Transport Protocols

Computer Network & Network Security System — Transport Layer, NEC licence examination syllabus (Nepal Engineering Council).

Transport Protocols: TCP and UDP

Two protocols at the same layer, because reliability is a cost — and not every application wants to pay it.

The layer below delivers packets between machines and makes no promises: packets may be lost, duplicated, or arrive out of order. The transport layer decides what to do about that, and TCP and UDP represent the two honest answers — fix it, or don't pretend to.

TCP — reliability, and what it costs

🤝 TCP — Transmission Control Protocol

Connection-orientedA handshake before any data, so both sides agree on starting sequence numbers and window sizes.

ReliableEvery byte is numbered and acknowledged; anything unacknowledged is retransmitted.

OrderedSequence numbers let the receiver reassemble in order regardless of arrival order.

Flow controlledThe receiver advertises how much buffer it has, so a fast sender cannot overrun a slow receiver.

Congestion controlledThe sender also backs off when the NETWORK is struggling, which is a different problem from the receiver struggling.

Flow control and congestion control are constantly confused, and the distinction is a favourite exam question. Flow control protects the receiver — it is advertised by the receiver in every acknowledgement. Congestion control protects the network in between — nobody advertises it, so the sender has to infer it from losses. Two different problems, two different mechanisms, both present in TCP.

The TCP segment header

TCP header — the fields worth being able to name: Source port 16 bits who sent it Destination port 16 bits which process receives it Sequence number 32 bits position of the FIRST byte in this segment Acknowledgement no. 32 bits the next byte expected FROM the other side Header length 4 bits because options make the header variable Flags (SYN ACK FIN RST PSH URG) control bits Window size 16 bits receiver's free buffer — this IS flow control Checksum 16 bits error detection over header and data Urgent pointer 16 bits Minimum header 20 bytes, more with options.

Two details carry most of the marks. The sequence number counts bytes, not segments — which is why TCP can retransmit part of a segment or coalesce several. And the acknowledgement number is the next byte expected, not the last byte received, so an acknowledgement of 5001 means "I have everything up to 5000, send 5001 next".

UDP — no promises, and a much smaller header

⚡ UDP — User Datagram Protocol

ConnectionlessNo handshake. The first packet carries data.

UnreliableNo acknowledgements, no retransmission, no ordering.

HeaderSource port, destination port, length, checksum — 8 bytes total.

🔧 UDP is not TCP with the features broken. It is the right choice whenever a late packet is worse than a missing one. In a voice call, retransmitting a lost 20-millisecond fragment means it arrives after the moment it belonged to — so TCP would faithfully deliver something already useless while stalling everything behind it. DNS uses UDP for a different reason: a query and its reply fit in one packet each, so a three-packet handshake would triple the cost of a one-packet exchange.

Choosing between them

Use TCP when: every byte matters and arriving late is acceptable → file transfer, web pages, email Use UDP when: arriving late is worse than not arriving → live audio and video, gaming or the exchange is a single short round trip → DNS Note: an application CAN build reliability on UDP itself, and several do — the point is that it gets to choose which guarantees it pays for, instead of taking all of them.

Commonly confused

Reliable and fast. TCP is not slower per packet. It is slower to recover, because recovery means waiting for a retransmission.

Connectionless and unreliable. Related but distinct: connectionless means no setup, unreliable means no guarantee. UDP is both; the terms are not synonyms.

UDP has no checksum. It does. What it lacks is anything to do about a failed one, so a corrupt datagram is simply discarded.

Rapid revision

TCP — connection-oriented, reliable, ordered, flow and congestion controlled, 20-byte minimum header.

UDP — connectionless, unreliable, 8-byte header, four fields.

Sequence numbers count bytes; the acknowledgement number is the next byte expected.

Flow control protects the receiver; congestion control protects the network.

Syllabus points

TCP: features, segment format
UDP: features, datagram format
TCP vs UDP

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