#467: Expect: 100-continue and "final" status codes

Mark Nottingham <mnot@mnot.net> Fri, 17 May 2013 02:49 UTC

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From: Mark Nottingham <mnot@mnot.net>
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Date: Fri, 17 May 2013 12:47:27 +1000
Cc: Ken Murchison <murch@andrew.cmu.edu>, ietf-http-wg@w3.org
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References: <CACuKZqGmrDiNQvG0SVw=XXcy_n-BBxK-pnp+ar7uAbnwkumRag@mail.gmail.com> <51780FBA.3080706@andrew.cmu.edu> <20130424170638.GD19750@1wt.eu>
To: Willy Tarreau <w@1wt.eu>
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Subject: #467: Expect: 100-continue and "final" status codes
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Looking at this again, I think one of the problems is that people misunderstand what e/c is for.

The current definition of 100-continue in requests doesn't help:

> 100-continue
> 
> 	• The request includes a payload body and the client will wait for a 100 (Continue) response after sending the request header section but before sending the payload body. The 100-continue expectation does not use any expect-params.


<https://svn.tools.ietf.org/svn/wg/httpbis/draft-ietf-httpbis/latest/p2-semantics.html#header.expect>

"will wait for" is misleading here; the client might send the body before getting the 100 response. This should really say something like:

"""
100-continue

* The request includes a payload body and, after sending the request header section, the client will wait before some period of time before sending it, to give the server an opportunity to reject the request with a final status code. The server can shorten the wait time by sending a 100 (Continue) response.
"""

It then goes on:

> The primary purpose of the 100 (Continue) status code (Section 6.2.1) is to allow a client that is sending a request message with a payload to determine if the origin server is willing to accept the request (based on the request header fields) before the client sends the payload body. In some cases, it might either be inappropriate or highly inefficient for the client to send the payload body if the server will reject the message without looking at the body.

Again, I think this is misleading. It should say something like:

"""
The 100-continue expectation and 100 (Continue) status code (Section 6.2.1) are useful when a request that has a large body might be rejected by the server; for example, if the request requires authorization (ref to p7). In these situations, clients will often pause between sending the request headers and its body, to give the server an opportunity to refuse the request. 

In cases where the request is successful, this can cause a needless delay, as the client waits to time out (a typical period is one second). If the client has send the 100-continue expectation, the server can use the 100 (Continue) status code to indicate that the request is not going to be rejected, thereby avoiding the remainder of this delay period.

Note that this mechanism does not change the request message parsing algorithm; in particular, whether or not a final response status code is sent, the client still needs to send a complete request message. As such, if a final status code is received, clients will often choose to close the connection, rather than send a complete request (e.g., if it is length-delimited).
"""

If we can agree on that, I think it'll help guide the rest of the discussion here and in the other E/C related issues:
  http://trac.tools.ietf.org/wg/httpbis/trac/ticket/458
  http://trac.tools.ietf.org/wg/httpbis/trac/ticket/468

Am I on track?

Cheers,



On 25/04/2013, at 3:06 AM, Willy Tarreau <w@1wt.eu>; wrote:

> On Wed, Apr 24, 2013 at 01:00:42PM -0400, Ken Murchison wrote:
>>> 2. If the client receives a final status code instead of 100 
>>> (Continue), it
>>> should stop sending request body if it is doing so; it must close the
>>> connection after the response is received.
>> 
>> I don't understand point #2.  If the client submits a request with 
>> Expect:100-continue, I would assume that the client MUST NOT send any 
>> part of the body until it receives 100 (Continue) from the server.  If 
>> the server rejects the request based on the headers (with 412, 415, 417, 
>> etc) there should be no body data in the pipe for either the client or 
>> server to worry about, correct?
> 
> In fact the client can decide that it's been waiting too long for 100
> and decides to send anyway (because some old servers or intermediaries
> do not know about Expect and will wait).
> 
> So what is generally done is that the client sends the headers, waits a
> bit then starts to send data if the server does not respond.
> 
> Implementation of expect+100 seems to be a real mess at some places,
> but when it works it prove to be quite useful.
> 
> Willy
> 
> 

--
Mark Nottingham   http://www.mnot.net/