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GrumpyOldTroll / draft-jholland-quic-multicast

Work in progress to propose a multicast extension to quic.
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Add something like a MC_RESERVE_SESSIONIDS frame. #9

Closed GrumpyOldTroll closed 2 years ago

GrumpyOldTroll commented 2 years ago

From Max in issue #4 comment: https://github.com/GrumpyOldTroll/draft-jholland-quic-multicast/pull/8#discussion_r854656985

There should probably be a mechanic to force clients to not use some connection IDs for unicast connections. I think only doing it in the MC_SESSION_PROPERTIES frame is too late, it should probably be done immediately after the handshake of the unicast connection. Its basically a frame telling the client "Do not use any of these connection IDs for your unicast streams as we might have a multicast session that uses this ID. If you (by sheer bad luck) already use one of them for a unicast connection (i.e. the initially created one), issue a new connection ID and retire the old one immediately."

GrumpyOldTroll commented 2 years ago

Capturing the other notes from the thread: I guess session properties should implicitly reserve the session id, but also a MC_RESERVE_SESSIONIDS frame to reserve a list of IDs without sending their properties.

I think if there is a collision it doesn't matter until the server issues a JOIN, but if the client has not yet migrated connection ID it can refuse the join with an "ID Collision" reason and we don't really need other signaling about it I think? Actually it occurs to me the server should already know if the connection ID is in use, and can determine when it has closed to retry the join?

GrumpyOldTroll commented 2 years ago

Another option here is just an empty MC_CHANNEL_PROPERTIES, if there's space constraints and to NOT add this frame. I think if you use a 0 content field and a 0 from packet number it's only 2 bytes of overhead. We can make a note that server can do this to reserve channel id space even before a channel is active, and that clients SHOULD migrate away from colliding connection IDs and avoid creating colliding connection ids whenever they see a MC_CHANNEL_PROPERTIES, and then we don't need a new frame.