Question

1. Describe the service TCP provides with respect to flow control and explain the mechanisms it uses to overcome the deficiencies of IP.

Answer 1

TCP provides a reliable communication channel over an unreliable network.When we send data from one system to another, packets may be lost, they may be transmitted out of order, the network can be congested or the receiver end can be overloaded. While establishing a TCP connection with our software,we usually don’t need to deal with this complexity, we just write some data to a socket and TCP makes sure the packets are delivered correctly to the receiver node.  

• In TCP, the flow control ensures that the sender is not distressing the receiver by sending packets faster than its limit.Flow control is like the congestion control over the receiving end.
• TCP ensures that the data packets delivered are in the exact order as they are sent from the sender end.TCP makes it possible with the help of send buffer and receive buffer.
• TCP uses the Receive Window (rwnd) to control the amount of data that can be delivered from the send buffer.
• TCP provides the ‘ack’ message concept, to acknowledge the sender that it receives the packet correctly and also the value of current window receive window in order to let the send buffer know whether or not it can send the next packet or not.
• TCP uses the sliding window protocol to control the number of bytes it can have in one packet i.e. it can easily understand the number of packets it has to send to empty the send buffer.
• Deadlock situations like one when the zero window situation or the ack message is lost, TCP enables the persist timer, which sends the ‘WindowProbe’ message in order to check the availability of the receiving window to receive the packets.

The basic deficiencies (or limitations ) of the IP are :

• No Error Reporting : IP does not have error reporting mechanisms with it. For example, Suppose that the lifetime of a packet is completed while traveling on the Internet and in time, the value of the Time to Live (TIL) field in the packet becomes 0, then in this situation, the packet will be discarded. But there is no mechanism for the receiver to tell the sender that the following packet has been lost. TCP handles such errors by sending the ‘ack’ message mechanism with a closed socket to the sender. Thus, sender resends the message until it receives a success message.
• No Communication : In Internet Protocol, there is no way to establish the communication between the two devices, whenever the two devices need to communicate, say for the situation when one device needs to check whether or not the device is alive. At the transport layer, TCP uses the network socket interface(NSI) to handle all handshaking and transmission details and present an abstraction of the network connection to the application.