OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection (S-BFD) Target Discriminators
Extracted elements (14)
S-BFD uses 4-octet discriminators, unique within an administrative domain, to identify network targets, eliminating many BFD negotiations. OSPF floods these discriminators so S-BFD initiators can map target nodes to discriminators and construct probes without per-session negotiation.
When multiple S-BFD Discriminators are advertised, how a given discriminator is mapped to a specific use case is explicitly left out of scope for this document.
Routers that do not recognize the S-BFD Discriminator TLV Type will silently ignore the TLV as specified in RFC 7770, and therefore will not learn S-BFD Discriminators via OSPF. This introduces no interoperability issues.
A change in information in the S-BFD Discriminator TLV MUST NOT trigger any SPF computations at a receiving router.
If the flooding scope is area local, the S-BFD Discriminator TLV MUST be carried within an OSPFv2 type 10 Router Information LSA or an OSPFv3 Router Information LSA with the S1 bit set and the S2 bit clear.
If the flooding scope is the entire IGP domain, the S-BFD Discriminator TLV MUST be carried within an OSPFv2 type 11 Router Information LSA or an OSPFv3 Router Information LSA with the S1 bit clear and the S2 bit set.
The S-BFD Discriminator TLV is OPTIONAL. Upon receipt, a router may decide to install the S-BFD Discriminator in the BFD target identifier table.
When the S-BFD reflector is deactivated, the OSPF speaker MUST originate a new Router Information LSA that no longer includes the corresponding S-BFD Discriminator TLV (if other TLVs remain), or MUST send an empty RI LSA or purge it by prematurely aging it if no other TLVs remain.
In the presence of multiple instances of the OSPFv2/OSPFv3 Router Information LSA, the S-BFD Discriminators for an OSPF router are the union of all discriminators advertised in all instances of the S-BFD Discriminator TLV in all advertised non-MaxAge OSPF Router Information LSAs.
S-BFD Discriminator TLV information for a specific target identifier is only considered current and usable when the advertising router is itself reachable via OSPF calculated paths in the same area of the LSA. For domain-wide flooding from a remote area, the mechanism in RFC 5250 Section 5 should be used.
The S-BFD Discriminator TLV (type 11) is a new OSPF Router Information (RI) TLV defined to flood S-BFD discriminator values associated with a target network identifier. It applies to both OSPFv2 and OSPFv3.
IANA assigned TLV codepoint 11 for the S-BFD Discriminator TLV in the 'OSPF Router Information (RI) TLVs' registry defined by RFC 7770.
The security of S-BFD Discriminator distribution relies entirely on the security of OSPF. OSPF provides no encryption for LSA privacy, but this is not a concern because there is no need to hide discriminator values used to reach S-BFD reflectors.
The S-BFD Discriminator TLV carries a Type field (value 11), a Length field (total octets of discriminator values, 4 octets per discriminator with no padding), and one or more 4-octet Discriminator values. It is carried within the OSPF Router Information LSA.