



Network Work group                                              N. Kumar
Internet-Draft                                                    NVIDIA
Intended status: Standards Track                            C. Pignataro
Expires: 13 November 2026                North Carolina State University
                                                                 M. Chen
                                                     Huawei Technologies
                                                               G. Mirsky
                                                             Independent
                                                             12 May 2026


          Bit Index Explicit Replication (BIER) Ping and Trace
                        draft-ietf-bier-ping-23

Abstract

   Bit Index Explicit Replication (BIER) is a multicast forwarding
   architecture designed to simplify and optimize multicast delivery.

   This document specifies the mechanism and basic BIER OAM packet
   format that can be used to perform failure detection and isolation on
   the BIER data plane without any dependency on other layers, like the
   IP layer.

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
   and may be updated, replaced, or obsoleted by other documents at any
   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 13 November 2026.

Copyright Notice

   Copyright (c) 2026 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
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
     2.1.  Terminology and Acronyms  . . . . . . . . . . . . . . . .   4
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   3.  BIER OAM  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  BIER OAM Message Format . . . . . . . . . . . . . . . . .   5
     3.2.  BIER Echo Request/Reply Message Format  . . . . . . . . .   6
     3.3.  Return Code . . . . . . . . . . . . . . . . . . . . . . .   9
     3.4.  BIER OAM TLVs . . . . . . . . . . . . . . . . . . . . . .  10
       3.4.1.  Original SI-BitString TLV . . . . . . . . . . . . . .  11
       3.4.2.  Target SI-BitString TLV . . . . . . . . . . . . . . .  12
       3.4.3.  Incoming SI-BitString TLV . . . . . . . . . . . . . .  13
       3.4.4.  Downstream Detailed Mapping TLV . . . . . . . . . . .  14
       3.4.5.  Responder BFER TLV  . . . . . . . . . . . . . . . . .  19
       3.4.6.  Responder BFR TLV . . . . . . . . . . . . . . . . . .  19
       3.4.7.  Ingress Interface TLV . . . . . . . . . . . . . . . .  20
       3.4.8.  Erroneous Echo Request TLV  . . . . . . . . . . . . .  21
   4.  BIER Ping and Traceroute Operations . . . . . . . . . . . . .  21
     4.1.  BIER OAM Processing . . . . . . . . . . . . . . . . . . .  21
     4.2.  BFER ECMP Discovery Within a BIER Domain with MPLS
           Underlay  . . . . . . . . . . . . . . . . . . . . . . . .  22
     4.3.  Sending BIER Echo Request . . . . . . . . . . . . . . . .  22
     4.4.  Receiving BIER Echo Request . . . . . . . . . . . . . . .  23
     4.5.  Sending Echo Reply  . . . . . . . . . . . . . . . . . . .  27
     4.6.  Receiving Echo Reply  . . . . . . . . . . . . . . . . . .  28
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
     5.1.  UDP Port Number . . . . . . . . . . . . . . . . . . . . .  28
     5.2.  BIER OAM as BIER NEXT Protocol  . . . . . . . . . . . . .  29
     5.3.  BIER OAM Registry Group . . . . . . . . . . . . . . . . .  29
     5.4.  BIER OAM Message Type . . . . . . . . . . . . . . . . . .  29
     5.5.  BIER Echo Request/Echo Reply Registries . . . . . . . . .  30
       5.5.1.  BIER Echo Request/Echo Reply Message Types  . . . . .  30
       5.5.2.  BIER Echo Reply Modes . . . . . . . . . . . . . . . .  30
       5.5.3.  BIER Echo Return Codes  . . . . . . . . . . . . . . .  31
     5.6.  Common Registration Procedures for TLVs and Sub-TLVs  . .  32
       5.6.1.  TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  33
       5.6.2.  Sub-TLVs for TLV Type 4 . . . . . . . . . . . . . . .  34



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   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  35
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  36
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  37
   Contributors' Addresses . . . . . . . . . . . . . . . . . . . . .  38
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Introduction

   [RFC8279] introduces and explains BIER architecture that provides
   optimal multicast forwarding through a "BIER domain" without
   requiring intermediate routers to maintain any multicast-related per-
   flow state.  BIER also does not require any explicit tree-building
   protocol for its operation.  A multicast data packet enters a BIER
   domain at a "Bit-Forwarding Ingress Router" (BFIR), and leaves the
   BIER domain at one or more "Bit-Forwarding Egress Routers" (BFERs).
   The BFIR router adds a BIER header to the packet.  The BIER header
   contains a bit-string in which each bit represents exactly one BFER
   to forward the packet to.  The set of BFERs to which the multicast
   packet needs to be forwarded is specified by setting the bits that
   correspond to those routers in the BIER header.  Similarly, the
   Initiator of the BIER OAM packet controls the set of BFRs to which
   the BIER OAM packet is addressed by setting bits in the BitString
   field of the BIER header that correspond to the BFR-ID values of
   those BFRs.

   Operations, Administration, and Maintenance (OAM) mechanisms are
   expected to support the detection of network failures.  After the
   detection, operators localize and characterize the network defect.  A
   query-based tool, e.g., ICMP [RFC0792] and LSP Ping [RFC8029],
   [RFC6425], is broadly used to detect and localize a network defect.
   Additionally, this mechanism can be used to check the consistency
   between the data and control planes.  This document describes the
   mechanism and basic BIER OAM packet format that can be used to
   perform failure detection and isolation on the BIER data plane
   without any dependency on other layers, like the IP layer.  The
   specification conforms to R-1 through R-3, R-5, and R-11 requirements
   listed in [I-D.ietf-bier-oam-requirements].  To conform to R-11, BIER
   Echo Request message is encapsulated in the BIER header [RFC8296]
   that uses the same values of BIFT-id, BSL, Entropy, and DSCP fields
   as in the BIER header of the monitored BIER flow.  Note that the BIER
   Echo Request/Reply protocol doesn't modify the content of the OAM
   field in the BIER header (Section 2 of [RFC8296]).







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2.  Conventions used in this document

2.1.  Terminology and Acronyms

   In this specification:

      The term "Initiator" is used interchangeably with the "Sender of a
      BIER Echo Request".

      An incoming interface, also referred to as ingress interface, is a
      BFR's interface on which it receives a BIER Echo Request packet.

      A downstream interface, also referred to as egress interface, is a
      BFR's interface over which the BIER Echo Request packet may be
      transmitted to reach the destination.

   BFER - Bit-Forwarding Egress Router

   BFIR - Bit-Forwarding Ingress Router

   BFR - Bit-Forwarding Router

   BIFT - Bit Index Forwarding Table

   BIER - Bit Index Explicit Replication

   DDMAP - Downstream Detailed Mapping TLV

   ECMP - Equal Cost Multi-Path

   OAM - Operation, Administration, and Maintenance

   SI - Set Identifier

   QTF - Querier Timestamp Format

   RTF - Responder Timestamp Format

   NTP - Network Time Protocol

   MTU - Maximum Transmission Unit

   DA - Downstream Address

   DIA - Downstream Interface Address

   DoS - Denial-of-Service




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   PTP - Precision Time Protocol

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.  BIER OAM

   BIER OAM is defined to stay within the BIER layer by directly
   following the BIER header without mandating the need for an IP
   header.  To produce information that is useful to an operator,
   information that statistically reflects conditions experienced by the
   monitored data flow, the operator must be able to ensure that active
   OAM packets, e.g., BIER Echo Request, traverse the set of links and
   nodes and receive the same forwarding treatment as the monitored
   flow.  Hence, all fields in the BIER header that affect packet
   forwarding (e.g., BFIR-id, BitString) must be set to the values
   applied to the monitored data flow.  [RFC8296] defines a 4-bit field
   as "Proto" to identify the payload following the BIER header.  When
   the payload is BIER OAM, the "Proto" field will be set to 5 as
   defined in [RFC8296].

