BMP Extension for Path Status TLV
draft-ietf-grow-bmp-path-marking-tlv-03

Network Working Group                                         C. Cardona
Internet-Draft                                                P. Lucente
Intended status: Standards Track                                     NTT
Expires: 25 October 2025                                     P. Francois
                                                               INSA-Lyon
                                                                   Y. Gu
                                                                  Huawei
                                                                 T. Graf
                                                                Swisscom
                                                           23 April 2025

                   BMP Extension for Path Status TLV
                draft-ietf-grow-bmp-path-marking-tlv-03

Abstract

   The BGP Monitoring Protocol (BMP) provides an interface for obtaining
   BGP path information, which is is conveyed through BMP Route
   Monitoring (RM) messages.  This document specifies a BMP extension to
   convey the status of a path after being processed by the BGP process.

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 RFC 2119 [RFC2119] RFC 8174 [RFC8174] when, and only when, they
   appear in all capitals, as shown here.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 25 October 2025.

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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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Path Status TLV encoding  . . . . . . . . . . . . . . . . . .   3
   3.  IANA encoding of Path Status TLV  . . . . . . . . . . . . . .   4
     3.1.  IANA path status codes  . . . . . . . . . . . . . . . . .   4
     3.2.  IANA reason codes . . . . . . . . . . . . . . . . . . . .   5
   4.  Implementation notes  . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Configuration of BMP path marking . . . . . . . . . . . .   7
     4.2.  Scalability and churn . . . . . . . . . . . . . . . . . .   7
     4.3.  Paths with no markings  . . . . . . . . . . . . . . . . .   7
     4.4.  Path markings applicability and consistency . . . . . . .   8
       4.4.1.  Significance of status and origin RIBs  . . . . . . .   8
     4.5.  Multiple TLVs assigned to the same route. . . . . . . . .   8
     4.6.  Enterprise-specific status  . . . . . . . . . . . . . . .   8
     4.7.  Multiple reason codes . . . . . . . . . . . . . . . . . .   9
   5.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Multiple paths with different path status, (e.g., the "best path",
   "backup path", "invalid", and so on), may co-exist for a given prefix
   in the BGP RIBs after being processed by the BGP decision process.
   The path status information is not carried in the BGP Update Message
   RFC4271 [RFC4271] or in the BMP Route Monitoring Message RFC7854
   [RFC7854].

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   External systems can use the path status for various applications.
   For example, operators commonly use path status during
   troubleshooting.  Having such status stored and tracked enables the
   development of tools that facilitate this process.  Optimization
   systems can consider path status in their process, e.g., as a
   validation source (since it can compare the calculated state to the
   actual outcome of the network, such as primary and backup path).
   Moreover, path status information can complement other centralized
   sources of data.  For example, flow collectors can leverage it to
   identify the exact forwarding paths, yielding more accurate traffic
   matrices than existing methods.

   This document defines a Path Status TLV to convey the BGP path status
   to a BMP server.  The BMP Path Status TLV is carried in the BMP Route
   Monitoring (RM) Message RFC7854 [RFC7854].

2.  Path Status TLV encoding

   The path status TLV follows the common header defined in
   [I-D.ietf-grow-bmp-tlv] and [I-D.ietf-grow-bmp-tlv-ebit].

      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
     +-------------------------------+-------------------------------+
     |            Common TLV Header (Variable bits)                  |
     +---------------------------------------------------------------+
     |                      Path Status (4 octets)                   |
     +---------------------------------------------------------------+
     | Reason Code (2 oct., opt.)    |
     +-------------------------------+

         Figure 2: Encoding of Path Status TLV

   *  The common TLV header that can encode IANA-registered TLV or
      Enterprise-specific markings using [I-D.ietf-grow-bmp-tlv-ebit].

   *  Path Status (4 Octets): indicates the path status of the BGP
      Update PDU encapsulated in the RM Message.  Path status are
      encoded using a bitmap where each bit position encodes a specific
      role of the path.  Multiple bits may be set when multiple path
      status apply to a path.  All zeros are reserved for paths with no
      marking.

   *  Reason Code (2 Octets, optional): indicates the reason of the path
      status indicated in the Path Status field.  The reason code field
      is optional.  If no reason code is carried, this field is not
      included.  If a reason code is carried, the reason code is
      indicated by a two-byte value.

