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Re: Summary of STATUS100 issue

From: David W. Morris <dwm@xpasc.com>
Date: Wed, 9 Jul 1997 23:10:58 -0700 (PDT)
To: Jeffrey Mogul <mogul@pa.dec.com>
Cc: http-wg@cuckoo.hpl.hp.com
Message-Id: <Pine.GSO.3.96.970709225506.26041E-100000@shell1.aimnet.com>
 

On Wed, 9 Jul 1997, Jeffrey Mogul wrote:

> During the editorial group teleconference on Friday, we discussed
> the STATUS100 issue (i.e., when/whether to send or wait for
> "100 (Continue)").  This is a summary of our understanding of
> that issue.  It was written before the discussion of whether
> "Expected" should be hop-by-hop or end-to-end, but I believe
> it is still accurate.
> 
> -Jeff (for the editorial group)
> 
> RFC2068 (section 8.2, "Message Transmission Requirements")
> introduces the use of a two-phase mechanism for certain requests,
> in which the client sends the request header, then waits for the
> server to reply with "100 (Continue)" before sending the request
> body.  This has led to a confusing discussion, in part because
> there are several underlying problems that this was expected to
> address.
> 
> Our understanding is that the two-phase mechanism solves two
> potential problems:
> 
> 	(1) A client sending a request message with a request body
> 	may wish to determine if the server is willing to accept
> 	the request (based on the request headers) before it
> 	transmits the body.  In some cases, it may either be
> 	inappropriate, or highly inefficient, for the client to
> 	send the body if the server will reject the message without
> 	looking at the body.
> 
> 	(2) There are race conditions that arise when pipelining
> 	several non-idempotent requests, due to the ambiguity that
> 	arises when the connection is terminated before the client
> 	has received responses to all of the requests it has sent.
> 	The use of the two-phase mechanism can reduce the exposure
> 	to such ambiguities, although it cannot entirely eliminate
> 	them.
> 
> Regarding problem #1, we believe that the proposal for an
> "Expected" request header resolves the issue.  The Expected header
> allows the client to be explicit about its intention to wait for a
> 100 (Continue) response, and so a server should only send it if the
> client expects it.  (And a client that does not "Expect" a 100
> Continue response need not wait for one.)  We also believe that the
> proposal adequately addresses the issue of compatibility with
> deployed implementations that conform to the current wording of
> RFC2068, although we would appreciate further experience on that.
> 
> Regarding problem #2, we observe that the most conservative
> solution is simply "do not pipeline non-idempotent requests."
> I.e., if your request stream consists of (conceptually) these two
> operations
> 	copy A to B
> 	copy C to A
> then you should not send the second operation before verifying that
> the first operation has completed successfully.  (Otherwise, you
> might end up with B being a copy of C, rather than of the original
> contents of A.)
> 
> What exactly counts as an idempotent request depends on the
> particular application.  However, a generic client may assume that
> GET and HEAD requests, except for those on URLs likely to cause
> side effects (see RFC2068 section 13.9, "Side Effects of GET and
> HEAD") are idempotent, and that PUT, POST, and DELETE requests are
> non-idempotent.  A client may also assume that after receiving a
> cachable response for a prior request, a repeat of the same method
> on the same URL is idempotent (since response cachability implies
> idempotency).
> 
> This is not a major restriction for "traditional" browser-like
> applications, since in most cases these would only pipeline GET
> requests (e.g., for imbedded images).  There is some concern that
> this would reduce the performance advantages of pipelining for
> other possible applications of HTTP (e.g., WEBDAV or printing).  We
> believe that the designers of such applications must be careful to
> understand the implications of pipelining for the correctness of
> their applications in the face of possible communication failures.
> It may be appropriate to layer, above HTTP, a mechanism that
> implements the semantics of "begin-transaction" and
> "end-transaction" operations, rather than depending on HTTP to
> provide so-called "ACID" (atomic, consistent, isolated and durable)
> semantics.  HTTP is not an ACID protocol.
> 
> We also observe that if a client is using the two-phase method
> (i.e., is sending "Expect: 100-continue") and has several
> non-idempotent operations to perform, it may safely pipeline the
> request header of request N+1 after sending the body of request N.

I don't agree! The only safe approach is to wait after sending the
whole request HEADERs and BODY before sending request header N+1.

Assume for example, under the RFC2068 last minute introduction of
pipelining w/o determination that the client was indeed talking to
an HTTP 1.1 server, that the request is sent, the 100 continue pause
times out and the body is sent. Pipelining the next headers could
result in a connection reset before the response is known.

(Of course, if pipelining wasn't allowed until the client knew it was
acceptable ....).

And just because request N is a POST using the 100Continue protocol, there 
is no reason to assume that request N+1 and N+2 and ... will also be
POSTs. I believe a client SHOULD wait for the response status to a
non-idempotent request prior initiating the next request. (NB. I
intentionally used the word 'status' as it is likely that request N+1's
headers could travel much earlier that complete receipt of the whole
'normal' response and body.)

> This message is a summary of our understanding of the situation.
> If no significant objections are raised, we will rewrite the
> relevant portions of the next draft of the HTTP/1.1 specification
> to incorporate the necessary changes (which will be far briefer
> than this message).
> 
> [My best guess about the "relevant portions" is that they would be
>  limited to these sections of RFC 2068:
> 
> 	8.2 Message Transmission Requirements
> 	10.1.1 100 Continue
> 	14.XXX Expected
> 
>  Also, section 9.1.2 (Idempotent Methods) may need some revisions.]

Dave Morris
Received on Wednesday, 9 July 1997 23:17:49 EDT

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