3.1.  BIER OAM Message Format

   The BIER OAM header format that follows the BIER header is displayed
   in Figure 1.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Ver  |MessageType|   Proto   |           Reserved            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   BIER OAM Message Length                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: BIER OAM Header

      Ver - a four-bit field that indicates the version of the BIER OAM
      header.  The value defined in this document is 1.  The version
      number is to be incremented whenever a change is made that affects
      the ability of an implementation to parse or process the BIER OAM
      header correctly.  For example, if syntactic or semantic changes
      are made to any of the fixed fields.




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      Message Type - a six-bit field that identifies OAM protocol.
      Values defined in this document are as in Table 1.

      Proto - a six-bit field.  This field is used to define whether
      there is any data packet immediately following the OAM payload.
      For example, the In-situ OAM Direct Export Option header [RFC9326]
      can be appended to the BIER OAM message, enabling the collection
      of the operational state and performance metrics.  This field MUST
      be set to 0 if no data packet follows the OAM payload.  Otherwise,
      the value is one from the IANA registry "BIER Next Protocol
      Identifiers" [IANA-Next-Protocol-Identifiers].

      Reserved - a two-octet field.  The value MUST be zeroed on
      transmission and ignored on receipt.

      BIER OAM Message Length - a four-octet field that reflects the
      length of the OAM message in octets, including the header and the
      Messsage Type Dependent Data.

3.2.  BIER Echo Request/Reply Message Format

   The format of the BIER Echo Request/Reply message, preceded by the
   BIER OAM header (Figure 1), is displayed in Figure 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
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------
  |  Ver  |   Echo Type   |   Proto   |         Reserved          |  BIER
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   OAM
  |               BIER Echo Request/Reply Length                  | Header
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+------
  |   QTF |   RTF |   Reply Mode  |  Return Code  |   Reserved2   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Sender's Handle                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Sequence Number                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Timestamp Sent                             |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                  Timestamp Received                           |
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                              TLVs                             ~
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: BIER Echo Request/Reply Format




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      Echo Type - a one-octet field.  Valid values are listed in
      Table 1.

      Proto field MUST be set to 0 for Echo Request/Reply header.

      Reserved - a fourteen-bit field.  The Reserved field MUST be
      zeroed on transmit and MUST be ignored on receipt.

      BIER Echo Request/Reply Length a four-octet field that reflects
      the length of the BIER Echo Request/Reply message in octets,
      including the BIER OAM header Section 3.

      QTF (Querier Timestamp Format) - a four-bit field.  When the field
      is set to 2, the Timestamp Sent field is (in seconds and
      picoseconds, according to the Initiator's clock) in the 64-bit
      long NTP format [RFC5905].  When the value of the QTF field is 3,
      the Timestamp Sent field is in the IEEE 1588-2008 (1588v2)
      Precision Time Protocol (PTP) [IEEE.1588.2008] 64-bit truncated
      version (48-bit sec + 16-bit nsec) format.

      RTF (Responder Timestamp Format) - a four-bit field.  When the
      field is set to 2, the Timestamp Received field is (in seconds and
      picoseconds, according to the Initiator's clock) in 64-bit long
      NTP format [RFC5905].  When field's value is 3, the format of the
      Timestamp Received is as defined in IEEE 1588-2008 (1588v2)
      Precision Time Protocol [IEEE.1588.2008] for 64-bit truncated
      version (48-bit sec + 16-bit nsec).  The Initiator MUST zero RTF
      in the Echo Request, and the Responder MUST ignore the value on
      receipt.

                       +=======+===================+
                       | Value | Description       |
                       +=======+===================+
                       |   1   | BIER Echo Request |
                       +-------+-------------------+
                       |   2   | BIER Echo Reply   |
                       +-------+-------------------+

                          Table 1: BIER Echo Type

   The sender of the BIER Echo Request might receive the BIER Echo Reply
   with RTF different from the Sender's QTF.  Thus, to calculate one-way
   delay, the Sender MUST be able to interpret both timestamp formats,
   i.e., NTP [RFC5905] and PTP [IEEE.1588.2008].  Although the use of
   different timestamp formats is permitted, it may cause ambiguity or
   even precision loss resulting from format conversion.  Thus, the use
   of homogeneous formats is RECOMMENDED.




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      The Reply Mode - a one-octet field.  The value MUST be set to one
      of the values from Table 2.

               +=======+===================================+
               | Value |            Description            |
               +=======+===================================+
               |   1   | Do not Reply                      |
               +-------+-----------------------------------+
               |   2   | Reply via an IPv4/IPv6 UDP packet |
               +-------+-----------------------------------+
               |   3   | Reply via a BIER packett          |
               +-------+-----------------------------------+

                          Table 2: BIER Reply Mode

      When Reply Mode is set to 1, the receiver will not send any reply.
      This mode can be used for unidirectional path validation.  When
      the Reply Mode is set to 2, the Responder Bit-Forwarding Router
      (BFR) encapsulates the Echo reply payload with the IP/UDP header.
      The Responder BFR uses the BFIR-id field in the BIER header to
      determine which IP address family to use in the IP/UDP
      encaspulation.  If the BFIR-id is associated with IPv4 and IPv6
      addresses, the Responder uses its local policy to select the
      address family.  When the Initiator intends to validate the return
      BIER path, the Reply Mode will be set to 3 so that the Responder
      BFR will encapsulate the Echo Reply with the BIER header.  Also,
      the Reply Mode "Reply via a BIER packet" can be used if the IP
      network is deemed less reliable compared to the BIER layer.

      Return Code - a one-octet field.  The value MUST be set to zero if
      the Type is "BIER Echo Request".  The Return Code MUST be set to
      zero by the Initiator of a BIER Echo Request, and ignored on its
      receipt.  The value of the Return Code field MUST be set to one of
      the values defined in Section 3.3, if the Type is "BIER Echo
      Reply".

      Reserved2 - a one-octet field.  The Reserved2 field MUST be zeroed
      on transmit and MUST be ignored on receipt.

      Sender's Handle - a four-octet field.  The Sender's Handle is
      filled by the Initiator, and returned unchanged by Responder BFR.
      This value can be used for matching the replies to the request
      (see Section 4.3).

      Sequence Number - a four-octet field.  The value of the field is
      assigned by the Initiator and can be used to detect any missed
      replies.




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      Timestamp - each field (Sent and Received) is an eight-octet
      field.  The Timestamp Sent is the time when the Echo Request is
      sent.  The Timestamp Received in Echo Reply is the time
      (accordingly to responding BFR clock) that the corresponding Echo
      Request was received.  The format depends on the QTF/RTF value.
      The Initiator MUST zero Timestamp Received in the Echo Request,
      and the Responder MUST ignore the value on receipt.

      TLVs - Carries the TLVs as defined in Section 3.4.

3.3.  Return Code

   The Responder uses the Return Code field to reply with a validity
   check or other error message to Initiator.  It does not carry any
   meaning in Echo Request and MUST be set to zero.  The Return Code can
   be one of the values in Table 3.

     +=======+======================================================+
     | Value | Description                                          |
     +=======+======================================================+
     |   0   | No return code                                       |
     +-------+------------------------------------------------------+
     |   1   | Malformed Echo Request received                      |
     +-------+------------------------------------------------------+
     |   2   | One or more of the TLVs is not supported             |
     +-------+------------------------------------------------------+
     |   3   | Replying BFR is the only BFER in header BitString    |
     +-------+------------------------------------------------------+
     |   4   | Replying BFR is one of the BFERs in header BitString |
     +-------+------------------------------------------------------+
     |   5   | Packet-Forward-Success                               |
     +-------+------------------------------------------------------+
     |   6   | Invalid Multipath Info Request                       |
     +-------+------------------------------------------------------+
     |   8   | No matching entry in the forwarding table            |
     +-------+------------------------------------------------------+
     |   9   | Set-Identifier Mismatch                              |
     +-------+------------------------------------------------------+
     |   10  | DDMAP Mismatch                                       |
     +-------+------------------------------------------------------+

                      Table 3: BIER Echo Return Code

   "No return code" will be used by Initiator in the Echo Request.  This
   value MUST NOT be used in Echo Reply.

