Network Working Group Rahul Aggarwal
Internet Draft Kireeti Kompella
Expiration Date: August 2005 Juniper Networks
Thomas D. Nadeau
George Swallow
Cisco Systems, Inc
BFD For MPLS LSPs
draft-ietf-bfd-mpls-01.txt
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Abstract
One desirable application of Bi-directional Forwarding Detection
(BFD) is to detect a MPLS LSP data plane failure. LSP-Ping is an
existing mechanism for detecting MPLS data plane failures and for
verifying the MPLS LSP data plane against the control plane. BFD can
be used for the former, but not for the latter. However the control
plane processing required for BFD control packets is relatively
smaller than the processing required for LSP-Ping messages. A
combination of LSP-Ping and BFD can be used to provide faster data
plane failure detection and/or make it possible to provide such
detection on a greater number of LSPs. This document describes the
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applicability of BFD in relation to LSP-Ping for this application. It
also describes procedures for using BFD in this environment.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [KEYWORDS].
1. Introduction
One desirable application of BFD is to track the liveliness of a
Multi Protocol Label Switched (MPLS) Label Switched Path (LSP). In
particular BFD can be used to detect a data plane failure in the
forwarding path of a MPLS LSP. LSP-Ping [LSP-PING] is an existing
mechanism for detecting MPLS LSP data plane failures and for
verifying the MPLS LSP data plane against the control plane. This
document describes the applicability of BFD in relation to LSP-Ping
for detecting MPLS LSP data plane failures. It also describes
procedures for using BFD in this environment.
2. Applicability
In the event of a MPLS LSP failing to deliver data traffic, it may
not always be possible to detect the failure using the MPLS control
plane. For instance the control plane of the MPLS LSP may be
functional while the data plane may be mis-forwarding or dropping
data. Hence there is a need for a mechanism to detect a data plane
failure in the MPLS LSP path [OAM-REQ].
2.1. BFD for MPLS LSPs: Motivation
LSP-Ping described in [LSP-Ping] is an existing mechanism for
detecting a MPLS LSP data plane failure. In addition LSP-Ping also
provides a mechanism for verifying the MPLS control plane against the
data plane. This is done by ensuring that the LSP is mapped to the
same Forwarding Equivalence Class (FEC) as the ingress.
BFD cannot be used for verifying the MPLS control plane against the
data plane. However BFD can be used to detect a data plane failure
in the forwarding path of a MPLS LSP. The LSP may be associated with
any of the following FECs:
a) RSVP IPv4/IPv6 Session [RSVP-TE]
b) LDP IPv4/IPv6 prefix [LDP]
c) VPN IPv4/IPv6 prefix [2547]
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d) Layer 2 VPN [L2-VPN]
e) Layer 2 Circuit ID [LDP-PW]
LSP-Ping includes extensive control plane verification. BFD on the
other hand was designed as a light-weight means of testing only the
data plane. As a result, LSP-Ping is computationally more expensive
than BFD for detecting MPLS LSP data plane faults. BFD is also more
suitable for being implemented in hardware or firmware due to its
fixed packet format. Thus the use of BFD for detecting MPLS LSP data
plane faults has the following advantages:
a) Support for fault detection for greater number of LSPs.
b) Fast detection. Detection with sub-second granularity is
considered as fast detection. LSP-Ping is intended to be used in an
environment where fault detection messages are exchanged in the order
of seconds. Hence its not appropriate for fast detection. BFD on the
other hand is designed for sub-second fault detection intervals.
Following are some potential cases when fast detection may be
desirable for MPLS LSPs:
1. In the case of a bypass LSP used for facility based link or
node protection [LSP-FR]. In this case the bypass LSP is essentially
being used as an alternate link to protect one or more LSPs. It
represents an aggregate and is used to carry data traffic belonging
to one or more LSPs when the link or the node being protected fails.
Hence fast failure detection of the bypass LSP may be desirable
particularly in the event of link or node failure when the data
traffic is moved to the bypass LSP.
2. MPLS Pseudo Wires (PW). Fast detection may be desired for MPLS
PWs depending on i) the model used to layer the MPLS network with the
layer 2 network. and ii) the service that the PW is emulating. For a
non-overlay model between the layer 2 network and the MPLS network
the provider may rely on PW fault detection to provide service status
to the end-systems. Also in that case interworking scenarios such as
ATM/Frame Relay interworking may force periodic PW fault detection
messages. Depending on the requirements of the service that the MPLS
PW is emulating, fast failure detection may be desirable. Use of BFD
for PWs is further described in [VCCV] and [OAM-MAP].
