| MPLS Working Group | G. Mirsky |
| Internet-Draft | ZTE |
| Intended status: Standards Track | J. Tantsura |
| Expires: August 10, 2017 | Individual |
| I. Varlashkin | |
| M. Chen | |
| Huawei | |
| February 6, 2017 |
Bidirectional Forwarding Detection (BFD) Directed Return Path
draft-ietf-mpls-bfd-directed-05
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 specific path for the reverse direction of the BFD session.
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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 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 [RFC4379] and proposes that it is to be used to instruct the egress BFD peer to use 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 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.
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].
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:
To address this scenario the egress BFD peer would be instructed to use a specific path for BFD control packets.
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.
The BFD Reverse Path TLV is an optional TLV within the LSP ping [RFC4379], [RFC6424]. 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 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. 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 3.2.
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 3.2. The egress BFD peer MAY establish the BFD session over IP network as defined in [RFC5884].
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.
This document defines the following Return Codes for MPLS LSP Echo Reply:
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.
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 |
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 (TBD3) | Too Many TLVs Detected. | This document |
| X (TBD4) | Failed to establish the BFD session. The specified reverse path was not found. | This document |
Security considerations discussed in [RFC5880], [RFC5884], and [RFC4379], apply to this document.
Authors greatly appreciate thorough review and the most helpful comments from Eric Gray and Carlos Pignataro.