Code change:
Delayed LGTM.
Section 5.1 of the draft says: For the second case above, I think it would be useful to have an explicit signal from the publisher to signal the end of the track. For the third case, the text seems to hint (via a section reference) that if the subscription abruptly terminates with an error, the publisher can send SUBSCRIBE ERROR, but that section says: The draft should clarify if this is allowed following a SUBSCRIBE OK. Also +1 to adding UNSUBSCRIBE ().
+1 we should allow sending a SUBSCRIBE ERROR after a SUBSCRIBE OK.
I was assuming that the catalog change would be the way a track end was indicated then the client would unsubscribe.
As an individual: This seems like a bit of a subtle signal and something more explicit using the Track ID would be preferable for me.
I tend to incline towards Catalog indicating the status the more i think of it. Catalog is source of truth for tracks. Having a catalog update that indicates the following Full Track Name lastobjectid field Also if the catalog is ending, since the producer is going away, then include another optional field finished or something These would be sufficient and a clear way to identify the status end to end.
I think putting the FIN group/sequence in the catalog is clean. It does prohibit relays from cleanly terminating a broadcast early or rewriting group/object sequences, but I think that's fine? However, the wrinkle is that a sender MAY choose to not transmit a group/object. While I don't think this is explicitly stated in the draft, it's a requirement given that relays can only cache data for a finite amount of time. The viewer could get stuck waiting for the FIN group/sequence as signaled in the catalog, even though it is not forthcoming as the relay did not receive the object or already evicted it from the cache.
As an individual: Having an explicit signal outside the catalog will make it easier for relays to clean up track related state. If a publisher is done with a track, they can update the catalog, but it will need to propagate to all subscribers, who in turn need to issue an unsubscribe. Only when all of them unsubscribe (which we still need) can a relay discard the entry in the forwarding table. If the publisher also sends an explicit signal closing off the track, the relay can immediately clean up the state.
We need few things. Cease the announcement This is an explicit signal from the publisher that it is no longer interested in sending media for the announced tracks. This is sent to the relay to trigger subscription cancellation downstream and clean up local relay state. Terminate the Subscription On subscribe end signal, each receiving entity will forward downstream and clean up local state. If it is the consumer, it stop media pipeline and so on. Unsubscribe For subscribers to explicitly indicate that they are no longer interested in a given track , they will send I am happy to write up a small PR as proposal if we think the general direction is fine.
What is the difference between Cancel the subscription and Unsubscribe?
Unsubscribe is an explicit action from the subscriber and Cancel is the signal from the publisher. May be we should call it ` instead ? I see cancel can be misleading
Got it. Terminate is better than Cancel.
We already have for the publisher to immediately terminate a subscription with a code/reason, so we could reuse that. However, the problem is when the termination occurs. The end of a broadcast would look like: There's a race, as the can arrive before the last objects due to retransmits or congestion control, severing the subscription early. Ideally we wait for all outstanding objects to get delivered and then terminate the subscription. In QUIC terminology, we need a in addition to our existing . The difficult part is determining which objects are still outstanding.
Do we want to distinguish a normal end of subscription (due to EOF), and an unexpected end (due to, e.g., a downstream server crashing)? We do that normally in QUIC (fin vs reset), so it might make sense to do that here.
Image the case where we have A sending to R sending to B. B subscribes to R. A sends ANNOUNCE to R. R subscribes to A. At this point media is flowing from A to R to B. A temporary looses WIFI connectivity and reconnects over 5G with new IP. A sends an ANNOUNCE to R, R subscribes to the new A and perhaps tries to send an error to original ANNOUNCE and end subscribe but A will not get that as that QUIC connection is gone. Once again data is flowing from A to R to B. I think it would be bad if the new ANNOUNCE, or an error on the QUIC connection from A to R caused a termination or error of the subscribe between B and R. I'm think it is better for B to unsubscribe if it is not receiving the media it wold expect.
