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PIM Assert Message Packing
draft-ietf-pim-assert-packing-11

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9466.
Authors Yisong Liu , Toerless Eckert , Mike McBride , Zheng Zhang
Last updated 2023-03-28 (Latest revision 2023-03-25)
Replaces draft-liu-pim-assert-packing
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Stream WG state Submitted to IESG for Publication
Document shepherd Stig Venaas
Shepherd write-up Show Last changed 2021-12-10
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Responsible AD Alvaro Retana
Send notices to stig@venaas.com, aretana.ietf@gmail.com
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Details
draft-ietf-pim-assert-packing-11
PIM                                                          Y. Liu, Ed.
Internet-Draft                                              China Mobile
Intended status: Standards Track                          T. Eckert, Ed.
Expires: 25 September 2023                                    M. McBride
                                                               Futurewei
                                                                Z. Zhang
                                                         ZTE Corporation
                                                           24 March 2023

                       PIM Assert Message Packing
                    draft-ietf-pim-assert-packing-11

Abstract

   In PIM-SM shared LAN networks, there is often more than one upstream
   router.  When PIM Sparse Mode (PIM-SM), including PIM Source
   Specific-Specific Multicast (PIM-SSM), is used, this can lead to
   duplicate IP multicast packets being forwarded by these PIM routers.
   PIM Assert messages are used to elect a single forwarder for each IP
   multicast traffic flow between these routers.

   This document defines a mechanism to send and receive information for
   multiple IP multicast flows in a single PackedAssert message.  This
   optimization reduces the total number of PIM packets on the LAN and
   can therefore speed up the election of the single forwarder, reducing
   the number of duplicate IP multicast packets incurred.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 25 September 2023.

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Copyright Notice

   Copyright (c) 2023 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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  PIM Assert Packing Hello Option . . . . . . . . . . . . .   5
     3.2.  Assert Packing Message Formats  . . . . . . . . . . . . .   5
     3.3.  PackedAssert Mechanism  . . . . . . . . . . . . . . . . .   6
       3.3.1.  Sending PackedAssert messages . . . . . . . . . . . .   7
         3.3.1.1.  Handling of reception-triggered assert
                 records.  . . . . . . . . . . . . . . . . . . . . .   8
         3.3.1.2.  Handling of timer expiry-triggered assert
                 records.  . . . . . . . . . . . . . . . . . . . . .   9
         3.3.1.3.  Beneficial delay in sending PackedAssert
                 messages  . . . . . . . . . . . . . . . . . . . . .   9
         3.3.1.4.  Handling Assert/PackedAssert message loss . . . .   9
         3.3.1.5.  Optimal degree of assert record packing . . . . .  10
       3.3.2.  Receiving PackedAssert messages . . . . . . . . . . .  10
   4.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  PIM Assert Packing Hello Option . . . . . . . . . . . . .  10
     4.2.  Assert Message Format . . . . . . . . . . . . . . . . . .  11
     4.3.  Simple PackedAssert Message Format  . . . . . . . . . . .  11
     4.4.  Aggregated PackedAssert Message Format  . . . . . . . . .  13
       4.4.1.  Source Aggregated Assert Record . . . . . . . . . . .  15
       4.4.2.  RP Aggregated Assert Record . . . . . . . . . . . . .  16
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Working Group considerations  . . . . . . . . . . . . . . . .  19
     8.1.  Open Issues . . . . . . . . . . . . . . . . . . . . . . .  19
     8.2.  Changelog . . . . . . . . . . . . . . . . . . . . . . . .  19
       8.2.1.  draft-ietf-pim-assert-packing-11  . . . . . . . . . .  19

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       8.2.2.  draft-ietf-pim-assert-packing-10  . . . . . . . . . .  20
       8.2.3.  draft-ietf-pim-assert-packing-09  . . . . . . . . . .  20
       8.2.4.  draft-ietf-pim-assert-packing-08  . . . . . . . . . .  21
       8.2.5.  draft-ietf-pim-assert-packing-07  . . . . . . . . . .  22
       8.2.6.  draft-ietf-pim-assert-packing-06  . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Use case examples  . . . . . . . . . . . . . . . . .  23
     A.1.  Enterprise network  . . . . . . . . . . . . . . . . . . .  24
     A.2.  Video surveillance  . . . . . . . . . . . . . . . . . . .  24
     A.3.  Financial Services  . . . . . . . . . . . . . . . . . . .  24
     A.4.  IPTV broadcast Video  . . . . . . . . . . . . . . . . . .  24
     A.5.  MVPN MDT  . . . . . . . . . . . . . . . . . . . . . . . .  25
     A.6.  Special L2 services . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   In PIM-SM shared LAN networks, there is typically more than one
   upstream router.  When duplicate data packets appear on the LAN, from
   different upstream routers, assert packets are sent from these
   routers to elect a single forwarder according to [RFC7761].  The PIM
   assert messages are sent periodically to keep the assert state.  The
   PIM assert message carries information about a single multicast
   source and group, along with the corresponding metric-preference and
   metric of the route towards the source or PIM Rendezvous Point (RP).

