Fault Management for EVPN networks
draft-gmsm-bess-evpn-bfd-00

Abstract

This document proposes a proactive, in-band network OAM mechanism to detect loss of continuity and miss-connection faults that affect unicast and multi-destination paths, used by Broadcast, unknown Unicast and Multicast traffic, in an EVPN network. The mechanisms proposed in the draft use the principles of the widely adopted Bidirectional Forwarding Detection protocol.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on January 7, 2017.

Copyright Notice

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Scope of the Document
• 3. Motivation for running BFD at the network layer of EVPN
• 4. Fault Detection of unicast traffic
• 5. Fault Detection of BUM traffic using ingress replication (MP2P)
• 6. Fault Detection of BUM traffic using P2MP tunnels (LSM)
• 7. BFD packet encapsulation
• 7.1. Using GAL/G-ACh encapsulation without IP headers
• 7.1.1. Ingress replication
• 7.1.1.1. Alternative encapsulation format
• 7.1.2. LSM
• 7.1.3. Unicast
• 7.1.3.1. Alternative encapsulation format
• 7.2. Using IP headers
• 8. Scalability Considerations
• 9. IANA Considerations
• 10. Security Considerations
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Authors' Addresses

1. Introduction

[I-D.salam-l2vpn-evpn-oam-req-frmwk] and [I-D.ooamdt-rtgwg-ooam-requirement] outlines the OAM requirements of Ethernet VPN networks [RFC7432]. This document proposes mechanisms for proactive fault detection at the network(overlay) OAM layer of EVPN. EVPN fault detection mechanisms need to consider unicast and Broadcast and unknown Unicast (BUM) traffic separately since they map to different FECs in EVPN, hence this document proposes different fault detection mechanisms to suit each type using the principles of[RFC5880],[RFC5884] and Point-to-multipoint BFD[I-D.ietf-bfd-multipoint] and [I-D.ietf-bfd-multipoint-active-tail].

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

2. Scope of the Document

This document proposes proactive fault detection for EVPN [RFC7432] using BFD mechanisms for:

• Unicast traffic.
• BUM traffic using Multi-point-to-Point (MP2P) tunnels (ingress replication).
• BUM traffic using Point-to-Multipoint (P2MP) tunnels (LSM).

This document does not discuss BFD mechanisms for:

• EVPN variants like PBB-EVPN [RFC7623]. This will be addressed in future versions.
• IRB solution based on EVPN [I-D.ietf-bess-evpn-inter-subnet-forwarding]. This will be addressed in future versions.
• EVPN using other encapsulations like VxLAN, NVGRE and MPLS over GRE [I-D.ietf-bess-evpn-overlay].
• BUM traffic using MP2MP tunnels will also be addressed in a future version of this document.

This specification describes procedures only for BFD asynchronous mode. BFD demand mode is outside the scope of this specification. Further, the use of the Echo function is outside the scope of this specification.

3. Motivation for running BFD at the network layer of EVPN

The choice of running BFD at the network layer of the OAM model for EVPN [I-D.salam-l2vpn-evpn-oam-req-frmwk] and [I-D.ooamdt-rtgwg-ooam-requirement] was made after considering the following:

• In addition to detecting link failures in the EVPN network, BFD sessions at the network layer can be used to monitor the successful programming of labels used for setting up MP2P and P2MP EVPN tunnels transporting Unicast and BUM traffic. The scope of reachability detection covers the ingress and the egress EVPN PE nodes and the network connecting them.
• Monitoring a representative set of path(s) or a particular path among the multiple paths available between two EVPN PE nodes could be done by exercising the entropy labels when they are used. However paths that cannot be realized by entropy variations cannot be monitored. Fault monitoring requirements outlined by [I-D.salam-l2vpn-evpn-oam-req-frmwk] are addressed by the mechanisms proposed by this draft.

Successful establishment and maintenance of BFD sessions between EVPN PE nodes does not fully guarantee that the EVPN service is functioning. For example, an egress EVPN-PE can understand the EVPN label but could switch data to incorrect interface. However, once BFD sessions in the EVPN Network Layer reach UP state, it does provide additional confidence that data transported using those tunnels will reach the expected egress node. When the BFD session in EVPN overlay goes down that can be used as indication of the Loss-of-Connectivity defect in the EVPN underlay that would cause EVPN service failure.

