What Is a Packet?
In networking, a packet is a small piece of a larger message. Data sent over computer networks*, such as the Internet, is divided into packets. These packets are then reassembled by the computer or device that receives them.
Suppose Alice is writing a letter to Bob, but Bob’s mail slot is only wide enough to accept envelopes the size of small index cards. Rather than writing her letter on regular paper and then trying to stuff it through the mail slot, Alice divides her letter into much shorter sections, each consisting of a few words, and writes these sections on index cards. She delivers the stack of cards to Bob, who puts them together to read the full letter.
This is similar to how packets work on the Internet. Suppose a user needs to download an image. The image file does not travel from a web server to the user’s computer in one piece. Instead, it is split into data packets, sent across the wires, cables, and radio waves of the Internet, and then reassembled by the user’s computer into the original image.
* A network is a group of two or more connected computers. The Internet is a network of networks – multiple networks around the world all interconnected with each other.
Why Use Packets?
In theory, it might be possible to send files and data over the Internet without dividing them into small packets of information. One computer could send data to another computer in the form of a long, unbroken stream of bits (small units of information, transmitted as electrical pulses that computers can interpret).
However, this approach quickly becomes impractical when there is more than one pair of computers. While the long stream of bits passes through the wires between two computers, no third computer can use those same wires to send information – it would have to wait its turn.
Unlike this approach, the Internet is a “packet-switching” network. Packet switching refers to the ability of network equipment to process packets independently of one another. This also means that packets can take different network routes to the same destination, as long as they all arrive at the destination. (In certain protocols, packets need to arrive at their final destinations in the correct order, even if each packet took a different route to get there.)
Because of packet switching, packets from multiple computers can travel through the same wires in any order. This allows multiple communications to take place over the same network equipment at the same time. As a result, billions of devices can exchange data on the Internet simultaneously, rather than just a handful.
What Is a Packet Header?
A packet header is a “label” of sorts, which provides information about the packet’s contents, origin, and destination.
When Alice sends her stack of index cards to Bob, the words on those cards alone will not give Bob enough context to read the message correctly. Alice needs to indicate the order in which the index cards go so that Bob does not read them out of order. She also needs to indicate that each one is from her, in case Bob is receiving messages from other people while she is delivering them. So Alice adds this information to the top of each index card, above the actual words of her message. On the first card she writes “Letter from Alice, 1 of 20,” on the second she writes “Letter from Alice, 2 of 20,” and so on.
Alice has created a miniature header for her cards so that Bob does not lose or mix them up. Similarly, all network packets include a header so that the device receiving them knows where the packets come from, what they are intended for, and how to process them.
Packets consist of two parts: the header and the payload. The header contains information about the packet, such as the source and destination IP addresses (an IP address is like a mailing address for a computer). The payload is the actual data. Returning to the image example, each of the thousands of packets that make up the image contains a payload, and the payload carries a small part of the image.
Where Do Packet Headers Come From?
In practice, packets actually contain more than one header, and each header is used by a different part of the networking process. Packet headers are attached by certain types of network protocols.
A protocol is a standardized way of formatting data so that any computer can interpret the data. Many different protocols make the Internet work. Some of these protocols add headers to packets that contain information associated with that protocol. At minimum, most packets that cross the Internet will include a Transmission Control Protocol (TCP) header and an Internet Protocol (IP) header.
What Are Packet Trailers and Footers?
Packet headers are placed at the beginning of each packet. Routers, switches, computers, and anything else that processes or receives a packet will see the header first. A packet can also contain trailers and footers attached at the end. Like headers, these contain additional information about the packet.
Only some network protocols attach trailers or footers to packets; most only attach headers. ESP (part of the IPsec suite) is one example of a network layer protocol that attaches trailers to packets.
What Is an IP Packet?
IP (Internet Protocol) is a network layer protocol concerned with routing. It is used to ensure that packets reach the correct destination.
Packets are sometimes defined by the protocol they use. A packet that includes an IP header can be referred to as an “IP packet.” An IP header contains important information about where the packet is from (its source IP address), where it is going (the destination IP address), how large the packet is, and how long network routers should continue forwarding the packet before dropping it. It may also indicate whether the packet can be fragmented, and includes information about reassembling fragmented packets.
Packets vs. Datagrams
A “datagram” is a piece of data sent over a packet-switched network. A datagram contains enough information to be routed from its source to its destination. Under this definition, an IP packet is one example of a datagram. In essence, a datagram is an alternative term for “packet.”
Network traffic is a term referring to the packets passing through a network, in the same way that automotive traffic refers to the cars and trucks traveling on roads.
However, not all packets are good or useful, and not all network traffic is safe. Attackers can create malicious network traffic – data packets designed to threaten or overwhelm a network. This can take the form of a distributed denial-of-service (DDoS) attack, exploitation of security vulnerabilities, or several other forms of cyberattacks.
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