OSPF extensions to advertise S-BFD Target Discriminator
Alcatel-Lucentmanav.bhatia@alcatel-lucent.comAlcatel-Lucenttrilok.ranganatha@alcatel-lucent.comCisco Systemscpignata@cisco.comHuawei Technologies aldrin.ietf@gmail.com
BFD Working Group
Internet Engineering Task ForceBFDseamless BFDnegotiation freelabel verificationsegment routingIPThis document defines a new OSPF Router Information (RI) TLV that
allows OSPF routers to flood the S-BFD discriminator values associated with a target network identifier.
This mechanism is applicable to
both OSPFv2 and OSPFv3.The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.
Seamless Bidirectional Forwarding Detection (S-BFD), specified in
,
is a simplified mechanism for using BFD with many negotiations eliminated.
This is achieved by using unique network-wide discriminators to identify
the Network Targets (e.g., IP addresses).
These S-BFD discriminators can be advertised by the IGPs, and this document concerns itself with OSPF.
Specifically, this document defines a new TLV (named the S-BFD Discriminator TLV) to be carried within the OSPF Router
Information LSA ().
This document, implicitly, defines a relationship between OSPF and S-BFD.
S-BFD assigns one or more Discriminators to each S-BFD reflector node.
OSPF, in turn, learns about these from S-BFD, and floods them in the newly
defined TLV. After this information is flooded, it is stored in all the OSPF
nodes such that S-BFD initiators can map out target nodes to target Discriminators, and can therefore construct the S-BFD probe.
This extension makes use of the Router Information (RI) Opaque LSA,
defined in , for both OSPFv2
and OSPFv3 , by defining a new
OSPF Router Information (RI) TLV: the S-BFD Discriminator TLV.
The S-BFD Discriminator TLV is OPTIONAL. Upon receipt
of the TLV, a router may decide to ignore this TLV or install the
S-BFD discriminator in BFD Target Identifier Table.
The format of the S-BFD Discriminator TLV is as follows: Type - S-BFD Discriminator TLV Type Length - Total length of the discriminator (Value field) in octets, not
including the optional padding. The Length is a multiple of 4 octets, and consequently specifies how many Discriminators are included in the TLV. Value - S-BFD network target discriminator value or values. Routers that do not recognize the S-BFD Discriminator TLV Type MUST ignore
the TLV. S-BFD discriminator is associated with the BFD Target Identifier type,
that allows demultiplexing to a specific task or service.
The flooding scope for S-BFD Discriminator information advertised through OSPF can be
limited to one or more OSPF areas, or can be
extended across the entire OSPF routing domain.
Note that the S-BFD session may be required to pan multiple areas, in which case the
flooding scope may comprise these areas. This could be the case for
an ABR, for instance, advertising the S-BFD Discriminator information within the
backbone area and/or a subset of its attached IGP area(s).
The S-BFD Discriminator TLV is advertised within OSPFv2 Router Information LSAs
(Opaque type of 4 and Opaque ID of 0) or OSPFv3 Router Information
LSAs (function code of 12), which are defined in . As such,
elements of procedure are inherited from those defined in .
In OSPFv2, the flooding scope is controlled by the opaque LSA type
(as defined in ) and in OSPFv3, by the S1/S2 bits (as
defined in ). If the flooding scope is area local, then 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, then
the S-BFD Discriminator TLV MUST be carried within an OSPFv2 type 11 Router
Information LSA or OSPFv3 Router Information LSA with the S1 bit
clear and the S2 bit set.
When the S-BFD Reflector is deactivated, the OSPF speaker advertising
this S-BFD Discriminator MUST originate a new Router Information LSA that no longer
includes the corresponding S-BFD Discriminator TLV, provided there are other TLVs in
the LSA. If there are no other TLVs in the LSA, it MUST either send
an empty Router Information LSA or purge it by prematurely ageing it.
For intra-area reachability, the S-BFD Discriminator TLV information regarding a specific target identifier is only considered
current and useable when the router advertising this information is
itself reachable via OSPF calculated paths in the same area of the
LSA in which the S-BFD Discriminator TLV appears.
In the case of domain-wide flooding, i.e., where the originator is sitting in a remote area, the mechanism described in section 5 of should be used.
A change in information in the S-BFD Discriminator TLV MUST NOT trigger any SPF
computation at a receiving router.
The S-BFD Discriminator TLV defined in this document does not introduce any
interoperability issues. A router not supporting the S-BFD Discriminator TLV will just silently ignore the
TLV as specified in .
This document defines OSPF extensions to distribute the S-BFD discriminator within an
administrative domain. Hence the security of the S-BFD discriminator distribution
relies on the security of OSPF.
OSPF provides no encryption mechanism for protecting the privacy of
LSAs and, in particular, the privacy of the S-BFD discriminator advertisement
information. This however is not a concern as there isn't any need to hide the
discriminator value that can be used to reach the Reflectors.
IANA has defined a registry for TLVs carried in the Router
Information LSA defined in .
IANA needs to assign a new TLV
codepoint for the S-BFD Discriminator TLV carried within the Router Information LSA.
The authors would like to thank Nobo Akiya, Les Ginsberg, Mach Chen and Peter Psenak for insightful comments and useful suggestions.