Multiplexing Using TCP Port Numbers
TCP provides a lot of features to applications, at the expense of requiring slightly more processing and overhead, as compared to UDP. However, TCP and UDP both use a concept called multiplexing. So, this section begins with an explanation of multiplexing with TCP and UDP. Afterward, the unique features of TCP and UDP are explored.
Multiplexing by TCP and UDP involves the process of how a computer thinks when receiving data. The computer might be running many applications, such as a web browser, an e-mail package, or an FTP client. TCP and UDP multiplexing enables the receiving computer to know which application to give the data to.
Some examples will help make the need for multiplexing obvious. The sample network consists of two PCs, labeled Hannah and Jessie. Hannah uses an application that she wrote to send advertisements that display on Jessie's screen. The application sends a new ad to Jessie every 10 seconds. Hannah uses a second application, a wire-transfer application, to send Jessie some money. Finally, Hannah uses a web browser to access the web server that runs on Jessie's PC. The ad application and wire-transfer application are imaginary, just for this example. The web application works just like it would in real life.
Figure 6-2 shows a figure of the example network, with Jessie running three applications:
■ A UDP-based ad application
■ A TCP-based wire-transfer application
■ A TCP web server application
Figure 6-2 Hannah Sending Packets to Jessie, with Three Applications Hannah
Web Server
-' Ad Application
Wire Application
Jessie [
Jessie [
I Received Three Packets, Each from the Same MAC and IP Address. What Application Should Get the Data in Each Packet?
|
Eth |
IP |
UDP |
Ad Data |
Jessie needs to know which application to give the data to, but all three packets are from the same Ethernet and IP address. You might think that Jessie could look at whether the packet contains a UDP or a TCP header, but, as you see in the figure, two applications (wire transfer and web) both are using TCP. TCP and UDP solve this problem by using a port number field in the TCP or UDP header, respectively. Each of Hannah's TCP and UDP segments uses a different destination port number so that Jessie knows which application to give the data to. Figure 6-3 shows an example. Multiplexing relies on the use of a concept called a socket. A socket consists of three things: an IP address, a transport protocol, and a port number. So, for a web server application on Jessie, the socket would be (10.1.1.2, TCP, port 80) because, by default, web servers use the well-known port 80. When Hannah's web browser connected to the web server, Hannah used a socket as well—possibly one like this: (10.1.1.1, TCP, 1030). Why 1030? Well, Hannah just needs a port number that is unique on Hannah, so Hannah saw that port 1030 was available and used it. In fact, hosts typically allocate dynamic port numbers starting at 1024 because the ports below 1024 are reserved for well-known applications, such as web services. Figure 6-3 Hannah Sending Packets to Jessie, with Three Applications Using Port Numbers to Multiplex Hannah Hannah Port 80 Web Server Port 800 Ad Server Port 20,100 Wire Application Port 80 Web Server Port 800 Ad Server Port 20,100 Wire Application Jessie f fifi I'll Look in the UDP or TCP Destination Port to Identify the Application!
Destination Port 80 Destination Port 80In Figure 6-3, Hannah and Jessie used three applications at the same time—hence, there were three socket connections open. Because a socket on a single computer should be unique, a connection between two sockets should identify a unique connection between two computers. The fact that each connection between two sockets is unique means that you can use multiple applications at the same time, talking to applications running on the same or different computers; multiplexing, based on sockets, ensures that the data is delivered to the correct applications. Figure 6-4 shows the three socket connections between Hannah and Jessie. Figure 6-4 Connections Between Sockets Hannah Jessie Jessie 
Port numbers are a vital part of the socket concept. Well-known port numbers are used by servers; other port numbers are used by clients. Applications that provide a service, such as FTP, Telnet, and web servers, open a socket using a well-known port and listen for connection requests. Because these connection requests from clients are required to include both the source and the destination port numbers, the port numbers used by the servers must be well known. Therefore, each server has a hard-coded, well-known port number, as defined in the well-known numbers RFC. On client machines, where the requests originate, any unused port number can be allocated. The result is that each client on the same host uses a different port number, but a server uses the same port number for all connections. For example, 100 Telnet clients on the same host computer would each use a different port number, but the Telnet server with 100 clients connected to it would have only 1 socket and, therefore, only 1 port number. The combination of source and destination sockets allows all participating hosts to distinguish between the source and destination of the data. (Look to www.rfc-editor.org to find RFCs such as the well-known numbers RFC 1700.) NOTE You can find all RFCs online at www.isi.edu/in-notes/rfcxxxx.txt, where xxxx is the number of the RFC. If you do not know the number of the RFC, you can try searching by topic at www.rfc-editor.org/cgi-bin/rfcsearch.html. Popular TCP/IP Applications Throughout your preparation for the CCNA INTRO and ICND exams, you will come across a variety of TCP/IP applications. You should at least be aware of some of the applications that can be used to help manage and control a network. The World Wide Web (WWW) application exists through web browsers accessing the content available on web servers, as mentioned earlier. While often thought of as an end-user application, you can actually use WWW to manage a router or switch by enabling a web server function in the router or switch, and using a browser to access the router or switch. The Domain Name System (DNS) allows users to use names to refer to computers, with DNS being used to find the corresponding IP addresses. DNS also uses a client/server model, with DNS servers being controlled by networking personnel, and DNS client functions being part of most any device that uses TCP/IP today. The client simply asks the DNS server to supply the IP address that corresponds to a given name. Simple Network Management Protocol (SNMP) is an application layer protocol used specifically for network device management. For instance, the Cisco Works network management software product can be used to query, compile, store, and display information about the operation of a network. In order to query the network devices, Cisco Works uses SNMP protocols. Traditionally, in order to move files to and from a router or switch, Cisco used Trivial File Transfer Protocol (TFTP). TFTP defines a protocol for basic file transfer - hence the word "trivial" to start the name of the application. Alternately, routers and switches can use File Transfer Protocol (FTP), which is a much more functional protocol, for transferring files. Both work well for moving files into and out of Cisco devices. FTP allows many more features, making it a good choice for the general end-user population, whereas TFTP client and server applications are very simple, making them good tools as imbedded parts of networking devices. Some of these applications use TCP, and some use UDP. As you will read later, TCP performs error recovery, whereas UDP does not. For instance, Simple Mail Transport Protocol (SMTP) and Post Office Protocol version 3 (POP3), both used for transferring mail, require guaranteed delivery, so they use TCP. Regardless of which transport layer protocol is used, applications use a well-known port number, so that clients know to which port to attempt to connect. Table 6-3 lists several popular applications and their well-known port numbers. Recommended ProgramsReaders' Questions
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