Understanding Incoming SIP ALG Call Support Using the SIP Registrar and NAT

Session Initiation Protocol (SIP) registration provides a discovery capability by which SIP proxies and location servers can identify the location or locations where users want to be contacted. A user registers one or more contact locations by sending a REGISTER message to the registrar. The To and Contact fields in the REGISTER message contain the address-of-record Uniform Resource Identifier (URI) and one or more contact URIs, as shown in Figure 19. Registration creates bindings in a location service that associates the address-of-record with the contact address or addresses.

The device monitors outgoing REGISTER messages, performs Network Address Translation (NAT) on these addresses, and stores the information in an Incoming NAT table. Then, when an INVITE message is received from outside the network, the device uses the Incoming NAT table to identify which internal host to route the INVITE message to. You can take advantage of SIP proxy registration service to allow incoming calls by configuring interface source NAT or NAT pools on the egress interface of the device. Interface source NAT is adequate for handling incoming calls in a small office, whereas we recommend setting up source NAT pools for larger networks or an enterprise environment.

Note: Incoming call support using interface source NAT or a source NAT pool is supported for SIP and H.323 services only.For incoming calls, JUNOS Software currently supports UDP and TCP only. Domain name resolution is also currently not supported; therefore, URIs must contain IP addresses, as shown in Figure 19.