Re: [quicwg/base-drafts] Add bidirectional streams on top of unidirectional (#720)

[Mike Bishop <[e-mail]>]

MikeBishop commented on this pull request.



> +This document doesn't proscribe the use of the Related Stream ID field, leaving
+decisions about the semantics of related streams to protocols that use QUIC.
+Protocols that use QUIC MUST either define the semantics of related streams or
+prohibit the use of the Related Stream ID field.  Protocols MAY define other
+uses for the Related Stream ID field, but are advised that complex relationships
+between streams could be poorly supported in generic QUIC implementations.  The
+INVALID_RELATED_STREAM error code can be used by protocols to signal receipt of
+invalid Related Stream ID field values, unless the protocol defines a more
+specific error code for the circumstances.
+
+The Related Stream ID for a stream only needs to be sent once for each (allowing
+for retransmissions due to loss).  Sending the field on all stream frames with
+an offset of 0 ensures that the value is received without duplicating the field
+unnecessarily.  If the value of the Related Stream ID changes for a stream, the
+endpoint that receives the conflicting value MUST treat this as a connection
+error of type INVALID_RELATED_STREAM.

I don't think the updated text explicitly says whether **NOT** setting the `R` bit on subsequent frames constitutes a change.  I think the simplest way to scope it is that if there's a related stream, `R` MUST be set on the zero-offset frame and MAY be set on other frames, but the value of the Related Stream ID MUST NOT change on subsequent frames if `R` is set.

If you receive frames out-of-order and `R` is set, you have the info early; if it's *not* set then you can't do anything with that yet because it might still have been set on some other frame.  You still have to wait for the zero-offset frame to conclude that it will never be set.

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