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MTRACE (8) | System administration commands and daemons | Unix Manual Pages | :man


mtrace - print multicast path from a source to a receiver


Finding the Last-Hop Router
Directing the Response
See Also


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


Assessing problems in the distribution of IP multicast traffic can be difficult. The mtrace utility utilizes a tracing feature implemented in multicast routers 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, 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. If the -g flag is specified, the source address defaults to the host running mtrace, and the receiver defaults to the router being addressed with the -g flag. In this case, there are no required parameters.

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.

The following options are available:

-e extrahops
Try tracing extrahops hops past a non-responding router.
-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 receiver.

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 request the response using multicast rather than attempting unicast for the last half of the tries.
-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.
-O Do not use the Router-Alert IP option on those requests which need it. Some versions of Cisco’s IOS cannot handle multicast traceroutes with IP options, so it may be necessary to use the -O flag if the last-hop router is a Cisco.
-p Listen passively for multicast responses from traces initiated by others. This works best when run on a multicast router.
-P Loop indefinitely collecting the path every 10 seconds (see -S stat_int) and printing it when it changes. Do not print any statistics.
-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 127, except for local queries to the "all routers" multicast group which use ttl 1.
-T "Tunnel statistics" mode; show loss rates for overall traffic. These statistics can be extremely misleading.
-U Always request the response using unicast rather than attempting multicast first.
-v Verbose mode; show hop times on the initial trace and statistics display. Also show the route that was used to forward the initial trace.
-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 utility 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 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 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 127, 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 two) are made with the reply address set to standard multicast address, mtrace.mcast.net ( with the ttl set to 32 more than what is needed to pass the thresholds seen so far along the path to the receiver. For each additional attempt, the ttl is increased by another 32 each time up to a maximum of 192. Since the desired router may not be able to send a multicast reply, the remainder of the attempts request that the response be sent via unicast to the host running mtrace. Alternatively, the multicast ttl may be set explicitly with the -t option, the initial multicast attempts can be forced to use unicast instead with the -U option, the final unicast attempts can be forced to use multicast instead with the -M option, or if you specify -UM , mtrace will first attempt using unicast and then multicast. 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 mtrace utility will try to query three (changed with the -e option) hops past a non-responding router, in the hopes that even though it is not capable of sending a response, it might be capable of forwarding the request on.


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, and the total ttl required for a packet to travel along this path. 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; total ttl of 6 required.

If a hop reports that it is using the default route to forward packets, the word [default] is printed after that hop. If the -v flag is supplied, the route being used to forward packets is printed in the form [18.26.0/24].

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 two sets of statistics. The first column contains the average packet rate for all traffic at each hop. The remaining 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. The first group of statistics may be expanded to include loss rates using the -T option. However, these numbers can be extremely misleading and require detailed knowledge of the routers involved to be interpreted properly.

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 or if no multicast group is specified.

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

Source Response Dest Overall Packet Statistics For Traffic From Packet To
| __/ rtt 125 msRateLost/Sent = Pct Rate
v / hop 65 ms ------- --------------------- mit.dart.net
|^ttl 1 0 pps 0/2 = --% 0 pps
v|hop 8 ms 0 pps 0/18 = 0% 0 pps bbn.dart.net
|^ttl 2 0 pps 0/2 = --% 0 pps
v|hop 12 ms 0 pps 0/18 = 0% 0 pps dc.dart.net
|^ttl 3 27 pps 0/2 = --% 0 pps
v|hop 34 ms26 pps 0/18 = 0% 0 pps la.dart.net
|^ttl 4 83 pps 0/2 = --% 0 pps
v|hop 11 ms79 pps 0/18 = 0% 0 pps cub.isi.edu
| \__ ttl 5 83 pps ?/2 0 pps
v \ hop -8 ms79 pps ?/18 0 pps
ReceiverQuery 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 "Output 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 Output 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


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



Created by Blin Media, 2008-2013