Transport Layer Services

COMP211 Lectures 2021-10-25

The transport layer provides logical communication between application processes running on different hosts.

• Sender - Breaks the application messages into segments and passes to the network layer.
• Receiver - Reassembles the segments into messages and passes them to the application layer.

There are two transport layer protocols available:

• TCP
• UDP

Network Layer vs. Transport Layer

The network layer is a logical communication between hosts.

This is similar to the postal service.

The transport layer extends this network service to provide a logical communication between processes:

• It relies on and enhances network layer services.

Transport Layer Actions

The sender:

1. Is passed an application-layer message.
2. Determines segment header fields values.
3. Creates a segment.
4. Passes the segment to IP.

The receiver:

1. Receives a segment from IP.
2. Checks header values.
3. Extracts the application-layer message.
4. Demultiplexes the message up to the application via a socket.

Two Principle Internet Transmission Protocols

TCP (Transmission Control Protocol):

• Reliable, in-order delivery.
• Congestion Control
• Flow Control
• Connection Setup

UDP (User Datagram Protocol)

• Unreliable, unordered delivery.
• No-frills extension of “best-effort” IP.

The following services are not available:

• Delay Guarantees
• Bandwidth Guarantees