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RFC 8487

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Mtrace Version 2: Traceroute Facility for IP Multicast

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Internet Engineering Task Force (IETF)                         H. Asaeda
Request for Comments: 8487                                          NICT
Category: Standards Track                                       K. Meyer
ISSN: 2070-1721                                                 Dell EMC
                                                             W. Lee, Ed.
                                                            October 2018


         Mtrace Version 2: Traceroute Facility for IP Multicast

Abstract

   This document describes the IP multicast traceroute facility, named
   Mtrace version 2 (Mtrace2).  Unlike unicast traceroute, Mtrace2
   requires special implementations on the part of routers.  This
   specification describes the required functionality in multicast
   routers, as well as how an Mtrace2 client invokes a Query and
   receives a Reply.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8487.

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Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Mtrace2 TLV Format  . . . . . . . . . . . . . . . . . . .   9
     3.2.  Defined TLVs  . . . . . . . . . . . . . . . . . . . . . .  10
       3.2.1.  Mtrace2 Query . . . . . . . . . . . . . . . . . . . .  10
       3.2.2.  Mtrace2 Request . . . . . . . . . . . . . . . . . . .  12
       3.2.3.  Mtrace2 Reply . . . . . . . . . . . . . . . . . . . .  12
       3.2.4.  IPv4 Mtrace2 Standard Response Block  . . . . . . . .  13
       3.2.5.  IPv6 Mtrace2 Standard Response Block  . . . . . . . .  18
       3.2.6.  Mtrace2 Augmented Response Block  . . . . . . . . . .  20
       3.2.7.  Mtrace2 Extended Query Block  . . . . . . . . . . . .  21
   4.  Router Behavior . . . . . . . . . . . . . . . . . . . . . . .  22
     4.1.  Receiving an Mtrace2 Query  . . . . . . . . . . . . . . .  22
       4.1.1.  Query Packet Verification . . . . . . . . . . . . . .  22
       4.1.2.  Query Normal Processing . . . . . . . . . . . . . . .  23
     4.2.  Receiving an Mtrace2 Request  . . . . . . . . . . . . . .  23
       4.2.1.  Request Packet Verification . . . . . . . . . . . . .  24
       4.2.2.  Request Normal Processing . . . . . . . . . . . . . .  24
     4.3.  Forwarding Mtrace2 Request  . . . . . . . . . . . . . . .  26
       4.3.1.  Destination Address . . . . . . . . . . . . . . . . .  26
       4.3.2.  Source Address  . . . . . . . . . . . . . . . . . . .  26
       4.3.3.  Appending Standard Response Block . . . . . . . . . .  26
     4.4.  Sending Mtrace2 Reply . . . . . . . . . . . . . . . . . .  27
       4.4.1.  Destination Address . . . . . . . . . . . . . . . . .  27
       4.4.2.  Source Address  . . . . . . . . . . . . . . . . . . .  27
       4.4.3.  Appending Standard Response Block . . . . . . . . . .  27
     4.5.  Proxying Mtrace2 Query  . . . . . . . . . . . . . . . . .  28
     4.6.  Hiding Information  . . . . . . . . . . . . . . . . . . .  28
   5.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .  29
     5.1.  Sending Mtrace2 Query . . . . . . . . . . . . . . . . . .  29
       5.1.1.  Destination Address . . . . . . . . . . . . . . . . .  29
       5.1.2.  Source Address  . . . . . . . . . . . . . . . . . . .  29
     5.2.  Determining the Path  . . . . . . . . . . . . . . . . . .  29
     5.3.  Collecting Statistics . . . . . . . . . . . . . . . . . .  29
     5.4.  Last-Hop Router (LHR) . . . . . . . . . . . . . . . . . .  30
     5.5.  First-Hop Router (FHR)  . . . . . . . . . . . . . . . . .  30
     5.6.  Broken Intermediate Router  . . . . . . . . . . . . . . .  30
     5.7.  Non-supported Router  . . . . . . . . . . . . . . . . . .  30
     5.8.  Mtrace2 Termination . . . . . . . . . . . . . . . . . . .  31
       5.8.1.  Arriving at Source  . . . . . . . . . . . . . . . . .  31
       5.8.2.  Fatal Error . . . . . . . . . . . . . . . . . . . . .  31
       5.8.3.  No Upstream Router  . . . . . . . . . . . . . . . . .  31
       5.8.4.  Reply Timeout . . . . . . . . . . . . . . . . . . . .  31
     5.9.  Continuing after an Error . . . . . . . . . . . . . . . .  31

