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

NAME
     mtrace - print multicast path from a source to a receiver

SYNOPSIS
     mtrace [-lMnpsv] [-g gateway] [-i if_addr] [-m max_hops] [-q nqueries]
            [-r host] [-S stat_int] [-t ttl] [-w waittime] source [receiver]
            [group]

DESCRIPTION
     Assessing problems in the distribution of IP multicast traffic can be
     difficult.  mtrace utilizes 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 re-
     verse 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 de-
     fault receiver is the host running mtrace, and the default group is
     "MBone Audio" (224.2.0.1), which is sufficient if packet loss statistics
     for a particular multicast group are not needed.  These two optional pa-
     rameters 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.

     The options are as follows:

     -g gateway
              Send the trace query via unicast directly to the multicast
              router gateway rather than multicasting the query.  This must be
              the last-hop router on the path from the intended source to the
              receiver.  NOTE: Read the BUGS section below.

     -i if_addr
              Use if_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 max_hops
              Set to the maximum number of hops that will be traced from the
              receiver back toward the source.  The default is 32 hops (infin-
              ity for the DVMRP routing protocol).

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

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

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

     -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 (224.0.1.32).

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

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

     -t ttl   Set the ttl (time-to-live, or number of hops) for multicast
              trace queries and responses.  The default is 64, except for lo-
              cal 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 waittime
              Set the time to wait for a trace response to waittime 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 ad-
     dress).  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 for-
     wards 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 re-
     turned.  That allows a partial path to be traced.

     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 incom-
     ing 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 to the receiver.  If the receiver is on the lo-
     cal subnet (as determined using the subnet mask), then the default method
     is to multicast the trace query to all-routers.mcast.net (224.0.0.2) 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 is 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 ad-
     dress 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
     -m 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 in-
     creasing 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 at-
     tempts request that the response be multicast to mtrace.mcast.net
     (224.0.1.32) 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.

EXAMPLES
     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 re-
     verse of the order from the to the For each hop, a line is printed show-
     ing the hop number (counted negatively to indicate that this is the re-
     verse path); the multicast routing protocol (DVMRP, MOSPF, PIM, etc.);
     the threshold required to forward data (to the previous hop in the list-
     ing 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 224.2.0.3
     Mtrace from 18.26.0.170 to 128.9.160.100 via group 224.2.0.3
     Querying full reverse path...
       0  oak.isi.edu (128.9.160.100)
      -1  cub.isi.edu (128.9.160.153)  DVMRP  thresh^ 1  3 ms
      -2  la.dart.net (140.173.128.1)  DVMRP  thresh^ 1  14 ms
      -3  dc.dart.net (140.173.64.1)  DVMRP  thresh^ 1  50 ms
      -4  bbn.dart.net (140.173.32.1)  DVMRP  thresh^ 1  63 ms
      -5  mit.dart.net (140.173.48.2)  DVMRP  thresh^ 1  71 ms
      -6  caraway.lcs.mit.edu (18.26.0.170)
     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, approxi-
     mately 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 be-
     low.  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
     18.26.0.170    128.9.160.100  All Multicast Traffic  From 18.26.0.170
          |       __/ rtt  125 ms  Lost/Sent = Pct  Rate    To 224.2.0.3
          v      /    hop   65 ms  ---------------------  ------------------
     18.26.0.144
     140.173.48.2   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
     140.173.48.1
     140.173.32.1   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
     140.173.32.2
     140.173.64.1   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
     140.173.64.2
     140.173.128.1  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
     140.173.128.2
     128.9.160.153  cub.isi.edu
          |      \__  ttl    5        833         83 pps     2         0 pps
          v         \ hop   -8 ms     8075        79 pps     18        0 pps
     128.9.160.100  128.9.160.100
       Receiver     Query Source

     Because the packet counts may be changing as the trace query is propagat-
     ing, there may be small errors (off by 1 or 2) in these statistics.  How-
     ever, 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:

         o   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.

         o   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 ex-
             ample above.

     Note that these negative losses may mask positive losses.

     In the example, there is also one negative hop time.  This simply indi-
     cates 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 mul-
     ticast traceroute function, so mtrace switched to hop-by-hop mode.  The
     "Route pruned" error code indicates that traffic for group 224.2.143.24
     would not be forwarded.

     oak.isi.edu 108# mtrace -g 140.173.48.2 204.62.246.73 \
                                butter.lcs.mit.edu 224.2.143.24
     Mtrace from 204.62.246.73 to 18.26.0.151 via group 224.2.143.24
     Querying full reverse path... * switching to hop-by-hop:
       0  butter.lcs.mit.edu (18.26.0.151)
      -1  jam.lcs.mit.edu (18.26.0.144)  DVMRP  thresh^ 1  33 ms  Route pruned
      -2  bbn.dart.net (140.173.48.1)  DVMRP  thresh^ 1  36 ms
      -3  dc.dart.net (140.173.32.2)  DVMRP  thresh^ 1  44 ms
      -4  darpa.dart.net (140.173.240.2)  DVMRP  thresh^ 16  47 ms
      -5  * * * noc.hpc.org (192.187.8.2) [mrouted 2.2] didn't respond
     Round trip time 95 ms

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

AUTHORS
     Implemented by Steve Casner based on an initial prototype written by Ajit
     Thyagarajan.  The multicast traceroute mechanism was designed by Van Ja-
     cobson 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.

BUGS
     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.

OpenBSD 4.7                      May 31, 2007                                5
