MPLS Working Group
Internet Engineering Task Force (IETF) G. Mirsky
Internet-Draft
Request for Comments: 9612 Ericsson
Intended status:
Category: Experimental J. Tantsura
Expires: 14 November 2024
ISSN: 2070-1721 NVIDIA
I. Varlashkin
Google
M. Chen
Huawei
13 May
July 2024
Bidirectional Forwarding Detection (BFD) Directed Return Path for MPLS
Label Switched Paths (LSPs)
draft-ietf-mpls-bfd-directed-31
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 path,
there may be a need to direct the egress BFD peer to use a specific
path for the reverse direction of the BFD session. This document
describes an extension to the MPLS Label Switched Path (LSP) echo
request that allows a BFD system to request that the remote BFD peer
transmits
transmit BFD control packets over the specified LSP.
Status of This Memo
This Internet-Draft document is submitted in full conformance with the
provisions of BCP 78 not an Internet Standards Track specification; it is
published for examination, experimental implementation, and BCP 79.
Internet-Drafts are working documents
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents valid approved by the IESG are candidates for a maximum any level of
Internet Standard; see Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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 14 November 2024.
https://www.rfc-editor.org/info/rfc9612.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used Used in this This document . . . . . . . . . . . . 3
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Control of the Reverse BFD Reverse Path . . . . . . . . . . . . . . . 4
3.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 4
3.2. Return Codes . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Failure Characterization . . . . . . . . . . . . . . . . 6
4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 7
5. Operational Considerations . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . . . 8
6.2. Return Code . . . . . . . . . . . . . . . . . . . . . . . 8 Codes
7. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9.
8. Normative References . . . . . . . . . . . . . . . . . . . . 10
10. Informative References . . . . . . . . . . . . . . . . . . . 11
Appendix A.
Acknowledgments . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[RFC5880], [RFC5881], and [RFC5883] established the Bidirectional
Forwarding Detection (BFD) protocol for IP networks. [RFC5884] and
[RFC7726] set rules for using BFD Asynchronous mode over MPLS Label
Switched Paths (LSPs), while not defining means to control the path
that an egress BFD system uses to send BFD control packets towards
the ingress BFD system.
When BFD is used to detect defects of the traffic-engineered LSP, the
path of the BFD control packets transmitted by the egress BFD system
toward the ingress may be disjoint from the monitored 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., notifications (i.e., false alarms, 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 system to use an explicit path for its BFD control packets
associated with a particular BFD session. The IANA has registered this
TLV will be allocated
from in the TLV and sub-TLV "TLVs" registry defined in [RFC8029]. by [RFC8029] (see Section 6.1).
As a special case, forward and reverse directions of the BFD session
can form a
bi-directional bidirectional co-routed associated channel.
The LSP ping extension, extension described in this document, document was developed and
implemented resulting from the operational experiment. The lessons
learned from the operational experiment enabled the use between
systems conforming to this specification. More implementations are Further implementation is
encouraged to understand better understand the operational impact of the
mechanism described in the document.
1.1. Conventions used Used in this This document
1.1.1. Terminology
BFD: Bidirectional Forwarding Detection
FEC: Forwarding Equivalency Equivalence Class
LSP: Label Switched Path
LSR: Label-Switching router Label Switching Router
1.1.2. 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 an explicitly routed unidirectional path,
e.g., path
(e.g., MPLS-TE LSP, LSP), BFD control packets in the forward direction
would be in-band using the mechanism defined in [RFC5884]. However,
the reverse direction of the BFD session would follow the shortest
path route, which could be completely or partially disjoint from the
forward path. This creates the potential for the failure of a
disjoint resource on the reverse path to trigger a BFD failure
detection, even though the forward path is unaffected.
If the reverse path is congruent with the forward path, the potential
for such false positives is greatly reduced. For this purpose, this
specification provides a means for the egress BFD peer to be
instructed to use a specific path for BFD control packets.
3. Control of the Reverse BFD Reverse Path
To bootstrap a BFD session over an MPLS LSP, LSP ping, defined in
[RFC8029], ping [RFC8029] MUST
be used with the BFD Discriminator TLV [RFC5884]. This document
defines a new TLV, the BFD Reverse Path TLV, that MAY contain
none, one zero or
more sub-TLVs that can be used to carry information about the reverse
path for the BFD session that is specified by the value in the 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;
included, then it MUST be treated as a malformed Echo Request, as
described in [RFC8029].
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 Type:
This two-octet field has a value of
TBD1 (to be assigned by IANA as requested in 16384 (see Section 6).
Length
Length:
This two-octet field is two octets long and defines the length in octets of the Reverse
Path field.
Reverse Path Path:
This field contains none, one, zero or more sub-TLVs. Only non-
multicast non-multicast
Target FEC Stack sub-TLVs (already defined, defined or to be defined in the
future) for TLV Types 1, 16, and 21 of MPLS LSP in the "Multiprotocol Label
Switching (MPLS) Label Switched Paths (LSPs) Ping
Parameters Parameters"
registry are permitted to be used in this field. Any
other sub-TLV Other sub-TLVs
MUST NOT be used. (This implies that Multicast multicast Target FEC Stack
sub-TLVs, i.e., p2mp and mp2mp, are not permitted in the Reverse
Path field.)
