Internet DRAFT - draft-ketant-lsr-ospf-bfd-strict-mode
draft-ketant-lsr-ospf-bfd-strict-mode
Link State Routing K. Talaulikar
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems, Inc.
Expires: May 4, 2020 A. Fu
Bloomberg
M. Rajesh
Juniper Networks
November 1, 2019
OSPF Strict-Mode for BFD
draft-ketant-lsr-ospf-bfd-strict-mode-03
Abstract
This document specifies the extensions to OSPF that enables a router
and its neighbor to signal their intention to use Bidirectional
Forwarding Detection (BFD) for their adjacency using link-local
advertisement between them. The signaling of this BFD enablement,
allows the router to block and not allow the establishment of
adjacency with its neighbor router until a BFD session is
successfully established between them. The document describes this
OSPF "strict-mode" of BFD establishment as a prerequisite to
adjacency formation.
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.
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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Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on May 4, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . 6
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 dataplane connectivity over links between them and to detect
faults in the bidirectional path between them. This capability is
leveraged by routing protocols like Open Shortest Path First (OSPFv2)
[RFC2328] and OSPFv3 [RFC5340] to detect connectivity failures for
their adjacencies and trigger the rerouting of traffic around this
failure more quickly than their periodic hello messaging based
detection mechanism.
The use of BFD for monitoring routing protocols adjacencies is
described in [RFC5882]. When BFD monitoring is enabled for OSPF
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adjacencies, the BFD session is bootstrapped based on the neighbor
address information discovered by the exchange of OSPF hello
messages. Faults in the bidirectional forwarding detected via BFD
then result in the bringing down of the OSPF adjacency. Note that it
is possible in some failure scenarios for the network to be in a
state such that the OSPF adjacency is capable of coming up, but the
BFD session cannot be established, and, more particularly, data
cannot be forwarded. In certain other scenarios, a degraded or poor
quality link may result in OSPF adjacency formation to succeed only
to result in BFD session establishment not being successful or the
BFD session going down frequently due to its faster detection
mechanism.
To avoid such situations which result in routing churn in the
network, it would be beneficial not to allow OSPF to establish a
neighbor adjacency until the BFD session is successfully established
and stabilized. However, this would preclude the OSPF operation in
an environment in which not all OSPF routers support BFD and are
enabled for BFD monitoring. A solution would be to block the
establishment of OSPF adjacencies if both systems are willing to
establish a BFD session but a BFD session cannot be established.
Such a mode of BFD use by OSPF is referred to as "strict-mode"
wherein BFD session establishment becomes a prerequisite for OSPF
adjacency coming up.
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 a BFD session in the "strict-mode". It
also introduces an extension for OSPFv3 link-local signaling of
interface IPv4 address when used for 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].
2. LLS B-bit Flag
A new B-bit is defined in the LLS Type 1 Extended Options and Flags
field. This bit is defined for the LLS block included in Hello
packets and indicates that BFD is enabled on the link and that the
router supports BFD strict-mode. Section 7 describes the position of
this new B-bit.
A router MUST include the LLS block with the LLS Type 1 Extended
Options and Flags TLV with the B-bit set its Hello messages when BFD
is enabled on the link.
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3. Local Interface IPv4 Address TLV
The Local Interface IPv4 Address TLV is a new LLS TLV meant for
OSPFv3 protocol operations for IPv4 AF instances [RFC5838]. It has
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: TBD, suggested value 21
Length: 4 octet
Local Interface IPv4 Address: The primary IPv4 address of the
local interface.
4. Procedures
A router supporting BFD strict-mode advertises this capability
through its hello messages as described in Section 2 above. When a
router supporting BFD strict-mode, detects a new neighbor router that
also supports BFD strict-mode, then it proceeds to establish
adjacency with that neighbor 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 BFD strict-mode is used:
Init (without BFD strict-mode)
In this state, an Hello packet has recently been seen 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 BFD strict-mode)
In this state, an Hello packet has recently been seen from the
neighbor. However, bidirectional communication has not yet been
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established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). A BFD session
establishment to the neighbor is requested, if not already done
(e.g. in the event of transition from 2-way state). All neighbors
in higher than Init state and those in Init state with BFD session
up are listed in the Hello packets sent from the associated
interface.
Whenever the neighbor state transitions to Down state, the removal of
the BFD session associated with that neighbor SHOULD be requested by
OSPF and the session re-setup SHOULD similarly be requested by OSPF
after transitioning into Init state. This may result in the deletion
and creation of BFD session respectively when OSPF is the only client
interested in BFD session to the neighbor address.
