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Versions: 00 draft-ietf-mboned-mtrace-v2

MBONED Working Group                                           H. Asaeda
Internet-Draft                                           Keio University
Expires: January 3, 2008                                       T. Jinmei
                                                     Toshiba Corporation
                                                               W. Fenner
                                                           AT&T Research
                                                               S. Casner
                                                     Packet Design, Inc.
                                                            July 2, 2007


         Mtrace Version 2: Traceroute Facility for IP Multicast
                    draft-asaeda-mboned-mtrace-v2-00

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 3, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).









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Abstract

   This document describes the IGMP and the ICMPv6 multicast traceroute
   facility.  Unlike unicast traceroute, multicast traceroute requires
   special packet types and implementations on the part of routers.
   This specification describes the required functionality in multicast
   routers, as well as how management applications can use the new
   router functionality.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  IPv4 Multicast Traceroute Header . . . . . . . . . . . . . . .  8
     4.1.  IGMP Type: 8 bits  . . . . . . . . . . . . . . . . . . . .  8
     4.2.  # hops: 8 bits . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Checksum: 16 bits  . . . . . . . . . . . . . . . . . . . .  8
     4.4.  Multicast Address  . . . . . . . . . . . . . . . . . . . .  9
     4.5.  Source Address . . . . . . . . . . . . . . . . . . . . . .  9
     4.6.  Destination Address  . . . . . . . . . . . . . . . . . . .  9
     4.7.  Response Address . . . . . . . . . . . . . . . . . . . . .  9
     4.8.  Resp TTL: 8 bits . . . . . . . . . . . . . . . . . . . . .  9
     4.9.  Query ID: 24 bits  . . . . . . . . . . . . . . . . . . . .  9
   5.  IPv4 Multicast Traceroute Response Data  . . . . . . . . . . . 10
     5.1.  Query Arrival Time: 32 bits  . . . . . . . . . . . . . . . 10
     5.2.  Incoming Interface Address . . . . . . . . . . . . . . . . 11
     5.3.  Outgoing Interface Address . . . . . . . . . . . . . . . . 11
     5.4.  Previous-Hop Router Address  . . . . . . . . . . . . . . . 11
     5.5.  Packet counts  . . . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Input packet count on incoming interface . . . . . . . . . 11
     5.7.  Output packet count on incoming interface  . . . . . . . . 11
     5.8.  Total number of packets for this source-group pair . . . . 11
     5.9.  Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 12
     5.10. Fwd TTL: 8 bits  . . . . . . . . . . . . . . . . . . . . . 12
     5.11. MBZ: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.12. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.13. Src Mask: 6 bits . . . . . . . . . . . . . . . . . . . . . 12
     5.14. Forwarding Code: 8 bits  . . . . . . . . . . . . . . . . . 13
   6.  IPv6 Multicast Traceroute Header . . . . . . . . . . . . . . . 15
     6.1.  ICMPv6 Type: 8 bits  . . . . . . . . . . . . . . . . . . . 16
     6.2.  # hops: 8 bits . . . . . . . . . . . . . . . . . . . . . . 16
     6.3.  Checksum: 16 bits  . . . . . . . . . . . . . . . . . . . . 16
     6.4.  Reserved: 32 bits  . . . . . . . . . . . . . . . . . . . . 16
     6.5.  Multicast Address  . . . . . . . . . . . . . . . . . . . . 16
     6.6.  Source Address . . . . . . . . . . . . . . . . . . . . . . 16
     6.7.  Destination Address  . . . . . . . . . . . . . . . . . . . 16



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     6.8.  Response Address . . . . . . . . . . . . . . . . . . . . . 16
     6.9.  Resp Hop Limit: 8 bits . . . . . . . . . . . . . . . . . . 17
     6.10. Query ID: 24 bits  . . . . . . . . . . . . . . . . . . . . 17
   7.  IPv6 Multicast Traceroute Response Data  . . . . . . . . . . . 18
     7.1.  Query Arrival Time: 32 bits  . . . . . . . . . . . . . . . 18
     7.2.  Incoming Interface ID: 32 bits . . . . . . . . . . . . . . 19
     7.3.  Outgoing Interface ID: 32 bits . . . . . . . . . . . . . . 19
     7.4.  Local Address  . . . . . . . . . . . . . . . . . . . . . . 19
     7.5.  Remote Address . . . . . . . . . . . . . . . . . . . . . . 19
     7.6.  Input packet count on incoming interface . . . . . . . . . 19
     7.7.  Output packet count on incoming interface  . . . . . . . . 20
     7.8.  Total number of packets for this source-group pair . . . . 20
     7.9.  Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 20
     7.10. Fwd Hop Limit: 8 bits  . . . . . . . . . . . . . . . . . . 20
     7.11. MBZ: 7 bits  . . . . . . . . . . . . . . . . . . . . . . . 20
     7.12. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 20
     7.13. Src Prefix Len: 8 bits . . . . . . . . . . . . . . . . . . 20
     7.14. Forwarding Code: 8 bits  . . . . . . . . . . . . . . . . . 20
     7.15. Reserved: 24 bit . . . . . . . . . . . . . . . . . . . . . 21
   8.  Router Behavior  . . . . . . . . . . . . . . . . . . . . . . . 22
     8.1.  Traceroute Query . . . . . . . . . . . . . . . . . . . . . 22
       8.1.1.  Packet Verification  . . . . . . . . . . . . . . . . . 22
       8.1.2.  Normal Processing  . . . . . . . . . . . . . . . . . . 22
     8.2.  Traceroute Request . . . . . . . . . . . . . . . . . . . . 22
       8.2.1.  Packet Verification  . . . . . . . . . . . . . . . . . 23
       8.2.2.  Normal Processing  . . . . . . . . . . . . . . . . . . 23
     8.3.  Traceroute Response  . . . . . . . . . . . . . . . . . . . 24
     8.4.  Forwarding Traceroute Requests . . . . . . . . . . . . . . 25
     8.5.  Sending Traceroute Responses . . . . . . . . . . . . . . . 25
       8.5.1.  Destination Address  . . . . . . . . . . . . . . . . . 25
       8.5.2.  TTL and Hop Limit  . . . . . . . . . . . . . . . . . . 25
       8.5.3.  Source Address . . . . . . . . . . . . . . . . . . . . 25
       8.5.4.  Sourcing multicast responses . . . . . . . . . . . . . 25
     8.6.  Hiding information . . . . . . . . . . . . . . . . . . . . 26
   9.  Using multicast traceroute . . . . . . . . . . . . . . . . . . 27
     9.1.  Sample client  . . . . . . . . . . . . . . . . . . . . . . 27
       9.1.1.  Sending initial query  . . . . . . . . . . . . . . . . 27
       9.1.2.  Determining the Path . . . . . . . . . . . . . . . . . 27
       9.1.3.  Collecting statistics  . . . . . . . . . . . . . . . . 27
     9.2.  Last hop router  . . . . . . . . . . . . . . . . . . . . . 28
     9.3.  First hop router . . . . . . . . . . . . . . . . . . . . . 28
     9.4.  Broken intermediate router . . . . . . . . . . . . . . . . 28
     9.5.  Mtrace termination . . . . . . . . . . . . . . . . . . . . 29
       9.5.1.  Arriving at source . . . . . . . . . . . . . . . . . . 29
       9.5.2.  Fatal error  . . . . . . . . . . . . . . . . . . . . . 29
       9.5.3.  No previous hop  . . . . . . . . . . . . . . . . . . . 29
       9.5.4.  Traceroute shorter than requested  . . . . . . . . . . 29
     9.6.  Continuing after an error  . . . . . . . . . . . . . . . . 29



