Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

back to the IP address of the proxy. This Record-Routeheader is forwarded

with the request, stored by the UA server, and included in the response sent

back to the UA client that originated the request. All future requests during the

session must now include a Routeheader that forces the request to route

through the proxy.

An example with one proxy that Record-Routes and another that does

not is shown in Figure 10.5.

In this example, proxy A needs to be included in all future SIP messaging

between the UAs, while proxy B does not. As a result, proxy A inserts a

Record-Routeheader, while proxy B does not add itself to the Record-

Routeheader. Therefore, the ACKand the BYErequests are routed through

proxy A but bypass proxy B. Note that the Routeheader always contains

information about the next hop, not the current hop. As a result, in this exam-

ple, since the ACKof Message 13 is sent to proxy A, the Routeheader does not

contain the URL of proxy A, but instead contains the URL of the next hop,

which is UA2. The same is true for the BYEof Message 15. After the last URL

in the Routeheader is used, the header is removed from the request, as it is in

Messages 14 and 16. Also note that the Routeheader is never present in

responses, because they are always routed back through the same set of prox-

ies taken by the request.

Figure 10.5 SIP proxy Record-Routeexample

Client Proxy A

1 INVITE

2 INVITE R-R : A

13 ACK

Proxy A Record-

Routes (R-R)

but Proxy B does

not.

The ACK and

BYE are routed

through Proxy A

but not Proxy B.

5 100 Trying

Proxy B Server

11 200 OK R-R : A

12 ACK Route : A

16 200 OK

Media Session

No More Media Session

Media Session

10 180 Ringing R-R : A

3 100 Trying

15 BYE

7 180 Ringing R-R : A

9 200 OK R-R : A

6 180 Ringing R-R : A

17 200 OK

8 200 OK R-R : A

14 BYE Route : A

4 INVITE R-R : A

182 Chapter 10