Subject: [Sip] REFER norefersub Max-Forwards question

From: Robert Sparks <[email-masked]>
Date: Tue, 2 Aug 2005 15:00:40 +0200

During today's meeting, we discussed whether the text
noting that Max-Forwards:0 can be used to avoid forking should
be included in the norefersub draft.

The text in question is:
   The best way that the REFER-Issuer can ensure that REFER doesn't get
    forked is by only sending a REFER with "Refer-Sub" with a Request-URI
    which has GRUU properties according to definitions of [5].

    If this is not known, the other way to ensure that forking will not
    occur is to ensure that there are no proxies between the REFER-Issuer
    and the REFER-Recipient.  This could be done by sending the REFER
    with a Max-Forwards: 0 header field.  Any proxy receiving this
    request will return a "483 Too Many Hops" response, indicating that
    it is not safe to use this extension.

Cullen expressed a concern that this looks like a recommendation from
the IETF that this is the way forking should be avoided in general. That
was never the intent to my knowledge, and on re-read, I agree that the
text as is can be interpreted that way.

I offer the following repair:

   If a REFER request to a given Request-URI might fork, the REFER-Issuer
   SHOULD NOT include the "Refer-Sub" header field. It is RECOMMENDED
   that a Request-URI that has GRUU properties according to the 
definitions
   of [5] be used to ensure the REFER request will not fork.

   In some specialized environments, the REFER-Issuer MAY use other
   techniques to be sure the REFER request will not fork, For example,
   in a constrained network where it is known with absolute certainty 
that
   all UAs can reach each other without the use of any proxy, the REFER-
   Issuer could include a Max-Forwards header field with a value of 0 to
   prevent forking (if, for some reason, a proxy found itself on the 
path of
   that request, it would return a 483 Too Many Hops response).

RjS

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