Question

Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size 1,200 bytes and a destination host address from the transport layer. The network layer then guarantees to deliver the segment to the transport layer at the destination host. Suppose many network application processes can be running at the destination host. a. Design the simplest possible transport-layer protocol that will get application data to the desired process at the destination host. Assume the operating system in the destination host has assigned a 4-byte port number to each running application process. b. Modify this protocol so that it provides a "return address" to the destination process. c. In your protocols, does the transport layer "have to do anything" in the core of the computer network?

Short answer

Minimal protocol includes destination and optional source ports. Transport layer does not interact with network core.

Step-by-step solution

Understand the Environment

Here, the network layer guarantees delivery of a 1200-byte segment to the transport layer at the destination host. The network layer handles addressing between hosts, but we need to design how the transport layer functions between applications (processes) at the destination.

Design a Basic Transport Protocol

To get data to the correct application, include the 4-byte destination port number (assigned by the operating system) in the transport layer header. This header encapsulates the data, specifying which application at the destination will receive it. The basic transport protocol requires only one field in the header: the destination port number.

Modify the Protocol for Return Addressing

To provide a 'return address', include an additional field in the transport layer header: the source port number (also 4 bytes). This allows the destination process to reply to the correct process at the source host, by sending responses back to this source port number.

Consider Network Core Involvement

The transport layer does not interact with the core of the computer network. Since the network layer guarantees delivery between hosts regardless of the network's internal workings, the transport layer manages communication between applications on those hosts without altering core network functions.

Key concepts

Port Number

A port number is like a mailbox number that ensures the right letters (or data) reach the right people (or applications). When a network host receives data, this data is intended for a specific application running on that host.
The port number helps guide the data to the correct application process.
Each application process is assigned a unique 4-byte port number by the operating system.

• The port number is used by the transport layer to direct incoming data to the correct application.
• This number acts as an identifier to distinguish different services on a host.
• Common applications have standardized port numbers (e.g., HTTP often uses port 80).

If you've ever seen ports like 80 or 443, these are just standard identifiers enabling systems to recognize what service to connect to, making data transfer seamless.

Network Layer

The network layer acts as the mailman of a network, ensuring your data reaches its intended address or destination.
It handles key responsibilities such as routing the data across different network devices.
Just like how a neighborhood map guides a mail truck, the network layer utilizes addresses to guide data.

• Responsible for end-to-end packet delivery across different networks.
• Takes care of logical addressing through IP addresses.
• Important in path determination and switching.

In the network hierarchy, it functions like the transport system, moving information between devices, while ensuring data packets correctly find their way from the source to the destination.

Application Processes

Application processes are the very reason networks exist — to allow software and applications to communicate across devices. Think of them as the programs or services that perform tasks, such as web browsers or email clients.
Each process is associated with specific port numbers to manage network communications.

• They run actively on devices, performing tasks users typically request.
• Each process listens on a specific port, waiting for communication or data.
• Applications can be categorized into client processes (requesting data) and server processes (providing data).

Within the network structure, application processes depend on the transport layer to effectively receive and send data to their target applications using the defined port numbers.

Network Host

A network host is like your home address in the digital world. It's any computer or device connected to a network that can send and receive data.
Every host, be it a server, personal computer, or smartphone, is capable of running multiple applications simultaneously.

• Each host is identified by a unique IP address to differentiate it from others on the network.
• Hosts communicate on a network using logical addresses (IP) and physical addresses (MAC).
• Depending on their role, hosts can be clients, servers, or both.

Think of network hosts as the endpoints in a network chain that exchange data and perform internet tasks, forming the backbone of our interconnected world.