Network Working Group R. Aggarwal
Internet Draft Juniper Networks
Intended Status: Standards Track
Updates: 1122 K. Kompella
Expiration Date: December 17, 2008 Juniper Networks
T. Nadeau
BT
G. Swallow
Cisco Systems, Inc.
June 20, 2008
BFD For MPLS LSPs
draft-ietf-bfd-mpls-07.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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.
Raggarwa [Page 1]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
Abstract
One desirable application of Bi-directional Forwarding Detection
(BFD) is to detect a Multi Protocol Label Switched (MPLS) Label
Switched Path (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 applicability of
BFD in relation to LSP-Ping for this application. It also describes
procedures for using BFD in this environment.
Table of Contents
1 Specification of requirements ......................... 3
2 Introduction .......................................... 3
3 Applicability ......................................... 3
3.1 BFD for MPLS LSPs: Motivation ......................... 3
3.2 Using BFD in Conjunction with LSP-Ping ................ 5
4 Theory of Operation ................................... 6
5 Initialization and Demultiplexing ..................... 7
6 Session Establishment ................................. 7
6.1 BFD Discriminator TLV in LSP-Ping ..................... 8
7 Encapsulation ......................................... 8
8 Security Considerations ............................... 9
9 IANA Considerations ................................... 10
10 Acknowledgments ....................................... 10
11 References ............................................ 10
11.1 Normative References .................................. 10
11.2 Informative References ................................ 10
12 Author's Address ...................................... 11
13 Intellectual Property Statement ....................... 12
14 Full Copyright Statement .............................. 12
Raggarwa [Page 2]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
1. Specification of requirements
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 [RFC2119].
2. Introduction
One desirable application of Bi-directional Forwarding Detection
(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 [RFC4379] 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 for detecting MPLS LSP
data plane failures.
3. 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 [RFC4377].
3.1. BFD for MPLS LSPs: Motivation
LSP-Ping described in [RFC4379] 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), at the egress, 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) Resource Reservation Protocol (RSVP) LSP_Tunnel IPv4/IPv6
Session [RFC3209]
b) Label Distribution Protocol (LDP) IPv4/IPv6 prefix [RFC5036]
c) Virtual Private Network (VPN) IPv4/IPv6 prefix [RFC4364]
Raggarwa [Page 3]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
d) Layer 2 VPN [L2-VPN]
e) Pseudowires based on PWid FEC and Generalized PWid FEC [RFC4447]
f) Border Gateway Protocol (BGP) labeled prefixes [RFC3107]
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, either for
diagnostic purposes or for infrequent periodic fault detection, in
the order of tens of seconds or minutes. 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 [RFC4090]. 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.
There may be other potential cases where fast failure detection is
desired for MPLS LSPs.
Raggarwa [Page 4]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
3.2. 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) or when the egress LSR
distributes an explicit null label to the penultimate hop router, 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 it possible to verify the control plane against the data plane
at the egress LSR.
b) Equal cost multi-path (ECMP) considerations. LSP-Ping traceroute
makes it possible to probe multiple alternate paths for LDP IP FECs.
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 ensuring that the LSP is mapped to the same
FEC, at the egress, as the ingress.
Raggarwa [Page 5]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
4. Theory of Operation
To use BFD for fault detection on a MPLS LSP a BFD session MUST be
established for that particular MPLS LSP. BFD control packets MUST be
sent along the same data path as the LSP being verified and are
processed by the BFD processing module of the egress LSR. If the LSP
is associated with multiple FECs, a BFD session SHOULD 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
perform the switchover exceeds the BFD fault detection interval a
fault will be declared 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. An implementation
SHOULD provide configuration options to control the BFD fault
detection interval.
If there are multiple alternate paths from an ingress LSR to an
egress LSR for a LDP IP FEC, LSP-Ping traceroute MAY be used to
determine each of these alternate paths. A BFD session SHOULD be
established for each alternate path that is discovered.
Periodic LSP-Ping echo request messages SHOULD be sent by the ingress
LSR to the egress LSR along the same data path as the LSP. This is to
periodically verify the control plane against the data plane by
ensuring that the LSP is mapped to the same FEC, at the egress, as
the ingress. The rate of generation of these LSP-Ping echo request
messages SHOULD be significantly less than the rate of generation of
the BFD control packets. An implemetation MAY provide configuration
options to control the rate of generation of the periodic LSP-Ping
echo request messages.
To enable fault detection procedures specified in this document, for
a particular MPLS LSP, this document requires the ingress and egress
LSRs to be configured. This includes configuration for supporting BFD
and LSP-Ping as specified in this document. It also includes
configuration that enables to the ingress LSR to determine the method
used by the egress LSR to identify OAM packets e.g. whether the TTL
of the innermost MPLS label needs to be set to 1 to enable the egress
LSR to identify the OAM packet. For fault detection for MPLS PWs,
this document assumes that the PW control channel type [RFC5085], is
Raggarwa [Page 6]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
configured and the support of LSP-Ping is also configured.
5. 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 or when the egress LSR
distributes an explicit null label to the penultimate hop router, or
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 MUST 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.
6. Session Establishment
A BFD session is boot-strapped using LSP-Ping. This specification
describes procedures only for BFD asynchronous mode. BFD demand mode
is outside the scope of this specification. Further the use of the
echo function is outside the scope of this specification. 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 MUST carry the local discriminator assigned by the
ingress LSR for the BFD session. This MUST subsequently be used as
the My Discriminator field in the BFD session packets sent by the
ingress LSR.
On receipt of the LSP-Ping echo request message, the egress LSR MUST
send a BFD control packet to the ingress LSR, if the validation of
the FEC in the LSP-Ping echo request message succeeds. This BFD
control packet MUST set the Your Discriminator field to the
discriminator received from the ingress LSR in the LSP-Ping echo
request message. The egress LSR MAY respond with a LSP-Ping echo
reply message that carries the local discriminator assigned by it for
the BFD session. The local discriminator assigned by the egress LSR
MUST be used as the My Discriminator field in the BFD session packets
sent by the egress LSR.
The ingress LSR follows the procedures in [BFD] to send BFD control
packets to the egress LSR in response to the BFD control packets
received from the egress LSR. The BFD control packets from the
ingress to the egress LSR MUST set use the local discriminator of the
egress LSR, in the Your Discriminator field. The egress LSR
demultiplexes the BFD session based on the received Your
Raggarwa [Page 7]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
Discriminator field. As mentioned above the egress LSR MUST send
control packets to the ingress LSR with the Your Discriminator field
set to the local discriminator of the ingress LSR. The ingress LSR
uses this to demultiplex the BFD session.
6.1. 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. IANA is requested to assign a type value of 15 to
this TLV. This TLV has 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.
If the BFD session is not in UP state, the periodic LSP-Ping echo
request messages MUST include the BFD discriminator TLV.
7. Encapsulation
BFD control packets sent by the ingress LSR MUST be 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 MAY be set to 1. This may
be necessary for certain FECs to enable the egress LSR's control
plane to receive the packet [RFC4379]. For MPLS PWs, alternatively,
the presence of a fault detection message may be indicated by setting
a bit in the control word [RFC5085].
The BFD control packet sent by the ingress LSR MUST be a UDP packet
with a well known destination port 3784 [BFD-IP] and a source port
assigned by the sender as per the procedures in [BFD-IP]. The source
IP address is a routable address of the sender. The destination IP
address MUST be randomly chosen from the 127/8 range for IPv4 and
from the 0:0:0:0:0:FFFF:7F00/104 range for IPv6 with the following
exception. If the FEC is a LDP IP FEC the ingress LSR may discover
multiple alternate paths to the egress LSR for this FEC using LSP-
ping traceroute. In this case the destination IP address, used in a
BFD session established for one such alternate path, is the address
in the 127/8 range for IPv4 or 0:0:0:0:0:FFFF:7F00/104 range for IPv6
discovered by LSP-ping traceroute [RFC4379] to exercise that
particular alternate path.
The motivation for using the address range 127/8 is the same as
specified in section 2.1 of [RFC4379]. This is an exception to the
behavior defined in [RFC1122].
Raggarwa [Page 8]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
The IP TTL or hop limit MUST be set to 1 [RFC4379].
BFD control packets sent by the egress LSR are UDP packets. The
source IP address is a routable address of the replier.
The BFD control packet sent by the egress LSR to the ingress LSR MAY
be routed based on the destination IP address as per the procedures
in [BFD-MHOP]. If this is the case the destination IP address MUST be
set to the source IP address of the LSP-Ping echo request message,
received by the egress LSR from the ingress LSR.
