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Internet Engineering Task Force Inter-Domain Multicast Routing Working Group
INTERNET-DRAFT                                                     W. Fenner
draft-ietf-idmr-traceroute-ipm-00.txt                             Xerox PARC
                                                                   S. Casner
                                                            Precept Software
                                                           November 13, 1995
                                                            Expires: 3/31/95


               A "traceroute" facility for IP Multicast.

Status of this Memo

This document is an Internet Draft.   Internet Drafts are working  docu-
ments of the Internet Engineering  Task Force (IETF), its  Areas, and
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                                Abstract

     This draft describes the IGMP multicast traceroute facility.  As
     the deployment of IP multicast has spread, it has become clear that
     a method for tracing the route that a multicast IP packet takes
     from a source to a particular receiver is absolutely required.
     Unlike unicast traceroute, multicast traceroute requires a special
     packet type and implementation on the part of routers.  This
     specification describes the required functionality.

This document is a product of the Inter-Domain Multicast Routing working
group within the Internet Engineering Task Force.  Comments are soli-
cited and should be addressed to the working group's mailing list at
idmr@cs.ucl.ac.uk and/or the author(s).





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Internet Draft   draft-ietf-idmr-traceroute-ipm-00.txt     November 1995


1.  Introduction

The unicast "traceroute" program allows the tracing of a path from one
machine to another, using mechanisms that already existed in IP.  Unfor-
tunately, no such existing mechanisms can be applied to IP multicast
paths. The key mechanism for unicast traceroute is the ICMP TTL exceeded
message, which is specifically precluded as a response to multicast
packets.  Thus, we specify the multicast "traceroute" facility to be
implemented in multicast routers and accessed by diagnostic programs.
While it is a disadvantage that a new mechanism is required, the multi-
cast traceroute facility can provide additional information about packet
rates and losses that the unicast traceroute cannot, and generally
requires fewer packets to be sent.

Goals:

+    To be able to trace the path that a packet would take from some
     source to some destination.

+    To be able to isolate packet loss problems (e.g., congestion).

+    To be able to isolate configuration problems (e.g., TTL threshold).

+    To minimize packets sent (e.g. no flooding, no implosion).

2.  Overview

Tracing from a source to a multicast destination is hard, since you
don't know down which branch of the multicast tree the destination lies.
This means that you have to flood the whole tree to find the path from
one source to one destination.  However, walking up the tree from desti-
nation to source is easy, as all existing multicast routing protocols
know the previous hop for each source.  Tracing from destination to
source can involve only routers on the direct path.

The party requesting the traceroute (which need be neither the source
nor the destination) sends a traceroute request packet to the last-hop
multicast router for the given destination.  The last-hop router adds a
response data block to the request packet containing its interface
addresses and packet statistics, and then forwards the request packet
via unicast to the router that it believes is the proper previous hop
for the given source.  Each hop adds its response data to the end of the
request packet, then unicast forwards it to the previous hop.  The first
hop router (the router that believes that packets from the source ori-
ginate on one of its directly connected networks) changes the packet
type to indicate a response packet and sends the completed response to
the response destination address.  The response may be returned before
reaching the first hop router if an error condition such as "no route"



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is encountered along the path.

3.  Request / Response header

The header for both requests and responses is as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    IGMP Type  |    # hops     |           checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Multicast Group Address                      |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
|                     Source Address                            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                   Destination Address                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Response Address                          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    resp ttl   |               Query ID                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3.1.  IGMP Type: 8 bits

     The IGMP type field is defined to be 0x1F for traceroute requests
     sent to the last hop router and forwarded hop by hop towards the
     source.  The IGMP type field is changed to 0x1E when the packet is
     completed and sent as a response from the first hop router to the
     querier.  Two codes are required so that multicast routers won't
     attempt to process a completed response in those cases where the
     initial query was issued from a router or the response is sent via
     multicast.

3.2.  # hops: 8 bits

     This field specifies the maximum number of hops that the requester
     wants to trace.  If there is some error condition in the middle of
     the path that keeps the traceroute request from reaching the
     first-hop router, this field can be used to perform an expanding-
     length search to trace the path to just before the problem.

3.3.  Checksum: 16 bits

     This is the standard IGMP checksum.






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3.4.  Group address

     This field specifies the group address to be traced.

3.5.  Source address

     This field specifies the IP address of the multicast source for the
     path being traced.  The traceroute request proceeds hop-by-hop from
     the intended multicast receiver towards this source.

3.6.  Destination address

     This field specifies the IP address of the multicast receiver for
     the path being traced.  The trace starts at this destination and
     proceeds toward the source.