   This document defines a mechanism to encode the information of
   multiple PIM Assert messages into a single PackedAssert message.
   This allows to send and receive information for multiple IP multicast
   flows in a single PackedAssert message without changing the PIM
   Assert state machinery.  It reduces the total number of PIM packets
   on the LAN and can therefore speed up the election of the single
   forwarder, reducing the number of duplicate IP multicast packets.
   This can particularly be helpful when there is traffic for a large
   number of multicast groups or SSM channels and PIM packet processing
   performance of the routers is slow.

1.1.  Requirements Language

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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1.2.  Terminology

   The reader is expected to be familiar with the terminology of
   [RFC7761].  The following lists the abbreviations repeated in this
   document.

   AT: Assert Timer

   RP: Rendezvous Point

   RPF: Reverse Path Forwarding

   SPT: Shortest Path Tree

   RPT: RP Tree

   DR: Designated Router

2.  Problem Statement

   PIM Asserts occur in many deployments.  See Appendix A for explicit
   examples and explanations of why it is often not possible to avoid.

   PIM assert state depends mainly on the network topology.  As long as
   there is a layer 2 network with more than 2 PIM routers, there may be
   multiple upstream routers, which can cause duplicate multicast
   traffic to be forwarded and assert process to occur.

   As the multicast services become widely deployed, the number of
   multicast entries increases, and a large number of assert messages
   may be sent in a very short period when multicast data packets
   trigger PIM assert processing in the shared LAN networks.  The PIM
   routers need to process a large number of PIM assert small packets in
   a very short time.  As a result, the device load is very large.  The
   assert packet may not be processed in time or even discarded, thus
   extending the time of traffic duplication in the network.

   The PIM Assert mechanism can only be avoided by designing the network
   to be without transit subnets with multiple upstream routers.  For
   example, an L2 ring between routers can sometimes be reconfigured to
   be a ring of point-to-point subnets connected by the routers.  These
   L2/L3 topology changes are undesirable though, when they are only
   done to enable IP multicast with PIM because they increase the cost
   of introducing IP multicast with PIM.

   These designs are also not feasible when specific L2 technologies are
   needed.  For example various L2 technologies for rings provide sub 50
   msec failover mechanisms, something not possible equally with an L5

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   subnet based ring.  Likewise, IEEE Time Sensitive Networking
   mechanisms would require an L2 topology that can not simply be
   replaced by an L3 topology.  L2 sub-topologies can also significantly
   reduce the cost of deployment.

3.  Specification

   This document defines three elements in support of PIM assert
   packing:

   1.  The PIM Assert Packing Hello Option.

   2.  The encoding of PackedAssert messages.

   3.  How to send and receive PackedAssert messages.

3.1.  PIM Assert Packing Hello Option

   The PIM Assert Packing Hello Option (Section 4.1) is used to announce
   support for the assert packing mechanisms specified in this document.
   PackedAssert messages (Section 3.2) MUST NOT be used unless all PIM
   routers in the same subnet announce this option.

3.2.  Assert Packing Message Formats

   The PIM Assert message, as defined in Section 4.9.6 of [RFC7761],
   describes the parameters of a (*,G) or (S,G) assert through the
   following information elements: Rendezvous Point Tree flag (R),
   Source Address, Group Address, Metric and Metric Preference.  This
   document calls this information an assert record.

   Assert packing introduces two new PIM Assert message encodings
   through the allocation and use of two flags in the PIM Assert message
   header [I-D.ietf-pim-rfc8736bis], the Packed (P) and the Aggregated
   (A) flags.

   If the (P)acked flag is 0, the message is a (non-packed) PIM Assert
   message as specified in [RFC7761].  See Section 4.2.  In this case,
   the (A) flag MUST be set to 0, and MUST be ignored on receipt.

   If the (P) flag is 1, then the message is called a PackedAssert
   message and the type and hence encoding format of the payload is
   determined by the (A) flag.

   If A=0, then the message body is a sequence of assert records.  This
   is called a "Simple PackedAssert" message.  See Section 4.3.

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   If A=1, then the message body is a sequence of aggregated assert
   records.  This is called an "Aggregated PackedAssert".  See
   Section 4.4.

   Two aggregated assert record types are specified.

   The "Source Aggregated Assert Record", see Section 4.4.1, encodes one
   (common) Source Address, Metric and Metric Preference as well as a
   list of one or more Group Addresses.  Source Aggregated Assert
   Records provide a more compact encoding than the Simple PackedAssert
   message format when multiple (S,G) flows share the same source S.  A
   single Source Aggregated Assert Record with n Group Addresses
   represents the information of assert records for (S,G1)...(S,Gn).

