Re: francois' comments and why RFC4474 not used in the field

Cullen,

These are "hotly debated" issues in the service provider community.  I
would hasten to say that nobody has appointed me to speak on that
community's behalf.  So the following views are my own.

You stated:
 
> I think this is exactly the right sort of question to be asking (and  
> we all agree they have reasons they are currently changing the SDP,  
> what we need to get at is why).
> 
> I think we also need to know about situations like
> >
> 
> 
>            UA A -> SP A -> SP B -> UA B.
>           Enterprise A -> SP A -> SP B -> SP C -> Enterprise B.
> 
> And reasons SP B would change it.


I think the rationale for SP B changing the SDP is the same as SP A and
C.  It comes down to addressing.  If their VoIP infrastructure is in a
private address space, as it frequently is, then they need NAT and SIP
ALG functionality at the edges of that space.  Other issues such as
topology hiding and media steering, are IMHO secondary in this debate.

More fundamentally I wonder how a path through SP B would ever be
constructed (assuming the arrows above refer to SIP flows).  For sure
you can do this with "policy routing" in SBCs, but using IETF preferred
routing mechanisms (e.g., ENUM, RFC-3263) it's not clear to me what
would motivate such routing.

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