



Network Work group                                              N. Kumar
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                            C. Pignataro
Expires: 22 May 2026                     North Carolina State University
                                                                 M. Chen
                                                     Huawei Technologies
                                                               G. Mirsky
                                                                Ericsson
                                                        18 November 2025


                          BIER Ping and Trace
                        draft-ietf-bier-ping-17

Abstract

   Bit Index Explicit Replication (BIER) is an 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 expressed by setting the bits that
   correspond to those routers in the BIER header.

   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.

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."




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   This Internet-Draft will expire on 22 May 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  BIER OAM  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  BIER OAM Message Format . . . . . . . . . . . . . . . . .   4
     3.2.  BIER Echo Request/Reply Message Format  . . . . . . . . .   6
     3.3.  Return Code . . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  BIER OAM TLV  . . . . . . . . . . . . . . . . . . . . . .   9
       3.4.1.  Original SI-BitString TLV . . . . . . . . . . . . . .  10
       3.4.2.  Target SI-BitString TLV . . . . . . . . . . . . . . .  10
       3.4.3.  Incoming SI-BitString TLV . . . . . . . . . . . . . .  11
       3.4.4.  Downstream Mapping TLV  . . . . . . . . . . . . . . .  12
       3.4.5.  Responder BFER TLV  . . . . . . . . . . . . . . . . .  16
       3.4.6.  Responder BFR TLV . . . . . . . . . . . . . . . . . .  17
       3.4.7.  Upstream Interface TLV  . . . . . . . . . . . . . . .  17
     3.5.  Multipath Entropy Data Encoding . . . . . . . . . . . . .  18
   4.  BIER Ping and Traceroute Operations . . . . . . . . . . . . .  18
     4.1.  BIER OAM Processing . . . . . . . . . . . . . . . . . . .  18
     4.2.  Per BFER ECMP Discovery . . . . . . . . . . . . . . . . .  19
     4.3.  Sending BIER Echo Request . . . . . . . . . . . . . . . .  19
     4.4.  Receiving BIER Echo Request . . . . . . . . . . . . . . .  20
     4.5.  Sending Echo Reply  . . . . . . . . . . . . . . . . . . .  22
     4.6.  Receiving Echo Reply  . . . . . . . . . . . . . . . . . .  23
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     5.1.  UDP Port Number . . . . . . . . . . . . . . . . . . . . .  23
     5.2.  BIER OAM Registry Group . . . . . . . . . . . . . . . . .  24
     5.3.  BIER OAM Message Type . . . . . . . . . . . . . . . . . .  24
     5.4.  BIER Echo Request/Echo Reply Registries . . . . . . . . .  25
       5.4.1.  BIER Echo Request/Echo Reply Message Types  . . . . .  25



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       5.4.2.  BIER Echo Reply Modes . . . . . . . . . . . . . . . .  26
       5.4.3.  BIER Echo Return Codes  . . . . . . . . . . . . . . .  27
     5.5.  Common Registration Procedures for TLVs and Sub-TLVs  . .  28
       5.5.1.  TLVs  . . . . . . . . . . . . . . . . . . . . . . . .  29
       5.5.2.  Sub-TLVs for TLV Type 4 . . . . . . . . . . . . . . .  30
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
   7.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  31
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  33
   Contributors' Addresses . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

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 expressed by setting the bits that
   correspond to those routers in the BIER header.

   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-4, and R-7 requirements
   listed in [I-D.ietf-bier-oam-requirements].

2.  Conventions used in this document

2.1.  Terminology

   BFER - Bit-Forwarding Egress Router

   BFIR - Bit-Forwarding Ingress Router



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   BFR - Bit-Forwarding Router

   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

   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.  [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 packet header format that follows the BIER header is
   displayed in Figure 1.



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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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Ver  | Message Type  |   Proto   |         Reserved          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        OAM Message Length                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                  Message Type Dependent Data                  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: BIER OAM Header

      Ver - a four-bit field that indicates the current version of the
      BIER OAM header.  The current value 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.

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

                   Value      Description
                   --------  ---------------
                     1        Echo Request
                     2        Echo Reply

      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 there is no data packet following the OAM payload.
      Value is one from the IANA registry "BIER Next Protocol
      Identifiers" [RFC8296].

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

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

      Message Type Dependant Data - a variable-length field that
      includes the OAM message identified by the value of the Message
      Type filed.





