Data into Small Packets

Data Packets:

We need to send messages from one end to another. A big file cannot be transmit. So we need to divide data into packets of fixed or variable size. The size of the packet is determined by the network and the governing protocol.

Depending on Layer and the type of network, packets may be referred to by another name 
* Segment (Transport Layer) 
* Datagram (Network Layer) 
* Frame (Datalink Layer)

Most packets are split into three parts:

1. Header - The header contains information. Such as source and destination address.
2. Payload - Also called the body or data of a packet. This is the actual data that the packet is delivering to the destination.
3. Trailer - The trailer, sometimes called the footer, typically contains a couple of bits that tell the receiving device that it has reached the end of the packet.

For an example, you send an email to a friend. The e-mail is about 3,018 Byte in size. According to IMP in 1969 (RFC7), the maximum massage length 8080 bits or 1010 Bytes. In this massage header and marking is 32 bits or 4 Byte. So data should be 1006 Bytes. Then it will make 3018 Byte mail into 3 packets.

This problem was solved. We can send data as packets. But really data reached at destination properly? Data can be corrupted or data can be lost on the way. So receiver may not get actual data. This is our 3^rd problem. For an Example

I sent - I have a dog

He/She received
One packet corrupted - I have a dgo
Or packet Lost - I a dog

This job is also done in transport layer. So all began to make the protocol that provides reliable data transfer. Few protocols are

* Stop & Wait
* Go back N
* Selective Repeat

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