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     9.7.  Multicast Traceroute and shared tree routing protocols . . 30
       9.7.1.  PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . 30
       9.7.2.  Bi-directional PIM . . . . . . . . . . . . . . . . . . 30
       9.7.3.  CBT  . . . . . . . . . . . . . . . . . . . . . . . . . 30
     9.8.  Protocol-specific considerations . . . . . . . . . . . . . 31
       9.8.1.  DVMRP  . . . . . . . . . . . . . . . . . . . . . . . . 31
       9.8.2.  PIM-DM . . . . . . . . . . . . . . . . . . . . . . . . 31
   10. Problem Diagnosis  . . . . . . . . . . . . . . . . . . . . . . 32
     10.1. Forwarding Inconsistencies . . . . . . . . . . . . . . . . 32
     10.2. TTL or hop limit problems  . . . . . . . . . . . . . . . . 32
     10.3. Packet loss  . . . . . . . . . . . . . . . . . . . . . . . 32
     10.4. Link Utilization . . . . . . . . . . . . . . . . . . . . . 33
     10.5. Time delay . . . . . . . . . . . . . . . . . . . . . . . . 33
   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
     11.1. Routing protocols  . . . . . . . . . . . . . . . . . . . . 34
     11.2. Forwarding codes . . . . . . . . . . . . . . . . . . . . . 34
     11.3. IPv6 mtrace  . . . . . . . . . . . . . . . . . . . . . . . 34
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 35
     12.1. Topology Discovery . . . . . . . . . . . . . . . . . . . . 35
     12.2. Traffic Rates  . . . . . . . . . . . . . . . . . . . . . . 35
     12.3. Unicast Replies  . . . . . . . . . . . . . . . . . . . . . 35
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 37
     14.2. Informative References . . . . . . . . . . . . . . . . . . 37
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
   Intellectual Property and Copyright Statements . . . . . . . . . . 40
























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1.  Introduction

   The unicast "traceroute" program allows the tracing of a path from
   one machine to another.  The key mechanism for unicast traceroute is
   the ICMP TTL exceeded message, which is specifically precluded as a
   response to multicast packets.  On the other hand, the multicast
   traceroute facility allows the tracing of an IP multicast routing
   paths.  In this document, we specify the new multicast "traceroute"
   facility to be implemented in multicast routers and accessed by
   diagnostic programs.  The new multicast traceroute facility, mtrace
   version 2, can provide additional information about packet rates and
   losses that the unicast traceroute cannot, and generally requires
   fewer packets to be sent.

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

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

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

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

   This document supports both IPv4 and IPv6 multicast traceroute
   facility.  The protocol design, concept, and program behavior are
   same between IPv4 and IPv6 mtrace, whereas the packet formats are
   different because of the different address family.  For instance, the
   query and response messages for IPv4 mtrace are implemented as IGMP
   messages [4], whereas IPv6 mtrace messages are of ICMPv6 messages
   [5].  This document clarifies the unique points or properties of each
   mtrace if exist.



















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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT","SHOULD", "SHOULD NOT", "RECOMMENDED","MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC 2119 [1].

   Since multicast traceroutes flow in the opposite direction to the
   data flow, we refer to "upstream" and "downstream" with respect to
   data, unless explicitly specified.

   Incoming interface:
   The interface on which traffic is expected from the specified source
   and group.

   Outgoing interface:
   The interface on which traffic is forwarded from the specified source
   and group toward the destination.  It is the interface on which the
   multicast traceroute Request was received.

   Previous-hop router:
   The router that is on the link attached to the Incoming Interface and
   is responsible for forwarding traffic for the specified source and
   group.

   Group state:
   It is the state in which a shared-tree protocol (e.g., PIM-SM [12])
   running on a router chooses the previous-hop router toward the core
   router (or RP) as its parent router.  In this state, source-specific
   state is not available for the corresponding multicast address on the
   router.

   Source-specific state:
   It is the state in which a routing protocol running on a router
   chooses the path that would be followed for a source-specific join.

