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   6.  Protocol-Specific Considerations  . . . . . . . . . . . . . .  32
     6.1.  PIM-SM  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     6.2.  Bidirectional PIM . . . . . . . . . . . . . . . . . . . .  32
     6.3.  PIM-DM  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     6.4.  IGMP/MLD Proxy  . . . . . . . . . . . . . . . . . . . . .  33
   7.  Problem Diagnosis . . . . . . . . . . . . . . . . . . . . . .  33
     7.1.  Forwarding Inconsistencies  . . . . . . . . . . . . . . .  33
     7.2.  TTL or Hop-Limit Problems . . . . . . . . . . . . . . . .  33
     7.3.  Packet Loss . . . . . . . . . . . . . . . . . . . . . . .  33
     7.4.  Link Utilization  . . . . . . . . . . . . . . . . . . . .  34
     7.5.  Time Delay  . . . . . . . . . . . . . . . . . . . . . . .  34
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     8.1.  "Mtrace2 Forwarding Codes" Registry . . . . . . . . . . .  35
     8.2.  "Mtrace2 TLV Types" Registry  . . . . . . . . . . . . . .  35
     8.3.  UDP Destination Port  . . . . . . . . . . . . . . . . . .  35
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
     9.1.  Addresses in Mtrace2 Header . . . . . . . . . . . . . . .  35
     9.2.  Verification of Clients and Peers . . . . . . . . . . . .  35
     9.3.  Topology Discovery  . . . . . . . . . . . . . . . . . . .  36
     9.4.  Characteristics of Multicast Channel  . . . . . . . . . .  36
     9.5.  Limiting Query/Request Rates  . . . . . . . . . . . . . .  37
     9.6.  Limiting Reply Rates  . . . . . . . . . . . . . . . . . .  37
     9.7.  Specific Security Concerns  . . . . . . . . . . . . . . .  37
       9.7.1.  Request and Response Bombardment  . . . . . . . . . .  37
       9.7.2.  Amplification Attack  . . . . . . . . . . . . . . . .  37
       9.7.3.  Leaking of Confidential Topology Details  . . . . . .  38
       9.7.4.  Delivery of False Information (Forged Reply Messages)  38
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     10.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

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1.  Introduction

   Given a multicast distribution tree, tracing hop-by-hop downstream
   from a multicast source to a given multicast receiver is difficult
   because there is no efficient and deterministic way to determine the
   branch of the multicast routing tree on which that receiver lies.  On
   the other hand, walking up the tree from a receiver to a source is
   easy, as most existing multicast routing protocols know the upstream
   router for each source.  Tracing from a receiver to a source can
   involve only the routers on the direct path.

   This document specifies the multicast traceroute facility named
   Mtrace version 2 or Mtrace2, which allows the tracing of an IP
   multicast routing path.  Mtrace2 is usually initiated from an Mtrace2
   client by sending an Mtrace2 Query to a Last-Hop Router (LHR) or to a
   Rendezvous Point (RP).  The RP is a special router where sources and
   receivers meet in Protocol Independent Multicast - Sparse Mode
   (PIM-SM) [5].  From the LHR/RP receiving the Query, the tracing is
   directed towards a specified source if a source address is specified
   and a source-specific state exists on the receiving router.  If no
   source address is specified or if no source-specific state exists on
   a receiving LHR, the tracing is directed toward the RP for the
   specified group address.  Moreover, Mtrace2 provides additional
   information such as the packet rates and losses, as well as other
   diagnostic information.  Mtrace2 is primarily intended for the
   following purposes:

   o  To trace the path that a packet would take from a source to a
      receiver.

   o  To isolate packet-loss problems (e.g., congestion).

   o  To isolate configuration problems (e.g., Time to live (TTL)
      threshold).

