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

controller to the controlled parties and to make the controlled SIP endpoints

exchange RTP media with each other. If the controller has the same identity as

one of the parties (for example, if the controller is just another device that is

associated with A in Figure 11.5), then no new authorization or identity issues

are caused by this. However, if the controller is a different identity, then this

scenario can appear to the parties involved as a man in the middle attack

(MitM). That is, B is exchanging signaling with the controller, but media with

A. Note that since the controller is not actually manipulating and modifying

the SDP, but is just cutting and pasting it from one message to another, it is

possible that the SDP bodies could be encrypted with S/MIME. A and B could

use this to securely exchange SRTP master keys to have an encrypted and

authenticated media session between them.

Figure 11.5 Basic third-party call control

Party A Party B

1 INVITE with no media SDP

2 200 OK with no media SDP

1. Controller sets up
   call to A using SDP
   with no media lines.

Controller and

party A can

be on same

desktop

4. Controller sets up
   call to B and gets SDP
   data from B.


5. Controller re-INVITEs
   A using SDP data from B.
   Party B receives the SDP
   from A in the ACK.
   Conversation between
   A and B.


6. Party A originates BYE
   to controller who in turn
   sends BYE to party B. The
   BYEs are confirmed and
   the call is terminated.

10 RTP Session from A to B

4 INVITE

8 ACK with SDP from A

12 BYE

7 200 OK with SDP from A

11 BYE

3 ACK

6 INVITE using SDP from B

9 ACK

14 200 OK

13 200 OK

5 200 OK with SDP from B

Controller

204 Chapter 11