Network Working Group                                            C. Lin
Internet Draft                                     New H3C Technologies
Intended status: Standards Track                                 Y. Liu
Expires: 26 April 2026                                     China Mobile
                                                                 Z. Ali
                                                    Cisco Systems, Inc.
                                                        20 October 2025


                       LDP Extensions for Flex-Algo
                      draft-lin-mpls-ldp-flex-algo-01


Abstract

   This document specifies extensions to LDP to support the use Flex-
   Algo, enabling Label Switched Paths (LSPs) to follow a specific
   Flex-Algo.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 April 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.

Lin, et al.             Expires 26 April 2026                 [Page 1]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025




Table of Contents


   1. Introduction...................................................2
      1.1. Terminology...............................................3
   2. Signaling Extensions...........................................3
      2.1. New Address Families: FA IP/FA IPv6.......................3
      2.2. LDP FEC Elements with FA IP AF............................4
      2.3. IGP IPA Mapping and Translation...........................5
      2.4. LDP Flex-Algo Capability Advertisement....................6
         2.4.1. Protocol Extension...................................6
         2.4.2. Procedures...........................................6
   3. FA Applicability on FEC-Based Features.........................7
      3.1. Typed Wildcard FEC Element................................7
      3.2. LSP Ping..................................................7
         3.2.1. New FEC Sub-Types....................................7
         3.2.2. FA LDP IPv4 FEC Sub-TLV..............................7
         3.2.3. FA LDP IPv6 FEC Sub-TLV..............................8
         3.2.4. Operation Considerations.............................8
   4. Error Handling.................................................9
      4.1. Error Notification for Invalid IPA ID.....................9
   5. IANA Considerations............................................9
      5.1. FA Capability TLV.........................................9
      5.2. New Status Code...........................................9
      5.3. New address families......................................9
   6. References....................................................10
      6.1. Normative References.....................................10
      6.2. Informative References...................................10
   Authors' Addresses...............................................10

1. Introduction

   Multi-Topology Routing (MTR) is a technology that enables service
   differentiation within an IP network. The Flexible Algorithm (FA) is
   another mechanism for creating a sub-topology within a topology
   using defined topology constraints and computation algorithms.

   To deploy Label Distribution Protocol (LDP) in a network that
   supports Flex-Algo, or other methods of signaling non-default IGP
   Algorithms (IPAs), it is necessary for LDP to become algorithm
   aware. This document specifies extensions to LDP to support the use
   of IPAs, enabling Label Switched Paths (LSPs) to follow a specific
   algorithm.

   [RFC7307] describes how to extend the LDP protocol to support Multi-
   Topology (MT), but it only uses one byte to carry the MT-ID and does
   not support IPAs. This document updates RFC 7307 by allowing LSPs to
   follow a specific topology and algorithm.

Lin, et al.             Expires 26 April 2026                 [Page 2]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


   This document defines how LDP can utilize a specific combination of
   topology and algorithm, referred to as Topology-Algorithm(TA). In
   this context, LDP's functionality is enhanced to ensure that the
   LSPs are constructed and maintained according to the specified TA
   parameters, thus optimizing the routing and resource utilization in
   diverse network environments.

   These enhancements to LDP provide the flexibility needed to support
   advanced topological and algorithmic configurations, making it a
   robust choice for modern, dynamic network infrastructures.

 1.1. Terminology

   This document uses terminologies defined in [RFC7307].

   o  MT-ID: A 16-bit value used to represent the Multi-Topology ID.

   o  Algorithm: A 1-octet value from the IGP Algorithm Types registry

      under IGP Parameters registry.

2. Signaling Extensions

2.1. New Address Families: FA IP/FA IPv6

   [RFC7307] defines MT IP and MT IPv6 address families, which are used
   to specify IPv4 and IPv6 prefixes within a topology scope, but they
   can only carry MT-ID information.

   To extend IP address families for Flex-Algo, two new Address
   Families named "FA IP" and "FA IPv6" are used to specify IPv4 and
   IPv6 prefixes within a Flex-Algo scope.



       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                     IPv4 Address                              |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    Reserved   |      IPA      |        MT-ID                  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: FA IP Address Family Format

Lin, et al.             Expires 26 April 2026                 [Page 3]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025




       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                     IPv6 Address                              |

      |                                                               |

      |                                                               |

      |                                                               |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    Reserved   |      IPA      |        MT-ID                  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 1: FA IPv6 Address Family Format



   IPA: A 1-octet value from the IGP Algorithm Types registry  under
   IGP Parameters registry.