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3.2.  BIER Echo Request/Reply Message Format

   The Echo Request/Reply header format 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 Req/Rep  |   Proto   |         Reserved          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Echo Request/Reply Length                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   QTF |   RTF |   Reply Mode  |  Return Code  |    Reserved   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sender's Handle                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Sequence Number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                    Timestamp Sent                             |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                  Timestamp Received                           |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              TLVs                             ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: BIER Echo Request/Reply Format

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

      QTF (Querier Timestamp Format) - a four-bit field.  When the field
      is set to 2, the Timestamp Sent field is (in seconds and
      microseconds, 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's format is the IEEE 1588-2008 (1588v2)
      Precision Time Protocol (PTP) [IEEE.1588.2008] format.  Any other
      value MAY be considered as a sanity check failure.

      RTF (Responder Timestamp Format) - a four-bit field.  When the
      field is set to 2, the Timestamp Received field is (in seconds and
      microseconds, according to the Initiator's clock) in 64-bit long
      NTP format [RFC5905].  When filed's value is 3, the format of the
      Timestamp Received is as defined in IEEE 1588-2008 (1588v2)
      Precision Time Protocol [IEEE.1588.2008].  Any other value MAY be
      considered as a sanity check failure.






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   The sender of the BIER Echo Request might receive the BIER Echo Reply
   with RTF different from the Sender's QTF, Thus, 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.

      The Reply Mode - a one-octet field.  The value MUST be set to one
      of the following values:

                   Value      Description
                   --------  ---------------
                     1        Do not Reply
                     2        Reply via IPv4/IPv6 UDP packet
                     3        Reply via BIER packet

      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.
      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 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 value of the Return Code
      filed MUST be set to one of the values defined in Section 3.3, if
      the Type is "BIER Echo Reply".

      Reserved - a one-octet field.  The Reserved 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.

      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 following:

           Value      Value Meaning
           ------    ---------------
            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

   "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.

   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 Bit-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 Bit-ID in addition to other Bit-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.



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   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 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 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 DDMAP TLV, a responding BFR will use "DDMAP
   Mismatch" to report the problem.

3.4.  BIER OAM TLV

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






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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].

   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.  This
   TLV MUST be included by the Initiator in the Echo Request packet.

   Any Initiator MUST include this TLV in the Echo Request 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:






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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 = 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.

   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.  This TLV MUST be included
   by Initiator in the BIER OAM packet if the Downstream Mapping TLV
   (Section 3.4.4) is included.

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:











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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 = 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
   Downstream Mapping TLV.

   An Initiator MUST NOT include this TLV in Echo Request.

3.4.4.  Downstream Mapping TLV

   This TLV has 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 = 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 Mapping TLV

   MTU  A two-octet field.  The value is the Maximum Transmission Unit
      (MTU) value of the egress interface.

   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 following values:

               Value     Address Type
              -------    ---------------
                  1       IPv4 Numbered
                  2       IPv4 Unnumbered
                  3       IPv6 Numbered
                  4       IPv6 Unnumbered

   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.





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   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 and Donstream
      Address Interface fileds as follows:

                  Value     DA Length     DIA Length
                 -------   ------------  ------------
                     1           4           4
                     2           4           4
                     3          16          16
                     4          16           4

      where:

      DA Length
         Downstream Address (DA) field Length

      DIA Length
         Downstream Interface Address (DIA) field Length

      If the Address Type is 1, the Downstream Address 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 2, the Downstream Address MUST be set to
      IPV4 BFR-Prefix of downstream BFR and Downstream Interface Address
      is set to the index assigned by upstream BFR to the interface.

      If the Address Type is 3, the Downstream Address MUST be set to
      IPV6 BFR-Prefix of downstream BFR and Downstream Interface Address
      is set to the downstream interface address.

      If the Address Type is 4, the Downstream Address MUST be set to
      IPv6 BFR-Prefix of downstream BFR and Downstream Interface Address
      is set to the index assigned by upstream BFR to the interface.

3.4.4.1.  Downstream Detailed Mapping Sub-TLVs

   This section defines the optional Sub-TLVs that can be included in
   Downstream Mapping TLV.





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                   Sub-TLV Type     Value
                   ------------  -------------
                       1         Multipath Entropy Data
                       2         Egress BitString

3.4.4.1.1.  Multipath Entropy Data

       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 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
      through the interface defined in the Downstream Mapping 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.

