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Versions: (draft-asaeda-mboned-mtrace-v2) 00 01 02 03 04 05 06 07 08 09 10 11

MBONED Working Group                                           H. Asaeda
Internet-Draft                                           Keio University
Intended status: Standards Track                               T. Jinmei
Expires: January 14, 2010                                            ISC
                                                               W. Fenner
                                                           Arastra, Inc.
                                                               S. Casner
                                                     Packet Design, Inc.
                                                           July 13, 2009


         Mtrace Version 2: Traceroute Facility for IP Multicast
                     draft-ietf-mboned-mtrace-v2-04

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  This document may contain material
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on January 14, 2010.

Copyright Notice



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   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.











































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Abstract

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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Packet Formats . . . . . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Mtrace2 TLV format . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Defined TLVs . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Mtrace2 Query Header . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  # hops: 8 bits . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Multicast Address  . . . . . . . . . . . . . . . . . . . . 10
     5.3.  Source Address . . . . . . . . . . . . . . . . . . . . . . 11
     5.4.  Destination Address  . . . . . . . . . . . . . . . . . . . 11
     5.5.  Query ID: 16 bits  . . . . . . . . . . . . . . . . . . . . 11
     5.6.  Client Port #  . . . . . . . . . . . . . . . . . . . . . . 11
   6.  IPv4 Mtrace2 Standard Response Block . . . . . . . . . . . . . 12
     6.1.  Query Arrival Time: 32 bits  . . . . . . . . . . . . . . . 12
     6.2.  Incoming Interface Address: 32 bits  . . . . . . . . . . . 13
     6.3.  Outgoing Interface Address: 32 bits  . . . . . . . . . . . 13
     6.4.  Previous-Hop Router Address: 32 bits . . . . . . . . . . . 13
     6.5.  Input packet count on incoming interface: 64 bits  . . . . 13
     6.6.  Output packet count on incoming interface: 64 bits . . . . 13
     6.7.  Total number of packets for this source-group pair: 64
           bits . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.8.  Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 14
     6.9.  Fwd TTL: 8 bits  . . . . . . . . . . . . . . . . . . . . . 14
     6.10. MBZ: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.11. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.12. Src Mask: 6 bits . . . . . . . . . . . . . . . . . . . . . 14
     6.13. Forwarding Code: 8 bits  . . . . . . . . . . . . . . . . . 14
   7.  IPv6 Mtrace2 Standard Response Block . . . . . . . . . . . . . 17
     7.1.  Query Arrival Time: 32 bits  . . . . . . . . . . . . . . . 17
     7.2.  Incoming Interface ID: 32 bits . . . . . . . . . . . . . . 17
     7.3.  Outgoing Interface ID: 32 bits . . . . . . . . . . . . . . 18
     7.4.  Local Address  . . . . . . . . . . . . . . . . . . . . . . 18
     7.5.  Remote Address . . . . . . . . . . . . . . . . . . . . . . 18
     7.6.  Input packet count on incoming interface . . . . . . . . . 18
     7.7.  Output packet count on incoming interface  . . . . . . . . 18
     7.8.  Total number of packets for this source-group pair . . . . 18