An implementation MUST NOT wait for BFD session establishment in Init
state unless BFD strict-mode is enabled on the router and the
specific neighbor indicates BFD strict-mode capability via its Hello
messages. When BFD is enabled, but the strict-mode of operation
cannot be used, then an implementation SHOULD start the BFD session
establishment only in 2-Way or higher state. This makes it possible
for router to operate a mix of BFD operation in strict-mode or normal
mode across different interfaces or even different neighbors on the
same multi-access LAN interface.
Once the OSPF state machine has moved beyond the Init state, any
change in the B-bit advertised in subsequent Hello messages 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 the Init
state). The disabling of BFD (or BFD strict-mode) on a router would
result in its not setting the B-bit in its subsequent Hello messages.
The disabling of BFD strict-mode has no change on the BFD operations
and would not result in bringing down of any established BFD session.
The disabling of BFD would result in the BFD session brought down due
to Admin reason 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 messages. If the adjacency is already
up (i.e. in its terminal state of Full or 2-way with non-DR routers
on a LAN) with a neighbor that also support BFD strict-mode, then an
implemantion SHOULD NOT bring this adjacency down and instead use the
BFD strict-mode of operations 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 BFD strict-mode
for its bring up.
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4.1. OSPFv3 IPv4 Address-Family Specifics
The multiple AF support in OSPFv3 [RFC5838] requires the use of IPv6
link-local address as 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 be 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 learnt only later in the
adjacency formation phase when the neighbor's Link-LSA is received.
This results in the setup of the BFD session either after the
adjacency is established or much later in the adjacency formation
sequence.
To enable the BFD operations in strict-mode, it is necessary for a
router to learn it's 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 messages for IPv4
AF instances makes this possible. Implementations that support
strict-mode of BFD operations for OSPFv3 IPv4 AF instances MUST
include the Local Interface IPv4 Address TLV in the LLS block of
their hello messages whenever the B-bit is set. A receiver MUST
ignore the B-bit (i.e. not operate in BFD strict mode) unless the
Local Interface IPv4 Address TLV is present in OSPFv3 Hello message
for IPv4 AF instances.
4.2. Graceful Restart Considerations
An implementation needs to handle scenarios where both graceful
restart (GR) and the strict-mode of BFD operations are deployed
together. The GR aspects discussed in [RFC5882] also apply with
strict-mode of operations. In addition to that, since the OSPF
adjacency formation is held up until the BFD session establishment in
the strict-mode of operation, the resultant delay in adajcency
formation may affect or break the GR based recovery. In such cases,
it is RECOMMENDED that the GR timers are setup such that they provide
sufficient time to cover 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 operating in BFD strict-mode 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.
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In network deployments with noisy links or those with packet loss,
BFD sessions may flap frequently. In such scenarions, OSPF strict-
mode for BFD may be deployed in conjunction with an BFD dampening or
hold-down mechanism to help avoid frequent adjacency flaps due BFD
causing routing churn.
6. Backward Compatibility
An implementation MUST support OSPF adjacency formation and
operations with a neighbor router that does not advertise the BFD
strict-mode 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 operations only in the
strict-mode in which case, OSPF adjacency with a neighbor that does
not support BFD strict-mode would not be established successfully.
Implementations MAY provide an option to disable BFD strict-mode
which results in the router not advertising the B-bit and BFD
operations being performed in the same way as before this
specification.
The signaling specified in this document happens at a link-local
level between routers on that link. A router which does not support
this specification would ignore the B-bit in the LLS block of hello
messages from its neighbors and continue to bootstrap BFD sessions,
if enabled, without holding back the OSPF adjacency formation. Since
the router which does not support this specification would not have
set the B-bit in the LLS block of its own hello messages, its
neighbor routers that support this specification would not use BFD
strict-mode with it. As a result, the behavior would be the same as
before this specification. Therefore, there are no backward
compatibility related issues or considerations that need to be taken
care of when implementing this specification.
7. IANA Considerations
This specification updates Link Local Signaling TLV Identifiers
registry.
Following values are requested for allocation:
o B-bit from "LLS Type 1 Extended Options and Flags" registry at bit
position 0x00000010.
o TBD (Suggested value 21) - Local Interface IPv4 Address TLV
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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 and Rajesh M.
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>.
[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>.
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[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@cisco.com
Peter Psenak
Cisco Systems, Inc.
Apollo Business Center
Mlynske nivy 43
Bratislava 821 09
Slovakia
Email: ppsenak@cisco.com
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Albert Fu
Bloomberg
USA
Email: afu14@bloomberg.net
Rajesh M
Juniper Networks
India
Email: mrajesh@juniper.net
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