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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 utilizes a tracing fea-
ture 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
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" (18.104.22.168), which
is sufficient if packet loss statistics for a particular
multicast group are not needed. These two optional param-
eters may be specified to test the path to some other
receiver in a particular group, subject to some con-
straints 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 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. There-
fore, 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
May 8, 1995 1
-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
-n Print hop addresses numerically rather than sym-
bolically and numerically (saves a nameserver
address-to-name lookup for each router found on
-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 multi-
cast address other than the one registered for
this purpose (22.214.171.124).
-s Print a short form output including only the mul-
ticast path and not the packet rate and loss
-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 traf-
fic. Instead, a tracing feature has been built into the
multicast routers. This technique has the advantage that
May 8, 1995 2
additional information about packet rates and losses can
be accumulated while the number of packets sent is mini-
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
If some multicast router along the path does not implement
the multicast traceroute feature or if there is some out-
age, 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 sepa-
rated 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 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
May 8, 1995 3
that the last-hop router is a member of another group, the
-g option may also be used to specify a different multi-
cast 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 (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 the last quarter of
the attempts (default is one), the ttl is increased by
another 32 each time up to a maximum of 192. Alterna-
tively, the ttl may be set explicity 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 pro-
gram) to see what kind of router it is.
The output of mtrace is in two sections. The first sec-
tion 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 pro-
tocol (DVMRP, MOSPF, PIM, etc.); the threshold required to
forward data (to the previous hop in the listing as indi-
cated 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:
May 8, 1995 4
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
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 synchro-
nized 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 statis-
tics 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 out-
put 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
188.8.131.52 184.108.40.206 All Multicast Traffic From 220.127.116.11
| __/ rtt 125 ms Lost/Sent = Pct Rate To 18.104.22.168
v / hop 65 ms --------------------- ------------------
| ^ ttl 1 0/6 = --% 0 pps 0/2 = --% 0 pps
May 8, 1995 5
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 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 sec-
onds. 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 multi-
cast 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 illus-
trates 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
May 8, 1995 6
-g option. In this example, the trace of the full reverse
path resulted in no response because there was a node run-
ning 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 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 Route 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.
mrouted(8), mrinfo(8), map-mbone(8), traceroute(8)
May 8, 1995 7
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
FreeBSD Sources for mtrace(8)
OpenBSD sources for mtrace(8)
Up to: Multicast - Implementation of multicast routing for communications and networking
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