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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 uses a tracing feature implemented in multicast
routers (mrouted version 3.3 and later) that is accessed via an exten-
sion 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
"MBone Audio" (126.96.36.199), which is sufficient if packet loss statis-
tics 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.
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
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 tar-
get mrouted has been verified to be 3.4 or newer
-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 send the response using multicast rather than attempting
-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.
-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 multicast address other than
the one registered for this purpose (188.8.131.52).
-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 64, except for
local queries to the "all routers" multicast group which use
-v Verbose mode; show hop times on the initial trace and statis-
-w n Set the time to wait for a trace response to n seconds (default
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 specif-
ically forbidden for multicast traffic. Instead, a tracing feature has
been built into the multicast routers. This technique has the advan-
tage 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 back-
wards 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 multi-
cast 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
(184.108.40.206) 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 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
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 (220.127.116.11) 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 mul-
ticast 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 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 pre-
vious 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 sys-
tem clock. A sample use and output might be:
oak.isi.edu 80# mtrace -l caraway.lcs.mit.edu 18.104.22.168
Mtrace from 22.214.171.124 to 126.96.36.199 via group 188.8.131.52
Querying full reverse path...
0 oak.isi.edu (184.108.40.206)
-1 cub.isi.edu (220.127.116.11) DVMRP thresh^ 1 3 ms
-2 la.dart.net (18.104.22.168) DVMRP thresh^ 1 14 ms
-3 dc.dart.net (22.214.171.124) DVMRP thresh^ 1 50 ms
-4 bbn.dart.net (126.96.36.199) DVMRP thresh^ 1 63 ms
-5 mit.dart.net (188.8.131.52) DVMRP thresh^ 1 71 ms
-6 caraway.lcs.mit.edu (184.108.40.206)
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 num-
ber 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 statis-
tics only for traffic forwarded from the specified source to the speci-
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 sta-
tistics 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
220.127.116.11 18.104.22.168 All Multicast Traffic From 22.214.171.124
| __/ rtt 125 ms Lost/Sent = Pct Rate To 126.96.36.199
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 be changing as the trace query is propa-
gating, there may be small errors (off by 1 or 2) in these statistics.
However, those errors should not accumulate, so the cumulative statis-
tics 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 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 exam-
ple, 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 imple-
ment the multicast traceroute function, so mtrace switched to hop-by-
hop mode. The "Route pruned" error code indicates that traffic for
group 188.8.131.52 would not be forwarded.
oak.isi.edu 108# mtrace -g 184.108.40.206 220.127.116.11 \
Mtrace from 18.104.22.168 to 22.214.171.124 via group 126.96.36.199
Querying full reverse path... * switching to hop-by-hop:
0 butter.lcs.mit.edu (188.8.131.52)
-1 jam.lcs.mit.edu (184.108.40.206) DVMRP thresh^ 1 33 ms Route pruned
-2 bbn.dart.net (220.127.116.11) DVMRP thresh^ 1 36 ms
-3 dc.dart.net (18.104.22.168) DVMRP thresh^ 1 44 ms
-4 darpa.dart.net (22.214.171.124) DVMRP thresh^ 16 47 ms
-5 * * * noc.hpc.org (126.96.36.199) [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 Fari-
nacci, and Deb Agrawal; it was implemented in mrouted by Ajit Thyagara-
jan and Bill Fenner. The option syntax and the output format of mtrace
are modeled after the unicast traceroute program written by Van Jacob-
mrouted(8), mrinfo(8), map-mbone(8), traceroute(8)
4.3 Berkeley Distribution May 8, 1995 MTRACE(8)