Transport (TCP/UDP)

Vocabulary

English	Meaning
UDP/ˌjuː diː ˈpiː/	A transport protocol providing datagrams and ports without guaranteeing delivery or order.
TCP/ˌtiː siː ˈpiː/	A transport protocol providing a reliable ordered byte stream with flow and congestion control.
three-way handshake	The SYN, SYN-ACK and ACK exchange normally used to establish a TCP connection.

Two postcards and a courier

UDP: 8-byte header — ports, length, checksum — a postcard. TCP: sequence numbers, acknowledgements, retransmission, flow and congestion control — a courier with a ledger.
The three-way handshake (SYN, SYN-ACK, ACK) opens a connection; FINs close it.

{exercise:uni-cs0904-two}

Practice

The UDP header carries —

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SYN consumes sequence 0; the server SYN-ACK carries its own 0. Client sends 100 bytes: seq 1, server ACKs 101. Sends 50: seq 101, ACK 151.
A 30-byte segment at seq 151 is lost: the cumulative ACK stays 151 until the timeout retransmits — then ACK 181. One ACK number summarises the whole receive state.

{exercise:uni-cs0904-ledger}

Practice

After SYN (seq 0), 100 bytes then 50 sent, the server ACKs —

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DNS fits in one UDP datagram: a request is one packet, and if it is lost the client re-asks. Video calls tolerate loss but not delay: a retransmitted frame due in 30 ms arrives too late to matter.
TCP guarantees order and delivery, never latency; flow control protects the receiver's buffer, congestion control protects the network — two different windows.

{exercise:uni-cs0904-choice}

Practice

A live video call prefers —

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