MTRACE(1M) MTRACE(1M)
mtrace - print multicast path from a source to a receiver
/usr/etc/mtrace [ -g gateway ] [ -i if_addr ] [ -l ] [ -M ] [ -m max_hops
] [ -n ] [ -p ] [ -q nqueries ] [ -r resp_dest ] [ -s ] [ -t ttl ] [ -w
waittime ] source [ receiver ] [ group ]
Assessing problems in the distribution of IP multicast traffic can be
difficult. mtrace utilizes a tracing feature 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 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" (224.2.0.1), 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.
-g gwy Send the trace query via unicast directly to the multicast router
gwy rather than multicasting the query. This must be the lasthop
router on the path from the intended source to the receiver.
CAUTION!! Version 3.3 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 newer than 3.3.
-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.
-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 routing protocol).
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-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 (not implemented yet).
-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 (224.0.1.32).
-s Print a short form output including only the multicast path and
not the packet rate and loss statistics.
-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.
-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 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.
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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
(224.0.0.2) 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 the
receiver.
Directing the Response [Toc] [Back]
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 (224.0.1.32) 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 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 program) to see what kind of router it is.
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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. A sample use and output might be:
oak.isi.edu 80# mtrace -l caraway.lcs.mit.edu 224.2.0.3
Mtrace from 18.26.0.170 to 128.9.160.100 via group 224.2.0.3
Querying full reverse path...
0 oak.isi.edu (128.9.160.100)
-1 cub.isi.edu (128.9.160.153) DVMRP thresh^ 1 3 ms
-2 la.dart.net (140.173.128.1) DVMRP thresh^ 1 14 ms
-3 dc.dart.net (140.173.64.1) DVMRP thresh^ 1 50 ms
-4 bbn.dart.net (140.173.32.1) DVMRP thresh^ 1 63 ms
-5 mit.dart.net (140.173.48.2) DVMRP thresh^ 1 71 ms
-6 caraway.lcs.mit.edu (18.26.0.170)
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 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.
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.
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Waiting to accumulate statistics... Results after 101 seconds:
Source Response Dest Packet Statistics For Only For Traffic
18.26.0.170 128.9.160.100 All Multicast Traffic From 18.26.0.170
| __/ rtt 125 ms Lost/Sent = Pct Rate To 224.2.0.3
v / hop 65 ms --------------------- ------------------
18.26.0.144
140.173.48.2 mit.dart.net
| ^ ttl 1 0/6 = --% 0 pps 0/2 = --% 0 pps
v | hop 8 ms 1/52 = 2% 0 pps 0/18 = 0% 0 pps
140.173.48.1
140.173.32.1 bbn.dart.net
| ^ ttl 2 0/6 = --% 0 pps 0/2 = --% 0 pps
v | hop 12 ms 1/52 = 2% 0 pps 0/18 = 0% 0 pps
140.173.32.2
140.173.64.1 dc.dart.net
| ^ 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
140.173.64.2
140.173.128.1 la.dart.net
| ^ 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
140.173.128.2
128.9.160.153 cub.isi.edu
| \__ ttl 5 833 83 pps 2 0 pps
v \ hop -8 ms 8075 79 pps 18 0 pps
128.9.160.100 128.9.160.100
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 seconds. There are two sources of larger errors, both of
which show up as negative losses:
o 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.
o 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.
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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 "Route pruned" error code indicates that traffic for group
224.2.143.24 would not be forwarded.
oak.isi.edu 108# mtrace -g 140.173.48.2 204.62.246.73 \
butter.lcs.mit.edu 224.2.143.24
Mtrace from 204.62.246.73 to 18.26.0.151 via group 224.2.143.24
Querying full reverse path... * switching to hop-by-hop:
0 butter.lcs.mit.edu (18.26.0.151)
-1 jam.lcs.mit.edu (18.26.0.144) DVMRP thresh^ 1 33 ms Route pruned
-2 bbn.dart.net (140.173.48.1) DVMRP thresh^ 1 36 ms
-3 dc.dart.net (140.173.32.2) DVMRP thresh^ 1 44 ms
-4 darpa.dart.net (140.173.240.2) DVMRP thresh^ 16 47 ms
-5 * * * noc.hpc.org (192.187.8.2) [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(1m), traceroute(1m)
PPPPaaaaggggeeee 6666 [ Back ]
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