[SIPForum-techwg] SIPConnect and IP addresses in Via headers

[Michael Procter]

I have been reading sf-draft-twg-IP_PBX_SP_Interop-sibley-sipconnect
version 5, and have a question about a statement in section 9 - Firewall
and NAT traversal.

The draft states that:
	Any IP address contained within the headers ... of SIP messages
exchanged between the Service Provider and the Enterprise networks
MUST be a publicly routable address.

My question is whether this is too strict.  The scenario I have in mind
is one where there is a UA connected to an IP-PBX that is implemented as
a proxy.  A message sent from the Enterprise network to the SP will
therefore have multiple Via headers (2 or more).  I can clearly see why
the topmost Via of the message at the point of egress must be publicly
routable, but why do any others need to be?

I can see why it might be desirable to only expose public IP addresses
(e.g. for topology hiding), but that doesn't seem to be what this
section is saying.

Can anyone shed light on what I am missing please?

Regards,

Michael