BFD for Multipoint Networks over Point-to-Multi-Point MPLS LSP
draft-mirsky-mpls-p2mp-bfd-10

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MPLS Working Group                                             G. Mirsky
Internet-Draft                                                 ZTE Corp.
Intended status: Standards Track                          April 17, 2020
Expires: October 19, 2020

     BFD for Multipoint Networks over Point-to-Multi-Point MPLS LSP
                     draft-mirsky-mpls-p2mp-bfd-10

Abstract

   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) using active tails with unsolicited
   notifications mode.  It also describes the applicability of LSP Ping,
   as in-band, and the control plane, as out-band, solutions to
   bootstrap a BFD session in this environment.

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
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   This Internet-Draft will expire on October 19, 2020.

Copyright Notice

   Copyright (c) 2020 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
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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
   networks.  [RFC8562] added two BFD session types - MultipointHead and
   MultipointTail.  Throughout this document, MultipointHead and
   MultipointTail refer to the value bfd.SessionType is set on a BFD
   system.  This document describes procedures for using such mode of
   BFD protocol to detect data plane failures in Multiprotocol Label
   Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths
   (LSPs).  The document also describes the applicability of out-band
   solutions to bootstrap a BFD session in this environment.

2.  Conventions used in this document

2.1.  Terminology

   MPLS: Multiprotocol Label Switching

   LSP: Label Switched Path

   BFD: Bidirectional Forwarding Detection

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   p2mp: Point-to-Multipoint

   FEC: Forwarding Equivalence Class

   G-ACh: Generic Associated Channel

   ACH: Associated Channel Header

   GAL: G-ACh Label

2.2.  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.

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.  The p2mp MPLS LSP label MAY provide the identification of
   the multipoint tree in case of inclusive p-tree or upstream assigned
   label in case of aggregate p-tree.  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
   MPLS LSP:

      UDP destination port MUST be set to 3784;

      destination IP address MUST be set to one of the loopback
      addresses from 127/8 range for IPv4 or to one of IPv4-mapped IPv4
      loopback addresses from ::ffff:127.0.0.0/104 range for IPv6.;

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   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
      that particular alternative path;

      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 that particular alternate path.

3.2.  Non-IP Encapsulation of Multipoint BFD

   In some environments, the overhead of extra IP/UDP encapsulations may
   be considered as overburden thus making the use of more compact G-ACh
   encapsulation attractive.  Also, the validation of the IP/UDP
   encapsulation of BFD control packet of p2mp BFD session may fail
   because of a problem not related to neither MPLS label stack nor to
   BFD.  Avoiding unnecessary encapsulation of p2mp BFD over 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 MUST use Generic Associated Channel (G-ACh) Label (GAL)
   (see [RFC5586]) at the bottom of the label stack followed by
   Associated Channel Header (ACH).  If BFD Control, PW-ACH
   encapsulation (without IP/UDP Headers) channel 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 6.  To provide
   the identity of the MultipointHead for the particular multipoint BFD
   session a Source Address TLV [RFC7212] MUST immediately follow a BFD
   control message.

4.  Bootstrapping Multipoint BFD

4.1.  LSP Ping

   LSP Ping is the part of on-demand OAM toolset 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 the
   egress, as the ingress.

   LSP Ping, as defined in [RFC6425], MAY be used to bootstrap
   MultipointTail.  If the LSP Ping used, it MUST include the Target FEC
   TLV and the BFD Discriminator TLV defined in [RFC5884].  The Target
   FEC TLV MUST use sub-TLVs defined in Section 3.1 [RFC6425].  It is
   RECOMMENDED setting the value of Reply Mode field to "Do not reply"
   [RFC8029] for the LSP Ping to bootstrap MultipointTail of the p2mp

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   BFD session.  Indeed, because BFD over a multipoint network is using
   BFD Demand mode, the LSP echo reply from a tail has no useful
   information to convey to the head, unlike in case of the BFD over a
   p2p MPLS LSP [RFC5884].  A MultipointTail that receives the LSP Ping
   that includes the BFD Discriminator TLV:

   o  MUST validate the LSP Ping;

   o  MUST associate the received BFD Discriminator value with the p2mp
      LSP;

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

   o  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.  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:

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   On an ingress LSR:

   o  bfd.SessionType is MultipointHead;

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

   On an egress LSR:

   o  bfd.SessionType is MultipointTail;

   o  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 the procedure
   of the active tail with unsolicited notifications for p2mp MPLS LSP
   as specified below.

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

   o  the Poll (P) bit is set;

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

   o  the Diagnostic (Diag) field set to Control Detection Time Expired
      value;

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

   o  BFD Control packet is encapsulated in IP/UDP with the destination
      IP address of the ingress LSR and the UDP destination port number
      set to 4784 per [RFC5883]

   o  these BFD Control packets are transmitted at the rate of one per
      second until either it receives the valid for this BFD session
      control packet with the Final (F) bit set from the ingress LSR or
      the defect condition clears.

   To improve the likelihood of notifying the ingress LSR of the failure
   of the p2mp MPLS LSP, the egress LSR SHOULD transmit three BFD
   Control packets defined above in short succession.

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   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.

4.3.  Control Plane

   BGP-BFD Attribute [I-D.ietf-bess-mvpn-fast-failover] MAY be used to
   bootstrap multipoint BFD session on a tail.

5.  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.  Also, the operator MAY consider the
   size of the packet the MultipointHead transmits periodically as using
   IP/UDP encapsulation adds up to 28 octets, which is more than 50% of
   BFD control packet length, comparing to G-ACh encapsulation.

6.  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

7.  Acknowledgements

   The author sincerely appreciates the comments received from Andrew
   Malis and thought stimulating questions from Carlos Pignataro.

8.  References

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8.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5586]  Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
              "MPLS Generic Associated Channel", RFC 5586,
              DOI 10.17487/RFC5586, June 2009,
              <https://www.rfc-editor.org/info/rfc5586>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [RFC5883]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
              June 2010, <https://www.rfc-editor.org/info/rfc5883>.

   [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, <https://www.rfc-editor.org/info/rfc5884>.

   [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,
              <https://www.rfc-editor.org/info/rfc6425>.

   [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,
              <https://www.rfc-editor.org/info/rfc6790>.

   [RFC7212]  Frost, D., Bryant, S., and M. Bocci, "MPLS Generic
              Associated Channel (G-ACh) Advertisement Protocol",
              RFC 7212, DOI 10.17487/RFC7212, June 2014,
              <https://www.rfc-editor.org/info/rfc7212>.

   [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,
              <https://www.rfc-editor.org/info/rfc7726>.

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   [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,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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>.

   [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, <https://www.rfc-editor.org/info/rfc8562>.

   [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,
              <https://www.rfc-editor.org/info/rfc8563>.

8.2.  Informative References

   [I-D.ietf-bess-mvpn-fast-failover]
              Morin, T., Kebler, R., and G. Mirsky, "Multicast VPN fast
              upstream failover", draft-ietf-bess-mvpn-fast-failover-10
              (work in progress), February 2020.

   [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,
              <https://www.rfc-editor.org/info/rfc4687>.

Author's Address

   Greg Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com

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