   The "RP Aggregated Assert Record", see Section 4.4.2, encodes one
   common Metric and Metric Preference as well as a list of "Group
   Records", each of which encodes a Group Address and a list of zero or
   more Source Addresses with a count.  This is called an "RP Aggregated
   Assert Record", because with standard RPF according to ([RFC7761]),
   all the Group Addresses that use the same RP will have the same
   Metric and Metric Preference.

   RP Aggregation Records provide a more compact encoding than the
   Simple PackedAssert message format for (*,G) flows.  The Source
   Address is optionally used by [RFC7761] assert procedures to indicate
   the source(s) that triggered the assert, otherwise the Source Address
   is set to 0 in the assert record.

   Both Source Aggregated Assert Records and RP Aggregated Assert
   Records also include the R flag which maintains its semantic from
   [RFC7761] but also distinguishes the encodings.  Source Aggregated
   Assert Records have R=0, as (S,G) assert records do in [RFC7761].  RP
   Aggregated Assert Records have R=1, as (*,G) assert records do in
   [RFC7761].

3.3.  PackedAssert Mechanism

   PackedAsserts do not change the [RFC7761] PIM assert state machine
   specification.  Instead, sending and receiving of PackedAssert
   messages as specified in the following subsections are logically new
   packetization options for assert records in addition to the (not
   packed) [RFC7761] Assert Message.  There is no change to the assert
   record information elements transmitted or their semantic.  They are
   just transmitted in fewer but larger packets and fewer total number
   of bytes used to encode the information elements.  In result, PIM
   routers should be able to send/receive assert records faster and/or
   with less processing overhead.

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3.3.1.  Sending PackedAssert messages

   When using assert packing, the regular [RFC7761] Assert message
   encoding with A=0 and P=0 is still allowed to be sent.  Routers are
   free to choose which PackedAssert message format they send - simple
   (Section 4.3) and/or aggregated (Section 4.4).

   *  When any PIM routers on the LAN have not signaled support for
      Assert Packing, implementations MUST send only Asserts and MUST
      NOT send PackedAsserts under any condition.

   *  Implementations SHOULD support sending of PackedAssert messages.
      It is out of scope of this specification for which conditions,
      such as data-triggered asserts or Assert Timer (AT) expiry-
      triggered asserts, or under which conditions (such as high load)
      an implementation will send PackedAsserts instead of Asserts.

   *  Implementations are expected to specify in documentation and/or
      management interfaces (such as a YANG model), which PackedAssert
      message formats they can send and under which conditions they will
      send them.

   *  Implementations SHOULD be able to indicate to the operator (such
      as through a YANG model) how many Assert and PackedAssert messages
      were sent/received and how many assert records were sent/received.

   *  Implementations that introduce support for PackedAsserts after
      support for Asserts SHOULD support configuration that disables
      PackedAssert operations.

   *  A configuration option SHOULD be available to disable PackedAssert
      operations.  Implementations that introduce support for assert
      packing from day one of their [RFC7761] implementation MAY omit
      this configuration option.

   When a PIM router has an assert record ready to send according to
   [RFC7761], it calls one of the following functions:

   *  send Assert(S,G) / send Assert(*,G) ([RFC7761], Section 4.2),

   *  Send Assert(S,G) / SendAssertCancel(S,G) ([RFC7761],
      Section 4.6.1),

   *  Send Assert(*,G) / Send AssertCancel(*,G) ([RFC7761],
      Section 4.6.2)

   *  send Assert(S,G) ([RFC7761], Section 4.8.2).

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   If sending of PackedAsserts is possible on the network, instead of
   sending an Assert message with an assert record, any of these calls
   MAY instead result in the PIM implementation remembering the assert
   record, and continueing with further processing for other flows which
   may result in additional assert records.

   PIM MUST then create PackedAssert messages from the remembered assert
   records and schedule them for sending according to the considerations
   of the following subsections.

3.3.1.1.  Handling of reception-triggered assert records.

   Avoiding additional delay because of assert packing compared to
   immediate scheduling of Assert messages is most critical for assert
   records that are triggered by reception of data or reception of
   asserts against which the router is in the "I am Assert Winner"
   state.  In these cases the router SHOULD send out an Assert or
   PackedAssert message containing this assert record as soon as
   possible to minimize the time in which duplicate IP multicast packets
   can occur.

   To avoid additional delay in this case, the router should employ
   appropriate assert packing and scheduling mechanisms, as explained
   here.

   Asserts/PackedAsserts created from reception-triggered assert records
   should be scheduled for serialization with a higher priority than
   those created from other reasons.  They should also bypass other PIM
   messages that can create significant bursts, such as PIM join/prune
   messages.

   When there is no reception-triggered Assert/PackedAssert messages
   currently being serialized on the interface or scheduled to be sent,
   the router should immediately generate and schedule an Assert or
   PackedAssert message without further assert packing.

