Subject: [Sipping] draft-ietf-sipping-update-pai-05 - forward compatibility mechanism

From: "Elwell, John" <[email-masked]>
Date: Fri, 15 Aug 2008 11:15:21 +0100

Draft-05 is now available at:
http://www.ietf.org/internet-drafts/draft-ietf-sipping-update-pai-05.txt

The only changes are the introduction of a forward compatibility
mechanism for additional URIs / URI schemes, as discussed during the
SIPPING session at IETF72. The changes can be found in section 2
(penultimate paragraph) and the new section 4.5. I believe this reflects
the consensus emerging from the meeting. The remainder is unchanged.

I have one slight doubt. What should a proxy do if it encounters URIs
that, according to the new provisions, are to be ignored? I believe in
this case those URIs should NOT be forwarded with the forwarded message.
My reasoning is that PAI is essentially a hop-by-hop mechanism, where
each hop has to trust the preceding hop before forwarding PAI on the
next hop. It does not seem reasonable to extend that trust to unexpected
URI schemes. This is reflected in the current text of 4.5 (for both PAI
and PPI). I am not sure that this argument applies to PPI, however.

Therefore I would particularly like comments on this proxy behaviour.

The other issue that arose in Dublin concerned the conditions under
which it is possible to assert an identity in a response. Draft-05 has
not changed that area. I will initiate a separate thread for that topic.

John

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