Re: notes on http2 draft

[William Chan (陈智昌) <[e-mail]>]

On Tue, May 21, 2013 at 1:00 PM, James M Snell <[e-mail]> wrote:

> Excellent review... a few brief responses...
>
> On Tue, May 21, 2013 at 8:01 AM, Patrick McManus <[e-mail]>
> wrote:
> >[snip]
> > 1.2
> >
> > message:A complete sequence of frames.
> >
> > I'm not sure that does a lot for me as a definition. It should at least
> say
> > something about them sharing the same stream ID.. or am I describing a
> > stream? Is a message a unidirectional stream? as I said.. it doesn't do a
> > lot as a definition. The document refers to "window update messages" and
> > "goaway messages" but I think it means frames in those cases.. it also
> talks
> > about "receiver of a message" sending WINDOW_UPDATE which makes it sound
> > like a message is any data frame (not the complete sequence of them)...
> and
> > then again we also talk about "HTTP Messages" which are something
> distinct..
> >
> > I suggest we scrub the term message from the document (and this section)
> > except when it refers to HTTP messages.
> >
>
> Agree. The term "message" used in the framing layer is confusing and
> meaningless and ought to be reserved specifically for the HTTP
> semantic layer discussion.
>
> >[snip[
> > Implementations MUST ignore unsupported and unrecognized frame types.
> >
> > I think invalid frame types should be session errors of the MUST NOT send
> > variety. I know the list has been talking about this lately and I haven't
> > had an opportunity to chime in. Be liberal in what you receive is
> overrated
> > and often fraught with security problems - we can rev the protocol with
> ALPN
> > at Internet scale in order to sanely guide extensions as needed. I know
> > design committees love open ended extensions so my view won't pervail,
> but
> > this is exactly the kind of thing that leads to interop doom.
> >
>
> There are generally three categories here depending on how you
> characterize the framing layer...
>
> 1. Malformed/Invalid Frames ... which yes, ought to be rejected with a
> connection error
> 2. Unknown protocol frames (hop-by-hop frames). Given that these may
> or may not affect the connection state, it would likely be better to
> reject these rather than silently ignore them and go with Roberto's
> option of requiring a new protocol version id when they're used.
> 3. Unknown data frames (end-to-end frames). These do not affect the
> connection state at all. These are safe for an endpoint to ignore and
> pass thru with the assumption that the a higher layer will deal with
> them.
>
> Right now, the spec draws absolutely no distinction between 2 and 3
> and just says to ignore everything you don't understand.
>
> Alternatively, if you view the framing layer as being essentially a
> black box to the higher application levels and require that any and
> all application data be transferred using DATA frames (ruling out the
> use of framing for anything other than hop-by-hop protocol level
> stuff) then I agree that MUST NOT send is significantly safer than
> MUST IGNORE.
>
> > 3.3.2
> >
> > Implementations with limited resources might not be capable of processing
> > large frame sizes. Such implementations MAY choose to place additional
> > limits on the maximum frame size. However, all implementations MUST be
> > capable of receiving and processing frames containing at least 8192
> octets
> > of data.
> >
> > I'm pretty confident we are just inventing complexity here for no good
> > reason. The tiny universe of implementations that can cope with 8KB but
> not
> > 64KB is not worth the complexity. One of the advantages of going with a
> 16
> > bit frame size is that it should be small enough for everything to handle
> > it.
> >
> > If there is reason to believe 64KB is too big for a population large
> enough
> > to care about (remembering of course that HTTP/2 is not a ticket to
> Internet
> > admission - if you're really so small that you can't read 64KB frames the
> > muxing of HTTP/2 is going to be a real challenge too; you're probably
> best
> > left using a different protocol. Just coap :)), then let's lower the max
> > frame size instead of reinventing Path MTU Discovery complexity. But I
> think
> > we can just drop this section and require everyone to deal with 64KB.
> >
>
> After experimenting with this further, I'm +1 on this for now but I
> want to see a lot more experimentation with flow control. Any
> configurable limits that we place on frame size is going to introduce
> a Path MTU problem and could force "reframing" to occur at various
> points in the message flow.
>

I'm +1 on this too. What experimentation would you like to see James? We've
been using flow control for awhile now in SPDY land.