   If there are one or more reception-triggered Assert/PackedAssert
   messages already serializing and/or scheduled to be serialized on the
   outgoing interface, then the router can use the time until the last
   of those messages will have finished serializing for PIM processing
   of further conditions that may result in additional reception-
   triggered assert records as well as packing of these assert records
   without introducing additional delay.

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3.3.1.2.  Handling of timer expiry-triggered assert records.

   Asserts triggered by expiry of the AT on an assert winner are not
   time-critical because they can be scheduled in advance and because
   the Assert_Override_Interval parameter of [RFC7761] already creates a
   3 second window in which such assert records can be sent, received,
   and processed before an assert loser's state would expire and
   duplicate IP multicast packets could occur.

   An example mechanism to allow packing of AT expiry-triggered assert
   records on assert winners is to round the AT to an appropriate
   granularity such as 100 msec.  This will cause AT for multiple (S,G)
   and/or (*,G) states to expire at the same time, thus allowing them to
   be easily packed without changes to the assert state machinery.

   AssertCancel messages have assert records with an infinite metric and
   can use assert packing as any other Assert.  They are sent on
   Override Timer (OT) expiry and can be packed for example with the
   same considerations as AT expiry-triggered assert records.

3.3.1.3.  Beneficial delay in sending PackedAssert messages

   Delay in sending PackedAsserts beyond what was discussed in prior
   subsections can still be beneficial when it causes the overall amount
   of (possible) duplicate IP multicast packets to decrease in a
   condition with large number of (S,G) and/or (*,G), compared to the
   situation in which an implementation only sends Assert messages.

   This delay can simply be used in implementations because it can not
   support the (more advanced) mechanisms described above, and this
   longer delay can be achieved by some simpler mechanism (such as only
   periodic generation of PackedAsserts) and still achieves an overall
   reduction in duplicate IP multicast packets compared to sending only
   Asserts.

3.3.1.4.  Handling Assert/PackedAssert message loss

   When Asserts are sent, a single packet loss will result only in
   continued or new duplicates from a single IP multicast flow.  Loss of
   (non AssertCancel) PackedAssert impacts duplicates for all flows
   packed into the PackedAssert and may result in the need for re-
   sending more than one Assert/PackedAssert, because of the possible
   inability to pack the assert records in this condition.  Therefore,
   routers SHOULD support mechanisms allowing for PackedAsserts and
   Asserts to be sent with an appropriate Differentiated Services Code
   Point (DSCP, [RFC2475]), such as Expedited Forwarding (EF), to
   minimize their loss, especially when duplicate IP multicast packets
   could cause congestion and loss.

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   Routers MAY support a configurable option for sending PackedAssert
   messages twice in short order (such as 50 msec apart), to overcome
   possible loss, but only when the following two conditions are met.

   1.  The total size of the two PackedAsserts is less than the total
       size of equivalent Assert messages,

   2.  The condition of the assert records flows in the PackedAssert is
       such that the router can expect that their reception by PIM
       routers will not trigger Assert/PackedAsserts replies.  This
       condition is true for example when sending an assert record while
       becoming or being Assert Winner (Action A1/A3 in [RFC7761]).

3.3.1.5.  Optimal degree of assert record packing

   The optimal target packing size will vary depending on factors
   including implementation characteristics and the required operating
   scale.  At some point, as the target packing size is varied from the
   size of a single non-packed Assert, to the MTU size, a size can be
   expected to be found where the router can achieve the required
   operating scale of (S,G) and (*,G) flows with minimum duplicates.
   Beyond this size, a further increase in the target packing size would
   not produce further benefits, but might introduce possible negative
   effects such as the incurrence of more duplicates on loss.

   For example, in some router implementations, the total number of
   packets that a control plane function such as PIM can send/receive
   per unit of time is a more limiting factor than the total amount of
   data across these packets.  As soon as the packet size is large
   enough for the maximum possible payload throughput, increasing the
   packet size any further may still reduce the processing overhead of
   the router, but may increase latency incurred in creating the packet
   in a way that may increase duplicates compared to smaller packets.

3.3.2.  Receiving PackedAssert messages

   Upon reception of a PackedAssert message, the PIM router logically
   converts its payload into a sequence of assert records that are then
   processed as if an equivalent sequence of Assert messages were
   received according to [RFC7761].

4.  Packet Formats

   This section describes the format of new PIM extensions introduced by
   this document.

4.1.  PIM Assert Packing Hello Option

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    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 = TBD         |      OptionLength = 0         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: PIM Assert Packing Hello Option

   The PIM Assert Packing Hello Option is a new option for PIM Hello
   Messages according to Section 4.9.2 of [RFC7761].

