In Figure 2. 3, ping sends one request each second, and produces one line of output for each response received. The output tells the size of the packet received, the sequence number, and the round-trip time in milliseconds. When the user interrupts the program, ping produces a summary that specifies the number of packets sent and received, packet loss, and the minimum, mean, and maximum round-trip times. The output in the figure also shows another interesting feature. Although the author specified www. sears. com as the destination computer, ping lists the computers name as seors. com. Chapter 29 discusses computer names in detail. For now, it is sufficient to know that www. sears. com is merely an alias for computer sears. com.
The output in Figure 2. 3 shows an average round-trip time of 49 milliseconds, typical when the Internet S not congested. As a comparison, Figure 2. 4 shows the roundtrip time from the author’s workstation to a site on the west coast (the University of California at Befkeley), and Figure 2. 5 shows the round-trip time for a site on the east coast (MIT in Cambridge, Massachusetts), all taken on the same day at nearly the same time. When the measurement was taken, the path to MIT was experiencing congestion later produced a much which increased delays substantially ; measurements a few hours lower round-trip time to the same destination.
PING amber. Berkeley. EEU 56 data bwes
64 bRes frem amber. Berkeley. EHJ (128. 32. 25. 12)
64 bytes frem amber. Berkeley. EHJ (128. 32. 25. 12)
64 bwes £ram amber. Berkeley. EDTJ (128. 32. 25. 12)
64 bytes £ram amber. Berkeley. EDU (128. 32. 25. 12)
64 bwes £rcm amber. Berw1ey. EIU (128. 32. 25. 12)
—-amber. Berkeley. EEIU pJNG Statistics—-
5 packets tranEntitted, 5 packets receiM, O% packet loss
rounjj-trip (ms) min/avg/max-53/53/55
Figure 2. 4 Example output from the ping program for destination
www. berkeley. edu, which is located on the west coast. round trip times are only slightly higher than those reported in Figure 2. 3.
PING DANDELICN-FATCH. MIT. EHJ 56 data bwes
64 bwes fram DANEMICN-FATCH. MIT. EDU (18. 181. 0. 31) iatFSeFO. time=176. Ms
64 byttes frem CN-EAKH. MIT. ED0 (18. 181. 0. 31) iweq=1. Time=234. Ms
64 bwes £rcm DANDELIA-rAICH. MIT. EEU (18. 181. 0. 31) larFsW2. Time-302. Ms
64 bwes £ram DANEMICN-FAITCH. MIT. EHJ (18. 181. 0. 31) iatF_seq3. Time-165. Rns
64 bFes frem ICN-EATCH. MIT. EHJ (18. 181. 0. 31) iatFeqa. Time=208. Rns
—-a-EATCH. MIT. EDU PING Statistics—-
5 packets transmitted, 5 packets receivecl, O% packet loss
round-trip (ms) min/avg/max-165/217/302
Figure 2. 5 Example output from the ping program for destination
www. mit. edu, a location on the east coast. round-trip times are significantly higher than in previous examples.
It may seem that the ping program is too simplistic to be useful. Even with options turned on, the round-trip times provide little information to the average user. For example, ping cannot explain why the time required to reach MIT is higher than the time to reach other distant locations. More important, it appears ping has little to otftr as a tool to debug network failures because output occurs only when a computer responds successfully. When no response is received, ping cannot help determine the reason. The remote computer could be turned off, disconnected from the network, its network interface could have failed, or it could be running software that does not respond to ping. The local computer could be disconnected from the network, the network to which the remote computer attaches could have failed, or the problem could be caused by the failure of an intermediate computer or network. Finally, ping sometimes fails because the network has become so congested with traffic that delays are unreasonably long. ping has no way to determine the cause of the problem.
iarrs € wo. Time-53. Ms
iarrsq1. Time=53. Ms
iarF €₩ 2. Time=54. Ms
iarr_sq3. Time-53. Ms
iww¢. Time-55. Ms
Companies configure their site to reject ping packets. The motivation for disabling ping is tible to a deizia/-of-service of Jloodizg attack in which so many ping packets arTive that the company’s networks and computers cannot respond to legitimate requests. To avoid such attacks, the company mefely rejects ping packets befofe they enter. You may be surprised to leafn that despite its limitations, ping is heavily used as a diagnostic tool. In fact, network administrators often fun ping as soon as they learn of a determine which parts of the network are still operating -x.. XXX h pix.= I ””esul..'”p them pinpoint the failure quickly.