   "Malformed Echo Request received" will be used by any BFR if the
   received Echo Request packet is not properly formatted.



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   When a receiver does not support any TLV included in the Echo
   Request, the Return code will be set to "One or more of the TLVs is
   not supported" carrying the respective TLVs.

   When the received header BitString in the Echo Request packet
   contains only its BFR-ID, "Replying BFR is the only BFER in header
   BitString" is set in the reply.  This value implies that the receiver
   is BFER, and the packet is not forwarded to any more neighbors.

   When the received header BitString in the Echo Request packet
   contains its BFR-ID in addition to other BFR-IDs, "Replying BFR is
   one of the BFERs in header BitString" is set in the reply.  This
   value implies that the Responder is a BFER and the packet is further
   forwarded to one or more neighbors.

   Any transit BFR will send the Echo Reply with "Packet-Forward-
   Success", if the TLV in the received Echo Request is understood and
   the forwarding table has forwarding entries for the BitString.  This
   behavior is demonstrated by a transit BFR during traceroute mode.

   When the Echo Request is received with multipath info
   (Section 3.4.4.1) for more than one BFER, the Return Code is set to
   "Invalid Multipath Info Request".

   If the BitString cannot be matched in the local forwarding table, the
   BFR will use "No matching entry in the forwarding table" in the
   reply.

   If the value of the BIFT-id field, representing a particular Bit
   Index Forwarding Table (see Section 6.4. of [RFC8279]), a.k.a. BIER-
   MPLS label, in the received Echo Request is not the one assigned for
   SI in Original SI-BitString TLV, "Set-Identifier Mismatch" is set in
   order to report the mismatch.

   If the BitString in Header-H does not match the BitString in Egress
   BitString Sub-TLV of Downstream Detailed Mapping (DDMAP) TLV, a
   responding BFR will use "DDMAP Mismatch" to report the problem.

3.4.  BIER OAM TLVs

   This section defines various TLVs that can be used in BIER OAM
   packet.  The TLVs (Type-Length-Value tuples) have the following
   format:








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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Type            |          Length               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                              Value                            ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Figure 3: Type-Length-Value Format Used in BIER Echo Request/Reply

   TLV Types are defined below.  A system that receives an Echo Request
   with unknown TLV Type with the value in the range 0 - 32767 MUST
   transmit an Echo Reply with the Return Code "One or more of the TLVs
   is not supported" (2).  Also, the Erroneous Echo Request TLV
   (Section 3.4.8) MUST be included in the BIER Echo Reply.  A system
   that receives an Echo Request with the value in the range 32768 -
   65535 MAY silently drop the packet.  Length is the length of the
   Value field in octets.  The Value field depends on the TLV Type.

3.4.1.  Original SI-BitString TLV

   The Original SI-BitString TLV carries the set of BFERs and carries
   the same BitString that the Initiator includes in the BIER header.
   This TLV has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 1              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Set ID     | Sub-domain ID |BS Len |      Reserved         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (first 32 bits)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (last 32 bits)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 4: The Format of the Original SI-BitString TLV

   Set ID - a one-octet field that is set to the value of the Set
   Identifier to which the BitString belongs.  This value is derived as
   defined in [RFC8279].





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   Sub-domain ID - a one-octet field that is set to the Sub-domain value
   to which BFER in BitString belongs.

   BS Len - a four-bit field that is set based on the length of
   BitString as defined in [RFC8296] reflected in four-octet words.

   Reserved - a twelve-bit field.  Its value MUST be zeroed on
   transmission and ignored on receipt.

   BitString - a variable length field.  The BitString field carries the
   set of BFR-IDs that Initiator will include in the BIER header.

   Any Initiator MUST include this TLV in the Echo Request packet.  A
   Responder MUST NOT include this TLV in the Echo Reply packet.

3.4.2.  Target SI-BitString TLV

   The Target SI-BitString TLV carries the set of BFERs from which the
   Initiator expects the reply.  This TLV has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 2              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Set ID     | Sub-domain ID |BS Len |       Reserved        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (first 32 bits)                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (last 32 bits)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 5: The Format of the Target SI-BitString TLV

   Set ID field is set to the Set Identifier to which the BitString
   belongs.This value is derived as defined in [RFC8279].

   Sub-domain ID is set to the Sub-domain value to which BFER in
   BitString belongs.

   BS Len is set based on the length of BitString as defined in
   [RFC8296]

   Reserved - the value MUST be zeroed on transmission and ignored on
   receipt.




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   The BitString field carries the set of BFR-IDs of BFER(s) that
   Initiator expects a response.  The BitString in this TLV may be
   different from the BitString in the BIER header and allows control of
   the BFER responding to the Echo Request.  If the DDMAP TLV
   (Section 3.4.4) is included in the BIER OAM Echo Request packet, one
   and only one Target BitString TLV MUST also be included in the
   packet.

3.4.3.  Incoming SI-BitString TLV

   The Incoming SI-BitString TLV will be included by Responder BFR in
   Reply message and copies the BitString from the BIER header of
   incoming Echo Request message.  This TLV has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 3              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Set ID     | Sub-domain ID |BS Len |       Reserved        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (first 32 bits)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (last 32 bits)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 6: The Format of the Incoming SI-BitString TLV

   Set ID field is set to the Set Identifier to which the BitString
   belongs.  This value is derived as defined in [RFC8279]

   Sub-domain ID is set to the Sub-domain value to which BFER in
   BitString belongs.

   BS Len is set based on the length of BitString as defined in
   [RFC8296].

   Reserved - the value MUST be zeroed on transmission and ignored on
   receipt.

   The BitString field copies the BitString from the BIER header of the
   incoming Echo Request.  A Responder BFR SHOULD include this TLV in
   Echo Reply if the Echo Request is received with the I flag set in
   DDMAP TLV.





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   An Initiator MUST NOT include this TLV in Echo Request.  A Responder
   BFR MUST include the Incoming SI-BitString TLV setting field values
   as specified in Section 4.5.

3.4.4.  Downstream Detailed Mapping TLV

   The Downstream Detailed Mapping object is an optional TLV that an
   Initiator MAY include in a BIER Echo Request message.  Only one DDMAP
   TLV MAY appear in an BIER Echo Request.  The presence of a DDMAP TLV
   is a request that the Responder MUST include in its BIER Echo Reply
   message DDMAP TLV for each interface over which this BIER OAM Echo
   Request could be forwarded.  The BFER received the BIER Echo Request
   MUST NOT include DDMAP TLV in its BIER Echo Reply.

   The format of the Downstream Detailed Mapping TLV shown in Figure 7.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 4              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               MTU             | Address Type  |     Flags     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Address (4 or 16 octets)             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Downstream Interface Address (4 or 16 octets)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Sub-TLVs Length        |                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      .                                                               .
      .                      List of Sub-TLVs                         .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 7: The Format of the Downstream Detailed Mapping TLV

   Maximum Transmission Unit (MTU)  A two-octet field.  The MTU is the
      size in octets of the largest BIER packet (including the BIER
      header) that fits on the interface to the downstream BFR.  The
      Initiator MUST zero the field, and the Responder ignores its value
      in the received BIER Echo Request.  The Responder sets the value
      in the BIER Echo Reply.

   Address Type  A one-octet field.  The Address Type indicates the
      address type and length of the IP address for the downstream
      interface.  The value of the Address Type field is set to one of
      the values listed in Table 4.  Any other value MUST be processed
      as invalid TLV.  The Initiator MUST set the Address Type to IPv4



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      Unnumbered in its BIER Echo Request.  The Responder MUST set the
      value in its BIER Echo reply according to the type and length of
      its downstream interface.

