Internet Engineering Task Force C. Pignataro
Internet-Draft Cisco
Intended status: Standards Track M. Bhatia
Expires: October 31, 2016 Ionos Networks
S. Aldrin
Huawei Technologies
T. Ranganath
April 29, 2016

OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection (S-BFD) Target Discriminators


This document defines a new OSPF Router Information (RI) TLV that allows OSPF routers to flood the Seamless Bidirectional Forwarding Detection (S-BFD) discriminator values associated with a target network identifier. This mechanism is applicable to both OSPFv2 and OSPFv3.

Requirements Language

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 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on October 31, 2016.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

Seamless Bidirectional Forwarding Detection (S-BFD), specified in [I-D.ietf-bfd-seamless-base], is a simplified mechanism for using Bidirectional Forwarding Detection (BFD) with many negotiations eliminated. This is achieved by using four-octet discriminators, unique within an administrative domain, to identify the Network Targets. 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 (RI) Link State Advertisement (LSA) ([RFC7770]).

1.1. Relationship Between OSPF and S-BFD

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.

When multiple S-BFD discriminators are advertised, how a given discriminator is mapped to a specific use case is out of scope for this document.

2. Implementation

This extension makes use of the Router Information (RI) Opaque LSA, defined in [RFC7770] , for both OSPFv2 [RFC2328] and OSPFv3 [RFC5340], 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 install the S-BFD discriminator in BFD Target Identifier Table.

In the presence of multiple instances of the OSPF/OSPFv3 RI 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 (see Section 2.1) in all advertised non-MaxAge OSPF RI LSAs.

2.1. S-BFD Discriminator TLV

The format of the S-BFD Discriminator TLV is as follows:

     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            |
   |                         Discriminator 1                       |
   |                    Discriminator 2 (Optional)                 |
   |                               ...                             |
   |                    Discriminator n (Optional)                 |

Type - S-BFD Discriminator TLV Type (TBD [to be filled in by IANA])

Length - Total length of the discriminator(s) that appear in the Value field, in octets. Each discriminator is 4 octets, so the Length is 4 times the number of discriminators included in the TLV. There is no optional padding for this field.

Discriminator(s) - The Value field of the TVL includes the S-BFD network target discriminator value or values.

Routers that do not recognize the S-BFD Discriminator TLV Type will ignore the TLV [RFC7770], and therefore will not learn S-BFD discriminators via OSPF.

2.2. Flooding Scope

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 [RFC7770]. As such, elements of procedure are inherited from those defined in [RFC7770].

In OSPFv2, the flooding scope is controlled by the opaque LSA type (as defined in [RFC5250]) and in OSPFv3, by the S1/S2 bits (as defined in [RFC5340]). 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 [RFC5250] should be used.

Although the S-BFD discriminators may change when enabling the S-BFD functionality or via an explicit configuration event, such changes are expected to occur very rarely. Such a change in the information will require S-BFD Discriminator TLV in OSPF to be advertised.

A change in information in the S-BFD Discriminator TLV MUST NOT trigger any SPF computation at a receiving router.

3. Backward Compatibility

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 [RFC7770].

4. Security Considerations

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.

5. IANA Considerations

IANA has defined a registry for TLVs carried in the Router Information LSA defined in [RFC7770]. IANA needs to assign a new TLV codepoint for the S-BFD Discriminator TLV carried within the Router Information LSA.

     Value      TLV Name                 Reference
     -----      --------                 ----------
     TBD        S-BFD                    (this document)

6. Acknowledgements

The authors would like to thank Nobo Akiya, Les Ginsberg, Mach Chen and Peter Psenak for insightful comments and useful suggestions.

7. References

7.1. Normative References

[I-D.ietf-bfd-seamless-base] Akiya, N., Pignataro, C., Ward, D., Bhatia, M. and J. Networks, "Seamless Bidirectional Forwarding Detection (S-BFD)", Internet-Draft draft-ietf-bfd-seamless-base-09, April 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, April 1998.
[RFC5340] Coltun, R., Ferguson, D., Moy, J. and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008.
[RFC7770] Lindem, A., Shen, N., Vasseur, JP., Aggarwal, R. and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, February 2016.

7.2. Informative References

[RFC5250] Berger, L., Bryskin, I., Zinin, A. and R. Coltun, "The OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC5250, July 2008.

Authors' Addresses

Carlos Pignataro Cisco Systems, Inc. EMail:
Manav Bhatia Ionos Networks EMail:
Sam Aldrin Huawei Technologies EMail:
Trilok Ranganath Nokia EMail: