Internet-Draft Multicast QUIC May 2022
Holland, et al. Expires 14 November 2022 [Page]
QUIC Working Group
Intended Status:
J. Holland
Akamai Technologies, Inc.
L. Pardue
M. Franke
TU Berlin

Multicast Extension for QUIC


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.

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 November 2022.

1. Introduction

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

The multicast data can only be consumed in conjunction with a unicast QUIC connection. When support for multicast is negotiated, the server can optionally advertise existence of one or more multicast channels that contain unidirectional data streams from server to client, and the client can optionally join the multicast channel and verify from integrity data the server provides that correct data is being received, then acknowledge the data from the multicast channel(s) over the unicast connection.

Enabling this can provide large scalability benefits for popular traffic over multicast-capable networks.

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

(S,G): A tuple of IP addresses identifying a source-specific multicast channel, as described in [RFC4607].

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. 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.

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 semantics. This however does not impose a requirement on how the underlying network stack has to handle the forwarding and delivery of multicast packets.

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.

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.

Since the network path for a channel is unidirectional, packets associated with a channel are acknowledged with MP_CHANNEL_ACK frames Section 11.7 instead of with ACK frames. Each channel has an independent packet number space.

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 graceeful degradation for endpoints that do not or cannot make use of the multicast functionality.

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 the client.

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 extesnsions 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_CHANNEL_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_CHANNEL_PROPERTIES 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_CLIENT_LIMITS frame adds permission for a different address family.

The Capabilities Field, Max Aggregate Rate, and Max Channel IDs are the same as in MC_CLIENT_LIMITS frames (Section 11.9) and provide the initial client values.

The AEAD Algorithms List field is in order of preference (most preferred occuring first) using values from the registry below. It lists the algorithms the client is willing to use to decrypt data in multicast channels, and the server MUST NOT send a MC_CHANNEL_JOIN to this client for any channels using unsupported algorithms:

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 a MC_CHANNEL_JOIN to this client for any channels using unsupported algorithms:

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_CLIENT_LIMITS Section 11.9 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.

The server asks the client to join channels with MC_CHANNEL_JOIN (Section 11.3) frames and to leave channels with MC_CHANNEL_LEAVE (Section 11.4) frames.

The server uses MC_CHANNEL_PROPERTIES (Section 11.2) frames 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 keys necessary to decode packets in the channel.

When the server has asked the client to join a channel, it also sends MC_CHANNEL_INTEGRITY frames (Section 11.5) 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 set of MC_CHANNEL_PROPERTIES (Section 11.2) frames containing the full set of data required, or for which it has not received a matching packet hash in an MC_CHANNEL_INTEGRITY (Section 11.5) frame.

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_CLIENT_LIMITS (Section 11.9) frame the server received.

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_CLIENT_CHANNEL_STATE (Section 11.11) frames. MC_CLIENT_CHANNEL_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 a unicast connection using MC_CHANNEL_ACK (Section 11.7) frames. Channels also will periodically contain PATH_CHALLENGE ([RFC9000] Section 19.17) frames, which cause clients to send MC_PATH_RESPONSE (Section 11.8) frames on the unicast connection in addition to their MC_CHANNEL_ACK frames.

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_CHANNEL_INTEGRITY (Section 11.5) 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_CHANNEL_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_CHANNEL_INTEGRITY frame.

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).

Since a server has access to all data in channels it uses, a server can always avoid stream ID collisions with the stream IDs carried in channels, and can usually (depending on the timing) avoid allowing channels to exceed the client's max_streams_uni by requesting that clients leave channels before their limits would be exceeded.

However, since clients can join later than a channel began, clients supporting the multicast extensions to QUIC should 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.

MC_CHANNEL_PROPERTIES can provide a recommended value for max_streams_uni to allow for uninterrupted transport using the multicast channel.

Servers also can send MC_CHANNEL_STREAM_BOUNDARY_OFFSET (Section 11.6) frames to indicate an application-layer boundary in a stream carried inside a channel. These frames enable new clients joining a channel to start receiving application data from the indicated stream as though the stream data at that offset had an offset of 0.

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 that the server is responsible for staying within via MC_CLIENT_LIMITS (Section 11.9) frames and their initial values from the transport parameter (Section 3). The server advertises the expected maxima of the values that can contribute toward client resource limits within a channel in MC_CHANNEL_PROPERTIES (Section 11.2) frames.

