draft-ietf-bfd-rfc5884-clarifications-04.txt   rfc7726.txt 
Internet Engineering Task Force V. Govindan Internet Engineering Task Force (IETF) V. Govindan
Internet-Draft K. Rajaraman Request for Comments: 7726 K. Rajaraman
Updates: 5884 (if approved) Cisco Systems Updates: 5884 Cisco Systems
Intended status: Standards Track G. Mirsky Category: Standards Track G. Mirsky
Expires: April 16, 2016 Ericsson ISSN: 2070-1721 Ericsson
N. Akiya N. Akiya
Big Switch Networks Big Switch Networks
S. Aldrin S. Aldrin
Google Google
October 14, 2015 January 2016
Clarifications to RFC 5884 Clarifying Procedures for Establishing BFD Sessions for
draft-ietf-bfd-rfc5884-clarifications-04 MPLS Label Switched Paths (LSPs)
Abstract Abstract
This document clarifies the procedures for establishing, maintaining This document clarifies the procedures for establishing, maintaining,
and removing multiple, concurrent BFD (Bidirectional Forwarding and removing multiple, concurrent BFD (Bidirectional Forwarding
Detection) sessions for a given <MPLS LSP, FEC> described in RFC5884. Detection) sessions for a given <MPLS LSP, FEC> as described in RFC
5884.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on April 16, 2016. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7726.
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Table of Contents Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 3 2. Theory of Operation . . . . . . . . . . . . . . . . . . . . . 3
2.1. Procedures for establishment of multiple BFD sessions . . 3 2.1. Procedures for Establishment of Multiple BFD Sessions . . 3
2.2. Procedures for maintenance of multiple BFD sessions . . . 4 2.2. Procedures for Maintenance of Multiple BFD Sessions . . . 4
2.3. Procedures for removing BFD sessions at the egress LSR . 4 2.3. Procedures for Removing BFD Sessions at the Egress LSR . 4
2.4. Changing discriminators for a BFD session . . . . . . . . 5 2.4. Changing Discriminators for a BFD Session . . . . . . . . 5
3. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 5 3. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Normative References . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6 5.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Background 1. Background
[RFC5884] defines the procedures to bootstrap and maintain BFD [RFC5884] defines the procedures to bootstrap and maintain BFD
sessions for a <MPLS FEC, LSP> using LSP ping. While Section 4 of sessions for an <MPLS LSP, FEC> using a Label Switched Path (LSP)
[RFC5884] specifies that multiple BFD sessions can be established for ping. While Section 4 of [RFC5884] specifies that multiple BFD
a <MPLS FEC, LSP> tuple, the procedures to bootstrap and maintain sessions can be established for an <MPLS LSP, FEC> tuple, the
multiple BFD sessions concurrently over a <MPLS FEC, LSP> are not procedures to bootstrap and maintain multiple BFD sessions
clearly specified. Additionally, the procedures of removing BFD concurrently over an <MPLS LSP, FEC> are not clearly specified.
sessions bootstrapped on the egress LSR are unclear. This document Additionally, the procedures of removing BFD sessions bootstrapped on
the egress Label Switching Router (LSR) are unclear. This document
provides those clarifications without deviating from the principles provides those clarifications without deviating from the principles
outlined in [RFC5884]. outlined in [RFC5884].
The ability for an ingress LSR to establish multiple BFD sessions for The ability for an ingress LSR to establish multiple BFD sessions for
a <MPLS FEC, LSP> tuple is useful in scenarios such as Segment an <MPLS LSP, FEC> tuple is useful in scenarios such as LSPs based on
Routing based LSPs or LSPs having Equal-Cost Multipath (ECMP). The Segment Routing [SEG-ROUTING] or LSPs having Equal-Cost Multipath
process used by the ingress LSR to determine the number of BFD (ECMP). The process used by the ingress LSR to determine the number
session(s) to be bootstrapped for a <MPLS FEC, LSP> tuple and the of BFD session(s) to be bootstrapped for an <MPLS LSP, FEC> tuple and
mechanism of constructing those session(s) are outside the scope of the mechanism used to construct those session(s) are outside the
this document. scope of this document.
1.1. Requirements Language 1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in "OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. [RFC2119].
2. Theory of Operation 2. Theory of Operation
2.1. Procedures for establishment of multiple BFD sessions 2.1. Procedures for Establishment of Multiple BFD Sessions
Section 4 of [RFC5884] specifies the procedure for bootstrapping BFD Section 4 of [RFC5884] specifies the procedure for bootstrapping BFD
sessions using LSP ping. It further states that "a BFD session sessions using LSP ping. It further states that "a BFD session
SHOULD be established for each alternate path that is discovered". SHOULD be established for each alternate path that is discovered."
