Updates:
Keywords: IGP, OSPF
Internet Engineering Task Force (IETF) K. Talaulikar, Ed.
Request for Comments: 9355 P. Psenak
Updates: 2328 Cisco Systems, Inc.
Category: Standards Track A. Fu
ISSN: 2070-1721 Bloomberg
M. Rajesh
Juniper Networks
February 2023
OSPF Bidirectional Forwarding Detection (BFD) Strict-Mode
Abstract
This document specifies the extensions to OSPF that enable an OSPF
router to signal the requirement for a Bidirectional Forwarding
Detection (BFD) session prior to adjacency formation. Link-Local
Signaling (LLS) is used to advertise the requirement for strict-mode
BFD session establishment for an OSPF adjacency. If both OSPF
neighbors advertise BFD strict-mode, adjacency formation will be
blocked until a BFD session has been successfully established.
This document updates RFC 2328 by augmenting the OSPF neighbor state
machine with a check for BFD session up before progression from Init
to 2-Way state when operating in OSPF BFD strict-mode.
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
https://www.rfc-editor.org/info/rfc9355.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
1.1. Requirements Language
2. LLS B-Bit Flag
3. Local Interface IPv4 Address TLV
4. Procedures
4.1. OSPFv3 IPv4 AF Specifics
4.2. Graceful Restart Considerations
5. Operations and Management Considerations
6. Backward Compatibility
7. IANA Considerations
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Bidirectional Forwarding Detection (BFD) [RFC5880] enables routers to
monitor data plane connectivity and to detect faults in the
bidirectional path between them. BFD is leveraged by routing
protocols like OSPFv2 [RFC2328] and OSPFv3 [RFC5340] to detect
connectivity failures for established adjacencies faster than the
OSPF Hello dead timer detection and to trigger rerouting of traffic
around the failure. The use of BFD for monitoring routing protocol
adjacencies is described in [RFC5882].
When BFD monitoring is enabled for OSPF adjacencies by the network
operator, the BFD session is bootstrapped based on the neighbor
address information discovered by the exchange of OSPF Hello packets.
Faults in the bidirectional forwarding detected via BFD then result
in the OSPF adjacency being brought down. A degraded or poor-quality
link may result in intermittent packet drops. In such scenarios,
implementations prior to the extensions specified in this document
may still get an OSPF adjacency established over such a link;
however, given the more aggressive monitoring intervals supported by
BFD, a BFD session may not get established and/or may flap. The
traffic forwarded over such a link would experience packet drops, and
the failure of the BFD session establishment will not enable fast
routing convergence. OSPF adjacency flaps may occur over such links
when OSPF brings up the adjacency only for it to be brought down
again by BFD.
To avoid the routing churn associated with these scenarios, it would
be beneficial not to allow OSPF to establish an adjacency until a BFD
session is successfully established and has stabilized. However,
this would preclude the OSPF operation in an environment where not
all OSPF routers support BFD and have it enabled on the link. A
solution is to block OSPF adjacency establishment until a BFD session
is established as long as both neighbors advertise such a
requirement. Such a mode of OSPF BFD usage is referred to as
"strict-mode". Strict-mode introduces signaling support in OSPF to
achieve the blocking of adjacency formation until BFD session
establishment occurs, as described in Section 4.1 of [RFC5882].
This document specifies the OSPF protocol extensions using Link-Local
Signaling (LLS) [RFC5613] for a router to indicate to its neighbor
the willingness to require BFD strict-mode for OSPF adjacency
establishment (refer to Section 2). It also introduces an extension
to OSPFv3 LLS of the interface IPv4 address (refer to Section 3) to
be used for the BFD session setup when OSPFv3 is used for an IPv4
Address Family (AF) instance.
This document updates [RFC2328] by augmenting the OSPF neighbor state
machine with a check for BFD session up before progression from Init
to 2-Way state when operating in OSPF BFD strict-mode.
The extensions and procedures for OSPF BFD strict-mode also apply for
adjacency over virtual links using BFD multi-hop [RFC5883]
procedures.