   *  OptionType TBD: PIM Packed Assert Capability Hello Option

   Including the PIM OptionType TBD indicates support for the ability to
   receive and process all the PackedAssert encodings defined in this
   document.

4.2.  Assert Message 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |7 6 5 4 3 2|A|P|           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Group Address (Encoded-Group format)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Source Address (Encoded-Unicast format)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                      Metric Preference                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Assert Message Format

   Figure 2 shows a PIM Assert message as specified in Section 4.9.6 of
   [RFC7761].  The Encoded-Group and Encoded-Unicast address formats are
   specified in Section 4.9.1 of [RFC7761] for IP and IPv6.

   This common header is showing the "7 6 5 4 3 2" Flag Bits as defined
   in Section 4 of [I-D.ietf-pim-rfc8736bis] and the location of the P
   and A flags, as described in Section 5.   As specified in
   Section 3.2, both flags in a (non-packed) PIM Assert message are
   required to be set to 0.

4.3.  Simple PackedAssert Message Format

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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |7 6 5 4 3 2|A|P|           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Zero       |                     Reserved                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Assert Record [1]                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Assert Record [2]                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   .                               .                               .
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Assert Record [M]                      .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: Simple PackedAssert Message Format

   *  PIM Version, Type, Checksum:

      As specified in Section 4.9.6 of [RFC7761].

   *  "7 6 5 4 3 2": IANA registry handled bits according to Section 4
      of [I-D.ietf-pim-rfc8736bis].

   *  Zero: Set to zero on transmission.  Serves to make non assert
      packing capable PIM routers fail in parsing the message instead of
      possible mis-parsing if this field was used.

   *  Reserved: Set to zero on transmission.  Ignored upon receipt.

   *  P: packed flag.  MUST be 1.

   *  A: aggregated flag.  MUST be 0.

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   *  M: The number of Assert Records in the message.  Derived from the
      length of the packet carrying the message.

   *  Assert Record: formatted according to {FIG-MESSAGE-SIMPLE}}, wich
      is the same as the PIM assert message body as specified in
      Section 4.9.6 of [RFC7761].  The number M of Assert Records is
      determined from the packet size.

   The format of each Assert Record is the same as the PIM assert
   message body as specified in Section 4.9.6 of [RFC7761].

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Group Address (Encoded-Group format)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Source Address (Encoded-Unicast format)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                      Metric Preference                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                          Figure 4: Assert Record

4.4.  Aggregated PackedAssert Message Format

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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |PIM Ver| Type  |7 6 5 4 3 2|A|P|           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Zero       |                     Reserved                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     Aggregated Assert Record [1]              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     Aggregated Assert Record [2]              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   .                               .                               .
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                     Aggregated Assert Record [M]              .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 5: Aggregated PackedAssert Message Format

   *  PIM Version, Type, Reserved, Checksum:

      As specified in Section 4.9.6 of [RFC7761].

   *  "7 6 5 4 3 2": IANA registry handled bits according to Section 4
      of [I-D.ietf-pim-rfc8736bis].

   *  P: packed flag.  MUST be 1.

   *  A: aggregated flag.  MUST be 1.

   *  Zero: Set to zero on transmission.  Serves to make non assert
      packing capable PIM routers fail in parsing the message instead of
      possible mis-parsing if this field was used.

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   *  Aggregated Assert Record: formatted according to Figure 5.  The
      number M of Aggregated Assert Records is determined from the
      packet size.

4.4.1.  Source Aggregated Assert Record

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                      Metric Preference                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Source Address (Encoded-Unicast format)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Number of Groups (N)   |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Group Address 1 (Encoded-Group format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Group Address 2 (Encoded-Group format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             .                                 |
   |                             .                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Group Address N (Encoded-Group format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 6: Source Aggregated Assert Record

   *  Reserved: Set to zero on transmission.  Ignored upon receipt.

   *  R: MUST be 0.

      R indicates both that the encoding format of the record is that of
      a Source Aggregated Assert Record, but also that all assert
      records represented by the Source Aggregated Assert Record have
      R=0 and are therefore (S,G) assert records according to the
      definition of R in [RFC7761], Section 4.9.6.

   *  Source Address, Metric Preference, Metric:

      As specified in Section 4.9.6 of [RFC7761].  Source Address MUST
      NOT be zero.

   *  Number of Groups:

      The number of Group Address fields.

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   *  Group Address:

      As specified in Section 4.9.6 of [RFC7761].

4.4.2.  RP Aggregated Assert Record

   An RP Aggregation Assert record aggregates (*,G) assert records with
   the same Metric Preference and Metric.  Typically this is the case
   for all (*,G) using the same RP, but the encoding is not limited to
   only (*,G) using the same RP because the RP address is not encoded as
   it is also not present in [RFC7761] assert records.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |R|                      Metric Preference                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Metric                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Number of Group Records (K) |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [1]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [2]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                               .                               |
   .                               .                               .
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   .                                                               .
   .                        Group Record [K]                       .
   .                                                               .
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 7: RP Aggregated Assert Record

   *  R: MUST be 1.

