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The following diagram shows the bandwidth admission control message flow for the network deployment described in section 4.1, where WAN Link1, between Network Site1 and Network Site2, has no bandwidth available for use. TURN Client1 is attempting to make a voice call to TURN Client2 over WAN link1. The voice call requires between 64 kilobytes and 128 kilobytes of network bandwidth.
Figure 5: Bandwidth admission control message flow when insufficient bandwidth is available
Client1 allocates a public transport from the TURN server following the procedures described in [MS-TURN] section 3.2.4.1. Client1’s relay transport address, allocated by the TURN server, is 192.0.2.20:55667. Client1 adds the relay transport address, along with its local transport address 10.0.0.1:12345, to the SDP of the SIP INVITE and sends the SIP INVITE to TURN Client2.
When Client2 receives the SIP INVITE from Client1, it knows both the local transport address and relay transport address of Client1. Client2 can now do a bandwidth check with the TURN server to verify the availability of network bandwidth between itself and Client1. It follows the procedure specified in section 3.2.4.1 to check for bandwidth admission control. It uses the following attributes in the Allocate Request carrying the Reservation Check message:
A Bandwidth Admission Control Message attribute with a value of "Reservation Check".
A Remote Site Address attribute containing Client1’s local transport address XOR’d with the TURN message Transaction ID (specified in [MS-TURN] section 2.2.1).
A Remote Relay Site Address attribute containing Client1’s relay transport address XOR’d with the TURN message Transaction ID (specified in [MS-TURN] section 2.2.1).
A Local Site Address attribute containing Client2’s local transport address XOR’d with the TURN message Transaction ID (specified in [MS-TURN] section 2.2.1).
A Bandwidth Reservation Amount attribute with a value of 64 kilobytes for the minimum send/receive bandwidth and 128 kilobytes for the maximum send/receive bandwidth.
A MS-Service Quality attribute with a stream type of "Audio" and a service quality of "best effort delivery".
A Location Profile attribute with the Peer-Location set to "Intranet", the Self-Location set to "Intranet" and Federation set to "Not Federated".
When the server (2) receives the Allocate Request message containing the Reservation Check, it follows the procedure specified in section 3.3.5.1 to determine if there is bandwidth available between Client1 and Client2. In this case, Client1’s local transport address maps to network site1, Client1’s relay transport address maps to network site1, Client2’s local transport address maps to network site2, and Client2’s relay transport address maps to network site1.
The server (2) checks the network path for each of the site addresses included in the Reservation Check request:
The server (2) checks the path between the Remote Site Address and the Remote Relay Site Address. Because both of these addresses map into the same network site, there is no bandwidth management constraints on that path and it is considered a valid network path. The server (2) marks the Remote Relay Site Address as valid for the full bandwidth amount, which is 128 kilobytes.
The server (2) checks the path between the Local Site Address and the Local Relay Site Address. In this case, the Local Site Address maps to network site2 but the Local Relay Site Address maps to network site1 and there is a bandwidth managed link, WAN link1, between these sites. The server (2) checks the amount of bandwidth requested in the Bandwidth Reservation Amount attribute against the amount of bandwidth currently available on WAN link1. Because all of the 1.54 megabytes that were available have been consumed by other calls, no bandwidth is available for a new call. The server (2) marks the Local Relay Site Address as invalid and sets the bandwidth value to zero.
The server (2) checks the path between the Local Site Address and the Remote Site Address. In this case, the Local Site Address maps to network site2 but the Remote Site Address maps to network site1 and there is a bandwidth managed link, WAN link1, between these sites. The server (2) checks the amount of bandwidth requested in the Bandwidth Reservation Amount attribute against the amount of bandwidth currently available on WAN link1. Because all of the 1.54 megabytes that was available has been consumed by other calls, no bandwidth is available for a new call. The server (2) marks both the Local Site Address and Remote Site Address as invalid and sets the bandwidth values to zero.
The server (2) replies to Client2 with an Allocate Response message and includes the following attributes:
A Bandwidth Admission Control Message attribute with a value of Reservation Check.
A Remote Site Address Response attribute with the Valid flag cleared (V="0") and a value of 0 kilobytes for the bandwidth amount values.
A Remote Relay Site Address Response attribute with a Valid flag set (V="1") and a value of 128 kilobytes for the bandwidth amount values.
A Local Site Address Response attribute with the Valid flag cleared (V="0") and a value of zero kilobytes for the bandwidth amount values.
A Local Relay Site Address Response attribute with the Valid flag cleared (V="0") and a value of zero kilobytes for the bandwidth amount values.
When Client2 receives the Allocate Response message with the Reservation Check results, it follows the procedure specified in section 3.2.5.1 to determine which transport addresses can be used for the media stream. Because all network paths out of the local site are marked as invalid, Client2 is not able to check connectivity with Client1. Client2 sends a SIP 488 Call denied because of bandwidth policy failure response message to Client1.
When Client1 receives the SIP 488 response, it notifies the user of the connection failure, indicating that no bandwidth was available to make the call.