CSCI 4550/8556 Computer Networks Comer, Chapter 2: Motivation and Tools

Introduction Motivation Service Tools for exploration

Historic Motivation Early computers were expensive. They were very large (compared to today’s machines). They were centralized and not easily moved. Programs took a relatively long time to run. It was not economically feasible to put computers in a large number of locations

ARPA The Advanced Research Project Agency initiated a project to connect researchers with the centralized computer systems. In doing so, they adopted new technologies: Packet switching Internetworking The result was a network that allowed remote access by users to the expensive resources.

Packet Switching In packet switching, data is transmitted in small, independent pieces. The source divides outgoing messages into packets. The destination reassembles the received packets into the original message. Each packet travels independently from the source to the destination. Each packet includes enough information for delivery. The packets may take different paths to the destination. Each packet can be retransmitted if necessary.

Internetworking There are many mutually incompatible network technologies in use today. No one technology is appropriate for every situation. Internetworking glues together networks of dissimilar technologies with routers. The result is a virtual network whose details are practically invisible to the individual computers connected to it.

History and Growth The ARPAnet began in late 1960s (not using TCP/IP). TCP/IP developed in late 1970s. ARPAnet switched to TCP/IP in early 1980s. Start of Internet Few hundred computers Few tens of networks

Growth Since 1981

Growth (Logarithmic Axis)

Probing the Internet Two common tools can be used to examine the Internet: ping - sends a message that is echoed by a remote computer back to the sending computer traceroute – attempts to identify and report the path taken by a packet as it travels to a remote computer

Ping Sends a packet to a remote computer system. The remote computer replies with an echo packet. The local computer reports receipt of the echo. Some systems (e.g. Windows) limit number of packets. Others (e.g. UNIX) may send until interrupted or until specified number of packets are sent. Report usually includes indication of how long the round trip required, and if any packets were lost.

Ping – Example 1 apollo> /usr/sbin/ping stanw.unomaha.edu PING stanw.ist.unomaha.edu ( ): 56 data bytes 64 bytes from : icmp_seq=0 ttl=126 time=2 ms 64 bytes from : icmp_seq=1 ttl=126 time=0 ms 64 bytes from : icmp_seq=2 ttl=126 time=0 ms 64 bytes from : icmp_seq=3 ttl=126 time=0 ms 64 bytes from : icmp_seq=4 ttl=126 time=0 ms ----stanw.ist.unomaha.edu PING Statistics packets transmitted, 5 packets received, 0% packet loss round-trip (ms) min/avg/max = 0/0/2 ms apollo>

Ping – Example 2 stanw]$ ping -c 5 cs002pc.unomaha.edu PING cs002pc.ist.unomaha.edu ( ) from : 56(84) bytes of data. 64 bytes from cs002pc.ist.unomaha.edu ( ): icmp_seq=0 ttl=248 time=940 usec 64 bytes from cs002pc.ist.unomaha.edu ( ): icmp_seq=1 ttl=248 time=6.797 msec 64 bytes from cs002pc.ist.unomaha.edu ( ): icmp_seq=2 ttl=248 time=6.393 msec 64 bytes from cs002pc.ist.unomaha.edu ( ): icmp_seq=3 ttl=248 time=6.382 msec 64 bytes from cs002pc.ist.unomaha.edu ( ): icmp_seq=4 ttl=248 time=6.605 msec --- cs002pc.ist.unomaha.edu ping statistics packets transmitted, 5 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.940/5.423/6.797/2.247 ms stanw]$

Traceroute Sends a series of packets along a path to the destination. Each successive packet identifies the next router along the path. Traceroute uses an expanding ring search. Each packet is usually sent three times. The list of packets received is reported at the local computer.

Traceroute – Example 1 stanw]# /usr/sbin/traceroute apollo.unomaha.edu traceroute to apollo.unomaha.edu ( ), 30 hops max, 38 byte packets ( ) ms ms ms 2 west-dist-rtr-back ( ) ms ms ms 3 apollo ( ) ms ms ms stanw]#

Traceroute – Example 2 stanw]$ /usr/sbin/traceroute apollo.unomaha.edu traceroute to apollo.unomaha.edu ( ), 30 hops max, 38 byte packets 1 unebpki.uneb.edu ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms ( ) ms ms ms 5 uncsn-uno ( ) ms ms ms 6 west-dist-rtr-main ( ) ms ms ms 7 apollo ( ) ms ms ms stanw]$

Web Access to Tools Ping and Traceroute Ping and Traceroute are available on the web (provided by Carnegie-Mellon University). To avoid some security problems, the University of Nebraska networks are configured to disallow ping and traceroute packets from “crossing the border” between the university networks and the rest of the Internet.