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      R indicates both that the encoding format of the record is that of
      an RP Aggregated Assert Record, and that all assert records
      represented by the RP Aggregated Assert Record have R=1 and are
      therefore (*,G) assert records according to the definition of R in
      [RFC7761], Section 4.9.6.

   *  Metric Preference, Metric:

      As specified in Section 4.9.6 of [RFC7761].

   *  Reserved: Set to zero on transmission.  Ignored upon receipt.

   *  Number of Group Records (K):

      The number of packed Group Records.  A record consists of a Group
      Address and a Source Address list with a number of sources.

   The format of each Group Record is:

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Group Address (Encoded-Group format)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Number of Sources (P)  |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Source Address 1 (Encoded-Unicast format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Source Address 2 (Encoded-Unicast format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             .                                 |
   |                             .                                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Source Address P (Encoded-Unicast format)        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                           Figure 8: Group Record

   *  Group Address and Reserved:

      As specified in Section 4.9.6 of [RFC7761].

   *  Reserved: Set to zero on transmission.  Ignored upon receipt.

   *  Number of Sources (P):

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      The Number of Sources is corresponding to the number of Source
      Address fields in the Group Record.  If this number is 0, the
      Group Record indicates one assert record with a Source Address of
      0.  If this number is not 0 and one of the (*,G) assert records to
      be encoded should have the Source Address 0, then 0 needs to be
      encoded as one of the Source Address fields.

   *  Source Address:

      As specified in Section 4.9.6 of [RFC7761].  But there can be
      multiple Source Address fields in the Group Record.

5.  IANA Considerations

   IANA is requested to assign a new code point from the "PIM-Hello
   Options" registry for the Packed Assert Capability as follows:

   +=======+========+=========================+================+
   | Value | Length |          Name           | Reference      |
   +=======+========+=========================+================+
   | TBD   |      0 | Packed Assert Capability| [This Document]|
   +=======+========+=========================+================+

                    Figure 9: IANA PIM-Hello Options ask

   IANA is requested to assign two Flag Bits in the Assert message from
   the "PIM Message Types" registry as follows:

   +======+========+=================+====================+
   | Type | Name   | Flag Bits       | Reference          |
   +======+========+=================+====================+
   |   5  | Assert | 2-7: Unassigned | [RFC3973][RFC7761] |
   |      |        |   1: Aggregated | [This Document]    |
   |      |        |   0: Packed     | [This Document]    |
   +======+========+=================+====================+

                   Figure 10: IANA PIM Message Types ask

   [RFC-Editor note: If IANA can not assign the requested two bits 0 and
   1, then the figures showing those two bits need to be fixed to show P
   and A in the actual locations IANA assigns - aka: the bits shown are
   "placeholders" according to the requested bits in this section.]

6.  Security Considerations

   The security considerations of [RFC7761] apply to the extensions
   defined in this document.

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   This document packs multiple assert records in a single message.  As
   described in Section 6.1 of [RFC7761], a forged Assert message could
   cause the legitimate designated forwarder to stop forwarding traffic
   to the LAN.  The effect may be amplified when using a PackedAssert
   message.

   Like other optional extensions of [RFC7761] that are active only when
   all routers indicate support for them, a single misconfigured or
   malicious router emitting forged PIM Hello messages can inhibit
   operations of this extension.

   Authentication of PIM messages such as explained in [RFC7761],
   Sections 6.2 and 6.3 can protect against the forged message attacks
   attacks.

7.  Acknowledgments

   The authors would like to thank: Stig Venaas for the valuable
   contributions of this document, Alvaro Retana for his thorough and
   constructive RTG AD review, Ines Robles for her Gen-ART review, Tommy
   Pauly for his transport area review, Robert Sparks for his SecDir
   review, Shuping Peng for her RtgDir review, John Scudder for his RTG
   AD review, Eric Vyncke for his INT AD review, Eric Kline for his INT
   AD review, Paul Wouter for his SEC AD review, Zaheduzzaman Sarker for
   his TSV AD review, Robert Wilton for his OPS AD review and Martin
   Duke for his TSV AD review.

8.  Working Group considerations

   [RFC-Editor: please remove this section].

8.1.  Open Issues

8.2.  Changelog

   This document is hosted starting with -06 on
   https://github.com/toerless/pim-assert-packing.

8.2.1.  draft-ietf-pim-assert-packing-11

   Comprehensive 2 round AD review by John Scudder.

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   Functional enhancement: add requirement for existing implementation
   to be able to disable assert packing so that any possible
   compatibility issues introduced (which we think will not happen) can
   be avoided when upgrading to a packedassert version of the software.
   Also to allow performance comparison.  No making a requirement for
   day 0 implementations because they may want to save the work of
   having a non-packed-assert code path.