A similar functionality for IS-IS is specified in [RFC6213].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. LLS B-Bit Flag
This document defines the B-bit in the LLS Type 1 Extended Options
and Flags. This bit is defined for the LLS block that is included in
Hello and Database Description (DD) packets. The B-bit indicates
that BFD is enabled on the link and that the router requests OSPF BFD
strict-mode. Section 7 describes the position of the B-bit.
A router MUST include the LLS block with the B-bit set in the LLS
Type 1 Extended Options and Flags in its Hello and DD packets when
OSPF BFD strict-mode is enabled on the link.
3. Local Interface IPv4 Address TLV
The Local Interface IPv4 Address TLV is an LLS TLV defined for OSPFv3
IPv4 AF instance [RFC5838] protocol operation as described in
Section 4.1.
It has the following format:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Local Interface IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
Type: 21
Length: 4 octets
Local Interface IPv4 Address: The primary IPv4 address of the local
interface.
4. Procedures
A router supporting OSPF BFD strict-mode advertises this capability
through its Hello packets as described in Section 2. When a router
supporting OSPF BFD strict-mode discovers a new neighbor router that
also supports OSPF BFD strict-mode, it will establish a BFD session
with that neighbor first before bringing up the OSPF adjacency as
described further in this section.
This document updates the OSPF neighbor state machine as described in
[RFC2328]. Specifically, the operations related to the Init state
are modified as described below when OSPF BFD strict-mode is used:
Init (without OSPF BFD strict-mode):
In this state, a Hello packet has recently been received from the
neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). All neighbors in this
state (or higher) are listed in the Hello packets sent from the
associated interface.
Init (with OSPF BFD strict-mode):
In this state, a Hello packet has recently been received from the
neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). BFD session establishment
with the neighbor is requested if it's not already completed
(e.g., in the event of transition from 2-Way state). Neighbors in
Init state or higher will be listed in Hello packets associated
with the interface if they either have a corresponding BFD session
established or have not advertised OSPF BFD strict-mode in the LLS
Type 1 Extended Options and Flags advertised in the Hello packet.
When the neighbor state transitions to Down state, the removal of the
BFD session associated with that neighbor is requested by OSPF;
subsequent BFD session establishment is similarly requested by OSPF
upon transitioning into Init state. This may result in BFD session
deletion and creation, respectively, when OSPF is the only client
interested in the BFD session with the neighbor address.
An implementation MUST NOT wait for BFD session establishment in Init
state unless OSPF BFD strict-mode is enabled by the operator on the
interface and the specific neighbor indicates OSPF BFD strict-mode
capability via the LLS Type 1 Extended Options and Flags advertised
in the Hello packet. When BFD is enabled, but OSPF BFD strict-mode
has not been signaled by both neighbors, an implementation SHOULD
start BFD session establishment only in 2-Way or greater state. This
makes it possible for an OSPF router to support BFD operation in both
strict-mode and normal mode across different interfaces or even
across different neighbors on the same multi-access interface.
Once the OSPF state machine has moved beyond the Init state, any
change in the B-bit advertised in subsequent Hello packets MUST NOT
result in any trigger in either the OSPF adjacency or the BFD session
management (i.e., the B-bit is considered only when in Init state).
Disabling BFD (or OSPF BFD strict-mode) on an OSPF interface would
result in it not setting the B-bit in the LLS Type 1 Extended Options
and Flags advertised in subsequent Hello packets. Disabling OSPF BFD
strict-mode has no effect on BFD operations and would not result in
the bringing down of any established BFD sessions. Disabling BFD
would result in the BFD session being brought down due to AdminDown
State (described in Section 3.2 of [RFC5882]); hence, it would not
bring down the OSPF adjacency.
When BFD is enabled on an interface over which we already have an
existing OSPF adjacency, it would result in the router setting the
B-bit in its subsequent Hello packets and the initiation of BFD
session establishment to the neighbor. If the adjacency is already
up (i.e., in its terminal state of Full or 2-Way with routers that
are not designated routers on a multi-access interface) with a
neighbor that also supports OSPF BFD strict-mode, then an
implementation SHOULD NOT bring this adjacency down into the Init
state to avoid disruption to routing operations and instead use the
OSPF BFD strict-mode wait only after a transition to Init state.