   MT-ID: A 2-octet field MT-ID (see Section 3.7 of [RFC4915],
   Section 7 of [RFC5120]) to special the topology.  If this field is
   set to zero, it means the default topology.



2.2. LDP FEC Elements with FA IP AF

   The following section specifies the format extensions of the
   existing LDP FEC elements to support Flex-Algo.  The "Address
   Family" of these FEC elements will be set to "FA IP" or "FA IPv6".

   The encoding of the Flex-Algo Prefix FEC element is as follows:

       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |  Prefix (2)   |  Address Family(FA IP/IPv6)   |     PreLen    |

Lin, et al.             Expires 26 April 2026                 [Page 4]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                     Prefix                                    |

      ~                                                               ~

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    Reserved   |      IPA      |        MT-ID                  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                  Figure 2: FA Prefix FEC Element Format



      The Flex-Algo Typed Wildcard FEC element encoding is as follows:

       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |Typed Wcard (5)|   FEC Type    |   Len = 6     |   AF = FA IP  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |  or FA IPv6   |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |    Reserved   |      IPA      |        MT-ID                  |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 4: Flex-Algo Typed Wildcard FEC Element

2.3. IGP IPA Mapping and Translation

   The non-reserved non-special IGP IPA values can be used and carried
   in LDP without the need for translation.  However, there is a need
   for translating reserved or special IGP IPA values to corresponding
   LDP IPAs.





Lin, et al.             Expires 26 April 2026                 [Page 5]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


2.4. LDP Flex-Algo Capability Advertisement

      2.4.1. Protocol Extension

   We specify a new LDP capability, named "Flex-Algo (FA)", which is
   defined in accordance with the LDP capability guidelines [RFC5561].
   The LDP "FA" capability can be advertised by an LDP speaker to its
   peers either during the LDP session initialization or after the LDP
   session is set up.  The advertisement is to announce the capability
   of the Label Switching Router (LSR) to support FA for the given IP
   address family.  An LDP speaker MUST NOT send messages containing FA
   FEC elements unless the peer has said it can handle it.

   The format of the FA Capability TLV is as follows:



       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

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |U|F|   Flex-Algo Cap.(IANA)    |            Length             |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |S| Reserved    |                                               |

      +-+-+-+-+-+-+-+-+                                               |

      ~                Typed Wildcard FEC element(s)                  ~

      |                                                               |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 5: Flex-Algo Capability TLV Format

   Where:

   o  Flex-Algo Capability: Capability TLV type (IANA assigned)

      2.4.2. Procedures

   To announce its Flex-Algo capability for an IP address family, LDP
   FEC type, an LDP speaker sends an "FA Capability" including the
   exact Typed Wildcard FEC element with the corresponding
   "AddressFamily" field (i.e., set to "FA IP" for IPv4 and set to "FA
   IPv6" for IPv6 address family), corresponding "FEC Type" field
   (i.e.,   set to "Prefix"), and corresponding "IPA".  To announce its
Lin, et al.             Expires 26 April 2026                 [Page 6]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


   Flex-Algo capability for both the IPv4 and IPv6 address family, or
   for multiple FEC types, or for multiple Flex-Algos, an LDP speaker
   sends an "FA Capability" with one or more FA Typed FEC elements in
   it.

3. FA Applicability on FEC-Based Features

3.1. Typed Wildcard FEC Element

   [RFC5918] extends base LDP and defines the Typed Wildcard FEC
   element framework.  The Typed Wildcard FEC element can be used in
   any LDP message to specify a wildcard operation/action for a given
   type of FEC.

   The FA extensions defined in this document do not require any
   extension to procedures for the Typed Wildcard FEC element, and
   these procedures apply as is to FA wildcarding.  The FA extensions,
   though, allow use of "FA IP" or "FA IPv6" in the Address Family
   field of the Typed Wildcard FEC element in order to use wildcard
   operations in the context of a given Flex-Algo.  The use of FA-
   scoped address family also allows us to specify Flex-Algo in these
   operations.

