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Re: STATUS100 Re: Proposed resolution

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Thu, 17 Jul 97 14:27:51 MDT
Message-Id: <9707172127.AA15536@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Responses to comments from Scott Lawrence:

      An observation: I find it interesting that the set of rules to limit
      use of 100 Continue seems to require such a long specification,
      given that the original mechanism was so simple...

H. L. Mencken once said "For every complex problem, there is a solution
that is simple, neat, and wrong."

Anyway, a lot of the lengthening that I added was to address the
complaints about ambiguity in the previous draft.
    
        Requirements for HTTP/1.1 or later origin servers:
        o  Upon receiving a request which includes an "Expect" request-header
           field with the "100-continue" expectation, an origin server must
           either respond with 100 (Continue) status and continue to read
           from the input stream, or respond with an error status. If it
           responds with an error status, it MAY close the transport (TCP)
           connection or it MAY continue to read and discard the rest of the
           request. It MUST NOT perform the requested method if it returns
           an error status.
    
  I would reword this to reflect that the 100 Continue response MUST
  be sent after the request _headers_ have been recieved, since the
  'request' includes the body.

You're right that it needs a re-wording (and one "must" should be
a "MUST"), but I don't think that the requirement is exactly that
the server MUST wait for the entire set of request headers.  It
could, for example, decide to send the 100 Continue as soon as it
sees the Expect header, if it has no intention of rejecting the
request based on other headers.

How about:
   o  Upon receiving a request which includes an "Expect" request-header
      field with the "100-continue" expectation, an origin server MUST
      either respond with 100 (Continue) status and continue to read
      from the input stream, or respond with an error status.  The
      origin server MUST NOT wait for the request body before sending
      the 100 (Continue) response.  If it
      responds with an error status, it MAY close the transport (TCP)
      connection or it MAY continue to read and discard the rest of the
      request. It MUST NOT perform the requested method if it returns
      an error status.

Note that since the decision between sending 100 (Continue) or 
some error status could take the server an arbitrary amount of
time, so we can't actually say how soon after receiving the request
headers the server must send a response.
    
        o  An origin server SHOULD NOT send a 100 (Continue) response if
           the request message does not include an "Expect" request-header
           field with the "100-continue" expectation, and MUST NOT send a
           100 (Continue) response if such a request comes from an HTTP/1.0
           (or earlier) client.
    
        o  An origin server SHOULD NOT send a 100 (Continue) response if
           has already received some or all of the request body for the
           corresponding request.
    
   I don't see the point of these two SHOULD NOTs, since the client
   MUST be prepared to accept an unexpected 100 response anyway.

We do have an obligation to limit the number of unnecessary bits that
travel over the Internet.  One might argue that adding
	HTTP/1.1 100 Continue<CR><LF><CR><LF>
to every response to a PUT or POST is insignificant, or one
might argue the contrary.  Anyway, the second rule seems innocuous
(since if the server is receiving the request body, manifestly
the client isn't waiting for the 100 Continue).  The first rule
might cause some delays for existing clients that wait for
100 Continue, if there are any.

      Arguing against these rules:
    
	- As noted elsewhere, existing 1.1 servers (yes, there are some)
	  won't have been coded to include these restrictions (since the
	  Expect header was only suggested a couple of weeks ago).
    
We shouldn't unnecessarily break any deployed systems.  On the other
hand, the use of words such as SHOULD NOT will ultimately be used to
determine whether an implementation complies with the HTTP/1.1
standard, and since that standard does not yet exist in its final
form, it's unlikely that any existing "1.1" server could actually
meet the final-standard rules for conditional compliance, anyway.

	- I think that it is poor design to encourage look-ahead in the
	  data stream to determine whether or not body has been received.
    
The language does not specifically require the server to look ahead
into the data stream.  However, if the server has actually read some
of the request body, I see no harm in encouraging efficient use of
the Internet.

