IP Telephony Cookbook by Saverio Niccolini, Jorg Ott, et al - HTML preview

Called party

Figure 2.17 SIP trapezoid

2.2.2.5.2 Dialogue identifiers

Dialogue identifiers consist of three parts, Call-Id, From tag and To tag, but it is not that clear why dialogue identifiers are created exactly this way and who contributes which part.

Call-ID is called call identifier. It must be a unique string that identifies a call. A call consists of one or more dialogues. Multiple user agents may respond to a request when a proxy along the path forks the request. Each user agent that sends a 2xx response, establishes a separate dialogue with the calling party. All such dialogues are part of the same call and have the same Call-ID.

A From tag is generated by the calling party and it uniquely identifies the dialogue in the calling party's user agent.

A To tag is generated by a called party and uniquely identifies it, just like the From tag is the dialogue in the called party's user agent.

This hierarchical dialogue identifier is necessary because a single call-invitation can create several dialogues and the calling party must be able to distinguish them.

{ 2.2.2.6 Typical SIP scenarios

This section gives a brief overview of typical SIP scenarios that usually make up the SIP traffic.

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[IP Telephony Cookbook] / Technological Background

2.2.2.6.1 Registration

Users must register themselves with a registrar to be reachable by other users. A registration comprises a REGISTER message followed by a 200 OK sent by the registrar if the registration was successful. Registrations are usually authorised so a 407 reply which can appear if the user did not provide valid credentials. Figure 2.18 shows an example of a registration.

User Agent

Registrar