Re: [sipcore] draft-ietf-sipcore-location-conveyance-04

Paul Kyzivat <pkyzivat@cisco.com> Wed, 27 October 2010 01:34 UTC

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Date: Tue, 26 Oct 2010 21:36:02 -0400
From: Paul Kyzivat <pkyzivat@cisco.com>
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To: "James M. Polk (E-mail)" <jmpolk@cisco.com>, Jon Peterson <jon.peterson@neustar.biz>
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Subject: Re: [sipcore] draft-ietf-sipcore-location-conveyance-04
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I took a quick look at the new version, and it seems to have cleanup a 
lot. But there *still* seems to be a problem with the syntax. (Interplay 
between ABNF and text.) From the text, I find:

    The placement of the "routing-allowed" header field parameter,
    strongly encouraged by [RFC5606], is outside the locationValue, and
    MUST always be last in the header field value. The routing-allowed
    parameter MUST be present, even when no locationValue is present.

This is questionably supported by the ABNF:

    Geolocation-header = "Geolocation" HCOLON Geolocation-value
    Geolocation-value  = ( locationValue [ COMMA locationValue ] )
                         / routing-param
    locationValue      =  LAQUOT locationURI RAQUOT
                           *(SEMI geoloc-param)
    locationURI        =  sip-URI / sips-URI / pres-URI
                           / http-URI / HTTPS-URI
                           / cid-url ; (from RFC 2392)
                           / absoluteURI ; (from RFC 3261)
    geoloc-param       =  generic-param;  (from RFC 3261)
    routing-param      =  "routing-allowed" EQUAL "yes" / "no"

It only works if you presume that there are separate occurrences of 
GeoLocation-header in the message, one with just locations, another with 
just the routing-param. The routing-param certainly can't be "last in 
the header field value" except in the degenerate sense, since it must be 
first and only in a Geolocation-header.

Below are some specific cases - both valid and invalid. In each case I 
show some headers bracketed by "...". That is intended to mean other 
headers in a message, but all in the same message. (And when I show 
multiple headers, there could be other non-geoloc headers interleaved.)

* The following are legal according to the above, and probably within 
expected usage:

...
Geolocation: cid:foo@example.com
Geolocation: routing-allowed=yes
...

...
Geolocation: cid:foo@example.com,cid:bar@example.com
Geolocation: routing-allowed=yes
...

...
Geolocation: routing-allowed=no
...

* IIUC the following are allowed by the above both syntactically and 
according to the text, but is presumably not *intended* to be valid. 
(Its legal because the in each instance of Geolocation-header its last.)

...
Geolocation: routing-allowed=no
Geolocation: routing-allowed=yes
...

The following is also allowed - I don't know if its intended or not:

...
Geolocation: routing-allowed=yes
Geolocation: cid:foo@example.com,cid:bar@example.com
...

* The following is allowed by the ABNF though disallowed by the text:

...
Geolocation: cid:foo@example.com,cid:bar@example.com
Geolocation: cid:baz@example.com
...

* The following is *not* allowed by the ABNF. I suspect it might be 
intended to be valid, but I'm far from sure about it:

...
Geolocation: cid:foo@example.com,routing-allowed=yes
...

In addition to that, the text is very explicit that:

    The routing-allowed
    parameter MUST be present, even when no locationValue is present.

but then it says:

    If no routing-allowed parameter
    is present in a SIP request, a SIP intermediary MAY insert this
    value with a RECOMMENDED value of "no" by default.

So its required, but we have rules to follow if its missing. But I don't 
understand how it *can* be required, since the sender of the request may 
not understand/support Geolocation and so won't include it. ISTM that in 
reality its only *required* if the is a locationValue present, and is 
otherwise optional. In that case, an intermediary that adds a 
locationValue not only MAY, but presumably MUST add a missing 
routing-param if it adds a locationValue.

I have never understood why the routing-param is required to be last. 
And as my examples show, its difficult/impossible to enforce this in ABNF.

	Thanks,
	Paul