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3.  IANA encoding of Path Status TLV

3.1.  IANA path status codes

               +============+=============================+
               |   Value    | Path Type                   |
               +============+=============================+
               | 0x00000001 | Invalid                     |
               +------------+-----------------------------+
               | 0x00000002 | Best                        |
               +------------+-----------------------------+
               | 0x00000004 | Nonselected                 |
               +------------+-----------------------------+
               | 0x00000008 | Primary                     |
               +------------+-----------------------------+
               | 0x00000010 | Backup                      |
               +------------+-----------------------------+
               | 0x00000020 | Non-installed               |
               +------------+-----------------------------+
               | 0x00000040 | Best-external               |
               +------------+-----------------------------+
               | 0x00000080 | Add-Path                    |
               +------------+-----------------------------+
               | 0x00000100 | Filtered in inbound policy  |
               +------------+-----------------------------+
               | 0x00000200 | Filtered in outbound policy |
               +------------+-----------------------------+
               | 0x00000400 | Stale                       |
               +------------+-----------------------------+
               | 0x00000800 | Suppressed                  |
               +------------+-----------------------------+

                   Table 1: IANA-Registered Path Status

   Table 1 includes a list of IANA status codes.  This list might be
   extended.  An explanation of each of the types is included next:

   *  An invalid route is a route that does not enter the BGP decision
      process as indicated in Section 9.1.2 of RFC4271 [RFC4271].

   *  The best route is defined in Section 9.1 of RFC4271 [RFC4271].

   *  Nonselected routes are those that are not selected in the BGP
      decision process.  Backup routes are considered nonselected, while
      the best and primary routes are not considered as nonselected.

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   *  A primary route is a path used for traffic forwarding.  See
      draft-ietf-rtgwg-bgp-pic [I-D.ietf-rtgwg-bgp-pic].  A prefix can
      have more than one primary path when multipath is configured
      draft-lapukhov-bgp-ecmp-considerations
      [I-D.lapukhov-bgp-ecmp-considerations].  The best path is also a
      primary path.

   *  A backup path is installed in the RIB, but it is not used until
      some or all primary paths become unreachable.  Backup paths are
      used for fast convergence in the event of primary path failures.

   *  A non-installed path refers to the route that is not installed
      into the IP routing table.

   *  The best external path is defined in draft-ietf-idr-best-external
      [I-D.ietf-idr-best-external].

   *  The advertisement of multiple paths for the same address prefix
      without the new paths implicitly replacing any previous ones, the
      add-path status is applied [RFC7911].

   *  Filtered in inbound policy routes are those that are filtered in
      the Adj-RIB-In policy

   *  Filtered in outbound policy routes are those that are filtered in
      the Adj-RIB-Out policy

   *  Stale routes refer to paths which have been declared stale by the
      BGP Graceful Restart mechanism, as described in Section 4.1 of
      [RFC4724].

   *  Suppressed routes refer to a path which has been declared
      suppressed by the BGP Route Flap Damping mechanism as described in
      Section 2.2 of [RFC2439].

3.2.  IANA reason codes

   Table 2 includes a list of IANA reason codes.  This list can be
   extended in future documents.  This document includes a brief
   explanation of each code and the path status they are directed to
   explain.  Please see Section 4.4 for notes on potentially
   inconsistencies on the path marking data

   *  Invalid routes due to AS loop and unresolvable nexthop are defined
      in Section 9.1.2 of [RFC4271].  These codes target routes of type
      "Invalid".

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   *  The reason codes starting with 'not preferred' are aimed at paths
      not selected as best, and describe the reason they were ranked
      lower in the decision process.  AIGP is explained in RFC7311
      [RFC7311].  The rest of the codes are described in
      Section 9.1.2.2. of [RFC4271]

             +========+=====================================+
             | Value  | Reason Code                         |
             +========+=====================================+
             | 0x0001 | Invalid due to AS loop              |
             +--------+-------------------------------------+
             | 0x0002 | Invalid due to unresolvable nexthop |
             +--------+-------------------------------------+
             | 0x0003 | Not preferred for local preference  |
             +--------+-------------------------------------+
             | 0x0004 | Not preferred for AS Path Length    |
             +--------+-------------------------------------+
             | 0x0005 | Not preferred for origin            |
             +--------+-------------------------------------+
             | 0x0006 | Not preferred for MED               |
             +--------+-------------------------------------+
             | 0x0007 | Not preferred for peer type         |
             +--------+-------------------------------------+
             | 0x0008 | Not preferred for IGP cost          |
             +--------+-------------------------------------+
             | 0x0009 | Not preferred for router ID         |
             +--------+-------------------------------------+
             | 0x000A | Not preferred for peer address      |
             +--------+-------------------------------------+
             | 0x000B | Not preferred for AIGP              |
             +--------+-------------------------------------+

                  Table 2: IANA-Registered Reason Codes

4.  Implementation notes

   The BMP path marking TLV remains optional within BMP implementations.

