Tcp Tutorial-3

Packet Sequencing-: All data being transferred on the net is broken down into packets at the source and joined together at the destination. The data is broken down into packets in a particular sequence at the source. This means that, for example, the first byte has the first sequence number and the second byte the second sequence number and so on. These packets are free to travel independently on the net, so sometimes, when the data packets reach the destination they arrive, out of sequence, which means that the

defines the order in which the hosts receive the data packets or messages. The application or the layer

running at the destination automatically builds up the data from the sequence number in each packet. The source system breaks the data to be transferred into smaller packets and assigns each packet a unique sequence number. When the destination gets the packets, it's starts rearranging the packets by reading the sequence numbers of each packet to make the data received usable.

For example, say you want to transfer a 18000 octet file. Not all networks can handle the entire 18000 octet packets at a time. So the huge file is broken down into smaller say 300 octet packets. Each packet has been assigned a unique sequence number. Now when the packets reach the destination the packets are put back together to get the usable data. Now during the transportation process, as the packets can move independently on the net, it is possible that the packet 5 will arrive at the destination before packet 4 arrives. In such a situation, the sequence numbers are used by the destination to rearrange the data packets in such a way that even if Data packet 5 arrived earlier, Packet 4 will always precede Packet 5.

A data can easily be corrupted while it is being transferred from the source to the destination. Now if a error control service is running then if it detects data corruption, then it asks the source to re-send the packets of data. Thus only non corrupted data reaches the destination. An error control service detects and controls the same two types of errors-:

1.) Data Loss 2.) Data Corruption

The Checksum values are used to detect if the data has been modified or corrupted during the transfer from source to destination or any corruption in the communication channel which may have caused data loss. Data Corruption is detected by the Checksum Values and by performing Cyclic Redundancy Checks



hence are rarely used.