3GPP Analysis revision -10 - SIP/SDP transition

[Pekka Savola <pekkas@netcore.fi>]

Hi again,

(co-chair hat on)

The 3GPP analysis document was at IESG evaluation again, mainly to 
iron out the SIP/SDP language.

The following was proposed by Allison Mankin.

Gonzalo Camarillo will be co-authoring an I-D which further elaborates 
on the SIP transition approach.

If you have serious objections to this, please voice them soon.  
Thanks!

==========
So here is a replacement for one paragraph in section 4.1.  With this
replacement I would remove my Discuss.

NEW:
    In an ALG approach, this edge would change the IP addresses
    transported in the SIP messages and the SDP payload of 
    those messages to the appropriate version. The SDP rewriting
    in this approach would have many drawbacks and would violate
    the protocol design of RFC 3261.  Moreover, this approach would not
    take advantage of SIP's ability to use proxy routing, nor of SDP's 
    ability to carry multiple alternative addresses. These intrinsic 
    features of SIP and SDP require a more detailed analysis, but they 
    will yield benefits. In addition, any SIP/SDP ALG approach would require 
    NAT-PT (with the issues described in Appendix A), because the IMS-side 
    IPv6addresses must be assigned IPv4 addresses for reachability from the 
    legacy IPv4 side shown in Figure 1. The approach based on intrinsic 
    SIP proxy routing would not require assignment of temporary IPv4 
    addresses to the IPv6 IMS endpoints; instead they would be reached 
    via an IPv4-side address of a SIP proxy acting for them. This SIP 
    proxy would be doing normal SIP processing. 

OLD:
   In a possible approach, this edge could contain a SIP ALG, which 
    would change the IP addresses transported in the SIP messages and 
    the SDP payload of those messages to the appropriate  version. This 
    approach would have the drawback (like other SDP rewriting 
    solutions) of impacting authentication mechanisms that may be 
    needed for other purposes. Moreover, this approach would not take 
    advantage of SIP's ability to use proxy routing, nor of SDP's 
    ability to carry multiple alternative addresses. These intrinsic 
    features of SIP and SDP require a more detailed analysis, but they 
    could yield benefits. The SIP ALG approach requires NAT-PT (with 
    the issues described in Appendix A), because the IMS-side IPv6 
    addresses must be assigned IPv4 addresses for reachability from the 
    legacy IPv4 side shown in Figure 1. The approach based on intrinsic 
    SIP proxy routing would not require assignment of temporary IPv4 
    addresses to the IPv6 IMS endpoints; instead they would be reached 
    via an IPv4-side address of a SIP proxy acting for them. This SIP 
    proxy would be doing normal SIP processing. 
============

(hat off)