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Unformatted text preview: ify a pattern of 0xc0, recalculate the
answer to the previous exercise. (Hint: Reread Section 2.4.)
7.4 Does the use of compressed SLIP (CSLIP, Section 2.5) affect the ping times that we
observed in Section 7.2?
7.5 Examine Figure 2.4. Do you expect any difference between a ping of the loopback address,
versus a ping of the host's Ethernet address? file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/pingprog.htm (13 of 13) [12/09/2001 14.46.45] Traceroute Program Traceroute Program
The Traceroute program, written by Van Jacobson, is a handy debugging tool that allows us
to further explore the TCP/IP protocols. Although there are no guarantees that two
consecutive IP datagrams from the same source to the same destination follow the same
route, most of the time they do. Traceroute lets us see the route that IP datagrams follow
from one host to another. Traceroute also lets us use the IP source route option.
The manual page states: "Implemented by Van Jacobson from a suggestion by Steve
Deering. Debugged by a cast of thousands with particularly cogent suggestions or fixes
from C. Philip Wood, Tim Seaver, and Ken Adelman." 8.2 Traceroute Program Operation
In Section 7.3 we described the IP record route option (RR). Why wasn't this used instead
of developing a new application? There are three reasons. First, historically not all routers
have supported the record route option, making it unusable on certain paths. (Traceroute
doesn't require any special or optional features at any intermediate routers.)
Second, record route is normally a one-way option. The sender enables the option and the
receiver has to fetch all the values from the received IP header and somehow return them to
the sender. In Section 7.3 we saw that most implementations of the Ping server (the ICMP
echo reply function within the kernel) reflect an incoming RR list, but this doubles the
number of IP addresses recorded (the outgoing path and the return path), which runs into the
limit described in the next paragraph. (Trac...
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