This requirement has been the source of some ambiguity as the This requirement has been the source of some ambiguity as the
procedures of establishing concurrent, multiple sessions have not procedures of establishing concurrent, multiple sessions have not
been explicitly specified. This ambiguity can also be attributed in been explicitly specified. This ambiguity can also be attributed in
part to the text in Section 7 of [RFC5884] forbidding either end to part to the text in Section 7 of [RFC5884] forbidding either end to
change local discriminator values in BFD control packets after the change local discriminator values in BFD control packets after the
session reaches the UP state. The following procedures are described session reaches the UP state. The following procedures are described
to clarify the ambiguity based on the interpretation of the authors's to clarify the ambiguity based on the interpretation of the authors'
reading of the referenced sections: reading of the referenced sections:
At the ingress LSR: At the ingress LSR:
MPLS LSP ping can be used to bootstrap multiple BFD sessions for a MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
given <MPLS FEC, LSP>. Each LSP ping MUST carry a different given <MPLS LSP, FEC>. Each LSP ping MUST carry a different
discriminator value in the BFD discriminator TLV [RFC4379]. discriminator value in the BFD discriminator TLV [RFC5884].
The egress LSR needs to perform the following: The egress LSR needs to perform the following:
If the validation of the FEC in the MPLS Echo request message If the validation of the Forwarding Equivalence Class (FEC) in the
succeeds, check the discriminator specified in the BFD MPLS Echo request message succeeds, check the discriminator
discriminator TLV of the MPLS Echo request. If there is no local specified in the BFD discriminator TLV of the MPLS Echo request.
session that corresponds to the (remote) discriminator received in If there is no local session that corresponds to the (remote)
the MPLS Echo request, a new session is bootstrapped and a local discriminator received in the MPLS Echo request, a new session is
discriminator is allocated. The validation of a FEC is a bootstrapped and a local discriminator is allocated. The
necessary condition to be satisfied to create a new BFD session at validation of a FEC is a necessary condition to be satisfied to
the egress LSR. However, the policy or procedure if any, to be create a new BFD session at the egress LSR. However, the policy
applied by the egress LSR before allowing a new BFD session to be or procedure, if any, to be applied by the egress LSR before
created is outside the scope of this document. Such policies or allowing a new BFD session to be created is outside the scope of
procedures could consider availability of system resources before this document. Such policies or procedures could consider
allowing a session to be created. When the egress LSR disallows availability of system resources before allowing a session to be
the creation of a BFD session due to policy, it MUST drop the MPLS created. When the egress LSR disallows the creation of a BFD
Echo request message. session due to policy, it MUST drop the MPLS Echo request message.
Ensure the uniqueness of the <MPLS FEC, LSP, Remote Ensure the uniqueness of the <MPLS LSP, FEC, Remote Discriminator>
Discriminiator> tuple. tuple.
Except for the clarification mentioned above, the remaining Except for the clarification mentioned above, the remaining
procedures ofBFD session establishment are as specified in procedures of BFD session establishment are as specified in
Sections 4-6 of [RFC5884]. Sections 4-6 of [RFC5884].
2.2. Procedures for maintenance of multiple BFD sessions 2.2. Procedures for Maintenance of Multiple BFD Sessions
Both the ingress LSR and egress LSR use the YourDiscriminator of the Both the ingress LSR and egress LSR use the Your Discriminator of the
received BFD packet to demultiplex BFD sessions. received BFD packet to demultiplex BFD sessions.
2.3. Procedures for removing BFD sessions at the egress LSR 2.3. Procedures for Removing BFD Sessions at the Egress LSR
[RFC5884] does not specify an explicit procedure for deleting BFD [RFC5884] does not specify an explicit procedure for deleting BFD
sessions. The procedure for removing a BFD session established by an sessions. The procedure for removing a BFD session established by an
out-of-band discriminator exchange using the MPLS LSP ping can out-of-band discriminator exchange using the MPLS LSP ping can
improve resource management (like memory etc.) especially in improve resource management (e.g., memory), especially in scenarios
scenarios involving thousands or more of such sessions. A few involving thousands or more of such sessions. A few observations are
observations are made here: made here:
The BFD session MAY be removed in the egress LSR if the BFD The BFD session MAY be removed in the egress LSR if the BFD
session transitions from UP to DOWN. This can either be done session transitions from UP to DOWN. This can either be done
immediately after the BFD session transitions from UP to DOWN or immediately after the BFD session transitions from UP to DOWN or
after the expiry of a configurable timer started after the BFD after the expiry of a configurable timer started after the BFD
session state transitions from UP to DOWN at the egress LSR to session state transitions from UP to DOWN at the egress LSR to
reduce flapping by adding hysteresis. reduce flapping by adding hysteresis.
