]> ZTE Corporation

Nanjing China chen.ran@zte.com.cn

ZTE Corporation

Nanjing China zhao.detao@zte.com.cn

Cisco Systems

Apollo Business Center Mlynske nivy 43 Bratislava 821 09 Slovakia ppsenak@cisco.com

Cisco Systems

India ketant.ietf@gmail.com

China Mobile

China gongliyan@chinamobile.com

RTG lsr 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.

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 () and the Flags field in the OSPFv2 Extended Prefix TLV (). 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 ). 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 ). 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.

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 when, and only when, they appear in all capitals, as shown here.

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.

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 . 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 ()
• External-Prefix TLV ()
• Intra-Area-Prefix TLV ()
• SRv6 Locator TLV

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.

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 and ).

IANA Considerations

OSPFv2

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

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 . The bits are to be allocated via IETF Review . 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.

OSPFv3

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

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 . The bits are to be allocated via IETF Review . 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.

Security Considerations Procedures and protocol extensions defined in this document do not affect the OSPFv2 or OSPFv3 security models. See for a discussion of OSPFv2 TLV-encoding considerations and for a discussion of OSPFv3 security.

References Normative References Informative References IANA IANA

Acknowledgements The authors would like to thank , , , and many others for their suggestions and comments. The authors would also like to thank for aligning the terminology with existing OSPF documents and for editorial improvements.