   The following figure shows a typical case of how Mtrace2 is used.
   FHR represents the first-hop router, LHR represents the last-hop
   router, and the arrow lines represent the Mtrace2 messages that are
   sent from one node to another.  The numbers before the Mtrace2
   messages represent the sequence of the messages that would happen.
   The source, receiver, and Mtrace2 client are typically hosts.

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                  2. Request                 2. Request
                    +----+                    +----+
                    |    |                    |    |
                    v    |                    v    |
   +--------+    +-----+                        +-----+    +----------+
   | Source |----| FHR |----- The Internet -----| LHR |----| Receiver |
   +--------+    +-----+            |           +-----+    +----------+
                     \              |             ^
                      \             |            /
                       \            |           /
                        \           |          /
                3. Reply \          |         / 1. Query
                          \         |        /
                           \        |       /
                            \  +---------+ /
                             v | Mtrace2 |/
                               | Client  |
                               +---------+

   When an Mtrace2 client initiates a multicast trace, it sends an
   Mtrace2 Query packet to an LHR or RP for a multicast group and,
   optionally, a source address.  The LHR/RP turns the Query packet into
   a Request.  The Request message type enables each of the upstream
   routers processing the message to apply different packet and message
   validation rules than those required for the handling of a Query
   message.  The LHR/RP then appends a Standard Response Block
   containing its interface addresses and packet statistics to the
   Request packet, then forwards the packet towards the source/RP.  The
   Request packet is either unicasted to its upstream router towards the
   source/RP or multicasted to the group if the upstream router's IP
   address is not known.  In a similar fashion, each router along the
   path to the source/RP appends a Standard Response Block to the end of
   the Request packet before forwarding it to its upstream router.  When
   the FHR receives the Request packet, it appends its own Standard
   Response Block, turns the Request packet into a Reply, and unicasts
   the Reply back to the Mtrace2 client.

   The Mtrace2 Reply may be returned before reaching the FHR under some
   circumstances.  This can happen if a Request packet is received at an
   RP or gateway, or when any of several types of error or exception
   conditions occur that prevent the sending of a Request to the next
   upstream router.

   The Mtrace2 client waits for the Mtrace2 Reply message and displays
   the results.  When not receiving an Mtrace2 Reply message due to
   network congestion, a broken router (see Section 5.6), or a non-
   responding router (see Section 5.7), the Mtrace2 client may resend
   another Mtrace2 Query with a lower hop count (see Section 3.2.1) and

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   repeat the process until it receives an Mtrace2 Reply message.  The
   details are specific to the Mtrace2 client and outside the scope of
   this document.

   Note that when a router's control plane and forwarding plane are out
   of sync, the Mtrace2 Requests might be forwarded based on the control
   states instead.  In this case, the traced path might not represent
   the real path the data packets would follow.

   Mtrace2 supports both IPv4 and IPv6.  Unlike the previous version of
   Mtrace, which implements its query and response as Internet Group
   Management Protocol (IGMP) messages [10], all Mtrace2 messages are
   UDP based.  Although the packet formats of IPv4 and IPv6 Mtrace2 are
   different because of the address families, the syntax between them is
   similar.

   This document describes the base specification of Mtrace2 that can
   serve as a basis for future proposals such as Mtrace2 for Automatic
   Multicast Tunneling (AMT) [16] and Mtrace2 for Multicast in MPLS/BGP
   IP VPNs (known as Multicast VPN (MVPN)) [15].  They are, therefore,
   out of the scope of this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [1] [7] when, and only when, they appear in all capitals, as
   shown here.  The key words indicate requirement levels for compliant
   Mtrace2 implementations.

2.1.  Definitions

   Since Mtrace2 Queries and Requests 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 data is expected to arrive from the
      specified source and group.

   Outgoing Interface:
      This is one of the interfaces to which data from the source or RP
      is expected to be transmitted for the specified source and group.
      It is also the interface on which the Mtrace2 Request was
      received.

