Secondary School of BatalhaPedro Perfeito

May 2009

Index

1. Introdution2. Output of a traceroute3. How a traceroute works (1)4. How a traceroute works (2)

Introdution

Traceroute is a network diagnostic tool found onmost operating systems, which is used fordetermining which path a packet has taken. Thetraceroute command can be used to draw up a mapof the routers found between a source machine anda target machine. The traceroute command isdifferent on each operating system.The traceroute command is: 

tracert name.of.the.machine

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Output of a tracerouteA traceroute's output describes the names and IP addresses of the chain of routers, each

preceded by a sequential number and minimum, average, and maximum response time.A rastrear a rota para cisco.netacad.net [128.107.229.50]

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Tracing a route to cisco.netacad.net [128.107.229.50] over a maximum of 30 hops:    1     *        *        *     O pedido excedeu o tempo.  2   127 ms   129 ms   139 ms  lis1-br1-gi6-2.cprm.net [195.8.10.201]   3   118 ms   139 ms   149 ms  lis1-cr1-po.cprm.net [195.8.0.221]   4   238 ms   259 ms   269 ms  was1-cr1-ge62.telecom.pt [206.223.115.82]   5   248 ms   249 ms   249 ms  ex2-g8-0s1.eqabva.sbcglobal.net [206.223.115.79]   6   318 ms   328 ms   320 ms  ded4-g1-3-0.sntc01.pbi.net [151.164.41.161]   7   299 ms   318 ms   319 ms  Cisco-Systems-1152786.cust-rtr.pacbell.net [64.161.0.62]   8   298 ms   319 ms   320 ms  sjc5-dmzbb-gw1-gig1-47.cisco.com [128.107.224.105]   9   298 ms   319 ms   310 ms  sjc12-dmzdc-gw2-gig1-1.cisco.com [128.107.224.18]  10   299 ms   319 ms   319 ms  cna-prod-nv.cisco.com [128.107.229.50] Route traced.

How a traceroute works (1)

Traceroute works thanks to the TTL field in IPpackets. Each IP packet has a time to live field(TTL) which is reduced each time it goes through arouter When this field reaches zero, the router determines that the packet has been goingaround in circles, terminates this packet and sendsan ICMP notification to the sender.

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How a traceroute works (2)

For this reason, traceroute sends packets to anunprivileged UDP port which is believed to beunused (port 33434 by default) with a TTL set to 1.The first router encountered will delete the packetand send an ICMP packet which includes the IPaddress of the router as well as the loop delay.Traceroute then increases the TTL field by 1 at atime, so as to obtain a reply from each router on thepathway, until it gets the reply "ICMP port

unreachable" from the target machine.

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That´s all… thanks for watching!

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