Internet DRAFT - draft-ietf-idr-bgp-bfd-strict-mode
draft-ietf-idr-bgp-bfd-strict-mode
IDR Workgroup M. Zheng
Internet-Draft Ciena
Updates: RFC4271 (if approved) A. Lindem
Intended status: Standards Track LabN Consulting, L.L.C.
Expires: 6 July 2024 J. Haas
Juniper Networks, Inc.
A. Fu
Bloomberg L.P.
3 January 2024
BGP BFD Strict-Mode
draft-ietf-idr-bgp-bfd-strict-mode-12
Abstract
This document specifies extensions to RFC4271 BGP-4 that enable a BGP
speaker to negotiate additional Bidirectional Forwarding Detection
(BFD) extensions using a BGP capability. This BFD Strict-Mode
Capability enables a BGP speaker to prevent a BGP session from being
established until a BFD session is established. It is referred to as
BGP BFD "strict-mode". BGP BFD strict-mode will be supported when
both the local speaker and its remote peer are BFD strict-mode
capable.
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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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."
This Internet-Draft will expire on 6 July 2024.
Copyright Notice
Copyright (c) 2024 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
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. BFD Strict-Mode Capability . . . . . . . . . . . . . . . . . 3
4. BGP BFD Strict-Mode Procedures . . . . . . . . . . . . . . . 3
4.1. Closing BGP Sessions . . . . . . . . . . . . . . . . . . 4
4.2. Stability Considerations . . . . . . . . . . . . . . . . 5
5. Manageability Considerations . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
Bidirectional Forwarding Detection BFD [RFC5882] enables routers to
monitor data plane connectivity and to detect faults in the
bidirectional forwarding path between them. This capability is
leveraged by routing protocols such as BGP [RFC4271] to rapidly react
to topology changes in the face of path failures.
The BFD interaction with BGP is specified in Section 10.2 of
[RFC5882]. When BFD is enabled for a BGP neighbor, faults in the
bidirectional forwarding detected by BFD result in session
termination. It is possible in some failure scenarios for the
network to be in a state such that a BGP session may be established
but a BFD session cannot be established. In some other scenarios, it
may be possible to establish a BGP session, but a degraded or poor-
quality link may result in the corresponding BFD session going up and
down frequently.
To avoid situations which result in routing churn and to minimize the
impact of network interruptions, it will be beneficial to disallow
BGP to establish a session until BFD session is successfully
established and has stabilized. We refer to this mode of operation
as BGP BFD "strict-mode". However, always using "strict-mode" would
preclude BGP operation in an environment where not all routers
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support BFD strict-mode or have BFD enabled. This document defines
BGP "strict-mode" operation as preventing BGP session establishment
until both the local and remove speakers have a stable BFD session.
The document also specifies the BGP protocol extensions for BGP
capability [RFC5492] for announcing BFD parameters including a BGP
speaker's support for "strict-mode", i.e., requiring a BFD session
for BGP session establishment.
2. 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.
3. BFD Strict-Mode Capability
The BGP Strict-Mode Capability [RFC5492] will allow a BGP speaker's
to advertise this capability. The capability is defined as follows:
Capability code: 74
Capability length: 0 octets
4. BGP BFD Strict-Mode Procedures
A BGP speaker which supports capabilities advertisement and has BFD
strict-mode enabled MUST include the BFD Strict-Mode Capability.
A BGP speaker which supports the BFD Strict-Mode Capability, examines
the list of capabilities present in the capabilities that the speaker
receives from its peer. If both the local and remote BGP speakers
include the BFD Strict-Mode Capability, the BGP finite state machine
does not transition to the Established state from OpenConfirm state
[RFC4271] until the BFD session is in the Up state (see below for
AdminDown state). This means that a KEEPALIVE message is not sent
nor is the KeepaliveTimer set.
If the BFD session does not transition to the Up state, and the
HoldTimer has been negotiated to a non-zero value, the BGP FSM will
close the session appropriately. If the HoldTimer has been
negotiated to a zero value, the session should be closed after a time
of X. This time X is referred as "BGP BFD Hold time". The proposed
default BGP BFD Hold time value is 30 seconds. The BGP BFD Hold time
value is configurable.
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If BFD session is in the AdminDown state, then the BGP finite state
machine will proceed normally without input from BFD. This means
that BFD session "AdminDown" state WILL NOT prevent the BGP state
transition from the OpenConfirm state to the Established state.
Once the BFD session has transitioned to the Up state, the BGP FSM
may proceed to transition from the OpenConfirm state to state
Established state. Once in the Established state, a KEEPALIVE
message is sent and a KeepaliveTimer for the BGP peer is started.
BGP strict-mode cannot be enabled unless BFD is configured for the
BGP peer. If BFD is removed for the BGP peer, then BGP strict-mode
will also be disabled.
Note that it is fully possible to have BFD enabled between the peers
without BGP strict-mode.
If either BGP peer has not advertised the BFD Strict-Mode Capability,
then a BFD session WILL NOT be required for the BGP session to reach
Established state. This does not preclude usage of BFD after BGP
session establishment [RFC5882].