The BFD session on the egress LSR MAY be removed by the ingress 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 LSR by using the BFD diagnostic code AdminDown(7) as specified in
skipping to change at page 4, line 44 skipping to change at page 5, line 9
The procedures to be followed at the egress LSR when BFD The procedures to be followed at the egress LSR when BFD
session(s) remain in the DOWN state for a significant amount of session(s) remain in the DOWN state for a significant amount of
time is a local matter. Such procedures are outside the scope of time is a local matter. Such procedures are outside the scope of
this document. this document.
All BFD sessions established with the FEC MUST be removed All BFD sessions established with the FEC MUST be removed
automatically if the FEC is removed. automatically if the FEC is removed.
The egress MUST use the discriminators exchanged when the session The egress MUST use the discriminators exchanged when the session
was brought UP, to indicate any session state change to the was brought UP to indicate any session state change to the
ingress. The egress SHOULD reset this to zero after transmitting ingress. The egress SHOULD reset this to zero after transmitting
bfd.detectMult number of packets if the BFD session transitions to bfd.detectMult number of packets if the BFD session transitions to
DOWN state. DOWN state.
2.4. Changing discriminators for a BFD session 2.4. Changing Discriminators for a BFD Session
The discriminators of a BFD session established over an MPLS LSP 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 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 removed after a graceful transition to AdminDown state using the BFD
diagnostic code AdminDown. A new session could be established with a diagnostic code AdminDown. A new session could be established with a
different discriminator. The initiation of the transition from the different discriminator. The initiation of the transition from the
Up to Down state can be done either by the ingress LSR or the egress UP to DOWN state can be done by either the ingress LSR or the egress
LSR. LSR.
3. Backwards Compatibility 3. Backwards Compatibility
The procedures clarified by this document are fully backward The procedures clarified by this document are fully backward
compatible with an existing implementation of [RFC5884]. While the compatible with an existing implementation of [RFC5884]. While the
capability to bootstrap and maintain multiple BFD sessions may not be capability to bootstrap and maintain multiple BFD sessions may not be
present in current implementations, the procedures outlined by this present in current implementations, the procedures outlined by this
document can be implemented as a software upgrade without affecting document can be implemented as a software upgrade without affecting
existing sessions. In particular, the egress LSR needs to support existing sessions. In particular, the egress LSR needs to support
multiple BFD sessions per <MPLS FEC, LSP> before the ingress LSR is multiple BFD sessions per <MPLS LSP, FEC> before the ingress LSR is
upgraded. upgraded.
4. Security Considerations 4. Security Considerations
This document clarifies the mechanism to bootstrap multiple BFD This document clarifies the mechanism to bootstrap multiple BFD
sessions per <MPLS FEC, LSP>. BFD sessions, naturally, use system sessions per <MPLS LSP, FEC>. BFD sessions, naturally, use system
and network resources. More BFD sessions means more resources will and network resources. More BFD sessions means more resources will
be used. It is highly important to ensure only minimum number of BFD be used. It is highly important to ensure that only a minimum number
sessions are provisioned per FEC, and bootstrapped BFD sessions are of BFD sessions are provisioned per FEC and that bootstrapped BFD
properly deleted when no longer required. Additionally security sessions are properly deleted when they are no longer required.
measures described in [RFC4379] and [RFC5884] are to be followed. 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 5. References
their review comments.
7. Normative References 5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol [RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379, Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006, DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>. <http://www.rfc-editor.org/info/rfc4379>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection [RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010, (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>. <http://www.rfc-editor.org/info/rfc5880>.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow, [RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label "Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884, Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
June 2010, <http://www.rfc-editor.org/info/rfc5884>. June 2010, <http://www.rfc-editor.org/info/rfc5884>.
5.2. Informative References
[SEG-ROUTING]
Filsfils, C., Ed., Previdi, S., Ed., Decraene, B.,
Litkowski, S., and R. Shakir, "Segment Routing
Architecture", Work in Progress, draft-ietf-spring-
segment-routing-07, December 2015.
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.
Authors' Addresses Authors' Addresses
Vengada Prasad Govindan Vengada Prasad Govindan
Cisco Systems Cisco Systems
Email: venggovi@cisco.com Email: venggovi@cisco.com
Kalyani Rajaraman Kalyani Rajaraman
Cisco Systems Cisco Systems
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