3.7.  Response Address

     This field specifies where the completed traceroute response packet
     gets sent.  It can be a unicast address or a multicast address, as
     explained in section 5.2.

3.8.  resp ttl: 8 bits

     This field specifies the TTL at which to multicast the response, if
     the response address is a multicast address.

3.9.  Query ID: 24 bits

     This field is used as a unique identifier for this traceroute
     request so that duplicate or delayed responses may be detected and
     to minimize collisions when a multicast response address is used.



















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4.  Response data

Each router adds a "response data" segment to the traceroute packet  be-
fore it forwards it on.  The response data looks like this:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Query Arrival Time                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Incoming Interface Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                  Outgoing Interface Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                 Previous-Hop Router Address                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Input packet count on incoming interface            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Output packet count on outgoing interface           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Total number of packets for this source-group pair       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Rtg Protocol  |    FwdTTL     |MBZ| Src Mask  | ForwardingErr |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


4.1.  Query Arrival Time

     The Query Arrival Time is a 32-bit NTP timestamp specifying the
     arrival time of the traceroute request packet at this router.  The
     32-bit form of an NTP timestamp consists of the middle 32 bits of
     the full 64-bit form; that is, the low 16 bits of the integer part
     and the high 16 bits of the fractional part.

4.2.  Incoming Interface Address

     This field specifies the address of the interface on which packets
     from this source are expected to arrive, or 0 if unknown.

4.3.  Outgoing Interface Address

     This field specifies the address of the interface on which packets
     from this source flow to the specified destination, or 0 if unk-
     nown.






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4.4.  Previous-Hop Router Address

     This field specifies the router from which this router expects
     packets from this source, or 0 if unknown.

4.5.  Input packet count on incoming interface

     This field contains the number of multicast packets received for
     all groups and sources on the incoming interface, or 0xffffffff if
     no count can be reported.

4.6.  Output packet count on outgoing interface

     This field contains the number of multicast packets that have been
     transmitted for all groups and sources on the outgoing interface,
     or 0xffffffff if no count can be reported.

4.7.  Total number of packets for this source-group pair

     This field counts the number of packets from the specified source
     forwarded by this router to the specified group, or 0xffffffff if
     no count can be reported.

4.8.  Rtg Protocol: 8 bits

     This field describes the routing protocol in use between this
     router and the previous-hop router.  Specified values include:

     1 - DVMRP
     2 - MOSPF
     3 - PIM
     4 - CBT

4.9.  FwdTTL: 8 bits

     This field contains the TTL that a packet is required to have
     before it will be forwarded over the outgoing interface.

4.10.  Src Mask: 6 bits

     This field contains the number of 1's in the netmask this router
     has for the source (i.e. a value of 24 means the netmask is
     0xffffff00)

4.11.  ForwardingErr: 8 bits

     This field contains a forwarding error code.  Specified values
     include:



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     0x00 No error
     0x01 Traceroute request arrived on an interface
          that is not the proper outgoing interface
          for this source,group,destination.
     0x02 This router has sent a prune upstream for the group.
     0x03 The next hop router has pruned the group.
     0x04 The group is subject to administrative scoping at this hop.
     0x05 This router has no route for the source.
     0x07 This router is not forwarding this source,group
          for an unspecified reason.
     0x81 There was not enough room to insert another response data block
          in the packet.
     0x82 The next hop router does not understand traceroute requests.

     Note that if a router discovers there is not enough room in a
     packet to insert its response, it puts the 0x81 error code in the
     previous router's ForwardingErr field, overwriting any error the
     previous router placed there.  It is expected that a multicast tra-
     ceroute client, upon receiving this error, will restart the trace
     at the last hop listed in the packet.

     The 0x80 bit of the ForwardingErr code is used to indicate a fatal
     error.  A fatal error is one that causes a router to be unable to
     forward this traceroute request on to the next hop.

     <<< Note that 0x01 and 0x05 should be fatal, but renumbering would
     be painful as backwards-compatibility is required.  It's not clear
     that an explicit fatal bit is required because a response issued
     when the number of hops has not reached the maximum indicates that
     the trace cannot go further.  Feedback from implementors is
     requested. >>>

5.  Using multicast traceroute

Several problems may arise when attempting to use multicast traceroute.

5.1.  Last hop router

     The traceroute querier may not know which is the last hop router,
     or that router may be behind a firewall that blocks unicast packets
     but passes multicast packets.  In these cases, the traceroute
     request should be multicasted to the group being traced (since the
     last hop router listens to that group).  All routers except the
     correct last hop router should ignore any multicast traceroute
     request received via multicast.