               +=======+==================================+
               | Value |           Address Type           |
               +=======+==================================+
               |   1   | IPv4 Numbered                    |
               +-------+----------------------------------+
               |   2   | IPv4 Unnumbered                  |
               +-------+----------------------------------+
               |   3   | IPv6 Global Unicast Address      |
               |       | (including Unique Local Address) |
               +-------+----------------------------------+
               |   4   | IPv6 Link-Local Address Only     |
               +-------+----------------------------------+

                        Table 4: The Address Types

   Flags  The Flags field has the following format:

                              0 1 2 3 4 5 6 7
                             +-+-+-+-+-+-+-+-+
                             |   Reserved  |I|
                             +-+-+-+-+-+-+-+-+

                      Figure 8: The Flags Field Format

   Reserved  A seven-bit field.  Its value MUST be zeroed on
         transmission and ignored on receipt.

   I     A one-bit field.  When I flag is set, the Responding BFR MUST
         include the Incoming SI- BitString TLV in Echo Reply message.

   Downstream Address and Downstream Interface Address
      each field is either four-octet or sixteen-octet, depending on the
      value of Address Type field.

      Note that values of the Address Type field are mapped to
      combinations of lengths of Downstream Address (DA) and Downstream
      Address Interface (DIA) fields as shown im Table 5.










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                      +=======+===========+============+
                      | Value | DA Length | DIA Length |
                      +=======+===========+============+
                      |   1   |     4     |     4      |
                      +-------+-----------+------------+
                      |   2   |     4     |     4      |
                      +-------+-----------+------------+
                      |   3   |     16    |     16     |
                      +-------+-----------+------------+
                      |   4   |     16    |     4      |
                      +-------+-----------+------------+

                      Table 5: The Address Type Lengths

      where:

      DA Length
         Downstream Address field Length

      DIA Length
         Downstream Interface Address field Length

      If the Address Type is "IPv4 Numbered" (1), the Downstream Address
      field MUST be set to IPv4 BFR-Prefix of downstream BFR and
      Downstream Interface Address is set to the downstream interface
      address.

      If the Address Type is "IPv4 Unnumbered" (2), the Downstream
      Address field MUST be set to IPv4 BFR-Prefix of downstream BFR and
      Downstream Interface Address is set to the index assigned by the
      responding BFR to the interface.

      If the Address Type is "IPv6 Global Unicast Address (including
      Unique Local Address)" (3), the Downstream Address MUST be set to
      IPv6 BFR-Prefix of downstream BFR and Downstream Interface Address
      is set to the downstream interface IPv6 Global Unicast Address
      (including Unique Local Address).

      If the Address Type is "IPv6 Link-Local Address Only" (4), the
      Downstream Address MUST be set to the IPv6 BFR-Prefix of the
      downstream BFR, and the Downstream Interface Address is set to the
      index assigned by the responding BFR to the interface.

   The Initiator MUST set the Downstream Address to 224.0.0.2, and the
   Downstream Interface Address MUST be set to 0 in the BIER Echo
   Request.  The Responder MUST ignore these values in the received BIER
   Echo Request.




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3.4.4.1.  Downstream Detailed Mapping Sub-TLVs

   This section defines the optional Sub-TLVs that can be included in
   DDMAP TLV in Table 6.

                    +=======+========================+
                    | Value |      Description       |
                    +=======+========================+
                    |   1   | Multipath Entropy Data |
                    +-------+------------------------+
                    |   2   |    Egress BitString    |
                    +-------+------------------------+

                         Table 6: Sub-TLV for the
                       Downstream Detailed Mapping
                                   TLV

   Any value other than listed in Table 6 MUST be considered as invalid,
   the Return Code set to Malformed Echo Request received (1).  Also,
   the Erroneous Echo Request TLV (Section 3.4.8) MUST be included in
   the BIER Echo Reply.

3.4.4.1.1.  MPLS Multipath Entropy Data Sub-TLV

   MPLS Multipath Entropy Data sub-TLV is applicable for BIER Echo
   Request packets encapsulated in MPLS.  Encoding of multipath
   information for other data planes, e.g., IPv6, is for further study.
   If the MPLS Multipath Entropy Data sub-TLV is present in the BIER
   Echo Request packet encapsulated in a non-MPLS data plane, it MUST be
   ignored by the responding BFR.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 1              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |M|   Reserved  | Multipath Type|                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
      |                                                               |
      |                  (Multipath Information)                      |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 9: The Format of the Multipath Data Blob

   M Flag  This flag is set to 0 if all packets will be forwarded out





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      through the interface defined in the DDMAP TLV.  When set to 1,
      Multipath Information will be defined by the Bit masked Entropy
      data.

   Reserved  The value MUST be zeroed on transmission and ignored on
      receipt.

  
      The interpretaion of the Multipath Type field and Multipath
      Entropy Data encoding options are the same defined in
      Section 3.4.1.1 of [RFC8029].

3.4.4.1.2.  Egress BitString Sub-TLV

   Responder BFR MAY include this Sub-TLV with the rewritten BitString
   in the downstream interface as defined in Section 6.1 of [RFC8279].

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 2              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Set ID     | Sub-domain ID |BS Len |       Reserved        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (first 32 bits)                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                                                               ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                BitString  (last 32 bits)                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: The Egress BitString Sub-TLV Format

   Set ID field is set to the Set Identifier to which the BitString
   belongs.  This value is derived as defined in [RFC8279].

   Sub-domain ID is set to the Sub-domain value to which BFER in
   BitString belongs.

   BS Len is set based on the length of BitString as defined in
   [RFC8296].

   Reserved - the value MUST be zeroed on transmission and ignored on
   receipt.

   The BitString field copies the rewritten BitString in the downstream
   interface as defined in Section 6.1 of [RFC8279].




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3.4.5.  Responder BFER TLV

   The BFER replying to the request MAY include the Responder BFER TLV
   in its BIER Echo Reply.  An Initiator MUST NOT include this TLV in
   Echo Request.  This TLV identifies the originator of BIER Echo Reply.
   This TLV has the following format:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Type = 5              |           Length = 4          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |           BFR-ID              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 11: The Responder BFER TLV Format

   Length  A two-octet field.  The value MUST be set to four.

   Reserved  A two-octet field.  The value MUST be zeroed on
      transmission and ignored on receipt.

   BFR-ID  A two-octet field.  The BFR-ID field carries the BFR-ID of
      the replying BFER.  This TLV MAY be included by the Responding
      BFER in the BIER Echo Reply packet.

3.4.6.  Responder BFR TLV

   Any transit BFR replying to the request MAY include the Responder BFR
   TLV in its BIER Echo Reply.  An Initiator MUST NOT include this TLV
   in Echo Request.  This is used to identify the replying BFR without
   BFR-ID.  This TLV has the following format:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLV Type = 6              |        Length = 8 or 20       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |          Address Type         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                       BFR-Prefix (4 or 16 bytes)              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 12: The Responder BFR TLV Format

   Length  The Length field, depending on the Address Type value - 8 or



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      20.

   Reserved  A two-octet field.  The value MUST be zeroed on
      transmission and ignored on receipt.

   Address Type  A two-octet field.  Set according to Table 7.  Any
      other value is invalid.

   BFR-Prefix  This field carries the local BFR-Prefix of the replying
      BFR.  This TLV MAY be included by Responding BFR in BIER Echo
      Reply packet.

