MTRACE(8) BSD System Manager's Manual MTRACE(8)
mtrace -- print multicast path from a source to a receiver
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
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
0.0.0.0, 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
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:
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
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 attempt-
ing 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 (infin-
ity 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 sta-
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 (22.214.171.124).
-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 sec-
onds (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
-v Verbose mode; show hop times on the initial trace and statistics
display. Also show the route that was used to forward the ini-
-w n Set the time to wait for a trace response to n seconds (default 3
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 specifi-
cally 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 accumu-
lated 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 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
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 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 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
(126.96.36.199) 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
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 (188.8.131.52) 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 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 num-
ber 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 isn't 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 previ-
ous 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 184.108.40.206
Mtrace from 220.127.116.11 to 18.104.22.168 via group 22.214.171.124
Querying full reverse path...
0 oak.isi.edu (126.96.36.199)
-1 cub.isi.edu (188.8.131.52) DVMRP thresh^ 1 3 ms
-2 la.dart.net (184.108.40.206) DVMRP thresh^ 1 14 ms
-3 dc.dart.net (220.127.116.11) DVMRP thresh^ 1 50 ms
-4 bbn.dart.net (18.104.22.168) DVMRP thresh^ 1 63 ms
-5 mit.dart.net (22.214.171.124) DVMRP thresh^ 1 71 ms
-6 caraway.lcs.mit.edu (126.96.36.199)
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 sup-
plied, the route being used to forward packets is printed in the form
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 aver-
age 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 cal-
culated 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 sec-
ond group shows the statistics only for traffic forwarded from the speci-
fied source to the specified group. The first group of statistics may be
expanded to include loss rates using the -T option. However, these num-
bers 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, 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
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
188.8.131.52 184.108.40.206 Packet 220.127.116.11 To 18.104.22.168
| __/ rtt 125 ms Rate Lost/Sent = Pct Rate
v / hop 65 ms ------- ---------------------
| ^ ttl 1 0 pps 0/2 = --% 0 pps
v | hop 8 ms 0 pps 0/18 = 0% 0 pps
| ^ ttl 2 0 pps 0/2 = --% 0 pps
v | hop 12 ms 0 pps 0/18 = 0% 0 pps
| ^ ttl 3 27 pps 0/2 = --% 0 pps
v | hop 34 ms 26 pps 0/18 = 0% 0 pps
| ^ ttl 4 83 pps 0/2 = --% 0 pps
v | hop 11 ms 79 pps 0/18 = 0% 0 pps
| \__ ttl 5 83 pps ?/2 0 pps
v \ hop -8 ms 79 pps ?/18 0 pps
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
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 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
``Output pruned'' error code indicates that traffic for group
22.214.171.124 would not be forwarded.
oak.isi.edu 108# mtrace -g 126.96.36.199 188.8.131.52 \
Mtrace from 184.108.40.206 to 220.127.116.11 via group 18.104.22.168
Querying full reverse path... * switching to hop-by-hop:
0 butter.lcs.mit.edu (22.214.171.124)
-1 jam.lcs.mit.edu (126.96.36.199) DVMRP thresh^ 1 33 ms Output pruned
-2 bbn.dart.net (188.8.131.52) DVMRP thresh^ 1 36 ms
-3 dc.dart.net (184.108.40.206) DVMRP thresh^ 1 44 ms
-4 darpa.dart.net (220.127.116.11) DVMRP thresh^ 16 47 ms
-5 * * * noc.hpc.org (18.104.22.168) [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.
map-mbone(8), mrinfo(8), mrouted(8), traceroute(8)
Statistics collection in passive mode doesn't always produce the same
output as when actively collecting data.
BSD May 8, 1995 BSD