2.1.1 Peer Download Transport
The Peer Download Transport is a peer-to-peer transport built on top of HTTP [RFC2616]. The client/server HTTP protocol is turned into a peer-to-peer transport by having each peer implement both a client and a server role. In a given transport session between two peers P1 and P2, the initiator peer acts as client, and the other peer acts as server. If P1 is the initiator of the transport session, P1 sends an HTTP request, and P2 replies by sending an HTTP response. Both the Retrieval Protocol request and response message types are included in the body of the HTTP messages. The payload of each such HTTP request or response consists solely of a single Retrieval Protocol message, with the response message prefixed with an additional length field (as defined in section 2.2.2) for reassembly purposes. A transport session between any two peers spans a single request-response sequence, and no context is kept within the transport across different transport sessions between those two peers.
Each peer implements the server role by reserving the URL under the root path of /116B50EB-ECE2-41ac-8429-9F9E963361B7/ and listening for POST requests on it.
The initiating/client-role peer P1 at IP address A1 initiates the transport of a given request-type Peer Retrieval Protocol message to peer P2 at IP address A2, by sending an HTTP POST request to the root path of /116B50EB-ECE2-41ac-8429-9F9E963361B7/.