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RFC 6807 - Population Count Extensions to Protocol Independent M


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Internet Engineering Task Force (IETF)                      D. Farinacci
Request for Comments: 6807                                   G. Shepherd
Category: Experimental                                         S. Venaas
ISSN: 2070-1721                                            Cisco Systems
                                                                  Y. Cai
                                                               Microsoft
                                                           December 2012

  Population Count Extensions to Protocol Independent Multicast (PIM)

Abstract

   This specification defines a method for providing multicast
   distribution-tree accounting data.  Simple extensions to the Protocol
   Independent Multicast (PIM) protocol allow a rough approximation of
   tree-based data in a scalable fashion.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  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).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6807.

Copyright Notice

   Copyright (c) 2012 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
   (http://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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Pop-Count-Supported Hello Option . . . . . . . . . . . . . . .  4
   3.  New Pop-Count Join Attribute Format  . . . . . . . . . . . . .  5
     3.1.  Options  . . . . . . . . . . . . . . . . . . . . . . . . .  8
       3.1.1.  Link Speed Encoding  . . . . . . . . . . . . . . . . . 10
     3.2.  Example Message Layouts  . . . . . . . . . . . . . . . . . 10
   4.  How to Use Pop-Count Encoding  . . . . . . . . . . . . . . . . 11
   5.  Implementation Approaches  . . . . . . . . . . . . . . . . . . 12
   6.  Caveats  . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 14
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   This document specifies a mechanism to convey accounting information
   using the Protocol Independent Multicast (PIM) protocol [RFC4601]
   [RFC5015].  Putting the mechanism in PIM allows efficient
   distribution and maintenance of such accounting information.
   Previous mechanisms require data to be correlated from multiple
   router sources.

   This mechanism allows a single router to be queried to obtain
   accounting and statistic information for a multicast distribution
   tree as a whole or any distribution sub-tree downstream from a
   queried router.  The amount of information is fixed and does not
   increase as multicast membership, tree diameter, or branching
   increases.

   The sort of accounting data this specification provides, on a per-
   multicast-route basis, are:

   1.  The number of branches in a distribution tree.

   2.  The membership type of the distribution tree, that is, Source-
       Specific Multicast (SSM) or Any-Source Multicast (ASM).

   3.  Routing domain and time zone boundary information.

   4.  On-tree node and tree diameter counters.

   5.  Effective MTU and bandwidth.

   This document defines a new PIM Join Attribute type [RFC5384] for the
   Join/Prune message as well as a new Hello option.  The mechanism is
   applicable to IPv4 and IPv6 multicast.

   This is a new extension to PIM, and it is not completely understood
   what impact collecting information using PIM would have on the
   operation of PIM.  This is an entirely new concept.  Many PIM
   features (including the core protocols) were first introduced in
   Experimental RFCs, and it seems appropriate to advance this work as
   Experimental.  Reports of implementation and deployment across whole
   distribution trees or within sub-trees (see Section 6) will enable an
   assessment of the desirability and stability of this specification.
   The PIM Working Group will then consider whether to move this work to
   the Standards Track.

   This document does not specify how an administrator or user can
   access this information.  It is expected that an implementation may
   have a command-line interface or other ways of requesting and

   displaying this information.  As this is currently an Experimental
   document, defining a MIB module has not been considered.  If the PIM
   Working Group finds that this should move on to Standards Track, a
   MIB module should be considered.

1.1.  Requirements Notation

   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 [RFC2119].

1.2.  Terminology

   This section defines the terms used in this document.

   Multicast Route:  An (S,G) or (*,G) entry regardless of whether the
      route is in ASM, SSM, or BIDIR mode of operation.

   Stub Link:  A link with members joined to the group via IGMP or
      Multicast Listener Discovery (MLD).

   Transit Link:  A link put in the oif-list (outgoing interface list)
      for a multicast route because it was joined by PIM routers.

   Note that a link can be both a Stub Link and a Transit Link at the
   same time.

