Question

Suppose end system A wants to send a large file to end system B. At a very high level, describe how end system A creates packets from the file. When one of these packets arrives to a packet switch, what information in the packet does the switch use to determine the link onto which the packet is forwarded? Why is packet switching in the Internet analogous to driving from one city to another and asking directions along the way?

Short answer

End system A breaks the file into packets, which are routed based on destination IP addresses in their headers. Packet switching in the Internet is like asking directions at each road intersection while traveling from city to city.

Step-by-step solution

Fragmenting the File into Packets

End system A breaks the large file into smaller, manageable packets. Each of these packets includes a portion of the actual file data and additional metadata, such as the packet sequence number, which helps assemble the packets correctly at the destination.

Packet Header Information

Each packet is assigned a header that contains essential routing information. Crucial fields in the header include the destination IP address, source IP address, and other protocol-specific information. This header guides the packet as it navigates through the network to reach end system B.

Routing via Packet Switch

Upon reaching a packet switch (router), the switch examines the destination IP address in the packet header. The switch uses this information along with its routing table to decide on the best next link for the packet to reach its final destination efficiently.

Analogous to Driving

Packet switching is analogous to driving from one city to another because, like a traveler who decides directions at each intersection based on signs or asking locals, each packet independently makes decisions at each hop. Routers act like traffic signs or locals giving directions, guiding packets to their next hop without a preconceived complete path laid out.

Key concepts

End Systems

In a computer network, end systems are devices where communication originates and terminates. These devices can be anything from computers, smartphones, to servers, and they play a crucial role in data transmission. The primary function of an end system is to send and receive data across a network. Each end system is identified by a unique IP address, which acts as a personal address, making sure data reaches the correct destination.

An end system involved in transmitting a file begins by breaking down the file into smaller, manageable packets. This process of fragmenting ensures that the network can efficiently handle data transfer, accommodating its capacity and constraints. The packets not only include pieces of the file but also metadata, such as sequence numbers, ensuring that the end system reassembles them correctly.

This process of creating packets and adding necessary information is crucial for the successful, error-free transmission of data from one end system to another. Without effective end systems, communication over networks wouldn't be feasible.

IP Addressing

Every device connected to a network is assigned a unique identifier known as an IP address. This address is akin to a mailing address for a house, guiding data packets from the sender to the recipient. An IP address ensures that the packets reach the intended end system without getting lost in the vast network of interconnections.

The IP address is a fundamental aspect of your packet header. Each packet contains the source IP address, stating who sent it, and the destination IP address, indicating where it should go. This addressing system is essential, as it allows routers and other network devices to forward packets toward their destination using predesignated paths.

Currently, there are two main versions of IP addresses: IPv4 and IPv6. While IPv4 uses a 32-bit address scheme, already reaching saturation, IPv6 was developed with 128-bit addresses to provide a more extensive pool of addresses, addressing the limitations in IPv4.

Routing

Routing is the process of directing data packets along pathways in a network, ensuring they reach their designated endpoint efficiently. When a packet reaches a packet switch, or router, it must decide where to send the packet next.

Routers use routing tables, essentially maps of the network, to make these decisions. The destination IP address embedded in the packet header is pivotal in this process. By consulting the routing table, the router can determine the most efficient path to the packet's final destination.

This decision-making process can be seen as a dynamic journey rather than a rigid predetermined path. Just like a traveler adjusting their route based on current traffic conditions and road signs, routers choose paths based on network conditions, making packet switches a flexible and adaptable means of data transmission.

Network Protocols

Network protocols are the agreed-upon rules and conventions that allow devices to communicate over a network. Without protocols, the vast diversity of devices and technologies would find it impossible to interact with one another.

Major network protocols form the foundation of the internet, ensuring that data is transmitted reliably and securely. Common protocols include TCP/IP, which governs how data is packaged and addressed, and HTTP/HTTPS, which rules how web data is transferred and presented to users.

TCP, or Transmission Control Protocol, is particularly crucial as it ensures that data packets arrive at their destination intact and in order. This protocol helps manage the flow of packets and initiates error checking, enhancing the reliability of data transmission. Understanding and mastering these protocols is vital for anyone diving into computer networks, as they are the backbone of efficient communication.