Essential Concepts - Network Packets

We have seen the topics that gives us the answers of the below questions :

• How a server is uniquely identified? -- IP Address
• What are the services provided between two servers? -- Protocols
• How to connect for a specific service? -- Ports

There are many other questions, one of which is : How the data is being transferred between two servers?

Consider that you have to transfer a PPT containing 15 slides froam a server to another. But the bandwidth is only of 5 slides. Now you divide the PPT into 3 PPTs each containing 5 slides and transfer them in a sequence. Also these PPTs are received in the same sequence at the receiving end and then combined to form the final PPT. Here, each PPT is called a packet and the final PPT is called data.

Packets and Packet Switching :

It turns out that everything you do on the Internet involves packets. For example, every Web page that you receive comes as a series of packets, and every e-mail you send leaves as a series of packets. Networks that ship data around in small packets are called packet switched networks. Packet is nothing but a part of divided data.

Packet switching is a digital networking communications method that groups all transmitted data into suitably sized blocks, called packets, which are transmitted via a medium that may be shared by multiple simultaneous communication sessions. Packet switching increases network efficiency, robustness and enables technological convergence of many applications operating on the same network.

A network packet (data packet, datagram or simply called as packet) is a basic unit of data sent from one host to another over a network. When data (such as a mail, message or a file) has to be transmitted between two hosts, it is fragmented into small structures called packets and are reassembled at the destination to make the original data chunk.

On the Internet, the network breaks an e-mail message into parts of a certain size in bytes. These are the packets. Each packet carries the information that will help it get to its destination -- the sender's IP address, the intended receiver's IP address, something that tells the network how many packets this e-mail message has been broken into and the number of this particular packet. The packets carry the data in the protocols that the Internet uses: Transmission Control Protocol/Internet Protocol (TCP/IP). Each packet contains part of the body of your message. A typical packet contains perhaps 1,000 or 1,500 bytes.

Each packet is then sent off to its destination by the best available route -- a route that might be taken by all the other packets in the message or by none of the other packets in the message. This makes the network more efficient. In short, all the packets travels through a common route in a sequence.

Parts of a single packet :

Most network packets are split into three parts :-

Header - The header contains instructions about the data carried by the packet. These instructions may include:

• Length of packet (some networks have fixed-length packets, while others rely on the header to contain this information)
• Synchronization (a few bits that help the packet match up to the network)
• Packet number (which packet this is in a sequence of packets)
• Protocol (on networks that carry multiple types of information, the protocol defines what type of packet is being transmitted: e-mail, Web page, streaming video)
• Destination address (where the packet is going)
• Originating address (where the packet came from)

Payload - Also called the body or data of a packet. This is the actual data that the packet is delivering to the destination. If a packet is fixed-length, then the payload may be padded with blank information to make it the right size.

 

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. It may also have some type of error checking.

The most common error checking used in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it works in certain computer networks: It takes the sum of all the 1s in the payload and adds them together. The result is stored as a hexadecimal value in the trailer. The receiving device adds up the 1s in the payload and compares the result to the value stored in the trailer. If the values match, the packet is good. But if the values do not match, the receiving device sends a request to the originating device to resend the packet.

­As an example, let's look at how an e-mail message might get broken into packets. Let's say that you send an e-mail to a friend. The e-mail is about 3,500 bits (3.5 kilobits) in size. The network you send it over uses fixed-length packets of 1,024 bits (1 kilobit). Let. The header of each packet is 96 bits long and the trailer is 32 bits long, leaving 896 bits for the payload. To break the 3,500 bits of message into packets, you will need four packets (divide 3,500 by 896). Three packets will contain 896 bits of payload and the fourth will have 812 bits. Here is what one of the four packets would contain:

• Each packet's header will contain the proper protocols, the originating address (the IP address of your computer), the destination address (the IP address of the computer where you are sending the e-mail) and the packet number (1, 2, 3 or 4 since there are 4 packets).
• Routers in the network will look at the destination address in the header and compare it to their lookup table to find out where to send the packet.
• Once the packet arrives at its destination, your friend's computer will strip the header and trailer off each packet and reassemble the e-mail based on the numbered sequence of the packets.