MPLS Working Group                                             G. Mirsky
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               G. Mishra
Expires: 13 14 August 2022                                     Verizon Inc.
                                                             D. Eastlake
                                                  Futurewei Technologies
                                                        10 February 2022

     BFD for Multipoint Networks over Point-to-Multi-Point MPLS LSP


   This document describes procedures for using Bidirectional Forwarding
   Detection (BFD) for multipoint networks to detect data plane failures
   in Multiprotocol Label Switching (MPLS) point-to-multipoint (p2mp)
   Label Switched Paths (LSPs) and Segment Routing (SR) point-to-
   multipoint policies with SR-MPLS data plane.

   It also describes the applicability of LSP Ping, as in-band, and the
   control plane, as out-band, solutions to bootstrap a BFD session.

   It also describes the behavior of the active tail for head

Status of This Memo

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   This Internet-Draft will expire on 13 14 August 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Multipoint BFD Encapsulation  . . . . . . . . . . . . . . . .   3
     3.1.  IP Encapsulation of Multipoint BFD  . . . . . . . . . . .   4
     3.2.  Non-IP Encapsulation of Multipoint BFD  . . . . . . . . .   4
   4.  Bootstrapping Multipoint BFD  . . . . . . . . . . . . . . . .   5
     4.1.  LSP Ping  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Control Plane . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Operation of Multipoint BFD with Active Tail over P2MP MPLS
           LSP . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  11

1.  Introduction

   [RFC8562] defines a method of using Bidirectional Detection (BFD)
   [RFC5880] to monitor and detect unicast failures between the sender
   (head) and one or more receivers (tails) in multipoint or multicast

   [RFC8562] added two BFD session types - MultipointHead and
   MultipointTail.  Throughout this document, MultipointHead and
   MultipointTail refer to the value of the bfd.SessionType is set on a
   BFD endpoint.

   This document describes procedures for using such modes of BFD
   protocol to detect data plane failures in Multiprotocol Label
   Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths
   (LSPs) and Segment Routing (SR) point-to-multipoint policies with SR-
   MPLS data plane
   The document also describes the applicability of out-band solutions
   to bootstrap a BFD session in this environment.

   It also describes the behavior of the active tail for head

2.  Conventions used in this document

2.1.  Terminology

   MPLS: Multiprotocol Label Switching

   LSP: Label Switched Path

   BFD: Bidirectional Forwarding Detection

   p2mp: Point-to-Multipoint

   FEC: Forwarding Equivalence Class

   G-ACh: Generic Associated Channel

   ACH: Associated Channel Header

   GAL: G-ACh Label

   LSR: Label Switching Router

   SR: Segment Routing

   SR-MPLS: SR with MPLS data plane

2.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

3.  Multipoint BFD Encapsulation

   [RFC8562] uses BFD in the Demand mode from the very start of a point-
   to-multipoint (p2mp) BFD session.  Because the head doesn't receive
   any BFD Control packet from a tail, the head of the p2mp BFD session
   transmits all BFD Control packets with the value of Your
   Discriminator field set to zero.  As a result, a tail cannot
   demultiplex BFD sessions using Your Discriminator, as defined in

   [RFC5880].  [RFC8562] requires that to demultiplex BFD sessions, the
   tail uses the source IP address, My Discriminator, and the identity
   of the multipoint tree from which the BFD Control packet was
   received.  If the BFD Control packet is encapsulated in IP/UDP, then
   the source IP address MUST be used to demultiplex the received BFD
   Control packet as described in Section 3.1.  The non-IP encapsulation
   case is described in Section 3.2.

3.1.  IP Encapsulation of Multipoint BFD

   [RFC8562] defines IP/UDP encapsulation for multipoint BFD over p2mp

   *  UDP destination port MUST be set to 3784;

   *  destination IP address MUST be set to the loopback address for IPv4, or the loopback address ::1/128 for IPv6
      [RFC4291].  Note that that is different from how the destination
      IP address selection is defined in Section 7 [RFC5884].  Firstly,
      because only one loopback address ::1/128 is defined in IPv6.  And
      also, it is recommended to use the Entropy Label [RFC6790] to
      discover multiple alternate paths in an MPLS network.  Using a
      single loopback address both for IPv4 and IPv6 encapsulation makes
      it consistent and more straightforward for an implementation.