   An implementation of the BMP path marking TLV may not fully support
   marking of all status defined in Table 1 or any future extensions.
   Similarly, an implementation may choose to support the inclusion of
   the reason code (for which support is also optional), without
   necessarily incorporating any of the reason codes defined in Table 2
   or future extensions.

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   This document refrains from defining mechanisms for signaling the
   status or reason codes an implementation supports.  This could be
   established through external means (e.g. documentation) or
   potentially addressed in a subsequent document.

   The remainder of this section covers additional points related to the
   implementation of the BMP Path marking TLV.

4.1.  Configuration of BMP path marking

   Implementations supporting the BMP path marking TLV should provide an
   option for enabling/disabling the Path Marking TLV over BMP
   monitoring sessions.  Furthermore, the configuration options for this
   TLV should provide the means to enable/disable the transmission of
   reason codes, if the reason codes are supported by the
   implementation.

4.2.  Scalability and churn

   The Path Marking TLV introduces metadata on the routes, which could
   increase the churn (Section 8.1.6 of RFC4098 [RFC4098]) of paths
   within the BMP session.  For instance, if path marking is configured,
   and a non-installed path changes status to a backup route, the device
   should send an update about this path with the new markings, even if
   its BGP attributes remain unchanged.  Enabling reason codes could
   additionally increase the churn.  Churn could be more pronounced
   during the start of a BGP session, where the device is processing all
   available routes.

   If churn is undesired, an implementation could make use of "state
   compression" to hide state until paths converge (Section 5 of
   [RFC7854]).  It could also initially send BMP routes without the path
   marking TLV, even if it were configured, and then add them once the
   implementation considers the path to be stable enough.  This document
   does not provide a definitive solution for churn since it depends on
   the capabilities of an implementation and the requirements of an
   operator.

4.3.  Paths with no markings

   Some BGP routes might not require any type of status or reasons.  For
   example, a path in Adj-RIB-In where the BGP best path decision has
   not been applied yet, falls under this category, since there is
   really nothing to mark for that path.  This document suggests
   applying an explicit marking of this route, by attaching a BMP path
   marking TLV with no bits set.  This will help BMP monitor stations to
   differentiate this case from those in which markings are not
   configured, or not yet attached by the device.

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4.4.  Path markings applicability and consistency

   The status and reason codes from Table 1 and Table 2 are included
   based on use cases from network operators and defined following the
   most relevant protocol references available.  While implementations
   are strongly encouraged to align with these code definitions, this
   document does not enforce strict validity rules for code combinations
   to accommodate the diversity of BGP implementations.

   The experience during testing of this TLV revealed scenarios where
   implementations might combine codes differently than originally
   anticipated.  For example, one test implementation marked routes with
   both 'Invalid' and 'Best' status bits set, which is contradictory
   from the point of view of [RFC4271], but made sense for their
   specific implementation.

   Operators should apply their own validation checks on the data from
   TLVs and discuss potential inconsistencies with their vendors, and
   raise bugs if applicable.

4.4.1.  Significance of status and origin RIBs

   This document refrains from imposing on any implementation the
   requirement to mark specific status from specific RIBs.  Some
   implementations might be able to mark some status over one RIB while
   others do it on others.  For instance, some might be able to mark
   Adj-RIB-In filtered routes when obtained from the Adj-RIB-In pre,
   while other could do it only from the Adj-RIB-In post.  To remove
   ambiguities in implementations, it is recommended that the meaning of
   status (and reason codes) does not depend on the origin RIB of a
   route.

4.5.  Multiple TLVs assigned to the same route.

   We advocate for the use of TLV grouping wherever feasible
   (Section 5.2.1. of [I-D.ietf-grow-bmp-tlv]).  The inclusion of all
   marking information within a single message is recommended.  In
   situations where multiple TLVs are associated with a single route,
   all markings and reasons will be applicable to that route.

4.6.  Enterprise-specific status

   Implementations introducing their own status and reason codes are
   advised to adhere to [I-D.ietf-grow-bmp-tlv-ebit] and use the
   enterprise-bit (ebit) for vendor-specific status and reasons.

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   For scenarios where a path state combines a standard status with an
   enterprise-specific reason code (or vice versa), the following
   alternatives are presented:

   *  Replication of the standard definitions within the enterprise-
      specific space, thus permitting direct marking within the same
      packet using the ebit.

   *  Assigning two TLVs to the same path(s): one containing the
      standard part and another housing the vendor-specific part.

