



Network Working Group                                           A. Gopal
Internet-Draft                                                 S. Venaas
Intended status: Experimental                        Cisco Systems, Inc.
Expires: 6 November 2026                                          F. Meo
                                                              5 May 2026


        PIM Flooding Mechanism and Source Discovery Enhancements
             draft-ietf-pim-pfm-forwarding-enhancements-05

Abstract

   The Protocol Independent Multicast (PIM) Flooding Mechanism (PFM)
   provides a generic hop-by-hop message exchange framework for
   distributing multicast information among PIM routers.  Existing PFM
   procedures enable efficient source discovery without reliance on
   Rendezvous Points, shared trees, or initial data registers.

   This document specifies enhancements to PFM forwarding behavior to
   improve efficiency and scalability.  In particular, it introduces
   mechanisms to reduce redundant message transmission over multiple
   parallel links and extends the encoding of multicast information
   through additional Type-Length-Value (TLV) structures and sub-TLVs to
   convey richer flow-related data.  These enhancements optimize
   control-plane overhead while preserving interoperability with
   existing PFM procedures, enabling more efficient dissemination of
   multicast state in PIM networks.

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
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   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 6 November 2026.







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Copyright Notice

   Copyright (c) 2026 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  . . . . . . . . . . . . . . . . . . . . . . . .   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
     2.2.  Group Source Info TLV Hello option  . . . . . . . . . . .   5
     2.3.  Considerations for using the Group Source Info TLV  . . .   6
   3.  PIM PFM forwarding optimization . . . . . . . . . . . . . . .   6
     3.1.  RFC 6395 Compliance . . . . . . . . . . . . . . . . . . .   7
     3.2.  PFM optimization Hello option . . . . . . . . . . . . . .   7
     3.3.  Sample PFM Topology . . . . . . . . . . . . . . . . . . .   8
     3.4.  PFM message handling with Relaxed-RPF enabled . . . . . .   8
     3.5.  Maintenance of PFM_OPT_IF . . . . . . . . . . . . . . . .   9
     3.6.  PFM message forwarding to sender  . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  11
   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 Reverse Path Forwarding (RPF)
   checking at each router.  This RPF check is defined in Section 3.4.1
   [RFC8364].  Periodic PFM messages are exchanged to keep the multicast
   information updated across the PIM domain (Section 3.4.2 [RFC8364])




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   The TLV defined in [RFC8364] for source discovery conveys only source
   and group information.  It does not provide a mechanism to include
   additional attributes describing a multicast flow.

   In addition, PFM messages are flooded on all PIM-enabled links.  When
   two routers maintain multiple PIM adjacencies, identical PFM messages
   are transmitted across each link.  Receivers perform RPF checks and
   discard duplicates as needed.  This behavior introduces unnecessary
   processing overhead, both periodically and upon source discovery.

   This document defines two independent enhancements to PFM message
   exchange:

   1.  A new TLV that supports Sub-TLVs, enabling the inclusion of
       additional flow-related information.  This enhancement is
       specified in Section 2.

   2.  An optimization for PFM message exchange across multiple PIM
       adjacencies between the same pair of routers.  By leveraging PIM
       Router-IDs [RFC6395], routers can identify such adjacencies and
       limit message transmission to a subset of links, reducing
       redundant processing.  This optimization applies to point-to-
       point links and does not alter behavior on shared LANs.  This
       enhancement is specified in Section 3.

   Implementations MAY support these enhancements independently;
   however, support for both is RECOMMENDED.

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

   PFM-SD:  PIM Flooding Mechanism and Source-Discovery








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2.  PIM PFM Sub-TLV

   PFM-SD [RFC8364] defines a Group Source Holdtime (GSH) TLV for
   announcing active sources.  The GSH TLV conveys only source and group
   information.  This document defines an extension that allows PIM
   routers to exchange additional information associated with multicast
   sources.

2.1.  Group Source Info TLV

   This document defines a new Group Source Info (GSI) TLV (Type TBD1).
   The GSI TLV is functionally similar to the GSH TLV but applies to a
   single (S,G) entry and supports the inclusion of Sub-TLVs to convey
   additional flow-specific information.  Support for the GSI TLV is
   advertised using a PIM Hello option (TBD2), as described in
   Section 2.2

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

   The format of the GSI TLV is as follows:

   T-bit (1 bit):  Indicates transitivity.  If set to 0, a router that
      does not support the TLV or any contained Sub-TLV MUST NOT forward
      the message.  If set to 1, the message MAY be forwarded even if
      unsupported TLVs or Sub-TLVs are present.



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   Type: (15 bits):  Set to TBD1.

   Length (16 bits):  The length, in octets, of the Value field.

   Group Address:  The multicast group address encoded as specified in
      Section 4.9.1 of [RFC7761].  The length of this field depends on
      the address family: 64 bits for IPv4 native encoding, 160 bits for
      IPv6.

