Multicast Extension for QUIC
draft-jholland-quic-multicast-04
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Jake Holland , Lucas Pardue , Max Franke | ||
| Last updated | 2024-01-09 | ||
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draft-jholland-quic-multicast-04
QUIC Working Group J. Holland
Internet-Draft Akamai Technologies, Inc.
Intended status: Experimental L. Pardue
Expires: 12 July 2024
M. Franke
TU Berlin
9 January 2024
Multicast Extension for QUIC
draft-jholland-quic-multicast-04
Abstract
This document defines a multicast extension to QUIC to enable the
efficient use of multicast-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/. Subscribe at
https://www.ietf.org/mailman/listinfo/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.
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Task Force (IETF). Note that other groups may also distribute
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions and Definitions . . . . . . . . . . . . . . . 4
2. Multicast Channel . . . . . . . . . . . . . . . . . . . . . . 4
3. Transport Parameters . . . . . . . . . . . . . . . . . . . . 6
4. Extension Overview . . . . . . . . . . . . . . . . . . . . . 7
4.1. Channel Management . . . . . . . . . . . . . . . . . . . 8
4.2. Client Response . . . . . . . . . . . . . . . . . . . . . 11
4.3. Data Carried in Channels . . . . . . . . . . . . . . . . 11
4.4. Stream Processing . . . . . . . . . . . . . . . . . . . . 12
5. Flow Control . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Congestion Control . . . . . . . . . . . . . . . . . . . . . 13
7. Data Integrity . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Packet Hashes . . . . . . . . . . . . . . . . . . . . . . 14
8. Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Connection Termination . . . . . . . . . . . . . . . . . . . 15
9.1. Stateless Reset . . . . . . . . . . . . . . . . . . . . . 16
9.2. Connection Migration . . . . . . . . . . . . . . . . . . 16
10. New Frames . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1. MC_ANNOUNCE . . . . . . . . . . . . . . . . . . . . . . 16
10.2. MC_KEY . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.3. MC_JOIN . . . . . . . . . . . . . . . . . . . . . . . . 22
10.4. MC_LEAVE . . . . . . . . . . . . . . . . . . . . . . . . 23
10.5. MC_INTEGRITY . . . . . . . . . . . . . . . . . . . . . . 23
10.6. MC_ACK . . . . . . . . . . . . . . . . . . . . . . . . . 24
10.7. MC_LIMITS . . . . . . . . . . . . . . . . . . . . . . . 25
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10.8. MC_RETIRE . . . . . . . . . . . . . . . . . . . . . . . 26
10.9. MC_STATE . . . . . . . . . . . . . . . . . . . . . . . . 26
11. Frames Carried in Channel Packets . . . . . . . . . . . . . . 28
12. Implementation and Operational Considerations . . . . . . . . 30
12.1. Constraints on Stream Data . . . . . . . . . . . . . . . 30
12.2. Application Use Cases . . . . . . . . . . . . . . . . . 31
12.3. Data Transport Use Cases . . . . . . . . . . . . . . . . 32
12.3.1. HTTP/3 Server Push . . . . . . . . . . . . . . . . . 32
12.3.2. HTTP/3 WebTransport Streams . . . . . . . . . . . . 32
12.3.3. Datagrams . . . . . . . . . . . . . . . . . . . . . 33
12.4. Graceful Degradation . . . . . . . . . . . . . . . . . . 34
12.4.1. Circuit Breakers . . . . . . . . . . . . . . . . . . 34
12.5. Server Scalability . . . . . . . . . . . . . . . . . . . 35
12.6. Address Collisions . . . . . . . . . . . . . . . . . . . 35
13. Security Considerations . . . . . . . . . . . . . . . . . . . 36
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
15.1. Normative References . . . . . . . . . . . . . . . . . . 37
15.2. Informative References . . . . . . . . . . . . . . . . . 38
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39
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
acknowledges 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 reliability. Thus, this specification has several
design goals:
* Re-use as much as possible the mechanisms and packet formats of
QUIC version 1
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* Provide flow control and congestion control mechanisms that work
with multicast traffic
* 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.
