Advertising Seamless Bidirectional Forwarding Detection (S-BFD) Discriminators in IS-IS
Extracted elements (10)
The flooding scope for S-BFD Discriminator advertisements is determined by the intended use: if S-BFD usage will be exclusively within a Level-1 area, area scope is appropriate; if it will span different Level-1 areas, domain scope is appropriate.
The IS-IS Router CAPABILITY TLV (RFC 4971) was chosen as the vehicle for advertising S-BFD Discriminators because S-BFD (RFC 7880) requires network nodes to know the BFD Discriminators each node has reserved, and IS-IS is one suitable distribution mechanism for this node-scoped information.
S-BFD Discriminators sub-TLVs are formatted as specified in RFC 5305 (IS-IS Extensions for Traffic Engineering) and are carried within the IS-IS Router CAPABILITY TLV as defined in RFC 4971.
Multiple S-BFD Discriminators sub-TLVs MAY be advertised by an IS. How a given discriminator is mapped to a specific use case when multiple are advertised is out of scope for this document.
The inclusion of an S-BFD Discriminators sub-TLV in a Router CAPABILITY TLV is optional. S-BFD Discriminator advertisements MAY be flooded within an area or throughout the domain using the procedures specified in RFC 4971.
A new sub-TLV (Type 20) for the IS-IS Router CAPABILITY TLV (TLV 242) is defined to advertise S-BFD Discriminators. The sub-TLV carries one or more 32-bit discriminator values and is formatted per RFC 5305.
IANA added value 20 (description: 'S-BFD Discriminators') to the 'Sub-TLVs for TLV 242' registry to accommodate the new sub-TLV defined by this document.
Advertising S-BFD Discriminators makes it possible for attackers to initiate S-BFD sessions using the advertised information. The vulnerabilities this poses and how to mitigate them are discussed in RFC 7880.
The new S-BFD Discriminators sub-TLV does not introduce any new security risks for IS-IS itself; existing IS-IS security mechanisms defined in RFC 5304 and RFC 5310 apply.
The S-BFD Discriminators sub-TLV encoding: Type field is 1 octet with value 20; Length field is 1 octet and must be a multiple of 4; each Discriminator Value field is 4 octets, allowing multiple discriminators to be packed in a single sub-TLV.