As a consequence, UA2 will send a BYEmessage to UA1 that contains the
reason header:Reason: Preemption; cause 1, text=”UA Preemption”The first call is terminated, and UA2 can send a 200 OK message to UA3 to
take the call.
Several reason codes have been proposed for the SIP reason header for pre-
emption as shown in Table 16.4.Linking SIP Preemption to IP Network and
Link Layer Preemption
As mentioned, in some special private networks using satellite links or on fru-
gal access links, preemption may also be required at the IP layers. Proposals
have been advanced to use various technologies at the IP layer such as Differ-
entiated Services, RSVP, and MPLS [13]. The elements required as precondi-
tions for 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 IP
layer 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 and
complexity as QoS for IP that is discussed in Chapter 18, “Quality of Service
for Real-Time Internet Communications.”Table 16.4 SIP Reason Codes for Preemption
CAUSE DEFAULT TEXT DESCRIPTION
1 UA Preemption The session has been preempted by a UA.
2 Reserved Resources The session has been preempted by an IP
Preempted occurence, such as RSVP preemption and not
by a link error.
3 Generic Preemption Designed to be used on the final leg to the
preempted UA to generalize the event.
4 Non-IP Preemption The preemption has occured in the non-IP
infrasructure, and the reason code is inserted
by the gateway.284 Chapter 16