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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. (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 only support source-specific multicast (SSM) and 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. To enable
clients to detect lost packets, packet numbers in channels MUST be
continuous. 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 12.4).
The server has access to all data transmitted on any multicast
channel it uses, and could optionally send this data with unicast
instead.
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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.
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 Parameters
Support for multicast extensions in a client is advertised by means
of QUIC transport parameters:
* name: multicast_server_support (TBD - experiments use 0xff3e808)
* name: multicast_client_params (TBD - experiments use 0xff3e800)
If a multicast_server_support transport parameter is not included,
clients MUST NOT send any frames defined in this document.
If a multicast_client_params transport parameter is not included,
servers MUST NOT send any frames defined in this document.
The multicast_server_support parameter is a 0-length value. Presence
indicates that multicast-capable clients MAY send frames defined in
this document, and SHOULD send MC_LIMITS (Section 10.7) frames as
appropriate when their capabilities or client-side limitations
change.
The multicast_client_params parameter has the structure shown below
in Figure 1.
multicast_client_params {
Reserved (6),
IPv6 Channels Allowed (1),
IPv4 Channels Allowed (1),
Max Aggregate Rate (i),
Max Channel IDs (i),
Hash Algorithms Supported (i),
Encryption Algorithms Supported (i),
Hash Algorithms List (16 * Hash Algorithms Supported),
Encryption Algorithms List (16 * Encryption Algorithms Supported)
}
Figure 1: multicast_client_params Format
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The Reserved, IPv6 Channels Allowed, IPv4 Channels Allowed, Max
Aggregate Rate, and Max Channel ID fields are identical to their
analogous fields in the MC_LIMITS frame (Section 10.7) and hold the
initial values.
A server MUST NOT send MC_ANNOUNCE (Section 10.1) frames with
addresses using an IP Family that is not allowed according to the
IPv4 and IPv6 Channels Allowed fields in the multicast_client_params,
unless and until a later MC_LIMITS (Section 10.7) frame adds
permission for a different address family.
The Encryption Algorithms List field is in order of preference (most
preferred occurring 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 MC_EXTENSION_ERROR.
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 MC_EXTENSION_ERROR:
* 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.
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* 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.
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. Sending an MC_ANNOUNCE before an MC_JOIN ensures the
client can establish the necessary state required to join and retire
any connection IDs that might collide with channel IDs. 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.
Note that MC_INTEGRITY frames MAY be carried in packets on multicast
channels, however such packets will not be accepted unless another
accepted MC_INTEGRITY frame contains its packet hash. Hashes of
packets containing hashes of other packets can thus form a Merkle
tree [MERKLE] with a root that is carried in the unicast connection.
See Section 7 for a more complete overview of the security issues
involved here.
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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 Section 2.1 of [RFC9000]).
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
MC_EXTENSION_ERROR.
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.
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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
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 "UNSYNCHRONIZED_PROPERTIES".
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.
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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:
* 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.)
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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.
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, as described in
Section 10.1 of [RFC9000], the client MUST terminate the
connection as described above.
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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.
9.2. Connection Migration
If the unicast connection migrated, e.g. due to a change of the NAT
binding or because the UE has changed to a different network, the
client properties might change. For example, the client might switch
from a network that supports both IPv6 and IPv4 multicast to a
network that only support IPv4. As such, it MUST immediately send an
MC_LIMITS frame after it has noticed that it migrated. The client
MAY rejoin any previously joined channels, if its limits still allow
it to. It MUST send MC_STATE(LEFT) frames with reason
LIMIT_VIOLATION for any channels it does not rejoin.
The server SHOULD take notice of migrating clients as the delay that
is being caused by rejoining a multicast group can lead to exceeding
the expected MAX_ACK_DELAY, which a server might interpret as a loss
of multicast connectivity. Instead, the server SHOULD treat all
multicast channels of a client whose unicast connection just migrated
as if it had just joined these channels initially and allow for ample
time before expecting the first MC_ACK frames.
