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
May 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 Attribute 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.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. Variable-Length Prefix Attribute Flags Sub-TLV
3. Backward Compatibility
4. IANA Considerations
4.1. OSPFv2
4.1.1. OSPFv2 Prefix Attribute Flags Sub-TLV
4.1.2. OSPFv2 Prefix Extended Flag Field Attribute Flags Registry
4.2. OSPFv3
4.2.1. OSPFv3 Prefix Attribute Flags Sub-TLV
4.2.2. OSPFv3 Prefix Extended Flag Field Attribute 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 attribute 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 Attribute 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 Attribute Flags Sub-TLV
This document defines a variable-length Prefix Attribute Flags sub-
TLV for OSPFv2 and OSPFv3. The sub-TLV specifies the variable-flag
fields variable-length
Prefix Attribute Flags field to advertise additional attributes
associated with OSPF prefixes. The advertisement and processing of
the existing fixed-
size fixed-size prefix attribute flags remain unchanged.
The format of the OSPFv2/OSPFv3 Prefix Attribute 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 Attribute Flags (Variable) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of OSPFv2/OSPFv3 Prefix Attribute Flags Sub-TLV
where:
Type (2 octets): 11 for OSPFv2 and 37 for OSPFv3
Length (2 octets): Variable, dependent on the included Prefix
Attribute Flags. Flags field. This indicates the length of the prefix
attribute flags Prefix
Attribute 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 Attribute 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 Attribute 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 Attribute 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 Attribute 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 Attribute Flags sub-TLV.
The OSPFv3 Prefix Attribute 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 Attribute 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 Attribute Flags sub-TLV does not introduce
any backward compatibility issues. An implementation that does not
recognize the OSPFv2/OSPFv3 Prefix Attribute Flags sub-TLV would
ignore the sub-TLV as per normal TLV processing operations (refer to
Section 6.3 2.3.2 of [RFC3630] and Section 2.3.2 6.3 of [RFC8362]).
4. IANA Considerations
4.1. OSPFv2
4.1.1. OSPFv2 Prefix Attribute Flags Sub-TLV
IANA has allocated the following codepoint in the "OSPFv2 Extended
Prefix TLV Sub-TLVs" registry:
+=======+===============================+===========+
| Value | Description | Reference |
+=======+===============================+===========+
| 11 | OSPFv2 Prefix Attribute Flags | RFC 9792 |
+-------+-------------------------------+-----------+
Table 1
4.1.2. OSPFv2 Prefix Extended Flag Field Attribute Flags Registry
IANA has created the "OSPFv2 Prefix Extended Flag Field" Attribute Flags" registry within
the "Open Shortest Path First v2 (OSPFv2) Parameters" registry group.
The registry defines the bits in the Prefix Attribute Flags field in
the OSPFv2 Prefix Attribute 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 Attribute Flags Sub-TLV
IANA has allocated the following codepoint in the "OSPFv3 Extended-
LSA Sub-TLVs" registry:
+=======+===============================+===========+
| Value | Description | Reference |
+=======+===============================+===========+
| 37 | OSPFv3 Prefix Attribute Flags | RFC 9792 |
+-------+-------------------------------+-----------+
Table 2
The entry in the "L2BM" field is "X".
4.2.2. OSPFv3 Prefix Extended Flag Field Attribute Flags Registry
IANA has created the "OSPFv3 Prefix Extended Flag Field" Attribute Flags" registry within
the "Open Shortest Path First v3 (OSPFv3) Parameters" registry group.
The registry defines the bits in the Prefix Attribute Flags field in
the OSPFv2 Prefix Attribute 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