Q2(b) · Telnet connection ports

IP delivers a packet toward a host interface. The transport layer delivers application data to a process endpoint, identified using a port. A socket endpoint is commonly described by IP address and port; the protocol also matters. TCP distinguishes connections by source IP/port and destination IP/port.

Swipe across the diagram to read its labels ↔

Diagram: Transport: from hosts to application processes. Labels: Transport: from hosts to application processes | Transport: from hosts to application processes | Host address selects the machine/interface | Example destination: 198.51.100.20. | Protocol + destination port select service | TCP port 443 and UDP port 53 identify different transport endpoints. | Demultiplex to the right socket | TCP clients share server port 443; their address/port tuples distinguish the connections.

1. At the sender, transport multiplexing accepts data from several application sockets and adds the appropriate header.
2. A server normally listens on a known service port; a client commonly chooses a temporary local port.
3. At the receiver, demultiplexing uses protocol/header information to deliver data to the appropriate socket.
4. A TCP connection is bidirectional and maintains state. Two clients can contact the same server port because their connection tuples differ. UDP demultiplexing is connectionless at the protocol level; applications can still keep their own state.