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     7.9.  Rtg Protocol: 8 bits . . . . . . . . . . . . . . . . . . . 19
     7.10. MBZ: 7 bits  . . . . . . . . . . . . . . . . . . . . . . . 19
     7.11. S: 1 bit . . . . . . . . . . . . . . . . . . . . . . . . . 19
     7.12. Src Prefix Len: 8 bits . . . . . . . . . . . . . . . . . . 19
     7.13. Forwarding Code: 8 bits  . . . . . . . . . . . . . . . . . 19
   8.  Mtrace2 Augmented Response Block . . . . . . . . . . . . . . . 20
   9.  Router Behavior  . . . . . . . . . . . . . . . . . . . . . . . 21
     9.1.  Traceroute Query . . . . . . . . . . . . . . . . . . . . . 21
       9.1.1.  Packet Verification  . . . . . . . . . . . . . . . . . 21
       9.1.2.  Normal Processing  . . . . . . . . . . . . . . . . . . 21
     9.2.  Mtrace2 Request  . . . . . . . . . . . . . . . . . . . . . 21
       9.2.1.  Packet Verification  . . . . . . . . . . . . . . . . . 22
       9.2.2.  Normal Processing  . . . . . . . . . . . . . . . . . . 22
     9.3.  Forwarding Mtrace2 Requests  . . . . . . . . . . . . . . . 24
     9.4.  Sending Mtrace2 Responses  . . . . . . . . . . . . . . . . 24
       9.4.1.  Destination Address  . . . . . . . . . . . . . . . . . 24
       9.4.2.  Source Address . . . . . . . . . . . . . . . . . . . . 24
     9.5.  Proxying Mtrace2 Queries . . . . . . . . . . . . . . . . . 24
     9.6.  Hiding Information . . . . . . . . . . . . . . . . . . . . 25
   10. Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 26
     10.1. Sending Mtrace2 Queries  . . . . . . . . . . . . . . . . . 26
     10.2. Determining the Path . . . . . . . . . . . . . . . . . . . 26
     10.3. Collecting Statistics  . . . . . . . . . . . . . . . . . . 26
     10.4. Last Hop Router  . . . . . . . . . . . . . . . . . . . . . 26
     10.5. First Hop Router . . . . . . . . . . . . . . . . . . . . . 27
     10.6. Broken Intermediate Router . . . . . . . . . . . . . . . . 27
     10.7. Mtrace2 Termination  . . . . . . . . . . . . . . . . . . . 27
       10.7.1. Arriving at source . . . . . . . . . . . . . . . . . . 27
       10.7.2. Fatal error  . . . . . . . . . . . . . . . . . . . . . 27
       10.7.3. No previous hop  . . . . . . . . . . . . . . . . . . . 27
       10.7.4. Traceroute shorter than requested  . . . . . . . . . . 28
     10.8. Continuing after an error  . . . . . . . . . . . . . . . . 28
   11. Protocol-Specific Considerations . . . . . . . . . . . . . . . 29
     11.1. PIM-SM . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.2. Bi-Directional PIM . . . . . . . . . . . . . . . . . . . . 29
     11.3. PIM-DM . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.4. IGMP/MLD Proxy . . . . . . . . . . . . . . . . . . . . . . 29
     11.5. AMT  . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   12. Problem Diagnosis  . . . . . . . . . . . . . . . . . . . . . . 31
     12.1. Forwarding Inconsistencies . . . . . . . . . . . . . . . . 31
     12.2. TTL or Hop Limit Problems  . . . . . . . . . . . . . . . . 31
     12.3. Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 31
     12.4. Link Utilization . . . . . . . . . . . . . . . . . . . . . 32
     12.5. Time Delay . . . . . . . . . . . . . . . . . . . . . . . . 32
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 33
     13.1. Forwarding Codes . . . . . . . . . . . . . . . . . . . . . 33
     13.2. UDP Destination Port and IPv6 Address  . . . . . . . . . . 33
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 34



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     14.1. Topology Discovery . . . . . . . . . . . . . . . . . . . . 34
     14.2. Traffic Rates  . . . . . . . . . . . . . . . . . . . . . . 34
     14.3. Limiting Query/Request Rates . . . . . . . . . . . . . . . 34
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 35
   16. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     16.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     16.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 38











































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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 multicast "traceroute"
   facility to be implemented in multicast routers and accessed by
   diagnostic programs.  The multicast traceroute described in this
   document named as mtrace version 2 or mtrace2 provides 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 mtrace2.  While the original IPv4
   multicast traceroute, mtrace, the query and response messages are
   implemented as IGMP messages [12], all mtrace2 messages are carried
   on UDP.  The packet formats of IPv4 and IPv6 mtrace2 are different
   because of the different address families, but the syntax is similar.




















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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 [8])
   running on a router chooses the previous-hop router toward the core
   router or Rendezvous Point (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.