4.7.  Multiple reason codes

   The path marking TLV was not designed to optimally hold more than one
   reason code per path.  However, if needed by a specific use case, the
   implementation can use two or more path markings TLVs for the same
   path listing the multiple reasons that apply to it.

5.  Acknowledgments

   We would like to thank Jeff Haas and Maxence Younsi for their
   valuable comments.

6.  IANA Considerations

   This document requests that IANA assign the following new parameters
   to the BMP parameters name space.

   Type = TBD1 (15 Bits): indicates that it is the IANA-registered Path
   Status TLV.

7.  Security Considerations

   Using the path status information may affect other applications which
   rely on this information for operational decisions.  Operators should
   secure BMP sessions and control access to TLV data to mitigate these
   risks.

8.  References

8.1.  Normative References

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   [I-D.ietf-grow-bmp-tlv]
              Lucente, P. and Y. Gu, "BMP v4: TLV Support for BGP
              Monitoring Prtoocol (BMP) Route Monitoring and Peer Down
              Messages", Work in Progress, Internet-Draft, draft-ietf-
              grow-bmp-tlv-16, 24 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-grow-
              bmp-tlv-16>.

   [I-D.ietf-grow-bmp-tlv-ebit]
              Lucente, P. and Y. Gu, "Support for Enterprise-specific
              TLVs in the BGP Monitoring Protocol", Work in Progress,
              Internet-Draft, draft-ietf-grow-bmp-tlv-ebit-06, 17
              January 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-grow-bmp-tlv-ebit-06>.

   [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>.

   [RFC2439]  Villamizar, C., Chandra, R., and R. Govindan, "BGP Route
              Flap Damping", RFC 2439, DOI 10.17487/RFC2439, November
              1998, <https://www.rfc-editor.org/info/rfc2439>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              DOI 10.17487/RFC4724, January 2007,
              <https://www.rfc-editor.org/info/rfc4724>.

   [RFC7311]  Mohapatra, P., Fernando, R., Rosen, E., and J. Uttaro,
              "The Accumulated IGP Metric Attribute for BGP", RFC 7311,
              DOI 10.17487/RFC7311, August 2014,
              <https://www.rfc-editor.org/info/rfc7311>.

   [RFC7854]  Scudder, J., Ed., Fernando, R., and S. Stuart, "BGP
              Monitoring Protocol (BMP)", RFC 7854,
              DOI 10.17487/RFC7854, June 2016,
              <https://www.rfc-editor.org/info/rfc7854>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

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   [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>.

8.2.  Informative References

   [I-D.ietf-idr-best-external]
              Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
              Gredler, "Advertisement of the best external route in
              BGP", Work in Progress, Internet-Draft, draft-ietf-idr-
              best-external-05, 3 January 2012,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              best-external-05>.

   [I-D.ietf-rtgwg-bgp-pic]
              Bashandy, A., Filsfils, C., and P. Mohapatra, "BGP Prefix
              Independent Convergence", Work in Progress, Internet-
              Draft, draft-ietf-rtgwg-bgp-pic-22, 20 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
              bgp-pic-22>.

   [I-D.lapukhov-bgp-ecmp-considerations]
              Lapukhov, P. and J. Tantsura, "Equal-Cost Multipath
              Considerations for BGP", Work in Progress, Internet-Draft,
              draft-lapukhov-bgp-ecmp-considerations-13, 6 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-lapukhov-bgp-
              ecmp-considerations-13>.

   [RFC4098]  Berkowitz, H., Davies, E., Ed., Hares, S., Krishnaswamy,
              P., and M. Lepp, "Terminology for Benchmarking BGP Device
              Convergence in the Control Plane", RFC 4098,
              DOI 10.17487/RFC4098, June 2005,
              <https://www.rfc-editor.org/info/rfc4098>.

Authors' Addresses

   Camilo Cardona
   NTT
   164-168, Carrer de Numancia
   08029 Barcelona
   Spain
   Email: camilo@ntt.net

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   Paolo Lucente
   NTT
   Siriusdreef 70-72
   2132 Hoofddorp
   Netherlands
   Email: paolo@ntt.net

   Pierre Francois
   INSA-Lyon
   Lyon
   France
   Email: Pierre.Francois@insa-lyon.fr

   Yunan Gu
   Huawei
   Huawei Bld., No.156 Beiqing Rd.
   Beijing
   100095
   China
   Email: guyunan@huawei.com

   Thomas Graf
   Swisscom
   Binzring 17
   CH-8045 Zurich
   Switzerland
   Email: thomas.graf@swisscom.com

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