The RS handles routing differently for SIP requests, responses, and subsequent messages.

Initial SIP Requests

When the RS receives a SIP request message, such as INVITE, it can use the Route field to send the message to its next destination. In general, this field is not present in the original message, so there are several other options available for routing. The message can route on the Request URI, or it can use DNS SRV or telephone number mapping (ENUM) to look up the routing. DNS SRV and ENUM are discussed later in this chapter.

SIP Responses

When the RS receives a SIP response message, such as 200 OK, it can use the address found in the Via field for routing, paying attention to the ingress address received from the tags. This method is explained in Chapter 11.


Aliases are alternate identities for users in the system. One aspiration of packet telephony is giving its users multiple addresses for calling one another—by phone number, email address, or an ID of the user’s invention. In traditional phone systems, there are internal dialing schemes that permit corporate users to call others within the enterprise through a truncated version of the local phone numbers. A corporate user in California might call a coworker in North Carolina by dialing a five-digit internal number. In many cases, this type of dialing bypasses the PSTN and transmits the call as packets over a managed backbone. We have prepared VOCAL to handle aliases, but the code has not been fully ...

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