A communication method where a data stream is divided into smaller packets and transmitted independently through the network. More efficient than leased lines.
Each packet contains a header and a data payload.
Steps:
- Data is divided into packets.
- Each packet is transmitted independently.
- Routers forward packets hop by hop.
- Packets reassemble at the destination host.
Packets from the same data stream may follow different routes. Two approaches exist: connectionless and connection-oriented.
Connectionless
Packets transmitted without establishing a connection beforehand. Also called a datagram service.
Properties:
- Each packet carries the full destination address.
- Routing decisions are made independently per packet.
- No connection setup required.
- No router state per connection.
- No built-in error control or QoS guarantees.
- Node failures have minimal effect; packets are rerouted independently.
Forwarding
Switching a packet from an input line to an output line. Each router maintains a forwarding table mapping destination addresses to next hops.
Connection-Oriented
A virtual circuit is established between source and destination before any packets are transmitted. Also called a connection-oriented network.
Properties:
- Connection setup required before data transfer.
- All packets follow the same pre-determined path.
- Packets addressed by CID, not full destination address.
- Routers maintain per-connection state.
- Network may provide error control and QoS guarantees.
- Node failure terminates all virtual circuits through that node.
Connection Identifier
Aka. CID. A unique identifier assigned to each virtual circuit on a given link. Used in place of full destination addresses for faster forwarding.
Each router maintains a forwarding table mapping incoming CIDs to outgoing CIDs. Packets are forwarded based on CID lookup alone.