Re: Design Issue: Frame Processing Model
Martin Thomson <martin.thomson@gmail.com> Thu, 25 April 2013 23:25 UTC
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Date: Thu, 25 Apr 2013 16:23:47 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: James M Snell <jasnell@gmail.com>
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Subject: Re: Design Issue: Frame Processing Model
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Semi-formally, the Tier 2/3 division is made. The 1/2 split hasn't been. We've been talking about a "framing layer", which refers to 1+2. I originally thought that the distinction would be 1=framing, 2=streams, 3=application (http). You can go further and split the streams layer into planes: control and data. But I agree, formalism only gets you so far. Otherwise, I think that you have it. I'm not sure whether the spec needs to say anything. Maybe you could save it for the book that you are writing on HTTP/2.0. On 25 April 2013 15:28, James M Snell <jasnell@gmail.com> wrote: > At the (very real) risk of adding a bit too much formalism to the > Frame processing model, I have noticed a number of areas in the > current -02 draft where references are made to an endpoint being > required to receive and accept frames but being permitted to ignore > them if necessary, etc. There is also a concern over where exactly in > the processing model steps such as header compression state management > ought to occur, whether or not that occurs before sending RST_STREAM > and GOAWAY frames, etc. > > In thinking it over, I think it would be very beneficial in the long > term for us to define specific processing levels or tiers for Frames. > Below is a strawman example: > > Tier 1: "Session Tier" > A frame received and parsed. This is where basic validation of the > frame syntax occurs and where state management based on frame > structure (e.g. compressed headers) happens. Any processing errors > that occur here are considered to be Session Errors and will typically > be related to incorrect protocol support, malformed frames, malformed > headers, etc. At this tier, frames are examined individually and not > yet processed as being part of a stream. > > Tier 2: "Stream Tier" > The next tier is to process the frame in context of a stream. This > is where we look at things like whether the frame has a valid known > stream identifier, whether the associated stream is open, half-closed, > closed or whatever. The errors that occur here can be Session or > Stream errors. > > Tier 3: "Application Tier" > The Frame data is passed on for application-level handling. All of > the basic parsing and stream validation has occurred already. This is > where we start applying HTTP specific semantics. The errors that occur > here are typically HTTP level errors with associated HTTP status > codes. > > Given these tiers, we can then begin speaking in very concrete terms > about what kinds of processing may be required at different points in > the session lifecycle. > > For instance: > - Protocol upgrade negotiation, SETTINGS frames, GOAWAY and flow > control are all handled in Tier 1. None of that ever passes on to > higher tiers. > - When we say things like, "an endpoint MUST be continue to accept > frames after a RST_STREAM", we're really saying that Tier 1 processing > must still occur, but that frames may not have to be passed on to Tier > 2 > - When we deal with HTTP specific semantics, we assume that all of > the Tier 1 and Tier 2 processing has been dealt with > > I believe these layers already informally exist in the model we have, > even if it's not entirely obvious in the current design. > > - James >
- Design Issue: Frame Processing Model James M Snell
- Re: Design Issue: Frame Processing Model Martin Thomson
- Re: Design Issue: Frame Processing Model James M Snell