10. New Frames
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.
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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 Protection 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).
* UDP Port: The 16-bit UDP Port of traffic for the channel.
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* Header Protection 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 Protection
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)
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)
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* 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
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.
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Secrets with even-valued Key Sequence Numbers have a Key Phase of 0
in the 1-RTT packet, and secrets with odd-valued Key Sequence 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.)
Should a client receive two different Keys with the same Key Sequence
Number and Channel ID, e.g. one over the unicast connection and one
over the multicast channel, it SHOULD close the connection with
reason MC_EXTENSION_ERROR.
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.
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* 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.
* 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 and the keys derived from them after
receiving new MC_KEY frames. Deleting old keys prevents later
compromise of a client from discovering an otherwise uncompromised
key, thus improving the chances of achieving forward secrecy for data
sent before a key rotation.
Client implementations MAY institute a delay before deleting secrets
to allow for decoding of packets for the channel that arrive shortly
after a new MC_KEY frame. For this experimental specification, it is
RECOMMENDED that clients delete old keys 10 seconds after receiving a
new key or after 3 seconds that elapse without receiving any new data
to decode with the old key, whichever is shorter. Clients MUST NOT
delay more than 60 seconds before deleting the old keys.
The delay values for this specification are somewhat arbitrary and
allow for implementation-dependent experimentation. One of the
target discoveries for experimental evaluation is to determine good
default delay values to use, and to understand whether there are use
cases that would benefit from a negotiation between server and client
to determine the delays to use dynamically. (A poor delay choice
results in either overhead from dropping packets instead of decoding
them with old keys for too short a delay or in extra forward secrecy
exposure time for too long a delay, and the purpose of the delays are
to bound the forward secrecy exposure without inducing unreasonable
overhead.)
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.
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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.
MC_JOIN Frame {
Type (i) = TBD-02 (experiments use 0xff3e802),
ID Length (8),
Channel ID (8..160),
MC_LIMITS Sequence Number (i),
MC_STATE Sequence Number (i),
MC_KEY Sequence Number (i)
}
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.
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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.
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.
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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.
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.
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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),
Reserved (6),
IPv6 Channels Allowed (1),
IPv4 Channels Allowed (1),
Max Aggregate Rate (i),
Max Channel IDs (i),
Max Joined Count (i),
}
Figure 10: MC_LIMITS Frame Format
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.
The 6 Reserved bits MUST be set to 0 by the client and MUST be
ignored by the server. These are reserved to advertise future
capabilities.
IPv6 Channels Allowed is a 1-bit field set to 1 if IPv6 channels can
be joined and 0 if IPv6 channels cannot be joined.
IPv4 Channels Allowed is a 1-bit field set to 1 if IPv4 channels can
be joined and 0 if IPv4 channels cannot be joined.
Max Aggregate Rate allowed across all joined channels is in Kibps.
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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 receiving an MC_RETIRE and retiring a channel, the client MUST
send a new MC_STATE frame with reason RETIRED to the server.
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 state LEFT, as state
RETIRED also implies the channel was left.
10.9. MC_STATE
MC_STATE frames (type=TBD-0b or TBD-0c) 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.
Frames of type TBD-0b are used for cases in which the reason for the
state change occur in the QUIC multicast layer while frames of type
TBD-0c are used for reasons that are application specific.
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MC_STATE frames are formatted as shown in Figure 12.
MC_STATE Frame {
Type (i) = TBD-0b..TBD-0c (experiments use 0xff3e80b and 0xff3e80c),
ID Length (8),
Channel ID (8..160),
Client Channel State Sequence Number (i),
State (8),
Reason Code (i),
Reason Phrase Length (i),
Reason Phrase (..)
