Transport Layer Services, Sockets, UDP Notes

Key Concepts of Transport Layer Services

  • Transport Layer Overview

    • Acts as an interface between the application layer and the network layer.
    • Responsible for process-to-process communication.

  • Segmentation and Reassembly

    • Data is divided into segments before sending (sender side).
    • Segments are reassembled back into messages upon receiving (receiver side).
    • Segments may arrive out of order.
    • Encapsulation: Adding headers as the packet moves down the layers.
    • Decapsulation: Removing headers as the packet moves up the layers.

Transport Layer Services

  • Logical Communication

    • Facilitates logical communication between application layer processes on different hosts.
    • Sender side: choses segments and sends it down to the network layer.
    • Receiver side: reconstructs the messages from segments.

  • Protocols in Transport Layer

    • TCP (Transmission Control Protocol):
    • Connection-oriented protocol.
    • Guarantees reliable, ordered delivery of segments.
    • Implements features like flow control, sequence numbers, and acknowledgments.
    • UDP (User Datagram Protocol):
    • Connectionless protocol.
    • Provides no guarantees on delivery, order, or data integrity.
    • Minimal services offered are process-to-process delivery and error checking.

  • Transport Protocol Functions

    • Extends the network-layer service into a process-to-process delivery service.
    • Implement multiplexing (sending) and demultiplexing (receiving) services.

Socket Programming

  • Definitions
    • Socket: Communication endpoints used by applications to communicate over the network.
    • Identified by IP address and port number.
    • Client-side: distinguishes between apps on the same machine.
    • Server-side: distinguishes between different protocols (e.g., HTTP on port 80).
    • Port Numbers:
    • Well-known ports: 0 to 1023 for standard application protocols.
    • Registered ports: 1024 to 49151 can be registered by vendors.
    • Dynamic ports: 49152 to 65535 for temporary or private uses.

Multiplexing & Demultiplexing

  • Transport-layer Multiplexing

    • Multiple segments distinguished using port numbers.
    • At sender end, data from multiple sockets handled and port numbers added.

  • Demultiplexing

    • At receiver end, tracks which segment belongs to which socket through its port number.

User Datagram Protocol (UDP)

  • Characteristics

    • Connectionless with best-effort service.
    • Susceptible to data loss and out-of-order delivery.
    • Favored for real-time applications like DNS, streaming, or those that require minimal overhead.

  • UDP Header Format

    • Contains source and destination ports, data length, checksum, and application data.
    • Simple structure leads to reduced latency.

  • Checksum Mechanism

    • A method to check for errors with sender performing one’s complement of the sum of segment words, and the receiver checks against the checksum value.

  • Application Contexts

    • Used when applications can tolerate some data loss or require fast transmission without the overhead of connection management.