2.  Pop-Count-Supported Hello Option

   A PIM router indicates that it supports the mechanism specified in
   this document by including the Pop-Count-Supported Hello option in
   its PIM Hello message.  Note that it also needs to include the Join-
   Attribute Hello option as specified in [RFC5384].  The format of the
   Pop-Count-Supported Hello option is defined to be:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          OptionType           |         OptionLength          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   OptionType = 29, OptionLength = 0.  Note that there is no option
   value included.  In order to allow future updates of this
   specification that may include an option value, implementations of
   this document MUST accept and process this option even if the length
   is non-zero.  Implementations of this specification MUST accept and
   process the option ignoring any option value that may be included.

3.  New Pop-Count Join Attribute Format

   When a PIM router supports this mechanism and has determined from a
   received Hello that the neighbor supports this mechanism, and also
   that all the neighbors on the interface support the use of join
   attributes, it will send Join/Prune messages that MAY include a Pop-
   Count Join Attribute.  The mechanism to process a PIM Join Attribute
   is described in [RFC5384].  The format of the new attribute is
   specified in the following.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|E| Attr_Type |    Length     |        Effective MTU          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Flags             |        Options Bitmap         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Options                            |
      .                               .                               .
      .                               .                               .
      .                               .                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The above format is used only for entries in the join-list section of
   the Join/Prune message.

   F bit:  0 (Non-Transitive Attribute).

   E bit:  As specified by [RFC5384].

   Attr_Type:  3.

   Length:  The minimum length is 6.

   Effective MTU:  This contains the minimum MTU for any link in the
      oif-list.  The sender of a Join/Prune message takes the minimum
      value for the MTU (in bytes) from each link in the oif-list.  If
      this value is less than the value stored for the multicast route
      (the one received from downstream joiners), then the value should
      be reset and sent in a Join/Prune message.  Otherwise, the value
      should remain unchanged.

      This provides the MTU supported by multicast distribution tree
      when examined at the first-hop router(s) or for sub-tree for any
      router on the distribution tree.

   Flags:  The flags field has the following format:

           0                   1
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  Unalloc/Reserved   |P|a|t|A|S|
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Unallocated/Reserved Flags:  The flags that are currently not
         defined.  If a new flag is defined and used by a new
         implementation, an old implementation should preserve the bit
         settings.  This means that a router MUST preserve the settings
         of all Unallocated/Reserved Flags in PIM Join messages received
         from downstream routers in any PIM Join sent upstream.

      S flag:  This flag is set if an IGMPv3 or MLDv2 report with an
         INCLUDE mode group record was received on any oif-list entry or
         the bit was set from any PIM Join message.  This bit should
         only be cleared when the above becomes untrue.

      A flag:  This flag is set if an IGMPv3 or MLDv2 report with an
         EXCLUDE mode group record, or an IGMPv1, IGMPv2, or MLDv1
         report, was received on any oif-list entry or the bit was set
         from any PIM Join message.  This bit should only be cleared
         when the above becomes untrue.

         A combination of settings for these bits indicate:

           A flag   S flag   Description
           ------   ------   --------------------------------------
             0        0      There are no members for the group.
                             ('Stub Oif-List Count' is 0)
             0        1      All group members are using SSM.
             1        0      All group members are using ASM.
             1        1      A mixture of SSM and ASM group members.

      t flag:  This flag is set if there are any manually configured
         tunnels on the distribution tree.  This means any tunnel that
         is not an auto-tunnel.  If a manually configured tunnel is in
         the oif-list, a router sets this bit in its Join/Prune
         messages.  Otherwise, it propagates the bit setting from
         downstream joiners.

      a flag:  This flag is set if there are any auto-tunnels on the
         distribution tree.  If an auto-tunnel is in the oif-list, a
         router sets this bit in its Join/Prune messages.  Otherwise, it
         propagates the bit setting from downstream joiners.  An example
         of an auto-tunnel is a tunnel set up by the Automatic Multicast
         Tunneling [AMT] protocol.

      P flag:  This flag is set by a router if all downstream routers
         support this specification.  That is, they are all PIM Pop-
         Count capable.  If a downstream router does not support this
         specification, it MUST be cleared.  This allows one to tell if
         the entire sub-tree is completely accounting capable.

