Re: INVALID_STREAM and STREAM_ALREADY_CLOSED

[William Chan (陈智昌) <[e-mail]>]

On Tue, Mar 26, 2013 at 1:34 PM, Martin Thomson <[e-mail]>wrote:

> On 26 March 2013 12:58, William Chan (陈智昌) <[e-mail]> wrote:
> > We used INVALID_STREAM to indicate the client received a SYN_STREAM with
> an
> > associated stream id that does not exist. Maybe we should just terminate
> the
> > session.
>
> There are five potential scenarios, all based on the highest stream
> identifier already seen:
>
> 1. Message with a stream ID that is higher than the last stream
> identifier by more than 2.
> Conclusion: No real problem here, just a few gaps in the chain that
> mean less efficient use fo .
>

No real problem, but unexpected and probably undesirable. Any reason to
allow this behavior? Should we make it a session error? Is that overly
strict?


> 2. Message with a stream ID that is lower (or equal to) the highest
> stream identifier.  That stream is still open.
> Conclusion: No problem, that's normal operation.
>
> 3. Message with a stream ID that is lower (or equal to) the highest
> stream identifier.  That stream is (half-)closed.
> Conclusion: STREAM_ALREADY_CLOSED.
>

Does the receiver need to keep the stream in its data structures after it's
closed? It's kind of nice just to throw state away ASAP. But you need that
state around to identify it as already closed.


> 4. Client generates a frame with an even numbered stream ID/server
> uses an odd stream ID. That stream hasn't been used by the peer yet.
> Conclusion: The sender of this frame is supposed to wait for its peer
> to send before using those streams.  This might warrant a separate
> error code.  Is this what INVALID_STREAM is intended to cover?
> Exception: RST_STREAM needs to be valid here to allow PUSH_PROMISE to
> work properly.
>

This is one reason for an invalid stream. What about the associated to
stream id in a PUSH_PROMISE?


>
> 5. Client generates a frame with an even numbered stream ID/server
> uses an odd stream ID. That stream ID has already been used.
> Conclusion: No problem, that's called a reply - normal operation.


Not normal if that stream has already been closed.


>
> > STREAM_ALREADY_CLOSED can be confusing since peers don't necessarily know
> > that it's already closed unless we maintain the set of old closed stream
> > ids.
>
> I never imagined that this would be the case:  If you track open
> streams, as you must, all streams that you aren't tracking are closed
> if they have an ID lower than the highest tracked stream ID.
>

I thought we had a reason why this didn't work, but I do not recall it, so
I'm going to believe you are right here.


>
> > And what if we receive DATA frames for non-existent streams? I guess we
> > shouldn't RST_STREAM a non-existent stream, since the stream id is
> invalid.
>
> Be very clear here.  I like to think of streams as having three
> states: unused, active, and closed.  (Note that you might consider
> these states to apply separately in both directions.)  What states
> correspond to 'non-existent'?  How does one convey an invalid stream
> ID?


I was thinking unused.


>
> On Tue, Mar 26, 2013 at 12:34 PM, Roberto Peon <[e-mail]> wrote:
> > I expect that we'll expand the error codes as more implementors find
> error
> > conditions that they want to be able to debug from a client debug
> > information, at which point we can re-add any of this.
>
> That is very much true..
>