We would like to point that the applicability of fast detection to
MPLS LSPs needs more study and operational input.
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2.2. Using BFD in Conjunction with LSP-Ping
BFD can be used for MPLS LSP data plane fault detection. However it
does not have all the funcitonality of LSP-Ping. In paticular it
cannot be used for verifying the control plane against the data
plane. LSP Ping performs the following functions that are outside the
scope of BFD:
a) Association of a LSP-Ping echo request message with a FEC. In
the case of Penultimate Hop Popping (PHP), for a single label stack
LSP, the only way to associate a fault detection message with a FEC
is by carrying the FEC in the message. LSP-Ping provides this
functionality. Next-hop label allocation also makes it necessary to
carry the FEC in the fault detection message as the label alone is
not sufficient to identify the LSP being verified. In addition to
this presence of the FEC in the echo request message makes is
possible to verify the control plane against the data plane at the
egress LSR.
b) ECMP considerations. LSP-Ping makes it possible to exercise
multiple alternate paths for a given LSP.
c) Traceroute. LSP-Ping supports traceroute for a FEC and it can be
used for fault isolation.
Hence BFD is used in conjunction with LSP-Ping for MPLS LSP fault
detection:
i) LSP-Ping is used for boot-strapping the BFD session as described
later in this document.
ii) BFD is used to exchange fault detection (i.e. BFD session)
packets at the required detection interval.
iii) LSP-Ping is used to periodically verify the control plane
against the data plane by re-synchronizing the MPLS LSP and FEC
mappings.
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3. Theory of Operation
To use BFD for fault detection on a MPLS LSP a BFD session is
established for that particular MPLS LSP. BFD control packets are
sent along the same data path as the LSP being verified and are
processed by the control plane of the egress LSR. If the LSP is
associated with multiple FECs, a BFD session is established for each
FEC. For instance this may happen in the case of next-hop label
allocation. Hence the operation is conceptually similar to the data
plane fault detection procedures of LSP-Ping.
If MPLS fast-reroute is being used for the MPLS LSP the use of BFD
for fault detection can result in false fault detections if the BFD
fault detection interval is less than the MPLS fast-reroute
switchover time. When MPLS fast-reroute is triggered because of a
link or node failure BFD control packets will be dropped until
traffic is switched on to the backup LSP. If the time taken to make
the switchover exceeds the BFD fault detection interval a fault will
be delcared even though the MPLS LSP is being locally repaired. To
avoid this the BFD fault detection interval should be greater than
the fast-reroute switchover time.
4. Initialization and Demultiplexing
A BFD session may be established for a FEC associated with a MPLS
LSP. As desribed above in the case of PHP and next-hop label
allocation the BFD control packet received by the egress LSR does not
contain sufficient information to associate it with a BFD session.
Hence the demultiplexing has to be done using the remote
discriminator field in the received BFD control packet. The exchange
of BFD discriminators for this purpose is described in the next
section.
5. Session Establishment
A BFD session is boot-strapped using LSP-Ping. The initiation of
fault detection for a particular <MPLS LSP, FEC> combination results
in the exchange of LSP-Ping echo request and echo reply packets, in
the ping mode, between the ingress and egress LSRs for that <MPLS
LSP, FEC>. To establish a BFD session a LSP-Ping echo request message
carries the local discriminator assigned by the ingress LSR for the
BFD session. This is subsequently used as the My Discriminator field
in the BFD session packets sent by the ingress LSR. The egress LSR
responds with a echo reply message that carries the local
discriminator assigned by it for the BFD session. This is
subsequently used as the My Discriminator field in the BFD session
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packets sent by the egress LSR.
Once the ingress LSR learns the local discriminator assigned by the
egress LSR for a given BFD session, it sends a BFD control packet to
the egress LSR with the Your Discriminator set to the local
discriminator of the egress LSR. The egress LSR demultiplexes the BFD
session based on the received Your Discriminator field. It sends
control packets to the ingress LSR with the Your Discriminator field
set to the local discriminator of the ingress LSR. The ingress LSR
can use this to demultiplex the BFD session.
