7. Computer networks

A. Transport layer services and protocols

The transport layer protocols provide the logical communication between applications/processes running in different hosts.

• These protocols run in the end systems and not in the network routers.
  • Sender: Breaks the application messages into Segments; passes it to network layers.
    • The segment is further encapsulated into network-layer packets called Datagrams.
      • The datagram is finally passed to the network router.
  • Receiver: Receives the segments and re-assembles the application messages; passes it to application layer.

There are different transport layer protocols, each with its own characteristics. The following are the most common ones:

• Transmission Control Protocol (TCP)
• User Datagram Protocol (UDP)

User Datagram Protocol (UDP)

“No frills,” “bare bones” Internet transport protocol. UDP is a connectionless protocol.

Connectionless:

• No handshaking between UDP sender and receiver.
• Each segment is handled independently of others.

Most often used for streaming multimedia applications.

Other uses: Domain name system (DNS), Simple network management protocol (SNMP)

Protocol specifications:

Few notable ports and protocols:

Port number	Protocol	Application
53	UDP, TCP	Domain name system (DNS)
80	TCP	HTTP (WWW)
443	TCP	HTTPS (SSL)
16, 384-32, 767	UDP	Voice and video

Transmission Control Protocol (TCP)

“Connection-oriented” transport protocol.

Properties:

• Point-to-point: One sender, one receiver.
• Connection-oriented: Handshaking in it’s sender. receiver state before data exchange.
• Full duplex: Bi-directional data flow in same connection
  • MSS: Maximum segment size
• Reliable, in-order byte stream: Segments are sent in order and acknowledged by receiver.
• Flow controlled: Segments are not sent if receiver is not ready.