| MPLS Working Group | G. Mirsky |
| Internet-Draft | ZTE Corp. |
| Intended status: Standards Track | January 5, 2019 |
| Expires: July 9, 2019 |
BFD for Multipoint Networks over Point-to-Multi-Point MPLS LSP
draft-mirsky-mpls-p2mp-bfd-06
This document describes procedures for using Bidirectional Forwarding Detection (BFD) for multipoint networks to detect data plane failures in Multiprotocol Label Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths (LSPs). It also describes the applicability of LSP Ping, as in-band, and the control plane, as out-band, solutions to bootstrap a BFD session in this environment.
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[I-D.ietf-bfd-multipoint] defines a method of using Bidirectional Detection (BFD) [RFC5880] to monitor and detect unicast failures between the sender (head) and one or more receivers (tails) in multipoint or multicast networks. This document describes procedures for using such mode of BFD protocol to detect data plane failures in Multiprotocol Label Switching (MPLS) point-to-multipoint (p2mp) Label Switched Paths (LSPs). The document also describes the applicability of out-band solutions to bootstrap a BFD session in this environment.
MPLS: Multiprotocol Label Switching
LSP: Label Switched Path
BFD: Bidirectional Forwarding Detection
p2mp: Point-to-Multipoint
FEC: Forwarding Equivalence Class
G-ACh: Generic Associated Channel
ACH: Associated Channel Header
GAL: G-ACh Label
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.
[I-D.ietf-bfd-multipoint] defines how the tail of multipoint BFD session demultiplexes received BFD control packet when Your Discriminator is not set, i.e., equals zero. Because [I-D.ietf-bfd-multipoint] uses BFD in Demand mode from the very start of the p2mp BFD session, the head of BFD multipoint session transmits all BFD control packets with Your Discriminator set to zero. As a result, a tail cannot demultiplex BFD sessions using Your Discriminator, as defined in [RFC5880]. [I-D.ietf-bfd-multipoint] requires that to demultiplex BFD sessions the tail uses the source IP address, My Discriminator and the identity of the multipoint tree from which the Multipoint BFD Control packet was received. The identification of the multipoint tree MAY be provided by the p2mp MPLS LSP label in case of inclusive p-tree or upstream assigned label in case of aggregate p-tree. If the BFD control packet is encapsulated in IP/UDP, then the source IP address MUST be used to demultiplex the received BFD control packet as described in Section 3.1. The non-IP encapsulation case is described in Section 3.2.
[I-D.ietf-bfd-multipoint] defines IP/UDP encapsulation for multipoint BFD over p2mp MPLS LSP:
This specification further clarifies that:
In some environments, the overhead of extra IP/UDP encapsulations may be considered as overburden and make using more compact G-ACh encapsulation attractive. Non-IP encapsulation for multipoint BFD over p2mp MPLS LSP MUST use Generic Associated Channel (G-ACh) Label (GAL) [RFC5586] at the bottom of the label stack followed by Associated Channel Header (ACH). Channel Type field in ACH MUST be set to MPLS-TP CV value (0x0023) [RFC6428]. To provide the identity of the MultipointHead for the particular multipoint BFD session this document defines new Source MEP ID IP Address type (TBA1) in Section 6.1. If the Length value is 4, then the Value field contains an IPv4 address. If the Length value is 16, then the Value field contains an IPv6 address. Any other value of the Length field MUST be considered as an error, and the BFD control packet MUST be discarded.
LSP Ping is the part of on-demand OAM toolset to detect and localize defects in the data plane, and verify the control plane against the data plane by ensuring that the LSP is mapped to the same FEC, at the egress, as the ingress.
LSP Ping, as defined in [RFC6425], MAY be used to bootstrap MultipointTail. If the LSP Ping used, it MUST include the Target FEC TLV and the BFD Discriminator TLV defined in [RFC5884]. The Target FEC TLV MUST use sub-TLVs defined in Section 3.1 [RFC6425]. It is RECOMMENDED setting the value of Reply Mode field to "Do not reply" [RFC8029] for the LSP Ping to bootstrap MultipointTail of the p2mp BFD session. A MaultipointTail that receives the LSP Ping that includes the BFD Discriminator TLV:
Besides bootstrapping a BFD session over a p2mp LSP, LSP Ping SHOULD be used to verify the control plane against the data plane periodically by checking that the p2mp LSP is mapped to the same FEC at the MultipointHead and all active MultipointTails. 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 because LSP Ping requires more processing to validate the consistency between the data plane and the control plane. An implementation MAY provide configuration options to control the rate of generation of the periodic LSP Ping Echo request messages.
BGP-BFD Attribute [I-D.ietf-bess-mvpn-fast-failover] MAY be used to bootstrap multipoint BFD session on a tail.
This document does not introduce new security aspects but inherits all security considerations from [RFC5880], [RFC5884], [RFC7726], [I-D.ietf-bfd-multipoint], [RFC8029], and [RFC6425].
Also, BFD for p2mp MPLS LSP MUST follow the requirements listed in section 4.1 [RFC4687] to avoid congestion in the control plane or the data plane caused by the rate of generating BFD control packets. An operator SHOULD consider the amount of extra traffic generated by p2mp BFD when selecting the interval at which the MultipointHead will transmit BFD control packets. Also, the operator MAY consider the size of the packet the MultipointHead transmits periodically as using IP/UDP encapsulation adds up to 28 octets, which is more than 50% of BFD control packet length, comparing to G-ACh encapsulation.
IANA is required to allocate value (TBD) for the Source MEP ID IP Address type from the "CC/CV MEP-ID TLV" registry which is under the "Pseudowire Associated Channel Types" registry.
| Value | Description | Reference |
|---|---|---|
| TBA1 | IP Address | This document |
The author sincerely appreciates the comments received from Andrew Malis.
| [RFC4687] | Yasukawa, S., Farrel, A., King, D. and T. Nadeau, "Operations and Management (OAM) Requirements for Point-to-Multipoint MPLS Networks", RFC 4687, DOI 10.17487/RFC4687, September 2006. |