Transport Layer – TCP & UDP Comprehensive Notes

Transport Layer: Scope

• Responsible for process-to-process (end-to-end) delivery of message segments.

• Operates above the network layer (host-to-host delivery).

Protocols at the Transport Layer

• Main TCP/IP protocols are:

  • TCP (Transmission Control Protocol): reliable, connection-oriented.

  • UDP (User Datagram Protocol): best-effort, connectionless.

TCP: Purpose & Philosophy

• Defined in RFC 793.

• Adds reliability and connection semantics to IP.

• Guarantees in-order, lossless, non-duplicated, and uncorrupted byte stream delivery; informs sender if not.

Core TCP Facilities

• Stream Data Transfer: Divides byte streams into segments, each byte numbered by a Sequence Number.

• Full-Duplex Operation: Bidirectional simultaneously.

• Multiplexing / Demultiplexing: Uses port numbers (16-bit) to identify processes. A full TCP connection is uniquely identified by $\langle \text{TCP},\,IP<em>S,\,Port</em>S,\,IP<em>D,\,Port</em>D \rangle$.

  • Well-known ports: $0\text{–}1023$.

• Logical Connections: Maintains state variables per stream.

• Reliability: Uses Positive ACK with Retransmission (retransmits on timer timeout) and Cumulative ACK (ACK value = next expected byte).

• Flow Control: Via Sliding Window, where the receiver advertises its window size (bytes it can accept).

TCP Segment Format

• Header size: $20\text{–}60$ bytes ($4 \times \text{HLEN}$ bytes).

• Key fields include Source/Destination Port, Sequence Number, Acknowledgment Number, Flags (URG, ACK, PSH, RST, SYN, FIN, CWR, ECE), Window, Checksum, Urgent Pointer.

TCP Connection Management

• Establishment: Three-way handshake (SYN, SYN+ACK, ACK).

• Termination: Four-step process (FIN, ACK, FIN, ACK).

UDP: Characteristics

• Defined in RFC 768.

• Connection-less, unreliable, no handshake, minimal 8-byte header.

• Suitable for real-time applications (VoIP, streaming) where retransmission is counterproductive.

UDP Datagram Format

• Header: Source Port, Destination Port, Length, Checksum.

• Maximum payload: $65507$ bytes.

Comparative Summary: TCP vs UDP

Ethical & Practical Considerations

• Protocol choice impacts network load.

• Security: Open ports require firewalling.

Key Numbers

• Port range: $0\text{–}65535$. Max UDP data: $65507$ bytes. TCP header: $20\text{–}60$ bytes.