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   Upstream router:
      The router, connecting to the Incoming Interface of the current
      router, which is responsible for forwarding data for the specified
      source and group to the current router.

   First-Hop Router (FHR):
      The router that is directly connected to the source the Mtrace2
      Query specifies.

   Last-Hop Router (LHR):
      A router that is directly connected to a receiver.  It is also the
      router that receives the Mtrace2 Query from an Mtrace2 client.

   Group state:
      The state a shared-tree protocol, such as Protocol Independent
      Multicast - Sparse Mode (PIM-SM) [5], uses to choose the upstream
      router towards the RP for the specified group.  In this state,
      source-specific state is not available for the corresponding group
      address on the router.

   Source-specific state:
      The state that is used to choose the path towards the source for
      the specified source and group.

   ALL-[protocol]-ROUTERS group:
      Link-local multicast address for multicast routers to communicate
      with their adjacent routers that are running the same routing
      protocol.  For instance, the IPv4 'ALL-PIM-ROUTERS' group is
      '224.0.0.13', and the IPv6 'ALL-PIM-ROUTERS' group is 'ff02::d'
      [5].

3.  Packet Formats

   This section describes the details of the packet formats for Mtrace2
   messages.

   All Mtrace2 messages are encoded in the Type/Length/Value (TLV)
   format (see Section 3.1).  The first TLV of a message is a message
   header TLV specifying the type of message and additional context
   information required for processing of the message and for parsing of
   subsequent TLVs in the message.  Subsequent TLVs in a message,
   referred to as Blocks, are appended after the header TLV to provide
   additional information associated with the message.  If an
   implementation receives an unknown TLV Type for any TLV in a message,
   it SHOULD ignore and silently discard the entire packet.  If the
   length of a TLV exceeds the available space in the containing packet,
   the implementation MUST ignore and silently discard the TLV and any
   remaining portion of the containing packet.

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   All Mtrace2 messages are UDP packets.  For IPv4, Mtrace2
   Query/Request/Reply messages MUST NOT be fragmented.  Therefore,
   Mtrace2 clients and LHRs/RPs MUST set the IP header do-not-fragment
   (DF) bit for all Mtrace2 messages.  For IPv6, the packet size for the
   Mtrace2 messages MUST NOT exceed 1280 bytes, which is the smallest
   Maximum Transmission Unit (MTU) for an IPv6 interface [8].  The
   source port is uniquely selected by the local host operating system.
   The destination port is the IANA-reserved Mtrace2 port number (see
   Section 8).  All Mtrace2 messages MUST have a valid UDP checksum.

   Additionally, Mtrace2 supports both IPv4 and IPv6, but not when
   mixed.  For example, if an Mtrace2 Query or Request message arrives
   as an IPv4 packet, all addresses specified in the Mtrace2 messages
   MUST be IPv4 as well.  The same rule applies to IPv6 Mtrace2
   messages.

3.1.  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

      Describes the format of the Value field.  For all the available
      types, please see Section 3.2.

   Length: 16 bits

      Length of Type, Length, and Value fields in octets.  Minimum
      length required is 4 octets.  The length MUST be a multiple of 4
      octets.  The maximum TLV length is not defined; however, the
      entire Mtrace2 packet length MUST NOT exceed the available MTU.

   Value: variable length

      The format is based on the Type value.  The length of the Value
      field is the Length field minus 3.  All reserved fields in the
      Value field MUST be transmitted as zeros and ignored on receipt.

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3.2.  Defined TLVs

   The following TLV Types are defined:

         Code         Type
         ====         ================================
         0x00         Reserved
         0x01         Mtrace2 Query
         0x02         Mtrace2 Request
         0x03         Mtrace2 Reply
         0x04         Mtrace2 Standard Response Block
         0x05         Mtrace2 Augmented Response Block
         0x06         Mtrace2 Extended Query Block

   Each Mtrace2 message MUST begin with either a Query, a Request, or a
   Reply TLV.  The first TLV determines the type of each Mtrace2
   message.  Following a Query TLV, there can be a sequence of optional
   Extended Query Blocks.  In the case of a Request or a Reply TLV, it
   is then followed by a sequence of Standard Response Blocks, each from
   a multicast router on the path towards the source or the RP.  In the
   case where more information is needed, a Standard Response Block can
   be followed by one or multiple Augmented Response Blocks.