3.2. LSP Ping

3.2.1. New FEC Sub-Types

   We define two new FEC types for LSP ping:

      o  FA LDP IPv4 FEC

      o  FA LDP IPv6 FEC

3.2.2. FA LDP IPv4 FEC Sub-TLV

   The format of the "FA LDP IPv4 FEC" sub-TLV to be used in a "Target
   FEC Stack" [RFC4379] is:

       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 (FA LDP IPv4 FEC)       |            Length = 12        |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                          IPv4 prefix                          |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Lin, et al.             Expires 26 April 2026                 [Page 7]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


      | Prefix Length |      IPA      |       MT-ID                   |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 5: FA LDP IPv4 FEC Sub-TLV



3.2.3. FA LDP IPv6 FEC Sub-TLV

   The format of the "FA LDP IPv6 FEC" sub-TLV to be used in a "Target
   FEC Stack" [RFC4379] is:

       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 (FA LDP IPv6 FEC)       |          Length = 24          |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      |                                                               |

      |                          IPv6 prefix                          |

      |                                                               |

      |                                                               |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      | Prefix Length |      IPA      |       MT-ID                   |

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 6: FA LDP IPv6 FEC Sub-TLV



3.2.4. Operation Considerations

   To detect data-plane failures using LSP ping for a specific Flex-
   Algo, the router will initiate an LSP ping request with the target
   FEC stack TLV containing the LDP FA IP Prefix Sub-TLV in the Echo
   Request packet.  The Echo Request packet is sent with the label
   bound to the IP Prefix in the Flex-Algo.  Once the Echo Request
   packet reaches the target router, it will process the packet and
   perform checks for the LDP FA IP Prefix sub-TLV present in the
   Target FEC Stack as described in [RFC4379] and respond according to
Lin, et al.             Expires 26 April 2026                 [Page 8]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


   the processing rules in [RFC4379].  For the case that the LSP ping
   with return path is not specified, the reply packet must go through
   the default Flex-Algo instead of the Flex-Algo where the Echo
   Request goes through.



4. Error Handling

4.1. Error Notification for Invalid IPA ID

   An LSR should respond with an "Invalid IPA ID" status code in the
   LDP Notification message when it receives an LDP message with a FEC
   element specifying an IPA that is not locally known or not
   supported.  The LSR MUST also discard the entire message before
   sending the Notification message.

5. IANA Considerations

 5.1. FA Capability TLV

   New LDP Capability TLV "Flex-Algo Capability" TLV is requested
   from the LDP Parameters registry "TLV Type Name Space".

 5.2. New Status Code

   A new status code, "Invalid IPA ID," was requested from the LDP
   Parameters registry "Status Code Name Space".

           Registry:

           Range/Value          Description

           --------------       ------------------------------

           TBD1                 Invalid IPA ID

 5.3. New address families

   New address families under the IANA registry "Address Family
   Numbers":

      Number       Description

      --------     ------------------------------------

      TBD2         FA IP: Flex-Algo IP version 4

      TBD3         FA IPv6: Flex-Algo IP version 6


Lin, et al.             Expires 26 April 2026                 [Page 9]

Internet-Draft      LDP Extensions for Flex-Algo          October 2025


6. References

 6.1. Normative References

   [RFC7307] Q. Zhao, Huawei Technology, K. Raza, C. Zhou, Cisco
             Systems, L. Fang, Microsoft, L. Li, China Mobile, D. King,
             Old Dog Consulting, "LDP Extensions for Multi-Topology",
             RFC 5286,DOI 10.17487/RFC7307, July 2014,<http://www.rfc-
             editor.org/info/rfc7307>.

   [RFC9658] IJ. Wijnands, Individual, M. Mishra, Ed., K. Raza, Cisco
             Systems, Inc., Z. Zhang, Juniper Networks,  A. Gulko,
             Edward Jones, "Multipoint LDP Extensions for Multi-
             Topology Routing", RFC 9658,DOI 10.17487/RFC9658, October
             2024,<http://www.rfc-editor.org/info/rfc9658>.



 6.2. Informative References

   TBD

Authors' Addresses

   Changwang Lin
   New H3C Technologies
   China

   Email: linchangwang.04414@h3c.com

   Yisong Liu
   China Mobile
   China

   Email: liuyisong@chinamobile.com

   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com











Lin, et al.             Expires 26 April 2026                [Page 10] 