   ALL-[protocol]-ROUTERS.MCAST.NET:
   It is a dedicated multicast address for a multicast router to
   communicate with other routers that are working with the same routing
   protocol.  For instance,the address of ALL-PIM-ROUTERS.MCAST.NET is
   '224.0.0.13' for IPv4 and 'ff02::d' for IPv6.











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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 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 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.  Packet Formats

   Mtrace2 message is encoded in TLV format.  If an implementation
   receives a TLV whose length exceeds the TLV length specified in the
   Length field, the TLV SHOULD be accepted but any additional data
   SHOULD be ignored.

4.1.  Mtrace2 TLV format

      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      |           Length              |   Value ....  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type (8 bits)

   Length (16 bits)

   Value (variable length)

4.2.  Defined TLVs

   The following TLV Types are defined:

              Code                       Type
             ======      ======================================
               1             Mtrace2 Query
               2             Mtrace2 Response
               3             Mtrace2 Standard Response Block
               4             Mtrace2 Augmented Response Block

   An mtrace2 message MUST contain one Mtrace2 Query or Response.  An
   mtrace2 message MAY contain one or multiple Mtrace2 Standard and
   Augmented Responses.  A multicast router that sends mtrace2 request
   MUST NOT contain multiple Mtrace2 Standard blocks but MAY contain
   multiple Augmented Response blocks.

   The type field is defined to be "0x1" for mtrace2 queries and
   requests.  The type field is changed to "0x2" 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 will not
   attempt to process a completed response in those cases where the
   initial query was issued from a router.







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5.  Mtrace2 Query Header

   The mtrace2 message is carried as a UDP packet.  The UDP source port
   is uniquely selected by the local host operating system.  The UDP
   destination port is the IANA reserved mtrace2 port number (see
   Section 13).  The UDP checksum MUST be valid in mtrace2 messages.

   The mtrace2 message includes the common mtrace2 Query header as
   follows.  The header is only filled in by the originator of the
   mtrace2 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
                                                     +-+-+-+-+-+-+-+-+
                                                     |    # hops     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Multicast Address                        |
     |                                                               |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                                                               |
     |                        Source Address                         |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Destination Address                      |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |             Query ID            |        Client Port #        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                 Figure 1

5.1.  # 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 mtrace2 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.

5.2.  Multicast Address

   This field specifies the 32 bits length IPv4 or 128 bits length IPv6
   multicast address to be traced, or is filled with "all 1" in case of
   IPv4 or with the unspecified address (::) in case of IPv6 if no
   group-specific information is desired.  Note that non-group-specific



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   mtrace2 MUST specify source address.

5.3.  Source Address

   This field specifies the 32 bits length IPv4 or 128 bits length IPv6
   address of the multicast source for the path being traced, or is
   filled with "all 1" in case of IPv4 or with the unspecified address
   (::) in case of IPv6 if no source-specific information is desired.
   Note that non-source-specific traceroutes may not be possible with
   certain multicast routing protocols.

5.4.  Destination Address

   This field specifies the 32 bits length IPv4 or 128 bits length IPv6
   address of the multicast receiver for the path being traced.  The
   trace starts at this destination and proceeds toward the traffic
   source.

5.5.  Query ID: 16 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.

5.6.  Client Port #

   Mtrace2 response is sent back to the address specified in a
   Destination Address field.  This field specifies the UDP port number
   the router will send Mtrace2 Response.  This client port number MUST
   NOT be changed by any router.





















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6.  IPv4 Mtrace2 Standard Response Block

   Each intermediate IPv4 router in a trace path appends "response data
   block" to the forwarded trace packet.  The standard response data
   block 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| |           |               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

6.1.  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)




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   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).

6.2.  Incoming Interface Address: 32 bits

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

6.3.  Outgoing Interface Address: 32 bits

   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.

6.4.  Previous-Hop Router Address: 32 bits

   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.

6.5.  Input packet count on incoming interface: 64 bits

   This field contains the number of multicast packets received for all
   groups and sources on the incoming interface, or "all 1" if no count
   can be reported.  This counter may have the same value as
   ifHCInMulticastPkts from the IF-MIB [14] for this interface.