   We will describe each message type in detail in the next few
   sections.

3.2.1.  Mtrace2 Query

   An Mtrace2 Query is originated by an Mtrace2 client, which sends an
   Mtrace2 Query message to the LHR.  The LHR modifies only the Type
   field of the Query TLV (to turn it into a "Request") before appending
   a Standard Response Block and forwarding it upstream.  The LHR and
   intermediate routers handling the Mtrace2 message when tracing
   upstream MUST NOT modify any other fields within the Query/Request
   TLV.  Additionally, intermediate routers handling the message after
   the LHR has converted the Query into a Request MUST NOT modify the
   Type field of the Request TLV.  If the actual number of hops is not
   known, an Mtrace2 client could send an initial Query message with a
   large # Hops (e.g., 0xff), in order to try to trace the full path.

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   An Mtrace2 Query message is shown 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |           Length              |    # Hops     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Multicast Address                        |
     |                                                               |
     +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
     |                                                               |
     |                        Source Address                         |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                    Mtrace2 Client Address                     |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            Query ID           |         Client Port #         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Length: 16 bits
      The Length field MUST be either 20 (i.e., 8 + 3 * 4 (IPv4
      addresses)) or 56 (i.e., 8 + 3 * 16 (IPv6 addresses)); if the
      length is 20, then IPv4 addresses MUST be assumed, and if the
      length is 56, then IPv6 addresses MUST be assumed.

   # Hops: 8 bits
      This field specifies the maximum number of hops that the Mtrace2
      client wants to trace.  If there are some error conditions in the
      middle of the path that prevent an Mtrace2 Reply from being
      received by the client, the client MAY issue another Mtrace2 Query
      with a lower number of hops until it receives a Reply.

   Multicast Address: 32 bits or 128 bits
      This field specifies an IPv4 or IPv6 address, which can be either:

      m-1:  a multicast group address to be traced or

      m-2:  all ones in case of IPv4 or the unspecified address (::) in
            case of IPv6 if no group-specific information is desired.

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   Source Address: 32 bits or 128 bits
      This field specifies an IPv4 or IPv6 address, which can be either:

      s-1:  a unicast address of the source to be traced or

      s-2:  all ones in case of IPv4 or the unspecified address (::) in
            case of IPv6 if no source-specific information is desired.
            For example, the client is tracing a (*,g) group state.

      Note that it is invalid to have a source-group combination of
      (s-2, m-2).  If a router receives such combination in an Mtrace2
      Query, it MUST silently discard the Query.

   Mtrace2 Client Address: 32 bits or 128 bits
      This field specifies the Mtrace2 client's IPv4 address or IPv6
      global address.  This address MUST be a valid unicast address;
      therefore, it MUST NOT be all ones or an unspecified address.  The
      Mtrace2 Reply will be sent to this address.

   Query ID: 16 bits
      This field is used as a unique identifier for this Mtrace2 Query
      so that duplicate or delayed Reply messages may be detected.

   Client Port #: 16 bits
      This field specifies the destination UDP port number for receiving
      the Mtrace2 Reply packet.

3.2.2.  Mtrace2 Request

   The Mtrace2 Request TLV is exactly the same as an Mtrace2 Query
   except for identifying the Type field of 0x02.

   When an LHR receives an Mtrace2 Query message, it turns the Query
   into a Request by changing the Type field of the Query from 0x01 to
   0x02.  The LHR then appends an Mtrace2 Standard Response Block (see
   Section 3.2.4) of its own to the Request message before sending it
   upstream.  The upstream routers do the same without changing the Type
   field until one of them is ready to send a Reply.

3.2.3.  Mtrace2 Reply

   The Mtrace2 Reply TLV is exactly the same as an Mtrace2 Query except
   for identifying the Type field of 0x03.

