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RE: #38 - HTTP2 min value for server supported max_concurrent_streams

From: Osama Mazahir <OSAMAM@microsoft.com>
Date: Tue, 26 Feb 2013 08:34:32 +0000
To: William Chan (³ÂÖDzı) <willchan@chromium.org>
CC: Yoav Nir <ynir@checkpoint.com>, Martin Thomson <martin.thomson@gmail.com>, Roberto Peon <grmocg@gmail.com>, "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
Message-ID: <B33F11E188FEAB49A7FAF38BAB08A2C001D34CA3@TK5EX14MBXW601.wingroup.windeploy.ntdev.microsoft.com>
Internet Explorer has similar gymnastics.  However, I don¡¯t think that is just cause to reinvent the same problems again.

In general, the problem we have is that one side initiates operations without knowing the peer¡¯s limits.  MaxConcurrentStreams is one example and negative flow control bytecounts is another (i.e. where the receiver is trying to advertise that it has small buffers but we shove data down its throat and dictate that it ¡°MUST be prepared to receive the entire amount¡± [1]).

Possible solutions include:

1.       Handshake Advertise: Advertise limits as part of handshake/negotiation.  That way, upon session start each side knows the other¡¯s limit and can guarantee that it won¡¯t violate it.  That way, we can simplify all parts of the protocol that are dealing with limit-exceed cases.

2.       Defaults and minimums: In the spec we pick some defaults and minimums so that each endpoint starts at a known initial state and each endpoint can thus guarantee that it won¡¯t violate the peer¡¯s limits.  The initial SETTINGS frame can grow those limits.  Thus, we can simplify/delete all the limit-exceed handling.

3.       Don¡¯t fix: Not really a ¡°solution¡±.  We write pages of protocol text describing the races, how to workaround limit-exceed cases, code and test it, and put that burden on all future implementers.

In short, I ask the WG to not summarily dismiss this issue.  We should devote some energy to ensure that the protocol is robust by-design.

[1] http://http2.github.com/http2-spec/#rfc.section.3.7.9.3



From: willchan@google.com [mailto:willchan@google.com] On Behalf Of William Chan (???)
Sent: Friday, February 22, 2013 3:57 PM
To: Osama Mazahir
Cc: Yoav Nir; Martin Thomson; Roberto Peon; ietf-http-wg@w3.org Group
Subject: Re: #38 - HTTP2 min value for server supported max_concurrent_streams

On Fri, Feb 22, 2013 at 3:45 PM, Osama Mazahir <OSAMAM@microsoft.com<mailto:OSAMAM@microsoft.com>> wrote:

As Martin said, 1 seems overly restrictive.

My major concern is not the value of the number, but that we have a minimum value and the default be the same as the minimum.  Otherwise, we leave the race hole open then we are just increasing complexity.

Do you feel like the complexity is that bad? In my experience, from implementing SPDY, it is not.


1.       Client will have to track negative allowance (because it did not know how many requests it allowed to send)

Isn't this easy? The client always has to track how many outstanding streams it has in order to respect the limit.


2.       Server has to promise that RST_STREAM due to max_concurrent_stream overflow did not have any side effects

o   The server should verb agnostic (i.e. GET vs POST) and just look at some streamCount variable.

o   Otherwise, client will have to pend all non-idempotent requests until it gets the SETTINGS frame from the server

Since RST_STREAM has an error code, this is easy to define.


3.       Client will have to resubmit the request into its queue to be sent when the allowance opens up
Clients already have to know how to do this due to the GOAWAY race. They also have to handle this in HTTP/1.X today. For example, if we get an error when reusing a persistent HTTP connection (e.g. TCP RST), we will resend the HTTP request over a new connection.

4.       If the ¡°blind¡± request(s) (i.e. sent before client received the SETTINGS frame) have entity-body then client

o   Must wait until the server¡¯s SETTINGS frame before sending entity-body OR

o   Be able to regenerate the entity-body when the ¡°blind¡± request is RST_STREAMed

*  This means the layer on top of client stack needs to be able to handle a ¡°retry¡± error and resubmit the entity-body OR

*  The client stack buffers all the entity-body, as it converts it into DATA frames, until it knows that the request won¡¯t get RST_STREAM due to max_concurrent_stream

o   Or just blow up and complain to the user

Again, clients already have to handle this.


In general, I would prefer if we made HTTP/2.0 to not have such races to begin with instead of piling on complexity to react to the races.

As someone with experience implementing a SPDY client, I do not believe this is a big burden. If you believe it is, I would like to hear why.



From: willchan@google.com<mailto:willchan@google.com> [mailto:willchan@google.com<mailto:willchan@google.com>] On Behalf Of William Chan (???)
Sent: Friday, February 22, 2013 2:36 PM
To: Yoav Nir
Cc: Martin Thomson; Roberto Peon; Osama Mazahir; ietf-http-wg@w3.org<mailto:ietf-http-wg@w3.org> Group
Subject: Re: #38 - HTTP2 min value for server supported max_concurrent_streams

We always have to examine what the choices end up being for which parties. If servers end up limiting parallelism, or requiring roundtrips to ramp up parallelism, then clients which want speed (browsers) will be incentivized to simply open up more connections to bypass the low parallelism limit or slow start.

Overall, I think it's better to tolerate the minor suboptimality of having servers RST_STREAM streams if they don't want so much parallelism, rather than incentivize browsers to open more connections.



On Fri, Feb 22, 2013 at 2:19 PM, Yoav Nir <ynir@checkpoint.com<mailto:ynir@checkpoint.com>> wrote:

On Feb 22, 2013, at 6:16 PM, Martin Thomson <martin.thomson@gmail.com<mailto:martin.thomson@gmail.com>> wrote:

> On 22 February 2013 05:18, Roberto Peon <grmocg@gmail.com<mailto:grmocg@gmail.com>> wrote:
>> Why 1?
>
> 1 seems a little restrictive, especially since 6 concurrent
> connections is the current expectation in many browsers.
Defaulting to 1 allows for a simple server that never has to handle multiple concurrent streams, one that can be implemented with much fewer lines of code, but is still compliant. Great for serving software updates, large files, CRLs, etc. Not so great for web pages.

Other servers will quickly raise the limit via a SETTINGS frame.

Yoav


Received on Tuesday, 26 February 2013 08:35:18 GMT

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