6.6.  Output packet count on incoming interface: 64 bits

   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 "all 1" if no count can be reported.  This
   counter may have the same value as ifHCOutMulticastPkts from the IF-
   MIB for this interface.

6.7.  Total number of packets for this source-group pair: 64 bits

   This field counts the number of packets from the specified source
   forwarded by this router to the specified group, or "all 1" if no
   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 [15] for this forwarding entry.



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6.8.  Rtg Protocol: 8 bits

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

              0    Unknown
              1    PIM
              2    PIM using special routing table
              3    PIM using a static route
              4    PIM using MBGP route
              5    PIM using state created by Assert processing
              6    Bi-directional PIM
              7    IGMP/MLD proxy
              8    AMT relay
              9    AMT gateway
             10    AMT gateway with IGMP/MLD proxy

   To obtain these values, multicast routers access to
   ipMcastRouteProtocol, ipMcastRouteRtProtocol, and ipMcastRouteRtType
   in IpMcastRouteEntry specified in IPMROUTE-STD-MIB [15], and combine
   these MIB values to recognize above routing protocol values.

6.9.  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.

6.10.  MBZ: 1 bit

   Must be zeroed on transmission and ignored on reception.

6.11.  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.

6.12.  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
   to 63 (0x3f).

6.13.  Forwarding Code: 8 bits

   This field contains a forwarding information/error code.  Section 9.2
   explains how and when the forwarding code is filled.  Defined values



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   are as follows;

     Value   Name            Description

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

     0x00   NO_ERROR        No error

     0x01   WRONG_IF        Mtrace2 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 Rendezvous Point or Core

     0x09   RPF_IF          Mtrace2 request arrived on the expected
                            RPF interface for this source and group.

     0x0A   NO_MULTICAST    Mtrace2 request arrived on an interface
                            which is not enabled for multicast.

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

     0x0C   REACHED_GW      Mtrace2 request arrived on a gateway (e.g.,
                            a NAT or firewall) that hides the
                            information between this router and the



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                            mtrace2 querier

     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
                            mtrace2 requests.

     0x83   ADMIN_PROHIB    Mtrace2 is administratively prohibited.

   Note that if a router discovers there is not enough room in a packet
   to insert its response, it puts the NO_SPACE error code in the
   previous router's Forwarding Code field, overwriting any error the
   previous router placed there.  After the router sends the response to
   the Destination Address in the header, the router continues the
   mtrace2 query by sending an mtrace2 request containing the same
   mtrace2 query header.  Section 9.3 and Section 10.8 include the
   details.

   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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7.  IPv6 Mtrace2 Standard Response Block

   Each intermediate IPv6 router in a trace path appends "response data
   block" to the forwarded trace packet.  The standard response data
   block 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  |     MBZ     |S|Src Prefix Len |Forwarding Code|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

7.1.  Query Arrival Time: 32 bits

   Same definition described in Section 6.1

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 [14] for
   this interface.  This field is carried in network byte order.



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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 [13] SHOULD NOT be used
   unless the router is only assigned link-local and unique local
   addresses.  If the router is only assigned link-local addresses, its
   link-local address can be specified in this field.

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 6.5

7.7.  Output packet count on incoming interface

   Same definition described in Section 6.6

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 "all 1" 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.



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   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 6.8

7.10.  MBZ: 7 bits

   Must be zeroed on transmission and ignored on reception.

7.11.  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.

7.12.  Src Prefix Len: 8 bits

   This field contains 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)

7.13.  Forwarding Code: 8 bits

   Same definition described in Section 6.13

























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8.  Mtrace2 Augmented Response Block

   In addition to the standard response block, a multicast router on the
   path will be able to add "augumented response block" when it sends
   the request to its upstream router or sends the response to the
   Destination Address.  This augmented response block is flexible to
   add various information.

      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              |           Value ....          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The augmented response block is always appended to mtrace2 TLV header
   (0x04).  The 16 bits Type filed of the augmented response block is
   defined for various purposes, such as diagnosis (as in Section 12)
   and protocol verification.  The packet length of the augmented
   response block is specified in the augmented response block TLV
   header as seen in Section 4.1.

