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           tracepath [ -n]  [ -l pktlen]  destination [ port]


           It traces path to destination discovering MTU along this path.  It uses
           UDP  port  port or some random port.  It is similar to traceroute, only
           does not not require superuser privileges and has no fancy options.
           tracepath6 is good replacement for traceroute6 and classic  example  of
           application  of  Linux  error  queues.  The situation with tracepath is
           worse, because commercial IP routers do not return  enough  information
           in  icmp  error  messages.  Probably, it will change, when they will be
           updated.  For now it uses Van Jacobson's trick, sweeping a range of UDP
           ports to maintain trace history.


           -n     Do not look up host names.  Only print IP addresses numerically.
           -l     Sets the initial packet length to pktlen instead  of  65536  for
                  tracepath or 128000 for tracepath6.


           root@mops:~ # tracepath6 3ffe:2400:0:109::2
            1?: [LOCALHOST]                              pmtu 1500
            1:                   0.411ms
            2:        asymm  1   0.390ms pmtu 1480
            2:  3ffe:2400:0:109::2               463.514ms reached
                Resume: pmtu 1480 hops 2 back 2
           The  first  column  shows TTL of the probe, followed by colon.  Usually
           value of TTL is obtained from reply from network, but  sometimes  reply
           does not contain necessary information and we have to guess it. In this
           case the number is followed by ?.
           The second column shows the network hop, which replied  to  the  probe.
           It  is  either  address of router or word [LOCALHOST], if the probe was
           not sent to the network.
           The rest of line shows miscellaneous information about path to the cor-
           respinding  hetwork  hop.  As rule it contains value of RTT.  Addition-
           ally, it can show Path MTU, when it changes.  If the path is asymmetric
           or  the  probe  finishes  before  it  reach  prescribed hop, difference
           between number of hops in forward and backward direction is shown  fol-
           loing  keyword async. This information is not reliable.  F.e. the third
           line shows asymmetry of 1, it is because the first probe with TTL of  2
           was rejected at the first hop due to Path MTU Discovery.
           The last line summarizes information about all the path to the destina-
           tion, it shows detected Path MTU, amount of hops to the destination and
           our guess about amount of hops from the destination to us, which can be
           enough  to  send  UDP datagrams to investigated destination using given


           tracepath is part of  iputils  package  and  the  latest  versions  are
           available  in source form at

    iputils-071127 23 July 2015 TRACEPATH(8)


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