   I'm thinking send a public key on the multicast channel and in the
   unicast channels send an individualized MAC signed with the private
   key and verify it with the public key, so that in addition to
   validating that the unicast server knows the contents of the
   multicast packets via the hashes it supplies, the multicast stream
   provides a way for the client to validate that the unicast stream is
   authorized to use it for data transport via proof they know the
   private key corresponding to the public key that arrived on the
   multicast channel.  Note this doesn't prevent unauthorized receipt of
   multicast data packts, but does prevent a quic server from lying when
   claiming a multicast data channel belongs to it, preventing legit
   receivers from consuming it.

   alternatively, can the multicast channel just periodically say what
   domain name is expected for the quic connection and get the same
   crypto guarantee of a proper sender via the domain's cert, which was
   already checked on the unicast channel?)

14.  IANA Considerations

   TODO: lots

15.  References

15.1.  Normative References

   [I-D.draft-ietf-mboned-ambi]
              Holland, J. and K. Rose, "Asymmetric Manifest Based
              Integrity", Work in Progress, Internet-Draft, draft-ietf-
              mboned-ambi-03, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mboned-
              ambi-03>.

   [I-D.draft-ietf-mboned-cbacc]
              Holland, J., "Circuit Breaker Assisted Congestion
              Control", Work in Progress, Internet-Draft, draft-ietf-



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              mboned-cbacc-04, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mboned-
              cbacc-04>.

   [I-D.draft-ietf-quic-multipath]
              Liu, Y., Ma, Y., Coninck, Q. D., Bonaventure, O., Huitema,
              C., and M. Kuehlewind, "Multipath Extension for QUIC",
              Work in Progress, Internet-Draft, draft-ietf-quic-
              multipath-01, 7 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              multipath-01>.

   [I-D.draft-krose-multicast-security]
              Rose, K. and J. Holland, "Security and Privacy
              Considerations for Multicast Transports", Work in
              Progress, Internet-Draft, draft-krose-multicast-security-
              02, 31 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-krose-
              multicast-security-02>.

   [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/rfc/rfc2119>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.

   [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/rfc/rfc8174>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC9000]  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/rfc/rfc9000>.

   [RFC9001]  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/rfc/rfc9001>.






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   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

   [RFC9221]  Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable
              Datagram Extension to QUIC", RFC 9221,
              DOI 10.17487/RFC9221, March 2022,
              <https://www.rfc-editor.org/rfc/rfc9221>.

15.2.  Informative References

   [I-D.draft-ietf-masque-h3-datagram]
              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", Work in Progress, Internet-Draft,
              draft-ietf-masque-h3-datagram-11, 17 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-masque-
              h3-datagram-11>.

   [I-D.draft-ietf-webtrans-http3]
              Frindell, A., Kinnear, E., and V. Vasiliev, "WebTransport
              over HTTP/3", Work in Progress, Internet-Draft, draft-
              ietf-webtrans-http3-02, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-
              webtrans-http3-02>.

   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
              <https://www.rfc-editor.org/rfc/rfc4607>.

   [RFC6726]  Paila, T., Walsh, R., Luby, M., Roca, V., and R. Lehtonen,
              "FLUTE - File Delivery over Unidirectional Transport",
              RFC 6726, DOI 10.17487/RFC6726, November 2012,
              <https://www.rfc-editor.org/rfc/rfc6726>.

   [RFC6968]  Roca, V. and B. Adamson, "FCAST: Object Delivery for the
              Asynchronous Layered Coding (ALC) and NACK-Oriented
              Reliable Multicast (NORM) Protocols", RFC 6968,
              DOI 10.17487/RFC6968, July 2013,
              <https://www.rfc-editor.org/rfc/rfc6968>.

   [RFC9114]  Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

Acknowledgments

   TODO acknowledge.





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Authors' Addresses

   Jake Holland
   Akamai Technologies, Inc.
   Email: jakeholland.net@gmail.com


   Lucas Pardue
   Email: lucaspardue.24.7@gmail.com


   Max Franke
   TU Berlin
   Email: mfranke@inet.tu-berlin.de





































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