PIM Flooding Mechanism and Source Discovery Enhancements
draft-ietf-pim-pfm-forwarding-enhancements-01

Network Working Group                                           A. Gopal
Internet-Draft                                                 S. Venaas
Intended status: Experimental                        Cisco Systems, Inc.
Expires: 7 May 2025                                               F. Meo
                                                         3 November 2024

        PIM Flooding Mechanism and Source Discovery Enhancements
             draft-ietf-pim-pfm-forwarding-enhancements-01

Abstract

   PIM Flooding Mechanism is a generic PIM message exchange mechanism
   that allows multicast information to be exchanged between PIM routers
   hop-by-hop.  One example is PIM Flooding Mechanism and Source
   Discovery which allows last hop routers to learn about new sources
   using PFM messages, without the need for initial data registers,
   Rendezvous Points or shared trees.

   This document defines a new TLV for announcing sources that allows
   for Sub-TLVs that can be used for providing various types of
   information.  This document also defines methodologies that enhance
   forwarding efficiency in PFM-SD deployments.

Status of This Memo

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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  PIM PFM Sub-TLV . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Group Source Info TLV . . . . . . . . . . . . . . . . . .   4
   3.  PIM PFM forwarding optimization . . . . . . . . . . . . . . .   5
     3.1.  RFC 6395 Compliance . . . . . . . . . . . . . . . . . . .   5
     3.2.  PFM optimization Hello option . . . . . . . . . . . . . .   6
   4.  PFM message handling with Optimizations in effect . . . . . .   7
     4.1.  PFM message handling with TLV Type TBD1 enabled . . . . .   7
     4.2.  PFM message handling with Relaxed-RPF enabled . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   PIM Flooding Mechanism [RFC8364] allows a PIM router in the network
   to originate a PFM message to distribute announcements of active
   sources to its PIM neighbors [RFC7761].  All PIM neighbors then
   process this PFM message and flood it further on their PIM-enabled
   links.  To prevent loops, the originator address as defined in
   Section 3.1 [RFC8364] is used for RPF checking at each router.  This
   RPF check is defined in Section 3.4.1 [RFC8364].  Periodic PFM
   messages are triggered, see Section 3.4.2 [RFC8364] and exchanged to
   keep the multicast information updated across the PIM domain.

   First of all, PFM-SD does not allow the distribution of anything
   except for the announcements of active sources.  However, it may be
   useful to provide additional information about flows in PFM [RFC8364]
   source announcements.

   Secondly, a PIM router will flood a PFM message on all its PIM
   enabled links.  It is the recipient's responsibility to perform RPF
   checks on all received PFM messages and then decide whether to accept

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   or drop a particular message.  This means that if two routers have
   PIM neighborships over more than one link, the same PFM messages are
   exchanged or dropped over more than one link between the same two
   routers.  This leads to extra processing at each PIM router,
   periodically, or every time a new source is discovered (in case of a
   PFM-SD implementation).  We can reduce the processing overhead for
   the router-pair having PIM neighborships over multiple links.

   This document discusses two new improvements in PFM message exchanges
   between PIM routers.

   1.  This document defines a new TLV for announcing sources that
       allows for Sub-TLVs that can be used providing various types of
       information.  This enhancement is discussed in detail in
       Section 2.  Section 3 discusses the PFM message forwarding
       mechanism when the network is heterogeneous in terms of PFM TLV
       support.

   2.  Utilizing the PIM Router-IDs [RFC6395], PIM routers can limit PFM
       message exchanges to only on ONE link per router-pair, even
       though these two PIM routers may maintain PIM neighborships over
       multiple links.  Note that this is applicable in cases where
       there are only two routers on each of the links between them -
       either a Point-Point link, or exactly 2 PIM neighbors on a LAN.
       In such cases, PFM can improve in performance by first
       identifying the PIM routers in the network using Router
       Identifiers [RFC6395] (Router-IDs) that are announced via PIM
       hellos.  This enhancement allows PFM to limit message exchanges
       to only those that are necessary and is discussed in detail in
       Section 3.

   Any existing PFM deployment MAY choose to implement one or both
   enhancements, however it is RECOMMENDED to implement both.

1.1.  Conventions Used in This Document

   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.

1.2.  Terminology

   RPF:  Reverse Path Forwarding

   FHR:  First-Hop Router

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   PFM-SD:  PIM Flooding Mechanism and Source-Discovery

2.  PIM PFM Sub-TLV

   PFM-SD [RFC8364] defines a Group Source Holdtime (GSH) TLV for
   announcing active sources.  However, it could be beneficial for PIM
   routers to exchange additional data about these sources.

