Link State Routing K. Talaulikar, Ed.
Internet-Draft P. Psenak
Updates: 2328 (if approved) Cisco Systems, Inc.
Intended status: Standards Track A. Fu
Expires: 9 April 2023 Bloomberg
M. Rajesh
Juniper Networks
6 October 2022
OSPF BFD Strict-Mode
draft-ietf-lsr-ospf-bfd-strict-mode-10
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 RFC2328 by augmenting the OSPF neighbor state
machine with a check for BFD session up before progression from Init
to Two-Way state when operating in OSPF BFD strict-mode.
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
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This Internet-Draft will expire on 9 April 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. LLS B-bit Flag . . . . . . . . . . . . . . . . . . . . . . . 4
3. Local Interface IPv4 Address TLV . . . . . . . . . . . . . . 4
4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. OSPFv3 IPv4 Address-Family Specifics . . . . . . . . . . 6
4.2. Graceful Restart Considerations . . . . . . . . . . . . . 7
5. Operations & Management Considerations . . . . . . . . . . . 7
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 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, in
implementations prior to the extensions specified in this document,
an OSPF adjacency may still get established over such a link but
given the more aggressive monitoring intervals supported by BFD, a
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BFD session may not get established and/or may flap over it. The
traffic that gets forwarded over such a link would experience packet
drops and the failure of the BFD session establishment would not
enable fast routing convergence. OSPF adjacency flaps may occur over
such links as 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 to not 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 both 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". It introduces the signaling support in OSPF to
achieve the blocking of adjacency formation until BFD session
establishment 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
for OSPFv3 Link-Local Signalling (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 Two-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 [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.
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2. LLS B-bit Flag
This document defines the B-bit in the LLS Type 1 Extended Options
and Flags field. This bit is defined for the LLS block included in
Hello and Database Description (DD) packets and 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 TLV 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
first with that neighbor before bringing up the OSPF adjacency as
described further in this section.
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This document updates the OSPF neighbor state machine as described in
[RFC2328]. Specifically, the operations related to the Init state
are modified as 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 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
Hello packet LLS Extended Options and Flags.
Whenever the neighbor state transitions to Down state, the removal of
the BFD session associated with that neighbor is requested by OSPF
and subsequent BFD session establishment is similarly requested by
OSPF upon transitioning into Init state. This may result in the
deletion and creation of the BFD session 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 its Hello LLS options. 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
state 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 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
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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 its subsequent Hello LLS
options. Disabling OSPF BFD strict-mode has no effect on BFD
operations and would not result in bringing down of any established
BFD sessions. Disabling BFD would result in the BFD session being
brought down due to Admin reason [RFC5882] and hence 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 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 non-DR 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 Address-Family Specifics
Multiple AF support 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 AF, it is required that BFD is used to monitor and verify
IPv4 data plane connectivity between the routers on the link and,
hence, the BFD session is setup 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 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 of 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 of their Hello packets
whenever the B-bit is also set in the LLS Options and Flags field. 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 IPv4 AF OSPFv3 instances.
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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
GR aspects discussed in section 3.3 of [RFC5882] also apply with OSPF
BFD strict-mode. Additionally, in OSPF BFD strict-mode, since the
OSPF adjacency formation is delayed until the BFD session
establishment, 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 & 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.
This in turn may cause routing churn. The use of OSPF BFD strict-
mode along with mechanisms such as hold-down (a delay in the initial
OSPF adjacency bringup following BFD session establishment) and/or
dampening (a delay in the OSPF adjacency bringup 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.
[I-D.ietf-ospf-yang] specifies the base OSPF YANG model. The
required configuration and operational elements for this feature are
expected to be introduce as augmentation to this base OSPF YANG
model.
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
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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 of 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 of 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 implementations 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.
IANA is requested to make permanent the following values that have
been assigned via early allocation:
o In the "LLS Type 1 Extended Options and Flags" registry, the B-bit
is assigned the bit position 0x00000010
o In the "Link Local Signaling TLV Identifiers (LLS Types)" registry,
the Type 21 is 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
failure to detect 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.
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9. 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
session for IPv4 AF instance with OSPFv3 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.
10. References
10.1. 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>.
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[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>.
10.2. Informative References
[I-D.ietf-ospf-yang]
Yeung, D., Qu, Y., Zhang, J., Chen, I., and A. Lindem,
"YANG Data Model for OSPF Protocol", Work in Progress,
Internet-Draft, draft-ietf-ospf-yang-29, 17 October 2019,
<https://www.ietf.org/archive/id/draft-ietf-ospf-yang-
29.txt>.
[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>.
Authors' Addresses
Ketan Talaulikar (editor)
Cisco Systems, Inc.
India
Email: ketant.ietf@gmail.com
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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
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