   The following augmented response block type is defined:

         Code                           Type
        ======    =================================================
         0x01        # Mtrace2 Standard Response Blocks Returned

   When the NO_SPACE error occurs, the router sends back the mtrace2
   response with contained data (i.e., all appended response blocks),
   and continues the mtrace2 query by sending an mtrace2 request as will
   be described in Section 9.3.  In this mtrace2 request, the router
   appends the augmented response block with the code "0x01" and the
   number of returned mtrace2 response blocks.  Every router between
   this router and the first-hop router can recognize the limit number
   of hops by referring this number and the # hops in the header.

   This document only defines the above augmented response block type
   and does not define other augmented response block types.  Specifing
   how to deal with diagnosis information will be also described in
   separate documents.











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9.  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.

9.1.  Traceroute Query

   An mtrace2 Query message is a traceroute message with no response
   blocks filled in, and uses TLV type 0x1 for IPv4 and IPv6 mtrace2.

9.1.1.  Packet Verification

   Upon receiving an mtrace2 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
   (S,G) 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 9.2

   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.

9.1.2.  Normal Processing

   When a router receives an mtrace2 Query and it determines that it is
   the proper last-hop router, it treats it like an mtrace2 Request and
   performs the steps listed in Section 9.2

9.2.  Mtrace2 Request

   An mtrace2 Request is a traceroute message with some number of
   response blocks filled in, and uses TLV type 0x1 for IPv4 and IPv6
   mtrace2.  Routers can tell the difference between Queries and
   Requests by checking the length of the packet.



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9.2.1.  Packet Verification

   If the mtrace2 Request does not come from an adjacent host or router,
   it MUST be silently ignored.  If the mtrace2 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.  The router's neighbor
   information, e.g.  ARP database or PIM neighbor list, should be used
   to determine whether the host or router is adjacent or not.

9.2.2.  Normal Processing

   When a router receives an mtrace2 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 mtrace2) or Outgoing
        Interface ID (for IPv6 mtrace2), Output Packet Count, and Fwd
        TTL (for IPv4 mtrace2).  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 address specified in the
        Destination Address field and continue the trace as described in
        Section 9.3.

   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, such as "dry-
        run".

        If using a shared-tree protocol and there is no source-specific
        state, or if no source-specific information is desired (i.e.,
        "all 1" for IPv4 or unspecified address (::) for IPv6), group
        state should be used.  If there is no group state or no group-
        specific information is desired, 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 state is available (e.g., this router has been
        receiving PIM Register messages from the first-hop router), note
        a code of REACHED_RP.





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   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 Section 9.3.

   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 mtrace2 is administratively prohibited or the previous hop
        router does not understand mtrace2 requests, note the
        appropriate forwarding code (ADMIN_PROHIB or OLD_ROUTER).  If
        mtrace2 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 Section 9.3.

   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 Rendezvous 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 mtrace2 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.  If this router is a gateway (e.g., a NAT or firewall) that hides
        the information between this router and the mtrace2 querier, it
        notes forwarding code REACHED_GW.

   11.  The packet is then sent on to the previous hop or the
        Destination Address as described in Section 9.3.




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9.3.  Forwarding Mtrace2 Requests

   If the Previous-hop router is known for this request and the number
   of response blocks is less than the number requested (i.e., the "#
   hops" field in mtrace2 header), 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
   Destination Address in the header.

   When the REACHED_GW code is noted, the router sends back the mtrace2
   response as in Section 9.4.  In addition to that, it must continue
   the mtrace2 query by proxying the original querier as in Section 9.5.

   When the NO_SPACE error occurs, the router sends back the mtrace2
   response with contained data and the NO_SPACE error code as in
   Section 9.4, and continues the mtrace2 query by sending an mtrace2
   request containing the same mtrace2 query header and its standard and
   augmented response blocks.  The corresponding augmented response
   block type is "# Mtrace2 Response Blocks Returned" described in
   Section 8.

9.4.  Sending Mtrace2 Responses

9.4.1.  Destination Address

   An mtrace2 Response must be sent to the address specified in the
   Destination Address field in the mtrace2 query header.

