Internet Communications Using SIP - RIPHAH INFORMATION ...

284 Chapter 16As a consequence, UA2 will send a BYE message to UA1 that contains thereason header:Reason: Preemption; cause 1, text=”UA Preemption”The first call is terminated, and UA2 can send a 200 OK message to UA3 totake the call.Several reason codes have been proposed for the SIP reason header for preemptionas shown in Table 16.4.Linking SIP Preemption to IP Network andLink Layer PreemptionAs mentioned, in some special private networks using satellite links or on frugalaccess links, preemption may also be required at the IP layers. Proposalshave been advanced to use various technologies at the IP layer such as DifferentiatedServices, RSVP, and MPLS [13]. The elements required as preconditionsfor SIP [14] can then link the preemption technology at the network layer,such as RSVP, with the preemption technology at the SIP application layer.A comprehensive framework that describes in detail the options for the IPlayer and also for various link layers (such as 802 networks, mobile networks,and cable links) is provided in [15].Preemption at the IP network layer has many of the same limitations andcomplexity as QoS for IP that is discussed in Chapter 18, “Quality of Servicefor Real-Time Internet Communications.”Table 16.4SIP Reason Codes for PreemptionCAUSE DEFAULT TEXT DESCRIPTION1 UA Preemption The session has been preempted by a UA.2 Reserved Resources The session has been preempted by an IPPreemptedoccurence, such as RSVP preemption and notby a link error.3 Generic Preemption Designed to be used on the final leg to thepreempted UA to generalize the event.4 Non-IP Preemption The preemption has occured in the non-IPinfrasructure, and the reason code is insertedby the gateway.