Multicast Extension for QUIC

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.¶

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.)¶

11.1. MC_CHANNEL_ANNOUNCE

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.¶

MC_CHANNEL_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 Key Length (i),
  Header Key (..),
}

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 https://www.iana.org/assignments/aead-parameters/aead-parameters.xhtml, 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].¶

  • Author's Note: I assume it's not better to use a TLS CipherSuite because there is no KDF stage for deriving these keys (they are a strict server-to-client advertisement), so the Hash part would be unused? (https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-4)¶

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.¶

11.2. MC_CHANNEL_PROPERTIES

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.¶

MC_CHANNEL_PROPERTIES Frame {
  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),
}

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 https://www.iana.org/assignments/aead-parameters/aead-parameters.xhtml. 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.¶

  • 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:¶

    • https://www.iana.org/assignments/named-information/named-information.xhtml#hash-alg¶
    • https://www.iana.org/assignments/tls-parameters/tls-parameters.xhtml#tls-parameters-18¶
    • (text-only): 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 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.¶

11.3. MC_CHANNEL_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.¶

MC_CHANNEL_JOIN Frame {
  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)
}

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.¶

11.4. MC_CHANNEL_LEAVE

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.¶

MC_CHANNEL_LEAVE Frame {
  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)
}

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

11.5. MC_CHANNEL_INTEGRITY

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.¶

MC_CHANNEL_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 (..)
}

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.¶

11.6. MC_CHANNEL_STREAM_BOUNDARY_OFFSET

MC_CHANNEL_STREAM_BOUNDARY_OFFSET frames are formatted as shown in Figure 7.¶

MC_CHANNEL_STREAM_BOUNDARY_OFFSET Frame {
  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.)¶

11.7. MC_CHANNEL_ACK

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.¶

MC_CHANNEL_ACK Frame {
  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 (..)],
}

11.8. MC_PATH_RESPONSE

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.¶

MC_PATH_RESPONSE Frame {
  Type (i) = TBD-08 (experiments use 0xffe38008),
  ID Length (8),
  Channel ID (8..160),
  Data (64)
}

11.9. MC_CLIENT_LIMITS

MC_CLIENT_LIMITS frames are formatted as shown in Figure 10.¶

MC_CLIENT_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),
}

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.¶

11.10. MC_CHANNEL_RETIRE

MC_CHANNEL_RETIRE frames are formatted as shown in Figure 11.¶

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

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

11.11. MC_CLIENT_CHANNEL_STATE

MC_CLIENT_CHANNEL_STATE frames are formatted as shown in Figure 12.¶

MC_CLIENT_CHANNEL_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)
}

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] - https://datatracker.ietf.org/doc/html/draft-liu-multipath-quic-04#section-9.1¶

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?¶