>
> > 3.4
> >
> > A "stream" is an independent, bi-directional sequence of frames
> >
> > Due to the (expected) compression requirements the frames aren't really
> > independent of other streams. I know this mistake comes up pretty
> commonly
> > on spdy-dev from new implementers of that protocol, so it probably helps
> to
> > avoid saying independent here. (section 5.3 does it too).
> >
>
> Well... streams themselves are independent of one another, it's the
> serialization of the individual frames that is dependent on a shared
> context state. I agree that this ought to be explained better.
>
> > 3.4.1
> >
> > Rather, new streams are established by sending a frame whose stream
> > identifier field references a previously unused stream identifier.
> >
> > That's a little too loose. Streams are created by the client through
> > HEADERS+PRIORITY (4.2.2) and by the server through PUSH_PROMISE. The text
> > as-is makes it sound more free flowing than that.
> >
> > The identifier of a newly established stream MUST be numerically greater
> > than all previously established streams from that endpoint within the
> > HTTP/2.0 connection, unless the identifier has been reserved using a
> > PUSH_PROMISE (Section 3.8.5) frame.
> >
> > Likewise, PUSH_PROMISE really creates the stream (4.3.1) from the
> server.. I
> > don't see a reason for the caveat here.
> >
>
> This is up for debate, apparently.
>
> I agree with you that H, H+P and P_P ought to be the only ways to
> create new streams but others (such as Roberto) have argued that any
> time you send any frame with an unused stream identifier, it
> initializes that stream.
>

Yes, this is up for debate. I agree with you guys too.


>
> >
> > 3.4.2
> >
> > I thought one of the takeaways at Tokyo was to define a change-priority
> > frame.. Can we do that now? is the intention to use a H+P without any
> > headers at any point in the stream? If so, I think that should be called
> out
> > so that server implementation's don't freak out at seeing H+P at strange
> > points in the stream.. and I think there is some language in 4.2.2 that
> > could be interpreted as meaning certain colon headers are required to be
> in
> > every H+P
> >
>
> Truthfully, I'd rather see us define a distinct PRIORITY frame and
> remove H+P from the picture altogether.
>
> To create the stream, I would send a HEADERS frame. Then to set the
> priority for the stream, I would send a separate PRIORITY frame. This
> would resolve any possible ambiguity and confusion with regards to
> when to use HEADERS vs HEADERS+PRIORITIES vs any new CHANGE-PRIORITY
> frame.
>
> Yes, I get the argument that sending HEADERS+PRIORITY vs. HEADERS and
> a PRIORITY frame allows creation of the stream and setting of the
> priority in a single operation, but the added complexity of having
> separate HEADERS, HEADERS+PRIORITY, CHANGE-PRIORITY frames just
> doesn't seem like a worth while tradeoff to me.
>

I don't view it as much added complexity. I think it just may be confusing
to people why you might have a combined HEADERS+PRIORITY frame vs separate
ones. It wasn't a problem earlier until we renamed the frames :P


>
> >
> > 3.8.5
> >
> > The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in
> > advance of streams the sender intends to initiate.
> >
> > I think I must have missed something on list about this. Why is the
> language
> > here general (peer, sender).. why define this so that clients can
> generate
> > PUSH_PROMISE ?
> >
>
> This also appears to be up for debate. There is a theoretical use case
> for clients sending PUSH_PROMISE frames and sending multiple
> associated streams to a server.. but, it's purely theoretical. For
> now, I think using more specific language that restricts PUSH_PROMISE
> use to the server is more than appropriate.
>

+1, restrict to server.


>
> >[snip]
> > 4.2.2
> >
> > User-agents MUST support gzip compression. Regardless of the
> Accept-Encoding
> > sent by the user-agent, the server may always send content encoded with
> gzip
> > or deflate encoding. [rfc.comment.11: Still valid?]
> >
> > I support that still being valid. This is an important performance
> property
> > to rely on.
> >
>
> Actually, this bit needs to be clarified, and I believe we ought to be
> able to simplify this significantly.
>
> For instance, for all DATA frames, we can introduce a GZIP flag (0x2)
> that indicates that the data contained in that frame has been
> compressed. Doing so really eliminates the need to specify anything at
> the HTTP semantic layer and eliminates the need for the
> accept-/transfer-/content-encoding header mechanism completely.
>
> - James
>
>