2.1.  Group Source Info TLV

   This document defines a new Group Source Info (GSI) TLV that is used
   similarly to the GSH TLV except that it only provides info for a
   single source, and includes additional information about the flow in
   Sub-TLVs.  Note that the support for this TLV Type TBD1 is advertised
   by PIM routers using Flag bit TBD4 in PIM Hello Option TBD2 and is
   discussed in detail in Section 3.2

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |T|         Type = TBD1         |          Length               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Group Address (Encoded-Group format)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Source Address (Encoded-Unicast format)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Holdtime           |        Type Sub-TLV 1         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Length Sub-TLV 1        |       Value Sub-TLV 1         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
      |                               .                               |
      |                               .                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               .                               |
      |                               .                               |
      |        Type Sub-TLV n         |       Length Sub-TLV n        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Value Sub-TLV n                                        |
      |                               .                               |
      |                               .                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   T:  If the Transitive bit is set to 0, a router MUST NOT forward the
      message unless it supports this TLV and all the Sub-TLVs that are
      present in the TLV in this message.  If the transitive bit is set
      to 1, it is forwarded even if the router does not support the TLV
      or all the Sub-TLVs present.

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   Type:  This TLV has type TBD.

   Length:  The length of the value in octets.

   Group Address:  The group that sources are to be announced for.  The
      format for this address is given in the Encoded-Group format in
      [RFC7761].

   Source Address:  The source address for the corresponding group.  The
      format for this address is given in the Encoded-Unicast address in
      [RFC7761].

   Holdtime:  The Holdtime (in seconds).

   Type Sub-TLV 1..n:  The TLV contains n Sub-TLVs, n MAY be 0.  The
      total length of the TLV (the Length field) is used to derive how
      many octets are used for Sub-TLVs.  It will be at least 4 * n
      octets if n Sub-TLVs are present.  Type Sub-TLV indicates the type
      of the Sub-TLV.  The allowed types are Sub-TLV types that are
      specifically defined for use in the Group Source Info TLV.

   Length Sub-TLV 1..n:  The length of the Sub-TLV Value field in
      octets.

   Value Sub-TLV 1..n:  The value of the Sub-TLV associated with the
      type and of the specified length.

3.  PIM PFM forwarding optimization

3.1.  RFC 6395 Compliance

   For the enhancements defined in this document to be adopted, all PIM
   routers MUST be compliant with RFC [RFC6395].  This means that PIM
   routers announce a unique domain-wide Router-ID in their PIM hellos.
   A PIM router announces the same 4-byte Router-ID in PIM hellos that
   it sends to all neighbors on all links.  It also caches the Router-
   IDs of its neighbors, when it receives Hellos from [RFC6395]
   Compliant PIM neighbors.  This can be used to determine that
   different PIM neighbors are really the same router.  In a VRF
   context, if the router has multiple interfaces with only one neighbor
   per interface, the router SHOULD check if those neighbors announce an
   RFC 6395 Router-ID.  If the router can see the same Router-ID for
   multiple neighbors, PFM message exchange is optimized.

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3.2.  PFM optimization Hello option

   A PIM router indicates that it supports enhancement mechanisms
   specified in this document by including the new PFM optimization
   Hello option.  When this optimization is included in the PIM hello,
   the router MUST also include the Router-ID Hello Option defined in
   [RFC6395] with a non-zero Router-ID.

       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 = TBD2       |           Length = 4          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            Flags                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 1: PFM optimization Hello option

   OptionType = TBD2

   OptionLength = 4

   Flags = TBD4, TBD5  The 4-octet field MUST be included in the PFM
      optimization Hello option.  The value MUST NOT be zero.  It
      indicates support for various optimizations described in this
      document.

   The "Flags" field indicates the PFM optimizations that the router
   supports.  When Flags has bit TBD4 set, it indicates that the router
   is capable of exchanging PFM TLVs of Type TBD1 [Section 2].

   When Flags has bit TBD5 set, it indicates that the router is capable
   of Relaxed-RPF optimization, which means that it will be relaxing the
   RPF check for PFM messages under conditions discussed in
   [Section 4.2].  It is referred to as the Relaxed-RPF optimization
   throughout the document.