   Options Bitmap:  This is a bitmap that shows which options are
      present.  The format of the bitmap is as follows:

            0                   1
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |T|s|m|M|d|n|D|z| Unalloc/Rsrvd |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Each one of the bits T, s, m, M, d, n, D and z is associated with
      one option, where the option is included if and only if the
      respective bit is set.  Included options MUST be in the same order
      as these bits are listed.  The bits denote the following options:

            bit     Option
           -----   ------------------------
             T      Transit Oif-List Count
             s      Stub Oif-List Count
             m      Minimum Speed Link
             M      Maximum Speed Link
             d      Domain Count
             n      Node Count
             D      Diameter Count
             z      TZ Count

      See Section 3.1 for details on the different options.  The
      unallocated bits are reserved.  Any unknown bits MUST be set to 0
      when a message is sent, and treated as 0 (ignored) when received.
      This means that unknown options that are denoted by unknown bits
      are ignored.

      By using this bitmap we can specify at most 16 options.  If there
      becomes a need for more than 16 options, one can define a new
      option that contains a bitmap that can then be used to specify
      which further options are present.  The last bit in the current
      bitmap could be used for that option.  However, the exact
      definition of this is left for future documents.

   Options:  This field contains options.  Which options are present is
      determined by the flag bits.  As new flags and options may be
      defined in the future, any unknown/reserved flags MUST be ignored,
      and any additional trailing options MUST be ignored.  See
      Section 3.1 for details on the options defined in this document.

3.1.  Options

   There are several options defined in this document.  For each option,
   there is also a related flag that shows whether the option is
   present.  See the Options Bitmap above for a list of the options and
   their respective bits.  Each option has a fixed size.  Note that
   there are no alignment requirements for the options, so an
   implementation cannot assume they are aligned.

   Transit Oif-List Count:  This is filled in by a router sending a
      Join/Prune message indicating the number of transit links on the
      multicast distribution tree.  The value is the number of oifs
      (outgoing interfaces) for the multicast route that have been
      joined by PIM plus the sum of the values advertised by each of the
      downstream PIM routers that have joined on this oif.  Length is 4
      octets.

   Stub Oif-List Count:  This is filled in by a router sending a Join/
      Prune message indicating the number of stub links (links where
      there are host members) on the multicast distribution tree.  The
      value is the number of oifs for the multicast route that have been
      joined by IGMP or MLD plus the sum of the values advertised by
      each of the downstream PIM routers that have joined on this oif.
      Length is 4 octets.

   Minimum Speed Link:  This contains the minimum bandwidth rate for any
      link in the oif-list and is encoded as specified in Section 3.1.1.
      The sender of a Join/Prune message takes the minimum value for
      each link in the oif-list for the multicast route.  If this value
      is less than the value stored for the multicast route (the
      smallest value received from downstream joiners), then the value
      should be reset and sent in a Join/Prune message.  Otherwise, the
      value should remain unchanged.  This, together with the Maximum

      Speed Link option, provides a way to obtain the lowest- and
      highest-speed links for the multicast distribution tree.  Length
      is 2 octets.

   Maximum Speed Link:  This contains the maximum bandwidth rate for any
      link in the oif-list and is encoded as specified in Section 3.1.1.
      The sender of a Join/Prune message takes the maximum value for
      each link in the oif-list for the multicast route.  If this value
      is greater than the value stored for the multicast route (the
      largest value received from downstream joiners), then the value
      should be reset and sent in a Join/Prune message.  Otherwise, the
      value should remain unchanged.  This, together with the Minimum
      Speed Link option, provides a way to obtain the lowest- and
      highest-speed links for the multicast distribution tree.  Length
      is 2 octets.

   Domain Count:  This indicates the number of routing domains the
      distribution tree traverses.  A router should increment this value
      if it is sending a Join/Prune message over a link that traverses a
      domain boundary.  For this to work, an implementation needs a way
      of knowing that a neighbor or an interface is in a different
      domain.  There is no standard way of doing this.  Length is 1
      octet.

