BFD for MPLS LSPs: Clarifying Procedures for Establishing BFD Sessions for MPLS Label Switched Paths (LSPs)
Spells out how several BFD liveness sessions can run side by side on one MPLS path, and how they get cleaned up.
Govindan, V., Rajaraman, K., Mirsky, G., Akiya, N., and S. Aldrin, "Clarifying Procedures for Establishing BFD Sessions for MPLS Label Switched Paths (LSPs)", RFC 7726, DOI 10.17487/RFC7726, January 2016, <https://www.rfc-editor.org/info/rfc7726>.
@misc{rfc7726,
series = {Request for Comments},
number = 7726,
howpublished = {RFC 7726},
publisher = {RFC Editor},
doi = {10.17487/RFC7726},
url = {https://www.rfc-editor.org/info/rfc7726},
author = {V. Govindan and K. Rajaraman and G. Mirsky and N. Akiya and S. Aldrin},
title = {{Clarifying Procedures for Establishing BFD Sessions for MPLS Label Switched Paths (LSPs)}},
pagetotal = 7,
year = 2016,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) V. Govindan
Request for Comments: 7726 K. Rajaraman
Updates: 5884 Cisco Systems
Category: Standards Track G. Mirsky
ISSN: 2070-1721 Ericsson
N. Akiya
Big Switch Networks
S. Aldrin
Google
January 2016
Clarifying Procedures for Establishing BFD Sessions for
MPLS Label Switched Paths (LSPs)
Abstract
This document clarifies the procedures for establishing, maintaining,
and removing multiple, concurrent BFD (Bidirectional Forwarding
Detection) sessions for a given <MPLS LSP, FEC> as described in RFC
5884.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Background . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
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. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Normative References . . . . . . . . . . . . . . . . . . 6
5.2. Informative References . . . . . . . . . . . . . . . . . 6
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Background
[RFC5884] defines the procedures to bootstrap and maintain BFD
sessions for an <MPLS LSP, FEC> using a Label Switched Path (LSP)
ping. While Section 4 of [RFC5884] specifies that multiple BFD
sessions can be established for an <MPLS LSP, FEC> tuple, the
procedures to bootstrap and maintain multiple BFD sessions
concurrently over an <MPLS LSP, FEC> are not clearly specified.
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
outlined in [RFC5884].
The ability for an ingress LSR to establish multiple BFD sessions for
an <MPLS LSP, FEC> tuple is useful in scenarios such as LSPs based on
Segment Routing [SEG-ROUTING] 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 an <MPLS LSP, FEC> tuple and
the mechanism used to construct 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].
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'
reading of the referenced sections:
At the ingress LSR:
MPLS LSP ping can be used to bootstrap multiple BFD sessions for a
given <MPLS LSP, FEC>. Each LSP ping MUST carry a different
discriminator value in the BFD discriminator TLV [RFC5884].
The egress LSR needs to perform the following:
If the validation of the Forwarding Equivalence Class (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 LSP, FEC, Remote Discriminator>
tuple.
Except for the clarification mentioned above, the remaining
procedures of BFD session establishment are as specified in
Sections 4-6 of [RFC5884].
2.2. Procedures for Maintenance of Multiple BFD Sessions
Both the ingress LSR and egress LSR use the Your Discriminator 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 (e.g., memory), 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.
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 by either 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 LSP, FEC> before the ingress LSR is
upgraded.
4. Security Considerations
This document clarifies the mechanism to bootstrap multiple BFD
sessions per <MPLS LSP, FEC>. BFD sessions, naturally, use system
and network resources. More BFD sessions means more resources will
be used. It is highly important to ensure that only a minimum number
of BFD sessions are provisioned per FEC and that bootstrapped BFD
sessions are properly deleted when they are no longer required.
Additionally, security measures described in [RFC4379] and [RFC5884]
are to be followed.
5. References
5.1. 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,
<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>.
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
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
Sam Aldrin
Google
Email: aldrin.ietf@gmail.com
AI ELI5
On this page
§1Background and Ambiguity in RFC 5884
RFC 5884 defines how to bootstrap and maintain BFD (Bidirectional Forwarding Detection) sessions for an <MPLS LSP, FEC> using an LSP ping. Section 4 of that RFC says multiple BFD sessions can be established for one such tuple, but the procedures for bootstrapping and maintaining them concurrently are not clearly specified. The procedures for removing sessions created on the egress Label Switching Router (LSR) are also unclear.