If the egress Label-Switching Router (LSR) LSR finds a multicast Target FEC Stack sub-TLV, it MUST
send an echo reply with the received BFD Reverse Path TLV, BFD
Discriminator TLV and set the Return Code to "Inappropriate 192 ("Inappropriate
Target FEC Stack sub-TLV present"
(Section present") (see Section 3.2). The BFD
Reverse Path TLV includes none, one zero or more sub-TLVs. However, the number
of sub-TLVs in the Reverse Path field MUST be limited. The default
limit is 128 sub-TLV entries, but an implementation MAY be able to
control that limit. An empty BFD Reverse Path TLV, i.e., TLV (i.e., a BFD
Reverse Path TLV with no sub-TLVs present, sub-TLVs) is used as withdrawal of to withdraw any previously
set reverse path for the BFD session identified in the BFD
Discriminator 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 policy-based
decision as described in Section 7 of [RFC5884], i.e., routed over an
IP network.
If the egress peer LSR cannot find the path specified in the BFD
Reverse Path TLV, it MUST send an Echo Reply with the received BFD
Discriminator TLV, BFD Reverse Path TLV, TLV and set the Return Code to "Failed
193 ("Failed to establish the BFD session. The specified reverse
path was not found"
(Section found.") (see Section 3.2). If an implementation
provides additional configuration options, these can control actions
at the egress BFD peer, including when the path specified in the BFD
Reverse Path TLV cannot be found. For example, optionally, if the
egress peer LSR cannot find the path specified in the BFD Reverse
Path TLV, it MAY establish the BFD session over an IP network, as
defined in [RFC5884]. Note that the return code Return Code required by the MUST
"MUST" clause in this paragraph does not preclude the session from
being established over a different path as discussed in the MAY "MAY"
clause.
The BFD Reverse Path TLV MAY be used in the bootstrapping process of a bootstrapping
the BFD session process as described in Section 6 of [RFC5884]. A system
that supports this specification MUST support using the BFD Reverse
Path TLV after the BFD session has been established. If a system
that supports this specification receives an LSP Ping with the BFD
Discriminator TLV and no BFD Reverse Path TLV even though the reverse
path for the specified BFD session has been was established according to the
previously received BFD Reverse Path TLV, the egress BFD peer MUST
transition to transmitting periodic BFD Control messages as
defined described
in Section 7 of [RFC5884]. If a BFD system that received an LSP Ping
with the BFD Reverse Path TLV does not support this specification, it
will "result in the Return Code of 2 ("One or more of the TLVs was
not understood") being sent in the echo response" (Section 3 of
[RFC8029]).
3.2. Return Codes
This document defines the following Return Codes for the MPLS LSP
Echo Reply:
*
"Inappropriate Target FEC Stack sub-TLV present" (TBD3). (192):
When a multicast Target FEC Stack sub-TLV is found in the received
Echo Request, the egress BFD peer sends an Echo Reply with the return
code
Return Code set to "Inappropriate 192 ("Inappropriate Target FEC Stack sub-TLV present"
present") to the ingress BFD peer peer, as described in Section 3.1.
*
"Failed to establish the BFD session. The specified reverse path
was not found" (TBD4). found." (193):
When a specified reverse path is unavailable, the egress BFD peer
sends an Echo Reply with the
return code Return Code set to "Failed 193 ("Failed to
establish the BFD session. The specified reverse path was not found"
found.") to the ingress BFD peer peer, as described in Section 3.1.
3.3. Failure Characterization
A failure detected by a BFD session that uses the BFD Reverse Path
TLV could be due to a change in the FEC used in the BFD Reverse Path
TLV. The ingress BFD peer, upon Upon detection of the network failure, the ingress BFD peer
MUST transmit the LSP Ping Echo request with the Return Path TLV to
verify whether the FEC is still valid. If the failure was caused by
the
a change in the FEC used for the reverse direction of the BFD
session, the ingress BFD peer MUST re-direct redirect the reverse path of the
BFD session using another FEC in the BFD Reverse Path TLV, TLV and notify
an operator.
4. Use Case Scenario
In the network presented in Figure 2, ingress LSR peer A monitors two
tunnels to the egress LSR peer H: A-B-C-D-G-H and A-B-E-F-G-H. To
bootstrap a BFD session to monitor the first tunnel, the ingress LSR peer
A includes a BFD Discriminator TLV with a Discriminator value (e.g.,
foobar-1). Peer A includes a BFD Reverse Path TLV referencing the
H-G-D-C-B-A tunnel to control the path from the egress LSR. To
bootstrap a BFD session to monitor the second tunnel, ingress LSR
peer A, A includes a BFD Discriminator TLV with a different
Discriminator value (e.g., foobar-2) [RFC7726] and a BFD Reverse Path
TLV that references the 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 egress LSR peer H to monitor a path to the ingress
LSR peer A, e.g., the H-G-D-C-B-A tunnel, then by looking up the list
of known Reverse Paths, reverse paths, it MAY find and use the existing BFD session.
