Subject: [midcom] need for PRR and PER

From: "Bob Penfield" <[email-masked]>
Date: Tue, 18 Mar 2003 21:59:30 -0500

I want to clarify what I was saying at the meeting.

Since RTP does not have to be symmetric, you need two policy rules. One for
the inbound RTP, and one for the outbound RTP.

Consider a SIP call from A to B.

    +------------[agent]-------------+
    |               |                |
   [A]=========I[middlebox]E========[B]

------ is SIP
====== is RTP

When the SIP INVITE traverses the agent, it has enough information to enable
the inbound flow (using a wildcard for the external source address+port B).
However, it does not have enough information to enable a policy rule for the
outbound flow. When the response traverses the agent, the outbound flow can
be enabled. The problem is that if by the time the response comes back, the
middlebox might not have enough resources (whatever they may be) to enable
the outbound flow, the call fails. If the outbound policy rule could be
created (without being enabled), then this sort of failure can be avoided.

With the existing semantics, a PRR would be used to reserve the outbound
flow and select the external address+port (E). A PER would then be used to
enable the inbound flow using (E) from the PRR response as the inbound
destination address+port (the external address+port). When the response came
back, a PER would be used to enabled the previously reserved rule for the
outbound flow.

It would be nice if both transactions could be completed in one
request/response, but that's a different problem.

cheers,
(-:bob

Robert F. Penfield
Chief Software Architect
Acme Packet, Inc.
130 New Boston Street
Woburn, MA 01801
[email-masked]

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