Bidirectional Forwarding Detection (BFD) Directed Return Path for MPLS Label Switched Paths (LSPs)

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Table of Contents

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, while not defining means to control the path an egress BFD system uses 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 system 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 case, forward and reverse directions of the BFD session can form a bi-directional co-routed associated channel.¶

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:¶

• 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 MAY contain none, one 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 BFD Discriminator 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
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   BFD Reverse Path TLV Type   |           Length              |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |                          Reverse Path                         |
 ~                                                               ~
 |                                                               |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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 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."¶

Suppose an operator planned network maintenance activity that possibly affects FEC used in the BFD Reverse Path TLV. In that case, 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 BFD session's failure. 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

8. Security Considerations

Security considerations discussed in [RFC5880], [RFC5884], [RFC7726], [RFC8029], and [RFC7110] apply to this document.¶

Authors' Addresses