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MTRACE(8)                   System Manager's Manual                  MTRACE(8)

       mtrace - print multicast path from a source to a receiver

       mtrace [ -g gateway ] [ -i if_addr ] [ -l ] [ -M ] [ -m max_hops ] [ -n
       ] [ -p ] [ -q nqueries ] [ -r resp_dest ] [ -s ] [ -S stat_int ] [ -t
       ttl ] [ -v ] [ -w waittime ] source [ receiver ] [ group ]

       Assessing problems in the distribution of IP multicast traffic can be
       difficult.  mtrace uses a tracing feature implemented in multicast
       routers (mrouted version 3.3 and later) that is accessed via an
       extension to the IGMP protocol.  A trace query is passed hop-by-hop
       along the reverse path from the receiver to the source, collecting hop
       addresses, packet counts, and routing error conditions along the path,
       and then the response is returned to the requestor.

       The only required parameter is the source host name or address.  The
       default receiver is the host running mtrace, and the default group is
       "MBone Audio" (, which is sufficient if packet loss
       statistics for a particular multicast group are not needed.  These two
       optional parameters may be specified to test the path to some other
       receiver in a particular group, subject to some constraints as detailed
       below.  The two parameters can be distinguished because the receiver is
       a unicast address and the group is a multicast address.

       NOTE: For Solaris 2.4/2.5, if the multicast interface is not the
       default interface, the -i option must be used to set the local address.

       -g gwy  Send the trace query via unicast directly to the multicast
               router gwy rather than multicasting the query.  This must be
               the last-hop router on the path from the intended source to the

               CAUTION!!   Versions 3.3 and 3.5 of mrouted will crash if a
                           trace query is received via a unicast packet and
                           mrouted has no route for the source address.
                           Therefore, do not use the -g option unless the
                           target mrouted has been verified to be 3.4 or newer
                           than 3.5.

       -i addr Use addr as the local interface address (on a multi-homed host)
               for sending the trace query and as the default for the receiver
               and the response destination.

       -l      Loop indefinitely printing packet rate and loss statistics for
               the multicast path every 10 seconds (see -S stat_int).

       -M      Always send the response using multicast rather than attempting
               unicast first.

       -m n    Set to n the maximum number of hops that will be traced from
               the receiver back toward the source.  The default is 32 hops
               (infinity for the DVMRP routing protocol).

       -n      Print hop addresses numerically rather than symbolically and
               numerically (saves a nameserver address-to-name lookup for each
               router found on the path).

       -q n    Set the maximum number of query attempts for any hop to n.  The
               default is 3.

       -p      Listen passively for multicast responses from traces initiated
               by others.  This works best when run on a multicast router.

       -r host Send the trace response to host rather than to the host on
               which mtrace is being run, or to a multicast address other than
               the one registered for this purpose (

       -s      Print a short form output including only the multicast path and
               not the packet rate and loss statistics.

       -S n    Change the interval between statistics gathering traces to n
               seconds (default 10 seconds).

       -t ttl  Set the ttl (time-to-live, or number of hops) for multicast
               trace queries and responses.  The default is 64, except for
               local queries to the "all routers" multicast group which use
               ttl 1.

       -v      Verbose mode; show hop times on the initial trace and
               statistics display.

       -w n    Set the time to wait for a trace response to n seconds (default
               3 seconds).

   How It Works
       The technique used by the traceroute tool to trace unicast network
       paths will not work for IP multicast because ICMP responses are
       specifically forbidden for multicast traffic.  Instead, a tracing
       feature has been built into the multicast routers.  This technique has
       the advantage that additional information about packet rates and losses
       can be accumulated while the number of packets sent is minimized.

       Since multicast uses reverse path forwarding, the trace is run
       backwards from the receiver to the source.  A trace query packet is
       sent to the last hop multicast router (the leaf router for the desired
       receiver address).  The last hop router builds a trace response packet,
       fills in a report for its hop, and forwards the trace packet using
       unicast to the router it believes is the previous hop for packets
       originating from the specified source.  Each router along the path adds
       its report and forwards the packet.  When the trace response packet
       reaches the first hop router (the router that is directly connected to
       the source's net), that router sends the completed response to the
       response destination address specified in the trace query.

       If some multicast router along the path does not implement the
       multicast traceroute feature or if there is some outage, then no
       response will be returned.  To solve this problem, the trace query
       includes a maximum hop count field to limit the number of hops traced
       before the response is returned.  That allows a partial path to be

       The reports inserted by each router contain not only the address of the
       hop, but also the ttl required to forward and some flags to indicate
       routing errors, plus counts of the total number of packets on the
       incoming and outgoing interfaces and those forwarded for the specified
       group.  Taking differences in these counts for two traces separated in
       time and comparing the output packet counts from one hop with the input
       packet counts of the next hop allows the calculation of packet rate and
       packet loss statistics for each hop to isolate congestion problems.

