Transport services and protocols provide logical communication between application processes running on different hosts.

In terms of what transport protocol does on end systems:

• Sender

  • Break application message into segments + pass to network layer

  

• Receiver

  • Reassemble segments into messages + pass to application layer

  

Comparing the transport layer to the network layer services and protocols:

• Network layer provides logical communication between hosts
• Transport layer provides logical communication between processes
  • i.e. transport layer relies on and enhances the network layer services

There are 2 principal Internet transport protocols:

• TCP - Transmission Control Protocol
  • Provides reliable, in-order delivery of packets
  • Congestion control
  • Flow control
  • Connection setup
• UDP - User Datagram Protocol
  • Unreliable, unordered delivery
  • No-frills extension of “best-effort” IP

Neither delay guarantees nor bandwidth guarantees are provided.