MPLS Working Group G. Mirsky
Internet-Draft J. Tantsura
Intended status: Standards Track Ericsson
Expires: September 3, 2016 I. Varlashkin
Google
M. Chen
Huawei
March 2, 2016
Bidirectional Forwarding Detection (BFD) Directed Return Path
draft-ietf-mpls-bfd-directed-02
Abstract
Bidirectional Forwarding Detection (BFD) is expected to monitor bi-
directional paths. When a BFD session monitors an explicit routed
path there is a need to be able to direct egress BFD peer to use
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. Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
3. Direct Reverse BFD Path . . . . . . . . . . . . . . . . . . . 4
3.1. Case of MPLS Data Plane . . . . . . . . . . . . . . . . . 4
3.1.1. BFD Reverse Path TLV . . . . . . . . . . . . . . . . 4
3.1.2. Static and RSVP-TE sub-TLVs . . . . . . . . . . . . . 5
3.1.3. Segment Routing: MPLS Data Plane Case . . . . . . . . 5
3.2. Segment Routing: IPv6 Data Plane Case . . . . . . . . . . 6
3.3. Bootstrapping BFD session with BFD Reverse Path over
Segment Routed tunnel . . . . . . . . . . . . . . . . . . 6
3.4. Return Codes . . . . . . . . . . . . . . . . . . . . . . 7
4. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5.1. TLV . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Sub-TLV . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.3. Return Codes . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
RFC 5880 [RFC5880], RFC 5881 [RFC5881], and RFC 5883 [RFC5883]
established the BFD protocol for IP networks and RFC 5884 [RFC5884]
set rules of using BFD asynchronous mode over IP/MPLS LSPs. These
four 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 an LSP is explicitly routed, if it is desired that
BFD control packets follow the same path in the reverse direction
(for support of common fault detection for explicitly routed
bidirectional co-routed LSPs, for example), it is likely that the
shortest return path to the ingress BFD peer may not follow the same
path as the LSP in the forward direction. The fact that BFD control
packets are not guaranteed to cross the same links and nodes in both
forward and reverse directions is a significant factor in producing
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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 [RFC4379] and proposes that it to be used to instruct the egress
BFD peer to use explicit path for its BFD control packets associated
with the particular BFD session. The TLV will be allocated from the
TLV and sub-TLV registry defined by RFC 4379 [RFC4379]. 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. Terminology
BFD: Bidirectional Forwarding Detection
MPLS: Multiprotocol Label Switching
LSP: Label Switching Path
LoC: Loss of Continuity
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
[RFC2119].
2. Problem Statement
BFD is best suited to monitor bi-directional co-routed paths. In
most cases, given stable environments, the forward and reverse
directions between two nodes are likely to be co-routed, thus
fulfilling the implicit BFD requirement. If 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 still follow the shortest path route and that might
lead to the following problem in detecting failures on a
unidirectional explicit path:
o a failure detection by ingress node on the reverse path cannot be
interpreted as bi-directional failure with all the certainty and
thus trigger, for example, protection switchover of the forward
direction without possibility of being a false positive defect
notification.
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To address this scenario the egress BFD peer should be instructed to
use a specific path for BFD control packets.
3. Direct Reverse BFD Path
3.1. Case of MPLS Data Plane
LSP ping, defined in [RFC4379], 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.
3.1.1. BFD Reverse Path TLV
The BFD Reverse Path TLV is an optional TLV within the LSP ping
protocol. 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 [RFC4379].
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
TB1 (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
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field. Exactly one sub-TLV MUST be included in the Reverse Path TLV.
If more than one sub-TLV is present in the Reverse Path TLV, then, in
order to avoid ambiguity of which of TLVs to use, the egress BFD peer
MUST send Echo Reply with the received Reverse Path TLVs and set the
Return Code to "Too Many TLVs Detected" Section 3.4.
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 3.4. The egress BFD
peer MAY establish the BFD session over IP network as defined in
[RFC5884].
3.1.2. 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.
3.1.3. Segment Routing: MPLS Data Plane Case
In addition to Static and RSVP-TE, Segment Routing with MPLS data
plane can be used to set an explicit path. In this case a new sub-
TLV is defined in this document as presented in Figure 2.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SegRouting MPLS sub-TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Entry 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Entry 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label Entry N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Segment Routing MPLS Tunnel sub-TLV
The Segment Routing Tunnel sub-TLV Type is two octets in length, and
has a value of TB2 (to be assigned by IANA as requested in
Section 5).
The egress LSR MUST use the Value field as label stack for BFD
control packets for the BFD session identified by the source IP
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address of the MPLS LSP Ping packet and the value in the BFD
Discriminator TLV. Label Entries MUST be in network order.
The Segment Routing Tunnel sub-TLV MAY be used in Reply Path TLV
defined in [RFC7110]
3.2. Segment Routing: IPv6 Data Plane Case
IPv6 can be used as the data plane of choice for Segment Routed
tunnels [I-D.previdi-6man-segment-routing-header]. In this case the
BFD Reverse Path TLV described in Section 3.1.1 can be used as well.
