Sip-Nat Interoperation - Cisco SPA-841 - Sipura VoIP Phone Administration Manual

Advanced Topics in SPA9000 Administration

SIP-NAT Interoperation

SIP-NAT Interoperation
NOTE
SPA9000 Voice System Administration Guide
If the SPA9000 is behind the NAT device, the private IP address of the SPA9000 is
not usable for communications with the SIP entities outside the private network.
If the ITSP offers an outbound NAT-Aware proxy, this discovers the public IP
address from the remote endpoint and eliminates the need to modify the SIP
message from the UAC.
The SPA9000 system must substitute the private IP address information with the
proper external IP address/port in the mapping chosen by the underlying NAT to
communicate with a particular public peer address/port. For this, the SPA9000
system needs to perform the following tasks:
•
Discover the NAT mappings used to communicate with the peer.
This can be done with the help of an external device, such as a STUN server. A
STUN server responds to a special NAT-Mapping-Discovery request by
sending back a message to the source IP address/port of the request, where
the message contains the source IP address/port of the original request. The
SPA9000 system can send this request when it first attempts to communicate
with a SIP entity over the Internet. It then stores the mapping discovery results
returned by the server.
•
Communicate the NAT mapping information to the external SIP entities.
If the entity is a SIP Registrar, the information should be carried in the Contact
header that overwrites the private address/port information. If the entity is
another SIP UA when establishing a call, the information should be carried in
the Contact header as well as in the SDP embedded in SIP message bodies.
The VIA header in outbound SIP requests might also need to be substituted
with the public address if the UAS relies on it to route back responses.
•
Extend the discovered NAT mappings by sending keep-alive packets.
Because the mapping is alive only for a short period, the SPA9000 system
continues to send periodic keep-alive packets through the mapping to extend
its validity as necessary.
A
172