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3.  Overview

   Given a multicast distribution tree, 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 destination to source
   is easy, as most 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 Query packet to the last-hop
   multicast router for the given destination.  The last-hop router
   turns the Query into a Request packet by adding a response data block
   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 and group.
   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 originate 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 a fatal error condition such as "no
   route" is encountered along the path.

   Multicast traceroute uses any information available to it in the
   router to attempt to determine a previous hop to forward the trace
   towards.  Multicast routing protocols vary in the type and amount of
   state they keep; multicast traceroute endeavors to work with all of
   them by using whatever is available.  For example, if a DVMRP router
   has no active state for a particular source but does have a DVMRP
   route, it chooses the parent of the DVMRP route as the previous hop.
   If a PIM-SM router is on the (*,G) tree, it chooses the parent
   towards the RP as the previous hop.  In these cases, no source/
   group-specific state is available, but the path may still be traced.














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4.  IPv4 Multicast Traceroute Header

   IPv4 mtrace includes the common packet header as follows.  The header
   is only filled in by the originator of the traceroute Query;
   intermediate routers MUST NOT modify any of the fields.

      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 Address                        |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                        Source Address                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Destination Address                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Response Address                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Resp TTL   |                  Query ID                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1.  IGMP Type: 8 bits

   The IGMP type field is defined to be 0x1F for traceroute queries and
   requests.  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.

4.2.  # 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-ring
   search to trace the path to just before the problem.

4.3.  Checksum: 16 bits

   The checksum is the 16-bit one's complement of the one's complement
   sum of the whole IGMP message (the entire IP payload) [8].  When
   computing the checksum, the checksum field is set to zero.  When
   transmitting packets, the checksum MUST be computed and inserted into
   this field.  When receiving packets, the checksum MUST be verified
   before processing a packet.



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4.4.  Multicast Address

   This field specifies the multicast address to be traced, or zero if
   no group-specific information is desired.  Note that non-group-
   specific traceroutes may not be possible with certain multicast
   routing protocols.

4.5.  Source Address

   This field specifies the IP address of the multicast source for the
   path being traced, or 0xffffffff if no source-specific information is
   desired.  Note that non-source-specific traceroutes may not be
   possible with certain multicast routing protocols.

4.6.  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 traffic source.

4.7.  Response Address

   This field specifies IP address to which the completed traceroute
   response packet gets sent.  It can be a unicast address or a
   multicast address, as explained in Section 8.2

4.8.  Resp TTL: 8 bits

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

4.9.  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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5.  IPv4 Multicast Traceroute Response Data

   Each intermediate router in a trace path appends "response data" to
   the forwarded trace packet.  The response data looks 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      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       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |               |               |M| |           |               |
     | Rtg Protocol  |    Fwd TTL    |B|S| Src Mask  |Forwarding Code|
     |               |               |Z| |           |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.1.  Query Arrival Time: 32 bits

   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.

   The following formula converts from a UNIX timeval to a 32-bit NTP
   timestamp:

      query_arrival_time
      = (tv.tv_sec + 32384) << 16 + ((tv.tv_usec << 10) / 15625)

   The constant 32384 is the number of seconds from Jan 1, 1900 to Jan
   1, 1970 truncated to 16 bits. ((tv.tv_usec << 10) / 15625) is a
   reduction of ((tv.tv_usec / 100000000) << 16).







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5.2.  Incoming Interface Address

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

5.3.  Outgoing Interface Address

   This field specifies the address of the interface on which packets
   from this source and group flow to the specified destination, or 0 if
   unknown.

5.4.  Previous-Hop Router Address

   This field specifies the router from which this router expects
   packets from this source.  This may be a multicast group (e.g.  ALL-
   [protocol]-ROUTERS.MCAST.NET) if the previous hop is not known
   because of the workings of the multicast routing protocol.  However,
   it should be 0 if the incoming interface address is unknown.

5.5.  Packet counts

   Note that these packet counts SHOULD be as up to date as possible.
   If packet counts are not being maintained on the processor that
   handles the traceroute request in a multi-processor router
   architecture, the packet SHOULD be delayed while the counters are
   gathered from the remote processor(s).  If this occurs, the Query
   Arrival Time should be updated to reflect the time at which the
   packet counts were learned.

5.6.  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.  This counter should have the same value as
   ifInMulticastPkts from the IF-MIB [10] for this interface.

5.7.  Output packet count on incoming interface

   This field contains the number of multicast packets that have been
   transmitted or queued for transmission for all groups and sources on
   the outgoing interface, or 0xffffffff if no count can be reported.
   This counter should have the same value as ifOutMulticastPkts from
   the IF-MIB for this interface.

5.8.  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



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   count can be reported.  If the S bit is set, the count is for the
   source network, as specified by the Src Mask field.  If the S bit is
   set and the Src Mask field is 63, indicating no source-specific
   state, the count is for all sources sending to this group.  This
   counter should have the same value as ipMcastRoutePkts from the
   IPMROUTE-STD-MIB [11] for this forwarding entry.

5.9.  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
             5    PIM using special routing table
             6    PIM using a static route
             7    DVMRP using a static route
             8    PIM using MBGP route
             9    CBT using special routing table
             10   CBT using a static route
             11   PIM using state created by Assert processing
             12   Bi-directional PIM

   Note that some of the routing protocols or functions are not
   supported or not used in either of IPv4 multicast nor IPv6 multicast.

5.10.  Fwd TTL: 8 bits

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

5.11.  MBZ: 1 bit

   Must be zeroed on transmission and ignored on reception.

5.12.  S: 1 bit

   This S bit indicates that the packet count for the source-group pair
   is for the source network, as determined by masking the source
   address with the Src Mask field.

5.13.  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).
   If the router is forwarding solely on group state, this field is set



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   to 63 (0x3f).

