Question

This problem is about query flooding in P2P networks. Here, we explore the reverse-path routing of the QueryHit messages in Gnutella. Suppose that Alice issues a Query message. Furthermore, suppose that Bob receives the Query message (which may have been forwarded by several intermediate peers) and has a file that matches the query.

5.1. As we know when a peer has a matching file, it sends a QueryHit message along the reverse path of the corresponding Query message. An alternative design would be for Bob to establish a direct TCP connection with Alice and send the QueryHit message over this connection. What are the advantages and disadvantages of such an alternative design?

5.2 In the Gnutella protocol, when the peer Alice generates a Query message, it inserts a unique ID in the message’s MessageID field. When the peer Bob has a match, it generates a QueryHit message using the same MessageID as the Query message. Describe how peers can use the MessageID field and local routing tables to accomplish reverse-path routing.

5.3. An alternative approach, which does not use message identifiers, is as follows. When a query message reaches a peer, before forwarding the message, the peer augments the query message with its IP address. Describe how peers can use this mechanism to accomplish reverse-path routing.

Answer 1

5.1.

The Advantage is: The QueryHit message is router by the underlying internet without passing through intermediate peers. Thus the delay in sending the message from Bob to Alice will be less.

The disadvantage is: Each peer that has a match would ask Alice to open a TCP connection. So, Alice may have to open hundreds of TCP connections for a given query.

5.2.

• When a QueryHit message enters a peer, the peer records the MessageID in a table along with an identifier of the TCP socket from which the message is arrived.
• When the same peer receives a QueryHit message with the same MessageID , it can index the table and determines the socket to which it should forward the message.

5.3.

• When the query message reaches Bob, it contains an ordered list of all the IP addresses of the peers the message passed through between Alice and Bob.
• When Bob sends back a QueryHit message, it will copy the ordered list into the message.
• When a peer receives the QueryHit message, it can use the list to determine the next peer in reverse path.