FIN, RESET_STREAM, RESET_STREAM_AT, STOP_SENDING

draft-ietf-moq-transport.md

@@ -2175,6 +2175,70 @@ The Object Status field is only sent if the Object Payload Length is zero.
 A publisher MUST NOT send an Object on a stream if its Object ID is less than a
 previously sent Object ID within a given group in that stream.
 
+### Closing Subgroup Streams
+
+Subscribers will often need to know if they have received all objects in a
+Subgroup, particularly if they serve as a relay or cache. QUIC and Webtransport
+streams provide signals that can be used for this purpose. Closing Subgroups
+promptly frees system resources and often unlocks flow control credit to open
+more streams.
+
+If a sender has delivered all objects in a Subgroup to the QUIC stream, except
+any objects before the beginning of a subscription, it MUST close the
+stream with a FIN.
+
+If a sender closes the stream before delivering all such objects to the QUIC
+stream, it MUST use a RESET_STREAM or RESET_STREAM_AT
+{{!I-D.draft-ietf-quic-reliable-stream-reset}} frame. This includes an open
+Subgroup exceeding its Delivery Timeout, early termination of subscription due to
+an UNSUBSCRIBE message, a sender's decision to abandon the subscription, or a
+SUBSCRIBE_UPDATE moving the end of the subscription to before the current Group.
+
+A sender might send all objects in a Subgroup and the FIN on a QUIC stream,
+and then reset the stream. In this case, the receiving application would receive
+the FIN if and only if all objects were received. If the application receives
+all data on the stream and the FIN, it can ignore any RESET_STREAM it receives.
+
+If a sender will not deliver any objects from a Subgroup, it MAY send
+a STREAM_HEADER_SUBGROUP on a new stream, with no objects, and
+then send RESET_STREAM_AT with a reliable_size equal to the length of the
+stream header. This explicitly tells the receiver there is an unsent Subgroup.
+
+Since SUBSCRIBEs always end on a group boundary, an ending subscription can
+always cleanly close all its subgroups. A sender that terminates a stream
+early for any other reason (e.g., to handoff to a different sender) MUST
+use RESET_STREAM or RESET_STREAM_AT. Senders SHOULD terminate a stream on
+Group boundaries to avoid doing so.
+
+An MoQT implementation that processes a stream FIN is assured it has received
+all objects in a subgroup from the start of the subscription. If a relay, it
+can forward stream FINs to its own subscribers once those objects have been
+sent. A relay MAY treat receipt of EndOfGroup, EndOfSubgroup, GroupDoesNotExist, or
+EndOfTrack objects as a signal to close corresponding streams even if the FIN
+has not arrived, as further objects on the stream would be a protocol violation.
+
+Similarly, an EndOfGroup message indicates the total maximum Object ID in the
+Group, so if all Objects in the Group have been received, a FIN can be sent on
+any stream where the entire subgroup has been sent. This might be complex to
+implement.
+
+Processing a RESET_STREAM or RESET_STREAM_AT means that there might be other
+objects in the Subgroup beyond the last one received. A relay might immediately
+reset the corresponding downstream stream, or it might attempt to recover the
+missing Objects in an effort send all the objects in the subgroups and the FIN. It also
+might send RESET_STREAM_AT with reliable_size set to the last object it has, so
+as to reliably deliver the objects it has while signaling that other objects
+might exist.
+
+A receiver MAY send a QUIC STOP_SENDING frame for a subgroup stream if the Group
+or Subgroup is no longer of interest to it. The sender SHOULD respond with
+RESET_STREAM or RESET_STREAM_AT. If RESET_STREAM_AT, note that the receiver has
+indicated no interest in the objects, so setting a reliable_size beyond the
+stream header is of questionable utility.
+
+RESET_STREAM and STOP_SENDING on SUBSCRIBE data streams have no impact on other
+Subgroups in the Group or the subscription, although applications might cancel all
+Subgroups in a Group at once.
 
 ### Fetch Header {#fetch-header}
 
@@ -2206,7 +2270,6 @@ Each object sent on a fetch stream after the FETCH_HEADER has the following form
 
 The Object Status field is only sent if the Object Payload Length is zero.
 
-
 ## Examples
 
 Sending a track on one stream: