IDR WorkGroup M. Zheng
Internet-Draft A. Lindem
Intended status: Standards Track Cisco Systems
Expires: October 5, 2019 April 3, 2019
BGP BFD Strict-Mode
draft-merciaz-idr-bgp-bfd-strict-mode-01
Abstract
This document specifies extensions to RFC4271 BGP-4 that enable a BGP
speaker to signal additional Bidirectional Forwarding Detection (BFD)
extensions using an optional parameter BFD capability. This BFD
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, Otherwise, a BGP speaker and its peer should not require a
BFD session for BGP session establishment.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 5, 2019.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. BFD Capability . . . . . . . . . . . . . . . . . . . . . . . 3
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Backward Compatibility . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 5
9. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 neighbor session until s 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 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
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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 Capability
The BGP Capability [RFC5492] for BFD parameters will allow a BGP
speaker's BFD capabilities including its support for BFD strict-mode.
This capability is defined as follows:
Capability code: TBD
Capability length: 1 octet
Capability value: Consists of 1 octet BFD flags as follows:
+--------------------------------------------------+
| BFD Flags (8 bits) |
+--------------------------------------------------+
The use and meaning of the fields are as follows:
BFD Flags: This field contains bit flags relating to BFD.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+
The most significant bit is defined as state of Strict-Mode ("Strict-
Mode", or "S") bit, which can be used by a BGP speaker to signal its
support for BFD Strict-mode. When set (value 1), this bit indicates
that the BGP speaker has the BFD "Strict-mode" enabled. If both
local BGP speaker and its peer have BFD strict-mode enabled, then BGP
session establishment will be prevented until a BFD session is
established between the peering addresses. A BGP speaker with BFD
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strict-mode enabled MUST advertise the BFD capability with "S" bit
set.
The remaining bits are reserved and SHOULD be set to zero by the
sender and MUST be ignored by the receiver.
4. Operation
A BGP speaker that supports capabilities advertisement sends an OPEN
message to its BGP peer, the message MAY include an Optional
Parameter, called Capabilities. The parameter lists the capabilities
supported by the speaker. By following BGP capabilities
advertisement procedures defined in [RFC5492], BFD capability
advertisement for strict-mode is advertised to BGP peers.
If both BGP speakers advertise the BFD capability with the strict-
mode bit set, then the BGP state machine will be held in OpenConfirm
state [RFC4271] until a BFD session is established or the BGP session
is terminated for some other reason (e.g., the BGP Hold time expires.
A BGP speaker which supports capabilities advertisement and has BFD
strict-mode enabled MUST include the BGP BFD capability with the "S"
Bit set in the BGP capabilities it advertises.
A BGP speaker which supports BFD capability advertisement, examines
the list of capabilities present in the Capabilities BFD Parameter
that the speaker receives from its peer. If both the local and
remote BGP speakers BFD strict-mode enabled, then the BGP session
will be held in OpenConfirm state until a BFD session is established
between the two BGP speaker or the BGP session terminates for some
other reason, e.g., the BGP hold timer expires. If either peer has
not advertised the BFD Capability with strict-mode enabled, 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].
5. Backward Compatibility
The BFD capability will not introduce any backward compatibility will
not result in any backward compatibility issues as long as the
procedures in [RFC5492] and Section 4 are followed. As per
[RFC5492], a BGP speaker which does not support the BFD capability
will ignore the BFD capability. If a BGP speaker advertising the
capability receives the Unsupported Capability NOTIFICATION message
and terminates the BGP session, the BGP speaker advertising the BFD
capability SHOULD simply attempt to reestablish the BGP session with
the BFD capability omitted.
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6. Security Considerations
This specification doesn't change the basic security model inherent
in [RFC4271]. However, it does introduce a new indirect attack
vector for BGP since it is now dependent on BFD.
7. IANA Considerations
This document defines a new BGP capability - BFD Capability. The
Capability Code for BFD Capability is TBD.
IANA is requested to establish a "BGP BFD Capability Flags" registry
within the "Border Gateway Protocol (BGP) Parameters" grouping. The
Registration Procedure should be Standards Action, the initial values
as follows:
+--------------+---------------+------------+---------------+
| Bit Position | Name | Short Name | Reference |
+--------------+---------------+------------+---------------+
| 0 | Strict-Mode | S | this document |
| 1-7 | Unassigned | | this document |
+--------------+---------------+------------+---------------+
8. Acknowledgement
The authors would like to acknowledge the review and inputs from
Shyam Sethuram, Mohammed Mirza, and Bruno Decraene.
9. 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>.
[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>.
[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>.
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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>.
Authors' Addresses
Mercia Zheng
Cisco Systems
821 Alder Drive,
MILPITAS, CALIFORNIA 95035
UNITED STATES
Email: merciaz@cisco.com
Acee Lindem
Cisco Systems
821 Alder Drive,
MILPITAS, CALIFORNIA 95035
UNITED STATES
Email: acee@cisco.com
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