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 a MC_CHANNEL_STATE_CHANGE 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 Property 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_CHANNEL_PROPERTY, clients SHOULD leave the stream with a PROTOCOL_ERROR/Limit Violated state change.

6. Congestion Control

The server maintains a full view of the traffic received by the client via the ACK frames coupled with the MC_CHANNEL_ACK (Section 11.7) frames.

Under sustained persistent loss, the server SHOULD instruct the client such that the aggregate rate of joined channels remains under the data rate successfully received by the client in the recent past.

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).

7.1. Packet Hashes

TODO: explanation and example for how to calculate the packet hash. Note that the hash is on the unencrypted packet because it checks against a specific packet number, which is protected by AEAD. (This approach also may help make better use of crypto hardware offload.)

9. 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 somehow still detect a stateless reset for a channel, they MUST ignore it.

11. New Frames


Once a server learns that a client supports multicast through its transport parameters, it can send one or multiple MC_CHANNEL_ANNOUNCE frames (type=TBD-11..TBD-22) to share information about available channels with the client. The MC_CHANNEL_ANNOUNCE frame contains the static properties of a channel that do not change during its lifetime.

MC_CHANNEL_ANNOUNCE frames are formatted as shown in Figure 2.

  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 Key Length (i),
  Header Key (..),
Figure 2: MC_CHANNEL_ANNOUNCE Frame Format

Frames of type TBD-10 are used for IPv4 and both Source and Group address are 32 bits long. Frames of type TBD-11 are used for IPv6 and both Source and Group address are 128 bits long.

MC_CHANNEL_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.
  • IP Family: Unset indicates IPv4, Set indicates IPv6 for both Source IP and Group IP.
  • 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 IP Family.
  • 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 IP Family.
  • UDP Port: The 16-bit UDP Port of traffic for the channel.
  • Header AEAD Algorithm: A value from, 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 Key Length: Provides the length of the Key field. It MUST match a valid key length for the Header AEAD Algorithm.
  • Header Key: A key for use with the Header AEAD Algorithm for protecting the header fields of 1-RTT packets in the channel as described in [RFC9001].

A client MUST NOT use the channel ID included in the MC_CHANNEL_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 a MC_CHANNEL_JOIN frame until the channel ID associated with it has been retired by the client.

As the properties in MC_CHANNEL_ANNOUNCE frames are immutable during the lifetime of a channel, a server SHOULD NOT send a MC_CHANNEL_ANNOUNCE frame for the same channel more than once to each client.

A server SHOULD send an MC_CHANNEL_ANNOUNCE frame for a channel before sending a MC_CHANNEL_PROPERTIES or MC_CHANNEL_JOIN frame for it.


An MC_CHANNEL_PROPERTIES frame (type=TBD-01) is sent from server to client, either with the unicast connection or in an existing joined multicast channel. The MC_CHANNEL_PROPERTIES frame consists of the properties of a channel that are mutable and might change during the course of its lifetime.

A server can send an update to a prior MC_CHANNEL_PROPERTIES frame with a new sequence number increased by one.

It is RECOMMENDED that servers set an Until Packet Number and send regular updates to the MC_CHANNEL_PROPERTIES before the packet numbers in the channel exceed that value.

MC_CHANNEL_PROPERTIES frames are formatted as shown in Figure 3.

  Type (i) = TBD-01 (experiments use 0xff3e801),
  ID Length (8),
  Channel ID (8..160),
  Properties Sequence Number (i),
  From Packet Number (i),
  Until Packet Number (i),
  AEAD Algorithm (16),
  Key Length (i),
  Key (..),
  Integrity Hash Algorithm (16),
  Max Rate (i),
  Max Idle Time (i),
  Max Streams (i),
  ACK Bundle Size (i),
Figure 3: MC_CHANNEL_PROPERTIES Frame Format

MC_CHANNEL_PROPERTIES 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.
  • Properties Sequence Number: Increases by 1 each time the properties for the channel are changed by the server. The client tracks the sequence number of the MC_CHANNEL_PROPERTIES frame that set its current value, and only updates the value and the packet number range on which it's applicable if the Properties Sequence Number is higher.
  • From Packet Number, Until Packet Number: The values in this MC_CHANNEL_PROPERTIES frame apply only to packets starting at From Packet Number and continuing for all packets up to and including Until Packet Number. If Until Packet Number is omitted it indicates the current property values for this channel have no expiration at (equivalent to the maximum value for packet numbers, or 2^62-1). If a packet number is received outside of any prior (From,Until) range, it has no applicable channel properties and MUST be dropped.
  • AEAD Algorithm: A value from The value MUST match a value provided in the "AEAD Algorithms List" of the transport parameter (see Section 3).
  • Key Length: Provides the length of the Key field. It MUST match a valid key length for the AEAD Algorithm.
  • Key: Used to protect the packet contents of 1-RTT packets for the channel as described in [RFC9001], with length given by Key Length. 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.
  • Integrity Hash Algorithm: The hash algorithm used in integrity frames.

