Transport Layer Ports and Connections

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• protocols at Layer 4 (Transport) are concerned with delivery of multiplexed application data
  • headers instruct a host what to do with the data in a packet
    • optionally, how to verify it is complete

Ports

• each application protocol is assigned a unique identification number called a port
  • host can operate multiple ports simultaneously
  • port ranges:
    • ports 0-1023
      • preassigned by the Internet Assigned Numbers Authority (IANA) to well-known server applications
    • port range 1024-49151
      • registered for other server applications
    • remaining ports up to 65,535
      • designated for private or dynamic use
• in addition to the server application needing a port,
  • each client application must assign its own port number to track its requests
  • client ports are called ephemeral ports or source ports

Info

• port number assignment guidelines are not always used
• early versions of Windows and UNIX/Linux used 1,024-5,000 for client ports
• modern linux kernels often use 32,768-60,999

Sockets & Connections

• a socket is a port number used in conjunction with a source IP address
  • each socket is bound to a software process
  • only one process can operate a socket at any one time
  • a connection is formed when a client socket requests a service from the server socket
    • uniquely identified by the combination of server port + IP address and client port + IP address
  • server socket can support multiple connections from a number of client sockets