RFC 9792: Prefix Flag Extension for 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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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)            //
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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:¶

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.¶

4. IANA Considerations

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.¶

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