   The Motivation section [RFC6790] lists several advantages of
   generating the entropy value by an ingress Label Switching Router
   (LSR) compared to when a transit LSR infers entropy using the
   information in the MPLS label stack or payload.  Thus this
   specification further clarifies that:

      if multiple alternative paths for the given p2mp LSP Forwarding
      Equivalence Class (FEC) exist, the MultipointHead SHOULD use
      Entropy Label [RFC6790] used for LSP Ping [RFC8029] to exercise
      those particular alternative paths;

      or the MultipointHead MAY use the UDP port number as discovered by
      LSP Ping traceroute [RFC8029] as the source UDP port number to
      possibly exercise those particular alternate paths.

3.2.  Non-IP Encapsulation of Multipoint BFD

   In some environments, the overhead of extra IP/UDP encapsulations may
   be considered burdensome, making the use of more compact G-ACh
   encapsulation attractive.  Also, the validation of the IP/UDP
   encapsulation of a BFD Control packet in a p2mp BFD session may fail
   because of a problem related to neither the MPLS label stack nor to
   BFD.  Avoiding unnecessary encapsulation of p2mp BFD over an MPLS LSP
   improves the accuracy of the correlation of the detected failure and
   defect in MPLS LSP.  Non-IP encapsulation for multipoint BFD over
   p2mp MPLS LSP (shown in Figure 1) MUST use Generic Associated Channel
   (G-ACh) Label (GAL) (see [RFC5586]) at the bottom of the label stack
   followed by an Associated Channel Header (ACH).  If a BFD Control
   packet in PW-ACH encapsulation (without IP/UDP Headers) is to be used
   in ACH, an implementation would not be able to verify the identity of
   the MultipointHead and, as a result, will not properly demultiplex
   BFD packets.  Hence, a new channel type value is needed.  The Channel
   Type field in ACH MUST be set to TBA1 value Section 7.  To provide
   the identity of the MultipointHead for the particular multipoint BFD
   session, a Source Address TLV [RFC7212] TLV, as defined in Section 4.1 [RFC7212],
   MUST immediately follow a BFD Control message.

       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
      |               LSP Label               |  TC |S|       TTL     |
      |                  GAL                  |  TC |1|       TTL     |
      |0 0 0 1|Version|   Reserved    |      Channel Type = TBA1      |
      ~                        BFD Control Message                    ~
      |     Type=0    |    Reserved   |            Length             |
      |      Reserved (16 bits)       |    Address Family (16 bits)   |
      ~                            Address                            ~

           Figure 1: VRRP Extension to Bootstrap P2MP BFD session

4.  Bootstrapping Multipoint BFD

4.1.  LSP Ping

   LSP Ping is the part of the on-demand OAM toolset used to detect and
   localize defects in the data plane and verify the control plane
   against the data plane by ensuring that the LSP is mapped to the same
   FEC at both egress and ingress endpoints.

   LSP Ping, as defined in [RFC6425], MAY be used to bootstrap
   MultipointTail.  If LSP Ping is used, it MUST include the Target FEC
   TLV and the BFD Discriminator TLV defined in [RFC5884].  For the case
   of p2mp MPLS LSP, the Target FEC TLV MUST use sub-TLVs defined in
   Section 3.1 [RFC6425].  For the case of p2mp SR policy with SR-MPLS
   data plane, an implementation of this specification MUST follow
   procedures defined in [RFC8287].  Setting the value of Reply Mode
   field to "Do not reply" [RFC8029] for the LSP Ping to bootstrap
   MultipointTail of the p2mp BFD session is RECOMMENDED.  Indeed,
   because BFD over a multipoint network uses BFD Demand mode, the LSP
   echo reply from a tail has no useful information to convey to the
   head, unlike in the case of the BFD over a p2p MPLS LSP [RFC5884].  A
   MultipointTail that receives an LSP Ping that includes the BFD
   Discriminator TLV:

   *  MUST validate the LSP Ping;

   *  MUST associate the received BFD Discriminator value with the p2mp

   *  MUST create a p2mp BFD session and set bfd.SessionType =
      MultipointTail as described in [RFC8562];

   *  MUST use the source IP address of LSP Ping, the value of BFD
      Discriminator from the BFD Discriminator TLV, and the identity of
      the p2mp LSP to properly demultiplex BFD sessions.

   Besides bootstrapping a BFD session over a p2mp LSP, LSP Ping SHOULD
   be used to verify the control plane against the data plane
   periodically by checking that the p2mp LSP is mapped to the same FEC
   at the MultipointHead and all active MultipointTails.  The rate of
   generation of these LSP Ping Echo request messages SHOULD be
   significantly less than the rate of generation of the BFD Control
   packets because LSP Ping requires more processing to validate the
   consistency between the data plane and the control plane.  An
   implementation MAY provide configuration options to control the rate
   of generation of the periodic LSP Ping Echo request messages.

4.2.  Control Plane

   The BGP-BFD Attribute [RFC9026] MAY be used to bootstrap multipoint
   BFD session on a tail.

5.  Operation of Multipoint BFD with Active Tail over P2MP MPLS LSP

   [RFC8562] defined how the BFD Demand mode can be used in multipoint
   networks.  When applied in MPLS, procedures specified in [RFC8562]
   allow an egress LSR to detect a failure of the part of the MPLS p2mp
   LSP from the ingress LSR.  The ingress LSR is not aware of the state
   of the p2mp LSP.  [RFC8563], using mechanisms defined in [RFC8562],
   defined an "active tail" behavior.  An active tail might notify the
   head of the detected failure and responds to a poll sequence
   initiated by the head.  The first method, referred to as Head
   Notification without Polling, is mentioned in Section 5.2.1
   [RFC8563], is the simplest of all described in [RFC8563].  The use of
   this method in BFD over MPLS p2mp LSP is discussed in this document.
   Analysis of other methods of a head learning of the state of an MPLS
   p2mp LSP is outside the scope of this document.

   As specified in [RFC8563] for the active tail mode, BFD variables
   MUST be as follows:

   On an ingress LSR:

   *  bfd.SessionType is MultipointHead;

   *  bfd.RequiredMinRxInterval is set to nonzero, allowing egress LSRs
      to send BFD Control packets.

   On an egress LSR:

   *  bfd.SessionType is MultipointTail;

   *  bfd.SilentTail is set to zero.

   In Section 5.2.1 [RFC8563] is noted that "the tail sends unsolicited
   BFD packets in response to the detection of a multipoint path
   failure" but without the specifics on the information in the packet
   and frequency of transmissions.  This document defines below the
   procedure of an active tail with unsolicited notifications for p2mp

   Upon detecting the failure of the p2mp MPLS LSP, an egress LSR sends
   BFD Control packet with the following settings:

   *  the Poll (P) bit is set;

   *  the Status (Sta) field set to Down value;

   *  the Diagnostic (Diag) field set to Control Detection Time Expired

   *  the value of the Your Discriminator field is set to the value the
      egress LSR has been using to demultiplex that BFD multipoint

   *  BFD Control packet MAY be encapsulated in IP/UDP with the
      destination IP address of the ingress LSR and the UDP destination
      port number set to 4784 per [RFC5883].  If non-IP encapsulation is
      used, then a BFD Control packet is encapsulated using PW-ACH
      encapsulation (without IP/UDP Headers) (0x0007) [RFC5885];

   *  these BFD Control packets are transmitted at the rate of one per
      second until either it receives a control packet valid for this
      BFD session with the Final (F) bit set from the ingress LSR or the
      defect condition clears; however to improve the likelihood of
      notifying the ingress LSR of the failure of the p2mp MPLS LSP, the
      egress LSR SHOULD initially transmit three BFD Control packets
      defined above in short succession.

