BMP Local Path-ID
draft-younsi-grow-local-path-id-03

Network Working Group                                          M. Younsi
Internet-Draft                                               P. Francois
Intended status: Standards Track                               INSA-Lyon
Expires: 4 January 2025                                       P. Lucente
                                                                     NTT
                                                             3 July 2024

                           BMP Local Path-ID
                   draft-younsi-grow-local-path-id-03

Abstract

   Intelligence is required to track BGP paths throughout the various
   RIBs and VRFs of a routing platform, due to potential attribute
   modifications and the use of BGP multipath.  This document introduces
   the option to identify a path within a router in order to ease
   correlation in monitoring.  A BMPv4 TLV is defined in order to
   communicate this locally significant identifier in monitoring
   messages.

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.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 4 January 2025.

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Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Local Path ID . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Local Path ID Properties  . . . . . . . . . . . . . . . .   3
     2.2.  Design Recommendation . . . . . . . . . . . . . . . . . .   4
   3.  Advertising the Local Path ID in BMP  . . . . . . . . . . . .   4
     3.1.  Local Path ID TLV . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Local Path ID TLV Sub-Types . . . . . . . . . . . . . . .   5
       3.2.1.  Local Path ID Value . . . . . . . . . . . . . . . . .   6
       3.2.2.  Unavailability Reason Codes . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   When using VRFs and/or BGP Multipath, multiple paths to the same
   destination may be shared among various routing information bases.
   From a collection perspective, tracking the identity of a path thus
   requires some form of modeling, which is subject to inaccuracy.  This
   aspect is exacerbated as path attributes may be modified in the
   process.  This is especially problematic when a PE using BGP
   multipath in VPN instances exports multiple paths for the same
   destination into the default VRF, which were learned from different
   peers.

   While BGP ADD-PATH [RFC7911] provides a way to identify paths in BGP
   multi-path scenarios, the scope of the ADD-PATH path-id is local to a
   single BGP peering session, and thus cannot be used to distinguish
   paths received over multiple sessions.

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   This document introduces a way to identify paths globally within a
   router, allowing operators to not resort to modeling when monitoring
   BGP paths on a router.  In Section 2, we introduce the concept of
   Local Path ID, which is an identifier of a path for a given NLRI,
   preserved through the import/export operations performed onto them.
   In Section 3, we introduce a BMPv4 TLV allowing to communicate the
   value of a Local Path ID on a BMP session.

2.  Local Path ID

   In this section, we define an identifier called Local Path ID, which
   allows to uniquely identify a path for a given NLRI on a router.

   According to this specification, a path to be advertised by BMP is
   provided with an associated Local Path ID.  The Local Path ID is an
   opaque numerical value with a few properties guaranteeing its
   utility.  The exact approach to generate a Local Path ID is however
   left for the implementation.

2.1.  Local Path ID Properties

   The Local Path ID of each path MUST be unique for a given NLRI.  We
   scope the identifier space to each NLRI to keep it a small value.
   Indeed, most Internet routers have at most a few tens of paths for a
   given NLRI.  While we put a minimum scope (the NLRI) for the
   identifier space, an implementor may decide to use a broader space
   for this unicity, as long as Local Path IDs are still unique for a
   given NLRI.  For example, Local Path IDs can be unique accross VRFs,
   even though they will have to be larger, as this does not violate the
   rule.

   The Local Path ID only has a meaning local to the router generating
   it.

   Once generated, the Local Path ID MUST be preserved between VRFs, and
   Routing Information Bases.  It is however not intended to be
   exchanged or synchronized between routers.

   To ensure traceability in monitoring, the Local Path ID SHOULD be
   transmitted when paths are redistributed between processes.  If the
   Local Path ID is not transmitted then the process receiving the path
   SHOULD allocate one.

   The value of 0 for a Local Path ID is reserved.

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2.2.  Design Recommendation

   In this section, we give general recommendations for the Local Path
   ID generation.  These recommendations may or may not be applicable
   depending on the platform, the implementation of BGP, etc.  The
   actual generation process of the Local Path ID does not matter as
   long as the the properties defined in Section 2.1 are respected.

   We recommend having the Local Path ID made of three concatenated
   parts: < process_id | vrf_id | path_discriminator >.

   The path_discriminator allows for differentiation between paths for a
   given NLRI, coming from the same table and process (with the same
   vrf_id and process_id).  The process originating the path is in
   charge of guaranteeing the uniqueness of the path_discriminator it
   produces for each path of its NLRIs.

   The vrf_id represents a unique identifier for the VRF in which the
   path to the NLRI is contained.  It allows leveraging the already
   existing routing table structures of most BGP implementations by
   having to guarantee the uniqueness of the path_discriminator only
   within the table.

   The process_id is the identifier of the process which produced,
   originated, or received a path.  The process_id allows
   differentiation between path IDs generated in BGP from path IDs
   generated in other processes like an IGP.  Redistributed IGP paths
   will then have a different Local Path ID no matter if BGP or another
   IGP has chosen the same path_discriminator value.  Using the
   process_id avoids requiring interprocess synchronization of
   path_discriminators or the use of a Local Path ID management process.