   The remaining bits in the Flags field are reserved and MUST
   transmitted as zero and MUST be ignored on receipt.  When a PIM hello
   with OptionType TBD1 is received from a PIM neighbor, the router MUST
   cache and/or update this information so that it can make forwarding
   and dropping decisions for PFM messages for that neighbor.  When
   OptionType TBD1 is included, the router MUST also cache the non-zero
   Router-ID of this neighbor.

   All PIM routers MUST track whether a specific optimization (e.g.,
   TBD4, TBD5) is supported by all PIM neighbors on each PIM interface.
   This tracking is beneficial in heterogeneous networks where only

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   certain routers support the new TLV Type TBD.  Additionally, it is
   RECOMMENDED that only Type TBD1 be used if support is available, to
   avoid unnecessary processing.  It is RECOMMENDED that routers use
   both optimizations defined in the document.

4.  PFM message handling with Optimizations in effect

   Consider a router that is advertising its capability to optimize PFM
   exchanges in the network with Hello Option TBD3 [Section 3.2] and
   Option Values TBD4 and TBD5 to all its PIM neighbors.  It MUST
   simultaneously track whether a specific optimization (e.g., TBD4,
   TBD5) is advertised by all PIM neighbors on each PIM interface.  This
   information, along with each neighbors' Router-IDs and optimization
   support on each interface will enable this router to make improved
   forwarding decisions for PFM messages.  It is left to the
   implementation to track neighbors' optimization support.  It is
   RECOMMENDED that routers use both optimizations defined in the
   document.  However, different message handling scenarios are
   discussed below.

4.1.  PFM message handling with TLV Type TBD1 enabled

   When a router sends a PIM Hello with OptionType TBD2 with Flag Bit
   TBD4 set, it is indicating to its PIM neighbor that it is capable of
   exchanging PFM TLVs of Type TBD1 which enables the router to send
   more information about each (S, G).

   Consider a router capable of exchanging PFM Type TBD1 TLVs.  It MUST
   do the following:

   *  It MUST advertise its capability by sending PIM Hello with
      OptionType TBD2 with Flag Bit TBD4 set.

   *  It MUST track whether all neighbors on each of its PIM interfaces
      support this new TLV.  Scope of this tracking is left to the
      implementation.  It MAY track this information even if the
      capability on itself is removed.

   *  If this router is an FHR, while originating a PFM message, it MUST
      originate a Type TBD1 TLV if all neighbors on the PIM interface
      support Type TBD.

   *  If this router is an FHR, while originating a PFM message, it MUST
      originate a Type 1 TLV [RFC8364] if at least 1 neighbor on the PIM
      interface does not support Type TBD.

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   *  On the receipt of a Type TBD1 TLV on a Type TBD1-capable
      intermediate router, this router MUST forward the PFM message as
      is on the PIM interfaces where all neighbors support this new
      type.

   *  If there are PIM interfaces where at least one router does not
      support the new TLV, an intermediate router that supports Type
      TBD1 MUST convert the Type TBD1 TLV to Type 1 TLV [RFC8364] and
      forward it on those interfaces.  The conversion mechanism is
      largely left to the implementation, however in a nutshell, the
      router MUST create and send TLV Type 1 with the source group and
      holdtime from the Type TBD1 and ignore the sub-tlvs.  Also, if
      there are multiple sources for the same group, then they should be
      put together in one TLV and sent as Type 1.

4.2.  PFM message handling with Relaxed-RPF enabled

   When a router sends a PIM Hello with OptionType TBD2 and Flag Bit
   TBD5 set, it signals to its PIM neighbor that it can optimize
   forwarding between them if they are the only two neighbors on all
   connecting links between them.

   Consider a topology where two PIM routers maintain multiple PIM
   neighborships over several links within the same PIM domain — and are
   the only two routers on these links - either a Point-to-Point link,
   or 2 PIM neighbors on a LAN.  From each router's point of view, there
   is a single neighbor on each link.  Traditionally, each of the
   routers will send out PFM messages out over all the links to its
   neighbor.  RPF checks are one of the commonly used ways to prevent
   loops, hence the recipient router performs an RPF check and accepts
   only on one link, thereby dropping packets from all the others.
   Since the sender does not know which link will be chosen as the RPF-
   source on the neighbor, it cannot choose one of the links, without
   knowing its neighbor's decision.

