W3C home > Mailing lists > Public > ietf-http-wg-old@w3.org > January to April 1998

Rigorously defining the interaction of conditional headers

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Wed, 04 Mar 98 15:18:51 PST
Message-Id: <9803042318.AA01009@acetes.pa.dec.com>
To: http-wg@cuckoo.hpl.hp.com
Dave Kristol in
   http://www.ics.uci.edu/pub/ietf/http/hypermail/1998q1/0402.html
brought up the problem that the current draft of the HTTP/1.1
specification is ambiguous or hard to interpret in several ways.

I believe we have now identified all of the actual ambiguities
and created "issues" for them:
    issue IM_IUS_412
	http://www.ics.uci.edu/pub/ietf/http/hypermail/1998q1/0409.html
    issue CACHE_CONDITIONAL
	http://www.ics.uci.edu/pub/ietf/http/hypermail/1998q1/0406.html
    issue HEAD_WITH_IMS
	(not visible in the archive yet)

This leaves the problem that it is still somewhat hard to read
the specification, especially if one is trying to figure out
how the various headers interact.

Dave and I have spent several days coming up with pseudo-code
that, we believe, correctly represents the specified behavior
in all possible cases.  It was agreed during this week's editorial
teleconference that the next draft should include such pseudo-code,
as an explanation of how to interpret the existing specification.

Since neither Dave nor I was able to get the right code on our
first tries, we would appreciate some review of this!

Because the cutoff date for Internet-Draft submission is Friday,
March 13, and Jim Gettys will need several days to actually generate
the draft in the required format, any comments must be received
as soon as possible (i.e., "read this today").

-Jeff (and Dave Kristol)

P.S.: Note that the somewhat repetitive format of this code
is intentional.  I found that the only way that I could be
sure that we had covered every possible case was to fully
expand the ELSE clauses of every IF statement.  Also, Dave
and I (mildly) disagreed on whether it is clearer to use
a few GOTO statements, or an auxilliary state variable which
is set in one place and tested in another.  My preference
was to use the GOTOs, to make the control flow as clear as possible.



NEW SECTION 13.3.5  Interpreting combinations of conditional headers

    HTTP/1.1 includes four request-header fields that can be used to
    make a request conditional: If-Match (section 14.24),
    If-Modified-Since (section 14.25), If-None-Match (section 14.26),
    and If-Unmodified-Since (section 14.28).  The protocol allows
    combinations of these four headers, and the proper interpretation
    of the meaning of the possible combinations has proved to be
    difficult to extract from the language of the specification.

    We therefore include a description, in pseudo-code, of one possible
    correct implementation of the logic for processing HTTP/1.1
    conditional headers in order to decide whether to perform the
    request, or to return a status code of 304 (Not Modified) or 412
    (Predoncondition Failed).

    Many other implementations are allowed by the HTTP/1.1
    specification, but their external behavior is required to conform
    to the behavior of the pseudo-code below, in all cases where it
    defines a specific behavior.

    Note that there is a fifth conditional header, If-Range (section
    14.26) that is not described here.  It does not make sense to
    combine the If-Range request-header field with other conditional
    request-header fields, and so there is no need to define its
    interaction with other such fields.
    
    /* Logic for processing HTTP/1.1 conditional headers
     *	(not including If-Range)
     *
     * Implicit inputs:
     *	The request message
     *	The selected resource
     *
     * Outputs:
     *  the response status code, if its value is defined by this logic
     *  otherwise, the decision to continue processing the request 
     */
    
    /*
     * Uses the following functions:
     *
     * BOOLEAN reqhdrs_include(STRING field_name)
     *	returns true if the request headers include the given field-name.
     *
     * STRING fieldval(STRING field_name)
     *	returns the request's field-value for the given field-name.
     *	(May involve combining multiple instances of the same header.)
     *
     * INT count_match_current_etag(STRING field_value)
     *	returns the number of entity tags in the field-value that
     *	match the current entity tag of the selected resource.
     *
     * BOOLEAN matches_lastmod(STRING field_value)
     *	returns true if the timestamp in the field-value matches
     *	the current last-modified timestamp of the selected resource.
     */
    
    /*
     * NOTE: all branches of the IF-THEN-ELSE statements are included
     * in the following nested conditional, to ensure that no cases
     * are omitted
     */
	IF reqhdrs_include("If-None-Match") THEN {
	  IF count_match_current_etag(fieldval("If-None-Match")) > 0 THEN {
	    IF reqhdrs_include("If-Modified-Since") THEN {
	      IF matches_lastmod(fieldval("If-Modified-Since")) THEN {
		IF req_method == "GET" OR req_method == "HEAD" THEN {
		  status_code = 304; RETURN;
		} ELSE {
		  /* WARNING: meaningless If-Modified-Since ? */
		  GOTO next_step;
		} ENDIF
	      } ELSE {	/* If-Modified-Since mismatch */
		GOTO next_step;
	      } ENDIF
	    } ELSE {	/* entity tags match, no If-Modified-Since */
	      /* 14.26 says MUST NOT perform method */
	      IF req_method == "GET" OR req_method == "HEAD" THEN {
		status_code = 304; RETURN;
	      } ELSE {
		status_code = 412; RETURN;
	      } ENDIF
	    } ENDIF 
	  } ELSE {	/* no entity tag matches, ignore If-Modified-Since */
	    GOTO next_step;
	  } ENDIF
	} ELSE {
	  /* Check for vanilla If-Modified-Since */
	  IF reqhdrs_include("If-Modified-Since") THEN {
	    IF matches_lastmod(fieldval("If-Modified-Since")) THEN {
	      IF req_method == "GET" OR req_method == "HEAD" THEN {
		status_code = 304; RETURN;
	      } ELSE {
		/* WARNING: meaningless If-Modified-Since ? */
		GOTO next_step;
	      } ENDIF
	    } ELSE {	/* If-Modified-Since mismatch */
	      GOTO next_step;
	    } ENDIF
	  } ELSE {
	    GOTO next_step;
	  } ENDIF
	} ENDIF
    
    next_step:
	/* If-Match */
	IF reqhdrs_include("If-Match") THEN {
	  IF count_match_current_etag(fieldval("If-Match")) == 0 THEN {
	    status_code = 412; RETURN;
	  } ENDIF
	} ENDIF
    
	/* If-Unmodified-Since */
	IF reqhdrs_include("If-Unmodified-Since") THEN {
	  IF NOT matches_lastmod(fieldval("If-Unmodified-Since")) THEN {
	    status_code = 412; RETURN;
	  } ENDIF
	} ENDIF
    
	/* continue processing the request */
    
Received on Wednesday, 4 March 1998 15:21:33 EST

This archive was generated by hypermail pre-2.1.9 : Wednesday, 24 September 2003 06:33:13 EDT