RFC7884: OSPF Extensions to Advertise Seamless Bidirectional Forwarding Detection (S-BFD) Target Discriminators
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Related keywords:
(BFD)
(IP)
(label verification)
(negotiation free)
(seamless BFD)
(segment routing)
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/rfc7884.
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RFC 7884 S-BFD Discriminators in OSPF July 2016
Copyright Notice
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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
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RFC 7884 S-BFD Discriminators in OSPF July 2016
1. Introduction
Seamless Bidirectional Forwarding Detection (S-BFD), specified in
[RFC7880], 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) [RFC7770].
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].
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 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.
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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 [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 this procedure are inherited from
those defined in [RFC7770].
The flooding scope is controlled by the Opaque LSA type (as defined
in [RFC5250]) in OSPFv2 and by the S1/S2 bits (as defined in
[RFC5340]) 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
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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 [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 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 [RFC7770].
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.
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5. IANA Considerations
IANA has defined a registry for TLVs carried in the Router
Information LSA defined in [RFC7770]. 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
<http://www.rfc-editor.org/info/rfc5340>.
[RFC7770] 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/RFC7770,
February 2016, <http://www.rfc-editor.org/info/rfc7770>.
[RFC7880] Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
Pallagatti, "Seamless Bidirectional Forwarding Detection
(S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
<http://www.rfc-editor.org/info/rfc7880>.
6.2. 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, <http://www.rfc-editor.org/info/rfc5250>.
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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
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