The layers above layer 3 handle flow controls, presentation and applications and as this is a network site we will not handle details about those here. Functions on those layers are always build in in the computers or in gateways with high level network functionality like firewall's. However there are some important issues that can be worth mention here.

TCP
Even if we call the protocol TCP/IP TCP is not always present. TCP (transmission control protocol) is as the name implies a function to check that the packets which is send to the next level is error free and in the correct sequence without missing packet. This function is critical for transferring data files and reproduce them correct. However for real time traffic like voice (IP telephony) and video the timing is more important than the continuously. In those applications TCP is replaced by UDP (User datagram protocol) which does not care about a missing packet but in lack of handshaking puts less timing overhead on the flow.

DNS
In the application layer there are some critical application for the network to work. One of those is the DNS, the domain name system. This is a globally build, hierarchical system that take care of the names used on the Internet and translate them into IP addresses that can be used in the network. There is a common application that can be used to check the name resolution, namely nslookup. If you open a cmd window or a terminal you can just write nslookup and you will get a new prompt. if you write down the name you want to lookup you will have an answer on who is resolving the name and the IP address. The DNS function is necessary for most Internet applications like web or mail. More on DNS another Technology page

Web
The world wide web is a quite simple system which uses pictures and links to direct to other information sources. The web is also using DNS as all referring are made with names. The web format is written in a special encoding called html and need a html reader to be able to present the code. The most common html readers today are Microsoft Internet explorer, Apples Safari and Mozillas Firefox. There is a hugh development ongoing with the web adding more functionality. More and more applications are using web readers as the user interface with the advantage to be independent of operating system.

Mail
The Internet has also a system for transferring mail to the correct receiver. The mail system uses also the DNS to find mail servers on the net. When you composed a mail on your Outlook application your client computer (PC, Mac) will send the mail to a mail server using a special protocol for mail delivery. The protocol used to send mail is SMTP (Simple mail transport protocol) while the client use POP (Post office protocol) or IMAP (Internet message access protocol) to fetch mail from the server. Imap gives slightly more functionality and can transfer folders etc.

The server then forward the mail to another server or lookup which server handle this domains mail in DNS. There is a special record in DNS which tells which servers are accepting mail to this domain called MX-pointer. You can use nslookup (previous chapter) to see those servers. When having the nslookup prompt just write "Set Q=MX" and enter the domain name (the domain name proceeding the @ sign in the mail address). The servers are listed in priority order and if none is available the mail is not possible to forward.