If BFD strict-mode is enabled or disabled for a BGP peer and the BGP
session state is not Established state, then the BGP will close the
session.
If BFD strict-mode is enabled or disabled for a BGP peer and the BGP
session state is in the Established state, the local BFD strict-mode
configuration will be modified but the session will remain in
Established state.
Since BFD strict-mode is only applicable during BGP session
establishment, this inconsistency would not have an impact on the
Established session unless the remote BGP peer is waiting in
OpenConfirm state. To avoid this situation, BFD strict-mode SHOULD
be modified consistently on both the local and remote BGP peers.
4.1. Closing BGP Sessions
When BGP sessions are closed according to the procedures in this
document, the session SHOULD be terminated with a NOTIFICATION
message with the Cease Code and the "BFD Down" Subcode.
([I-D.ietf-idr-bfd-subcode]) This informs the operator that
interaction with BFD is the root cause of the BGP session being
unable to move to the Established state.
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4.2. Stability Considerations
The use of BGP BFD strict-mode along with mechanisms such as hold-
down (a delay in the initial BGP Establishment state following BFD
session establishment) and/or dampening (a delay in the BGP
Establishment state following failure detected by BFD) may help
reduce the frequency of BGP session flaps and therefore reduce the
associated routing churn.
To avoid deadlock when utilizing both BFD hold-down and BGP BFD
strict-mode, when strict-mode is enabled for a peer, the BGP FSM MUST
be enabled. That is, BFD hold-down procedures MUST NOT prevent BGP
from establishing a connection with the remote BGP speaker.
If both the local and remote BGP speakers include the BFD Strict-Mode
Capability, the BGP state machine is permitted to transition to the
Established state from the OpenConfirm state after the locally
configured BFD hold-down interval is observed. That is, the BFD
session has been Up for the desired amount of time. Implementations
MAY start the BFD session associated with the BGP bfd-strict session
prior to the BGP FSM starting.
It is RECOMMENDED that the BFD hold-down intervals used with BFD
strict-mode, when configured, use similar values. Similarly, the
negotiated BGP holdtime SHOULD be long enough to account for the time
between the BGP FSM reaching the OpenConfirm state, the BFD hold-down
interval, and any delay for the BFD session being initiated. Failure
to do so can result in the BGP speaker that has transitioned to the
Established state expiring its BGP holdtime and closing the
connection. This is because the remote BGP speaker hasn't
transitioned to Established and begun sending KEEPALIVE messages.
A BGP speaker SHOULD log a message if it closes its session due to
hold timer expiration while waiting for the BFD hold-down interval.
The behavior of BGP speakers implementing BFD hold-down without
negotiating the BFD strict-mode feature is out of scope of this
document. However, the authors are aware that inconsistent behaviors
in BGP implementations for BFD hold-down without BGP BFD strict-mode
may result in BGP session deadlock.
5. Manageability Considerations
Auto-configuration is possible for the enabling BGP BFD strict-mode.
However, the configuration automation is out of the scope of this
document.
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To simplify troubleshoorting and avoid inconsistencies, it is
RECOMMENDED that BFD strict-mode configuration be consistent for both
BGP peers.
6. Security Considerations
The mechanism defined in this document interacts with the BGP finite
state machine when so configured. The security considerations of BFD
thus, become considerations for BGP-4 [RFC4271] so used. Given that
a BFD session is required for a BGP session, a Denial-of-Service
(DoS) attack on BGP can now be mounted by preventing a BFD session
between the BGP peers from being established or interrupting an
existing BFD session. The use of the BFD Authentication mechanism
defined in [RFC5880] is thus RECOMMENDED when used to protect BGP-4
[RFC4271].
7. IANA Considerations
This document defines the BGP BFD Strict-Mode Capability. The
Capability Code 74 has been assigned from the First-Come-First-Served
range (64-238) of the Capability Codes registry.
8. Acknowledgement
The authors would like to acknowledge the review and inputs from
Shyam Sethuram, Mohammed Mirza, Bruno Decraene, Carlos Pignataro,
Enke Chen, and Anup Kumar.
9. Normative References
[I-D.ietf-idr-bfd-subcode]
Haas, J., "A BGP Cease Notification Subcode For
Bidirectional Forwarding Detection (BFD)", Work in
Progress, Internet-Draft, draft-ietf-idr-bfd-subcode-05,
27 December 2022, <https://datatracker.ietf.org/doc/html/
draft-ietf-idr-bfd-subcode-05>.
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
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[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[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>.
[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>.
Authors' Addresses
Mercia Zheng
Ciena
3939 N. 1st Street
San Jose, CA 95134
United States
Email: merciaz.ietf@gmail.com
Acee Lindem
LabN Consulting, L.L.C.
301 Midenhall Way
Cary, NC 27513
United States
Email: acee.ietf@gmail.com
Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net
Albert Fu
Bloomberg L.P.
731 Lexington Avenue
New York, NY 10022
United States of America
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Email: afu14@bloomberg.net
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