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mtrace - Print multicast path from a source to a receiver  


/usr/bin/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]  


Sends the trace query 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.


Versions 3.3 and 3.5 of the mrouted daemon will crash if a trace query is received in a unicast packet and mrouted has no route for the source address. Therefore, do not use the -g flag unless the target version of mrouted is Version 3.4 or later than Version 3.5.

Specifies 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. Loops indefinitely printing the packet rate and loss statistics for the multicast path every 10 seconds. Use the -S flag to change the time interval. Sends the response using a multicast path rather than attempting a unicast first. Sets the maximum number of hops to be traced from the receiver back toward the source to max_hops. The default is 32 hops (infinity for the DVMRP routing protocol). Prints hop addresses numerically rather than symbolically and numerically (saves a nameserver address-to-name lookup for each router found on the path). Sets the maximum number of query attempts for any hop to nqueries. The default is 3. Listens passively for multicast responses from traces initiated by others. This works best when run on a multicast router. Sends 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 ( Prints a short form output including only the multicast path and not the packet rate and loss statistics. Sets the interval between statistics gathering traces to stat_int seconds. The default is 10 seconds. Sets 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. Prints a verbose form output, displaying hop times on initial trace and statistics. Sets the time to wait for a trace response to n seconds. The default is 3 seconds.



The mtrace program utilizes a tracing feature implemented in multicast routers (mrouted Version 3.3 and later) that is accessed by 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. The response is then returned to the requester.

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. You can specify the receiver and group parameters to test the path to some other receiver in a particular group, subject to some constraints as detailed in the following sections. The two parameters can be distinguished because the receiver is a unicast address and the group is a multicast address.  

How mtrace Works

The technique used by the traceroute tool to trace unicast network paths does not work for IP multicast because Internet Control Message Protocol (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 a multicast router along the path does not implement the multicast traceroute feature or if there is an outage, no response is 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 traced.

The reports inserted by each router contain the address of the hop, the ttl required to forward, some flags to indicate routing errors, and the 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), the default method is to multicast the trace query to ( 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 the same group as the receiver. Therefore you must 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 flag). If the last-hop router is known, it may also be addressed directly using the -g flag). Alternatively, if you want to trace a group that the receiver has not joined, but you know that the last-hop router is a member of another group, you can use the -g flag 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, explicitly specify the desired interface 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 flag), an asterisk (*) is printed and the probing switches to hop-by-hop mode. Trace queries are issued starting with a maximum hop count of 1 and increasing by 1 until the full path is traced or no response is received. At each hop, multiple probes are sent (default is 3, changed with -q flag). The first half of the attempts (default is 1) 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 ( with the ttl set to 32 more than that needed to pass the thresholds encountered so far along the path to the receiver. For the last quarter of the attempts (default is 1), the ttl is increased by another 32 each time up to a maximum of 192. Alternatively, you can set the ttl explicitly with the -t flag or the initial unicast attempts can be forced to use multicast instead with the -M flag. For each attempt, if no response is received within the timeout, an asterisk (*) is printed. After the specified number of attempts have failed, mtrace tries 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 that shows the hop number (counted negatively to indicate that this is the reverse path); the multicast routing protocol (DVMRP, MOSPF, or PIM); 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 that shows 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:

# mtrace -l Mtrace from to via group Querying full reverse path...
  0 (
 -1 (  DVMRP  thresh^ 1  3 ms
 -2 (  DVMRP  thresh^ 1  14 ms
 -3 (  DVMRP  thresh^ 1  50 ms
 -4 (  DVMRP  thresh^ 1  63 ms
 -5 (  DVMRP  thresh^ 1  71 ms
 -6 ( Round trip time 124 ms

The second section shows 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. In the right half of this section are 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. The first group shows the statistics for all traffic flowing out of the interface at one hop and into 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. With no options, the second section of the output is printed once, approximately 10 seconds after the initial trace. For each hop, one line is printed that shows the statistics over that 10-second period. If the -l flag is given, the second section repeats every 10 seconds and two lines are printed 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 flag 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  ---------------------  ------------------
     |     ^     ttl    1      0/6    = --%   0 pps   0/2  = --%  0 pps
     v     |     hop    8 ms   1/52   =  2%   0 pps   0/18 =  0%  0 pps
     |     ^     ttl    2      0/6    = --%   0 pps   0/2  = --%  0 pps
     v     |     hop   12 ms   1/52   =  2%   0 pps   0/18 =  0%  0 pps
     |     ^     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
     |     ^     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
     |      \__  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 change as the trace query propagates, there may be small errors (off by 1 or 2) in these statistics. However, those errors should not accumulate, so the cumulative statistics 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 multiaccess 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 some 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 previous example.

Note that these negative losses may mask positive losses.

In the example, there is also one negative hop time. This 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.

The following example shows a trace to a receiver that is not local; the query is sent to the last-hop router with the -g flag. 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 will not be forwarded.

# mtrace -g \
         Mtrace from to via group Querying full reverse path... * switching to hop-by-hop:
  0 (
 -1 (  DVMRP  thresh^ 1  33 ms  Route pruned
 -2 (  DVMRP  thresh^ 1  36 ms
 -3 (  DVMRP  thresh^ 1  44 ms
 -4 (  DVMRP  thresh^ 16  47 ms
 -5  * * * ( [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.  


Commands: map-mbone(8), mrinfo(8), mrouted(8), traceroute(8) delim off



How mtrace Works
Finding the Last-Hop Router
Directing the Response

This document was created by man2html, using the manual pages.
Time: 02:40:27 GMT, October 02, 2010