Question

Do the routers in both datagram networks and virtual-circuit networks use forwarding tables? If so, describe the forwarding tables for both classes of networks.

Short answer

Yes, both networks use forwarding tables; datagram networks use them for independent packet forwarding, while virtual-circuit networks use them to map circuit identifiers to next hops.

Step-by-step solution

Understanding Forwarding in Networks

In data communication, forwarding tables are used by routers to decide the direction that packets should take to reach their destination. These tables are essential components of both datagram (also known as connectionless) networks and virtual-circuit (connection-oriented) networks.

Forwarding Tables in Datagram Networks

In datagram networks, each packet is routed independently, and a forwarding table is used to determine the next hop for each packet based on the destination address. These tables are typically organized with destination IP addresses and associated next-hop addresses, allowing routers to forward packets toward their destination without prior knowledge of connection states or sequences.

Forwarding Tables in Virtual-Circuit Networks

In virtual-circuit networks, forwarding tables are used differently from datagram networks. When a virtual circuit is established, routers maintain forwarding tables that map a unique virtual circuit identifier (VCI) to the corresponding next-hop router and the output line. This means that once the initial setup is completed, packets are forwarded based on their VCI rather than their destination address, which helps manage connections more efficiently.

Key concepts

Datagram Networks

In datagram networks, routers handle packets individually. Each packet is treated independently and might take a different path to reach the destination. This kind of network is often referred to as a "connectionless" network which means that no pre-established path exists for data. The forwarding decision for each packet is made based on the packet’s destination address, which the router looks up in its forwarding table.

The forwarding table in a datagram network is organized with destination address prefixes mapped to the corresponding next-hop addresses. Because packet paths are dynamic and based on current network conditions, datagram forwarding tables need regular updates to reflect network topology changes efficiently.

• Each packet includes its own destination information.
• Routers use destination IP addresses for forwarding decisions.
• Path taken by packets can vary from one packet to another.
• No need for set-up before sending data.

Because of its flexibility, datagram networking is commonly used in the Internet today, where it supports scalability and robustness.

Virtual-Circuit Networks

Virtual-circuit networks operate differently because they use a dedicated circuit path for each session. When data is to be sent, a virtual circuit is first established across the network. This setup involves allocating a path and reserving resources for specific connections.

Routers in virtual-circuit networks make use of forwarding tables to map virtual circuit identifiers (VCIs) to their next hop and output line. This means that once a connection is established, every packet that is part of that circuit follows the same fixed path.

• Needs connection setup before data can begin transmission.
• Forwarding decisions are based on the virtual circuit identifier (VCI).
• All packets follow the same predetermined path.
• Ideal for stable connections where sequence and reliability are critical.

Virtual-circuit networks thus provide a more consistent route and can guarantee quality of service (QoS) better than datagram networks, making them suitable for telecommunications and private corporate networks.

Forwarding Tables

Forwarding tables are crucial for the functioning of both datagram and virtual-circuit networks, albeit they serve slightly different roles in each.

In datagram networks, forwarding tables map destination IP addresses to the next-hop IP address. This allows routers to quickly determine where to send a packet next, based simply on its destination information.

On the other hand, in virtual-circuit networks, forwarding tables map VCIs to output ports and the next-hop VCI. This facilitates a streamlined packet forwarding process since each VCI is associated with a predefined path.

• In datagram networks, tables are concerned mainly with destination addresses.
• In virtual-circuit networks, they deal with mapping VCIs to output lines.
• Tables may often require updates in datagram networks due to changing network paths.
• Virtual-circuit forwarding tables are mainly updated during connection setup and teardown.

In summary, forwarding tables act as the organizational backbone in networks, guiding packets efficiently to their destinations by maintaining critical mapping information.