   When an FHR or an RP receives an Mtrace2 Request message that is
   destined to itself, it appends an Mtrace2 Standard Response Block
   (see Section 3.2.4) of its own to the Request message.  Next, it
   turns the Request message into a Reply by changing the Type field of

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   the Request from 0x02 to 0x03 and by changing the UDP destination
   port to the port number specified in the Client Port Number field in
   the Request.  It then unicasts the Reply message to the Mtrace2
   client specified in the Mtrace2 Client Address field.

   There are a number of cases in which an intermediate router might
   return a Reply before a Request reaches the FHR or the RP.  See
   Sections 4.1.1, 4.2.2, 4.3.3, and 4.5 for more details.

3.2.4.  IPv4 Mtrace2 Standard Response Block

   This section describes the message format of an IPv4 Mtrace2 Standard
   Response Block.  The Type field is 0x04.

      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              |      MBZ      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Query Arrival Time                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Incoming Interface Address                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Outgoing Interface Address                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                   Upstream Router Address                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .           Input packet count on Incoming Interface            .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .           Output packet count on Outgoing Interface           .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .      Total number of packets for this source-group pair       .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          Rtg Protocol         |    Multicast Rtg Protocol     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Fwd TTL    |      MBZ      |S|   Src Mask  |Forwarding Code|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MBZ: 8 bits
      This field MUST be zeroed on transmission and ignored on
      reception.

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   Query Arrival Time: 32 bits
      The Query Arrival Time is a 32-bit Network Time Protocol (NTP)
      timestamp specifying the arrival time of the Mtrace2 Query or
      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 timespec (fractional part in
      nanoseconds) to a 32-bit NTP timestamp:

         query_arrival_time
         = ((tv.tv_sec + 32384) << 16) + ((tv.tv_nsec << 7) / 1953125)

      The constant 32384 is the number of seconds from Jan 1, 1900 to
      Jan 1, 1970 truncated to 16 bits.  ((tv.tv_nsec << 7) / 1953125)
      is a reduction of ((tv.tv_nsec / 1000000000) << 16), where "<<"
      denotes a logical left shift.

      Note that synchronized clocks are required on the traced routers
      to estimate propagation and queuing delays between successive
      hops.  Nevertheless, even without this synchronization, an
      application can still estimate an upper bound on cumulative one-
      way latency by measuring the time between sending a Query and
      receiving a Reply.

      Additionally, Query Arrival Time is useful for measuring the
      packet rate.  For example, suppose that a client issues two
      Queries, and the corresponding Requests R1 and R2 arrive at router
      X at time T1 and T2, then the client would be able to compute the
      packet rate on router X by using the packet-count information
      stored in the R1 and R2 and using the time T1 and T2.

   Incoming Interface Address: 32 bits
      This field specifies the address of the interface on which packets
      from the source or the RP are expected to arrive, or 0 if unknown
      or unnumbered.

   Outgoing Interface Address: 32 bits
      This field specifies the address of the interface on which packets
      from the source or the RP are expected to transmit towards the
      receiver, or 0 if unknown or unnumbered.  This is also the address
      of the interface on which the Mtrace2 Query or Request arrives.

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   Upstream Router Address: 32 bits
      This field specifies the address of the upstream router from which
      this router expects packets from this source.  This MAY be a
      multicast group (e.g., ALL-[protocol]-ROUTERS group) if the
      upstream router is not known because of the workings of the
      multicast routing protocol.  However, it MUST be 0 if the Incoming
      Interface address is unknown or unnumbered.

   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 ones if
      no count can be reported.  This counter may have the same value as
      ifHCInMulticastPkts from the Interfaces Group MIB (IF-MIB) [9] for
      this interface.

   Output packet count on Outgoing 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 ones if no count can be
      reported.  This counter may have the same value as
      ifHCOutMulticastPkts from the IF-MIB [9] for this interface.

   Total number of packets for this source-group pair: 64 bits
      This field counts the number of packets from the specified source
      forwarded by the router to the specified group, or all ones if no
      count can be reported.  If the S bit is set (see below), the count
      is for the source network, as specified by the Src Mask field (see
      below).  If the S bit is set and the Src Mask field is 127,
      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 IP Multicast MIB [14] for this
      forwarding entry.

