Neon-komputadór

TCP/IP Upper Layer: Applications

The Application Layer is an assortment of network-aware software that sends information to and from the TCP and UDP ports. They are no by no means equivalent in either their design or complexity. Applications that connect with TCP/IP include network services, such as file and print services, name resolution services and redirectors. A local operating system may have it's only components that assist users with network access, the most common being NetBIOS. However, including these operating and vendor-specific options there a range of key Applications which are directly part of TCP/IP which can be differentiated as Connectivity Utilities, File Transfer Utilities, Remote Utilities and Internet Utilities.

Connectivity utilities include IPConfig, which displays a TCP/IP configuration settings, Ping, a simple utility for testing connectivity, AR/RARP, which allows the user to view the cache that contains the physical address of a remote or local computer, Traceroute, that traces the path of a data package, Route, that allows editing or additions to a router table and Netstate that displays IP, UDP, TCP and ICMP statistics. Among the File Transfer Utilities are FTP (File Transfer Protocol) transfers files from one computer to another using TCP, TFTP (Trivial File Transfer Protocol) which uses UDP instead and RCp (Remote Copy Protocol). Among Remote Utilities is Telnet, using for opening a remote terminal window, Rexec, a utility that runs commands on a remote computer, Rsh (Remote Shell) which invokes a the shell on a remote computer to execute a command and Finger, which displays user information.

Testing Your Connection

Many of the TCP/IP applications are relatively easy to use even for an inexperienced end-user. By opening the MS-DOS command prompt in any version of Windows, or the Linux console, a simple connectivity test can be carried out. For example, if an Internet browser doesn't appear to be working there is a quick way to determine whether the problem is with the local area network, our connection through the UNDP and the Ministry of Telecommunications or the configuration of the browser through the 'ping' utility. The 'ping' utility can also determine if we're just having a slow (sometimes very slow) connection. At the c:\ prompt the following are recommended:

ping 127.0.0.1

If this fails there is something seriously wrong. It means that your computer can't connect with itself, let alone the rest of the world!

ping 10.1.1.1

This is the address of our router. If this ping fails there there's probably a problem with your connection to the local area network. If there's a problem with the router we're all in trouble!

ping 202.160.244.15

This site is actually hosted in East Timor at the very point where the connection is made to Singapore and then to the outside world. If no packets are returned from a ping to this site, then the problem lies with the connection at the UNDP/Ministry of Telecommunications. Nothing can be done from this Ministry.

If all of these ping tests are successful check the rate and quantity of the pings on the outside site that are successful - usually between 750-1000 ms per 32 byte packet is the norm. If it is a lot higher than this (say 1300+) or there is a lot of packet loss, then you'll know that the connection from East Timor to the outside world is just particularly poor in this instance.

If all the pings are successful and of a reasonable speed and packet loss and you still can't connect then it's clear that there's something wrong with your network configuration - either the IP address of your computer is conflicting with another machine, or your webbrowser isn't configured properly. In either case, it's time to contact a system administrator.

It is however, in the field of Internet Utilities that most users are familiar with Internet connectivity. Here we find the variety of web browsers (Internet Explorer, Netscape, Mozilla, NSCA Mosiac, Opera, Lynx etc) using HyperText Transfer Protocol (http) and derivatives, usenet newsreaders (Outlook Express, Agent, Newswatcher, Threaded Internet News) using the Network News Transfer Protocol (NNTP) , email clients (Outlook, Lotus Notes, PINE) and servers that operate through the Simple Mail Transfer Protocol (SMTP), directory services (whois, the American Registry of Internet Numbers, the Asia-Pacific Network Information Center etc) as well as the by-passed Archie and Gopher utilities which respectively provided indexes of anonymous FTP sites and an information utility. From a user's perspective, Archie and Gopher were both by-passed by the evolution of the web. Also worthy of inclusion here is Internet Relay Chat (IRC), Mult-User Domains (MUDs) and ICQ. IRC was originally written in 1988 as a replacement for the Unix “talk” program, a multi-user text message system based on public or private channels. MUDs allow multiple users to interact in a (currently) text-based virtual reality. The first MUD was designed in 1978 and now there are several hundred MUDs in existence. About 70% of MUDs are based on fantasy-mythic historical themes. ICQ (“I Seek You”) is a vendor-provided simple message service for computer-to-computer or computer-to-'phone text messages.

Many of these applications will be discussed in detail Chapter IV of this manual along with information concerning the Network Use Policy (which ensures that the Ministry network is used productively and for work-related tasks) and the Standard Software Policy, which denies, or at the very least questions, access to some of the services listed above. At this point however, it's worth noting that some of these services can be very fruitful for gaining - or contributing - information about a particular topic and others can be exhausting and a complete waste of time. For example, the World Wide Web can be useful, but be careful of where the source material is coming from. For the sort of work that we do in the Ministry, usually only reputable news agencies, government or university sites should be considered. Apart from some technical and moderated information groups, newsgroups should be avoided. This also goes for web-based newsgroups such as those hosted through Yahoo! or Topica. There is no perceived need for ICQ, IRC or MUD clients.

Whenever possible, users should use the email system instead of webmail. Whilst at the time of writing, this only operates through the internal email system, it is hoped that in the very near future that access will be provided to the outside world as well. Email clients are a lot faster than webmail, especially if the information is being sent within the Ministry! The reason for this is quite simple. An internal email only requires connection to our local server, whereas a webmail - along with the extra graphics and so forth - requires connection to wherever the webmail server is located (usually the United States). Along the same lines, users should always avoid using pictures and attaching formatted text (e.g., MS-Word) documents to their emails. Whenever possible always use plain text. The bandwidth savings are enormous.