Question

What are the two most important network-layer functions in a datagram network? What are the three most important network-layer functions in a virtualcircuit network?

Short answer

In datagram networks: routing and forwarding. In virtual-circuit networks: connection setup, data transfer, and connection teardown.

Step-by-step solution

Understanding Datagram Networks

In a datagram network, each packet is treated independently and the network doesn't need to establish a connection beforehand. Key functions are determining the route and forwarding the packet efficiently along that route.

Identifying Key Functions in Datagram Networks

The two most important network-layer functions in a datagram network are: 1) Routing, which involves determining the optimal path each packet must take from the source to the destination. 2) Forwarding, which is the process of moving packets from an incoming link to an appropriate outgoing link based on the routing decision.

Understanding Virtual-Circuit Networks

In a virtual-circuit network, a connection is established between the sender and receiver before data is transmitted. Packets follow this established route, ensuring a predetermined path for communication.

Identifying Key Functions in Virtual-Circuit Networks

The three most important network-layer functions in a virtual-circuit network are: 1) Connection setup, which establishes the path through the network before any packets are sent, 2) Data transfer, which refers to the actual movement of data along the established path, and 3) Connection teardown, which involves ending the connection and freeing associated resources after data transmission is complete.

Key concepts

Datagram Networks

A datagram network is similar to how you might send a series of letters through postal mail. Each letter travels independently, finding its own path to the destination. There is no pre-established route, which means datagram networks treat each packet on its own. This offers flexibility, as each packet can take a different path to avoid congestion. However, it may result in packets arriving out of order.

Two critical functions of the network layer in datagram networks are:

• Routing: This is akin to providing each packet with a map. Routing involves deciding the optimal path through the network to reach its destination. The path is chosen dynamically, meaning it's determined on-the-fly for each packet.
• Forwarding: Once a packet arrives at a router, forwarding ensures that the packet moves along the determined path. It involves taking the packet from the incoming link and sending it to the correct outgoing link based on the routing decision.

These functions work together to deliver packets efficiently, although the lack of a predetermined path can lead to variability in delivery times.

Virtual-Circuit Networks

Virtual-circuit networks function more like a train where a specific track or path is reserved in advance. Before data is sent, a path is established between the sender and receiver. This creates a "virtual circuit" which all packets follow, ensuring that they arrive in order.

In virtual-circuit networks, three key network-layer functions are:

• Connection setup: Before sending packets, a dedicated circuit is established through the network routers. This involves negotiating and reserving resources along the path.
• Data transfer: After the path is established, packets are sent along this pre-defined route. This facilitates organized transmission, reducing uncertainty as packets follow the same sequence.
• Connection teardown: Once the data transfer is complete, the path is dismantled. Resources allocated for the virtual circuit are freed up and can be used for other connections.

This system can improve reliability and consistency but requires time to set up and tear down connections.

Routing

Routing plays a pivotal role in both datagram and virtual-circuit networks. It's the method of determining the path that packets need to take to reach their destination. Think of it as planning a road trip; different routes may need to be chosen depending on traffic and road closures.

In datagram networks, routing decisions are made independently for each packet. This allows networks to respond to changes, such as congestion or failures, dynamically. However, this dynamic nature means that there's no guarantee the packets will arrive in order.

In contrast, virtual-circuit networks establish a fixed path for data to follow, providing a stable and predictable route. Routing is done during the connection setup phase, ensuring that each packet follows the same path, arriving in sequence. This method can help manage network traffic and ensure smooth data flows in large and complex networks.

Forwarding

Forwarding is the process that keeps data moving through the network. It involves receiving a packet on one link and sending it out on another toward its destination. Think of it as a junction where packets are directed through different lanes based on their destination.

• In Datagram Networks: Each packet is forwarded independently. Routers use the destination address to determine the outgoing link, considering the best available path at that moment.
• In Virtual-Circuit Networks: Forwarding is a little different as the path is predefined during the connection setup. Routers forward packets along the established circuit, ensuring all packets follow the same route, maintaining order and reliability.

This forwarding mechanism is essential as it allows packets to traverse from the source to the destination while using the available network infrastructure efficiently. It ensures that data reaches its destination, whether through dynamic routing of datagram networks or the established path of virtual circuits.