Network Working Group Rahul Aggarwal
Internet Draft Kireeti Kompella
Expiration Date: April 2004 Juniper Networks
BFD For MPLS LSPs
draft-raggarwa-mpls-bfd-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as ``work in progress.''
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
Abstract
One desirable application of BFD is to detect a MPLS LSP data plane
failure. LSP-Ping [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 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 applicability of BFD in relation to LSP-Ping
for this application. It also describes procedures for using BFD in
this environment.
draft-raggarwa-mpls-bfd-00.txt [Page 1]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
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 user traffic, it may
not always be possible to detect the failure using the MPLS control
plane. Hence there is a need for a mechanism to detect a data plane
failure in the MPLS LSP path.
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]
d) Layer 2 VPN [L2-VPN]
e) Layer 2 Circuit ID [LDP-PW]
LSP-Ping is a more computationally expensive mechanism than BFD for
detecting MPLS LSP data plane faults. The control plane processing
draft-raggarwa-mpls-bfd-00.txt [Page 2]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
required at the egress/ingress LSRs for processing LSP-Ping echo
request/reply messages is greater than the processing required for
BFD control packets. 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. Since
BFD is a more lightweight mechanism than LSP-Ping it may be possible
to provide data plane fault detection for a 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 node
protection [LSP-FR]. In this case the bypass LSP is essentially being
used as an alternate link. It represents an aggregate and is used to
carry a set of other LSPs. Hence fast failure detection of the bypass
LSP may be desirable.
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
We would like to point that the applicability of fast detection to
MPLS LSPs needs more study and operational input.
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
draft-raggarwa-mpls-bfd-00.txt [Page 3]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
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.
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.
draft-raggarwa-mpls-bfd-00.txt [Page 4]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
4. Initialization and Demultiplexing
A BFD session is established for each 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. 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 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.
draft-raggarwa-mpls-bfd-00.txt [Page 5]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
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 IP TTL is set to
255. The BFD control packet sent by the egress LSR to the ingress is
routed based on the destination IP address.
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.
draft-raggarwa-mpls-bfd-00.txt [Page 6]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
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. Thanks also to Yakov Rekhter for his comments on this
document.
10. Normative References
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding Detection",
draft-katz-ward-bfd-01.txt, August 2003.
[LSP-Ping] K. Kompella et. al., "Detecting MPLS Data Plane Failures",
draft-ietf-mpls-lsp-ping-03.txt
[RFC] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[VCCV] T. Nadeau, R. Aggarwal, "Pseudo Wire (PW) Virtual Circuit
Connection Verification ((VCCV)",
draft-ietf-pwe3-vccv-00.txt
[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, January
[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-03.txt
Informative References
[BFD-IP] D. Katz, D. Ward, "BFD for IPv4 and IPv6 (Single Hop)",
draft-katz-ward-bfd-v4v6-1hop-00.txt
draft-raggarwa-mpls-bfd-00.txt [Page 7]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
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
IPR Notice
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
draft-raggarwa-mpls-bfd-00.txt [Page 8]
Internet Draft draft-raggarwa-mpls-bfd-00.txt October 2003
Full Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
draft-raggarwa-mpls-bfd-00.txt [Page 9]