  • 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 Idle Time: The maximum expected idle time of the channel. If this amount of time passes in a joined channel without data received, clients SHOULD leave the channel with reason Max Idle Time Exceeded.
  • Max Streams: The maximum stream ID that might appear in the channel. If a client joined to this channel can raise its Max Streams limit up to or above this value it SHOULD do so, otherwise it SHOULD leave or decline join for the channel with Max Streams Exceeded.
  • ACK Bundle Size:nThe minimum number of ACKs a client should send in a single QUIC packet. If the max_ack_delay would force a client to send a packet that only consists of MC_CHANNEL_ACK frames, it SHOULD instead wait with sending until at least the specified number of acknowledgements have been collected. However, the Client MUST send any pending acknowledgements at least once per Max Idle Time to prevent the Server from perceiving the channel as interrupted.

From Packet Number and Until Packet Number are used to indicate the packet number (Section 17.1 of [RFC9000]) the 1-RTT packets received) over which these values are applicable.

A From Packet Number without an Until Packet Number has an unspecified termination.

If new property values appear and are different from prior values, the From Packet Number implicitly sets the Until Packet Number of the prior property value equal to one below the new From Packet Number for all the changed properties.

The properties of a channel MAY change during its lifetime. As such, a server SHOULD NOT send properties for channels except those the client has joined or will be imminently asked to join.


An MC_CHANNEL_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_CHANNEL_PROPERTIES.

A client cannot join a multicast channel without first receiving a MC_CHANNEL_ANNOUNCE and MC_CHANNEL_PROPERTIES frame which together set all the values for a channel.

If a client receives a MC_CHANNEL_JOIN for a channel for which it has not received both, it MUST respond with a MC_CLIENT_CHANNEL_STATE with State "Declined Join" and reason "Missing Properties". The server MAY send another MC_CHANNEL_JOIN after retransmitting the MC_CHANNEL_PROPERTIES and receiving an acknowledgement indicating receipt of the MC_CHANNEL_ANNOUNCE.

MC_CHANNEL_JOIN frames are formatted as shown in Figure 4.

  Type (i) = TBD-02 (experiments use 0xff3e802),
  MC_CLIENT_LIMIT Sequence Number (i),
  MC_CLIENT_CHANNEL_STATE Sequence Number (i),
  MC_CHANNEL_PROPERTIES Sequence Number (i),
  ID Length (8),
  Channel ID (8..160)
Figure 4: MC_CHANNEL_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_CHANNEL_PROPERTIES not yet received by the client or one or more missing updates to MC_CLIENT_LIMITS or MC_CLIENT_CHANNEL_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 a MC_CLIENT_CHANNEL_STATE with "Declined Join" and a reason.


An MC_CHANNEL_LEAVE frame (type=TBD-03) is sent from server to client in either the unicast connection or a channel.

MC_CHANNEL_LEAVE frames are formatted as shown in Figure 5.

  Type (i) = TBD-03 (experiments use 0xff3e803),
  MC_CLIENT_CHANNEL_STATE Sequence Number (i),
  ID Length (8),
  Channel ID (8..160),
  After Packet Number (i)
Figure 5: MC_CHANNEL_LEAVE Frame Format

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


MC_CHANNEL_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_CHANNEL_INTEGRITY frames are formatted as shown in Figure 6.

  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 6: MC_CHANNEL_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_CHANNEL_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 Channel Properties.

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


MC_CHANNEL_STREAM_BOUNDARY_OFFSET frames are formatted as shown in Figure 7.

  Type (i) = TBD-06 (experiments use 0xff3e806),
  ID Length (8),
  Channel ID (8..160),
  Stream ID (i),
  Stream Offset (i)

Clients must discard data before Stream Offset, and should start accepting stream data at Stream Offset as though it's a new stream with offset 0.

