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Re: Can the response entity be transmitted before all the request entity has been read?

From: Scott Lawrence <scott@skrb.org>
Date: Sun, 14 Mar 2004 19:10:07 -0500
To: Jamie Lokier <jamie@shareable.org>
Cc: Joris Dobbelsteen <joris.dobbelsteen@mail.com>, WWW WG <ietf-http-wg@w3.org>
Message-Id: <1079309406.26913.33.camel@localhost.localdomain>

On Sun, 2004-03-14 at 11:49, Jamie Lokier wrote:
> Joris Dobbelsteen wrote:
> > Requests and responses are *always* fully completed. There is no way of
> > canceling a request/response half-way, except by closing the connection.
> 
> Thank you.  I had managed to figure that out, more recently than my
> mail on the subject :)
> 
> > Just ignore the 100 response. The protocol behaves exactly with or without
> > it.
> 
> I can't ignore it: I'm writing a HTTP/1.1 server.  I must send the 100
> response at the appropriate time, to prevent the client delay.  Also
> it's a protocol requirement: section 8.2.3.  OTOH I don't want to send
> it if I'm generating an error response.  Pretty much what 100 is for.

100 is an attempt to optimize away some of the dumb things that happen
with > 299 responses.  It was an attempt to prevent the pipe from
getting filled with stuff the server doesn't want to read.   So, yes,
you had the right idea.

The qualifiers and exceptions all derive from a desire to be backward
compatible; since there were clients and servers out there using the 1.1
version number that didn't do this, we decided to leave in exceptions
for them. 

> I delay sending the 100 response until the application (which sits on
> top of the server) asks to read any of the request entity, which makes sense.
> 
> The complications arose from the question of what to do when the
> server application doesn't read any of the request entity.
> Conservatively the HTTP server just reads it anyway, but I was trying
> to apply a close to optimal set of rules.  The obvious thing to do is
> not send 100, so that the client might not sent the entity.
> 
> But I've looked a bit harder, and RFC 2616 section 8.2.3 states:
> 
>       - Upon receiving a request which includes an Expect request-header
>         field with the "100-continue" expectation, an origin server MUST
>         either respond with 100 (Continue) status and continue to read
>         from the input stream, or respond with a final status code. The
>         origin server MUST NOT wait for the request body before sending
>         the 100 (Continue) response. If it responds with a final status
>         code, it MAY close the transport connection or it MAY continue
>         to read and discard the rest of the request.  It MUST NOT
>         perform the requested method if it returns a final status code.
> 
> That final MUST NOT indicates that if my server application generates
> a non-error response, i.e. a response which indicates it _did_ perform
> the requested method, and the server application doesn't ever ask for
> any of the request entity from the HTTP server, the HTTP server must
> _still_ send 100.
> 
> Ok.
> 
> Furthermore, the HTTP server _must_ read _all_ of the request entity.
> It cannot abort, because there is no way to know when the client
> application has read the response.  It is possible to know when the
> client's TCP stack has acknowledged all the response, but the client
> _application_ might not see that data until it has sent all the request.

Right - no matter how long you wait, the client might see your failure
to read the body as an error, so you've got to read it, regardless of
whether or not you send a success status.
Received on Sunday, 14 March 2004 19:10:11 GMT

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