   An ingress LSR that has received the BFD Control packet, as described
   above, sends the unicast IP/UDP encapsulated BFD Control packet with
   the Final (F) bit set to the egress LSR.

6.  Security Considerations

   This document does not introduce new security aspects but inherits
   all security considerations from [RFC5880], [RFC5884], [RFC7726],
   [RFC8562], [RFC8029], and [RFC6425].

   Also, BFD for p2mp MPLS LSP MUST follow the requirements listed in
   section 4.1 [RFC4687] to avoid congestion in the control plane or the
   data plane caused by the rate of generating BFD Control packets.  An
   operator SHOULD consider the amount of extra traffic generated by
   p2mp BFD when selecting the interval at which the MultipointHead will
   transmit BFD Control packets.  The operator MAY consider the size of
   the packet the MultipointHead transmits periodically as using IP/UDP
   encapsulation, which adds up to 28 octets, more than 50% of the BFD
   Control packet length, comparing to G-ACh encapsulation.

7.  IANA Considerations

   IANA is requested to allocate value (TBA1) from its MPLS Generalized
   Associated Channel (G-ACh) Types registry.

            | Value |      Description       | Reference     |
            | TBA1  | Multipoint BFD Session | This document |

                Table 1: Multipoint BFD Session G-ACh Type

8.  Acknowledgements

   The authors sincerely appreciate the comments received from Andrew
   Malis, Italo Busi, Shraddha Hegde, and thought stimulating questions
   from Carlos Pignataro.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <>.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <>.

   [RFC5885]  Nadeau, T., Ed. and C. Pignataro, Ed., "Bidirectional
              Forwarding Detection (BFD) for the Pseudowire Virtual
              Circuit Connectivity Verification (VCCV)", RFC 5885,
              DOI 10.17487/RFC5885, June 2010,

   [RFC6425]  Saxena, S., Ed., Swallow, G., Ali, Z., Farrel, A.,
              Yasukawa, S., and T. Nadeau, "Detecting Data-Plane
              Failures in Point-to-Multipoint MPLS - Extensions to LSP
              Ping", RFC 6425, DOI 10.17487/RFC6425, November 2011,

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, DOI 10.17487/RFC6790, November 2012,

   [RFC7212]  Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
              Associated Channel (G-ACh) Advertisement Protocol",
              RFC 7212, DOI 10.17487/RFC7212, June 2014,

   [RFC7726]  Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S.
              Aldrin, "Clarifying Procedures for Establishing BFD
              Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726,
              DOI 10.17487/RFC7726, January 2016,

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,

   [RFC8562]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) for
              Multipoint Networks", RFC 8562, DOI 10.17487/RFC8562,
              April 2019, <>.

   [RFC8563]  Katz, D., Ward, D., Pallagatti, S., Ed., and G. Mirsky,
              Ed., "Bidirectional Forwarding Detection (BFD) Multipoint
              Active Tails", RFC 8563, DOI 10.17487/RFC8563, April 2019,

9.2.  Informative References

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

   [RFC4687]  Yasukawa, S., Farrel, A., King, D., and T. Nadeau,
              "Operations and Management (OAM) Requirements for Point-
              to-Multipoint MPLS Networks", RFC 4687,
              DOI 10.17487/RFC4687, September 2006,

   [RFC9026]  Morin, T., Ed., Kebler, R., Ed., and G. Mirsky, Ed.,
              "Multicast VPN Fast Upstream Failover", RFC 9026,
              DOI 10.17487/RFC9026, April 2021,

Authors' Addresses

   Greg Mirsky


   Gyan Mishra
   Verizon Inc.


   Donald Eastlake, 3rd
   Futurewei Technologies
   2386 Panoramic Circle
   Apopka,  FL 32703
   United States of America