   Source Address:  The unicast source address encoded as specified in
      Section 4.9.1 of [RFC7761].  The length of this field depends on
      the address family: 48 bits for IPv4 native encoding, 160 bits for
      IPv6.

   Holdtime (16 bits):  The lifetime, in seconds, for the advertised
      (S,G) information.

   Sub-TLVs:  Zero or more Sub-TLVs MAY be included.  Each Sub-TLV
      consists of:

      Type (16 bits):  The Sub-TLV Type.  Values for this field are
         assigned by IANA.

      Length (16 bits):  The length, in octets, of the Value field.  The
         length may be 0 if no value is present.

      Value:  The content of the Sub-TLV, whose format is determined by
         the Sub-TLV Type.

2.2.  Group Source Info TLV Hello option

   A PIM router indicates support for the GSI TLV defined in this
   document by including the Group Source Info TLV Hello option in PIM
   Hello messages.  The format of the Hello option is as follows:


       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       OptionType = TBD2       |           Length = 0          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   OptionType (16 bits):  TBD2

   OptionLength (16 bits):  0

   The presence of this option signifies that the router supports the
   GSI TLV.



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2.3.  Considerations for using the Group Source Info TLV

   All PIM routers MUST track which neighbors advertise support for the
   GSI TLV via the Hello option Section 2.2.  This tracking is
   beneficial in heterogeneous networks where only certain routers
   support the new TLV Type TBD1.  If GSI TLV is supported, use of the
   GSI TLV (Type TBD1) is RECOMMENDED.

   A router that supports the GSI TLV MUST:

   *  Advertise its capability by including the Hello option (OptionType
      TBD2) in PIM Hello messages.

   *  Track, per PIM interface, whether all neighbors support the GSI
      TLV.  The scope and persistence of this state are implementation-
      specific.  An implementation MAY retain this state even if local
      capability is disabled.

   *  If acting as a First Hop Router (FHR), originate a Type TBD1 TLV
      when all neighbors on the outgoing interface support Type TBD1.

   *  If acting as an FHR, originate a Type 1 TLV [RFC8364] when any
      neighbor on the outgoing interface does not support Type TBD1.

   *  Upon receipt of a Type TBD1 TLV, MUST forward the PFM message
      unchanged on interfaces where all neighbors support Type TBD1.

   *  For interfaces with at least one neighbor that does not support
      Type TBD1, convert each Type TBD1 TLV to a Type 1 TLV [RFC8364]
      and forward only on those interfaces.  The conversion MUST
      preserve the group, source, and holdtime fields, and MUST ignore
      Sub-TLVs.  Multiple (S,G) entries for the same group SHOULD be
      aggregated into a single Type 1 TLV.  However, it MUST still send
      Type TBD1 TLV on all interfaces where the neighbors do support it.

   *  A PFM message MAY contain both Type 1 and Type TBD1 TLVs.  When
      forwarding to neighbors that do not support Type TBD1, all Type
      TBD1 TLVs MUST be converted to Type 1 TLVs.

3.  PIM PFM forwarding optimization











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3.1.  RFC 6395 Compliance

   To apply the forwarding optimization defined in this document, PIM
   routers MUST advertise a Router-ID as specified in [RFC6395].  Within
   a PIM VRF, a router MUST use the same 4-octet Router-ID in PIM Hello
   messages on all interfaces and MUST cache Router-IDs learned from
   neighbors.  Within a PIM VRF, a router MUST identify interfaces with
   a single neighbor sharing the same Router-ID, indicating multiple
   adjacencies to that neighbor.  This identification is necessary for
   applying the forwarding optimization defined in this document.
   Router-IDs are assumed to be unique within the PIM domain.  If this
   assumption is violated, the optimization defined in this document
   MUST NOT be applied.

3.2.  PFM optimization Hello option

   A PIM router indicates support for the forwarding optimization by
   including the PFM Optimization Hello option (OptionType TBD3) in PIM
   Hello messages.


       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 = TBD3       |           Length = 0          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   OptionType (16 bits):  TBD3

   OptionLength (16 bits):  0

   A router that supports this optimization MUST track, per interface,
   whether all neighbors support the option.  This tracking is
   beneficial in heterogeneous networks where only certain routers
   support the optimization.

   For each learned Router-ID, the router MUST maintain a set of
   interfaces, denoted as PFM_OPT_IF, that satisfy both of the following
   conditions:

   *  The neighbor with this Router-ID is the sole PIM neighbor on this
      interface.

   *  The neighbor is advertising the PFM optimization option TBD3 on
      this interface.

   PFM message exchange MAY be optimized on interfaces in the PFM_OPT_IF
   set.  This is discussed in Section 3.4.



