3.1.4.2.1 Sending the Describe Request

The Describe request MUST adhere to the syntax specified in section 2.2.2.1.

In addition, the common processing steps specified in section 3.1.5.1 MUST be followed when sending the Describe request.

If the client implementation supports the pipelined mode of the protocol, the Describe request SHOULD include a pipeline-experiment (section 2.2.1.4.15) token on the Pragma header, with the value set to 1, as specified in 2.2.1.4.14. Clients that use HTTP 1.0 MUST NOT send the pipeline-experiment token.

If the client implementation would like to use $P (Packet-Pair) packets to measure the bottleneck bandwidth between the client and the server, and if the value of the Server-Version variable is not uninitialized and is less than 5.0, and if the value of the Server-features variable indicates that the com.microsoft.wm.srvppair feature is supported, then the client SHOULD specify the packet-pair-experiment (section 2.2.1.4.14) token on the Pragma header with the value set to 1. Otherwise, the packet-pair-experiment token SHOULD NOT be specified. For example, a client that intends to select all of the streams in the ASF file, or that does not support the stream-switch-entry token, is not likely to have any use for $P packets and thus does not need to specify the packet-pair-experiment token, as specified in 2.2.1.4.14.

After having sent the request, the client MUST process the rules  in section 3.1.5.2.