Introduction to the transport layer

The transport layer provides process-to-process communication between applications running on different hosts.

It is implemented in end systems (not in network routers).

Household analogy

Kids in two households send letters to each other:

• the postal service delivers letters between houses:
  • it provides house to house communication
• Ann and Bill, who manage mail inside their respective houses, deliver letters to the correct kids:
  • they provide person to person communication

This analogy explains how the transport layer relates to the network layer:

• hosts = houses
• application messages = letters
• network-layer protocol = postal service
• processes = kids
• transport-layer protocol = Ann and Bill

What does the transport layer do?

A transport protocol:

• moves messages from application processes to the network layer (and vice versa)
• is a delivery service between application processes
  • also known as multiplexing and demultiplexing
• doesn't have any say about how the messages are moved within the network core
• can offer reliable data transfer service to an application
  • even when the underlying network protocol is unreliable (loses, garbles, or duplicates packets)
• can use encryption
  • even when the network layer cannot guarantee the confidentiality of transport-layer segments

Transport protocol guarantees

Each transport-layer protocol may offer guarantees:

Guarantee	Description
Reliable data transfer	packets can get lost within a computer network a protocol that guarantees data delivery service is said to provide reliable data transfer
Throughput	the rate at which the sending process can deliver bits to the receiving process applications that have throughput requirements are said to be bandwidth-sensitive applications elastic applications (e-mail, file transfer, and Web transfers) can make use of as much, or as little, throughput as happens to be available
Timing	example guarantee: every bit that the sender pumps into the socket arrives at the receiver's socket no more than 100 msec later
Security	encrypt/decrypt all data transmitted

Throughput and timing guarantees are services not provided by today's Internet transport protocols.

UDP and TCP

The transport layer makes two transport protocols available to applications:

1. UDP (User Datagram Protocol)
   • provides an unreliable connectionless service to the invoking application
2. TCP (Transmission Control Protocol)
   • provides a reliable, connection-oriented service to the invoking application
   • can be enhanced at the application layer with TLS to provide security services

Service models provided by UDP and TCP:

• transport-layer multiplexing and demultiplexing (UDP, TCP)
• error/integrity checking (UDP, TCP)
• reliable data transfer (TCP only)
• congestion control (TCP only)

Segment vs datagram

Internet literature refers to:

• TCP packets as segments
• UDP packets as datagrams
• but also uses the term datagrams for the network-layer packets

It is less confusing:

• to refer to both TCP and UDP packets as segments
• to reserve the term datagram for the network-layer packet