



Internet Engineering Task Force (IETF)                           R. Chen
Request for Comments: 9792                                       D. Zhao
Category: Standards Track                                ZTE Corporation
ISSN: 2070-1721                                                P. Psenak
                                                           K. Talaulikar
                                                           Cisco Systems
                                                                 L. Gong
                                                            China Mobile
                                                               June 2025


              Prefix Flag Extension for OSPFv2 and OSPFv3

Abstract

   Each OSPF prefix can be advertised with an 8-bit field to indicate
   specific properties of that prefix.  However, all the OSPFv3 Prefix
   Options bits have already been assigned, and only a few bits remain
   unassigned in the Flags field of the OSPFv2 Extended Prefix TLV.

   This document solves this problem by defining a variable-length
   Prefix Extended Flags sub-TLV for OSPF.  This sub-TLV is applicable
   to OSPFv2 and OSPFv3.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9792.

Copyright Notice

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Table of Contents

   1.  Introduction
     1.1.  Requirements Language
   2.  Variable-Length Prefix Extended Flags Sub-TLV
   3.  Backward Compatibility
   4.  IANA Considerations
     4.1.  OSPFv2
       4.1.1.  OSPFv2 Prefix Extended Flags Sub-TLV
       4.1.2.  OSPFv2 Prefix Extended Flags Registry
     4.2.  OSPFv3
       4.2.1.  OSPFv3 Prefix Extended Flags Sub-TLV
       4.2.2.  OSPFv3 Prefix Extended Flags Registry
   5.  Security Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Each OSPF prefix can be advertised with an 8-bit field to indicate
   specific properties of that prefix.  This is done using the OSPFv3
   Prefix Options (Appendix A.4.1.1 of [RFC5340]) and the Flags field in
   the OSPFv2 Extended Prefix TLV (Section 2.1 of [RFC7684]).  The rest
   of this document refers to these 8-bit fields in both OSPFv2 and
   OSPFv3 as the "existing fixed-size prefix flags".

   However, all the OSPFv3 Prefix Options bits have already been
   assigned (see the "OSPFv3 Prefix Options (8 bits)" IANA registry
   [IANA-OSPFv3-PO]).  Also, at the time of publication of this
   document, only 5 bits remain unassigned in the Flags field of the
   OSPFv2 Extended Prefix TLV (see the "OSPFv2 Extended Prefix TLV
   Flags" IANA registry [IANA-OSPFv2-EPF]).

   This document solves the problem of insufficient flag bits for the
   signaling of prefix properties in OSPF by defining a variable-length
   Prefix Extended Flags sub-TLV for OSPFv2 and OSPFv3.

1.1.  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.

2.  Variable-Length Prefix Extended Flags Sub-TLV

   This document defines a variable-length Prefix Extended Flags sub-TLV
   for OSPFv2 and OSPFv3.  The sub-TLV specifies the variable-length
   Prefix Extended Flags field to advertise additional attributes
   associated with OSPF prefixes.  The advertisement and processing of
   the existing fixed-size prefix flags remain unchanged.

   The format of the OSPFv2/OSPFv3 Prefix Extended Flags sub-TLV is
   shown 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             |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   //                 Prefix Extended Flags (Variable)            //
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Figure 1: Format of OSPFv2/OSPFv3 Prefix Extended Flags Sub-TLV

   where:

   Type (2 octets):  11 for OSPFv2 and 37 for OSPFv3

   Length (2 octets):  Variable, dependent on the included Prefix
      Extended Flags field.  This indicates the length of the Prefix
      Extended Flags field in octets.  The length MUST be a multiple of
      4 octets.  If the length is not a multiple of 4 octets, the Link
      State Advertisement (LSA) is malformed and MUST be ignored.

   Prefix Extended Flags (Variable):  The extended flag field.  This
      field contains a variable number of flags, grouped in 4-octet
      blocks.  The bits are numbered starting from bit 0 as the most
      significant bit of the first 32-bit block.  If the length of the
      Prefix Extended Flags field exceeds 4 octets, numbering for the
      additional bits picks up where the previous 4-octet block left
      off.  For example, the most significant bit in the fifth octet of
      an 8-octet Prefix Extended Flags field is referred to as bit 32.
      Currently, no bits are defined in this document.

   Unassigned bits MUST be set to zero on transmission and MUST be
   ignored on receipt.

   An implementation MUST limit the length of the sub-TLV so as to
   signal the bits that are set to 1.  Defined prefix flags that are not
   transmitted due to being beyond the transmitted length MUST be
   treated as being set to 0.

   The OSPFv2 Prefix Extended Flags sub-TLV is advertised as a sub-TLV
   of the OSPFv2 Extended Prefix TLV defined in [RFC7684].  Additional
   OSPFv2 prefix flags SHOULD be allocated from the unused bits in the
   Flags field of the OSPFv2 Extended Prefix TLV prior to allocating
   flags in the OSPFv2 Prefix Extended Flags sub-TLV.

   The OSPFv3 Prefix Extended Flags sub-TLV is advertised as a sub-TLV
   of the following OSPFv3 TLVs:

   *  Inter-Area-Prefix TLV (Section 3.4 of [RFC8362])

   *  External-Prefix TLV (Section 3.6 of [RFC8362])

   *  Intra-Area-Prefix TLV (Section 3.7 of [RFC8362])

   *  SRv6 Locator TLV [RFC9513]

   When multiple instances of the OSPFv2/OSPFv3 Prefix Extended Flags
   sub-TLVs are received within the same TLV, an implementation MUST use
   only the first occurrence of the sub-TLV and MUST ignore all
   subsequent instances of the sub-TLV.  Errors SHOULD be logged subject
   to rate limiting.

