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Unformatted text preview: The following output verifies our calculation: file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (5 of 13) [12/09/2001 14.46.45] Chapter 7. Ping Program svr4 % ping -a slip
PING slip: 56 data bytes
64 bytes from slip (22.214.171.124): icmp_seq=0. time=1480.
64 bytes from slip (126.96.36.199): icmp_seq=1. time=1480.
64 bytes from slip (188.8.131.52): icmp_seq=2. time=1480.
64 bytes from slip (184.108.40.206): icmp_seq=3. time=1480.
----slip PING Statistics---5 packets transmitted, 4 packets received, 20% packet loss
round-trip (ms) min/avg/max = 1480/1480/1480 Ms
Ms (The -s option is required for SVR4 to send one request every second.) The round-trip time is
almost 1.5 seconds but the program is still sending out each ICMP echo request at 1-second
intervals. This means there are two outstanding echo requests (sent at time 0 and time 1) before
the first reply comes back (at time 1.480). That's also why the summary line says one packet has
been lost. It really hasn't been lost, it's probably still on its way back.
We'll return to this slow SLIP link in Chapter 8 when we examine the traceroute program.
Dialup SLIP Links
The conditions change with a dialup SLIP link since we now have modems on each end of the
link. The modems being used between the systems sun and netb provide what is called V.32
modulation (9600 bits/sec), V.42 error control (also called LAP-M), and V.42bis data
compression. This means that our simple calculations, which were fairly accurate for a
hardwired link where we knew all the parameters, become less accurate.
Numerous factors are at work. The modems introduce some latency. The size of the packets
may decrease with the data compression, but the size may then increase to a multiple of the
packet size used by the error control protocol. Also the receiving modem can't release received
data bytes until the cyclic redundancy character (the checksum) has been verified. Finally, we're
dealing with a computer's asynchronous serial interface on each end, and many operating
systems read these interfaces only at fixed intervals, or when a certain number of characters
have been received.
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