5.14.  Forwarding Code: 8 bits

   This field contains a forwarding information/error code.  Defined
   values are as follows;

     Value   Name            Description

     -----  --------------  -------------------------------------------

     0x00   NO_ERROR        No error

     0x01   WRONG_IF        Traceroute request arrived on an interface
                            to which this router would not forward for
                            this source,group,destination.

     0x02   PRUNE_SENT      This router has sent a prune upstream which
                            applies to the source and group in the
                            traceroute request.

     0x03   PRUNE_RCVD      This router has stopped forwarding for this
                            source and group in response to a request
                            from the next hop router.

     0x04   SCOPED          The group is subject to administrative
                            scoping at this hop.

     0x05   NO_ROUTE        This router has no route for the source or
                            group and no way to determine a potential
                            route.

     0x06   WRONG_LAST_HOP  This router is not the proper last-hop
                            router.

     0x07   NOT_FORWARDING  This router is not forwarding this source,
                            group out the outgoing interface for an
                            unspecified reason.

     0x08   REACHED_RP      Reached Rendez-vous Point or Core

     0x09   RPF_IF          Traceroute request arrived on the expected
                            RPF interface for this source, group.

     0x0A   NO_MULTICAST    Traceroute request arrived on an interface
                            which is not enabled for multicast.

     0x0B   INFO_HIDDEN     One or more hops have been hidden from this



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                            trace.

     0x81   NO_SPACE        There was not enough room to insert another
                            response data block in the packet.

     0x82   OLD_ROUTER      The previous-hop router does not understand
                            traceroute requests.

     0x83   ADMIN_PROHIB    Traceroute is administratively prohibited.

   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 Forwarding Code field, overwriting any error the previous
   router placed there.  A multicast traceroute client, upon receiving
   this error, MAY restart the trace at the last hop listed in the
   packet.

   The 0x80 bit of the Forwarding Code is used to indicate a fatal
   error.  A fatal error is one where the router may know the previous
   hop but cannot forward the message to it.































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6.  IPv6 Multicast Traceroute Header

   IPv6 mtrace includes the common packet header as follows.  Because of
   the specification of the IPv6 address, all IPv6 addresses used in
   each field consume 128 bits length.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Type     |    # hops     |           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Reserved                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                      Multicast Address                        *
     |                                                               |
     *                                                               *
     |                                                               |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                                                               |
     *                                                               *
     |                                                               |
     *                        Source Address                         *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                      Destination Address                      *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                       Response Address                        *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Resp Hop Limit |                  Query ID                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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6.1.  ICMPv6 Type: 8 bits

   The ICMPv6 type field is defined to be MTRACE6_QRYREQ (TBD (see
   Section 11)) for traceroute queries and requests.  The ICMPv6 type
   field is changed to MTRACE6_RESP (TBD) 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.

6.2.  # hops: 8 bits

   Same definition described in Section 4.2

6.3.  Checksum: 16 bits

   As defined in [2], the checksum is the 16-bit one's complement of the
   one's complement sum of the entire ICMPv6 message, starting with the
   ICMPv6 message type field, and prepended with a "pseudo-header" of
   IPv6 header fields.

6.4.  Reserved: 32 bits

   Initialized to zero by the sender; ignored by receivers.

6.5.  Multicast Address

   Same definition described in Section 4.4

6.6.  Source Address

   This field specifies the IPv6 address of the multicast source for the
   path being traced, or is filled with the unspecified address (::) if
   no source-specific information is desired.  Note that non-source-
   specific traceroutes may not be possible with certain multicast
   routing protocols.

6.7.  Destination Address

   Same definition described in Section 4.6

6.8.  Response Address

   Same definition described in Section 4.7







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6.9.  Resp Hop Limit: 8 bits

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

6.10.  Query ID: 24 bits

   Same definition described in Section 4.9











































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7.  IPv6 Multicast Traceroute Response Data

   Each intermediate router in a trace path appends "response data" to
   the forwarded trace packet.  The response data looks 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Query Arrival Time                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Incoming Interface ID                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Outgoing Interface ID                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                         Local Address                         *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                         Remote Address                        *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Input packet count on incoming interface            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Output packet count on outgoing interface           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Total number of packets for this source-group pair       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Rtg Protocol  | Fwd Hop Limit |     MBZ     |S|Src Prefix Len |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Forwarding Code|                   Reserved                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.1.  Query Arrival Time: 32 bits

   Same definition described in Section 5.1







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7.2.  Incoming Interface ID: 32 bits

   This field specifies the interface ID on which packets from this
   source and group are expected to arrive, or 0 if unknown.  This ID
   should be the value taken from InterfaceIndex of the IF-MIB for this
   interface.  This field is carried in network byte order.

7.3.  Outgoing Interface ID: 32 bits

   This field specifies the interface ID on which packets from this
   source and group flow to the specified destination, or 0 if unknown.
   This ID should be the value taken from InterfaceIndex of the IF-MIB
   for this interface.  This field is carried in network byte order.

7.4.  Local Address

   This field specifies a global IPv6 address that uniquely identifies
   the router.  A unique local unicast address [7] SHOULD NOT be used
   unless the node is only assigned link-local and unique local
   addresses.  [TBD: What if the node is only assigned link-local
   addresses?  It should be very unlikely case, but is possible even for
   a properly working router.]

   Note that since interface indices used in the Incoming and Outgoing
   Interface ID fields are node-local information, a global identifier
   is needed to specify the router.

7.5.  Remote Address

   This field specifies the address of the previous-hop router, which,
   in most cases, is a link-local unicast address for the queried source
   and destination addresses.

   Although a link-local address does not have enough information to
   identify a node, it is possible to detect the previous-hop router
   with the assistance of Incoming Interface ID and the current router
   address (i.e., Local Address).

