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Re: multi-host virtual sites for HTTP 1.2

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Wed, 10 Jul 96 15:56:18 MDT
Message-Id: <9607102256.AA12020@acetes.pa.dec.com>
To: Paul Leach <paulle@microsoft.com>
Cc: "'http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com'" <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
I realize that there is some debate about the necessity of a
mechanism such as Paul's proposed "Referral" header, but I tend
to agree that it allows things that cannot be done with DNS
multiple bindings, NFS cross-mounts, or clever proxies.

In particular, it looks like it would allow a single logical "site"
to be distributed geographically around the Internet, which might
be a Very Good Thing for such potential hot spots as the 1996
US presidential election server.

I consider it potentially quite important that Paul's proposal
allows more than one referral-URI.  This not only allows replication
for performance, it also allows replication for fault-tolerance.
The use of multiple DNS bindings, for example, doesn't help much
if one of the actual servers is dead (or your path through the
Internet to that server is broken).

On the other hand, Paul's description ("append [the suffix of
the original URI] to one of the referrals") might not be sufficiently
well specified.  I.e., if a client chooses one of the referral-URIs,
and then after a few successful transactions that server seems to
die, should the client re-negotiate with the original server?
Pick another referral-URI?  Keep trying?  The behavior of the
client in this case has some potential implications for the
failure semantics, especially if the client is updating a
database at the server end.

Paul's proposed BNF was:

	Referral	= "Referral" ":" prefix 1#referral-URI
	prefix  	= absoluteURI
	referral	= absoluteURI

I assume that the last line should have been:
	referral-URI	= absoluteURI

I'd like to suggest an elaboration, to:

	Referral	= "Referral" ":" prefix 1#referral-info
	prefix  	= absoluteURI
	referral-info	= referral-URI delta-seconds metric
	referral-URI	= absoluteURI
	metric		= 1*DIGIT

Thus, a referral would include not only a new URI, but a maximum
age ("time-to-live" in DNS terms) and a cost metric.  The client
algorithm would then be
	choose from the unexpired referral-infos (those whose
	delta-seconds is less than the Age of the response)
	the referral-URI with the lowest metric.
Or maybe that should be "highest metric"?  I dunno.

We could perhaps get rid of the expiration (delta-seconds) parameter if
we made it explicit that a referral lasts only as long as the
Cache-control: max-age of the response that carries it.  But
you need an expiration mechanism of some sort, or else these bindings
are impossible to revoke.

We might want to modify that client algorithm so that "unexpired
referral-info" includes "which has not been unresponsive recently".
I.e., if a server is playing hard-to-GET, drop it from the list.

-Jeff
Received on Wednesday, 10 July 1996 16:14:09 EDT

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