   Rtg Protocol: 16 bits
      This field describes the unicast routing protocol running between
      this router and the upstream router, and it is used to determine
      the Reverse Path Forwarding (RPF) interface for the specified
      source or RP.  This value should have the same value as
      ipMcastRouteRtProtocol from the IP Multicast MIB [14] for this
      entry.  If the router is not able to obtain this value, all 0's
      must be specified.

   Multicast Rtg Protocol: 16 bits
      This field describes the multicast routing protocol in use between
      the router and the upstream router.  This value should have the
      same value as ipMcastRouteProtocol from the IP Multicast MIB [14]
      for this entry.  If the router cannot obtain this value, all 0's
      must be specified.

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   Fwd TTL: 8 bits
      This field contains the configured multicast TTL threshold, if
      any, of the Outgoing Interface.

   S: 1 bit
      If this bit is set, it 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.

   Src Mask: 7 bits
      This field contains the number of 1's in the netmask the 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 127 (0x7f).

   Forwarding Code: 8 bits
      This field contains a forwarding information/error code.  Values
      with the high-order bit set (0x80-0xff) are intended for use with
      conditions that are transitory or automatically recovered.  Other
      Forwarding Code values indicate a need to fix a problem in the
      Query or a need to redirect the Query.  Sections 4.1 and 4.2
      explain how and when the Forwarding Code is filled.  Defined
      values are as follows:

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   Value  Name            Description
   -----  --------------  ----------------------------------------------
   0x00   NO_ERROR        No error.
   0x01   WRONG_IF        Mtrace2 Request arrived on an interface
                          for which this router does not perform
                          forwarding for the specified group to the
                          source or RP.
   0x02   PRUNE_SENT      This router has sent a prune upstream that
                          applies to the source and group in the
                          Mtrace2 Request.
   0x03   PRUNE_RCVD      This router has stopped forwarding for this
                          source and group in response to a Request
                          from the downstream router.
   0x04   SCOPED          The group is subject to administrative
                          scoping at this router.
   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 LHR.
   0x07   NOT_FORWARDING  This router is not forwarding this source and
                          group out the Outgoing Interface for an
                          unspecified reason.
   0x08   REACHED_RP      Reached the Rendezvous Point.
   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
                          that 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
                          Mtrace2 client.
   0x0D   UNKNOWN_QUERY   A non-transitive Extended Query Type was
                          received by a router that does not support
                          the type.
   0x80   FATAL_ERROR     A fatal error is one where the router may
                          know the upstream router but cannot forward
                          the message to it.
   0x81   NO_SPACE        There was not enough room to insert another
                          Standard Response Block in the packet.
   0x83   ADMIN_PROHIB    Mtrace2 is administratively prohibited.

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3.2.5.  IPv6 Mtrace2 Standard Response Block

   This section describes the message format of an IPv6 Mtrace2 Standard
   Response Block.  The Type field is also 0x04.

      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              |      MBZ      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      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         |    Multicast Rtg Protocol     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |              MBZ 2          |S|Src Prefix Len |Forwarding Code|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MBZ: 8 bits
      This field MUST be zeroed on transmission and ignored on
      reception.

   Query Arrival Time: 32 bits
      Same definition as in IPv4.

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   Incoming Interface ID: 32 bits
      This field specifies the interface ID on which packets from the
      source or RP are expected to arrive, or 0 if unknown.  This ID
      should be the value taken from InterfaceIndex of the IF-MIB [9]
      for this interface.

   Outgoing Interface ID: 32 bits
      This field specifies the interface ID to which packets from the
      source or RP are expected to transmit, or 0 if unknown.  This ID
      should be the value taken from InterfaceIndex of the IF-MIB [9]
      for this interface.

   Local Address: 128 bits
      This field specifies a global IPv6 address that uniquely
      identifies the router.  A unique local unicast address [12] 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.

   Remote Address: 128 bits
      This field specifies the address of the upstream router, which, in
      most cases, is a link-local unicast address for the upstream
      router.

