 send indication, another newly defined STUN message containing the channel number chosen by the client (2 in Figure 12.9) together TURN client TURN server Allocate request Allocate response Send indication (ch 2) (DATA) Channel confirm Send indication (ch 5) (DATA) Channel confirm DATA over channel 2 DATA over channel 5 FIGURE 12.9 A TURN operation. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security with the transport address of the peer stored in the PEER-ADDRESS- ATTRIBUTE. The send indication can also be used to send encapsulated data to the peer, placing the data to be delivered in a DATA attribute. If UDP is used between STUN client and server, the client continues sending data to the peer encapsulated in send indication messages until it receives a channel confirmation message from the server confirming the channel assign- ment. After the confirmation, the client can send data to the STUN server simply by adding a framing header in front of the packets. The confirma- tion, not used if TCP is deployed between STUN server and client, is needed because since UDP messages may be received out of order, the client must make sure that the server knows about a channel before starting to send pack- ets over it. When the enabled peer starts sending data, the server forwards them to the client using data indications until the first one is confirmed by the client (the server reserves channel 5 in Figure 12.9). When the STUN server receives a channel confirmation by the server, it forwards data received by the peer on the assigned channel simply adding a framing header in front of the packets. 12.5 SIP-Specific NAT Traversal Techniques This section describes how the NAT traversal techniques described above are used to support SIP UAs behind NATs, providing a general overview of the approaches adopted; more details can be found in [10]. 12.5.1 Signaling Layer: Ensuring Correct Delivery of a Response to a SIP Request Every host that forwards a SIP message adds a Via header field to t