   Some rewrite to increase readibility, subdivided 3.3.1 into multiple
   subsections to better structure it.

   3.3.1 improved core requirements - added requirement for counters to
   show assert/packedassert operations, documentation (e.g.: YANG) for
   what/how it can send, config option to disable packedasserts.
   Refined text: Bulletized cases of asserts in rfc7761,

   Subdivided 3.3.1 into multiple subsections for readability improved
   text based on review.  Added reference for DSCP.

   3.3.1.5 Added explicit example of improvement because of packet size/
   throughput limits of router.

   Added notion of attacks by wrong hellos to security section.

   Eric Vyncke review:

   Appendix A: Better elaboration of L2 ring vs L3 ring benefits.  Nits.

   Paul Wouter review:

   Changed explanation of number "M" of records to be inline with
   formatting of other data (sections 4.3 and 4.4).

   Some nits.

8.2.2.  draft-ietf-pim-assert-packing-10

   Fixed up Reserved field of PackedAsserts to get back to 32 bit
   alignment of the following fields (was down to 16 bits).  Sorry, had
   a misinterpretation reading rfc7761, though there ws something that
   had only made it 16 bit aligned.  Anyhow.  Only this one change, 8 ->
   24 bit for this field.

8.2.3.  draft-ietf-pim-assert-packing-09

   For details of review discussion/replies, see review reply emails in
   (https://github.com/toerless/pim-assert-packing/tree/main/emails)j

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   review Alvaro Retana: Reintroduced ref to PIM-DM, fixed typos,
   downgraded MAY->may for "sufficient".

   IANA Expert Review / Stig Venaas:

   Removed Count field from message headers as it complicates parsing
   and is unnecessary.  Added "Zero" field to make packed asserts
   received by a non-packed-assert-capable-router guaranteed to fail
   ("Reserved" address family type).

   Changed from RFC8736 to RFC8736bis so that we can use the word
   "Unassigned" in the IANA table.

   Review Shuping Peng

   Changed explanation of how assert packing works from "layer" to
   "alternative to packetization via PIM Assert Message.  Fixed various
   typos, expanded term etc..

   Review Robert Sparks:

   Moved Intro explanations of how one could avoid asserts (but how its
   problematic) to appendix.  Applied textual nits found.  Removed
   quotes around term "sufficient" for easier readbility.

   Review Tommy Paul:

   Transport related concern explained in reply, but no additional
   explanations in text because the question referred to basic PIM
   behavior expected to be understood by readers: No discovery of loss/
   trigger for retransmission, just restransmission of same message
   element after discover of ongoing duplicates and/or expiry of timers.

8.2.4.  draft-ietf-pim-assert-packing-08

   Included changes from review of Alvaro Retana
   (https://mailarchive.ietf.org/arch/msg/pim/
   GsKq9bB2a6yDdM9DvAUGCWthdEI)

   Please see the following emails discussing the changes:

   https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
   emails/07-alvaro-review-reply.txt

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8.2.5.  draft-ietf-pim-assert-packing-07

   Included changes from review of Alvaro Retana
   (https://mailarchive.ietf.org/arch/msg/pim/
   Cp4o5glUFge2b84X9CQMwCWZjAk/)

   Please see the following emails discussing the changes:

   https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
   emails/05-alvaro-review-reply.txt

   https://raw.githubusercontent.com/toerless/pim-assert-packing/main/
   emails/07-pim-wg-announce.txt

8.2.6.  draft-ietf-pim-assert-packing-06

   This version was converted from txt format into markdown for better
   editing later, but is otherwise text identical to -05.  It was posted
   to DataTracker to make diffs easier.

   Functional changes:

9.  References

9.1.  Normative References

   [I-D.ietf-pim-rfc8736bis]
              Venaas, S. and A. Retana, "PIM Message Type Space
              Extension and Reserved Bits", Work in Progress, Internet-
              Draft, draft-ietf-pim-rfc8736bis-00, 2 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pim-
              rfc8736bis-00>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7761]  Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
              Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
              Multicast - Sparse Mode (PIM-SM): Protocol Specification
              (Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
              2016, <https://www.rfc-editor.org/info/rfc7761>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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9.2.  Informative References

   [RFC2475]  Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
              and W. Weiss, "An Architecture for Differentiated
              Services", RFC 2475, DOI 10.17487/RFC2475, December 1998,
              <https://www.rfc-editor.org/info/rfc2475>.

   [RFC3973]  Adams, A., Nicholas, J., and W. Siadak, "Protocol
              Independent Multicast - Dense Mode (PIM-DM): Protocol
              Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973,
              January 2005, <https://www.rfc-editor.org/info/rfc3973>.