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3.3.  Sample PFM Topology

                            Router C
                               |
                               | LAN 1
                               |
        Router A --------------------------------- Router B
           |                                          |
           |----------------- Link L1 ----------------|
           |                                          |
           |----------------- Link L2 ----------------|
           |                                          |
           |----------------- Link L3 ----------------|
           |__________________________________________|
                               |
                               | LAN 2
                               |
                            Router D

               Figure 1: Four Router Network Topology Example

3.4.  PFM message handling with Relaxed-RPF enabled

   When two routers maintain multiple adjacencies and are the only
   neighbors on those links, PFM messages are typically transmitted on
   all links and filtered by RPF checks at the receiver.  This results
   in redundant processing.  If both routers advertise Router-IDs and
   support the optimization, each router forms a PFM_OPT_IF set
   containing eligible interfaces.

   When Relaxed-RPF is enabled:

   *  A sender MUST select a single interface from its PFM_OPT_IF set
      for PFM transmission to that neighbor.  The selection method is
      implementation-specific.

   *  On shared LANs, the sender MUST send PFM messages as normal since
      optimization cannot be applied when there are more than two
      routers on the network segment.

   *  A receiver that supports Relaxed-RPF MUST:

      -  Determine the expected RPF interface using standard procedures.

      -  Accept a PFM message received on any interface in the
         PFM_OPT_IF set if both sender and receiver support the
         optimization.




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      -  Otherwise, perform standard RPF validation.

   Referring to Figure 1, when Router A originates or forwards a PFM
   message, it MUST transmit the message on exactly one of links L1, L2,
   or L3.  This behavior reduces processing overhead on point-to-point
   links.  The selection of the interface from the PFM_OPT_IF set is
   implementation-specific.  Router A also MUST send the message on both
   LAN 1 and LAN 2 to ensure Routers C and D receive the message.

3.5.  Maintenance of PFM_OPT_IF

   Routers MUST update the PFM_OPT_IF set upon neighbor or capability
   changes:

   *  Neighbor Addition: If the new neighbor is the sole neighbor on the
      interface and advertises both a Router-ID and the optimization
      option, the interface MUST be added to the corresponding
      PFM_OPT_IF set.  If no set exists, it MUST be created.  If a
      second neighbor appears on the interface, the interface MUST be
      removed from the PFM_OPT_IF set.

   *  Neighbor Removal: If exactly one neighbor remains and it
      advertises both a Router-ID and the optimization option, the
      interface MUST be added to the PFM_OPT_IF set for that Router-ID.
      If no set exists, it MUST be created.

   *  Router-ID Changes: If a neighbor starts advertising a Router-ID
      and satisfies all conditions, the interface MUST be added to the
      PFM_OPT_IF set.  If a neighbor stops advertising a Router-ID, the
      interface MUST be removed from the PFM_OPT_IF set for that Router-
      ID.  If the set becomes empty, it MUST be deleted.

   *  Optimization Capability Changes: If a neighbor starts advertising
      the optimization option and satisfies all conditions, the
      interface MUST be added to the PFM_OPT_IF set.  If a neighbor
      stops advertising the optimization option, the interface MUST be
      removed from the PFM_OPT_IF set for that Router-ID.  If the set
      becomes empty, it MUST be deleted.

   These procedures apply during topology changes, configuration
   updates, and software upgrades or downgrades.  Routers MUST maintain
   accurate PFM_OPT_IF state for each Router-ID.









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3.6.  PFM message forwarding to sender

   When the TBD3 optimization is enabled on a PIM router, the router
   MUST NOT forward a PFM message on a link if both of the following
   conditions are true: (1) the link has only one neighbor, and (2) that
   neighbor's Router-ID matches the Router-ID of the router that
   originated the PFM message.  It is sufficient for the neighbor to
   advertise only the Router-ID, without any additional optimization
   options, since this information alone ensures the message is not sent
   back to its original sender, thereby reducing unnecessary PFM message
   forwarding.

4.  Security Considerations

   When it comes to general PIM message security, see [RFC7761].  For
   PFM security see [RFC8364].  This optimization relies on correct
   Router-ID and capability advertisement in PIM Hellos, as well as
   general PIM hello integrity.  For the new PFM TLV, the security
   considerations are the same as for the existing PFM TLV defined in
   [RFC8364].

5.  IANA Considerations

   This document requires the assignment of a new PFM TLV Type TBD1 in
   the "PIM Flooding Mechanism Message Types" registry.

           PIM Flooding Mechanism Message Types

         Type            Name           Reference
       -----------------------------------------------
         TBD1       Group Source Info   [This document]

   Also, a new registry "PIM Flooding Mechanism Group Source Info
   Message 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:

           PIM Flooding Mechanism
         Group Source Info Sub-TLV Types

         Type            Name           Reference
       -----------------------------------------------
           0-32767      Unassigned

   This document requires the assignment of two new PIM Hello Options:






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                         PIM Hello Options

         Value   Length   Name              Reference
       --------------------------------------------------
         TBD2      0     GSI TLV support   [This document]
         TBD3      0     PFM Optimization  [This document]


6.  Acknowledgments

7.  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>.

   [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



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   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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