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Re: Of HTTP/1.1 persistent connections and TCP Keepalive timers

From: James Lacey <James.Lacey@Motorola.com>
Date: Thu, 02 Nov 2000 13:13:13 -0600
Message-ID: <3A01BCC9.B7755930@Motorola.com>
To: Carl Kugler/Boulder/IBM <kugler@us.ibm.com>
CC: Fred Bohle <fbohle@neonsys.com>, James Lacey <James_Lacey-CJL023@email.mot.com>, http-wg <http-wg@hplb.hpl.hp.com>
Carl Kugler/Boulder/IBM wrote:

> I stand corrected.  But then, why was Content-Length added in HTTP/1.0 and
> Tranfer-Encoding: chunked in HTTP/1.1?

Content-Length was added so that persistent connections would be possible.

Without a content length, neither the client or the server would know
where the message ended. You have to realize that TCP is a stream and
it is up to the users of the stream to impose some kind of framing protocol
onto the stream.

Transfer-Encoding: chunked was added so that servers could return
possibly large dynamic content in chunks, as it was generated, so that
the server did not have to buffer up all of the dynamic content before
sending it back. With chunking you can also have persistent connections
w/o the need of a Content-Length header field.

One curious think about chunking that I do not understand is that
HTTP proxies are required to move the chunked (any?) encoding before
forwarding the response.

>
>
>      -Carl
>
> "Fred Bohle" <fbohle@neonsys.com> on 11/02/2000 11:27:24 AM
>
> To:   Carl Kugler/Boulder/IBM@IBMUS
> cc:   James Lacey <James.Lacey@Motorola.com>, http-wg
>       <http-wg@hplb.hpl.hp.com>
> Subject:  Re: Of HTTP/1.1 persistent connections and TCP Keepalive timers
>
> Fred Bohle@NEON
> 11/02/2000 12:27 PM
>
>      We seem to be diverging into TCP coding.  A read will return zero
> length
> when the other end has issued a normal close (and all the data has been
> read).
> A read will return -1 when the connection is ReSeT, or there is a
> connection time-out
> of any sort.  So the server can too tell the difference between end of data
> and
> a connection failure.
>
> Fred
>
> From: Carl Kugler/Boulder/IBM <kugler@us.ibm.com> on 11/02/2000 11:59 AM
>
> To:   James Lacey <James.Lacey@Motorola.com>
> cc:   http-wg <http-wg@hplb.hpl.hp.com>
>
> Subject:  Re: Of HTTP/1.1 persistent connections and TCP Keepalive timers
>
> ...
> >
> >For example, suppose the  server sends back an HTTP response to the client
> >that does not have a Content-Length: header field and that it is not
> >chunked.
> >
> >Then the only way the client knows that it has read the
> >entire response off of the pipe is when the server closes the connection.
> >When the server closes the connection the client will receive a
> >zero-byte read which is socket layer's indication that the pipe
> >is broken.
> >
> This is not good.  If this is the server's normal behavior, the client has
> no way to distinguish a dropped connection from end of file.  So the client
> can never be sure it received an entire message.
>
> If this is the server's behavior for error conditions, this is still not
> good unless the server either waits for the entire request (could be a
> humongous POST and/or a very slow connection), or only closes one half of
> the connection (which, BTW, is impossible in Java, except maybe in the
> latest releases).  Otherwise, the client might get a RST while transmitting
> the request, and will then never see the error response.
>
> ...
> >
> >I've done some snoops on browsers that are GETting HTML pages with
> >lots of embedded links that are in the same realm as the original HTML
> >page.
> >
> >Despite this obvious opportunity to take advantage of persistent
> connections
> >the browser opens  a connection for each subsequent GET.
> >
> I was looking at IE's (version 5 somthing) traffic yesterday, and it seems
> to send two requests per connection.
>
>      -Carl
Received on Thursday, 2 November 2000 19:14:22 EST

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