Computer Networks · TEE · papers
Q4(b) · TCP socket port numbers
2023-11-22 · AI-only Sem V variant; incomplete recovered scan
Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.
Q4(b). Clients A and B initiate Telnet sessions with server S. Give possible source/destination ports for B→S and S→B. If A and B are on the same host, can both connections use the same source port to the same server? Explain. [5 marks]
Answer
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.
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.
- At the sender, transport multiplexing accepts data from several application sockets and adds the appropriate header.
- A server normally listens on a known service port; a client commonly chooses a temporary local port.
- At the receiver, demultiplexing uses protocol/header information to deliver data to the appropriate socket.
- 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.
| Identifier | Purpose |
|---|---|
| MAC address | Delivery on a local link |
| IP address | Network-layer interface/addressing |
| Port number | Transport-layer application endpoint |
| Domain name | Application-level naming resolved through DNS |
Yaad rakho: IP host tak; port process tak.
| Direction | Source port | Destination port |
|---|---|---|
| Client→server | Example49152 | 23 (Telnet) |
| Server→client | 23 | 49152 |
Different simultaneous connections from the same client IP to the same server IP and port23 require different client ports, e.g.49152 and49153. Otherwise the TCP four-tuples would be identical.