   Node Count:  This indicates the number of routers on the distribution
      tree.  Each router will sum up all the Node Counts from all
      joiners on all oifs and increment by 1 before including this value
      in the Join/Prune message.  Length is 1 octet.

   Diameter Count:  This indicates the longest length of any given
      branch of the tree in router hops.  Each router that sends a Join
      increments the max value received by all downstream joiners by 1.
      Length is 1 octet.

   TZ Count:  This indicates the number of time zones the distribution
      tree traverses.  A router should increment this value if it is
      sending a Join/Prune message over a link that traverses a time
      zone.  This can be a configured link attribute, or using other
      means to determine the time zone is acceptable.  Length is 1
      octet.

3.1.1.  Link Speed Encoding

   The speed is encoded using 2 octets as follows:

            0                   1
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           | Exponent  |    Significand    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Using this format, the speed of the link is Significand * 10 ^
   Exponent kbps.  This allows specifying link speeds with up to 3
   decimal digits precision and speeds from 1 kbps to 10 ^ 67 kbps.  A
   computed speed of 0 kbps means the link speed is < 1 kbps.

   Here are some examples of how this is used:

            Link Speed     Exponent     Significand
           ------------   ----------   -------------
            500 kbps       0            500
            500 kbps       2              5
            155 Mbps       3            155
             40 Gpbs       6             40
            100 Gpbs       6            100
            100 Gpbs       8              1

3.2.  Example Message Layouts

   Here, we will give a few examples to illustrate the use of flags and
   options.

   A minimum-size message has no option flags set and looks like this:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|E| Attr_Type |  Length = 6   |        Effective MTU          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Unalloc/Reserved   |P|a|t|A|S|0|0|0|0|0|0|0|0| Unalloc/Rsrvd |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A message containing all the options defined in this document would
   look like this:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|E| Attr_Type |  Length = 18  |        Effective MTU          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Unalloc/Reserved   |P|a|t|A|S|1|1|1|1|1|1|1|1| Unalloc/Rsrvd |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Transit Oif-List Count                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Stub Oif-List Count                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Minimum Speed Link       |      Maximum Speed Link       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Domain Count |  Node Count   | Diameter Count|    TZ Count   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A message containing only Stub Oif-List Count and Node Count would
   look like this:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|E| Attr_Type |  Length = 9   |        Effective MTU          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Unalloc/Reserved   |P|a|t|A|S|0|1|0|0|0|1|0|0| Unalloc/Rsrvd |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Stub Oif-List Count                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Node count   |
      +-+-+-+-+-+-+-+-+

4.  How to Use Pop-Count Encoding

   A router supporting this mechanism MUST, unless administratively
   disabled, include the PIM Join Attribute option in its PIM Hellos.
   See [RFC5384] and "PIM-Hello Options" on [PIM-REG] for details.

   It is RECOMMENDED that implementations allow for administrative
   control of whether to make use of this mechanism.  Implementations
   MAY also allow further control of what information to store and send
   upstream.

   It is very important to note that any changes to the values
   maintained by this mechanism MUST NOT trigger a new Join/Prune
   message.  Due to the periodic nature of PIM, the values can be
   accurately obtained at 1-minute intervals (or whatever Join/Prune
   interval used).

   When a router removes a link from an oif-list, it needs to be able to
   reevaluate the values that it will advertise upstream.  This happens
   when an oif-list entry is timed out or a Prune is received.

   It is RECOMMENDED that the Join Attribute defined in this document be
   used only for entries in the join-list part of the Join/Prune
   message.  If the attribute is used in the prune-list, an
   implementation MUST ignore it and process the Prune as if the
   attribute were not present.

   It is also RECOMMENDED that join suppression be disabled on a LAN
   when Pop-Count is used.

   It is RECOMMENDED that, when triggered Join/Prune messages are sent
   by a downstream router, the accounting information not be included in
   the message.  This way, when convergence is important, avoiding the
   processing time to build an accounting record in a downstream router
   and processing time to parse the message in the upstream router will
   help reduce convergence time.  If an upstream router receives a Join/
   Prune message with no accounting data, it SHOULD NOT interpret the
   message as a trigger to clear or reset the accounting data it has
   cached.