    ...
        o  If an origin server receives a request that does not include an
           "Expect" request-header field with the "100-continue"
           expectation, and the request includes a request body, and the
           server responds with an error status before reading the entire
           request body from the transport connection, then the server
           SHOULD NOT close the transport connection until it has read the
           entire request, or until the client closes the connection.
           Otherwise, the client may not reliably receive the response
           message.
    
   Does this amount to a rule for the purpose of avoiding bugs in some
   TCP implementations?  I can live with this rule since it is not a
   MUST.

Yes.  I'd appreciate it if Henrik Frystyk or Jim Gettys could review
this rule, since I included it based on my understanding of their
implementation experience and suggestions.
    
        For compatibility with RFC 2068, a server MAY send a 100 (Continue)
        status in response to an HTTP/1.1 PUT or POST request that does not
        include an "Expect" request-header field with the "100-continue"
        expectation.  This exception, the purpose of which is to minimize
        any client processing delays associated with an undeclared wait for
        100 (Continue) status, applies only to HTTP/1.1 requests, and not to
        requests with any other HTTP-version value.
    
   I think that making this a special case allowed _only_ for HTTP/1.1
   is going too far.  We already have the requirement that 100 Continue
   must be accepted by the client anyway.  We're talking about as few
   as 'HTTP/1.1 100CRLF" - 14 bytes - 23 if you send the ' Continue'.
   What justification is there for the complexity of this restriction?

See above.  But I agree with you that this is a judgement call.
I'm just worried that we may not need to send these extra bytes,
and we have the chance now to discourage 23*N extra bytes over
the Internet for many years to come (where N is a large number!)

What do other people think?
    
   Furthermore, I think that the above rule, which is stated using
   'MAY', is really a 'MUST NOT':
 
     A server using any revision of HTTP later than HTTP/1.1 MUST NOT
     send a 100 (Continue) status in response to an HTTP/1.1 PUT or
     POST request that does not include an "Expect" request-header
     field with the "100-continue" expectation.

I don't think the HTTP/1.1 spec can actually define the behavior
of an implementation of a later version of HTTP, without also
at least defining that behavior for an implementation of HTTP/1.1.
This is basically what you said, I think:
   I don't believe that this rule on future versions of the protocol
   meets that test.
but part of my proposal which you are paraphrasing here doesn't
have the same problem.  (Perhaps, though, I need to change
"Requirements for HTTP/1.1 or later XXX" to "Requirements for
HTTP/1.1 XXX".)

At any rate, your proposed change doesn't address the RFC2068
compatibility issue, which is that *existing* clients might
be waiting for 100 (Continue) without having sent the Expect
header.  (Unless you are assuming that all such deployed clients
are gone by the time that HTTP/1.2 arrives.)
 
     Requirements for HTTP/1.1 or later proxies:
  ...
     o  If the proxy knows that the version of the next-hop server is
	HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
	respond with a 419 (Expectation Failed) status.
 
   How is the client to know that repeating the request using HTTP/1.0 could
   resolve this situation?  Should clients always back off to 1.0 if
   they receive a 419?

If you send "Expect: 100-continue" and you get a 419 (Expectation
failed) response, then you know that there is no guarantee that
you will get a 100 (Continue) response before you send the request
body, no longer how long you wait.

At this point, the user-agent has two choices:
	(1) fall back to the HTTP/1.0-like behavior (this is NOT
	the same thing as falling back to HTTP/1.0), i.e., send
	the request body without waiting.
	(2) tell the user that the request cannot be accomplished.

My guess is that, in reality, relatively few (if any) general-purpose
applications are actually going to require "Expect: 100-continue" in
the first place, and so this is really only an issue for applications
in which the need for server pre-confirmation is actually significant.
When we write the HTTP/1.1 spec, we can't presume to decide for these
applications whether it is semantically necessary to get the server's
pre-confirmation (i.e., 100 response).  All we can do is to give them
a relatively unambiguous signal about whether it will be available
or not.  The application implementor has to make the final decision.

-Jeff
Received on Thursday, 17 July 1997 14:46:10 EDT

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