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Re: Accept-Transfer header field (was HTTP/1.1 Issues: TRAILER_FIELDS)

From: Roy T. Fielding <fielding@kiwi.ics.uci.edu>
Date: Tue, 18 Nov 1997 17:03:33 -0800
To: Jeffrey Mogul <mogul@pa.dec.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9711181717.aa11856@paris.ics.uci.edu>
Jeffrey Mogul writes:
>Roy Fielding writes:
>    Qvalues are not useful for transfer encodings -- the coding must not
>    be lossy, and the vast portion of work is being performed on the server
>    side, so the server should be capable of choosing which one is best.
>    
>We can quibble about whether these should be called "qvalues" or
>something else, but some sort of preference weighting is most
>definitely useful.  Generally, there are both costs and benefits
>associated with transfer-codings (such as compression) and there
>isn't always a fixed tradeoff to be made.  For example, I tried
>running a large file through both compress and gzip.  The output
>of gzip was about 25% smaller than the output of compress, but
>it took almost 4 times as much CPU time to do the compression.
>
>On the other hand, it took about 33% longer, and slightly more memory,
>to decompress the output of compress.  So, depending on parameters such
>as network bandwidth and client CPU performance (and perhaps client RAM
>availability), the server is not necessarily capable of choosing the
>most appropriate transfer-coding without some help from the client.

Let me put it this way.  There are a bunch of parameters one might
give to the deflate process such that it controls various performance
aspects of the compression algorithm.  These parameters are known
by the server, and will be set according to the server's needs.  The
only thing the server will care about is whether the client is capable
of decoding the final result.  It doesn't matter what preference the
client assigns beyond that yes/no criteria -- the server's performance
needs are paramount.

In other words, I agree that there are tradeoffs, but the client has
no input in deciding those tradeoffs (nor should it).  That doesn't
mean the server will necessarily ignore the client's needs -- it just
means the server will decide what is needed based on its own observation
of the connection throughput, typical client capabilities, and the
current resource constraint/load on the server itself.  A simple
ordering or relative quality value given by the client says nothing
of importance to the server, and thus provides no useful input to
its decision.

As such, sending additional information in every request, which adds
its own latency problems, is wrong.  We are at the stage in the
protocol developent where new features need demonstrated benefit before
they get added to the specification, and the only demonstrated benefit
here is the desire to indicate that the client understands a new
transfer-coding.

>One more thing: I couldn't find a place in the spec where it
>says that transfer-codings "must not be lossy".  In fact, the
>TransSend project at UC Berkeley has demonstrated the utility
>of lossy codings in some applications, and I'm not sure we should
>be banning these.

Those are not transfer-codings.  If the transmission is capable of
losing data, then it is losing the data associated with the other
metadata on that response.  In other words, a lossy encoding performs
a transform on the content, and therefore must be a content coding.

>    Likewise, chunked and identity are always required -- there is no
>    reasonable use for refusal based on lack-of-encoding.  Thus, the only
>    feature we actually need is the ability to request a given
>    transfer-encoding be used.
>    
>I disagree.  With respect to "chunked", we could presumably change
>	All HTTP/1.1 applications MUST be able to receive and decode
>        the "chunked" transfer coding,
>to
>	All HTTP/1.1 applications MUST be able to receive and decode
>        the "chunked" transfer coding, except for clients that
>	explicitly reject this transfer coding using a qvalue of 0
>	in an Accept-Transfer request header field.
>
>I'm not necessarily saying that we should do this, but it seems
>safe to do so, and it might pay off for implementors of clients
>with limited code space.

No, we can't do that.  That has been written in stone (i.e., deployed code).
It is also not desirable from a robustness perspective -- HTTP needs an
indicator of message length in order to differentiate between complete
and incomplete responses.  HTTP is unsafe for data transfer without that
information, so we don't give the compliant application any choice.

>With respect to "identity", I believe that the argument has already
>been made (the last time we debated the coding issue) that a client
>might want to suppress the transfer of a large response if it cannot
>be compressed first.  (The user might want to choose a different
>link instead, for example.)

I have yet to see or hear of a client that wishes to duplicate connection
setup, round-trip request/response, and shutdown costs just to insist on
a given transfer-coding.  Even if such a client were to exist, it would be
more efficient to just close the connection if the response was not
encoded to its satisfaction (after all, that is what it would have to
do in your scenario if it gets back an error response indicating that
the encoding is unavailable on that connection).  In practice, I doubt
that anyone would ever implement such a client.

>    Note that we must also include a requirement that chunked be the
>    last encoding applied if there is more than one.
>
>Is this really true?  I'm not sure that it would be a major win,
>but why not allow a server to apply compression after chunking?
>It would probably improve the overall compression ratio.  (I.e.,
>you generally get a better ratio when compressing a large file
>than when compressing a small prefix of the same file.)  What
>goes wrong if we allow this?

I suppose it would not be a problem if the encoding was size-delimited.

....Roy
Received on Tuesday, 18 November 1997 17:29:08 EST

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