MPLS Working Group G. Mirsky
Internet-Draft ZTE
Intended status: Standards Track J. Tantsura
Expires: December 15, 2017 Individual
I. Varlashkin
Google
M. Chen
Huawei
June 13, 2017
Bidirectional Forwarding Detection (BFD) Directed Return Path
draft-ietf-mpls-bfd-directed-07
Abstract
Bidirectional Forwarding Detection (BFD) is expected to be able to
monitor wide variety of encapsulations of paths between systems.
When a BFD session monitors an explicitly routed unidirectional path
there may be a need to direct egress BFD peer to use a specific path
for the reverse direction of the BFD session.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Requirements Language . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Control of the Reverse BFD Path . . . . . . . . . . . . . . . 3
3.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 3
3.2. Static and RSVP-TE sub-TLVs . . . . . . . . . . . . . . . 4
3.3. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5
4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 5
5.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . 6
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
[RFC5880], [RFC5881], and [RFC5883] established the BFD protocol for
IP networks. [RFC5884] and [RFC7726] set rules of using BFD
asynchronous mode over IP/MPLS LSPs. These standards implicitly
assume that the egress BFD peer will use the shortest path route
regardless of route being used to send BFD control packets towards
it.
For the case where a LSP is explicitly routed it is likely that the
shortest return path to the ingress BFD peer would not follow the
same path as the LSP in the forward direction. The fact that BFD
control packets are not guaranteed to follow the same links and nodes
in both forward and reverse directions is a significant factor in
producing false positive defect notifications, i.e. false alarms, if
used by the ingress BFD peer to deduce the state of the forward
direction.
This document defines the BFD Reverse Path TLV as an extension to LSP
Ping [RFC8029] and proposes that it is to be used to instruct the
egress BFD peer to use an explicit path for its BFD control packets
associated with a particular BFD session. The TLV will be allocated
from the TLV and sub-TLV registry defined in [RFC8029]. As a special
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case, forward and reverse directions of the BFD session can form a
bi-directional co-routed associated channel.
1.1. Conventions used in this document
1.1.1. 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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Problem Statement
When BFD is used to monitor unidirectional explicitly routed path,
e.g. MPLS-TE LSP, BFD control packets in forward direction would be
in-band using the mechanism defined in [RFC5884] and [RFC5586]. But
the reverse direction of the BFD session would follow the shortest
path route and that might lead to the problem in detecting failures
on a unidirectional explicit path as described below:
o a failure detection by ingress node on the reverse path cannot be
interpreted as bi-directional failure unambiguously and thus
trigger, for example, protection switchover of the forward
direction without possibility of being a false positive.
To address this scenario the egress BFD peer would be instructed to
use a specific path for BFD control packets.
3. Control of the Reverse BFD Path
LSP ping, defined in [RFC8029], uses BFD Discriminator TLV [RFC5884]
to bootstrap a BFD session over an MPLS LSP. This document defines a
new TLV, BFD Reverse Path TLV, that MUST contain a single sub-TLV
that can be used to carry information about the reverse path for the
BFD session that is specified by value in BFD Discriminator TLV.
3.1. BFD Reverse Path TLV
The BFD Reverse Path TLV is an optional TLV within the LSP ping
[RFC8029]. However, if used, the BFD Discriminator TLV MUST be
included in an Echo Request message as well. If the BFD
Discriminator TLV is not present when the BFD Reverse Path TLV is
included, then it MUST be treated as malformed Echo Request, as
described in [RFC8029].
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The BFD Reverse Path TLV carries information about the path onto
which the egress BFD peer of the BFD session referenced by the BFD
Discriminator TLV MUST transmit BFD control packets. The format of
the BFD Reverse Path TLV is as presented in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BFD Reverse Path TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Path |
~ ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: BFD Reverse Path TLV
BFD Reverse Path TLV Type is 2 octets in length and has a value of
TBD1 (to be assigned by IANA as requested in Section 5).
Length field is 2 octets long and defines the length in octets of the
Reverse Path field.
Reverse Path field contains a sub-TLV. Any Target FEC sub-TLV
(already defined, or to be defined in the future) for TLV Types 1,
16, and 21 of MPLS LSP Ping Parameters registry MAY be used in this
field. None, one or more sub-TLVs MAY be included in the BFD Reverse
Path TLV. If none sub-TLVs found in the BFD Reverse Path TLV, the
egress BFD peer MUST revert to using the default, i.e., over IP
network, reverse path.
If the egress LSR cannot find the path specified in the Reverse Path
TLV it MUST send Echo Reply with the received Reverse Path TLV and
set the Return Code to "Failed to establish the BFD session. The
specified reverse path was not found" Section 3.3. The egress BFD
peer MAY establish the BFD session over IP network as defined in
[RFC5884].
3.2. Static and RSVP-TE sub-TLVs
When an explicit path on an MPLS data plane is set either as Static
or RSVP-TE LSP respective sub-TLVs defined in [RFC7110] MAY be used
to identify the explicit reverse path for the BFD session.
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3.3. Return Codes
This document defines the following Return Codes for MPLS LSP Echo
Reply:
o "Failed to establish the BFD session. The specified reverse path
was not found", (TBD4). When a specified reverse path is not
available at the egress BFD peer, an Echo Reply with the return
code set to "Failed to establish the BFD session. The specified
reverse path was not found" MUST be sent back to the ingress BFD
peer Section 3.1.
4. Use Case Scenario
In the network presented in Figure 2 node A monitors two tunnels to
node H: A-B-C-D-G-H and A-B-E-F-G-H. To bootstrap a BFD session to
monitor the first tunnel, node A MUST include a BFD Discriminator TLV
with Discriminator value (e.g. foobar-1) and MAY include a BFD
Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap a
BFD session to monitor the second tunnel, node A MUST include a BFD
Discriminator TLV with a different Discriminator value (e.g. foobar-
2) [RFC7726] and MAY include a BFD Reverse Path TLV that references
H-G-F-E-B-A tunnel.
C---------D
| |
A-------B G-----H
| |
E---------F
Figure 2: Use Case for BFD Reverse Path TLV
If an operator needs node H to monitor a path to node A, e.g.
H-G-D-C-B-A tunnel, then by looking up list of known Reverse Paths it
MAY find and use the existing BFD session.
5. IANA Considerations
5.1. BFD Reverse Path TLV
The IANA is requested to assign a new value for BFD Reverse Path TLV
from the "Multiprotocol Label Switching Architecture (MPLS) Label
Switched Paths (LSPs) Ping Parameters - TLVs" registry, "TLVs and
sub-TLVs" sub-registry.
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+----------+----------------------+---------------+
| Value | Description | Reference |
+----------+----------------------+---------------+
| X (TBD1) | BFD Reverse Path TLV | This document |
+----------+----------------------+---------------+
Table 1: New BFD Reverse Type TLV
5.2. Return Code
The IANA is requested to assign a new Return Code value from the
"Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
Ping Parameters" registry, "Return Codes" sub-registry, as follows
using a Standards Action value.
+----------+----------------------------------------+---------------+
| Value | Description | Reference |
+----------+----------------------------------------+---------------+
| X (TBD4) | Failed to establish the BFD session. | This document |
| | The specified reverse path was not | |
| | found. | |
+----------+----------------------------------------+---------------+
Table 2: New Return Code
6. Security Considerations
Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
and [RFC8029], apply to this document.
7. 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>.
[RFC5586] Bocci, M., Ed., Vigoureux, M., Ed., and S. Bryant, Ed.,
"MPLS Generic Associated Channel", RFC 5586,
DOI 10.17487/RFC5586, June 2009,
<http://www.rfc-editor.org/info/rfc5586>.
[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>.
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[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<http://www.rfc-editor.org/info/rfc5881>.
[RFC5883] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for Multihop Paths", RFC 5883, DOI 10.17487/RFC5883,
June 2010, <http://www.rfc-editor.org/info/rfc5883>.
[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>.
[RFC7110] Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
"Return Path Specified Label Switched Path (LSP) Ping",
RFC 7110, DOI 10.17487/RFC7110, January 2014,
<http://www.rfc-editor.org/info/rfc7110>.
[RFC7726] 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,
<http://www.rfc-editor.org/info/rfc7726>.
[RFC8029] Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
Switched (MPLS) Data-Plane Failures", RFC 8029,
DOI 10.17487/RFC8029, March 2017,
<http://www.rfc-editor.org/info/rfc8029>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
Appendix A. Acknowledgments
Authors greatly appreciate thorough review and the most helpful
comments from Eric Gray and Carlos Pignataro.
Authors' Addresses
Greg Mirsky
ZTE
Email: gregimirsky@gmail.com
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Jeff Tantsura
Individual
Email: jefftant.ietf@gmail.com
Ilya Varlashkin
Google
Email: Ilya@nobulus.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
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