3.  Advertising the Local Path ID in BMP

   The Local Path ID is to be included in BMPv4 Route Monitoring
   messages [I-D.ietf-grow-bmp-tlv] as a BMPv4 TLV, called "Local Path
   ID TLV".  This TLV can carry multiple types of data which are
   discriminated by the "TLV Sub-Type" field.  The "TLV Sub-Type" field
   can take the values given in Table 1.  When a Local Path ID is
   allocated for a path, the router includes it using the "TLV Sub-Type"
   value 0x00 (Local Path ID Value).  When a Local Path ID is not
   allocated, the router includes a "TLV Sub-Type" value 0x01
   (Unavailability Reason Code) that describes why the Local Path ID is
   not available for the path.  In practice, this means that a Local
   Path ID enabled router SHOULD always associate a Path ID TLV with a
   path advertised in BMP.

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   On the collector side, a path should be identified using the Local
   Path ID Value when provided, and only resort to network modeling when
   the Local Path ID is not available.

3.1.  Local Path ID TLV

   The encoding of the "Local Path ID TLV" is illustrated in Figure 1.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Type (2 octets)        |       Length (2 octets)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Index (2 octets)       |   Sub-Type    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                        Value (variable)                       ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                  Figure 1

   Type:  set to TBD1

   Length:  the length of the "TLV Sub-Type" field + the length of the
      "Value" field, in bytes

   Index:  index of the NLRI in the BGP Update PDU as described by
      [I-D.ietf-grow-bmp-tlv].  The Index MUST refer to a single NLRI
      (no Group TLV).

   Sub-Type:  a one byte TLV Sub-Type, as listed in Table 1

   Value:  the value corresponding to the TLV Sub-Type as described in
      Section 3.2

3.2.  Local Path ID TLV Sub-Types

   The Table 1 defines the list of TLV Sub-Types and references the
   section defining their associated values.

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     +======+============================+==========+===============+
     | Code | Name                       | Length   | Section       |
     +======+============================+==========+===============+
     | 0x00 | Local Path ID Value        | Variable | Section 3.2.1 |
     +------+----------------------------+----------+---------------+
     | 0x01 | Unavailability Reason Code | 2 bytes  | Section 3.2.2 |
     +------+----------------------------+----------+---------------+

                   Table 1: Local Path ID TLV Sub-Types

3.2.1.  Local Path ID Value

   When the Local Path ID is available for a path, the router exports
   the Local Path ID using a "Local Path ID TLV" with "TLV Sub-Type" =
   0x00.

   The Local Path ID Value contains the raw bytes of the generated Local
   Path ID (Section 2).  The Length field is thus set to the length, in
   bytes, of the Local Path ID, plus one (for the TLV Sub-Type field).

3.2.2.  Unavailability Reason Codes

   An implementation enabled for Local Path ID usage MUST notify if a
   Local Path ID is unavailable (for any reason) by using the "TLV Sub-
   Type" = 0x01.

   When "TLV Sub-Type" is 0x01, the TLV Value is a 2-byte error code
   from Table 2.  The Length field of the "Local Path ID TLV" is thus
   set to 3 in this case.

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      +======+================+=====================================+
      | Code | Reason         | Description                         |
      +======+================+=====================================+
      | 0x00 | Unknown Reason | A unknown error occurred during the |
      |      |                | allocation of the Local Path ID     |
      +------+----------------+-------------------------------------+
      | 0x01 | Origin process | A path is imported from another     |
      |      | did not        | process, the latter does not        |
      |      | provide a      | provide a Local Path ID for the     |
      |      | Local Path ID. | imported path and the receiving     |
      |      |                | process did not allocate one either |
      +------+----------------+-------------------------------------+
      | 0x02 | All Local Path | A Local Path ID could not be        |
      |      | ID have been   | allocated for a path because the    |
      |      | allocated.     | entire range of Local Path IDs is   |
      |      |                | in use                              |
      +------+----------------+-------------------------------------+

             Table 2: Local Path ID Unavailability Reason Codes

4.  IANA Considerations

   This document requests that IANA assigns the following new parameters
   to the "BMP Route Monitoring TLVs" [I-D.ietf-grow-bmp-tlv] registry

   *  Type = TBD1: Local Path ID TLV type.  The value of this TLV is
      defined in Section 3

   This document also requests the definition of a "Local Path ID Reason
   Codes" registry in the "BMP Parameters" namespace, seeded as follows:

   *  Type = 0: Unknown Reason.  Set to 0 when an unknown error occurred
      during the allocation of the Local Path ID.

   *  Type = 1: Origin Process did not provide a Local Path ID.  Set to
      1 when a path is imported from another process, the latter does
      not provide a Local Path ID for the imported path and the
      receiving process did not allocate one either.

   *  Type = 2: All Local Path ID have been allocated.  Set to 2 when a
      Local Path ID could not be allocated for a path because the entire
      range of Local Path IDs is in use.

5.  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>.

   [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>.

   [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>.

   [I-D.ietf-grow-bmp-tlv]
              Lucente, P. and Y. Gu, "BMP v4: TLV support for BMP Route
              Monitoring and Peer Down Messages", Work in Progress,
              Internet-Draft, draft-ietf-grow-bmp-tlv-13, 23 October
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              grow-bmp-tlv-13>.

Authors' Addresses

   Maxence Younsi
   INSA-Lyon
   Villeurbanne
   France
   Email: maxence.younsi@insa-lyon.fr

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

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

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