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5.2.  First hop router

     The traceroute querier may not be unicast reachable from the first
     hop router.  In this case, the querier should set the traceroute
     response address to a multicast address, and should set the
     response TTL to a value sufficient for the response from the first
     hop router to reach the querier.  It may be appropriate to start
     with a small TTL and increase in subsequent attempts until a suffi-
     cient TTL is reached, up to an appropriate maximum (such as 192).

     The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET, as the standard
     multicast group for multicast traceroute responses.

5.3.  Broken intermediate router

     A broken intermediate router might simply not understand traceroute
     packets, and drop them.  The querier would then get no response at
     all from its traceroute requests.  It should then perform a search
     by setting the number of responses field until it gets a response
     (both linear and binary search are options, but binary is likely to
     be slower because a failure requires waiting for a timeout).

6.  Problem Diagnosis

6.1.  Forwarding Inconsistencies

     The forwarding error code can tell if a group is unexpectedly
     pruned or administratively scoped.

6.2.  TTL problems

     By taking the maximum of (hops from source + forwarding TTL thres-
     hold) over all hops, you can discover the TTL required for the
     source to reach the destination.

6.3.  Congestion

     By taking two traces, you can find packet loss information by com-
     paring the difference in input packet counts to the difference in
     output packet counts at the previous hop.  On a point-to-point
     link, any difference in these numbers implies packet loss.  Since
     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.
     However, these errors will not accumulate if multiple traces are
     taken to expand the measurement period.  On a shared link, the
     count of input packets can be larger than the number of output
     packets at the previous hop, due to other routers or hosts on the
     link injecting packets.  This appears as "negative loss" which may



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     mask real packet loss.

     In addition to the counts of input and output packets for all mul-
     ticast traffic on the interfaces, the response data includes a
     count of the packets forwarded by a node for the specified source-
     group pair.  Taking the difference in this count between two traces
     and then comparing those differences between two hops gives a meas-
     ure of packet loss just for traffic from the specified source to
     the specified receiver via the specified group.  This measure is
     not affected by shared links.

     On a point-to-point link that is a multicast tunnel, packet loss is
     usually due to congestion in unicast routers along the path of that
     tunnel.  On native multicast links, loss is more likely in the out-
     put queue of one hop, perhaps due to priority dropping, or in the
     input queue at the next hop.  The counters in the response data do
     not allow these cases to be distinguished.  Differences in packet
     counts between the incoming and outgoing interfaces on one node
     cannot generally be used to measure queue overflow in the node
     because some packets may be routed only to or from other interfaces
     on that node.

     In the multicast extensions for SunOS 4.1.x from Xerox PARC, both
     the output packet count and the packet forwarding count for the
     source-group pair are incremented before priority dropping for rate
     limiting occurs and before the packets are put onto the interface
     output queue which may overflow.  These drops will appear as (posi-
     tive) loss on the link even though they occur within the router.

     In release 3.3/3.4 of the multicast extensions, a multicast packet
     generated on a router will be counted as having come in an inter-
     face even though it did not.  This can create the appearance of
     negative loss even on a point-to-point link.

     In releases up through 3.5/3.6, packets were not counted as input
     on an interface if the reverse-path forwarding check decided that
     the packets should be dropped.  That causes the packets to appear
     as lost on the link if they were output by the upstream hop.  This
     situation can arise when two routers on the path for the group
     being traced are connected by a shared link, and the path for some
     other group does not flow between those two routers because the
     downstream router receives packets for the other group on another
     interface, but the upstream router is the elected forwarder to
     other routers or hosts on the shared link.

6.4.  Link Utilization

     Again, with two traces, you can divide the difference in the input



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     or output packet counts at some hop by the difference in time
     stamps from the same hop to obtain the packet rate over the link.
     If the average packet size is known, then the link utilization can
     also be estimated to see whether packet loss may be due to the rate
     limit or the physical capacity on a particular link being exceeded.

6.5.  Time delay

     If the routers have synchronized clocks, you can estimate propaga-
     tion and queueing delay from the differences between the timestamps
     at successive hops.

7.  Acknowledgments

This specification started largely as a transcription of Van Jacobson's
slides from the 30th IETF, and the implementation in mrouted 3.3 by Ajit
Thyagarajan.  Van's original slides credit Steve Casner, Steve Deering,
Dino Farinacci and Deb Agrawal.  A multicast traceroute client, mtrace,
has been implemented by Ajit Thyagarajan and Steve Casner.

8.  Security Considerations

Security issues are not discussed in this memo.




























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9.  Authors' Addresses

   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Phone: +1 415 812 4816
   Email: fenner@parc.xerox.com

   Stephen L. Casner
   Precept Software, Inc.
   21580 Stevens Creek Blvd, Suite 207
   Cupertino, CA 95014
   Email: casner@precept.com





































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