
Chapter 1 Using the Linksys Voice System
SPA9000 Architecture
SIP-NAT Interoperation
In the case of SIP, the addresses where messages/data should be sent to a SPA9000 system are embedded in the SIP messages sent by the device. If the SPA9000 system is sitting behind a NAT device, the private IP address assigned to it is not usable for communications with the SIP entities outside the private network.
Note If the ITSP offers an outbound
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
•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
Because the mapping is alive only for a short period, the SPA9000 system continues to send periodic
SPA9000 Architecture
This section describes the basic architecture, function, and configuration options for the SPA9000. It includes the following topics:
•Architectural Components, page
•Multicast Addressing and Group Paging, page
•Configuration Options, page
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| Linksys SPA9000 Administrator Guide |
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| Document Version 3.01 |
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