A server must ensure that data beginning at the given stream offsets could equivalently begin a new stream, and are safe for clients to start processing in order to use this. (Well-suited for boundaries of http server push objects, for example, which otherwise would need to start a new stream per object in order to be usable by late joiners.)


Client->Server on unicast connection.

(TODO: Is it possible to reuse 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 2022-04-12 they're identical.)

MC_CHANNEL_ACK frames are formatted as shown in Figure 8.

  Type (i) = TBD-07 (experiments use 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 8: MC_CHANNEL_ACK Frame Format


MC_PATH_RESPONSE frames are sent from client to server in a unicast connection in response to an PATH_CHALLENGE received in a channel. Like PATH_RESPONSE but includes a channel id.

MC_PATH_RESPONSE frames are formatted as shown in Figure 9.

  Type (i) = TBD-08 (experiments use 0xffe38008),
  ID Length (8),
  Channel ID (8..160),
  Data (64)
Figure 9: MC_PATH_RESPONSE Frame Format


MC_CLIENT_LIMITS frames are formatted as shown in Figure 10.

  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_CLIENT_LIMITS Frame Format

The sequence number is implicitly 0 before the first MC_CLIENT_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 reserved and have properties. Retired Channel IDs don't count against this value.

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


MC_CHANNEL_RETIRE frames are formatted as shown in Figure 11.

  Type (i) = TBD-0a (experiments use 0xff3e80a),
  ID Length (8),
  Channel ID (8..160)
Figure 11: MC_CHANNEL_RETIRE Frame Format

Retires a channel by id. (We can't use RETIRE_CONNECTION_ID because we don't have a coherent sequence number.)


MC_CLIENT_CHANNEL_STATE frames are formatted as shown in Figure 12.

  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_CLIENT_CHANNEL_STATE Frame Format

State has these defined values:

  • 0x1: Left
  • 0x2: Declined Join
  • 0x3: Joined

If State is Joined, 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
  • 0x4: Property Violation
  • 0x5: Unsynchronized Properties
  • 0x6: ID Collision
  • 0x10: Held Down
  • 0x11: Max Idle Time Exceeded
  • 0x12: Max Rate Exceeded
  • 0x13: High Loss
  • 0x14: Spurious Traffic
  • 0x1000000-0x3fffffff: Application-specific Reason

A client might receive multicast packets that it can not associate with any channel ID. If these are addressed to an (S,G) that is used for reception in one or more known channels, it MAY leave these channels with reason "Spurious traffic".

(TODO: Or should we try to reuse PATH_ABANDON and/or PATH_STATUS? I don't think they're sufficient, but maybe?): - [I-D.draft-ietf-quic-multipath] -

The things server needs to know for state changes could maybe be inferred from ack responses but explicit seems better, allowing for a more proactive response under strain?

12. Frames Carried in Channel Packets

MC 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 from MC_CHANNEL_PROPERTIES for the corresponding channel.

Data packet hashes will also be sent in MC_CHANNEL_INTEGRITY frames, as keys cannot be trusted for integrity due to giving them to too many receivers, as 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 cannot.


  • PADDING Frames ([RFC9000] Section 19.1)
  • 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)
  • PATH_CHALLENGE Frames ([RFC9000] Section 19.17)
  • MC_CHANNEL_LEAVE (however, join must come over unicast?)
  • MC_CHANNEL_INTEGRITY (not for this channel, only for another)

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, not 0-starting

    • 19.15. NEW_CONNECTION_ID Frames
    • 19.16. RETIRE_CONNECTION_ID Frames
  • 19.18. PATH_RESPONSE Frames
  • 19.19. CONNECTION_CLOSE Frames
  • 19.20. HANDSHAKE_DONE Frames

14. Security Considerations

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?)

16. References

16.1. Normative References

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-00, , <>.
Rose, K. and J. Holland, "Security and Privacy Considerations for Multicast Transports", Work in Progress, Internet-Draft, draft-krose-multicast-security-02, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, , <>.
Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, , <>.
Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable Datagram Extension to QUIC", RFC 9221, DOI 10.17487/RFC9221, , <>.

16.2. Informative References

Holbrook, H. and B. Cain, "Source-Specific Multicast for IP", RFC 4607, DOI 10.17487/RFC4607, , <>.


TODO acknowledge.

Authors' Addresses

Jake Holland
Akamai Technologies, Inc.
Lucas Pardue
Max Franke
TU Berlin