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Re: two-phase send concerns

From: Roy T. Fielding <fielding@avron.ICS.UCI.EDU>
Date: Sun, 10 Dec 1995 15:50:48 -0800
To: Jeffrey Mogul <mogul@pa.dec.com>
Cc: http working group <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
Message-Id: <9512101550.aa18614@paris.ics.uci.edu>
> Roy has taken a pessimistic approach to the byte-deluge problem.
> In particular, every use of every method with a two-phase requirement
> must pay either the arbitrary timeout period OR at least one
> round-trip time.  In other words, by taking this approach, we
> build in "extra" delay to every POST (etc.) invocation for millions
> of users for many years.  Hmm.

Yes, this is an excellent analysis of the tradeoffs.  However, it fails
to consider that all of the two-phase methods are not speed-dependent:
it is far more important to get it right the first time (before data
is sent across the wire, if possible) then it is to save one or two
round-trips.  Given that, it is okay to be pessimistic and
pay for that round-trip every time.

> So here's an alternate suggestion (mostly as a way to start people
> thinking about additional suggestions, although I think it would
> work).
> 
> 	If the server doesn't want to receive the large body,
> 	it immediately replies with its 4xx or 5xx response,
> 	and immediately closes (not resets) the connection.
> 
> 	If the client manages to read the 4xx or 5xx response,
> 	it must honor it and should reflect it to the user.
> 
> 	If the client simply sees the transport connection
> 	disappear, then the language in section 1.4, "Overall
> 	Operation",
> 	   Both clients and servers must be capable of handling cases
> 	   where either party closes the connection prematurely, due to
> 	   user action, automated time-out, or program failure. In any
> 	   case, the closing of the connection by either or both
> 	   parties always terminates the current request, regardless of
> 	   its status.
> 	applies with one variation:
> 	   if the aborted request is a POST, PUT, PATCH, etc. (i.e.,
> 	   any request with a non-empty entity body) then if the
> 	   client repeats the aborted request, it MUST include
> 	   the request-header:
> 		Connection: two-phase
> 	   and then follow Roy's two-phase protocol: wait for
> 	   a response or for an specified timeout period before
> 	   proceeding.
> 
> 	A client MAY use
> 		Connection: two-phase
> 	at other times, on its own initiative, but MUST use it
> 	when repeating a request because of a prematurely-closed
> 	connection.

Yes, this might work also -- it depends on whether or not we are willing
to send a bunch of data downstream before getting the response.  Slow start
may not apply here, since this request may be in the middle of a persistent
connection.

> We might also want to do this:
> 	Servers rejecting an entity-body for reasons of size
> 	must respond with 
> 		413 Request too large
> 
> 	Once a client has received a 413 response from a server,
> 	it SHOULD always use
> 		Connection: two-phase
> 	and the two-phase protocol with that server in the future.
> 	This is actually not a big cost, because we know that
> 	a server which has sent us a 413 response is an HTTP 1.1
> 	server, and so the client should (almost?) never need to wait
> 	much longer than one RTT for the two-phase mechanism
> 	to complete.

Yep, we should assign 413 for that error, and use Retry-After for
cases when it is a temporary injunction -- both good ideas.

> Several people have pointed out that 5 seconds seems like a
> poor choice for a time.  It appears to be a compromise between
> "give the server enough time to respond" and "don't delay too
> long waiting for HTTP 1.0 servers, which won't send a Continue
> response."  But in the optimistic approach, we only invoke the
> N-second wait in the (hopefully) rare case of a prematurely
> closed connection.  So N could be somewhat larger than 5 (say,
> 10 or 20) without impacting the normal case.

I don't think "poor choice" is the word -- it is an arbitrary choice
based on typical overall request timeouts (30 seconds) and user
annoyability (anything less than 10 seconds is probably okay) and the
"normal" round-trip time of poor network connections over long
distances.
Received on Sunday, 10 December 1995 15:56:57 EST

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