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Unformatted text preview: ted values are 180 ms.
7.3 (86 + 48) bytes divided by 960 bytes/sec, times 2 gives 279.2 ms. The additional 48
bytes are because the final 48 bytes of the 56 bytes in the data portion must be escaped:
0xc0 is the SLIP END character.
7.4 CSLIP only compresses the TCP and IP headers for TCP segments. It has no effect on
the ICMP messages used by ping.
7.5 On a SPARCstation ELC a ping of the loopback address yields an RTT of 1.310 ms,
while a ping of the host's Ethernet address yields an RTT of 1.460 ms. This difference is
the additional processing done by the Ethernet driver, to determine that the datagram is
really destined for the local host. You need a version of ping that outputs microsecond
resolution to measure this. Chapter 8
8.1 If an incoming datagram has a TTL of 0, doing the decrement and then test would set
the TTL to 255 and let the datagram continue. Although a router should never receive a
datagram with a TTL of 0, it has occurred.
8.2 We noted that traceroute stores 12 bytes of data in the data portion of the UDP
datagram, part of which is the time the datagram was sent. From Figure 6.9, however, we
see that ICMP only returns the first 8 bytes of the IP datagram that was in error, and we
noted there that this is the 8-byte UDP header. Therefore the time value stored by
traceroute is not returned in the ICMP error message. traceroute saves the time
when it sends a packet, and when an ICMP reply is received, fetches the current time and file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_d.htm (4 of 20) [12/09/2001 14.48.03] Appendix D: Solutions to Selected Exercises subtracts the two value to get the RTT.
Recall from Chapter 7 that ping stored the time in the outgoing ICMP echo request and
this data was echoed by the server. This allowed ping to print the correct RTT, even if
the packets were returned out of order.
8.3 The first line of output is correct and identifies R1. The next probe starts with a TTL
of 2, and this is decremented by R1. When R2 receives this it decrements the TTL from 1
to 0 but incorrectly forwards it to R3. R3 sees that the incoming TTL is 0 an...
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