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Re: END_SEGMENT? (#397)

From: Daniel Sommermann <dcsommer@fb.com>
Date: Tue, 17 Jun 2014 09:23:11 -0700
Message-ID: <53A06B6F.5010102@fb.com>
To: Jeff Pinner <jpinner@twitter.com>, <K.Morgan@iaea.org>
CC: HTTP Working Group <ietf-http-wg@w3.org>, <C.Brunhuber@iaea.org>
Apologies for the (very) late response on this topic. My high level 
question here is if this is strictly needed for interop with 1.1. If 
not, I don't think it warrants the extra complexity and constraints on 
intermediary behavior.

On 04/17/2014 09:25 AM, Jeff Pinner wrote:
> On Thu, Apr 17, 2014 at 12:52 AM, <K.Morgan@iaea.org 
> <mailto:K.Morgan@iaea.org>> wrote:
>     What is the purpose of END_SEGMENT?
> END_SEGMENTS allows the layering of record-based semantics onto the 
> framing layer.
> Consider this use case in HTTP/1.1. An API provides a stream of data 
> using HTTP/1.1 chunked encoding. Each chunk contains a single record, 
> and the length of this record is indicated by the chunk length. In 
> HTTP/1.1 the length of these chunks was unconstrained, but when 
> translating into HTTP/2, these chunks must be segmented into multiple 
> data frames to fit within the frame size limit.
> If the chunk delineation was meaningful, then END_SEGMENT allows this 
> meaning to be preserved.

I'm not exactly sure how END_SEGMENT alone can be used to preserve chunk 
semantics. If you are proxying from 2 to chunked 1.1, you would need 
some other flag to know to add transfer-encoding: chunked and then other 
flags to know the chunk lengths. Roberto implied in an earlier message 
that END_SEGMENT is needed to preserve chunk boundaries.

> The flag is also useful for layering other record-based protocols on 
> top of HTTP.

Couldn't such protocols include their own delimiters? If a protocol 
running on HTTP/2 is record based, the record delimiters should be 
incorporated into the contents of the DATA frame. Otherwise, this seems 
like an unnecessary layering violation.

I'm curious who plans to use this feature and why END_SEGMENT is the 
only way to do it. While not exactly a DOS vulnerability for proxies, 
placing requirements on the framing layer limits how proxies can respond 
in low memory situations (e.g. a memory-constrained proxy might need to 
forward a partially received DATA frame as 2 smaller frames). The "MUST 
NOT coalesce" requirement in particular seems out of place. As with any 
feature, the complexity is also a potential source of bugs.

The other point brought up is that many HTTP APIs will not expose this 
segment information, potentially making it difficult to conform to the 
"intermediaries MUST NOT coalesce" requirement.
Received on Tuesday, 17 June 2014 16:23:45 UTC

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