To specify the reverse path of a BFD session for an IPv6 explicitly
routed path the BFD Discriminator TLV MUST be used along with the BFD
Reverse Path TLV. The BFD Reverse Path TLV in IPv6 network MUST
include the Segment Routing IPv6 Tunnel sub-TLV.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SegRouting IPv6 sub-TLV Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 Prefix |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| IPv6 Prefix |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Segment Routing IPv6 Tunnel sub-TLV
The Segment Routing IPv6 Tunnel sub-TLV Type is two octets in length,
and has a value of TB3 (to be assigned by IANA as requested in
Section 5).
3.3. Bootstrapping BFD session with BFD Reverse Path over Segment
Routed tunnel
As discussed in [I-D.kumarkini-mpls-spring-lsp-ping] introduction of
Segment Routing network domains with an MPLS data plane adds three
new sub-TLVs that MAY be used with Target FEC TLV. Section 6.1
addresses use of the new sub-TLVs in Target FEC TLV in LSP ping and
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LSP traceroute. For the case of LSP ping the
[I-D.kumarkini-mpls-spring-lsp-ping] states that:
"Initiator MUST include FEC(s) corresponding to the destination
segment. "
"Initiator, i.e. ingress LSR, MAY include FECs corresponding to some
or all of segments imposed in the label stack by the ingress LSR to
communicate the segments traversed. "
When LSP ping is used to bootstrap BFD session this document updates
the statement and defines that LSP Ping MUST include the FEC
corresponding to the destination segment and SHOULD NOT include FECs
corresponding to some or all of other segments imposed by the ingress
LSR. Operationally such restriction would not cause any problem or
uncertainty as LSP ping with FECs corresponding to some or all
segments or traceroute that validate the segment route MAY precede
the LSP ping that bootstraps the BFD session.
3.4. Return Codes
This document defines the following Return Codes for MPLS LSP Echo
Reply:
o "Too Many TLVs Detected", (TBD4). When more than one Reverse Path
TLV found in the recieved Echo Request by the egress BFD peer, an
Echo Reply with the return code set to "Too Many TLVs Detected"
MUST be sent to the ingress BFD peer Section 3.1.1.
o "Failed to establish the BFD session. The specified reverse path
was not found", (TBD5). 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 3.1.1.
4. Use Case Scenario
In the network presented in Figure 4 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.
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C---------D
| |
A-------B G-----H
| |
E---------F
Figure 4: 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. 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 |
+----------+----------------------+---------------+
| X (TBD1) | BFD Reverse Path TLV | This document |
+----------+----------------------+---------------+
Table 1: New BFD Reverse Type TLV
5.2. Sub-TLV
The IANA is requested to assign two new sub-TLV types from
"Multiprotocol Label Switching Architecture (MPLS) Label Switched
Paths (LSPs) Ping Parameters - TLVs" registry, "Sub-TLVs for TLV
Types 1, 16, and 21" sub-registry.
+----------+-------------------------------------+---------------+
| Value | Description | Reference |
+----------+-------------------------------------+---------------+
| X (TBD2) | Segment Routing MPLS Tunnel sub-TLV | This document |
| X (TBD3) | Segment Routing IPv6 Tunnel sub-TLV | This document |
+----------+-------------------------------------+---------------+
Table 2: New Segment Routing Tunnel sub-TLV
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5.3. Return Codes
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) | Too Many TLVs Detected. | This document |
| X (TBD5) | Failed to establish the BFD session. | This document |
| | The specified reverse path was not | |
| | found. | |
+----------+----------------------------------------+---------------+
Table 3: New Return Code
6. Security Considerations
Security considerations discussed in [RFC5880], [RFC5884], and
[RFC4379], apply to this document.
7. Acknowledgements
Authors greatly appreciate thorough review and the most helpful
comments from Eric Gray.
8. Normative References
[I-D.kumarkini-mpls-spring-lsp-ping]
Kumar, N., Swallow, G., Pignataro, C., Akiya, N., Kini,
S., Gredler, H., and M. Chen, "Label Switched Path (LSP)
Ping/Trace for Segment Routing Networks Using MPLS
Dataplane", draft-kumarkini-mpls-spring-lsp-ping-05 (work
in progress), January 2016.
[]
Previdi, S., Filsfils, C., Field, B., Leung, I., Linkova,
J., Kosugi, T., Vyncke, E., and D. Lebrun, "IPv6 Segment
Routing Header (SRH)", draft-previdi-6man-segment-routing-
header-08 (work in progress), October 2015.
[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>.
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[RFC4379] Kompella, K. and G. Swallow, "Detecting Multi-Protocol
Label Switched (MPLS) Data Plane Failures", RFC 4379,
DOI 10.17487/RFC4379, February 2006,
<http://www.rfc-editor.org/info/rfc4379>.
[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>.
[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>.
Authors' Addresses
Greg Mirsky
Ericsson
Email: gregory.mirsky@ericsson.com
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Jeff Tantsura
Ericsson
Email: jeff.tantsura@ericsson.com
Ilya Varlashkin
Google
Email: Ilya@nobulus.com
Mach(Guoyi) Chen
Huawei
Email: mach.chen@huawei.com
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