   This may be a multicast group (e.g., ALL-[protocol]-
   ROUTERS.MCAST.NET) if the previous hop is not known because of the
   workings of the multicast routing protocol.  However, it should be
   the unspecified address (::) if the incoming interface address is
   unknown.

7.6.  Input packet count on incoming interface

   Same definition described in Section 5.6




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7.7.  Output packet count on incoming interface

   Same definition described in Section 5.7

7.8.  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.  If the S bit is set, the count is for the
   source network, as specified by the Src Prefix Len field.  If the S
   bit is set and the Src Prefix Len field is 255, indicating no source-
   specific state, the count is for all sources sending to this group.
   This counter should have the same value as ipMcastRoutePkts from the
   IPMROUTE-STD-MIB for this forwarding entry.

7.9.  Rtg Protocol: 8 bits

   Same definition described in Section 5.9

   Note that some of the routing protocols or functions are not
   supported or not used in IPv6 multicast.

7.10.  Fwd Hop Limit: 8 bits

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

7.11.  MBZ: 7 bits

   Must be zeroed on transmission and ignored on reception.

7.12.  S: 1 bit

   This S bit indicates that the packet count for the source-group pair
   is for the source network, as determined by masking the source
   address with the Src Prefix Len field.

7.13.  Src Prefix Len: 8 bits

   This field contains the decimal number of the prefix length this
   router has for the source.  If the router is forwarding solely on
   group state, this field is set to 255 (0xff)

7.14.  Forwarding Code: 8 bits

   Same definition described in Section 5.14





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7.15.  Reserved: 24 bit

   Initialized to zero by the sender; ignored by receivers.
















































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8.  Router Behavior

   All of these actions are performed in addition to (NOT instead of)
   forwarding the packet, if applicable.  E.g. a multicast packet that
   has TTL or the hop limit remaining MUST be forwarded normally, as
   MUST a unicast packet that has TTL or the hop limit remaining and is
   not addressed to this router.

8.1.  Traceroute Query

   A traceroute Query message is a traceroute message with no response
   blocks filled in, and uses IGMP type 0x1F for IPv4 mtrace or ICMPv6
   type MTRACE6_QRYREQ (TBD) for IPv6 mtrace.

8.1.1.  Packet Verification

   Upon receiving a traceroute Query message, a router must examine the
   Query to see if it is the proper last-hop router for the destination
   address in the packet.  It is the proper last-hop router if it has a
   multicast-capable interface on the same subnet as the Destination
   Address and is the router that would forward traffic from the given
   source onto that subnet.

   If the router determines that it is not the proper last-hop router,
   or it cannot make that determination, it does one of two things
   depending if the Query was received via multicast or unicast.  If the
   Query was received via multicast, then it MUST be silently dropped.
   If it was received via unicast, a forwarding code of WRONG_LAST_HOP
   is noted and processing continues as in Section 8.2

   Duplicate Query messages as identified by the tuple (IP Source, Query
   ID) SHOULD be ignored.  This MAY be implemented using a simple 1-back
   cache (i.e. remembering the IP source and Query ID of the previous
   Query message that was processed, and ignoring future messages with
   the same IP Source and Query ID).  Duplicate Request messages MUST
   NOT be ignored in this manner.

8.1.2.  Normal Processing

   When a router receives a traceroute Query and it determines that it
   is the proper last-hop router, it treats it like a traceroute Request
   and performs the steps listed in Section 8.2

8.2.  Traceroute Request

   A traceroute Request is a traceroute message with some number of
   response blocks filled in, and also uses IGMP type 0x1F for IPv4
   mtrace or ICMPv6 type MTRACE6_QRYREQ (TBD) for IPv6 mtrace.  Routers



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   can tell the difference between Queries and Requests by checking the
   length of the packet.

8.2.1.  Packet Verification

   If the traceroute Request is not addressed to this router, or if the
   Request is addressed to a multicast group which is not a link-scoped
   group (i.e. 224/24 for IPv4, FFx2::/16 [3] for IPv6), it MUST be
   silently ignored.

8.2.2.  Normal Processing

   When a router receives a traceroute Request, it performs the
   following steps.  Note that it is possible to have multiple
   situations covered by the Forwarding Codes.  The first one
   encountered is the one that is reported, i.e. all "note forwarding
   code N" should be interpreted as "if forwarding code is not already
   set, set forwarding code to N".

   1.   If there is room in the current buffer (or the router can
        efficiently allocate more space to use), insert a new response
        block into the packet and fill in the Query Arrival Time,
        Outgoing Interface Address (for IPv4 mtrace) or Outgoing
        Interface ID (for IPv6 mtrace), Output Packet Count, and Fwd TTL
        or Fwd Hop Limit.  If there was no room, fill in the response
        code "NO_SPACE" in the *previous* hop's response block, and
        forward the packet to the requester as described in "Forwarding
        Traceroute Requests".

   2.   Attempt to determine the forwarding information for the source
        and group specified, using the same mechanisms as would be used
        when a packet is received from the source destined for the
        group.  State need not be instantiated, it can be "phantom"
        state created only for the purpose of the trace.

        If using a shared-tree protocol and there is no source-specific
        state, or if the source is specified as 0xFFFFFFFF, group state
        should be used.  If there is no group state or the group is
        specified as 0, potential source state (i.e. the path that would
        be followed for a source-specific Join) should be used.  If this
        router is the Core or RP and no source-specific information is
        available, note an error code of REACHED_RP.

   3.   If no forwarding information can be determined, the router notes
        an error code of NO_ROUTE, sets the remaining fields that have
        not yet been filled in to zero, and then forwards the packet to
        the requester as described in "Forwarding Traceroute Requests".




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   4.   Fill in the Incoming Interface Address, Previous-Hop Router
        Address, Input Packet Count, Total Number of Packets, Routing
        Protocol, S, and Src Mask from the forwarding information that
        was determined.

