3.2.5.1 Receiving a Bandwidth Admission Control Check Response Message

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When a client receives an Allocate Response message with a Transaction ID (specified in [MS-TURN] section 2.2.1) that matches the Transaction ID (specified in [MS-TURN] section 2.2.1) of an Allocate Request message that included a Bandwidth Admission Control Reservation Check message, it proceeds as follows:

  • The response MUST contain a Remote Site Address Response attribute, as specified in section 2.2.8.

  • If the original request message contained a Remote Relay Site Address attribute, the response MUST contain a Remote Relay Site Address Response attribute, as specified in section 2.2.9.

  • The response MUST contain a Local Site Address Response attribute, as specified in section 2.2.10.

  • If the TURN server allocated a relay transport address on behalf of the client, the response MUST contain a Local Relay Site Address Response attribute, as specified in section 2.2.11.

If the response message is valid, the client MUST check the flags for each of the included site address response attributes. The server (2) marks the Valid (V) flag of each of the site address response attributes, based on the available bandwidth over three possible network paths:

  • Local Site Address to the Remote Site Address.

  • Local Site Address to the Local Relay Site Address, if present.

  • Remote Site Address to the Remote Relay Site Address, if present.

The rules for checking the network paths are as follows:

  • The network path between the Local Site Address and the Remote Site Address is only available if both the Local Site Address Response attribute and Remote Site Address Response attribute are flagged as valid (V="1"). If either of these attributes are marked as invalid (V="0"), the network path is not available because of bandwidth constraints.

  • If either of Local Site Address or Remote Site Address attributes are marked as invalid (V="0") and the PSTN Failover flag is set (F="1") the bandwidth policy allows redirecting the connection over PSTN. The client SHOULD consider the network path to the PSTN gateway as a valid network path.

  • If the Local Relay Site Address Response attribute is present and it is flagged as valid (V="1"), the Local Site Address – Local Relay Site Address network path is available. If the Local Relay Site Address Response attribute is not present or if it is flagged as invalid (V="0"), the Local Site Address – Local Relay Site Address network path is not available.

  • If the Remote Relay Site Address Response attribute is present and it is flagged as valid (V="1"), the Remote Site Address – Remote Relay Site Address network path is available. If the Remote Relay Site Address Response attribute is not present or if it is flagged as invalid (V="0"), the Remote Site Address – Remote Relay Site Address network path is not available.

A full check failure occurs when there is no valid network path out of either the Local Site Address or the Remote Site Address. A partial connection failure occurs if any path out of either the Local Site Address or the Remote Site Address is not available. The client SHOULD use all valid network paths to explore connectivity options with the peer. The client MUST NOT use any network paths that are marked as invalid.