Reliable communication at Layer 4

Reliable communication at Transport Layer

IP service model is the best effort delivery model. This clearly indicates that IP (Internet Protocol) makes its best efforts in order to deliver data between communicating hosts but it does not gives any kind of guarantee.  IP model therefore, does not guarantee orderly delivery of segments / data and it does not guarantee integrity of data in the segments.

A transport layer protocols provide a reliable data transfer to the application layer even though the network through which the data is travelling, is unreliable that is, network protocol losses, garbles and duplicate packets.

Transmission control protocol allows TCP connections for communication between hosts traversing through a congested network to share the link bandwidth equally. Whereas, UDP (User Datagram Protocol) traffic is unregulated as UDP transport can send data at any data rate.

Extending host to host delivery to process to process delivery is called application multiplexing and demultiplexing.

Just to Remember: Segments are the data chunks, grouped at transport layer by the help of TCP protocol, and whereas data grouped at transport layer by UDP protocol is known as datagram.

For reliable transport layer function:

1. Feedback from the receiver is very important for the sender to re-transmit the packet.
2. Re-transmission of packet may also occur if the timer expires.
3. Corruption within the received packet.

CRC (Cyclic redundancy check)

Continuing point number 3 from above: What if the packet received at the other side is corrupted?

technique is used at the receiver end in order to make sure that the packet received are corrupted or not. Before transmitting a packet, the sender calculates a checksum, usually 16 or 32 bits, attaches that checksum in that packet and then transmits the packet. When the packet is received by the other end, it rechecks and calculates the checksum in order to see if there is an error in any bit or not. After comparing the checksum, if the bits were correct then an acknowledgement is sent by the receiver.

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