   [RFC6037]  Rosen, E., Ed., Cai, Y., Ed., and IJ. Wijnands, "Cisco
              Systems' Solution for Multicast in BGP/MPLS IP VPNs",
              RFC 6037, DOI 10.17487/RFC6037, October 2010,
              <https://www.rfc-editor.org/info/rfc6037>.

   [RFC7431]  Karan, A., Filsfils, C., Wijnands, IJ., Ed., and B.
              Decraene, "Multicast-Only Fast Reroute", RFC 7431,
              DOI 10.17487/RFC7431, August 2015,
              <https://www.rfc-editor.org/info/rfc7431>.

   [RFC7490]  Bryant, S., Filsfils, C., Previdi, S., Shand, M., and N.
              So, "Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)",
              RFC 7490, DOI 10.17487/RFC7490, April 2015,
              <https://www.rfc-editor.org/info/rfc7490>.

Appendix A.  Use case examples

   The PIM Assert mechanism can only be avoided by designing the network
   to be without transit subnets with multiple upstream routers.  For
   example, an L2 ring between routers can sometimes be reconfigured to
   be a ring of point-to-point subnets connected by the routers.  These
   L2/L3 topology changes are undesirable though, when they are only
   done to enable IP multicast with PIM because they increase the cost
   of introducing IP multicast with PIM.

   These L3 ring designs are specifically undesirable, when particular
   L2 technologies are needed.  For example various L2 technologies for
   rings provide sub 50 msec failover mechanisms that will benefit IP
   unicast and multicast alike without any added complexity to the IP
   layer (forwarding or routing).  If such L2 rings where to be replaced
   by L3 rings just to avoid PIM asserts, then this would result in the
   need for a complex choice of of a sub 50 msec IP unicast failover
   solutions as well as a sub 50 msec IP multicast failover solution.
   The mere fact that by operating at the IP layer, different solutions
   for IP unicast and multicast are required makes them more difficult
   to operate, they typically require more expensive hardware and

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   therefore most often, they are not even available on the target
   equipment, such as [RFC7490] with IP repair tunnels for IP unicast or
   [RFC7431] for IP multicast.

   Likewise, IEEE Time Sensitive Networking mechanisms would require an
   L2 topology that can not simply be replaced by an L3 topology.  L2
   sub-topologies can also significantly reduce the cost of deployment.

   The following subsections give examples of the type of network and
   use-cases in which subnets with asserts have been observerd or are
   expected to require scaling as provided by this specification.

A.1.  Enterprise network

   When an Enterprise network is connected through a layer-2 network,
   the intra-enterprise runs layer-3 PIM multicast.  The different sites
   of the enterprise are equivalent to the PIM connection through the
   shared LAN network.  Depending upon the locations and amount of
   groups there could be many asserts on the first-hop routers.

A.2.  Video surveillance

   Video surveillance deployments have migrated from analog based
   systems to IP-based systems oftentimes using multicast.  In the
   shared LAN network deployments, when there are many cameras streaming
   to many groups there may be issues with many asserts on first-hop
   routers.

A.3.  Financial Services

   Financial services extensively rely on IP Multicast to deliver stock
   market data and its derivatives, and current multicast solution PIM
   is usually deployed.  As the number of multicast flows grow, there
   are many stock data with many groups may result in many PIM asserts
   on a shared LAN network from publisher to the subscribers.

A.4.  IPTV broadcast Video

   PIM DR deployments are often used in host-side network for IPTV
   broadcast video services.  Host-side access network failure scenario
   may be benefitted by assert packing when many groups are being used.
   According to [RFC7761] the DR will be elected to forward multicast
   traffic in the shared access network.  When the DR recovers from a
   failure, the original DR starts to send traffic, and the current DR
   is still forwarding traffic.  In the situation multicast traffic
   duplication maybe happen in the shared access network and can trigger
   the assert progress.

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A.5.  MVPN MDT

   As described in [RFC6037], MDT (Multicast Distribution Tree) is used
   as tunnels for MVPN.  The configuration of multicast-enabled VRF (VPN
   routing and forwarding) or interface that is in a VRF changing may
   cause many assert packets to be sent in a same time.

A.6.  Special L2 services

   Additionally, future backhaul, or fronthaul, networks may want to
   connect L3 across an L2 underlay supporting Time Sensitive Networks
   (TSN).  The infrastructure may run DetNet over TSN.  These transit L2
   LANs would have multiple upstreams and downstreams.  This document is
   taking a proactive approach to prevention of possible future assert
   issues in these types of environments.

Authors' Addresses

   Yisong Liu (editor)
   China Mobile
   China
   Email: liuyisong@chinamobile.com

   Toerless Eckert (editor)
   Futurewei
   United States of America
   Email: tte@cs.fau.de

   Mike McBride
   Futurewei
   United States of America
   Email: michael.mcbride@futurewei.com

   Zheng(Sandy) Zhang
   ZTE Corporation
   China
   Email: zhang.zheng@zte.com.cn

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