HP VCX Software SIP Sessions, SIP in 3Com VCX IP Telecommuting Module, Establishing a SIP session

Appendix A. More About SIP

SIP Sessions

Establishing a SIP session

You start a call (a session) by sending a request to the address of the person you want to communicate with. The format of the address is <sip:user@host>, where user can be a user name or a telephone number, and host can be a domain name (e.g. example.com) or a numerical IP address (e.g. 172.15.253.12). This means that it usually looks a lot like a standard email address. In this request information about which media streams the client wants to send/receive and what ports should be used is also included.

The SIP client sends this request to its default SIP proxy. This proxy resolves the SIP domain in DNS, and sends the request to the SIP registrar for that domain. The proxy also adds information stating that the request was routed through the proxy, thus ensuring that the reply will be routed the same way.

The registrar for the domain looks up the user to see where he is registered, and forwards the request to the machine in question. The SIP client on this machine alerts the user, indicating that someone wants to initiate a SIP session. The user confirms that he, too, wants the SIP session. The client sends a reply with necessary information about what ports should be used by this client for sending and receiving media streams.

The first client receives the reply and sends a confirmation packet. After this, the media streams can be sent.

SIP in 3Com VCX IP Telecommuting Module

SIP Routing Order

Here, the order for SIP routing decisions is listed. Sometimes you need to know this in order to configure the Telecommuting Module to make it work the way you want. The Telecom- muting Module searches for the first matching setting in the list.

1.The Telecommuting Module checks that the SIP method in the packet is allowed ac- cording to the settings under SIP Methods.

2.The Telecommuting Module checks that the SIP packet is allowed according to the settings under Sender IP Filter Rules.

3.The Telecommuting Module checks that the SIP packet is allowed according to the settings under Header Filter Rules.

4.The Telecommuting Module checks if the SIP packet contains a Route header which determines the next destination.

5.If VoIP Survival is enabled and active, the Telecommuting Module checks if the SIP packet is addressed to a user under a monitored domain.

6.The Telecommuting Module checks for the SIP domain of the Request-URI in the DNS Override For SIP Requests table.

7.The Telecommuting Module checks for the SIP user from the Request-URI among lo- cally registered users and users listed in the Static Registrations table.

325