  
      Multipath Information Entropy Data is encoded as defined in
      Section 3.5.

3.4.4.1.2.  Egress BitString Sub-TLV

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














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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 = 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 outgoing
   interface as defined in Section 6.1 of [RFC8279].

3.4.5.  Responder BFER TLV

   The BFER replying to the request MAY include the Responder BFER TLV.
   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.



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   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.  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 BFD 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.

   Address Type  A two-octet field.  Set to 1 for IPv4 or 2 for IPv6.

   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.

3.4.7.  Upstream Interface TLV

   The BFR replying to the request MUST include the Upstream Interface
   TLV.  This TLV identifies the incoming interface and the BIER-MPLS
   label in the incoming Echo Request.  This TLV has 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TLV Type = 7              |        Length = 8 or 20       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         Reserved              |          Address Type         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                 Upstream Address (4 or 16 bytes)              ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 13: The Upstream 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.

   Address Type  A two-octet field.  Set to 1 for IPv4 numbered, 2 for
      IPv4 Unnumbered, 3 for IPv6 numbered, or 4 for IPv6 Unnumbered.

   Upstream Address
      As defined in Section 3.4.4

3.5.  Multipath Entropy Data Encoding

   The size of the Entropy field in the BIER header is 20 bits, as
   defined in Section 2 of [RFC8296].  This encoding is the same as the
   Multipath Type 9 encoding, defined in Section 3.4.1.1.1 of [RFC8029].

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 sent to the BIER control plane for OAM
   processing if one of the following conditions is true:

   *  The receiving BFR is a BFER.

   *  TTL of BIER-MPLS Label (Section 2.1.1.1 [RFC8296]) expired.

   The use of the Router Alert label to be deprecated as proposed in
   [RFC9570].



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4.2.  Per BFER ECMP Discovery

   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.

   The Initiator MUST include Multipath Entropy Data Sub-TLV in
   Downstream Mapping TLV.  It MUST also include the BFER in BitString
   TLV to which the Multipath query is performed.

   Any transit BFR will reply 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 SHOULD increment the Sequence Number by 1 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.

   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" SHOULD unset the respective BFR-id from Target SI-
   BitString for any subsequent Echo Request.



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   The Initiator MAY include Downstream Mapping TLV (Section 3.4.4) in
   the Echo Request to query additional information from transit BFRs
   and BFERs.  In case of ECMP discovery, the Initiator MUST include the
   Multipath Entropy Data Sub-TLV and SHOULD 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 and further encapsulates with BIER-MPLS
   label.  In ping mode, the BIER-MPLS Label TTL MUST be set to 255.  In
   traceroute mode, the BIER-MPLS Label TTL 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.

   Any BFR on receiving Echo Request MUST perform the basic sanity
   check.  If the BFR cannot parse the OAM Dependent data payload
   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.  If the packet sanity check is fine, it SHOULD 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 Upstream Interface TLV.

   BIER-Label-L
      The BIER-MPLS Label received as the top label of the received 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
      Multipath Entropy Data Sub-TLV.

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



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   BFR will populate the Interface-I with the identifier of the
   interface over which the Echo Request is received, the top label in
   the MPLS stack of the received Echo Request to BIER-Label-L, and the
   BIER header to BIER-Header.  If the received Echo Request carries
   Target SI-BitString TLV, a BFR SHOULD run the boolean AND operation
   between BitString in 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 BIER-Label-L does not correspond to the local label
      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.

   This step 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.

   *  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.

   *  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.

   This step 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 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
      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.



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   *  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 Downstream Mapping 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.

   This step 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
      Downstream Mapping 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 SHOULD 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 BIER-Label-L.  Responder BFR SHOULD
   include the Upstream 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 outgoing interface over which the packet
      will be replicated and include the respective Multipath Entropy
      Data Sub-TLV.  For each outgoing interface, the respective Egress
      BitString MUST be included in DDMAP TLV.

      If the received Echo Request had DDMAP without Multipath Entropy



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      Data Sub-TLV, Responder BFR MUST include DDMAP as defined in
      Section 3.4.4 for each outgoing interface over which the packet
      will be replicated.  For each outgoing 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
   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 is any local address of the responder, and
   the destination IP is derived from the BFIR-id of the BIER header
   [RFC8296] of 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 Downstream Mapping, the Initiator SHOULD
   copy the same 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.



