OSPF Strict-Mode for BFD
draft-ietf-lsr-ospf-bfd-strict-mode-04
Link State Routing K. Talaulikar
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc.
Expires: April 25, 2022 A. Fu
Bloomberg
M. Rajesh
Juniper Networks
October 22, 2021
OSPF Strict-Mode for BFD
draft-ietf-lsr-ospf-bfd-strict-mode-04
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 of strict-mode
for BFD session establishment for OSPF adjacency. If both OSPF
neighbors advertise the strict-mode for BFD, 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 April 25, 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 . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 and trigger the
rerouting of traffic around the failure faster than with OSPF hello
packet monitoring.
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. Note that it is possible
in some failure scenarios for the network to be in a state such that
an OSPF adjacency can be established but a BFD session cannot be
established and maintained. In certain other scenarios, a degraded
or poor quality link will allow OSPF adjacency formation to succeed
but the BFD session establishment will fail or the BFD session will
flap. In this case, traffic that gets forwarded over such a link may
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experience packet drops while the failure of the BFD session
establishment would not enable fast routing convergence if the link
were to go down or flap.
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 in which not
all OSPF routers support BFD and are enabled for BFD 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".
This document specifies the OSPF protocol extensions using link-local
signaling (LLS) [RFC5613] for a router to indicate to its neighbor
the willingness to establish its adjacency using the strict-mode for
BFD. It also introduces an extension for OSPFv3 link-local signaling
of the interface IPv4 address when used for an IPv4 address-family
(AF) instance to enable discovery of the IPv4 addresses for BFD
session setup.
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 strict-mode for BFD.
Section 7 describes the position of the B-bit.
A router MUST include the LLS block with the LLS Type 1 Extended
Options and Flags TLV with the B-bit set in its Hello and DD packets
when strict-mode for BFD is enabled on the link.
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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 strict-mode for BFD advertises this capability
through its Hello packets as described in Section 2. When a router
supporting strict-mode for BFD discovers a new neighbor router that
also supports strict-mode for BFD, then 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 as
below when strict-mode for BFD is used:
Init (without strict-mode for BFD)
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 strict-mode for BFD)
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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 the Hello packets associated
with the interface if they either have a corresponding BFD session
established or have not advertised strict-mode for BFD 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 strict-mode for BFD is enabled on the router and the
specific neighbor indicates strict-mode for BFD capability via its
Hello LLS options. When BFD is enabled, but the strict-mode for
operation has not be signaled by both neighbors, then an
implementation SHOULD start the 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 strict-mode for BFD) on an OSPF router would result
in it not setting the B-bit in its subsequent Hello LLS options.
Disabling strict-mode for BFD has no effect on the BFD operations and
would not result in bringing down of any established BFD session.
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
strict-mode for BFD, then an implementation SHOULD NOT bring this
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adjacency down but instead use the strict-mode for BFD operation
after the next transition into Init state. However, if the adjacency
is not up, then an implementation MAY bring such an adjacency down so
it can use the strict-mode for BFD 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
the 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 enable operation in strict-mode for BFD, 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 the OSPFv3 Hello packets for IPv4
AF instances makes this possible. Implementations that support
strict-mode for BFD operation 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 BFD strict mode) when the Local Interface IPv4 Address TLV is not
present in OSPFv3 Hello message for IPv4 AF OSPFv3 instances.
4.2. Graceful Restart Considerations
An implementation needs to handle scenarios where both graceful
restart (GR) and the strict-mode for BFD operation are deployed
together. The GR aspects discussed in [RFC5882] also apply with
strict-mode for BFD operation. Additionally, in strict-mode for BFD
operation, 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 operating in strict-mode for BFD and
perform logging operations on state transitions to 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 links or those with packet loss,
BFD sessions may flap frequently. In such scenarios, OSPF strict-
mode for BFD may be deployed in conjunction with a BFD dampening or
hold-down mechanism to avoid frequent adjacency flaps that cause
routing churn.
6. Backward Compatibility
An implementation MUST support OSPF adjacency formation and
operations with a neighbor router that does not advertise the strict-
mode for BFD capability - both when that neighbor router does not
support BFD and when it does support BFD but not in the strict-mode
of operation as described in this document. Implementations MAY
provide an option to specifically enable BFD operation only in the
strict-mode. In this case, an OSPF adjacency with a neighbor that
does not support strict-mode for BFD would not be established
successfully. Implementations MAY provide an option to disable
strict-mode for BFD 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 the
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 that support this specification would not use strict-mode for
BFD with such OSPF routers. As a result, the behavior would be the
same as before this specification. Therefore, there are no backward
compatibility issues or implementations considerations beyond what is
specified herein.
7. IANA Considerations
This specification updates Link Local Signaling TLV Identifiers
registry.
Following values have been assigned via early allocation:
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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 and Balaji Ganesh.
The authors would like to acknowledge Dylan van Oudheusden for
highlighting the problems in using 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>.
[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>.
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[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>.
[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
Cisco Systems, Inc.
India
Email: ketant.ietf@gmail.com
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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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