However, if the adjacency is not up, then an implementation MAY bring
such an adjacency down so it can use the OSPF BFD strict-mode for its
adjacency establishment.
4.1. OSPFv3 IPv4 AF Specifics
Support for multiple AFs in OSPFv3 [RFC5838] requires the use of an
IPv6 link-local address as the source address for Hello packets, even
when forming adjacencies for IPv4 AF instances. In most deployments
of OSPFv3 IPv4 AFs, it is required that BFD is used to monitor and
verify IPv4 data plane connectivity between the routers on the link;
hence, the BFD session is set up using IPv4 neighbor addresses. The
IPv4 neighbor address on the interface is learned only later in the
adjacency formation process when the neighbor's Link-LSA (Link State
Advertisement) is received. This results in the setup of the BFD
IPv4 session either after the adjacency is established or later in
the adjacency formation sequence.
To operate in OSPF BFD strict-mode, it is necessary for an OSPF
router to learn its neighbor's IPv4 link address during the Init
state of adjacency formation (ideally, when it receives the first
Hello). The use of the Local Interface IPv4 Address TLV (as defined
in Section 3) in the LLS block advertised in OSPFv3 Hello packets for
IPv4 AF instances makes this possible. Implementations that support
OSPF BFD strict-mode for OSPFv3 IPv4 AF instances MUST include the
Local Interface IPv4 Address TLV in the LLS block advertised in their
Hello packets whenever the B-bit is also set in the LLS Type 1
Extended Options and Flags. A receiver MUST ignore the B-bit (i.e.,
not operate in strict-mode for BFD) when the Local Interface IPv4
Address TLV is not present in OSPFv3 Hello messages for OSPFv3 IPv4
AF instances.
4.2. Graceful Restart Considerations
An implementation needs to handle scenarios where both graceful
restart (GR) and the OSPF BFD strict-mode are deployed together. The
graceful restart aspects related to process restart scenarios
discussed in Section 3.3 of [RFC5882] also apply with OSPF BFD
strict-mode. Additionally, since the OSPF adjacency formation is
delayed until the BFD session establishment in OSPF BFD strict-mode,
the resultant delay in adjacency formation may affect or break the
GR-based recovery. In such cases, it is RECOMMENDED that the GR
timers are set such that they provide sufficient time to allow for
normal BFD session establishment delays.
5. Operations and Management Considerations
An implementation SHOULD report the BFD session status along with the
OSPF Init adjacency state when OSPF BFD strict-mode is enabled and
support logging operations on neighbor state transitions that include
the BFD events. This allows an operator to detect scenarios where an
OSPF adjacency may be stuck waiting for BFD session establishment.
In network deployments with noisy or degraded links with intermittent
packet loss, BFD sessions may flap, resulting in OSPF adjacency
flaps. In turn, this may cause routing churn. The use of OSPF BFD
strict-mode along with mechanisms such as hold-down (a delay in
bringing up the initial OSPF adjacency following BFD session
establishment) and/or dampening (a delay in bringing up the OSPF
adjacency following failure detected by BFD) may help reduce the
frequency of adjacency flaps and therefore reduce the associated
routing churn. The details of these mechanisms are outside the scope
of this document.
[RFC9129] specifies the base OSPF YANG module. The required
configuration and operational elements for this feature are expected
to be introduced as augmentation to this base OSPF YANG module.
6. Backward Compatibility
An implementation MUST support OSPF adjacency formation and
operations with a neighbor router that does not advertise the OSPF
BFD strict-mode capability: both when that neighbor router does not
support BFD and when it does support BFD but does not signal the OSPF
BFD strict-mode as described in this document. Implementations MAY
provide a local configuration option to force BFD operation only in
OSPF BFD strict-mode (i.e, adjacency will not come up unless BFD
session is established). In this case, an OSPF adjacency with a
neighbor that does not support OSPF BFD strict-mode would not be
established successfully. Implementations MAY provide a local
configuration option to enable BFD without the OSPF BFD strict-mode,
which results in the router not advertising the B-bit and BFD
operation being performed in the same way as prior to this
specification.
The signaling specified in this document happens at a link-local
level between routers on that link. A router that does not support
this specification would ignore the B-bit in the LLS block advertised
in Hello packets from its neighbors and continue to establish BFD
sessions (if enabled) without delaying the OSPF adjacency formation.
