Internet Applications: Telnet, Ping and Traceroute

1. Internet Applications: Telnet, Ping and Traceroute

2. 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. Computer Networks Applications

3. 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 . Computer Networks Applications

4. 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. Computer Networks Applications

5. 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 Computer Networks Applications

6. 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 Computer Networks Applications

7. 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 Computer Networks Applications

8. 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? Computer Networks Applications

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

10. Running telnet (cont.) • chmod <rights> <file name> 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 <command name>---displays the functionality of the command given as parameter; • Ex man chmod • more <file_name> --- displays the content of the file given as parameter; Computer Networks Applications

11. Running telnet (cont.) • cat <files> --- concatenates the files given as parameters; • lpr <file-name> --- prints the file given as parameter at the default printer Computer Networks Applications

12. 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 Computer Networks Applications

13. 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 <host name> sends a datagram every second and displays the round trip time (the time between sending a message and receiving a response); Computer Networks Applications

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

15. 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. Computer Networks Applications

16. 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: Computer Networks Applications

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