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Unformatted text preview: ime=390. ms
64 bytes from (22.214.171.124): icmp_seq=14. time=110. ms
64 bytes from (126.96.36.199): icmp_seq=15. time=170. ms
64 bytes from (188.8.131.52): icmp_seq=16. time=100. ms
^? type interrupt key to stop
----vangogh.CS.Berkeley.EDU PING Statistics---17 packets transmitted, 8 packets received, 52% packet loss
round-trip (ms) min/avg/max = 100/501/1780
Either the echo requests or the echo replies for sequence numbers 1, 2, 3, 4, 6, 10, II, 12, and 13
were lost somewhere. Note also the large variance in the round-trip times. (This high packet loss
rate of 52% is an anomaly. This is not normal for the Internet, even on a weekday afternoon.)
It is also possible across WANs to see packets duplicated (the same sequence number printed
two or more times), and to see packets reordered (sequence number N +1 printed before
sequence number N).
Hardwired SLIP Links
Let's look at the round-trip times encountered over SLIP links, since they often run at slow
asynchronous speeds, such as 9600 bits/sec or less. Recall our serial line throughput calculations
in Section 2.10. For this example we'll set the speed of the hardwired SLIP link between hosts
bsdi and slip to 1200 bits/sec.
We can estimate the round-trip time as follows. First, notice from the example Ping output
shown earlier that by default it sends 56 bytes of data in the ICMP message. With a 20-byte IP
header and an 8-byte ICMP header this gives a total IP datagram size of 84 bytes. (We can
verify this by running tcpdump -e and seeing the Ethernet frame sizes.) Also, from Section 2.4
we know that at least two additional bytes are added: the END byte at the beginning and end of
the datagram. It's also possible for additional bytes to be added by the SLIP framing, but that
depends on the value of each byte in the datagram. At 1200 bits/sec with 8 bits per byte, I start
bit, and I stop bit, the rate is 120 bytes per second, or 8.33 ms per byte. Our estimate is then (86
x 8.33 x 2), or 1433 ms. (The multiplier of 2 is because we are calculating the round-trip time.)...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12