   5.   If traceroute is administratively prohibited or the previous hop
        router does not understand traceroute requests, note the
        appropriate forwarding code (ADMIN_PROHIB or OLD_ROUTER).  If
        traceroute is administratively prohibited and any of the fields
        as filled in step 4 are considered private information, zero out
        the applicable fields.  Then the packet is forwarded to the
        requester as described in "Forwarding Traceroute Requests".

   6.   If the reception interface is not enabled for multicast, note
        forwarding code NO_MULTICAST.  If the reception interface is the
        interface from which the router would expect data to arrive from
        the source, note forwarding code RPF_IF.  Otherwise, if the
        reception interface is not one to which the router would forward
        data from the source to the group, a forwarding code of WRONG_IF
        is noted.

   7.   If the group is subject to administrative scoping on either the
        Outgoing or Incoming interfaces, a forwarding code of SCOPED is
        noted.

   8.   If this router is the Rendez-vous Point or Core for the group, a
        forwarding code of REACHED_RP is noted.

   9.   If this router has sent a prune upstream which applies to the
        source and group in the traceroute Request, it notes forwarding
        code PRUNE_SENT.  If the router has stopped forwarding
        downstream in response to a prune sent by the next hop router,
        it notes forwarding code PRUNE_RCVD.  If the router should
        normally forward traffic for this source and group downstream
        but is not, it notes forwarding code NOT_FORWARDING.

   10.  The packet is then sent on to the previous hop or the requester
        as described in Section 8.4.

8.3.  Traceroute Response

   A router must forward all traceroute response packets normally, with
   no special processing.  If a router has initiated a traceroute with a
   Query or Request message, it may listen for Responses to that
   traceroute but MUST still forward them as well.






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8.4.  Forwarding Traceroute Requests

   If the Previous-hop router is known for this request and the number
   of response blocks is less than the number requested, the packet is
   sent to that router.  If the Incoming Interface is known but the
   Previous-hop router is not known, the packet is sent to an
   appropriate multicast address on the Incoming Interface.  The
   appropriate multicast address may depend on the routing protocol in
   use, MUST be a link-scoped group (i.e. 224/24 for IPv4, FF02::/16 for
   IPv6), MUST NOT be ALL-SYSTEMS.MCAST.NET (224.0.0.1) for IPv4 and All
   Nodes Address (FF02::1) for IPv6, and MAY be ALL-ROUTERS.MCAST.NET
   (224.0.0.2) for IPv4 or All Routers Address (FF02::2) for IPv6 if the
   routing protocol in use does not define a more appropriate group.
   Otherwise, it is sent to the Response Address in the header, as
   described in Section 8.5.

   Note that it is not an error for the number of response blocks to be
   greater than the number requested; such a packet should simply be
   forwarded to the requester as described in Section 8.5.

8.5.  Sending Traceroute Responses

8.5.1.  Destination Address

   A traceroute response must be sent to the Response Address in the
   traceroute header.

8.5.2.  TTL and Hop Limit

   If the Response Address is unicast, the router inserts its normal
   unicast TTL or hop limit in the IP header, and may use any of its
   interface addresses as the source address.  If the Response Address
   is multicast, the router copies the Response TTL or hop limit from
   the traceroute header into the IP header.

8.5.3.  Source Address

   If the Response Address is unicast, the router may use any of its
   interface addresses as the source address.  Since some multicast
   routing protocols forward based on source address, if the Response
   Address is multicast, the router MUST use an address that is known in
   the multicast routing topology if it can make that determination.

8.5.4.  Sourcing multicast responses

   When a router sources a multicast response, the response packet MUST
   be sent on a single interface, then forwarded as if it were received
   on that interface.  It MUST NOT source the response packet



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   individually on each interface, in order to avoid duplicate packets.

8.6.  Hiding information

   Information about a domain's topology and connectivity may be hidden
   from multicast traceroute requests.  The exact mechanism is not
   specified here; however, the INFO_HIDDEN forwarding code may be used
   to note that, for example, the incoming interface address and packet
   count are for the entrance to the domain and the outgoing interface
   address and packet count are the exit from the domain.  The source-
   group packet count may be from either router or not specified
   (0xffffffff).







































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9.  Using multicast traceroute

9.1.  Sample client

   This section describes the behavior of an example multicast
   traceroute client.

9.1.1.  Sending initial query

   When the destination of the mtrace is the machine running the client,
   the mtrace Query packet can be sent to the ALL-ROUTERS.MCAST.NET
   (224.0.0.2) for IPv4 or All Routers Address (FF02::2) for IPv6.  This
   will ensure that the packet is received by the last-hop router on the
   subnet.  Otherwise, if the proper last-hop router is known for the
   mtrace destination, the Query could be unicasted to that router.
   Otherwise, the Query packet should be multicasted to the group being
   queried; if the destination of the mtrace is a member of the group,
   this will get the Query to the proper last-hop router.  In this final
   case, the packet should contain the Router Alert option [9], to make
   sure that routers that are not members of the multicast group notice
   the packet.

   See also Section 9.2 on determining the last-hop router.

9.1.2.  Determining the Path

   The client could send a small number of initial query messages with a
   large "# hops" field, in order to try to trace the full path.  If
   this attempt fails, one strategy is to perform a linear search (as
   the traditional unicast traceroute program does); set the "# hops"
   field to 1 and try to get a response, then 2, and so on.  If no
   response is received at a certain hop, the hop count can continue
   past the non-responding hop, in the hopes that further hops may
   respond.  These attempts should continue until a user-defined timeout
   has occurred.

   See also Section 9.3 and Section 9.4 on receiving the results of a
   trace.