   Finding the Last-Hop Router
       The trace query must be sent to the multicast router which is the last
       hop on the path from the source to the receiver.  If the receiver is on
       the local subnet (as determined using the subnet mask), then the
       default method is to multicast the trace query to all-routers.mcast.net
       ( with a ttl of 1.  Otherwise, the trace query is multicast
       to the group address since the last hop router will be a member of that
       group if the receiver is.  Therefore it is necessary to specify a group
       that the intended receiver has joined.  This multicast is sent with a
       default ttl of 64, which may not be sufficient for all cases (changed
       with the -t option).  If the last hop router is known, it may also be
       addressed directly using the -g option).  Alternatively, if it is
       desired to trace a group that the receiver has not joined, but it is
       known that the last-hop router is a member of another group, the -g
       option may also be used to specify a different multicast address for
       the trace query.

       When tracing from a multihomed host or router, the default receiver
       address may not be the desired interface for the path from the source.
       In that case, the desired interface should be specified explicitly as
       the receiver.

   Directing the Response
       By default, mtrace first attempts to trace the full reverse path,
       unless the number of hops to trace is explicitly set with the -m
       option.  If there is no response within a 3 second timeout interval
       (changed with the -w option), a "*" is printed and the probing switches
       to hop-by-hop mode.  Trace queries are issued starting with a maximum
       hop count of one and increasing by one until the full path is traced or
       no response is received.  At each hop, multiple probes are sent
       (default is three, changed with -q option).  The first half of the
       attempts (default is one) are made with the unicast address of the host
       running mtrace as the destination for the response.  Since the unicast
       route may be blocked, the remainder of attempts request that the
       response be multicast to mtrace.mcast.net ( with the ttl set
       to 32 more than what's needed to pass the thresholds seen so far along
       the path to the receiver.  For the last quarter of the attempts
       (default is one), the ttl is increased by another 32 each time up to a
       maximum of 192.  Alternatively, the ttl may be set explicitly with the
       -t option and/or the initial unicast attempts can be forced to use
       multicast instead with the -M option.  For each attempt, if no response
       is received within the timeout, a "*" is printed.  After the specified
       number of attempts have failed, mtrace will try to query the next hop
       router with a DVMRP_ASK_NEIGHBORS2 request (as used by the mrinfo
       program) to see what kind of router it is.

       The output of mtrace is in two sections.  The first section is a short
       listing of the hops in the order they are queried, that is, in the
       reverse of the order from the source to the receiver.  For each hop, a
       line is printed showing the hop number (counted negatively to indicate
       that this is the reverse path); the multicast routing protocol (DVMRP,
       MOSPF, PIM, etc.); the threshold required to forward data (to the
       previous hop in the listing as indicated by the up-arrow character);
       and the cumulative delay for the query to reach that hop (valid only if
       the clocks are synchronized).  This first section ends with a line
       showing the round-trip time which measures the interval from when the
       query is issued until the response is received, both derived from the
       local system clock.  A sample use and output might be:

       oak.isi.edu 80# mtrace -l caraway.lcs.mit.edu
       Mtrace from to via group
       Querying full reverse path...
         0  oak.isi.edu (
        -1  cub.isi.edu (  DVMRP  thresh^ 1  3 ms
        -2  la.dart.net (  DVMRP  thresh^ 1  14 ms
        -3  dc.dart.net (  DVMRP  thresh^ 1  50 ms
        -4  bbn.dart.net (  DVMRP  thresh^ 1  63 ms
        -5  mit.dart.net (  DVMRP  thresh^ 1  71 ms
        -6  caraway.lcs.mit.edu (
       Round trip time 124 ms

       The second section provides a pictorial view of the path in the forward
       direction with data flow indicated by arrows pointing downward and the
       query path indicated by arrows pointing upward.  For each hop, both the
       entry and exit addresses of the router are shown if different, along
       with the initial ttl required on the packet in order to be forwarded at
       this hop and the propagation delay across the hop assuming that the
       routers at both ends have synchronized clocks.  The right half of this
       section is composed of several columns of statistics in two groups.
       Within each group, the columns are the number of packets lost, the
       number of packets sent, the percentage lost, and the average packet
       rate at each hop.  These statistics are calculated from differences
       between traces and from hop to hop as explained above.  The first group
       shows the statistics for all traffic flowing out the interface at one
       hop and in the interface at the next hop.  The second group shows the
       statistics only for traffic forwarded from the specified source to the
       specified group.