3.  Backward Compatibility

   The OSPFv2/OSPFv3 Prefix Extended Flags sub-TLV does not introduce
   any backward compatibility issues.  An implementation that does not
   recognize the OSPFv2/OSPFv3 Prefix Extended Flags sub-TLV would
   ignore the sub-TLV as per normal TLV processing operations (refer to
   Section 2.3.2 of [RFC3630] and Section 6.3 of [RFC8362]).

4.  IANA Considerations

4.1.  OSPFv2

4.1.1.  OSPFv2 Prefix Extended Flags Sub-TLV

   IANA has allocated the following codepoint in the "OSPFv2 Extended
   Prefix TLV Sub-TLVs" registry:

           +=======+==============================+===========+
           | Value | Description                  | Reference |
           +=======+==============================+===========+
           | 11    | OSPFv2 Prefix Extended Flags | RFC 9792  |
           +-------+------------------------------+-----------+

                                 Table 1

4.1.2.  OSPFv2 Prefix Extended Flags Registry

   IANA has created the "OSPFv2 Prefix Extended Flags" registry within
   the "Open Shortest Path First v2 (OSPFv2) Parameters" registry group.
   The registry defines the bits in the Prefix Extended Flags field in
   the OSPFv2 Prefix Extended Flags sub-TLV as specified in Section 2.
   The bits are to be allocated via IETF Review [RFC8126].  Each bit
   definition will include:

   *  Bit number (counting from bit 0 as the most significant bit of the
      first block)

   *  Description

   *  Reference

   No bits are currently defined.  Bits 0-31 are to be initially marked
   as "Unassigned".  The flags defined in this document may use either a
   single bit or multiple bits to represent a state, as determined by
   the specific requirements of the document defining them.  IANA will
   add subsequent blocks of 32 bits upon exhaustion of the preceding
   32-bit block.

4.2.  OSPFv3

4.2.1.  OSPFv3 Prefix Extended Flags Sub-TLV

   IANA has allocated the following codepoint in the "OSPFv3 Extended-
   LSA Sub-TLVs" registry:

        +=======+==============================+======+===========+
        | Value | Description                  | L2BM | Reference |
        +=======+==============================+======+===========+
        | 37    | OSPFv3 Prefix Extended Flags |  X   | RFC 9792  |
        +-------+------------------------------+------+-----------+

                                  Table 2

4.2.2.  OSPFv3 Prefix Extended Flags Registry

   IANA has created the "OSPFv3 Prefix Extended Flags" registry within
   the "Open Shortest Path First v3 (OSPFv3) Parameters" registry group.
   The registry defines the bits in the Prefix Extended Flags field in
   the OSPFv2 Prefix Extended Flags sub-TLV as specified in Section 2.
   The bits are to be allocated via IETF Review [RFC8126].  Each bit
   definition will include:

   *  Bit number (counting from bit 0 as the most significant bit of the
      first block)

   *  Description

   *  Reference

   No bits are currently defined.  Bits 0-31 are to be initially marked
   as "Unassigned".  The flags defined in this document may use either a
   single bit or multiple bits to represent a state, as determined by
   the specific requirements of the document defining them.  IANA will
   add subsequent blocks of 32 bits upon exhaustion of the preceding
   32-bit block.

5.  Security Considerations

   Procedures and protocol extensions defined in this document do not
   affect the OSPFv2 or OSPFv3 security models.  See Section 5 of
   [RFC7684] for a discussion of OSPFv2 TLV-encoding considerations and
   Section 7 of [RFC8362] for a discussion of OSPFv3 security.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              DOI 10.17487/RFC3630, September 2003,
              <https://www.rfc-editor.org/info/rfc3630>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
              Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
              Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
              2015, <https://www.rfc-editor.org/info/rfc7684>.

   [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>.

   [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>.

   [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
              F. Baker, "OSPFv3 Link State Advertisement (LSA)
              Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
              2018, <https://www.rfc-editor.org/info/rfc8362>.

   [RFC9513]  Li, Z., Hu, Z., Talaulikar, K., Ed., and P. Psenak,
              "OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)",
              RFC 9513, DOI 10.17487/RFC9513, December 2023,
              <https://www.rfc-editor.org/info/rfc9513>.

6.2.  Informative References

   [IANA-OSPFv2-EPF]
              IANA, "OSPFv2 Extended Prefix TLV Flags",
              <https://www.iana.org/assignments/ospfv2-parameters>.

   [IANA-OSPFv3-PO]
              IANA, "OSPFv3 Prefix Options (8 bits)",
              <https://www.iana.org/assignments/ospfv3-parameters>.

Acknowledgements

   The authors would like to thank Shraddha Hegde, Changwang Lin, Tom
   Petch, and many others for their suggestions and comments.

   The authors would also like to thank Acee Lindem for aligning the
   terminology with existing OSPF documents and for editorial
   improvements.

Authors' Addresses

   Ran Chen
   ZTE Corporation
   Nanjing
   China
   Email: chen.ran@zte.com.cn


   Detao Zhao
   ZTE Corporation
   Nanjing
   China
   Email: zhao.detao@zte.com.cn


   Peter Psenak
   Cisco Systems
   Apollo Business Center
   Mlynske nivy 43
   821 09 Bratislava
   Slovakia
   Email: ppsenak@cisco.com


   Ketan Talaulikar
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com


   Liyan Gong
   China Mobile
   China
   Email: gongliyan@chinamobile.com