9.1.3.  Collecting statistics

   After a client has determined that it has traced the whole path or as
   much as it can expect to (see Section 9.5), it might collect
   statistics by waiting a short time and performing a second trace.  If
   the path is the same in the two traces, statistics can be displayed
   as described in Section 10.3 and Section 10.4.





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9.2.  Last hop router

   The mtrace 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 mtrace 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 mtrace request received via multicast.
   Mtrace requests which are multicasted to the group being traced must
   include the Router Alert option [9].

   Another alternative is to unicast to the trace destination.
   Traceroute requests which are unicasted to the trace destination must
   include the Router Alert option, in order that the last-hop router is
   aware of the packet.

   If the traceroute querier is attached to the same router as the
   destination of the request, the traceroute request may be multicasted
   to ALL-ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All Routers Address
   (FF02::2) for IPv6 if the last-hop router is not known.

9.3.  First hop router

   The mtrace 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 (or
   hop limit) 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 sufficient TTL
   is reached, up to an appropriate maximum (such as 192).

   The IANA has assigned 224.0.1.32, MTRACE.MCAST.NET as the default
   multicast group for IPv4 mtrace responses, and will assign
   MTRACE6_RESPADDR (TBD) for IPv6 mtrace responses.  Other groups may
   be used if needed, e.g. when using mtrace to diagnose problems with
   the IANA-assigned group.

9.4.  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 hop-by-
   hop 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).





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9.5.  Mtrace termination

   When performing an expanding hop-by-hop trace, it is necessary to
   determine when to stop expanding.

9.5.1.  Arriving at source

   A trace can be determined to have arrived at the source if the
   Incoming Interface of the last router in the trace is non-zero, but
   the Previous Hop router is zero.

9.5.2.  Fatal error

   A trace has encountered a fatal error if the last Forwarding Error in
   the trace has the 0x80 bit set.

9.5.3.  No previous hop

   A trace can not continue if the last Previous Hop in the trace is set
   to 0.

9.5.4.  Traceroute shorter than requested

   If the trace that is returned is shorter than requested (i.e. the
   number of Response blocks is smaller than the "# hops" field), the
   trace encountered an error and could not continue.

9.6.  Continuing after an error

   When the NO_SPACE error occurs, the client might try to continue the
   trace by starting it at the last hop in the trace.  It can do this by
   unicasting to this router's outgoing interface address, keeping all
   fields the same.  If this results in a single hop and a "WRONG_IF"
   error, the client may try setting the trace destination to the same
   outgoing interface address.

   If a trace times out, it is likely to be because a router in the
   middle of the path does not support multicast traceroute.  That
   router's address will be in the Previous Hop field of the last entry
   in the last reply packet received.  A client may be able to determine
   (via mrinfo or SNMP [7][11]) a list of neighbors of the non-
   responding router.  If desired, each of those neighbors could be
   probed to determine the remainder of the path.  Unfortunately, this
   heuristic may end up with multiple paths, since there is no way of
   knowing what the non-responding router's algorithm for choosing a
   previous-hop router is.  However, if all paths but one flow back
   towards the non-responding router, it is possible to be sure that
   this is the correct path.



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9.7.  Multicast Traceroute and shared tree routing protocols

   When using shared-tree routing protocols like PIM-SM and CBT, a more
   advanced client may use multicast traceroute to determine paths or
   potential paths.

9.7.1.  PIM-SM

   When a multicast traceroute reaches a PIM-SM RP and the RP does not
   forward the trace on, it means that the RP has not performed a
   source-specific join so there is no more state to trace.  However,
   the path that traffic would use if the RP did perform a source-
   specific join can be traced by setting the trace destination to the
   RP, the trace source to the traffic source, and the trace group to 0.
   This trace Query may be unicasted to the RP.

9.7.2.  Bi-directional PIM

   Bi-directional PIM [14] is a variant of PIM-SM that builds bi-
   directional shared trees connecting multicast sources and receivers.
   Along the bi-directional shared trees, multicast data is natively
   forwarded from sources to the RPA (Rendezvous Point Address) and from
   the RPA to receivers without requiring source-specific state.  In
   contrast to PIM-SM, RP always has the state to trace.

   A Designated Forwarder (DF) for a given RPA is in charge of
   forwarding downstream traffic onto its link, and forwarding upstream
   traffic from its link towards the RPL (Rendezvous Point Link) that
   the RPA belongs to.  Hence mtrace reports DF addresses or RPA along
   the path.

9.7.3.  CBT

   When a multicast traceroute reaches a CBT [13] Core, it must simply
   stop since CBT does not have source-specific state.  However, a
   second trace can be performed, setting the trace destination to the
   traffic source, the trace group to the group being traced, and the
   trace source to the Core (or to 0, since CBT does not have source-
   specific state).  This trace Query may be unicasted to the Core.
   There are two possibilities when combining the two traces:

9.7.3.1.  No overlap

   If there is no overlap between the two traces, the second trace can
   be reversed and appended to the first trace.  This composite trace
   shows the full path from the source to the destination.





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9.7.3.2.  Overlapping paths

   If there is a portion of the path that is common to the ends of the
   two traces, that portion is removed from both traces.  Then, as in
   the no overlap case, the second trace is reversed and appended to the
   first trace, and the composite trace again contains the full path.

   This algorithm works whether the source has joined the CBT tree or
   not.

9.8.  Protocol-specific considerations

9.8.1.  DVMRP

   DVMRP's dominant router election and route exchange guarantees that
   DVMRP routers know whether or not they are the last-hop forwarder for
   the link and who the previous hop is.

9.8.2.  PIM-DM

   Routers running PIM Dense Mode do not know the path packets would
   take unless traffic is flowing.  Without some extra protocol
   mechanism, this means that in an environment with multiple possible
   paths with branch points on shared media, multicast traceroute can
   only trace existing paths, not potential paths.  When there are
   multiple possible paths but the branch points are not on shared
   media, the previous hop router is known, but the last hop router may
   not know that it is the appropriate last hop.