5.1. BFD Discriminator TLV in LSP-Ping
LSP-Ping echo request and echo reply messages carry a BFD
discriminator TLV for the purpose of session establishment as
described above. This TLV has a type TBD and a length of 4. The value
contains the 4 byte local discriminator that the LSR sending the LSP-
Ping message associates with the BFD session.
6. Encapsulation
BFD control packets sent by the ingress LSR are encapsulated in the
MPLS label stack that corresponds to the FEC for which fault
detection is being performed. If the label stack has a depth greater
than one, the TTL of the inner MPLS label maybe set to 1. This may be
necessary for certain FECs to enable the egress LSR's control plane
to receive the packet [LSP-Ping]. For MPLS PWs, alternatively, the
presence of a fault detection message may be indicated by setting a
bit in the control word [VCCV].
The BFD control packet sent by the ingress LSR MUST be a UDP packet
with a well known destination port TBD and a source port assigned by
the sender. The source IP address is a routable address of the
sender. The destination IP address is a (randomly chosen) address
from 127/8. The IP TTL is set to 1.
BFD control packets sent by the egress LSR are UDP packets. The
source IP address is a routable address of the replier; the source
port is the well-known UDP port TBD. The destination IP address and
UDP port are copied from the source IP address and UDP port of the
control packet received from the ingress LSR. The BFD control packet
sent by the egress LSR to the ingress LSR may be encapsulated in a
MPLS label stack and the presence of the fault detection message is
indicated as described above. This may be the case if the FEC for
which the fault detection is being perfomed corresponds to a bi-
directional LSP or a MPLS PW. This may also be the case when there is
a return LSP from the egress LSR to the ingress LSR. It may also be
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routed based on the destination IP address [BFD-MHOP].
7. Security Considerations
Security considerations discussed in [BFD] and [LSP-Ping] apply to
this document.
8. IANA Considerations
This document introduces a BFD discriminator TLV in LSP-Ping. This
has to be assigned from the TLV type registry maintained by IANA.
9. Acknowledgments
We would like to thank Yakov Rekhter, Dave Katz and Ina Minei for
contributing to the discussions that formed the basis of this
document and for their comments. Thanks to Dimitri Papadimitriou for
his comments and review.
10. References
10.1. Normative References
[BFD] Katz, D., and Ward, D.,
"Bidirectional Forwarding Detection",
draft-ietf-bfd-base-00.txt.
[LSP-Ping] K. Kompella et. al., "Detecting MPLS Data Plane Failures",
draft-ietf-mpls-lsp-ping-07.txt
[RFC] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[BFD-IP] D. Katz, D. Ward, "BFD for IPv4 and IPv6 (Single Hop)",
draft-ietf-bfd-v4v6-1hop-01.txt
[BFD-MHOP] D. katz, D. Ward, "BFD for Multihop Paths",
draft-ietf-bfd-multihop-00.txt
[VCCV] T. Nadeau, R. Aggarwal, "Pseudo Wire (PW) Virtual Circuit
Connection Verification ((VCCV)",
draft-ietf-pwe3-vccv-03.txt
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[RSVP-TE] Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
tunnels", RFC 3209, December 2001.
[LDP] Andersson, L., et al, "LDP Specification", RFC 3036.
[2547] E. Rosen, Y. Rekhter, "BGP/MPLS IP VPNs",
draft-ietf-l3vpn-rfc2547bis-01.txt
[L2-VPN] K. Kompella, et. al., "Layer 2 VPNs Over Tunnels",
draft-kompella-ppvpn-l2vpn-03.txt
[LDP-PW] L. Martini et. al.,"Pseudowire Setup and Maintenance
using LDP",
draft-ietf-pwe3-control-protocol-08.txt
[OAM-MAP] Nadeau, T., Morrow, M., Busschbach, P., et. al,
Pseudo Wire (PW) OAM Message Mapping,
draft-nadeau-pwe3-oam-msg-map-05.txt, January 2004
[OAM-REQ] Nadeau, T., et. al, "OAM Requirements for MPLS
Networks", draft-ietf-mpls-oam-requirements-02.txt,
June 2003.
11. Author Information
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: rahul@juniper.net
Kireeti Kompella
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: kireeti@juniper.net
Thomas D. Nadeau
Cisco Systems, Inc.
300 Beaver Brook Road
Boxboro, MA 01719
Phone: +1-978-936-1470
Email: tnadeau@cisco.com
George Swallow
Cisco Systems, Inc.
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300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: swallow@cisco.com
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14. Acknowledgement
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