Code change:
This matches agreement at the last interim. We should separate this into a different draft, but for now this will make the draft match implementation.
Partially addresses . cc NAME NAME
Http3Transport currently assumes that server-initiated bidirectional streams and datagrams can only be used for WebTransport. I suggest we prefix each server-initiated bidirectional stream and datagram with a new HTTP/3 frame type for WebTransport. This way, a future standard could define it's own frame type to run alongside WebTransport.
Using an H3 frame type for server initiated bidi streams would also make the design more consistent, instead of the asymmetric design there is today. It costs a byte per bidi stream, but it seems worth it for consistency and flexibility. For datagrams, I believe that we'd want to be able to coexist with other H3 extensions, such as MASQUE, and I believe that's possible without any extra overhead by adding a Session -> Datagram flow ID mapping when a WebTransport session is created.
If it's a single byte, it's more of a stream type (like the unidirectional stream-type) than a frame type, right? There's a discussion Victor pointed me to in , where it's suggested that datagrams belonging to a WebTransport session use the Flow ID = to the Session ID. It would be nice to avoid the extra state and lookup.
Ah yes, it's more of a stream type. In that case, I'm unsure of whether there's a good way of eliminating the asymmetry. The only option I can think of is to not use a stream type for server bidi streams and only use the frame, like for the client initiated bidi streams? Thanks for the pointer to , it looks like there are a few potential designs for datagrams flow IDs. If in doubt, simpler is probably better. I'll think more about this one.
I agree we should use the same mechanics for client and server bidi streams.
After thinking about this more, not having a stream type doesn't seem so bad to me and it does provide symmetry. Stream types seem like an obvious design choice, but I believe HTTP/3 is sufficiently constrained that one could infer the stream type based on the first frame on the stream. For example, the control stream MUST start with SETTINGS. In retrospect, having both stream types and frame types is a bit awkward, and it likely would have been cleaner to only have frame types, but that ship has sailed.
After having implemented this for hackathon, I can confirm that the lack of stream preface on server-bidi streams is annoying and requires special casing (I actually simulate receiving one). Let's please add one. I took a bit of a shortcut and treated the stream preface as both a stream type AND a frame type. After peeking at the unidirectional stream input to discover that it's WT, I let the parser run on that and treat it as a frame type, making it symmetric with the bidi case. Is it possible to define a reserved frame type with the same value as the stream type?
Given HTTP/3 does not have a stream type varint for bidi streams, I think a potential way forward is to say that the first frame on the bidi stream can indicate the 'type' of the stream, but that there is no explicit type identifier on the wire. Feel free to ignore the below comment, as it's apparently unchanged from January Looking back, SETTINGS is required to be the first frame on the control stream, so the control stream type is a bit redundant and the same is true for QPACK and I believe could be true for server push? Obviously we're not going to redesign HTTP/3 stream and frame types at this stage in the process, but making bidi streams consistent for both perspectives seems preferable to me.
Discussed at IETF 110. There seems to be a general agreement that we should make server-initiated bidi streams use the same encoding as client-initiated ones (that is to say, frames). There was some interest expressed in splitting definition of server-initiated bidi streams into their own draft; NAME and NAME are to reach out to httpbis about the appropriate venue for that.
Just bringing some of the discussion from here: HTTP should provide some consistent guidance for the format of HTTP/3 extension streams. Client-bidi extension streams have to start with a frame or series of frames, to be compatible for H3. One of these frames has to identify the type of stream. This is how WT is currently specified. Unidirectional streams all start with a Stream Type, but after that are inconsistent in H3 the control stream is: Stream Type + Frames QPACK Streams are: Stream Type + QPACK instructions Push streams are: Stream Type + Preface (Push ID) + Frames Stream Type seems like a more general extension mechanism, since it can be used to completely decouple from HTTP/3 frame parsing.
I find stream types to be less extensible than frames: frames allow you to do anything that stream types do, while additionally providing more opportunity for extensibility. I would prefer server-initiated bidirectional streams to behave like client-initiated bidirectional streams instead of unidirectional streams, as that makes implementations simpler.
January Me said: So I think I'm arguing against myself and I'm going to stop. My other comments on this thread we that I didn't like was the asymmetry of WT uni streams, which currently are . My past self seemed to be advocating that we change WT uni streams to or perhaps . Do we want general guidance about how extensions should use Stream Type vs Frames on unidirectional streams -- specifically that there should be at least one frame?
The place to put that guidance would have been draft-ietf-quic-http, and I think that ship has sailed. For unidirectional streams, I like your framing proposal as that enables extension frames down the road.
Well, not too late to put out a BCP or something, especially if there's going to be some kind of formal guidance on server-initiated bidi streams. I'll open a separate issue about changing the WT uni framing.
I think this specific issue was fixed by . Please reopen if I'm wrong.

Related discussion:
text: MTUs can vary.  TODO: Describe how the path MTU can be computed, specifically propagation across HTTP proxies. <del> 4.5. </del> <ins> 4.4. </ins> In WebTransport over HTTP/3, the client MAY send its SETTINGS frame, as well as multiple WebTransport CONNECT requests, WebTransport data