}
Figure 12: MC_STATE Frame Format
State has these defined values:
* 0x1: LEFT
* 0x2: DECLINED_JOIN
* 0x3: JOINED
* 0x4: RETIRED
If a server receives an undefined value, it SHOULD close the
connection with reason MC_EXTENSION_ERROR.
If State is JOINED or RETIRED, the Reason Code MUST be
REQUESTED_BY_SERVER (0x1).
If State is LEFT or DECLINED_JOIN, for frames of type TBD-0b the
Reason Code field is set to one of:
* 0x0: UNSPECIFIED_OTHER
* 0x1: REQUESTED_BY_SERVER
* 0x2: ADMINISTRATIVE_BLOCK
* 0x3: PROTOCOL_ERROR
* 0x4: PROPERTY_VIOLATION
* 0x5: UNSYNCHRONIZED_PROPERTIES
* 0x6: ID_COLLISION
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* 0x10: HELD_DOWN
* 0x12: MAX_RATE_EXCEEDED
* 0x13: HIGH_LOSS
* 0x14: EXCESSIVE_SPURIOUS_TRAFFIC
* 0x15: MAX_STREAMS_EXCEEDED
* 0x16: LIMIT_VIOLATION
(Author's note TODO: consider whether that these reasons should be
added to the QUIC Transport Error Codes registry (Section 22.5 of
[RFC9000]) instead of defining a new registry specific to multicast.)
For frames of type TBD-0c, the Reason Code is left to the
application, as described in Section 20.2 of [RFC9000]
The Reason Phrase field, in combination with the Reason Phrase Length
field, can optionally be used to give further details for the state
change.
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].
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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.
Should a not permitted frame arrive on a multicast channel, the
connection MUST be closed with a connection error of type
MC_EXTENSION_ERROR.
Permitted:
* PADDING Frames (Section 19.1 of [RFC9000] )
* PING Frames (Section 19.2 of [RFC9000] )
* RESET_STREAM Frames (Section 19.4 of [RFC9000] )
* STREAM Frames (Section 19.8 of [RFC9000] )
* DATAGRAM Frames (both types) (Section 4 of [RFC9221])
* 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
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- 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
* 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. Constraints on Stream Data
Note that when a newly connected client joins a channel, the client
will only be able to receive application data carried in stream
frames delivered on that channel when they have received the stream
data starting from offset 0 of the stream.
This usually means that new streams must be started for application
data carried in channel packets whenever there might be new clients
that have joined since an earlier stream started. If the server
deems it convenient, it could also send preceding data for that
stream over the unicast connection to catch the client up.
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With broadcast video, this usually means a new stream is necessary
for every video segment or group of video frames since new clients
will join throughout the broadcast, whereas for video conferencing,
it could be possible to start a new stream whenever new clients join
the conference without needing a new stream per object.
12.2. Application Use Cases
There are several known applications that could benefit from using
multicast QUIC, either with their own custom application-layer
transport or with one of the transports discussed in Section 12.3. 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
existing error recovery techniques (e.g. techniques based on the
forward error correction (FEC) framework in [RFC6363]).
- 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
- Using QUIC datagrams in place of UDP packets 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, but adding encryption, packet-level authentication,
and congestion control as provided by QUIC.
* 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 an interoperable system such
as these QUIC extensions that can fall back to unicast
transparently as needed, for example if there are a few 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.
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12.3. Data Transport Use Cases
This section outlines considerations for some known transport
mechanisms that are worth highlighting as potentially useful with
multicast QUIC.
12.3.1. 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.
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? (Or is there a better approach?)
12.3.2. HTTP/3 WebTransport Streams
WebTransport over HTTP/3 is defined in
[I-D.draft-ietf-webtrans-http3].
Popular data that can be sent with server-initiated streams and
carried over WebTransport is a good use cases for multicast transport
because the same server-to-client data can be pushed to many
different receivers on a multicast channel.
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A QUIC connection using HTTP/3 and WebTransport can use multicast
channels to deliver WebTransport server-initiated streams.
