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Re: What is Content-Length?

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 11 Dec 97 17:39:26 PST
Message-Id: <9712120139.AA29199@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
I agree with John Franks and Roy Fielding.  There seems to be no way to
make Transfer-Encoding work, in the general case, without some
resolution to this problem.  We went through a discussion of *why*
Transfer-Encoding is necessary a few weeks ago, and if we are going to
go ahead with the TE header (to make non-chunked transfer-codings
feasible), then we really can't avoid solving the length problem as
well.

Here's a sketch of a specific proposal:

   (1) The definition in 7.2.2:
	The length of an entity-body is the length of the message-body after
	any transfer codings have been removed.
    should be retained.  At first I thought "maybe we should rename
    this the 'entity-length'", but we already use that non-terminal (in
    Content-Range) to mean something different.  Which maybe should be
    changed to use "instance-length", since the "entity-length" used
    with a Content-Range has nothing to do with the length of the
    entity-body.  But I digress.
   
   (2) We add this definition (somewhere)
	The transfer-length of a message is the length of the message-body
	as it appears in the message; that is, after any transfer codings
	have been applied.

   (3) We add a new message-header

	Transfer-Length = "Transfer-Length" ":" transfer-length

   (4) We add a requirement that a non-identity transfer-coding MUST
   either be self-delimiting or be accompanied by a Transfer-Length header.
   
   (5) We generalize this statement in section 4.4 (Message Length)
       Messages MUST NOT include both a Content-Length header field and
       the "chunked" transfer coding. If both are received, the
       Content-Length MUST be ignored.
   so that it reads
       Messages MUST NOT include both a Content-Length header field and
       a non-identity transfer coding. If both are received, the
       Content-Length MUST be ignored.

   (6) In 13.5.2 (Non-modifiable Headers), where it currently says
	A cache or non-caching proxy MUST NOT modify any of the following
	fields in a response:

         .  Expires
         .  Content-Length

       but it may add any of these fields if not already present.
   if one interprets "modify" to include "delete", then this would seem
   to prevent a proxy from adding *any* transfer-coding that changes
   the message length(!), including chunked!  And so this looks like a
   potential ambiguity.  I think we need to add something like:

       The Content-Length header MAY be deleted, but if so, the proxy
       MUST provide some other means for the recipient to discover the
       length of the entity (i.e., a self-delimiting transfer-coding
       such as chunked, a Transfer-length header, or non-persistent
       connection with no transfer-coding.)  A Content-Length header
       MUST be added if the proxy provides no other means for the
       recipient for discovering the length of the entity.

   In other words, I guess Content-Length really is a hop-by-hop
   header, but there are some rules to follow about if and how a proxy
   regenerates it for the next hop.

This all means that, in section 4.4, the statement:
     3. If a Content-Length header field (section 14.14) is present, its
        decimal value in OCTETs represents the length of the message-body.
is still essentially correct.  That is, if you see a Content-Length,
then the length of the entity-body is the same as the length of the
message-body.  Since (if there is no Transfer-Encoding) we expect
the recipient to use Content-Length to find the end of the body, this
statement is true and useful.

The introduction of Transfer-Length along these lines won't cause
*any* problems for deployed implementations, since nobody should
be sending a non-self-delimiting transfer coding unless the recipient
has, by using the TE header, indicated that it can accept such a
transfer coding.

I'm not familiar enough with Digest Authentication to know if this
clarification requires some debugging of the D. A. spec.

-Jeff

P.S.: Actually, instead of "Transfer-Length", which is 15 characters,
I would suggest a header name such as "TLen".  After all, the obvious
non-self-delimiting transfer codings are for the purpose of data
compression, so why add unnecessary bytes?
Received on Thursday, 11 December 1997 17:42:59 EST

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