|MTRACE(8)||System Manager's Manual||MTRACE(8)|
mtraceutilizes a tracing feature implemented in multicast routers (
mroutedversion 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" (126.96.36.199), 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.
The options are as follows:
mtraceis being run, or to a multicast address other than the one registered for this purpose (188.8.131.52).
traceroutetool 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.
-toption). If the last hop router is known, it may also be addressed directly using the
-goption). 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
-goption 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.
mtracefirst attempts to trace the full reverse path, unless the number of hops to trace is explicitly set with the
-moption. If there is no response within a 3 second timeout interval (changed with the
-moption), 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
-qoption). The first half of the attempts (default is one) are made with the unicast address of the host running
mtraceas 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 (184.108.40.206) 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
-toption and/or the initial unicast attempts can be forced to use multicast instead with the
-moption. For each attempt, if no response is received within the timeout, a "*" is printed. After the specified number of attempts have failed,
mtracewill try to query the next hop router with a DVMRP_ASK_NEIGHBORS2 request (as used by the
mrinfoprogram) to see what kind of router it is.
mtraceis 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 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... 0 oak.isi.edu (188.8.131.52) -1 cub.isi.edu (184.108.40.206) DVMRP thresh^ 1 3 ms -2 la.dart.net (220.127.116.11) DVMRP thresh^ 1 14 ms -3 dc.dart.net (18.104.22.168) DVMRP thresh^ 1 50 ms -4 bbn.dart.net (22.214.171.124) DVMRP thresh^ 1 63 ms -5 mit.dart.net (126.96.36.199) DVMRP thresh^ 1 71 ms -6 caraway.lcs.mit.edu (188.8.131.52) 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
-s. option is set.
Waiting to accumulate statistics... Results after 101 seconds: Source Response Dest Packet Statistics For Only For Traffic 184.108.40.206 220.127.116.11 All Multicast Traffic From 18.104.22.168 | __/ rtt 125 ms Lost/Sent = Pct Rate To 22.214.171.124 v / hop 65 ms --------------------- ------------------ 126.96.36.199 188.8.131.52 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 184.108.40.206 220.127.116.11 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 18.104.22.168 22.214.171.124 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 126.96.36.199 188.8.131.52 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 184.108.40.206 220.127.116.11 cub.isi.edu | \__ ttl 5 833 83 pps 2 0 pps v \ hop -8 ms 8075 79 pps 18 0 pps 18.104.22.168 22.214.171.124 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:
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
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 126.96.36.199 would not be forwarded.
oak.isi.edu 108# mtrace -g 188.8.131.52 184.108.40.206 \ butter.lcs.mit.edu 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
mroutedby Ajit Thyagarajan and Bill Fenner. The option syntax and the output format of
mtraceare modeled after the unicast traceroute(8) program written by Van Jacobson.
mroutedwill crash if a trace query is received via a unicast packet and
mroutedhas no route for the source address. Therefore, do not use the
-goption unless the target
mroutedhas been verified to be 3.4 or newer than 3.5.
|September 8, 2014||OpenBSD-current|