Data Networking Features Supported - Linksys SPA941 Administration Manual

DRAFT
In the case of SIP, the addresses where messages/data should be sent to an SPA are embedded in the
SIP messages sent by the device. If the SPA is sitting behind a NAT, the private IP address assigned to it
is not usable for communications with the SIP entities outside the private network. The SPA 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
SPA needs to perform the following tasks:
Discover the NAT mappings used to communicate with the peer. This could be done with the help of
some external device. For example a server could be deployed on the external network such that the
server will respond to a special NAT-Mapping-Discovery request by sending back a message to the
source IP address/port of the request, where the message will contain the source IP address/port of the
original request. The SPA can send such a request when it first attempts to communicate with a SIP
entity in the public network and 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. Since the mapping is only alive for
short period, the SPA continues to send periodic keep-alive packets through the mapping to extend its
validity as necessary.
Note: NAT Mapping does not have to applied globally on the phone. It can be enabled or disabled per
Extension. Some service providers offer their own solution of NAT traversal; NAT mapping should be
disabled on the SPA941 for extensions configured for these service providers.

• Data Networking Features Supported

The SPA941 supports the following Data Networking Features:
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