                         +=======+==============+
                         | Value | Address Type |
                         +=======+==============+
                         |   1   | IPv4 Address |
                         +-------+--------------+
                         |   2   | IPv6 Address |
                         +-------+--------------+

                        Table 7: The Address Types

3.4.7.  Ingress Interface TLV

   The BFR replying to the request MUST include the Ingress Interface
   TLV.  This TLV identifies the incoming interface on which the Echo
   Request was received.  This TLV has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     TLV Type = 7              |        Length = 8 or 20       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Reserved              |          Address Type         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~           Ingress Interface Address (4 or 16 bytes)           ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 13: The Ingress Interface TLV Format

   Length  The Length field, depending on the Address Type value - 8 or
      20.

   Reserved  A two-octet field.  The value MUST be zeroed on
      transmission and ignored on receipt.




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   Address Type  A two-octet field.  Set its value according to Table 4.

   Ingress Interface Address
      A four or sixteen-octet-long field.  It lists an address
      associated with the interface on which the BIER Echo Request
      received.

3.4.8.  Erroneous Echo Request TLV

   The BFER replying to the request MAY include the Erroneous Echo
   Request TLV.  This TLV provides information about the type and
   location of the problem in the BIER Echo Request.  This TLV has the
   following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Type = 8              |       Length = variable       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                               Pointer                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             As much of invoking BIER Echo Request             |
      ~            as possible without exceeding path MTU             ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 14: The Erroneous Echo Request TLV Format

   Pointer  A four-octet field that identifies the octet offset within
      the received BIER Echo Request message where the error was
      detected.  The Pointer will point beyond the end of the BIER Echo
      Reply message if the field in error is beyond what can fit in the
      resulting packet.

4.  BIER Ping and Traceroute Operations

   This section describes aspects of BIER ping and traceroute
   operations.

4.1.  BIER OAM Processing

   A BIER OAM packet MUST be punted to the BIER control plane for OAM
   processing if one of the following conditions is true:

   *  The receiving BFR is a BFER.

   *  TTL of the BIER header (Section 2.1.1.1 of [RFC8296]) expired.




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   *  Hop Limit in the IPv6 header (Section 2 of
      [I-D.ietf-bier-bierin6]) expired.

   The use of the Router Alert label has been deprecated by [RFC9570].

   Processing of the received BIER OAM packet with an unknown value of
   the Message Type field (Figure 1) is stopped, and the event MUST be
   logged through the rate-controlling system.

   A transit BFR, i.e., one that does not punt the BIER OAM packet to
   the BIER control plane, forwards the BIER OAM packet according to the
   rules specified in Section 6.5 of [RFC8279].

4.2.  BFER ECMP Discovery Within a BIER Domain with MPLS Underlay

   As defined in [RFC8279], BIER follows the unicast forwarding path and
   allows load balancing over ECMP paths between BFIR and BFER.  BIER
   OAM is expected to support ECMP path discovery between a BFIR and a
   given BFER and MUST support path validation and failure detection of
   any particular ECMP path between BFIR and BFER.

   [RFC8296] proposes the BIER header with the Entropy field that can be
   leveraged to exercise all ECMP paths.  The Initiator/BFIR will use a
   traceroute message to query each hop about the Entropy information
   for each of the downstream paths.  To avoid complexity, it is
   suggested that the ECMP query is performed per BFER by carrying the
   required information in the BIER OAM message.

   When an operator performs BFER ECMP discovery within a BIER domain
   over MPLS underlay, the Initiator MUST include MPLS Multipath Entropy
   Data Sub-TLV in DDMAPTLV.  It MUST also include the BFER in the
   BitString TLV to which the Multipath query is performed.

   Any transit BFR will transmit the BIER Echo Reply to the Initiator
   with Bit-masked Entropy for each downstream path as defined in
   [RFC8029].

4.3.  Sending BIER Echo Request

   The Initiator MUST set the Message Type as 1 and Return Code as 0.
   The Proto field in the OAM packet MUST be set to 0.  The choice of
   the Sender's Handle and Sequence Number is a local matter to the
   Initiator and the Initiator SHOULD monotonically increase the
   Sequence Number, e.g., increment it by one for every, subsequent Echo
   Request.  The QTF field is set to Initiator's local timestamp format,
   and the TimeStamp Sent field is set to the time that the Echo Request
   is sent.




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   The Initiator MUST include Original SI-BitString TLV.  The Initiator
   MUST NOT include more than one Original SI-BitString TLV.  The
   Initiator infers the Set Identifier value and Sub-domain ID value
   from the respective BitString that will be included in the BIER
   header of the packet and includes the values in "SI" and Sub-Domain
   ID fields, respectively.

   In Ping mode, the Initiator MAY include Target SI-BitString TLV to
   control the responding BFER(s) by listing all the BFERs from which
   the Initiator expects a response.  In the traceroute mode, the
   Initiator MAY include Target SI-BitString TLV to control the path
   trace towards any specific BFER or set of BFERs.  The Initiator on
   receiving a reply with the Return code "Replying BFR is the only BFER
   in the header BitString" or "Replying router is one of the BFERs in
   header BitString" MUST unset the respective BFR-ID from Target SI-
   BitString for any subsequent Echo Request.

   The Initiator MAY include DDMAP TLV (Section 3.4.4) in the Echo
   Request to query additional information from transit BFRs and BFERs.
   In case of ECMP discovery within a BIER domain with the MPLS
   underlay, the Initiator MUST include the MPLS Multipath Entropy Data
   Sub-TLV and MUST set the Target SI-BitString TLV carrying a specific
   BFER ID.

   The Initiator MUST encapsulate the OAM packet with the BIER header
   and MUST set the Proto as 5.  In ping mode, the TTL field in the BIER
   header MUST be set to 255.  In traceroute mode, the TTL in the BIER
   header is set successively, starting from 1 and MUST stop sending the
   Echo Request if it receives a reply with Return code as "Replying
   router is the only BFER in BIER header BitString" from all BFER
   listed in Target SI-BitString TLV.

4.4.  Receiving BIER Echo Request

   Sending a BIER OAM Echo Request to control plane for payload
   processing is triggered as mentioned in Section 4.1.















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   Any BFR on receiving an Echo Request MUST perform the basic sanity
   check, including, but not limited to, checking values of the fields
   with a priori known values, e.g., Ver, Type and Length if any TLV is
   present.  If, at any stage of processing the received BIER Echo
   Request, the BFR encounters an error, it MUST stop processing and
   transmit BIER Echo Reply with the Return Code set accordingly.  If
   the BFR cannot parse the OAM packet completely because the value in
   the OAM Message Length field is incorrect, BFR MUST send Echo Reply
   with Return Code set to "Malformed Echo Request received" if the OAM
   Message Length is incorrect.  The Erroneous Echo Request TLV
   (Section 3.4.8) MUST be included in the BIER Echo Reply.  If the
   packet sanity check is fine, it MUST initiate the below set of
   variables:

   Reply-Flag
      This flag is initially set to 1.

   Interface-I
      The incoming interface on which the Echo Request was received.
      This MAY be used to validate the Downstream Detailed Mapping TLV
      (DDMAP) info and populate the Ingress Interface TLV.

   BIFT-id-L
      The BIFT-id field in the BIER header of the received BIER Echo
      Request.  This MAY be used to validate if the packet is traversing
      the desired Set Identifier and sub-domain path.

   Header-H
      The BIER header of the received Echo Request.  It can be used to
      validate the DDMAP info and to populate the Incoming SI-BitString
      TLV.  Also, it can be used to perform entropy calculation
      considering a different field in the header and replying with MPLS
      Multipath Entropy Data Sub-TLV.

   Best-return-code
      contains the Return Code for the echo reply packet as currently
      best known.  As the algorithm progresses, this code may change
      depending on the results of further checks that it performs.

   BFR MUST initialize the internal, to the implementation, Best-return-
   code variable to the null value.

