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TCP/IP Ports: Transport Layer (TCP/UDP) Addressing
(Page 3 of 3)
Summary of Port Use for Datagram Transmission and Reception
So, to summarize, here's the basics of how transport-layer addressing (port addressing) works in TCP and UDP:
- Sending Datagrams: An application specifies
the source and destination port it wishes to use for the communication.
These are encoded into the TCP or UDP header, depending on which transport
layer protocol the application is using. When TCP or UDP passes data
to IP, IP indicates the protocol type appropriate for TCP or UDP in
the Protocol field of the IP datagram. The source and destination
port numbers are encapsulated as part of the TCP or UDP message, within
the IP datagram's data area.
- Receiving Datagrams: The IP software receives the datagram, inspects the Protocol field and decides to which protocol the datagram belongs (in this case, TCP or UDP, but of course there are other protocols that use IP directly, such as ICMP). TCP or UDP receives the datagram and passes its contents to the appropriate process based on the destination port number.
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Key Concept: Application process multiplexing and demultiplexing in TCP/IP is implemented using the IP Protocol field and the UDP/TCP Source Port and Destination Port fields. Upon transmission, the Protocol field is given a number to indicate whether TCP or UDP was used, and the port numbers are filled in to indicate the sending and receiving software process. The device receiving the datagram uses the Protocol field to determine whether TCP or UDP was used, and then passes the data to the software process indicated by the Destination Port number.
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Note: As an aside, I should point out that the term port has many meanings aside from this one in TCP/IP. For example, a physical outlet in a network device is often called a port. Usually one can discern whether the “port” in question refers to a hardware port or a software port from context, but you may wish to watch out for this.
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Version 3.0 - Version Date: September 20, 2005
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