   When traffic is flowing, PIM Dense Mode routers know whether or not
   they are the last-hop forwarder for the link (because they won or
   lost an Assert battle) and know who the previous hop is (because it
   won an Assert battle).  Therefore, multicast traceroute is always
   able to follow the proper path when traffic is flowing.

















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10.  Problem Diagnosis

10.1.  Forwarding Inconsistencies

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

10.2.  TTL or hop limit problems

   By taking the maximum of (hops from source + forwarding TTL (or hop
   limit) threshold) over all hops, you can discover the TTL required
   for the source to reach the destination.

10.3.  Packet loss

   By taking two traces, you can find packet loss information by
   comparing 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 propagating,
   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 mask real packet
   loss.

   In addition to the counts of input and output packets for all
   multicast 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 measure
   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 output
   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.






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10.4.  Link Utilization

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

10.5.  Time delay

   If the routers have synchronized clocks, it is possible to estimate
   propagation and queuing delay from the differences between the
   timestamps at successive hops.  However, this delay includes control
   processing overhead, so is not necessarily indicative of the delay
   that data traffic would experience.



































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11.  IANA Considerations

   The following new assignments can only be made via a Standards Action
   as specified in [6].

11.1.  Routing protocols

   The IANA is responsible for allocating new Routing Protocol codes.
   The Routing Protocol code is somewhat problematic, since in the case
   of protocols like CBT and PIM it must encode both a unicast routing
   algorithm and a multicast tree-building protocol.  The space was not
   divided into two fields because it was already small and some
   combinations (e.g.  DVMRP) would be wasted.

11.2.  Forwarding codes

   New Forwarding codes must only be created by an RFC that modifies
   this document's Section 9, fully describing the conditions under
   which the new forwarding code is used.  The IANA may act as a central
   repository so that there is a single place to look up forwarding
   codes and the document in which they are defined.

11.3.  IPv6 mtrace

   The IANA should allocate ICMPv6 types (MTRACE6_QRYREQ and
   MTRACE6_RESP) for IPv6 multicast traceroute upon publication of the
   first RFC.  Additionally, the well-known multicast address
   (MTRACE6_RESPADDR) intended for default use by IPv6 multicast
   traceroute should be registered and defined by the first RFC
   published.





















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12.  Security Considerations

12.1.  Topology Discovery

   mtrace can be used to discover any actively-used topology.  If your
   network topology is a secret, mtrace may be restricted at the border
   of your domain, using the ADMIN_PROHIB forwarding code.

12.2.  Traffic Rates

   mtrace can be used to discover what sources are sending to what
   groups and at what rates.  If this information is a secret, mtrace
   may be restricted at the border of your domain, using the
   ADMIN_PROHIB forwarding code.

12.3.  Unicast Replies

   The "Response address" field may be used to send a single packet (the
   traceroute Reply packet) to an arbitrary unicast address.  It is
   possible to use this facility as a packet amplifier, as a small
   multicast traceroute Query may turn into a large Reply packet.






























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13.  Acknowledgements

   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.  The original
   multicast traceroute client, mtrace (version 1), has been implemented
   by Ajit Thyagarajan, Steve Casner and Bill Fenner.

   The idea of unicasting a multicast traceroute Query to the
   destination of the trace with Router Alert set is due to Tony
   Ballardie.  The idea of the "S" bit to allow statistics for a source
   subnet is due to Tom Pusateri.






































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14.  References

14.1.  Normative References

   [1]   Bradner, S., "Key words for use in RFCs to indicate requirement
         levels", RFC 2119, March 1997.

   [2]   Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
         Specification", RFC 2460, December 1998.

   [3]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

   [4]   Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
         Thyagarajan, "Internet Group Management Protocol, Version 3",
         RFC 3376, October 2002.

   [5]   Conta, A., Deering, S., and M. Gupta, "Internet Control Message
         Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6)
         Specification", RFC 4443, March 2006.

   [6]   Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", RFC 2434, October 1998.

14.2.  Informative References

   [7]   Draves, R. and D. Thaler, "Default Router Preferences and More-
         Specific Routes", RFC 4191, November 2005.

   [8]   Braden, B., Borman, D., and C. Partridge, "Computing the
         Internet Checksum", RFC 1071, September 1988.

   [9]   Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

   [10]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB",
         RFC 2863, June 2000.

   [11]  McWalter, D., Thaler, D., and A. Kessler, "IP Multicast MIB",
         draft-ietf-mboned-ip-mcast-mib-05.txt (work in progress),
         March 2007.

   [12]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
         "Protocol Independent Multicast - Sparse Mode (PIM-SM):
         Protocol Specification (Revised)", RFC 4601, August 2006.

   [13]  Ballardie, T., "Core Based Trees (CBT version 2) Multicast
         Routing -- Protocol Specification --", RFC 2189,
         September 1997.



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   [14]  Handley, M., Kouvelas, I., and T. Speakman, "Bi-directional
         Protocol Independent Multicast (BIDIR-PIM)",
         draft-ietf-pim-bidir-09.txt (work in progress), February 2007.
















































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Authors' Addresses

   Hitoshi Asaeda
   Keio University
   Graduate School of Media and Governance
   Fujisawa, Kanagawa  252-8520
   Japan

   Email: asaeda@wide.ad.jp


   Tatsuya Jinmei
   Toshiba Corporation
   Corporate Research & Development Center
   Kawasaki, Kanagawa  212-8582
   Japan

   Email: jinmei@isl.rdc.toshiba.co.jp


   William C. Fenner
   AT&T Research
   Menlo Park, CA  94025
   US

   Email: fenner@research.att.com


   Stephen L. Casner
   Packet Design, Inc.
   Palo Alto, CA  94304
   US

   Email: casner@packetdesign.com

















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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
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Acknowledgment

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