[AusNOG] IPv4 Exhaustion

[Edwin Groothuis]

On Fri, Aug 01, 2008 at 03:18:52PM +0930, Matthew Moyle-Croft wrote:
> Not quite sure of your point Geoff.
> 
> SIP has IP address details in the SDP - so if you have an ATA behind a  
> NAT gateway then either the NAT gateway needs to understand how to do  
> fixups or you have a Session Border Controller that does it (or both  
> if you have a broken attempt at NAT fixup in your CPE).
> 
> ATAs behind NAT gateways is a VERY common deployment mechanism for  
> consumer VOIP.   Engin, for example, pretty much has it as a business  
> model.

There are two reasons why SIP via NAT is a bad idea:

- Indeed, the SDP packet has the IP address of the media termination
  points in it. Which will give the famous one way audio if it
  doesn't get through. STUN to the rescue? Re-invite to the rescue?

- The other one is that the "world", at least the SIP registrar,
  needs to be able to contact the SIP end-point, which means that
  the NAT gateway needs to remember that the packet for its IP
  address on port N needs to be forwarded to the IP address of the
  SIP end-point on port M. If the entry on the NAT gateway times
  out before the SIP end-point registers again, then the call can't
  be setup. Re-registration with short time-intervals (1 minute? 5
  minutes?) of the SIP end-point to keep the NAT table refreshed
  is the solution here.


One application, two different NAT problems :-)


Edwin

-- 
Edwin Groothuis      |            Personal website: http://www.mavetju.org
edwin at mavetju.org    |              Weblog: http://www.mavetju.org/weblog/