   If the Relaxed-RPF optimization is advertised by both routers, the
   sender MUST choose one of the links and send and forward PFM messages
   to its neighbor using only that link.  The sender MUST do this only
   when the receiver is capable of the Relaxed-RPF optimization.
   Otherwise, the messages may be dropped because of RPF failures.  The
   mechanism to choose a link is left to the implementation.

   When a router that supports the Relaxed-RPF optimization receives a
   PFM message, it MUST first verify if the sender supports Relaxed-RPF
   optimization.  If true, the receiver MUST relax its RPF check and
   accept the message.  Additionally, the receiver MUST record the
   sender's router ID to prevent forwarding the message back to the
   sender on any other link.  However, if the sender does not advertise

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   the Relaxed-RPF optimization specified in this document and the
   receiver has enabled Relaxed-RPF, the receiver SHOULD NOT relax its
   RPF check, as the sender will still transmit messages across all
   connecting links.

   The optimization mechanism relies heavily on a router's insight into
   whether all neighbors on each PIM interface support the TLV Type TBD1
   and/or Relaxed-RPF optimization.  Therefore the following scenarios
   MUST be handled:

   Adding a new neighbor on any link:  Appropriate logic SHOULD be
      implemented to handle new neighbor additions so that extra
      messages are not forwarded to the same neighbor, as well as
      ensuring that a new neighbor quickly gets the correct state.  A
      router MUST track whether a specific optimization (e.g., TBD4,
      TBD5) is supported by all PIM neighbors when a new neighbor is
      added.

   Existing neighbor on a new link:  When a new neighbor is detected
      announcing support for the optimization and announcing a non-zero
      Router-ID, and it is the only neighbor on the link, a PIM router
      needs to check if there is an existing neighbor on another link
      with the same Router-ID.  A mechanism SHOULD be implemented to
      prevent PFM messages sent on this link.

   Neighbor removal on a link:  A router MUST track whether a specific
      optimization (e.g., TBD4, TBD5) is supported by all PIM neighbors
      when an existing neighbor is removed.

   Software Downgrade Considerations:  TBD

5.  Security Considerations

   When it comes to general PIM message security, see [RFC7761].  For
   PFM security see [RFC8364].

   This document defines a new format allowing for additional flow
   information.  One concern is what happens if wrong information is
   provided by accident, or intentionally in a spoofed message by an
   attacker.  The impact depends on what information is provided.

   TBD any security considerations for forwarding optimizations.

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6.  IANA Considerations

   This document requires the assignment of a new PFM TLV Type TBD1 in
   the "PIM Flooding Mechanism Message Types" registry.  Also, a new
   registry "PFM Group Source Info Sub-Types" registry needs to be
   created.  Assignments for the new registry are to be made according
   to the policy "IETF Review" as defined in [RFC8126].  The initial
   content of the registry should be:

    Sub-Type         Name                  Reference
   ------------------------------------------------------
        0        Reserved               [this document]
     1-65535     Unassigned

   This document requires the assignment of a new PIM Hello Option TBD2
   with OptionLength 4, and a 32-bit OptionValue "Flags" for indicating
   the PFM optimization Hello options in the PIM-Hello Options Registry.

   This document requires the assignment of the following new values for
   OptionValue "Flags" for the PIM Hello Option(TBD2).

    Flags         Name                    Reference
   ------------------------------------------------------
        0      Reserved                  [this document]
     TBD4      Type TBD1 TLV support
     TBD5      Relaxed-RPF optimization
               support
   Remaining
      bits     Reserved

7.  Acknowledgments

8.  Normative References

   [RFC6395]  Gulrajani, S. and S. Venaas, "An Interface Identifier (ID)
              Hello Option for PIM", RFC 6395, DOI 10.17487/RFC6395,
              October 2011, <https://www.rfc-editor.org/info/rfc6395>.

   [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>.

   [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>.

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   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8364]  Wijnands, IJ., Venaas, S., Brig, M., and A. Jonasson, "PIM
              Flooding Mechanism (PFM) and Source Discovery (SD)",
              RFC 8364, DOI 10.17487/RFC8364, March 2018,
              <https://www.rfc-editor.org/info/rfc8364>.

Authors' Addresses

   Ananya Gopal
   Cisco Systems, Inc.
   Tasman Drive
   San Jose,  CA 95134
   United States of America
   Email: ananygop@cisco.com

   Stig Venaas
   Cisco Systems, Inc.
   Tasman Drive
   San Jose,  CA 95134
   United States of America
   Email: svenaas@cisco.com

   Francesco Meo
   Email: fran.meo@gmail.com

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