Or the BFD control packet sent by the egress LSR to the ingress LSR
MAY be encapsulated in a MPLS label stack. In this case 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. In this case the destination IP address MUST be
randomly chosen from the 127/8 range for IPv4 and from the
0:0:0:0:0:FFFF:7F00/104 range for IPv6.
The BFD control packet sent by the egress LSR MUST have a well known
destination port 4784, if it is routed [BFD-MHOP], or it MUST have a
well known destination port 3784 [BFD-IP] if it is encapsulated in a
MPLS label stack. The source port MUST be assigned by the egress LSR
as per the procedures in [BFD-IP].
Note that once the BFD session for the MPLS LSP is UP, either end of
the BFD session MUST NOT change the source IP address and the local
discriminator values of the BFD control packets it generates, unless
it first brings down the session. This implies that a LSR MUST ignore
BFD packets for a given session, that is demultiplexed using the
received Your Discriminator field, if the session is in UP state and
if the My Discriminator or the Source IP address fields of the
received packet do not match the values associated with the session.
8. Security Considerations
Security considerations discussed in [BFD], [BFD-MHOP] and [RFC4379]
apply to this document. For BFD control packets sent by the ingress
LSR or when the BFD control packet sent by the egress LSR are
encapsulated in a MPLS label stack, MPLS security considerations
apply. These are discussed in [MPLS-SEC]. When BFD control packets
sent by the egress LSR are routed the authentication considerations
discussed in [BFD-MHOP] should be followed.
Raggarwa [Page 9]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
9. 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.
IANA is requested to assign a value of 15 to this TLV.
10. 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. Thanks to Carlos Pignataro for his comments
and review.
11. References
11.1. Normative References
[BFD] Katz, D., and Ward, D., "Bidirectional Forwarding
Detection",
draft-ietf-bfd-base-08.txt.
[BFD-IP] D. Katz, D. Ward, "BFD for IPv4 and IPv6 (Single Hop)",
draft-ietf-bfd-v4v6-1hop-08.txt
[RFC4379] K. Kompella et. al., "Detecting MPLS Data Plane Failures",
RFC 4379, February 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
11.2. Informative References
[BFD-MHOP] D. katz, D. Ward, "BFD for Multihop Paths",
draft-ietf-bfd-multihop-06.txt
[RFC5085] T. Nadeau, C. Pignataro, "Pseudo Wire (PW) Virtual Circuit
Connectivity Verification ((VCCV): A Control Channel
for Pseudowires", RFC 5085
[RFC3209] Awduche, D., et. al, "RSVP-TE: Extensions to RSVP for LSP
Raggarwa [Page 10]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
tunnels", RFC 3209, December 2001.
[RFC4090] P. Pan, et. al., "Fast Reroute Extensions to RSVP-TE for
LSP Tunnels", May 2005.
[RFC5036] Andersson, L., et al, "LDP Specification", RFC 5036.
[RFC4364] E. Rosen, Y. Rekhter, "BGP/MPLS IP VPNs", RFC 4364,
February 2006.
[L2-VPN] K. Kompella, et. al., "Layer 2 VPNs Over Tunnels",
draft-kompella-ppvpn-l2vpn-03.txt
[RFC4447] L. Martini et. al.,"Pseudowire Setup and Maintenance
using LDP", RFC 4447, April 2006.
[RFC3107] Y. Rekhter, E. Rosen, "Carrying Label Information in
BGP-4",
RFC 3107, May 2001
[RFC4377] Nadeau, T., et. al, "OAM Requirements for MPLS
Networks", RFC 4377, February 2006.
[MPLS-SEC] L. fang, ed, "Security Framework for MPLS and GMPLS
Networks", draft-ietf-mpls-mpls-and-gmpls-security-framework-02.txt
12. Author's Address
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
BT
BT Centre
81 Newgate Street
EC1A 7AJ
London
Email: tom.nadeau@bt.com
Raggarwa [Page 11]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
George Swallow
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: swallow@cisco.com
13. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
14. Full Copyright Statement
Copyright (C) The IETF Trust (2008). This document is subject to the
rights, licenses and restrictions contained in BCP 78, and except as
set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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,
Raggarwa [Page 12]
Internet Draft draft-ietf-bfd-mpls-07.txt June 2008
Raggarwa [Page 13]
