Thursday, June 22, 2017

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

At the next OSI Reference Model layer (layer 4) is a set of protocols specifying virtual “ports” at transmitting and receiving devices through which data is communicated. The purpose of these virtual ports is to manage multiple types of data transactions to and from the same IP address, such as in the case of a personal computer accessing a web page (using HTTP) and sending an email message (using SMTP) at the same time. An analogy to help understand the role of ports is to think of multiple packages delivered to different people at a common address such as a business office. The mailing address for the office is analogous to the IP address of a computer exchanging data over a network: it is how other computers on the network “find” that computer. The persons’ names or department numbers written on the different packages are analogous to virtual ports on the computer: “places” where specific messages are directed once they arrive at the common address.

Transmission Control Protocol ( TCP) and User Datagram Protocol (UDP) are two methods used to manage “ports” on a DTE device, with TCP being the more complex (and robust) of the two. Both TCP and UDP must rely on IP addressing to specify which devices send and receive data, which is why you will often see these protocols listed in conjunction with IP (e.g. TCP/IP and UDP/IP). TCP and UDP are both useless on their own: a protocol specifying port locations without an IP address would be as meaningless as a package placed in the general mail system with just a name or a department number but no street address. Conversely, Internet Protocol anticipates the presence of a higher-level protocol such as TCP or UDP by reserving a portion of its “datagram” (packet) bit space for a “protocol” field1 to specify which high-level protocol generated the data in the IP packet.

TCP is a complex protocol specifying not only which virtual “ports” will be used at the sending and receiving devices, but also how packet transmission will be guaranteed. A segment of data sent via TCP will be error-checked to guard against corruption, marked with special bit flags if of an “urgent” priority, and otherwise marked and labeled in such a way that a high degree of communication reliability is assured. A simplified analogy for TCP’s actions might be certified mail in the United States postal system, where certain extra steps are taken to ensure delivery and receipt of a parcel.

UDP is a much simpler protocol, lacking many of the data-integrity features of TCP. It is quite common to see UDP applied in industrial settings, where communication takes place over much smaller networks than the world-wide Internet. Another reason UDP is more common in industrial applications is that it is easier to implement in the “embedded” computer hardware at the heart of many industrial devices. The TCP algorithm requires greater computational power and memory capacity than the UDP algorithm, and so it is much easier to engineer a single-chip computer (i.e. microcontroller) to implement UDP than it would be to implement TCP.

Using another utility program on a personal computer called netstat (available for both Microsoft Windows and UNIX operating systems) to check active connections2, we see the various IP addresses and their respective port numbers (shown by the digits following the colon after the IP address) as a list, organized by TCP connections and UDP connections:



This netstat utility program is available for both Microsoft and Unix operating systems to check active connections


Many different port numbers have been standardized for different applications at OSI Reference Model layers above 4 (above that of TCP or UDP). Port 25, for example, is always used for SMTP (Simple Mail Transfer Protocol) applications. Port 80 is used by HTTP (HyperText Transport Protocol), a layer-7 protocol used to view Internet “web” pages. Port 107 is used by TELNET applications, a protocol whose purpose it is to establish command-line connections between computers for remote administrative work. Port 22 is used by SSH, a protocol similar to TELNET but with significantly enhanced security. Port 502 is designated for use with Modbus messages communicated over TCP/IP.


1Both IPv4 and IPv6 reserve eight bits for this purpose.

2In this particular case, I typed netstat -an to specify all (a) ports with numerical (n) IP addresses and port numbers shown.


Click here to go the next page, The HART Digital/Analog Hybrid Standard

Click here to go back to the previous page, Internet Protocol (IP)

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