Because each router along the path is required to decrement the packet??™s TTL
by at least 1 before forwarding the packet, the TTL is effectively a hop counter. When the
TTL on a packet reaches zero (0), the router sends an ICMP ???Time Exceeded??? message
back to the source computer.
TRACERT sends the ?¬?rst echo packet with a TTL of 1 and increments the TTL by 1 on each
subsequent transmission until the destination responds or until the maximum TTL is
reached. The ICMP ???Time Exceeded??? messages that intermediate routers send back show
the route. Note, however, that some routers silently drop packets with expired TTL values,
and these packets are invisible to TRACERT.
TRACERT prints out an ordered list of the intermediate routers that return ICMP ???Time
Exceeded??? messages. Using the -d option with the tracert command instructs TRACERT
not to perform a DNS lookup on each IP address, so that TRACERT reports the IP address
of the near-side interface of the routers. Figure 1-88 shows a traceroute to yahoo.com.
Exploring the Packet Delivery Process 103
Figure 1-88 Performing a Traceroute
The syntax for a windows traceroute is as follows:
ttrraacceerrtt -d -h maximum_hops --jj HostList --ww Timeout target_host
The following are the parameters associated with the windows traceroute command:
?– -d: Speci?¬?es to not resolve addresses to hostnames
?– -h maximum_hops: Speci?¬?es the maximum number of hops to search for the target
?– -j HostList: Speci?¬?es loose source route along the host list
?– -w Timeout: Waits the number of milliseconds speci?¬?ed by timeout for each reply
?– target_host: Speci?¬?es the name or IP address of the target host
Summary of Exploring the Packet Delivery Process
The following summarizes the key points that were discussed in this lesson.
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