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Transport Layer

Short Notes

The Transport Layer provides end-to-end communication between processes on different hosts.

Major Protocols

  • TCP: Reliable, byte-stream, connection-oriented.
  • UDP: Unreliable, datagram, connectionless.

Key Theories & Formulas

1. Port Numbers

  • Standard range: 0-1023 (Well-known ports).
  • Total range: 0-65535 (16 bits).

2. Error and Flow Control

  • Uses checksums for error detection.
  • Uses sliding window for flow control.

Example Problems

Problem: Which protocol is used for real-time video streaming?

  • Result: UDP (due to low latency requirements).

Hardest GATE Questions

Topic: Multiplexing and Socket Addressing Tricky Question (GATE 2011/2015/2018): A host has one IP but multiple applications (HTTP, FTP, SMTP) running. How are incoming packets delivered to the correct application?

  • Analysis: Port Multiplexing. Each application listens on a unique port number. The 5-tuple (Src IP, Src Port, Dest IP, Dest Port, Protocol) uniquely identifies a connection.
  • The "Trap": UDP Checksum is optional in IPv4 but mandatory in IPv6.
  • Hard Aspect: TCP Timers.
  • Retransmission Timeout (RTO) calculation using smoothed RTT (SRTT).
  • \(SRTT = (1-\alpha)SRTT_{old} + \alpha RTT_{new}\).
  • Complexity: Comparison of cumulative ACK (TCP) vs individual ACK.
  • TCP ACKs indicate the "next expected byte", not the "last received byte"

References