9.4.2.  Source Address

   An mtrace2 Response must be sent with the address of the router's
   reception interface.

9.5.  Proxying Mtrace2 Queries

   When a gateway (e.g., a NAT or firewall) that needs to block unicast
   packets to the mtrace2 querier or hide information between the
   gateway and the mtrace2 querier receives mtrace2 query from an
   adjacent host or mtrace2 request from an adjacent router, it sends
   back the mtrace2 response with contained data and the REACHED_GW code
   to the address specified in the Destination Address field in the



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   mtrace2 query header.

   At the same time, the gateway prepares a new mtrace2 query message.
   The gateway uses the original mtrace2 query header as the base for
   the new mtrace2 query; it sets the Destination Address to its
   Incoming Interface address and the Client Port # to its own port
   (which may be the same as the mtrace2 port as the gateway is
   listening on that port), and decreases # hops according to the number
   of standard response blocks in the returned mtrace2 response from the
   gateway.  The mtrace2 query message is sent to the previous-hop
   router or to an appropriate multicast address on the Incoming
   Interface.

   When the gateway receives the mtrace2 response from the first-hop
   router or any intermediate router, it MUST forward the mtrace2
   response back to the mtrace2 querier with the original mtrace2 query
   header.

9.6.  Hiding Information

   Information about a domain's topology and connectivity may be hidden
   from multicast traceroute requests.  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
   (all 1).
























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10.  Client Behavior

10.1.  Sending Mtrace2 Queries

   When the destination of the mtrace2 is the machine running the
   client, the mtrace2 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 mtrace2 destination, the Query could be
   unicasted to that router.

   See also Section 10.4 on determining the last-hop router.

10.2.  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 10.5 and Section 10.6 on receiving the results of a
   trace.

10.3.  Collecting Statistics

   After a client has determined that it has traced the whole path or as
   much as it can expect to (see Section 10.7), 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 12.3 and Section 12.4.

10.4.  Last Hop Router

   The mtrace2 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 mtrace2 request should
   be multicasted to ALL-ROUTERS.MCAST.NET (224.0.0.2) for IPv4 or All
   Routers Address (FF02::2) for IPv6.  All routers except the correct
   last hop router should ignore any mtrace2 request received via
   multicast.  Mtrace2 requests which are multicasted to the group being
   traced must include the Router Alert option[6][7].




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   Another alternative is to unicast to the trace destination.  Mtrace2
   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.

10.5.  First Hop Router

   The IANA assigned 224.0.1.32, MTRACE.MCAST.NET as the default
   multicast group for IPv4 mtrace responses, in order to support mtrace
   queriers that are not unicast reachable from the first hop router.
   However, mtrace2 does not reserve any IPv4/IPv6 multicast addresses
   for mtrace2 responses.  Every mtrace2 response is sent to the unicast
   address specified in the Destination Address field of the mtrace2
   query header.

10.6.  Broken Intermediate Router

   A broken intermediate router might simply not understand mtrace2
   packets, and drop them.  The querier would then get no response at
   all from its mtrace2 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).

10.7.  Mtrace2 Termination

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

10.7.1.  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.

10.7.2.  Fatal error

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

10.7.3.  No previous hop

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






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10.7.4.  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.

10.8.  Continuing after an error

   When the NO_SPACE error occurs, as described in Section 9.3, the
   multicast routers sends back the mtrace2 response to the address
   specified in the Destination Address field in the mtrace2 query
   header.  In this case, the mtrace2 client may receive multiple
   mtrace2 responses from different routers (along the path).  After the
   client receives multiple mtrace2 response messages, it integrates
   (i.e. constructs) them as a single mtrace2 response message.

   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 response packet received.  A client may be able to
   determine (via mrinfo or SNMP [13][15]) 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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11.  Protocol-Specific Considerations

11.1.  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.

11.2.  Bi-Directional PIM

   Bi-directional PIM [9] 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 mtrace2 reports DF addresses or RPA along
   the path.

11.3.  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.

