PIM Source Fanout Trace for multicast flow
draft-mankamana-pim-source-fanout-trace-00

PIM Working Group                                              M. Mishra
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                            20 July 2026
Expires: 21 January 2027

               PIM Source Fanout Trace for multicast flow
               draft-mankamana-pim-source-fanout-trace-00

Abstract

   Mtrace version 2 traces an IP multicast path by walking from a last-
   hop router or rendezvous point toward the source.  That model is
   efficient for a single receiver path, but it does not directly answer
   the operational question of where a multicast source fans out
   downstream without issuing separate traces from many receiver-side
   locations.

Status of This Memo

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   This Internet-Draft will expire on 21 January 2027.

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Table of Contents

   1.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
   6.  Capability Discovery  . . . . . . . . . . . . . . . . . . . .   3
   7.  PIM SFTrace Message . . . . . . . . . . . . . . . . . . . . .   3
   8.  SFTrace TLV Format  . . . . . . . . . . . . . . . . . . . . .   3
   9.  Router Behavior . . . . . . . . . . . . . . . . . . . . . . .   3
   10. Relationship to Mtrace2 . . . . . . . . . . . . . . . . . . .   3
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   4
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   13. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .   4
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   4
   15. Normative References  . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   4

1.  Problem Statement

   [RFC8487] defines Mtrace version 2 (Mtrace2), which allows a
   multicast path to be traced by starting at a Last-Hop Router (LHR) or
   Rendezvous Point (RP) and walking upstream toward the source.  This
   receiver-to-source model is useful for diagnosing a single receiver
   path, but it does not directly provide source-rooted visibility into
   the downstream multicast fanout tree.

   Operationally, there are cases where an operator needs to start from
   the source side and discover the downstream fanout for an exact
   multicast flow.  Examples include finding all receiver branches for a
   high-rate IPTV flow, identifying the branch where forwarding stops,
   or validating whether a source-rooted tree reaches all expected
   receiver edges.  With only the receiver-to-source model, the operator
   must know candidate receivers or last-hop routers in advance and
   issue many separate traces.  This is inefficient and can miss
   branches that are unknown to the operator.

   This limitation also affects automated tree health monitoring.  If an
   operator wants to continuously probe the health of a multicast tree,
   an Mtrace2-only approach requires originating one or more receiver-
   side probes for each candidate branch during every polling interval.
   As the number of sources, groups, receivers, VRFs, and polling
   frequency grow, this can result in N probes every T seconds, with
   each probe requiring control-plane processing on routers along the
   traced path.  Repeated per-branch probing can therefore become CPU
   intensive and does not scale as a general mechanism for monitoring
   whole-tree health.

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   A source-to-receiver fanout tracing mechanism is therefore needed to
   complement Mtrace2 by allowing an operator to discover downstream
   forwarding branches, termination points, and failure points for an
   exact multicast flow from the source-side root.  The desired approach
   is one where a single source-rooted probe can provide enough
   information about the downstream tree to identify reachable branches
   and failure points, while reducing the amount of repeated control-
   plane work required from the network.

2.  Requirements Language

   To be added.

3.  Terminology

   To be added.

4.  Applicability

   To be added.

5.  Protocol Overview

   To be added.

6.  Capability Discovery

   To be added.

7.  PIM SFTrace Message

   To be added.

8.  SFTrace TLV Format

   To be added.

9.  Router Behavior

   To be added.

10.  Relationship to Mtrace2

   To be added.

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11.  Security Considerations

   To be added.

12.  IANA Considerations

   To be added.

13.  Open Issues

   To be added.

14.  Acknowledgements

   To be added.

15.  Normative References

   [RFC8487]  Asaeda, H., Meyer, K., and W. Lee, Ed., "Mtrace Version 2:
              Traceroute Facility for IP Multicast", RFC 8487,
              DOI 10.17487/RFC8487, October 2018,
              <https://www.rfc-editor.org/info/rfc8487>.

Author's Address

   Mankamana Mishra
   Cisco Systems
   821 Alder Drive
   Milpitas, CA 95035
   United States of America
   Email: mankamis@cisco.com

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