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Re: Content-Transfer-Encoding "packet"

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Mon, 24 Jul 95 16:11:58 MDT
Message-Id: <9507242311.AA14626@acetes.pa.dec.com>
To: Rick Troth <TROTH@ua1vm.ua.edu>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
    >In other words, the result looks like a normal message, with
    >the Content-Length computed by the unpacketizer.  In actuality,
    >it is often faster to read packet-size + 1 bytes, slurping in
    >the next packet-size as part of the prior packet read.
    
	    That's an optimization that works on some systems;
    not on others.   Keep the protocol free from platform specifics.

Although I strongly favor the use of ASCII encodings of "packet"
lengths (perhaps we could use a term besides "packet", which will
cause untold confusion for years to come), I agree that it might
be nice to use an encoding that occupies a fixed number of bytes.
It's not exactly "platform-specific" to optimize things in this
way; it's basically a tradeoff between these two algorithms:

[*** Anti-paranoia warning: I'm writing these examples using UNIX
 *** APIs for concreteness, NOT because this is the the only API
 *** I care about.  So don't flame me about that, please.]

   Naive algorithm:

	while (not done)
		fgets(length, sizeof(length), stream);
		n = decode(length);
		fread(buffer, n, 1, stream);
	end

   Clever algorithm:

	fgets(length, sizeof(length), stream);
	n = decode(length)
	while (not done)
		fread(buffer, n+sizeof(length), 1, stream);
		n = decode(&buffer[n]);
	end

The clever algorithm uses M+1 instead of 2*M reads from the
input stream.  This is especially important if your code
doesn't want to read past the end of the input stream (cf.
the NCSA httpd server), in which case you would write this
using read() system calls instead of using stdio buffering.

Note also that if we use a simple ASCII encoding, you need
to parse the input stream to figure out where the end of
the "packet" length string is.  If we use an encoding that
takes a fixed number of columns, then you don't need to parse
anything, you just grab a bunch of bytes.

Since I also don't see any reason to limit the "packet" size
to just 256 bytes, or to any small value, I recommend that the
length field be fairly large.  For example, suppose that the
standard says "use 6 digits + CR + LF".  Then this leaves
the following data nicely aligned (even for 64-bit machines),
and allows almost a million bytes per "packet".  Better yet,
use a hexadecimal encoding; this allows >16 million bytes, and
is much easier to encode and decode.

Example:
	0 0 0 0 0 8 CR LF
	eight bytes of data
	1 0 0 0 0 0 CR LF
	1,048,576 bytes of data
	0 0 0 0 0 0 CR LF

-Jeff
Received on Monday, 24 July 1995 16:17:52 EDT

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