Understanding Source and Destination Port Numbers

Transport Layer

• Enables the management of lost data and flow control

UDP: Communicating with Low Overhead

• A simple protocol providing basic transport layer functions
• Has lower overhead than TCP due to lack of connection-oriented and sophisticated retransmission, sequencing, and flow control mechanisms
• Reliability needs to be implemented elsewhere if required

UDP: Datagram Reassembly

• Connectionless, with no session establishment before communication
• Transaction-based, sending data without establishing a session
• Many applications using UDP send small amounts of data fitting in one segment
• UDP does not care about datagram order

UDP Server Processes and Requests

• Different types of port numbers:
  • Well-known ports (0-1023)
  • Registered ports (1024-49151)
  • Dynamic or private ports (49152-65535)
• Dynamic or private ports are usually assigned dynamically to client applications when initiating a connection

Using Both TCP and UDP

• netstat command:
  • Verifies active TCP connections on a networked host
  • Lists protocol, local address and port number, destination address and port number, and connection state
  • Used to examine open connections on a host when performance appears compromised

TCP Connection Establishment and Termination

• A connection is established before data exchange
• Sessions are closed and connection terminated after communication is completed
• Connection and session mechanisms enable TCP's reliability function

TCP Three-Way Handshake

• Establishes the presence of the destination device on the network
• Verifies the destination device has an active service and is accepting requests on the destination port number
• Informs the destination device that the source client intends to establish a communication session on that port number

Identifying Conversations

• Port numbers:
  • Source port number: number for a communication associated with the originating application on the local host
  • Destination port number: number for a communication associated with the destination application on the remote host
• Socket: combination of the transport layer port number and the network layer IP address assigned to the host
• Example: 192.168.1.20:80

Summary

• Transport layer provides data network needs by:
  • Tracking individual communications between applications on source and destination hosts
  • Segmenting data and managing each piece
  • Reassembling segments into streams of application data
  • Identifying different applications
  • Performing flow control between end users
  • Enabling error recovery
  • Initiating a session
• UDP and TCP are common transport layer protocols
• UDP datagrams and TCP segments have headers prefixed to the data that include a source port number and destination port number
• TCP does not pass data to the network until it knows that the destination is ready to receive it
• TCP uses mechanisms of the three-way handshake, timers, acknowledgments, and dynamic windowing to achieve reliable features