CS610 Computer Networks GDB Solution, Spring 2014

Scenario:

“For communication across Internet, TCP/IP protocol suit is used for achieving universal services and routers are used to connect different networks. TCP is one of the major transport protocol used in TCP/IP protocol suit which provides reliable data delivery service to applications.

Suppose two applications use TCP to establish a connection, communicate, terminate the connection, and then open a new connection. Further suppose a FIN message sent to shut down the first connection is duplicated and delayed until the second connection has been established”

Requirements:
a.     If a copy of the old FIN is delivered, will TCP terminate the new connection? Why or why not?
b.     Can a router send request to another router for TCP connection? Give reason.

Answer: No
The message for the new TCP connection would be required to have identical source & destination IP addresses, protocol port numbers and TCP sequence numbers for the earlier message to have an effect.
The host waits 2 * max_segment_life time before reallocating the port from the old connection.
Most of the s/w implementation of TCP allocate sequence number depending upon the system clock time, which will be different for two subsequent messages.

Suppose two programs use TCP to establish a connection, communicate, terminate the connection, and then open a new connection. Further suppose a FIN message sent to shut down the first connection is duplicated and delayed until the second connection has been established. If a copy of the old FIN is delivered, will TCP terminate the new connection? Why?
Answer: 
TCP will not terminate the new connection since each connection will have a unique sequence number to be distinguished from other connections.

Ans to part 1:
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TCP uses a port. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number. The port number, added to a computer's IP address, completes the destination address for a communications session. That is, data packets are routed across the network to a specific destination IP address, and then, upon reaching the destination computer, are further routed to the specific process bound to the destination port number.
So,a copy of the old FIN will not be able to terminate the new connection as port number in new connection will be different.

Ans to part 2:
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 No, a router can not send request to another router for TCP connection?
Reason:TCP connections are for communication between applications but routers do not run applications.