11.4.  IGMP/MLD Proxy

   When a mtrace2 Query packet reaches an incoming interface of IGMP/MLD
   Proxy [10], it put a WRONG_IF (0x01) value in Forwarding Code of



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   mtrace2 standard response block (as in Section 6.13) and sends the
   mtrace2 response back to the Destination Address.  When a mtrace2
   Query packet reaches an outgoing interface of IGMP/MLD proxy, it is
   forwarded through its incoming interface towards the upstream router.

11.5.  AMT

   AMT [11] provides the multicast connectivity to the unicast-only
   inter-network.  To do this, multicast packets being sent to or from a
   site are encapsulated in unicast packets.  When a mtrace2 query
   packet reaches an AMT pseudo-interface of an AMT gateway, the AMT
   gateway encapsulats it to a particular AMT relay reachable across the
   unicast-only infrastructure.  Then the AMT relay decapsulates the
   mtrace2 query packet and forwards the mtrace2 request to the
   appropriate multicast router.




































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12.  Problem Diagnosis

12.1.  Forwarding Inconsistencies

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

12.2.  TTL or Hop Limit Problems

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

12.3.  Packet Loss

   By taking two traces, it is possible to find packet loss information
   by comparing the difference in input packet counts to the difference
   in output packet counts for the specified source-group address pair
   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 mtrace2 query is propagating, there may be small
   errors (off by 1 or 2 or more) 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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12.4.  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.

12.5.  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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13.  IANA Considerations

   The following new assignments can only be made via a Standards Action
   as specified in [4].

13.1.  Forwarding Codes

   New Forwarding codes must only be created by an RFC that modifies
   this document's Section 10, 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.

13.2.  UDP Destination Port and IPv6 Address

   The IANA should allocate UDP destination port for multicast
   traceroute version 2 upon publication of the first RFC.


































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14.  Security Considerations

14.1.  Topology Discovery

   Mtrace2 can be used to discover any actively-used topology.  If your
   network topology is a secret, mtrace2 may be restricted at the border
   of your domain, using the ADMIN_PROHIB forwarding code.

14.2.  Traffic Rates

   Mtrace2 can be used to discover what sources are sending to what
   groups and at what rates.  If this information is a secret, mtrace2
   may be restricted at the border of your domain, using the
   ADMIN_PROHIB forwarding code.

14.3.  Limiting Query/Request Rates

   Routers should limit mtrace2 queries and requests by ignoring the
   received messages.  Routers MAY randomly ignore the received messages
   to minimize the processing overhead, i.e., to keep fairness in
   processing queries.






























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15.  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.

   For the mtrace version 2 specification, extensive comments were
   received from Yiqun Cai, Liu Hui, Bharat Joshi, Shinsuke Suzuki,
   Achmad Husni Thamrin, and Cao Wei.


































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16.  References

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

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

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

   [7]   Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
         RFC 2711, October 1999.

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

   [9]   Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
         "Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
         RFC 5015, October 2007.

   [10]  Fenner, B., He, H., Haberman, B., and H. Sandick, "Internet
         Group Management Protocol (IGMP) / Multicast Listener Discovery
         (MLD)-Based Multicast Forwarding ("IGMP/MLD Proxying")",
         RFC 4605, August 2006.

   [11]  Thaler, D., Talwar, M., Aggarwal, A., Vicisano, L., and T.
         Pusateri, "Automatic IP Multicast Without Explicit Tunnels
         (AMT)", draft-ietf-mboned-auto-multicast-08.txt (work in
         progress), October 2007.

16.2.  Informative References

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




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

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

   [15]  McWalter, D., Thaler, D., and A. Kessler, "IP Multicast MIB",
         RFC 5132, December 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
   URI:   http://www.sfc.wide.ad.jp/~asaeda/


   Tatuya Jinmei
   Internet Systems Consortium
   Redwood City, CA  94063
   US

   Email: Jinmei_Tatuya@isc.org


   William C. Fenner
   Arastra, Inc.
   Menlo Park, CA  94025
   US

   Email: fenner@fenron.com


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

   Email: casner@packetdesign.com

















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