       These statistics are shown on one or two lines for each hop.  Without
       any options, this second section of the output is printed only once,
       approximately 10 seconds after the initial trace.  One line is shown
       for each hop showing the statistics over that 10-second period.  If the
       -l option is given, the second section is repeated every 10 seconds and
       two lines are shown for each hop.  The first line shows the statistics
       for the last 10 seconds, and the second line shows the cumulative
       statistics over the period since the initial trace, which is 101
       seconds in the example below.  The second section of the output is
       omitted if the -s option is set.

       Waiting to accumulate statistics... Results after 101 seconds:

         Source       Response Dest  Packet Statistics For  Only For Traffic  All Multicast Traffic  From
            |       __/ rtt  125 ms  Lost/Sent = Pct  Rate    To
            v      /    hop   65 ms  ---------------------  ------------------   mit.dart.net
            |     ^     ttl    1      0/6    = --%   0 pps   0/2  = --%  0 pps
            v     |     hop    8 ms   1/52   =  2%   0 pps   0/18 =  0%  0 pps   bbn.dart.net
            |     ^     ttl    2      0/6    = --%   0 pps   0/2  = --%  0 pps
            v     |     hop   12 ms   1/52   =  2%   0 pps   0/18 =  0%  0 pps   dc.dart.net
            |     ^     ttl    3      0/271  =  0%  27 pps   0/2  = --%  0 pps
            v     |     hop   34 ms  -1/2652 =  0%  26 pps   0/18 =  0%  0 pps  la.dart.net
            |     ^     ttl    4     -2/831  =  0%  83 pps   0/2  = --%  0 pps
            v     |     hop   11 ms  -3/8072 =  0%  79 pps   0/18 =  0%  0 pps  cub.isi.edu
            |      \__  ttl    5        833         83 pps     2         0 pps
            v         \ hop   -8 ms     8075        79 pps     18        0 pps
         Receiver     Query Source

       Because the packet counts may be changing as the trace query is
       propagating, there may be small errors (off by 1 or 2) in these
       statistics.  However, those errors should not accumulate, so the
       cumulative statistics line should increase in accuracy as a new trace
       is run every 10 seconds.  There are two sources of larger errors, both
       of which show up as negative losses:

              ⊕  If the input to a node is from a multi-access network with
                 more than one other node attached, then the input count will
                 be (close to) the sum of the output counts from all the
                 attached nodes, but the output count from the previous hop on
                 the traced path will be only part of that.  Hence the output
                 count minus the input count will be negative.
              ⊕  In release 3.3 of the DVMRP multicast forwarding software for
                 SunOS and other systems, a multicast packet generated on a
                 router will be counted as having come in an interface even
                 though it did not.  This creates the negative loss that can
                 be seen in the example above.

       Note that these negative losses may mask positive losses.

       In the example, there is also one negative hop time.  This simply
       indicates a lack of synchronization between the system clocks across
       that hop.  This example also illustrates how the percentage loss is
       shown as two dashes when the number of packets sent is less than 10
       because the percentage would not be statistically valid.

       A second example shows a trace to a receiver that is not local; the
       query is sent to the last-hop router with the -g option.  In this
       example, the trace of the full reverse path resulted in no response
       because there was a node running an old version of mrouted that did not
       implement the multicast traceroute function, so mtrace switched to hop-
       by-hop mode.  The "Route pruned" error code indicates that traffic for
       group would not be forwarded.

       oak.isi.edu 108# mtrace -g \
       Mtrace from to via group
       Querying full reverse path... * switching to hop-by-hop:
         0  butter.lcs.mit.edu (
        -1  jam.lcs.mit.edu (  DVMRP  thresh^ 1  33 ms  Route pruned
        -2  bbn.dart.net (  DVMRP  thresh^ 1  36 ms
        -3  dc.dart.net (  DVMRP  thresh^ 1  44 ms
        -4  darpa.dart.net (  DVMRP  thresh^ 16  47 ms
        -5  * * * noc.hpc.org ( [mrouted 2.2] didn't respond
       Round trip time 95 ms

       Implemented by Steve Casner based on an initial prototype written by
       Ajit Thyagarajan.  The multicast traceroute mechanism was designed by
       Van Jacobson with help from Steve Casner, Steve Deering, Dino
       Farinacci, and Deb Agrawal; it was implemented in mrouted by Ajit
       Thyagarajan and Bill Fenner.  The option syntax and the output format
       of mtrace are modeled after the unicast traceroute program written by
       Van Jacobson.

       mrouted(8), mrinfo(8), map-mbone(8), traceroute(8)

4.3 Berkeley Distribution         May 8, 1995                        MTRACE(8)