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Re: What is Content-Length?

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Fri, 12 Dec 97 14:16:18 PST
Message-Id: <9712122216.AA01780@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/4929
After thinking about this some more, I believe we've (almost) all
missed an obvious solution.

We're concerned about how the sender specifies the actual length
of the message body when a transfer-coding is used.

Dave Morris has been saying "redefine the encoding to include a header
(within the output of the encoding process) which gives the length."
Which I had been thinking of as a bad idea, because it means that
HTTP would be in the business of specifying the format of the
message bodies.

But then I remembered something that Dave had written earlier:
    We introduced CHUNKed transfer encoding because it is difficult for
    some servers to know the content length prior to beginning to send
(in this case, he was discussing trailers, but it's an important

And we already have the ability to specify multiple transfer codings
in the Transfer-Encoding header.

So: Instead of defining a Transfer-Length header, we could simply
add this rule:

	If a message is sent on a persistent connection using
	a transfer-coding that does not exactly preserve the
	length of the data being encoding, then the "chunked"
	transfer-coding MUST be used, and MUST be the last
	transfer-coding applied.

I.e., instead of sending

	HTTP/1.1 200 OK
	Date: Thu, 11 Dec 1997 20:33:51 GMT
	Transfer-Encoding: compress
	Transfer-Length: 12345
	... compressed data ...

a server could send

	HTTP/1.1 200 OK
	Date: Thu, 11 Dec 1997 20:33:51 GMT
	Transfer-Encoding: compress, chunked
	... compressed data ...

[Note: 12345 = 0x3039]

Since the chunked encoding by definition includes the "transfer
length", we don't need another header field.  And all HTTP/1.1
implementations are required to support ("receive and decode")
the chunked transfer-coding, so we don't need to argue about
whether this is interoperable.

We don't have to require a chunked transfer-coding when the
end-of-message is marked by end-of-connection (not that we
want to encourage it, of course) since current HTTP/1.0 practice
suggests that this basically works.

Aside from that, I don't have much to complain about Paul Leach's
"simplified" proposal, although I'm not sure it would really be
any simpler once all the definitions are made consistent.

When Paul writes:
    Under these rules, Content-Length is still logically end-to-end --
    the header may not physically be present, but its value if it is
    ever present is well-defined end-to-end and the same end-to-end.
I'm not sure it makes sense to talk about a header being "end-to-end"
if it isn't actually transmitted on some hops.  The content-length
*value* is certainly end-to-end (although this is really an
entity-length value), but I'm not sure we should be calling a
header end-to-end if it is possible to remove it.  After all,
the definition in 13.5.1 says:
       .  End-to-end headers, which must be transmitted to the ultimate
	  recipient of a request or response. End-to-end headers in
	  responses must be stored as part of a cache entry and
	  transmitted in any response formed from a cache entry.

Do you really want to change this definition?  Or admit that
Content-Length is another category?

Received on Friday, 12 December 1997 14:18:17 UTC

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