Link State Routing K. Talaulikar
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc.
Expires: August 14, 2022 A. Fu
Bloomberg
M. Rajesh
Juniper Networks
February 10, 2022
OSPF BFD Strict-Mode
draft-ietf-lsr-ospf-bfd-strict-mode-05
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 the BFD strict-mode, adjacency formation will be
blocked until a BFD session has been successfully established.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on August 14, 2022.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. LLS B-bit Flag . . . . . . . . . . . . . . . . . . . . . . . 3
3. Local Interface IPv4 Address TLV . . . . . . . . . . . . . . 4
4. Procedures . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. OSPFv3 IPv4 Address-Family Specifics . . . . . . . . . . 6
4.2. Graceful Restart Considerations . . . . . . . . . . . . . 6
5. Operations & Management Considerations . . . . . . . . . . . 7
6. Backward Compatibility . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
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 protocols
adjacencies is described in [RFC5882].
When BFD monitoring is enabled for OSPF adjacencies, 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, sometimes, an OSPF
adjacency may still get established over such a link but given the
more aggressive monitoring intervals supported by BFD, a 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
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routing convergence. OSPF adjaceny 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
establishement 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. It also introduces an extension for OSPFv3 Link-Local
Signalling (LLS) of the interface IPv4 address to be used for the BFD
session setup when OSPFv3 is used for an IPv4 address-family (AF)
instance.
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.
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.
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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. 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 octet
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.
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.
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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 SHOULD be requested by
OSPF and subsequent BFD session establishment SHOULD similarly be
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 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 be signaled by both neighbors, an implementation SHOULD start BFD
session establishment only in 2-Way state or higher 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
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. 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
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BFD strict-mode, then an implementation SHOULD NOT bring this
adjacency down but instead use the OSPF BFD strict-mode 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 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 for 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.
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 [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.
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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.
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 specifically enable BFD
operation in OSPF BFD strict-mode only. 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 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 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.
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7. IANA Considerations
This specification updates Link Local Signaling TLV Identifiers
registry.
The following values have been assigned via early allocation:
o B-bit from "LLS Type 1 Extended Options and Flags" registry at bit
position 0x00000010.
o Type 21 - 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] SHOULD also be used to protect the contents of the LLS
block.
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, and Muthu Arul Mozhi Perumal.
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.
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>.
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[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>.
10.2. 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>.
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Authors' Addresses
Ketan Talaulikar
Cisco Systems, Inc.
India
Email: ketant.ietf@gmail.com
Peter Psenak
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
Bratislava 821 09
Slovakia
Email: ppsenak@cisco.com
Albert Fu
Bloomberg
USA
Email: afu14@bloomberg.net
Rajesh M
Juniper Networks
India
Email: mrajesh@juniper.net
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