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   Service Name BIER Echo Port

   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

5.2.  BIER OAM Registry Group

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

5.3.  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 - Reserved

      1-31 - IETF Review

      32-62 - First Come, First Served

      63 - Reserved















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         +========+==============================+===============+
         | Value  |         Description          | Reference     |
         +========+==============================+===============+
         | 0      |           Reserved           | This document |
         +--------+------------------------------+---------------+
         | 1      | BIER Echo Request/Echo Reply | This document |
         +--------+------------------------------+---------------+
         | 2 - 31 |          Unassigned          | This document |
         +--------+------------------------------+---------------+
         | 32-62  |          Unassigned          | This document |
         +--------+------------------------------+---------------+
         | 63     |           Reserved           | This document |
         +--------+------------------------------+---------------+

                       Table 1: BIER OAM Message Type

5.4.  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.

5.4.1.  BIER Echo Request/Echo Reply Message Types

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

      Registry Name: BIER Echo Request/Echo Reply Message Types

      Assignment Policy:

      0 - Reserved

      1 - 175 - IETF Review

      176 - 239 - First Come, First Served

      240-251 - Experimental

      252-254 - Private Use

      255 - Reserved










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             +===========+===================+===============+
             | 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 - 239 |     Unassigned    | This document |
             +-----------+-------------------+---------------+
             | 240 - 251 |    Experimental   | This document |
             +-----------+-------------------+---------------+
             | 252 - 254 |    Private Use    | This document |
             +-----------+-------------------+---------------+
             | 255       |      Reserved     | This document |
             +-----------+-------------------+---------------+

               Table 2: BIER Echo Request/Echo Reply Message
                                   Types

5.4.2.  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:

      0 - Reserved

      1 - 175 - IETF Review

      176 - 239 - First Come, First Served

      240 - 251 - Experimental

      252 - 254 - Private Use

      255 - Reserved








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     +===========+===================================+===============+
     | 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 BIER packet       | This document |
     +-----------+-----------------------------------+---------------+
     | 4 - 175   |             Unassigned            | This document |
     +-----------+-----------------------------------+---------------+
     | 176 - 239 |             Unassigned            | This document |
     +-----------+-----------------------------------+---------------+
     | 240 - 251 |            Experiemntal           | This document |
     +-----------+-----------------------------------+---------------+
     | 252 - 254 |            Private Use            | This document |
     +-----------+-----------------------------------+---------------+
     | 255       |              Reserved             | This document |
     +-----------+-----------------------------------+---------------+

                       Table 3: BIER Echo Reply Modes

5.4.3.  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 - Reserved

      1 - 191 IETF Review

      192 - 251 - First Come, First Served

      252 - 254 - Private Use

      255 - Reserved









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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 |
       | 191     |                                 |               |
       +---------+---------------------------------+---------------+
       | 192-251 |            Unassigned           | This document |
       +---------+---------------------------------+---------------+
       | 252-254 |           Private Use           | This document |
       +---------+---------------------------------+---------------+
       | 255     |             Reserved            | This document |
       +---------+---------------------------------+---------------+

                      Table 4: BIER Echo Return Codes

5.5.  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    | his range is for TLVs and sub- |
      |             | Action       | TLVs that can be silently      |
      |             |              | dropped if not recognized.     |
      +-------------+--------------+--------------------------------+
      | 49162-64507 | RFC Required | his 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,  | his range is for TLVs and sub- |
      |             | First Served | TLVs that can be silently      |
      |             |              | dropped if not recognized.     |
      +-------------+--------------+--------------------------------+

                               Table 5: TLVs

5.5.1.  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 5.5



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   The TLVs requested by this document for the IANA consideration are
   listed in Table 6.

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

                               Table 6: TLVs

5.5.2.  Sub-TLVs for TLV Type 4

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

      Registry Name: Sub-TLVs for Type 4

      Assignment Policy: Section 5.5











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

                               Table 7: TLVs

6.  Security Considerations

   The security consideration for BIER Ping is similar to ICMP [RFC0792]
   or 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.  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.

8.  References

8.1.  Normative References





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   [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>.

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




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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>.

   [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-20, 3 November 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-bier-
              oam-requirements-20>.

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
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC 27709
   United States of America
   Email: naikumar@cisco.com



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   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
   Ericsson
   Email: gregimirsky@gmail.com





































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