Subject: [Simple] NAT traversal in MSRP

From: "Achint Aggarwal" <[email-masked]>
Date: Wed, 2 Mar 2011 11:15:01 +0530

Sec 8.5 of RFC 4975 states the following:

"Previous versions of this document included a mechanism to negotiate
   the direction for any required TCP connection.  The mechanism was
   loosely based on the Connection-Oriented Media (COMEDIA) [26] work
   done by the MMUSIC working group.  The primary motivation was to
   allow MSRP sessions to succeed in situations where the offerer could
   not accept connections but the answerer could.  For example, the
   offerer might be behind a NAT, while the answerer might have a
   globally routable address.
 
   The SIMPLE working group chose to remove that mechanism from MSRP, as
   it added a great deal of complexity to connection management.
   Instead, MSRP now specifies a default connection direction.  The
   party that sent the original offer is responsible for connecting to
   its peer."

Does this mean that in order to resolve NAT traversal issue in MSRP,
using MSRP relays is the only solution??

In other words does this indicates that if MSRP relays are not used then
COMEDIA has to be used (which as per rfc 4975 is deprecated)?? 

Regards

Achint Aggarwal

---

From: Ben Campbell <[email-masked]>
Date: Wed, 2 Mar 2011 00:07:16 -0600

On Mar 1, 2011, at 11:45 PM, Achint Aggarwal wrote:

> “Previous versions of this document included a mechanism to negotiate
>    its peer.”
> Does this mean that in order to resolve NAT traversal issue in MSRP, using MSRP relays is the only solution??
> In other words does this indicates that if MSRP relays are not used then COMEDIA has to be used (which as per rfc 4975 is deprecated)??

The MSRP relay is the primary way the work group intended one to get around NATs. You are likely going to need a relay, or something like it, to get around the "both clients behind NATs" problem (depending on the NAT behavior, of course.) This is true even with COMEDIA. COMEDIA would, however increase the number of scenarios where one peer is using a routable address and the other behind a NAT that can work without a relay.

COMEDIA was not deprecated so much as ruled out-of-scope because of timing issues. It happens that  RFC 6135 was just published this week. That RFC specifies the use of COMEDIA with MSRP.

Thanks!

Ben.