5.  Implementation Approaches

   This section offers some non-normative suggestions for how Pop-Count
   may be implemented.

   An implementation can decide how the accounting attributes are
   maintained.  The values can be stored as part of the multicast route
   data structure by combining the local information it has with the
   joined information on a per-oif basis.  So, when it is time to send a
   Join/Prune message, the values stored in the multicast route can be
   copied to the message.

   Or, an implementation could store the accounting values per oif and,
   when a Join/Prune message is sent, it can combine the oifs with its
   local information.  Then, the combined information can be copied to
   the message.

   When a downstream joiner stops joining, accounting values cached must
   be evaluated.  There are two approaches that can be taken.  One is to
   keep values learned from each joiner, so when the joiner goes away,
   the count/max/min values are known and the combined value can be
   adjusted.  The other approach is to set the value to 0 for the oif,
   and then start accumulating new values as subsequent Joins are
   received.

   The same issue arises when an oif is removed from the oif-list.
   Keeping per-oif values allows you to adjust the per-route values when
   an oif goes away.  Or, alternatively, a delay for reporting the new
   set a values from the route can occur while all oif values are zeroed
   (where accumulation of new values from subsequent Joins cause
   repopulation of values and a new max/min/count can be reevaluated for
   the route).

6.  Caveats

   This specification requires each router on a multicast distribution
   tree to support this specification or else the accounting attributes
   for the tree will not be known.

   However, if there is a contiguous set of routers downstream in the
   distribution tree, they can maintain accounting information for the
   sub-tree.

   If there is a set of contiguous routers supporting this specification
   upstream on the multicast distribution tree, accounting information
   will be available, but it will not represent an accurate assessment
   of the entire tree.  Also, it will not be clear how much of the
   distribution tree the accounting information covers.

7.  IANA Considerations

   A new PIM-Hello Option type, 29, has been assigned by IANA.  Although
   the length is specified as 0 in this specification, non-zero length
   is allowed, so IANA has listed the length as being variable.

   A new PIM Join Attribute type, 3, has been assigned by IANA.

8.  Security Considerations

   The use of this specification requires some additional processing of
   PIM Join/Prune messages.  However, the additional amount of
   processing is fairly limited, so this is not believed to be a
   significant concern.

   The use of this mechanism includes information like the number of
   receivers.  This information is assumed to not be of a sensitive
   nature.  If an operator has concerns about revealing this information
   to upstream routers or other routers/hosts that may potentially
   inspect this information, there should be a way to disable the
   mechanism or, alternatively, more detailed control of what
   information to include.

9.  Acknowledgments

   The authors would like to thank John Zwiebel, Amit Jain, and Clayton
   Wagar for their review comments on the initial versions of this
   document.  Adrian Farrel did a detailed review of the document and
   proposed textual changes that have been incorporated.  Further review
   and comments were provided by Thomas Morin and Zhaohui (Jeffrey)
   Zhang.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.

   [RFC5015]  Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
              "Bidirectional Protocol Independent Multicast (BIDIR-
              PIM)", RFC 5015, October 2007.

   [RFC5384]  Boers, A., Wijnands, I., and E. Rosen, "The Protocol
              Independent Multicast (PIM) Join Attribute Format",
              RFC 5384, November 2008.

10.2.  Informative References

   [AMT]      Bumgardner, G., "Automatic Multicast Tunneling", Work
              in Progress, June 2012.

   [PIM-REG]  IANA, "Protocol Independent Multicast (PIM) Parameters",
              <http://www.iana.org/assignments/pim-parameters>.

Authors' Addresses

   Dino Farinacci
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: dino@cisco.com

   Greg Shepherd
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: gjshep@gmail.com

   Stig Venaas
   Cisco Systems
   Tasman Drive
   San Jose, CA  95134
   USA

   EMail: stig@cisco.com

   Yiqun Cai
   Microsoft
   1065 La Avenida
   Mountain View, CA  94043
   USA

   EMail: yiqunc@microsoft.com

 

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