   BFR will populate the Interface-I with the identifier of the
   interface over which the Echo Request is received.  The value from
   the BIFT-id field of the BIER header of the received BIER Echo
   Request is copied to BIFT-id-L, and the BIER header is copied to
   Header-H.  If the received Echo Request carries Target SI-BitString
   TLV, a BFR MUST run the boolean AND operation between BitString in



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   Header-H and BitString in Target SI-BitString TLV.  If the resulting
   BitString is all-zero, reset Reply-Flag=0 and go to Section 4.5.
   Else:

   *  If the BIFT-id-L does not correspond to the BIFT-id assigned for
      {sub-domain, BitStringLen, SI} in Original SI- BitString TLV, Set
      the Best-return-code to "Set-Identifier Mismatch" and Go to
      Section 4.5.

   The step above allows the detection of a synchronization problem in
   the upstream BFR between BIER-Label and {sub-domain, BitStringLen,
   SI} that might cause an unintended packet leak between sub-domains.

   *  If the value in the Reply Mode field is unknown, the Receiver MUST
      set Reply-Flag=0 and go to Section 4.5.

   *  The Receiver sets the Best-return-code to "Malformed Echo Request
      received" if the value of the QTF field is neither 2, nor 3.
      Also, the Erroneous Echo Request TLV (Section 3.4.8) MUST be
      included in the BIER Echo Reply.  Go to Section 4.5.

   *  The Receiver sets the Best-return-code to "Malformed Echo Request
      received" if none or more than one Original SI-BitString TLV found
      in the received BIER Echo Request.  Also, the Erroneous Echo
      Request TLV (Section 3.4.8) MUST be included in the BIER Echo
      Reply.  Go to Section 4.5.

   *  The Receiver sets the Best-return-code to "Malformed Echo Request
      received" if DDMAP TLV is present in the BIER Echo Request message
      and none or more than one Target SI-BitString TLVs found.  Also,
      the Erroneous Echo Request TLV (Section 3.4.8) MUST be included in
      the BIER Echo Reply.  Go to Section 4.5.

   *  The Receiver sets the Best-return-code to "Malformed Echo Request
      received" if the Incoming S_-BitString TLV is present in the BIER
      Echo Request message.  Also, the Erroneous Echo Request TLV
      (Section 3.4.8) MUST be included in the BIER Echo Reply.  Go to
      Section 4.5.

   *  Set the Best-return-code to "One or more of the TLVs is not
      supported" if any of the TLVs in the Echo Request message is not
      supported.  Go to Section 4.5.









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   *  If the BitString in Header-H does not match the BitString in
      Egress BitString Sub-TLV of DDMAP TLV, set the Best-return-code to
      "DDMAP Mismatch" and go to Section 4.5.  When there are more than
      one DDMAP TLV in the received Request packet, the Downstream
      Address and Downstream Interface Address should be matched with
      Interface-I to identify the right DDMAP TLV and then perform the
      BitString match.

   The step above allows the detection of a deviation between the BIER
   control plane and the BIER forwarding plane in the upstream node that
   may result in a forwarding loop or packet duplication.

   *  Set the Best-return-code to "Invalid Multipath Info Request", when
      the DDMAP TLV carries MPLS Multipath Entropy Data Sub-TLV, and if
      the Target SI-BitString TLV in the received Echo Request carries
      more than 1 BFER id.  Go to Section 4.5.  Else, list the ECMP
      downstream neighbors to reach BFR-ID in Target SI-BitString TLV,
      calculate the Entropy considering the BitString in Header-H and
      MPLS Multipath Entropy Data Sub-TLV from received Echo Request.
      Store the Data for each Downstream interface in a temporary
      variable.  Set the Best-return-code to 5 (Packet-Forward-Success)
      and goto Section 4.5

   This step instructs the node to calculate the Entropy Data for each
   downstream interface to reach the BFER in Target SI-BitString TLV by
   considering the Incoming BitString and Entropy Data.

   *  Set the Best-return-code to "Replying router is the only BFER in
      BIER header BitString", and go to Section 4.5 if the Responder is
      BFER and there are no more bits in the BIER header BitString left
      for forwarding.

   *  Set the Best-return-code to "Replying router is one of the BFERs
      in BIER header BitString", and include DDMAP TLV if the Responder
      is BFER and there are more bits in BitString left for forwarding.
      Also, include the Multipath information as defined in Section 4.2
      if the received Echo Request carries Multipath Entropy Data Sub-
      TLV.  Go to Section 4.5.

   *  Set the Best-return-code to "No matching entry in the forwarding
      table", if the forwarding lookup, defined in Section 6.5 of
      [RFC8279] does not match any entry for the received BitString in
      BIER header.








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   The step above allows the detection of the missing BFR-ID in the
   node's BIER forwarding table.  It is difficult to detect the absence
   of the BFR-ID if the Request includes more than one BFR-IDs in the
   BitString and so may need to include the BFER-id that is not
   responding to detect such failure.

   *  Set the Best-return-code to "Packet-Forward-Success", and include
      DDMAP TLV.  Go to Section 4.5.

4.5.  Sending Echo Reply

   If Reply-Flag=0, BFR MUST release the variables and MUST NOT send any
   response to the Initiator.  If Reply-Flag=1, proceed as below:

   The Responder BFR MUST include the BitString from Header-H to
   Incoming SI-BitString TLV and include the Set ID, Sub-domain ID and
   BS Len that corresponds to BIFT-id-L.  Responder BFR MUST include the
   Ingress Interface TLV and populate the address from Interface-I.

   When the Best-return-code is "Replying BFR is one of the BFERs in
   header BitString", it MUST include Responder BFER TLV.

      If the received Echo Request had DDMAP with Multipath Entropy Data
      Sub-TLV, Responder BFR MUST include DDMAP as defined in
      Section 3.4.4 for each downstream interface over which the packet
      will be replicated and include the respective Multipath Entropy
      Data Sub-TLV.  For each downstream interface, the respective
      Egress BitString MUST be included in DDMAP TLV.

      If the received Echo Request had DDMAP without Multipath Entropy
      Data Sub-TLV, Responder BFR MUST include DDMAP as defined in
      Section 3.4.4 for each downstream interface over which the packet
      will be replicated.  For each downstream interface, respective
      Egress BitString MUST be included in DDMAP TLV.

   When the Best-return-code is "Replying BFR is the only BFER in header
   BitString", it MUST include Responder BFER TLV.

   The Responder MUST set the Message Type as 2 and Return Code as Best-
   return-code.  The Proto field MUST be set to 0.

   The Echo Reply can be sent as BIER-encapsulated, or IP/UDP
   encapsulated, depending on the Reply Mode in the received Echo
   Request.  When the Reply Mode in the received Echo Request is set to
   3, Responder appends the BIER header listing the BitString with BFIR
   ID (from Header-H), sets the Proto to 5, and sets the BFIR as 0.
   When the Reply Mode in the received Echo Request is set to 2,
   Responder encapsulates with the IP/UDP header.  The UDP destination



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   port MUST be set to TBD1 (Section 5.1), and the source port MAY be
   set to TBD1 or other value selected from the Dynamic range of port
   numbers.  The source IP address is any non-link-local address
   associated with the Responder, and the destination IP address is
   derived from the BFIR-id of the BIER header [RFC8296] in the received
   Echo Request.

4.6.  Receiving Echo Reply

   The Initiator, upon receiving the Echo Reply, will use the Sender's
   Handle to match with Echo Request sent.  If no match is found, the
   Initiator MUST ignore the Echo Reply.

   If receiving Echo Reply has DDMAP TLV, the Initiator MUST copy the
   TLV to subsequent Echo Request(s).

   If one of the Echo Reply is received with Return Code as "Replying
   BFR is one of the BFERs in header BitString", it SHOULD reset the
   BFR-ID of the Responder from Target SI-BisString TLV in subsequent
   Echo Request.  This step helps avoid any BFR that is both BFER and
   transit BFR to respond with Echo Reply continuously.