5. Operational Considerations
When an explicit path is set either as either Static or RSVP-TE LSP,
corresponding sub-TLVs, defined sub-TLVs (defined in [RFC7110], [RFC7110]) MAY be used to identify
the explicit reverse path for the BFD session. If a particular set
of sub-TLVs composes the Return Path TLV [RFC7110] and does not
increase the length of the Maximum Transmission Unit for the given
LSP, that set can be safely used in the BFD Reverse Path TLV. If any
of the sub-TLVs defined in [RFC7110] sub-TLVs are used in the BFD Reverse Path
TLV, then the periodic verification of the control plane against the
data plane, as recommended in Section 4 of [RFC5884], MUST use the
Return Path TLV, as per [RFC7110], with that sub-TLV. By using the
LSP Ping with the 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. session at the
egress BFD node. Selection and control of the rate of the LSP Ping
with the Return Path TLV follows the recommendation of in [RFC5884]: "The
| 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." messages.
Suppose an operator planned a network maintenance activity that
possibly affects the FEC used in the BFD Reverse Path TLV. In that
case, the operator can avoid the unnecessary disruption by 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 BFD session's failure. An
operator will be notified as described in Section 3.3.
6. IANA Considerations
6.1. BFD Reverse Path TLV
The
IANA is requested to assign a new has assigned the following value for the BFD Reverse Path TLV
from the 16384-31739 range in the "TLVs" registry of subregistry within the
"Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs)
Ping Parameters" registry.
+=======+=======+=============+====================================+
|Type |TLV |Reference
+=======+=========+===========+====================================+
| Type | TLV | Reference | Sub-TLV Registry |
| |Name | Name | | |
+=======+=======+=============+====================================+
+=======+=========+===========+====================================+
| 16384 | BFD | RFC 9612 | (TBD1)|BFD |This document| Only non-multicast sub-TLV sub-TLVs |
| |Reverse| | Reverse | | (already defined or to be defined |
| |Path | Path | | in the future) at |
| |TLV | | [https://www.iana.org/assignments/ |
| | | | mpls-lsp-ping-parameters/mpls-lsp- |
| | | | ping-parameters.xml#sub-tlv- |
| | | | 1-16-21] | | | | | (https://www.iana.org/assignments/ <https://www.iana.org/assignments/ |
| | | | mpls-lsp-ping-parameters/mpls-lsp- |
| | | | ping-parameters.xml#sub-tlv- |
| | | | 1-16-21) 1-16-21> are permitted to be used |
| | | | in this field. Any other sub-TLV Other sub-TLVs |
| | | | MUST NOT be used. |
+-------+-------+-------------+------------------------------------+
+-------+---------+-----------+------------------------------------+
Table 1: New BFD Reverse Type Path TLV
6.2. Return Code
The Codes
IANA is requested to assign new has assigned the following Return Code values from the range 192-247 of
range in the "Return Codes" registry of subregistry within the "Multi-Protocol "Multiprotocol
Label Switching (MPLS) Label Switched Paths (LSPs) Ping Parameters", as in
Table 2.
+=========+=============================+===============+ Parameters"
registry.
+=======+===========================================+===========+
| Value | Description Meaning | Reference |
+=========+=============================+===============+
+=======+===========================================+===========+
| (TBD3) 192 | Inappropriate Target FEC Stack sub-TLV | This document RFC 9612 |
| | Stack sub-TLV present. present | |
+---------+-----------------------------+---------------+
+-------+-------------------------------------------+-----------+
| (TBD4) 193 | Failed to establish the BFD | This document |
| | session. The specified | RFC 9612 |
| | specified reverse path was not found. | |
+---------+-----------------------------+---------------+
+-------+-------------------------------------------+-----------+
Table 2: New Return Code
8. Codes
7. Security Considerations
Security considerations discussed in [RFC5880], [RFC5884], [RFC7726],
[RFC8029], and [RFC7110] apply to this document.
The BFD Reverse Path TLV may be exploited as an attack vector by
inflating the number of included sub-TLVs. The number of sub-TLVs
MUST be limited to mitigate that threat. The default limit for the
number of sub-TLVs is set in Section 3.1 to 128. 128 (see Section 3.1). An
implementation MAY use a mechanism to control that limit.
9.
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>.
[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>.
10. Informative References
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Appendix A.
Acknowledgments
The authors greatly appreciate a the thorough review reviews and the most helpful
comments from Eric Gray and Carlos Pignataro. The authors much
appreciate the help of Qian Xin, who provided information about the
implementation of this specification.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregimirsky@gmail.com
Jeff Tantsura
NVIDIA
Email: jefftant.ietf@gmail.com
Ilya Varlashkin
Google
Email: imv@google.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com