Internet DRAFT - draft-grmas-bfd-rfc5884-clarifications
draft-grmas-bfd-rfc5884-clarifications
Internet Engineering Task Force V. Govindan
Internet-Draft Cisco Systems
Updates: 5884 (if approved) K. Rajaraman
Intended status: Standards Track G. Mirsky
Expires: April 6, 2015 Ericsson
N. Akiya
Cisco Systems
S. Aldrin
Huawei Technologies
October 3, 2014
Clarifications to RFC 5884
draft-grmas-bfd-rfc5884-clarifications-00
Abstract
This document clarifies the procedures for establishing, maintaining
and removing multiple, concurrent BFD sessions for a given <MPLS LSP,
FEC> described in RFC5884.
Status of This Memo
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This Internet-Draft will expire on April 6, 2015.
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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 . . . 3
2.3. Procedures for removing BFD sessions at the egress LSR . 4
2.4. Changing discriminators for a BFD session . . . . . . . . 4
3. Backwards Compatibility . . . . . . . . . . . . . . . . . . . 4
4. Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. Normative References . . . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
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 6 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 discriminator (remote) received in
the MPLS Echo request, a new session is bootstrapped and a local
discriminator is allocated. Since the BFD local discriminator of
either ends cannot change as long as the session is in the UP
state, a new discriminator received in the LSP ping unambiguously
conveys the intent of the LSR ingress to bootstrap a new BFD
session for the FEC specified in the LSP ping.
Ensure the uniqueness of the <MPLS FEC, LSP, Remote
Discriminiator> tuple.
The remaining procedures of session establishment are as specified
in [RFC5884].
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.
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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
options are possible here:
The BFD session MAY be removed in the egress LSR if the BFD
session transitions from UP to DOWN. This can be done after the
expiry of a configurable timer started after the BFD session state
transitions from UP to DOWN at the egress LSR.
The BFD session on the egress LSR MAY be gracefully removed by the
ingress LSR by using the BFD diagnostic code AdminDown(7)
specified in [RFC5880]. When the ingress LSR wants to gracefully
remove a session, it MAY transmit BFD packets containing the
diagnostic code AdminDown(7) detectMultiplier number of times.
Upon receiving such a packet, the egress LSR MAY remove the BFD
session gracefully, without triggering a change of state.
Ed Note: The procedures to be followed at the egress LSR when the BFD
session never transitions to UP from DOWN state are yet to be
clarified
Regardless of the option chosen to proceed, all BFD sessions
established with the FEC MUST be removed automatically if the FEC is
removed.
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
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multiple BFD sessions per <MPLS FEC, LSP> before the ingress LSR is
upgraded.
4. Encapsulation
The encapsulation of BFD packets are the same as specified by
[RFC5884].
5. 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.
6. IANA Considerations
This document does not make any requests to IANA.
7. Acknowledgements
The authors would like to thank Mudigonda Mallik, Rajaguru Veluchamy
and Carlos Pignataro of Cisco Systems for their review comments.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
February 2006.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010.
[RFC5884] Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
"Bidirectional Forwarding Detection (BFD) for MPLS Label
Switched Paths (LSPs)", RFC 5884, June 2010.
Authors' Addresses
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Vengada Prasad Govindan
Cisco Systems
Email: venggovi@cisco.com
Kalyani Rajaraman
Ericsson
Email: kalyani.rajaraman@ericsson.com
Gregory Mirsky
Ericsson
Email: gregory.mirsky@ericsson.com
Nobo Akiya
Cisco Systems
Email: nobo@cisco.com
Sam Aldrin
Huawei Technologies
Email: aldrin.ietf@gmail.com
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