However, because the WebTransport Session ID is a client-specific
value, the bytes that carry the WebTransport Session ID value within
the stream would need to be carried over unicast, since it's not the
same for different clients.
For this situation, note that the Session ID is a variable length
integer, and that a variable length integer can be encoded in any
size that's big enough to hold it. In particular, it's possible to
use the largest size of any Session IDs of any of the WebTransport
sessions of any clients (or 8 octets, the maximum size for a variable
length integer), and that all clients receiving stream data on a
channel will need to use the same size for the Session ID so that the
rest of the stream data will be at the same offset for every client.
12.3.3. Datagrams
DATAGRAM frames ([RFC9221]) can be carried in multicast channels, and
can be a good way to deliver popular content to receivers. Doing so
can align well with existing multicast UDP-based applications, since
a datagram API in a QUIC application offers similar functionality to
a UDP API for sending and receiving packets.
However, at the time of this writing (version -05 of
[I-D.draft-ietf-masque-h3-datagram]) multicast channels generally
cannot deliver HTTP/3 datagrams, including WebTransport datagrams
(version -02 of [I-D.draft-ietf-webtrans-http3]), since the demuxing
of WebTransport datagrams uses a Session ID based on a client-
specific value (the HTTP/3 Session ID comes from the Stream ID of the
client-initiated stream that issued the initial extended CONNECT
request).
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 server-chosen Session
ID value for demuxing WebTransport datagrams (and HTTP/3 datagrams in
general).
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 avoids
collision with any client-initiated stream ID values, it 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, the server would be able to use the same channel to
carry the identical complete datagrams, including the server-chosen
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Session ID, to multiple receivers that the server asks to join the
same channel. Such a change could either replace the current client-
chosen definition for Session ID in server-to-client datagrams, or
could add new HTTP/3 frame types that allow a server-chosen Session
ID when the client has advertised support for this extended
functionality.
12.4. 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.
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 12.3.1) to use
unmodified existing HTTP-based video players with multicast QUIC, in
practice it may require extra development 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.
12.4.1. 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.
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12.5. Server Scalability
Use of QUIC multicast channels can provide large scalability gains,
but there still will be significant scaling requirements 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.
12.6. 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
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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
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 packets, but does prevent a quic server from lying
when claiming a multicast data channel belongs to it, preventing
legit receivers from consuming it.
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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: MC_EXTENSION_ERROR error code
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-
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., De Coninck, Q., Bonaventure, O., Huitema,
C., and M. Kühlewind, "Multipath Extension for QUIC", Work
in Progress, Internet-Draft, draft-ietf-quic-multipath-06,
23 October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-quic-multipath-06>.
[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-
06, 27 December 2023,
<https://datatracker.ietf.org/doc/html/draft-krose-
multicast-security-06>.
[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>.
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[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>.
[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-http3-08, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-http3-08>.
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[MERKLE] Merkle, R., "Secrecy, Authentication, and Public Key
Systems", Computer Science Series, UMI Research Press,
ISBN: 9780835713849 , 1983.
[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>.
[RFC6363] Watson, M., Begen, A., and V. Roca, "Forward Error
Correction (FEC) Framework", RFC 6363,
DOI 10.17487/RFC6363, October 2011,
<https://www.rfc-editor.org/rfc/rfc6363>.
[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
Thanks to Louis Navarre on his comments and text contributions to the
multipath and FEC sections.
Thanks to Martin Duke, Sam Hurst, Kyle Rose, Michael Welzl and Momoka
Yamamoto for their helpful reviews and comments.
This work has been supported by the Federal Ministry of Education and
Research of Germany in the programme of “Souverän. Digital.
Vernetzt.” Joint project 6G-RIC, project identification number (PIN):
FKZ 16KISK030
TODO acknowledge.
Authors' Addresses
Jake Holland
Akamai Technologies, Inc.
Email: jakeholland.net@gmail.com
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Lucas Pardue
Email: lucaspardue.24.7@gmail.com
Max Franke
TU Berlin
Email: mfranke@inet.tu-berlin.de
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