1.3 Overview

The Peer Content Caching and Retrieval (PCCR): Discovery Protocol is based on the Web Services Dynamic Discovery (WS-Discovery) Protocol [WS-Discovery], referred to as WSD, which uses multicast to discover and locate services over a peer-to-peer (P2P)  network. There are two modes of operations in WSD: client-initiated Probes and service-initiated announcements; both are sent through IP multicast to a predefined group. The PCCR Discovery Protocol uses the client-initiated Probes to query peer nodes for content.

In the PCCR Discovery Protocol the peers looking for content are the WSD clients, sending out multicast WSD Probe messages with the hashes of the content. The peers that are serving the content take the server role of WSD, listening to Probes and replying to the querying clients with unicast WSD ProbeMatch messages if the content hashes match the ones that are cached locally. To avoid the situation where multiple server peers overwhelm a client with ProbeMatch messages, per the WS-Discovery protocol, each server peer waits for a backoff timer to expire before sending the ProbeMatch. The value of the backoff timer is set randomly between 1 millisecond and a maximum back-off time.