Subject: [midcom] framework examples

From: "Bob Penfield" <[email-masked]>
Date: Mon, 21 May 2001 15:31:39 -0400

I have a few issues with the SIP examples in section 7.

Having the SIP Proxy be part of the private domain might be a little more
realistic. I doubt people are going to let external MIDCOM agents control
their private Middleboxes.

For the example in 7.1, the request to the middlebox to permit the
Pri-to-ext flow should be done before the INVITE is sent on to the private
SIP phone (assuming the INVITE has an SDP payload). In SIP, the calling
party (external SIP phone in the example) must be ready to receive the media
when it sends the INVITE with a session description. If the called party
(internal phone) assumes this and sends "early media" (as described in 2.3.4
of the scenarios draft) before sending the 200 OK response, the firewall
will block these packets.

For the example in 7.2, I don't believe there is any need to modify SDP
parameters in the ACK. Since the calling party is external, 'Ea' would be a
global IP address that would not need to be changed. However, some of the
SIP headers (To, From, Via, Contact) would require modification if they had
IP addresses in them. Also, typically the SDP payload is sent with the
INVITE as opposed to the ACK. The BYE request does not usually have an SDP
payload, but you might have to modify SIP headers.

For the example in 7.3, the notations of addresses and ports with RTP and
F-RTP are a bit confusing. Shouldn't the Create NAT bind RTPx to F-RTPx?
Also, In 7.1, there are 2 "permit" requests. Here they are combined into
one. Is there really a difference?

Also, could the "Create NAT" request imply a "permit" (why would you create
a NAT if you we're going to allow the packets to flow?). This is probably a
requirements issue, but I believe that most people feel the number of
messages between a MIDCOM agent and a middlebox should be minimized in order
to maximize performance.

(-:bob

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

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