The RFC names two scenarios where several sessions are useful: LSPs based on Segment Routing and LSPs that use Equal-Cost Multipath (ECMP). How the ingress decides how many sessions to create, and how it builds them, is out of scope.
An older rulebook said you could have more than one check-in on a road, but it never explained how. This note fills in the missing steps.
§2.1Establishing Multiple BFD Sessions
RFC 5884 says a BFD session SHOULD be established for each alternate path that is discovered. The ambiguity comes partly from Section 7 of RFC 5884, which forbids either end from changing its local discriminator in BFD control packets once the session is UP. The authors describe the procedures below as their reading of those sections.
At the ingress LSR, MPLS LSP ping can bootstrap multiple BFD sessions for a given <MPLS LSP, FEC>. Each LSP ping MUST carry a different discriminator value in the BFD discriminator TLV.
At the egress LSR, if validation of the FEC in the MPLS Echo request succeeds, it checks the discriminator in the BFD discriminator TLV. If no local session corresponds to that remote discriminator, a new session is bootstrapped and a local discriminator is allocated. Successful FEC validation is necessary to create a session, but any further policy, such as checking system resources, is outside the scope. If policy disallows the session, the egress MUST drop the MPLS Echo request.
The egress must also keep the <MPLS LSP, FEC, Remote Discriminator> tuple unique. The rest of session establishment follows Sections 4 to 6 of RFC 5884.
Each check-in gets its own name tag. When a new name tag shows up, the other side starts a new check-in for it, unless it is too busy to take on more.
- MPLS Echo requestCarries a BFD discriminator TLV with a different discriminator for each session.
- BFD control packetsA new session with its own local discriminator is created if FEC validation succeeds and policy allows.
§2.2Maintaining Sessions with Your Discriminator
Once several sessions are running, both the ingress and the egress LSR use the Your Discriminator field of each received BFD packet to demultiplex the sessions, meaning to decide which session the packet belongs to.
Every message has the name tag of the check-in it belongs to, so each side knows which conversation to put it in.
§2.3Removing Sessions at the Egress
RFC 5884 gives no explicit procedure for deleting BFD sessions. Removing sessions created by out-of-band discriminator exchange through LSP ping can help resource management, for example memory, when thousands of such sessions exist. The RFC makes several observations.
- The egress MAY remove a session when it moves from UP to DOWN, either immediately or after a configurable timer that adds hysteresis to reduce flapping.
- The ingress MAY remove a session on the egress by sending the diagnostic code
AdminDown(7)from RFC 5880. To avoid triggering a state change at the egress, it MAY send BFD packets with StateDown(1)and diagnosticAdminDown(7)detectMultiplier times, after which the egress MAY remove the session without a state change. - What the egress does when sessions stay DOWN for a long time is a local matter.
- All BFD sessions established with a FEC MUST be removed automatically if the FEC is removed.
- The egress MUST use the discriminators exchanged when the session came UP to signal any state change, and SHOULD reset them to zero after sending bfd.detectMult packets if the session goes DOWN.
Old check-ins that nobody needs should be tidied away so they do not clog up memory. Either side can say goodbye politely, and if the road itself is closed, all its check-ins go too.
§2.4Changing Discriminators
The discriminators of a BFD session over an MPLS LSP cannot be changed while it is UP. To use different ones, the session can be removed after a graceful transition to AdminDown using the AdminDown diagnostic code, and a new session established with a different discriminator. Either the ingress or the egress LSR can start the UP to DOWN transition.
You cannot swap a check-in's name tag in the middle of it. You end it nicely and start a new one with a new tag.
§3Backwards Compatibility
The clarified procedures are fully backward compatible with existing implementations of RFC 5884. Current implementations may lack the ability to bootstrap and maintain multiple sessions, but the procedures can be added as a software upgrade without affecting existing sessions. The egress LSR needs to support multiple BFD sessions per <MPLS LSP, FEC> before the ingress LSR is upgraded.
Nothing that already works breaks. Upgrade the receiving side first, then the sending side.
§4Security Considerations
BFD sessions use system and network resources, and more sessions use more. The RFC says it is highly important to provision only the minimum number of sessions per FEC and to delete bootstrapped sessions properly when they are no longer required. The security measures in RFC 4379 and RFC 5884 are still to be followed.
Too many check-ins waste effort, so only make as many as you need and clean them up afterwards.