Internet DRAFT - draft-ietf-bfd-rfc5884-clarifications
draft-ietf-bfd-rfc5884-clarifications
Internet Engineering Task Force V. Govindan
Internet-Draft K. Rajaraman
Updates: 5884 (if approved) Cisco Systems
Intended status: Standards Track G. Mirsky
Expires: April 16, 2016 Ericsson
N. Akiya
Big Switch Networks
S. Aldrin
Google
October 14, 2015
Clarifications to RFC 5884
draft-ietf-bfd-rfc5884-clarifications-04
Abstract
This document clarifies the procedures for establishing, maintaining
and removing multiple, concurrent BFD (Bidirectional Forwarding
Detection) sessions for a given <MPLS LSP, FEC> described in RFC5884.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on April 16, 2016.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Procedures for establishment of multiple BFD sessions . . 3
2.2. Procedures for maintenance of multiple BFD sessions . . . 4
2.3. Procedures for removing BFD sessions at the egress LSR . 4
2.4. Changing discriminators for a BFD session . . . . . . . . 5
3. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Background
[RFC5884] defines the procedures to bootstrap and maintain BFD
sessions for a <MPLS FEC, LSP> using LSP ping. While Section 4 of
[RFC5884] specifies that multiple BFD sessions can be established for
a <MPLS FEC, LSP> tuple, the procedures to bootstrap and maintain
multiple BFD sessions concurrently over a <MPLS FEC, LSP> are not
clearly specified. Additionally, the procedures of removing BFD
sessions bootstrapped on the egress LSR are unclear. This document
provides those clarifications without deviating from the principles
outlined in [RFC5884].
The ability for an ingress LSR to establish multiple BFD sessions for
a <MPLS FEC, LSP> tuple is useful in scenarios such as Segment
Routing based LSPs or LSPs having Equal-Cost Multipath (ECMP). The
process used by the ingress LSR to determine the number of BFD
session(s) to be bootstrapped for a <MPLS FEC, LSP> tuple and the
mechanism of constructing those session(s) are outside the scope of
this document.
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
[RFC2119].
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2. Theory of Operation
2.1. Procedures for establishment of multiple BFD sessions
Section 4 of [RFC5884] specifies the procedure for bootstrapping BFD
sessions using LSP ping. It further states that "a BFD session
SHOULD be established for each alternate path that is discovered".
This requirement has been the source of some ambiguity as the
procedures of establishing concurrent, multiple sessions have not
been explicitly specified. This ambiguity can also be attributed in
part to the text in Section 7 of [RFC5884] forbidding either end to
change local discriminator values in BFD control packets after the
session reaches the UP state. The following procedures are described
to clarify the ambiguity based on the interpretation of the authors's
reading of the referenced sections:
At the ingress LSR:
MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
given <MPLS FEC, LSP>. Each LSP ping MUST carry a different
discriminator value in the BFD discriminator TLV [RFC4379].
The egress LSR needs to perform the following:
If the validation of the FEC in the MPLS Echo request message
succeeds, check the discriminator specified in the BFD
discriminator TLV of the MPLS Echo request. If there is no local
session that corresponds to the (remote) discriminator received in
the MPLS Echo request, a new session is bootstrapped and a local
discriminator is allocated. The validation of a FEC is a
necessary condition to be satisfied to create a new BFD session at
the egress LSR. However, the policy or procedure if any, to be
applied by the egress LSR before allowing a new BFD session to be
created is outside the scope of this document. Such policies or
procedures could consider availability of system resources before
allowing a session to be created. When the egress LSR disallows
the creation of a BFD session due to policy, it MUST drop the MPLS
Echo request message.
Ensure the uniqueness of the <MPLS FEC, LSP, Remote
Discriminiator> tuple.
Except for the clarification mentioned above, the remaining
procedures ofBFD session establishment are as specified in
Sections 4-6 of [RFC5884].
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2.2. Procedures for maintenance of multiple BFD sessions
Both the ingress LSR and egress LSR use the YourDiscriminator of the
received BFD packet to demultiplex BFD sessions.
2.3. Procedures for removing BFD sessions at the egress LSR
[RFC5884] does not specify an explicit procedure for deleting BFD
sessions. The procedure for removing a BFD session established by an
out-of-band discriminator exchange using the MPLS LSP ping can
improve resource management (like memory etc.) especially in
scenarios involving thousands or more of such sessions. A few
observations are made here:
The BFD session MAY be removed in the egress LSR if the BFD
session transitions from UP to DOWN. This can either be done
immediately after the BFD session transitions from UP to DOWN or
after the expiry of a configurable timer started after the BFD
session state transitions from UP to DOWN at the egress LSR to
reduce flapping by adding hysteresis.
The BFD session on the egress LSR MAY be removed by the ingress
LSR by using the BFD diagnostic code AdminDown(7) as specified in
[RFC5880]. When the ingress LSR wants to remove a session without
triggering any state change at the egress, it MAY transmit BFD
packets indicating the State as Down(1), diagnostic code
AdminDown(7) detectMultiplier number of times. Upon receiving
such a packet, the egress LSR MAY remove the BFD session, without
triggering a change of state.
The procedures to be followed at the egress LSR when BFD
session(s) remain in the DOWN state for a significant amount of
time is a local matter. Such procedures are outside the scope of
this document.
All BFD sessions established with the FEC MUST be removed
automatically if the FEC is removed.
The egress MUST use the discriminators exchanged when the session
was brought UP, to indicate any session state change to the
ingress. The egress SHOULD reset this to zero after transmitting
bfd.detectMult number of packets if the BFD session transitions to
DOWN state.
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2.4. Changing discriminators for a BFD session
The discriminators of a BFD session established over an MPLS LSP
cannot be changed when it is in UP state. The BFD session could be
removed after a graceful transition to AdminDown state using the BFD
diagnostic code AdminDown. A new session could be established with a
different discriminator. The initiation of the transition from the
Up to Down state can be done either by the ingress LSR or the egress
LSR.
3. Backwards Compatibility
The procedures clarified by this document are fully backward
compatible with an existing implementation of [RFC5884]. While the
capability to bootstrap and maintain multiple BFD sessions may not be
present in current implementations, the procedures outlined by this
document can be implemented as a software upgrade without affecting
existing sessions. In particular, the egress LSR needs to support
multiple BFD sessions per <MPLS FEC, LSP> before the ingress LSR is
upgraded.
4. Security Considerations
This document clarifies the mechanism to bootstrap multiple BFD
sessions per <MPLS FEC, LSP>. BFD sessions, naturally, use system
and network resources. More BFD sessions means more resources will
be used. It is highly important to ensure only minimum number of BFD
sessions are provisioned per FEC, and bootstrapped BFD sessions are
properly deleted when no longer required. Additionally security
measures described in [RFC4379] and [RFC5884] are to be followed.
5. IANA Considerations
This document does not make any requests to IANA.
6. Acknowledgements
The authors would like to thank Marc Binderberger for performing
thorough reviews and providing valuable suggestions.
The authors would like to thank Mudigonda Mallik, Rajaguru Veluchamy
and Carlos Pignataro of Cisco Systems for their review comments.
The authors would like to thank Alvaro Retana and Scott Bradner for
their review comments.
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7. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
June 2010, <http://www.rfc-editor.org/info/rfc5884>.
Authors' Addresses
Vengada Prasad Govindan
Cisco Systems
Email: venggovi@cisco.com
Kalyani Rajaraman
Cisco Systems
Email: kalyanir@cisco.com
Gregory Mirsky
Ericsson
Email: gregory.mirsky@ericsson.com
Nobo Akiya
Big Switch Networks
Email: nobo.akiya.dev@gmail.com
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Sam Aldrin
Google
Email: aldrin.ietf@gmail.com
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