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MPLS Working Group G. Mirsky
Internet-Draft ZTE
Intended status: Standards Track J. Tantsura
Expires: February 22, 2020 Nuage Networks
I. Varlashkin
Google
M. Chen
Huawei
August 21, 2019
Bidirectional Forwarding Detection (BFD) Directed Return Path
draft-ietf-mpls-bfd-directed-12
Abstract
Bidirectional Forwarding Detection (BFD) is expected to be able to
monitor a 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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."
This Internet-Draft will expire on February 22, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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. Return Codes . . . . . . . . . . . . . . . . . . . . . . 5
4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 5
5. Operational Considerations . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 6
6.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
[RFC5880], [RFC5881], and [RFC5883] established the BFD protocol for
IP networks. [RFC5884] and [RFC7726] set rules for using BFD
asynchronous mode over IP/MPLS LSPs. These standards do not define
means to control the path selection at the egress BFD peer to send
BFD control packets towards the ingress BFD system.
For the case when BFD is used to detect defects of the traffic
engineered LSP the path the BFD control packets transmitted by the
egress BFD system toward the ingress may be disjoint from 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 may be one of the factors contributing to
producing false positive defect notifications, i.e., false alarms, at
the ingress BFD peer. Ensuring that both directions of the BFD
session use co-routed paths may, in some environments, improve the
determinism of the failure detection and localization.
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
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from the TLV and sub-TLV registry defined in [RFC8029]. As a special
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 explicitly routed unidirectional path,
e.g., MPLS-TE LSP, BFD control packets in forward direction would be
in-band using the mechanism defined in [RFC5884]. 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 an explicit
unidirectional path as described below:
o detection by an ingress node of a failure on the reverse path may
not be unambiguously interpreted as the failure of the path in the
forward direction.
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
To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in
[RFC8029], MUST be used with BFD Discriminator TLV [RFC5884]. 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 the 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 two octets in length and has a value of
TBD1 (to be assigned by IANA as requested in Section 6).
Length field is two octets long and defines the length in octets of
the Reverse Path field.
Reverse Path field contains a sub-TLV. Any non-multicast Target FEC
Stack 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. Multicast Target FEC Stack sub-TLVs, i.e., p2mp
and mp2mp, SHOULD NOT be included in Reverse Path field. If the
egress LSR finds multicast Target Stack sub-TLV, it MUST send echo
reply with the received Reverse Path TLV, BFD Discriminator TLV and
set the Return Code to "Inappropriate Target FEC Stack sub-TLV
present" Section 3.2. None, one or more sub-TLVs MAY be included in
the BFD Reverse Path TLV. If no sub-TLVs are found in the BFD
Reverse Path TLV, the egress BFD peer MUST revert to using the local
policy based decision as described in Section 7 [RFC5884], i.e.,
routed over IP network.
If the egress LSR cannot find the path specified in the Reverse Path
TLV it MUST send Echo Reply with the received BFD Discriminator TLV,
Reverse Path TLV and set the Return Code to "Failed to establish the
BFD session. The specified reverse path was not found" Section 3.2.
An implementation MAY provide configuration options to define action
at the egress BFD peer. For example, if the egress LSR cannot find
the path specified in the Reverse Path TLV it MAY establish the BFD
session over IP network as defined in [RFC5884].
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3.2. Return Codes
This document defines the following Return Codes for MPLS LSP Echo
Reply:
o "Inappropriate Target FEC Stack sub-TLV present", (TBD3). When
multicast Target FEC Stack sub-TLV found in the received Echo
Request by the egress BFD peer, an Echo Reply with the return code
set to "Inappropriate Target FEC Stack sub-TLV present" MUST be
sent to the ingress BFD peer Section 3.1.
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 the list of known Reverse
Paths it MAY find and use the existing BFD session.
5. Operational Considerations
When an explicit path is set either as Static or RSVP-TE LSP,
corresponding sub-TLVs, defined in [RFC7110], MAY be used to identify
the explicit reverse path for the BFD session. If any of defined in
[RFC7110] sub-TLVs used in BFD Reverse Path TLV, then the periodic
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verification of the control plane against the data plane, as
recommended in Section 4 [RFC5884], MUST use the Return Path TLV, as
per [RFC7110], with that sub-TLV. By using the LSP Ping with Return
Path TLV, an operator monitors whether at the egress BFD node the
reverse LSP is mapped to the same FEC as the BFD session. Selection
and control of the rate of LSP Ping with Return Path TLV follows the
recommendation of [RFC5884]: "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 implementation MAY
provide configuration options to control the rate of generation of
the periodic LSP Ping Echo request messages."
If an operator planned network maintenance activity that possibly
affects FEC used in the BFD Reverse Path TLV, the operator MUST avoid
the unnecessary disruption using the LSP Ping with a new FEC in the
BFD Reverse Path TLV. But in some scenarios, proactive measures
cannot be taken. Because the frequency of LSP Ping messages will be
lower than the defect detection time provided by the BFD session. As
a result, a change in the reverse-path FEC will first be detected as
the failure of the BFD session. In such a case, the ingress BFD node
SHOULD immediately transmit the LSP Ping Echo request with Return
Path TLV to verify whether the FEC is still valid. If the failure
was caused by the change in the FEC used for the reverse direction of
the BFD session, the ingress BFD node SHOULD bootstrap a new BFD
session using another FEC in BFD Reverse Path TLV.
6. IANA Considerations
6.1. 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.
+--------+----------------------+---------------+
| Value | Description | Reference |
+--------+----------------------+---------------+
| (TBD1) | BFD Reverse Path TLV | This document |
+--------+----------------------+---------------+
Table 1: New BFD Reverse Type TLV
6.2. Return Code
The IANA is requested to assign a new Return Code value from the
"Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
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Ping Parameters" registry, "Return Codes" sub-registry, as follows
using a Standards Action value.
+--------+----------------------------------------------+-----------+
| Value | Description | Reference |
+--------+----------------------------------------------+-----------+
| (TBD3) | Inappropriate Target FEC Stack sub-TLV | This |
| | present. | document |
| (TBD4) | Failed to establish the BFD session. The | This |
| | specified reverse path was not found. | document |
+--------+----------------------------------------------+-----------+
Table 2: New Return Code
7. Security Considerations
Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
and [RFC8029], apply to this document.
8. 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,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://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, <https://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, <https://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,
<https://www.rfc-editor.org/info/rfc7110>.
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[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,
<https://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,
<https://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, <https://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
Jeff Tantsura
Nuage Networks
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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