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IETF RFC 7884
Last modified on Wednesday, July 13th, 2016
 Permanent link to RFC 7884
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Internet Engineering Task Force (IETF) C. Pignataro
Request for Comments: 7884 Cisco
Category: Standards Track M. Bhatia
ISSN: 2070-1721 Ionos Networks
S. Aldrin
Huawei Technologies
T. Ranganath
Nokia
July 2016
OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection
(S-BFD) Target Discriminators
 Abstract
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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7884.
Pignataro, et al. Standards Track PAGE 1 
RFC 7884 S-BFD Discriminators in OSPF July 2016
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Relationship between OSPF and S-BFD ........................3
2. Implementation ..................................................3
2.1. S-BFD Discriminator TLV ....................................4
2.2. Flooding Scope .............................................4
3. Backward Compatibility ..........................................5
4. Security Considerations .........................................5
5. IANA Considerations .............................................6
6. References ......................................................6
6.1. Normative References .......................................6
6.2. Informative References .....................................6
Acknowledgements ...................................................7
Authors' Addresses .................................................7
Pignataro, et al. Standards Track PAGE 2
RFC 7884 S-BFD Discriminators in OSPF July 2016
1. Introduction
Seamless Bidirectional Forwarding Detection (S-BFD), specified in
[RFC 7880], is a simplified mechanism for using BFD with many
negotiations eliminated. This is achieved by using 4-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) [RFC 7770].
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 [RFC 2119].
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 [RFC 7770], for both OSPFv2 [RFC 2328] and OSPFv3 [RFC 5340]
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 the BFD
target identifier table.
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 (see Section 2.1) in all advertised non-MaxAge OSPF
Router Information LSAs.
Pignataro, et al. Standards Track PAGE 3
RFC 7884 S-BFD Discriminators in OSPF July 2016
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 (11)
Length - This field represents the total length of the
discriminator(s) that appears in the Value field, in octets. Each
discriminator is 4 octets, so the Length is four times the number
of discriminators included in the TLV. There is no optional
padding for this field.
Discriminator(s) - The Value field of the TLV 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 [RFC 7770] 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
[RFC 7770]. As such, elements of this procedure are inherited from
those defined in [RFC 7770].
The flooding scope is controlled by the Opaque LSA type (as defined
in [RFC 5250]) in OSPFv2 and by the S1/S2 bits (as defined in
[RFC 5340]) in OSPFv3. 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
Pignataro, et al. Standards Track PAGE 4
RFC 7884 S-BFD Discriminators in OSPF July 2016
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
a particular 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 aging it.
For intra-area reachability, the S-BFD Discriminator TLV information
regarding a specific target identifier is only considered current and
usable when the router advertising that 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 [RFC 5250] 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 changes in information will
require that the S-BFD Discriminator TLV in OSPF be advertised.
A change in information in the S-BFD Discriminator TLV MUST NOT
trigger any SPF computations 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 [RFC 7770].
4. Security Considerations
This document defines OSPF extensions to distribute the S-BFD
Discriminator within an administrative domain. Hence, the security
of 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. However, this is not a concern, as there
isn't any need to hide the Discriminator value that can be used to
reach the reflectors.
Pignataro, et al. Standards Track PAGE 5
RFC 7884 S-BFD Discriminators in OSPF July 2016
5. IANA Considerations
IANA has defined a registry for TLVs carried in the Router
Information LSA defined in [RFC 7770]. IANA has assigned a new TLV
codepoint (11) for the S-BFD Discriminator TLV in the "OSPF Router
Information (RI) TLVs" registry.
Value TLV Name Reference
----- -------- ----------
11 S-BFD RFC 7884
Discriminator
6. References
6.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC 2328, April 1998,
<http://www.rfc-editor.org/info/RFC 2328>.
[RFC 5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC 5340, July 2008,
<http://www.rfc-editor.org/info/RFC 5340>.
[RFC 7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC 7770,
February 2016, <http://www.rfc-editor.org/info/RFC 7770>.
[RFC 7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC 7880, July 2016,
<http://www.rfc-editor.org/info/RFC 7880>.
6.2. Informative References
[RFC 5250] Berger, L., Bryskin, I., Zinin, A., and R. Coltun, "The
OSPF Opaque LSA Option", RFC 5250, DOI 10.17487/RFC 5250,
July 2008, <http://www.rfc-editor.org/info/RFC 5250>.
Pignataro, et al. Standards Track PAGE 6
RFC 7884 S-BFD Discriminators in OSPF July 2016
Acknowledgements
The authors would like to thank Nobo Akiya, Les Ginsberg, Mach Chen,
and Peter Psenak for insightful comments and useful suggestions.
Authors' Addresses
Carlos Pignataro
Cisco Systems, Inc.
Email: cpignata@cisco.com
Manav Bhatia
Ionos Networks
Email: manav@ionosnetworks.com
Sam Aldrin
Huawei Technologies
Email: aldrin.ietf@gmail.com
Trilok Ranganath
Nokia
Email: trilok.ranganatha@nokia.com
Pignataro, et al. Standards Track PAGE 7
RFC TOTAL SIZE: 13612 bytes
PUBLICATION DATE: Wednesday, July 13th, 2016
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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