error messages. In the example, the return path is
“prithuladhungel@hotmail.com”.
18. With download and delete, after a user retrieves its messages from a POP server,
the messages are deleted. This poses a problem for the nomadic user, who may
want to access the messages from many different machines (office PC, home PC,
etc.). In the download and keep configuration, messages are not deleted after the
user retrieves the messages. This can also be inconvenient, as each time the user
retrieves the stored messages from a new machine, all of non-deleted messages
will be transferred to the new machine (including very old messages).
19. Yes an organization’s mail server and Web server can have the same alias for a
host name. The MX record is used to map the mail server’s host name to its IP
address.
20. It is not necessary that Bob will also provide chunks to Alice. Alice has to be in
the top 4 neighbors of Bob for Bob to send out chunks to her; this might not occur
even if Alice is provides chunks to Bob throughout a 30-second interval.
21. Alice will get her first chunk as a result of she being selected by one of her
neighbors as a result of an “optimistic unchoke,” for sending out chunks to her.
22. The overlay network in a P2P file sharing system consists of the nodes
participating in the file sharing system and the logical links between the nodes.
There is a logical link (an “edge” in graph theory terms) from node A to node B if
there is a semi-permanent TCP connection between A and B. An overlay network
does not include routers. With Gnutella, when a node wants to join the Gnutella
network, it first discovers (“out of band”) the IP address of one or more nodes
already in the network. It then sends join messages to these nodes. When the node
receives confirmations, it becomes a member of the of Gnutella network. Nodes
maintain their logical links with periodic refresh messages.
23. It is a hybrid of client server and P2P architectures:
a) There is a centralized component (the index) like in the case of a client
server system.
b) Other functions (except the indexing) do not use any kind of central
server. This is similar to what exists in a P2P system.
24. Whenever a user, say Alice wants to locate another user in the instant messaging
system, say Bob, her node would send out a query message asking for the
(unique) username (Bob) into the overlay network, which would be flooded into
the network in the same manner a query requesting the location of a resource (eg:
a file in Gnutella network) is flooded. If the user with the username (Bob) is
online, as the query reaches the node where the user is online from, the node
sends back the query response to the original query source (Alice). In this way,