5.  IANA Considerations

   The terms used in the IANA Considerations below are intended to be
   consistent with [RFC8126].

5.1.  UDP Port Number

   This document requests a UDP port TBD1 to be allocated by IANA for
   BIER Echo.

   Service Name bier-echo

   Transport Protocol UDP, TCP

   Assignee IESG iesg@ietf.org

   Contact IETF Chair chair@ietf.org

   Description The UDP destination port number for the IP/UDP
   encapsulated BIER Echo Reply message.

   Reference This document

   Port Number TBD1





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5.2.  BIER OAM as BIER NEXT Protocol

   IANA is requested to update the BIER Next Protocol Identifiers
   registry as follows:

                  +=======+=============+===============+
                  | Value | Description | Reference     |
                  +=======+=============+===============+
                  | 5     |  OAM Packet | This document |
                  +-------+-------------+---------------+

                  Table 8: BIER OAM as BIER Next Protocol

5.3.  BIER OAM Registry Group

   IANA is requested to create and maintain the "BIER OAM" registry
   group containing the registries listed below.

5.4.  BIER OAM Message Type

   IANA is requested to create in the BIER OAM Message Type registry in
   the BIER OAM registry group as follows:

      Registry Name: BIER OAM Message Type.

      Assignment Policy:

      0-58 - IETF Review

      59 - 61 - Experimental Use (Reserved, not to be assigned)

      62 - 63 - Private Use (Reserved, not to be assigned)

        +=========+===============================+===============+
        | Value   |          Description          | Reference     |
        +=========+===============================+===============+
        | 0       |            Reserved           | This document |
        +---------+-------------------------------+---------------+
        | 1       |  BIER Echo Request/Echo Reply | This document |
        +---------+-------------------------------+---------------+
        | 2 - 58  |           Unassigned          | This document |
        +---------+-------------------------------+---------------+
        | 59 - 61 | Reserved for Experimental Use | This document |
        +---------+-------------------------------+---------------+
        | 62 - 63 |    Reserved for Private Use   | This document |
        +---------+-------------------------------+---------------+

                       Table 9: BIER OAM Message Type



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5.5.  BIER Echo Request/Echo Reply Registries

   IANA is requested to create three BIER Echo Request/Echo Reply
   registries in the BIER OAM registry group, as described below.

5.5.1.  BIER Echo Request/Echo Reply Message Types

   IANA is requested to create in the in the BIER OAM registry group the
   BIER Echo Types registry as follows:

      Registry Name: BIER Echo Types

      Assignment Policy:

      0 - 247 - IETF Review

      248 - 251 - Experimental Use (Reserved, not to be assigned)

      252 - 255 - Private Use (Reserved, not to be assigned)

       +===========+===============================+===============+
       | Value     |          Description          | Reference     |
       +===========+===============================+===============+
       | 0         |            Reserved           | This document |
       +-----------+-------------------------------+---------------+
       | 1         |       BIER Echo Request       | This document |
       +-----------+-------------------------------+---------------+
       | 2         |        BIER Echo Reply        | This document |
       +-----------+-------------------------------+---------------+
       | 3 - 175   |           Unassigned          | This document |
       +-----------+-------------------------------+---------------+
       | 176 - 247 |           Unassigned          | This document |
       +-----------+-------------------------------+---------------+
       | 248-251   | Reserved for Experimental Use | This document |
       +-----------+-------------------------------+---------------+
       | 252-255   |    Reserved for Private Use   | This document |
       +-----------+-------------------------------+---------------+

                         Table 10: BIER Echo Types

5.5.2.  BIER Echo Reply Modes

   IANA is requested to create in the BIER OAM registry group the new
   BIER Echo Reply Mode registry as follows:

      Registry Name: BIER Echo Reply Mode

      Assignment Policy:



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      0 - 247 - IETF Review

      248 - 251 - Experimental Use (Reserved, not to be assigned)

      252 - 255 - Private Use (Reserved, not to be assigned)

     +===========+===================================+===============+
     | Value     |            Description            | Reference     |
     +===========+===================================+===============+
     | 0         |              Reserved             | This document |
     +-----------+-----------------------------------+---------------+
     | 1         |            Do Not Reply           | This document |
     +-----------+-----------------------------------+---------------+
     | 2         | Reply via an IPv4/IPv6 UDP Packet | This document |
     +-----------+-----------------------------------+---------------+
     | 3         |      Reply via a BIER packet      | This document |
     +-----------+-----------------------------------+---------------+
     | 4 - 191   |             Unassigned            | This document |
     +-----------+-----------------------------------+---------------+
     | 192 - 247 |             Unassigned            | This document |
     +-----------+-----------------------------------+---------------+
     | 248-251   |   Reserved for Experimental Use   | This document |
     +-----------+-----------------------------------+---------------+
     | 252-255   |      Reserved for Private Use     | This document |
     +-----------+-----------------------------------+---------------+

                      Table 11: BIER Echo Reply Modes

5.5.3.  BIER Echo Return Codes

   IANA is requested to create in the BIER OAM registry group the new
   BIER Echo Return Codes registry as follows:

      Registry Name: BIER Echo Return Codes

      Assignment Policy:

      0 - 247 - IETF Review

      248 - 251 - Experimental Use (Reserved, not to be assigned)

      252 - 255 - Private Use (Reserved, not to be assigned)









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       +=========+=================================+===============+
       | Value   |           Description           | Reference     |
       +=========+=================================+===============+
       | 0       |          No Return Code         | This document |
       +---------+---------------------------------+---------------+
       | 1       | Malformed Echo Request received | This document |
       +---------+---------------------------------+---------------+
       | 2       |  One or more of the TLVs is not | This document |
       |         |            supported            |               |
       +---------+---------------------------------+---------------+
       | 3       |  Replying BFR is the only BFER  | This document |
       |         |       in header BitString       |               |
       +---------+---------------------------------+---------------+
       | 4       |    Replying BFR is one of the   | This document |
       |         |    BFERs in header BitString    |               |
       +---------+---------------------------------+---------------+
       | 5       |      Packet-Forward-Success     | This document |
       +---------+---------------------------------+---------------+
       | 6       | Invalid Multipath Info Request  | This document |
       +---------+---------------------------------+---------------+
       | 7       |            Unassigned           | This document |
       +---------+---------------------------------+---------------+
       | 8       | No matching entry in the        | This document |
       |         | forwarding table                |               |
       +---------+---------------------------------+---------------+
       | 9       | Set-Identifier Mismatch         | This document |
       +---------+---------------------------------+---------------+
       | 10      | DDMAP Mismatch                  | This document |
       +---------+---------------------------------+---------------+
       | 11 -    |            Unassigned           | This document |
       | 247     |                                 |               |
       +---------+---------------------------------+---------------+
       | 248-251 |  Reserved for Experimental Use  | This document |
       +---------+---------------------------------+---------------+
       | 252-255 |     Reserved for Private Use    | This document |
       +---------+---------------------------------+---------------+

                      Table 12: BIER Echo Return Codes

5.6.  Common Registration Procedures for TLVs and Sub-TLVs

   This section describes registration procedures for Type registries in
   BIER Echo Request/Reply TLVs and sub-TLVs.