Having explicit messages reflecting the actions would be my inclination as well. I thought it might be beneficial to explain the role of the above proposed messages with few flow diagrams Here is the happy path flow (it ignores Subscribe_OK and response flows to keep the flow focussed)
Summary thoughts: Want to support disconnection without propagation to all subscribers (ie: If there were two publishers) Need a way to cleanly unsubscribe from a track (UNSUBSCRIBE) - Like STOPSENDING? Need a way to indicate that a relay is no longer the place to subscribe to for a track (ie: UNANNOUNCE) But UNANNOUNCE does not indicate an unsubscription. Need a way to indicate the track is FIN'd and ending, could use the catalog or put in the protocol (or both) Likely want a way to drain a relay, such as an HTTP GOAWAY style functionality. (Could be SUBSCRIBETERMINATE) Timeouts do need to be handled, even if they're not the ideal/clean case. Want to know whether the relay wants to stop publishing or the origin did, so a client can decide whether to re-sub. NOTE: Objects can be dropped, hard to know if they'll ever arrive. Cullen will summarize a proposal. Note that the relay doesn't really need to know about tracks or what its subscribing to. An open issue is how to do FIN, whether it as part of Objects, only the catalog, a separate message, etc.
Rough notes on proposal in the call: The catalog updates and says what the last message in a track is. Catalogs are a way that endpoints can say there will never be more on a track. The annouce has an equivalent un-Annouce message. It changes routing of future subscribes but does not change state of existing subscribes. The entity that send an subscribe and send an un-subscribe The entity that receives a subscribe can send a go-away which will let the entity that sent the subscribe go do something new ( like subscribe to something else ). We might need to figure out what data goes in the go away to given info that the clients enough info to do the right thing. Timeout can happen, and connections disappearing, and we need to sort out what impact that has but that is not the primary way to cause flow of media to change. Update: Key issue here is when the relays see the go away, do they drop the related forwarding state for the subscription, or do they just forward the update to the subscribers and wait for the subscribers to remove the subscription.
NAME NAME thanks for capturing the notes. I will work on set of small PRs to capture some of the items from the summary.
The point I was trying to clarify on the last call was that catalog already has a mechanism for a publisher to communicate to all subscribers that the track it has advertised as being available is no longer available. A clean FIN if you like. We can, as has been pointed out, create a control message which says the same thing. But then we have a weird asymmetry in which a publisher notifies clients of track availability through a catalog update but then uses control messages to do the reverse. For good design with clean encapsulation, we should seek symmetry in operation. We should either use control messages to advertise the availability/unavailability of tracks , or use catalogs, but not a hybrid of both. In my understanding of moq-transport, control messages have utility that is scoped to the current hop. They negotiate the format in which data will be exchanged on that hop and issue commands to subscribe/unsubscribe/announce etc. Those commands have no applicability to any other hops on the distribution chain. A catalog however is a contract between a publishers and all subscribers. Unlike control messages, it holds data that is not interpreted hop-by-hop and is intended for targets that can be many hops away. I think relays should be as simple as possible. In my conception, they should not understand whether tracks are being published or not. Their job is to move objects. Their view of the world is around objects and they do not need to understand what tracks are. While of course we can build all sorts of track intelligence in to relays, I think if we can build a base protocol which is agnostic to tracks, then it will be scalable and robust, and cleverness with tracks can evolve in the streaming formats without affecting deployed networks of relays.
Discussed in Boston: Agree we should add a SUBSCRIBE FIN message with the "final offset"

Related discussion:
text: subscribers for each full track name.  Each new OBJECT belonging to the track is forwarded to each active subscriber, dependent on the congestion response.  A subscription remains active until it expires, <del> until the publisher of the track stops producing objects or there is a subscription error (see message-subscribe-error). </del> <ins> until the publisher of the track terminates the track with a SUBSCRIBE_FIN (see message-subscribe-fin) or a SUBSCRIBE_RST (see message-subscribe-rst). </ins> Relays MAY aggregate authorized subscriptions for a given track when multiple subscribers request the same track.  Subscription