Datagram Approach and Virtual-Circuit Approach

Definition: Packet-switched networks can deliver packets using either a datagram (connectionless) approach or a virtual-circuit (connection-oriented) approach.

1. Datagram approach

Each packet is treated independently. No connection setup is required, each packet carries the destination address, and routers make forwarding decisions packet by packet. Packets may take different routes and may arrive out of order.

• Setup: Not required.
• Routing: Decided independently for each packet.
• Addressing: Full destination address in each packet.
• Router state: No per-connection virtual-circuit state.
• Ordering: Not guaranteed.
• Example: IP networks use datagram packet delivery.

2. Virtual-circuit approach

A logical path is established before data transfer. Routers keep forwarding-table entries, and packets carry a virtual-circuit identifier (VCI). Packets normally follow the established path; the connection is released when communication ends.

Phases:

1. Setup: Choose a path and install forwarding entries at the routers.
2. Data transfer: Forward packets using the VCI and established path.
3. Teardown: Remove the virtual-circuit entries and release resources.

Comparison

Aspect	Datagram	Virtual circuit
Connection setup	Not required	Required
Routing	Per packet	Path established before transfer
Packet header	Full destination address	VCI/label
Router state	No per-connection VC state	Per-connection forwarding state
Packet order	May be out of order	Usually maintained along the path
Failure handling	Packets may be rerouted	Circuit may need re-establishment
Examples	IP datagram delivery	X.25, ATM; MPLS uses label-switched paths

Conclusion: Datagram delivery offers flexibility without connection setup. Virtual circuits require setup and router state but provide a logical path and simpler per-packet identifiers.