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      +=============+==============+================================+
      | Range       | Registration | Note                           |
      |             | Procedures   |                                |
      +=============+==============+================================+
      | 0-16383     | Standards    | This range is for TLVs and     |
      |             | Action       | sub-TLVs that require an error |
      |             |              | message if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 16384-31739 | RFC Required | This range is for TLVs and     |
      |             |              | sub-TLVs that require an error |
      |             |              | message if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 31740-31743 | Experimental | Not to be assigned.            |
      |             | Use          |                                |
      +-------------+--------------+--------------------------------+
      | 31744-32767 | First Come,  | This range is for TLVs and     |
      |             | First Served | sub-TLVs that require an error |
      |             |              | message if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 32768-49161 | Standards    | This range is for TLVs and     |
      |             | Action       | sub-TLVs that can be silently  |
      |             |              | dropped if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 49162-64507 | RFC Required | This range is for TLVs and     |
      |             |              | sub-TLVs that can be silently  |
      |             |              | dropped if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 64508-64511 | Experimental | Not to be assigned.            |
      |             | Use          |                                |
      +-------------+--------------+--------------------------------+
      | 64512-65535 | First Come,  | This range is for TLVs and     |
      |             | First Served | sub-TLVs that can be silently  |
      |             |              | dropped if not recognized.     |
      +-------------+--------------+--------------------------------+

                               Table 13: TLVs

5.6.1.  TLVs

   IANA is requested to create in the BIER OAM registry group a registry
   for the Type field of top-level TLVs. as well as sub-registries for
   the associated sub-TLVs.  Note that the meaning of a sub-TLV is
   scoped by the TLV.  The number of spaces for the sub-TLVs of various
   TLVs is independent.

      Registry Name: TLVs

      Assignment Policy: Section 5.6



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   The TLVs requested by this document for the IANA consideration are
   listed in Table 14.

   +======+==================+===============+=========================+
   | Type |     TLV Name     | Reference     | Sub-TLV Registry        |
   +======+==================+===============+=========================+
   | 0    |     Reserved     | This          |                         |
   |      |                  | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 1    |   Original SI-   | This          | No Sub-TLVs             |
   |      |    BitString     | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 2    |    Target SI-    | This          | No Sub-TLVs             |
   |      |    BitString     | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 3    |   Incoming SI-   | This          | No Sub-TLVs             |
   |      |    BitString     | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 4    |    Downstream    | This          | Link the Sub-TLVs for   |
   |      | Detailed Mapping | document      | TLV Type 4 sub-registry |
   +------+------------------+---------------+-------------------------+
   | 5    |  Responder BFER  | This          | No Sub-TLVs             |
   |      |                  | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 6    |  Responder BFR   | This          | No Sub-TLVs             |
   |      |                  | document      |                         |
   +------+------------------+---------------+-------------------------+
   | 7    |     Ingress      | This          | No Sub-TLVs             |
   |      |    Interface     | document      |                         |
   +------+------------------+---------------+-------------------------+

                               Table 14: TLVs

5.6.2.  Sub-TLVs for TLV Type 4

   IANA is requested to create in the registry for the Type 4
   (Downstream Detailed Mapping) a sub-registry Sub-TLVs for Type 4.

      Registry Name: Sub-TLVs for Type 4

      Assignment Policy: Section 5.6










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          +======+=============================+===============+
          | Type |         Sub-TLV Name        | Reference     |
          +======+=============================+===============+
          | 0    |           Reserved          | This document |
          +------+-----------------------------+---------------+
          | 1    | MPLS Multipath Entropy Data | This document |
          +------+-----------------------------+---------------+
          | 2    |       Egress BitString      | This document |
          +------+-----------------------------+---------------+

                              Table 15: TLVs

6.  Security Considerations

   The security considerations of [RFC8296], and through it of
   [RFC8279], apply to this specification.

   The security considerations for BIER Ping are similar to ICMP
   [RFC0792], ICMPv6 [RFC4443], and LSP Ping [RFC8029], [RFC6425].  As
   with ICMP or LSP Ping, BFR can be exposed to Denial-of-Service (DoS)
   attacks, and it is RECOMMENDED to regulate the BIER Ping packet flow
   to the control plane.  A rate limiter SHOULD be applied to avoid any
   attack.  Specifically, a rate limiter SHOULD be applied to the well-
   known UDP port defined in Section 5.1.  Although using BIER Echo
   Request in a DoS amplification attack is theoretically possible,
   spoofing BFIR ID in the BIER Header presents itself as a serious
   challenge.  As a result, this threat is not a big concern.

   As with ICMP or LSP Ping, a traceroute can be used to obtain network
   information.  It is RECOMMENDED that the implementation checks the
   integrity of BFIR of the Echo messages against any locally secured
   list before processing the message further.

   In some BIER environments, transmitting a single BIER Echo Request
   message can result in the sender receiving an overwhelming number of
   BIER Echo Reply messages.  In that case, an operator MAY choose to
   address the BIER Echo Request to a subset of BFERs rather than to all
   BFERs in the domain.

7.  Acknowledgement

   The authors would like to thank Antoni Przygienda, Eric Rosen, Faisal
   Iqbal, Jeffrey (Zhaohui) Zhang, and Shell Nakash for their review and
   comments.

   The authors would like to thank Mankamana Mishra for his thorough
   review and comments.




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8.  References

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>.

   [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>.

   [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
              "Network Time Protocol Version 4: Protocol and Algorithms
              Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
              <https://www.rfc-editor.org/info/rfc5905>.

   [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>.

   [RFC8279]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Przygienda, T., and S. Aldrin, "Multicast Using Bit Index
              Explicit Replication (BIER)", RFC 8279,
              DOI 10.17487/RFC8279, November 2017,
              <https://www.rfc-editor.org/info/rfc8279>.

   [RFC8296]  Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
              Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
              for Bit Index Explicit Replication (BIER) in MPLS and Non-
              MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
              2018, <https://www.rfc-editor.org/info/rfc8296>.

   [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>.

   [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>.





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   [IEEE.1588.2008]
              "Standard for a Precision Clock Synchronization Protocol
              for Networked Measurement and Control Systems",
              IEEE Standard 1588, March 2008.

   [IANA-Next-Protocol-Identifiers]
              IANA, "IANA BIER Next Protocol Identifiers Registry",
              <https://www.iana.org/assignments/bier/bier.xhtml#bier-
              next-protocol-identifiers>.

8.2.  Informative References

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC9326]  Song, H., Gafni, B., Brockners, F., Bhandari, S., and T.
              Mizrahi, "In Situ Operations, Administration, and
              Maintenance (IOAM) Direct Exporting", RFC 9326,
              DOI 10.17487/RFC9326, November 2022,
              <https://www.rfc-editor.org/info/rfc9326>.

   [RFC9570]  Kompella, K., Bonica, R., and G. Mirsky, Ed., "Deprecating
              the Use of Router Alert in LSP Ping", RFC 9570,
              DOI 10.17487/RFC9570, May 2024,
              <https://www.rfc-editor.org/info/rfc9570>.

   [I-D.ietf-bier-oam-requirements]
              Mirsky, G., Nainar, N. K., Chen, M., and S. Pallagatti,
              "Operations, Administration and Maintenance (OAM)
              Requirements for Bit Index Explicit Replication (BIER)
              Layer", Work in Progress, Internet-Draft, draft-ietf-bier-
              oam-requirements-21, 23 November 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              oam-requirements-21>.










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   [I-D.ietf-bier-bierin6]
              Zhang, Z., Zhang, Z. J., Wijnands, I., Mishra, M. P.,
              Bidgoli, H., and G. S. Mishra, "Supporting BIER in IPv6
              Networks (BIERin6)", Work in Progress, Internet-Draft,
              draft-ietf-bier-bierin6-13, 23 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              bierin6-13>.

Contributors' Addresses

   Nobo Akiya
   Big Switch Networks
   Japan
   Email: nobo.akiya.dev@gmail.com


   Lianshu Zheng
   Individual Contributor
   China
   Email: veronique_cheng@hotmail.com


Authors' Addresses

   Nagendra Kumar
   NVIDIA
   Email: nagendrakumar.nainar@gmail.com


   Carlos Pignataro
   North Carolina State University
   United States of America
   Email: cpignata@gmail.com, cmpignat@ncsu.edu


   Mach Chen
   Huawei Technologies
   Email: mach.chen@huawei.com


   Greg Mirsky
   Independent
   Email: gregimirsky@gmail.com








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