Ping and Pong optimizations

Since the original protocol specification ([Gnu00]) used network resources very inefficiently [Gnu02] states some optimizations for Ping and Pong messages. To avoid unnecessary propagation of Ping broadcasts caching of Pong messages is deployed.

If a Gnutella host conforms to the following requirements it is considered Pong caching compatible and must provide the following header when establishing a connection. Possibly a higher number can be given if the caching scheme has a higher version number.

Pong-Caching: 0.1

1. All Pong messages sent should have a high probability of referring to connectable hosts and provide a good distribution of hosts across the network.
2. The bandwidth used by Ping and Pong messages should be minimized.
3. If a Ping message is received, with TTL greater than 1 and it was at least one second since another Ping was received on that connection, a servent must (if possible) respond with a number of Pong messages. The number of messages sent in response may vary but 10 is a reasonable number according to [Gnu02].
4. An incoming Ping message with a TTL field of 1 and a Hops field of 0 or 1 must always be replied to with a Pong message, since the Ping is used to probe the remote host of a connection.
5. An incoming Ping message with a TTL field of 2 and a Hops field of 0 is called a ``Crawler Ping'' and is used to scan the network. Pong messages should be sent in reply listing the host receiving the Ping and all its' neighbours.