Determining Whether The Router Uses Symmetric Or Asymmetric Nat - Linksys SPA9000 Administration Manual

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.

Determining Whether the Router Uses Symmetric or Asymmetric NAT

STUN does not work on routers with symmetric NAT. With symmetric NAT, IP addresses are
mapped from one internal IP address and port to one external, routable destination IP address
and port. If another packet is sent from the same source IP address and port to a different
destination, then a different IP address and port number combination is used. This method is
restrictive because an external host can send a packet to a particular port on the internal host
only if the internal host first sent a packet from that port to the external host.
This procedure assumes that a syslog server is configured and is ready
Note
to receive syslog messages. For more information about logging, see
"Collecting System Logs and Debug Information, " on page
1. Make sure you do not have firewall running on your PC that could block the syslog port
(port 514 by default).
2. Connect to the SPA9000 administration web server, and choose Admin access with
Advanced settings. (See
page 24).
3. To enable debugging on the SPA9000, complete the following tasks:
a. Click Voice tab > System.
Linksys Voice System Administration Guide
"Connecting to the SPA9000 Administration Web Server, " on
SIP-NAT Interoperation
31.
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