Internet Applications: Telnet, Ping and Traceroute

Spring 2002Computer Networks Applications Timesharing Systems Before Internet: Multiple terminals attach to a single, time sharing computers; a user interacts with the (timesharing) computer through its terminal; Each individual user can choose which application to run; Although applications are run remotely, input and output for the applications are displayed on the user terminal; The timeshared computer can run several applications sequentially.

Spring 2002Computer Networks Applications Timesharing Systems (cont.) Because several users may work on a timesharing machine, the system requires: An account be made for each legit user, which is given a unique name, called login identifier; Each user has a password associated with its account; In order to access the timeshared machine, a user has to enter its login identifier and password---the process is called logging in or login. After logging in, a user can run any application; When a user is done, he logs out.

Spring 2002Computer Networks Applications Remote login (telnet) Achieves the same functionality as conventional login: A user invokes telnet on a remote machine The remote machine asks for a login and a password The user can work on the remote machine as it does on a terminal connected directly with a timeshared machine.

Spring 2002Computer Networks Applications Telnet Follows the client-server paradigm: The local applications is the client The remote application is the server The server has to be running before any client can begin Any data entered by the client is transmitted to the remote computer; Any data produced by an application started by the user in question on the remote machine is displayed on the local machine

Spring 2002Computer Networks Applications Telnet (cont.) Uses TCP/IP for communication between client and server; Telnet protocol part of TCP/IP specification: How to submit login and password information How to terminate a session (type exit, for ex.) How to abort a session

Spring 2002Computer Networks Applications Telnet benefits Allows for performing computation on different machines; Allows for a local user to use applications residing only on the remote machine ex. applications designed for a specific type of computer (ex Unix applications); Once a user is connected, he can work on the remote machine as if it were local

Spring 2002Computer Networks Applications Telnet benefits (cont.) Allows many users to access specific applications without having to modify the applications EX1: a database applications, say Oracle, running on special computers, say IBM machines, cannot be installed on PCs running Microsoft Windows. However, any user on PC can use the database provided it has a valid account on the remote machine where the database is installed. Should the database software run before the client application started in order for a user to connect to it?

Spring 2002Computer Networks Applications Running telnet Type telnet pegasus to connect; Enter your login name and password Commands that you can issue on pegasus: mkdir --- creates a new directory; ex mkdir public_html cd ---makes the current directory the one specified in the command; Ex: cd public_html ls ---list the files in the current directory;

Spring 2002Computer Networks Applications Running telnet (cont.) chmod change the rights to the file; Ex: chmod 644 index.html ---- makes file index.html readable and writable by owner, and readable by others; chmod 755 ~/public_html/ --- makes the directory public_html modifiable by the user, and readable by everybody else. man ---displays the functionality of the command given as parameter; Ex man chmod more --- displays the content of the file given as parameter;

Spring 2002Computer Networks Applications Running telnet (cont.) cat --- concatenates the files given as parameters; lpr --- prints the file given as parameter at the default printer

Spring 2002Computer Networks Applications A cautionary note… When working remote, telnet uses the display, keyboard, and mouse on the user’s local computer, However the remote applications can only interact with the files and I/O devices of the remote computer

Spring 2002Computer Networks Applications Probing tools: ping Sends messages to a remote host given as parameter and reports the result. Ex: entering ping cs.rutgers.edu will display the status of that host, ex “alive” ping -s sends a datagram every second and displays the round trip time (the time between sending a message and receiving a response);

Spring 2002Computer Networks Applications Ping Ex: try ping –s cs.columbia.edu ping –s cs.stanford.edu What do you notice?

Spring 2002Computer Networks Applications Ping (cont.) The average time increases with the distance; Values of ~40 ms are most common Values of hundreds of ms and/or packet loss mean that the path to the destination is congested; However when no response is received ping cannot determine the reason; Possible reasons for failure: network failure or congestion, the remote computer can be off, or disconnected from the network, or discard ping messages.

Spring 2002Computer Networks Applications Probing tools: traceroute Used to determine the intermediate computers along the path to a remote destination; Ex: traceroute dandelion-patch.mit.edu may produce the following result:

Spring 2002Computer Networks Applications Traceroute (cont.) Each line---corresponds to an intermediate computer or hop;