| MPLS Working Group | G. Mirsky |
| Internet-Draft | J. Tantsura |
| Intended status: Standards Track | Ericsson |
| Expires: March 11, 2016 | I. Varlashkin |
| M. Chen | |
| Huawei | |
| September 8, 2015 |
Bidirectional Forwarding Detection (BFD) Directed Return Path
draft-ietf-mpls-bfd-directed-01
Bidirectional Forwarding Detection (BFD) is expected to monitor bi-directional paths. When a BFD session monitors in its forward direction an explicitly routed path there is a need to be able to direct egress BFD peer to use specific path as 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. All 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. As result, if the ingress BFD peer sends its BFD control packets over explicit path that is diverging from the best route, then reverse direction of the BFD session is likely not to be on co-routed bi-directional path with the forward direction of the BFD session. And because BFD control packets are not guaranteed to cross the same links and nodes in both directions detection of Loss of Continuity (LoC) defect in forward direction may demonstrate positive negatives.
This document defines the extension to LSP Ping [RFC4379], BFD Reverse Path TLV, 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 bi-directional co-routed associated channel.
BFD: Bidirectional Forwarding Detection
MPLS: Multiprotocol Label Switching
LSP: Label Switching Path
LoC: Loss of Continuity
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].
BFD is best suited to monitor bi-directional co-routed paths. In most cases, given stable environments, the forward and reverse direction between two nodes is likely to be co-routed, this fulfilling the implicit BFD requirements. If BFD is used to monitor unidirectional explicitly routed paths, e.g. MPLS-TE LSPs, its 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 the unidirectional explicit path:
To address these challenges the egress BFD peer should be instructed to use specific path for its 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 reverse path for the specified in BFD Discriminator TLV session.
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 the specified path that BFD control packets of the BFD session referenced in the BFD Discriminator TLV are required to follow. 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 value to be assigned by IANA.
Length is 2 octets in length and defines the length in octets of the Reverse Path field.
Reverse Path field contains a sub-TLV. Any Target FEC sub-TLV, already or in the future defined, from IANA sub-registry Sub-TLVs for TLV Types 1, 16, and 21 of MPLS LSP Ping Parameters registry MAY be used in this field. Only one sub-TLV MUST be included in the Reverse Path TLV. If more than one sub-TLVs are present in the Reverse Path TLV, then only the first sub-TLV MUST be used and the rest MUST be silently discarded.
If the egress LSR cannot find 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 LSR MAY establish the BFD session over IP network according to [RFC5884].
When explicit path on MPLS data plane set either as Static or RSVP-TE LSP respective sub-TLVs defined in [RFC7110] identify explicit return path.
In addition to Static and RSVP-TE, Segment Routing with MPLS data plane can be used to set 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 will be allocated by IANA.
The egress LSR MUST use the Value field as label stack for BFD control packets for the BFD session identified by source IP address and value in BFD Discriminator TLV.
The Segment Routing Tunnel sub-TLV MAY be used in Reply Path TLV defined in [RFC7110]
IPv6 can be data plane of choice for Segment Routed tunnels [I-D.previdi-6man-segment-routing-header]. In such networks the BFD Reverse Path TLV described in Section 3.1.1 can be used as well. To specify reverse path of a BFD session in IPv6 environment the BFD Discriminator TLV MUST be used along with the BFD Reverse Path TLV. The BFD Reverse Path TLV in IPv6 network MUST include 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
As discussed in [I-D.kumarkini-mpls-spring-lsp-ping] introduction of Segment Routing network domains with MPLS data plane adds three new sub-TLVs that may be used with Target FEC TLV. Section 6.1 addresses use of new sub-TLVs in Target FEC TLV in LSP ping and 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 this 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 segment 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 MAY precede the LSP ping that bootstraps the BFD session.
This document defines the following Return Codes:
In 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 BFD session to monitor the first tunnel, node A MUST include BFD Discriminator TLV with Discriminator value foobar-1 and MAY include BFD Reverse Path TLV that references H-G-D-C-B-A tunnel. To bootstrap BFD session to monitor the second tunnel, node A MUST include BFD Discriminator TLV with Discriminator value foobar-2 [I-D.ietf-bfd-rfc5884-clarifications] and MAY include BFD Reverse Path TLV that references H-G-F-E-B-A tunnel.
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 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 existing BFD sessions.
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 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 |
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) | 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.