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

      Note that this may be a multicast group (e.g., ALL-[protocol]-
      ROUTERS group) if the upstream router is not known because of the
      workings of a multicast routing protocol.  However, it should be
      the unspecified address (::) if the Incoming Interface address is
      unknown.

   Input packet count on Incoming Interface: 64 bits
      Same definition as in IPv4.

   Output packet count on Outgoing Interface: 64 bits
      Same definition as in IPv4.

   Total number of packets for this source-group pair: 64 bits
      Same definition as in IPv4, except if the S bit is set (see
      below), 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

Top      ToC       Page 20 
      for all sources sending to this group.  This counter should have
      the same value as ipMcastRoutePkts from the IP Multicast MIB [14]
      for this forwarding entry.

   Rtg Protocol: 16 bits
      Same definition as in IPv4.

   Multicast Rtg Protocol: 16 bits

      Same definition as in IPv4.

   MBZ 2: 15 bits
      This field MUST be zeroed on transmission and ignored on
      reception.

   S: 1 bit
      Same definition as in IPv4, except the Src Prefix Len field is
      used to mask the source address.

   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).

   Forwarding Code: 8 bits
      Same definition as in IPv4.

3.2.6.  Mtrace2 Augmented Response Block

   In addition to the Standard Response Block, a multicast router on the
   traced path can optionally add one or multiple Augmented Response
   Blocks before sending the Request to its upstream router.

   The Augmented Response Block is flexible for various purposes such as
   providing diagnosis information (see Section 7) and protocol
   verification.  Its Type field is 0x05, and its format is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |           Length              |      MBZ      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Augmented Response Type    |           Value ....          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MBZ: 8 bits
      This field MUST be zeroed on transmission and ignored on
      reception.

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   Augmented Response Type: 16 bits
      This field specifies the type of various responses from a
      multicast router that might need to communicate back to the
      Mtrace2 client as well as the multicast routers on the traced
      path.

      The Augmented Response Type is defined as follows:

         Code      Type
         ======    ==============================================
         0x0001    # of the returned Standard Response Blocks

      When the NO_SPACE error occurs on a router, the router should send
      the original Mtrace2 Request received from the downstream router
      as a Reply back to the Mtrace2 client and continue with a new
      Mtrace2 Request.  In the new Request, the router adds a Standard
      Response Block followed by an Augmented Response Block with 0x01
      as the Augmented Response Type, and the number of the returned
      Mtrace2 Standard Response Blocks as the Value.

      Each upstream router recognizes the total number of hops the
      Request has traced so far by adding this number and the number of
      the Standard Response Block in the current Request message.

      This document only defines one Augmented Response Type in the
      Augmented Response Block.  The description on how to provide
      diagnosis information using the Augmented Response Block is out of
      the scope of this document and will be addressed in separate
      documents.

   Value: variable length
      The format is based on the Augmented Response Type value.  The
      length of the Value field is Length field minus 6.

3.2.7.  Mtrace2 Extended Query Block

   There may be a sequence of optional Extended Query Blocks that follow
   an Mtrace2 Query to further specify any information needed for the
   Query.  For example, an Mtrace2 client might be interested in tracing
   the path the specified source and group would take based on a certain
   topology.  In this case, the client can pass in the multi-topology ID
   as the value for an Extended Query Type (see below).  The Extended
   Query Type is extensible, and the behavior of the new types will be
   addressed by separate documents.

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   The Mtrace2 Extended Query Block's Type field is 0x06 and is
   formatted 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |           Length              |      MBZ    |T|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Extended Query Type      |           Value ....          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   MBZ: 7 bits
      This field MUST be zeroed on transmission and ignored on
      reception.

   T-bit (Transitive Attribute): 1 bit
      If the TLV Type is unrecognized by the receiving router, then this
      TLV is either discarded or forwarded along with the Query,
      depending on the value of this bit.  If this bit is set, then the
      router MUST forward this TLV.  If this bit is clear, the router
      MUST send an Mtrace2 Reply with an UNKNOWN_QUERY error.

   Extended Query Type: 16 bits
      This field specifies the type of the Extended Query Block.

   Value: 16 bits
      This field specifies the value of this Extended Query.



(page 22 continued on part 2)

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