Since a router that does not support this specification would not
have set the B-bit in the LLS block advertised in its own Hello
packets, its neighbor routers supporting this specification would not
use OSPF BFD strict-mode with such OSPF routers. As a result, the
behavior would be the same as without this specification. Therefore,
there are no backward compatibility issues or implementation
considerations beyond what is specified herein.
7. IANA Considerations
This specification makes the following updates under the "Open
Shortest Path First (OSPF) Link Local Signaling (LLS) - Type/Length/
Value Identifiers (TLV)" parameters.
* In the "LLS Type 1 Extended Options and Flags" registry, the B-bit
has been assigned the bit position 0x00000010.
* In the "Link Local Signaling TLV Identifiers (LLS Types)"
registry, the Type 21 has been assigned to the Local Interface
IPv4 Address TLV.
8. Security Considerations
The security considerations for "OSPF Link-Local Signaling" [RFC5613]
also apply to the extension described in this document.
Inappropriate use of the B-bit in the LLS block of an OSPF Hello
message could prevent an OSPF adjacency from forming or lead to the
failure of detecting bidirectional forwarding failures. If
authentication is being used in the OSPF routing domain [RFC5709]
[RFC7474], then the Cryptographic Authentication TLV [RFC5613] MUST
also be used to protect the contents of the LLS block.
9. References
9.1. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://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,
<https://www.rfc-editor.org/info/rfc5340>.
[RFC5613] Zinin, A., Roy, A., Nguyen, L., Friedman, B., and D.
Yeung, "OSPF Link-Local Signaling", RFC 5613,
DOI 10.17487/RFC5613, August 2009,
<https://www.rfc-editor.org/info/rfc5613>.
[RFC5838] Lindem, A., Ed., Mirtorabi, S., Roy, A., Barnes, M., and
R. Aggarwal, "Support of Address Families in OSPFv3",
RFC 5838, DOI 10.17487/RFC5838, April 2010,
<https://www.rfc-editor.org/info/rfc5838>.
[RFC5882] Katz, D. and D. Ward, "Generic Application of
Bidirectional Forwarding Detection (BFD)", RFC 5882,
DOI 10.17487/RFC5882, June 2010,
<https://www.rfc-editor.org/info/rfc5882>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, DOI 10.17487/RFC5709, October
2009, <https://www.rfc-editor.org/info/rfc5709>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <https://www.rfc-editor.org/info/rfc5883>.
[RFC6213] Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV",
RFC 6213, DOI 10.17487/RFC6213, April 2011,
<https://www.rfc-editor.org/info/rfc6213>.
[RFC7474] Bhatia, M., Hartman, S., Zhang, D., and A. Lindem, Ed.,
"Security Extension for OSPFv2 When Using Manual Key
Management", RFC 7474, DOI 10.17487/RFC7474, April 2015,
<https://www.rfc-editor.org/info/rfc7474>.
[RFC9129] Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
"YANG Data Model for the OSPF Protocol", RFC 9129,
DOI 10.17487/RFC9129, October 2022,
<https://www.rfc-editor.org/info/rfc9129>.
Acknowledgements
The authors would like to acknowledge the review and inputs from Acee
Lindem, Manish Gupta, Balaji Ganesh, Les Ginsberg, Robert Raszuk,
Gyan Mishra, Muthu Arul Mozhi Perumal, Russ Housley, and Wes
Hardaker.
The authors would like to acknowledge Dylan van Oudheusden for
highlighting the problems in using OSPF BFD strict-mode for BFD
sessions for OSPFv3 IPv4 AF instances and Baalajee S for his
suggestions on the approach to address it.
The authors would like to thank John Scudder for his AD review and
suggestions to improve the document.
Authors' Addresses
Ketan Talaulikar (editor)
Cisco Systems, Inc.
India
Email: ketant.ietf@gmail.com
Peter Psenak
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
821 09 Bratislava
Slovakia
Email: ppsenak@cisco.com
Albert Fu
Bloomberg
United States of America
Email: afu14@bloomberg.net
Rajesh M
Juniper Networks
India
Email: mrajesh@juniper.net