4. Fault Detection of unicast traffic

The mechanisms specified in BFD for MPLS LSPs [RFC5884] [RFC7726] can be applied to bootstrap and maintain BFD sessions for unicast EVPN traffic. The discriminators required for de-multiplexing the BFD sessions MUST be exchanged using EVPN LSP ping specifying the Unicast EVPN FEC [I-D.jain-bess-evpn-lsp-ping] before establishing the BFD session. This is needed since the MPLS label stack does not contain enough information to disambiguate the sender of the packet. The usage of MPLS entropy labels take care of addressing the requirement of monitoring various paths of the multi-path server layer network [RFC6790]. Each unique realizable path between the participating PE routers MAY be monitored separately when entropy labels are used. The multi-path connectivity between two PE routers MUST be tracked by at least one representative BFD session, in which case the granularity of fault-detection would be coarser. The PE node receiving the EVPN LSP ping MUST allocate BFD discriminators using the procedures defined in [RFC7726]. Note that once the BFD session for the EVPN label is UP, either end of the BFD session MUST NOT change the local discriminator values of the BFD Control packets it generates, unless it first brings down the session as specified in [RFC5884].

5. Fault Detection of BUM traffic using ingress replication (MP2P)

Ingress replication uses separate MP2P tunnels for transporting BUM traffic from the ingress PE (head) to a set of one or more egress PEs (tails). The fault detection mechanism proposed by this document takes advantage of the fact that a unique copy is made by the head for each tail. Another key aspect to be considered in EVPN is the advertisement of the inclusive multicast route. The BUM traffic flows from a head node to a particular tail only after the head receives the inclusive multicast route containing the BUM EVPN label (downstream allocated) corresponding to the MP2P tunnel. The head-end PE performing ingress replication MUST initiate an EVPN LSP ping using the inclusive multicast FEC [I-D.jain-bess-evpn-lsp-ping] upon receiving an inclusive multicast route from a tail to bootstrap the BFD session. There MAY exist multiple BFD sessions between a head PE an individual tail due to the usage of entropy labels [RFC6790] for an inclusive multicast FEC. The PE node receiving the EVPN LSP ping MUST allocate BFD discriminators using the procedures defined in [RFC7726]. Note that once the BFD session for the EVPN label is UP, either end of the BFD session MUST NOT change the local discriminator values of the BFD Control packets it generates, unless it first brings down the session as specified in [RFC5884].

6. Fault Detection of BUM traffic using P2MP tunnels (LSM)

TBD.

7. BFD packet encapsulation

7.1. Using GAL/G-ACh encapsulation without IP headers

7.1.1. Ingress replication

The packet contains the following labels: LSP label (transport) when not using PHP, the optional entropy label, the BUM label and the SH label[RFC7432] (where applicable). The G-ACh type is set to TBD. The G-Ach payload of the packet MUST contain the L2 header (in overlay space) followed by the IP header encapsulating the BFD packet. The MAC address of the inner packet is used to validate the <EVI, MAC> in the receiving node. The discriminator values of BFD are obtained through negotiation through the out-of-band EVPN LSP ping.

7.1.1.1. Alternative encapsulation format

A new TLV can be defined as proposed in Sec 3 of [RFC6428] to include the EVPN FEC information as a TLV following the BFD Control packet. The format of the TLV can be reused from the EVPN Inclusive Multicast sub-TLV proposed by Fig 2 of [I-D.jain-bess-evpn-lsp-ping].

A new type (TBD3) to indicate the EVPN Inclusive Multicast SubTLV is requested from the "CC/ CV MEP-ID TLV" registry [RFC6428].

  
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 1|Version|     Flags     |       BFD CV Code Point TBD2  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                  BFD Control Packet                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~               EVPN Inclusive Multicast TLV                    ~
   |                 (Type = TBD3)                                 | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 1: BFD-EVPN CV Message for EVPN Multicast 
                  (Ingress Replication)

7.1.2. LSM

TBD.

7.1.3. Unicast

The packet contains the following labels: LSP label (transport) when not using PHP, the optional entropy label and the EVPN Unicast label. The G-ACh type is set to TBD. The G-Ach payload of the packet MUST contain the L2 header (in overlay space) followed by the IP header encapsulating the BFD packet. The MAC address of the inner packet is used to validate the <EVI, MAC> in the receiving node. The discriminator values of BFD are obtained through negotiation through the out-of-band EVPN ping.

7.1.3.1. Alternative encapsulation format

A new TLV can be defined as proposed in Sec 3 of [RFC6428] to include the EVPN FEC information as a TLV following the BFD Control packet. The format of the TLV can be reused from the EVPN MAC sub-TLV proposed by Fig 1 of [I-D.jain-bess-evpn-lsp-ping]. A new type (TBD4) to indicate the EVPN MAC SubTLV is requested from the "CC/ CV MEP-ID TLV" registry [RFC6428].

  
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 1|Version|     Flags     |       BFD CV Code Point TBD2  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                  BFD Control Packet                           ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                           EVPN MAC Sub TLV                    ~
   |                 (Type = TBD4)                                 | 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 2: BFD-EVPN CV Message for EVPN Unicast

7.2. Using IP headers

The encapsulation option using IP headers will not be suited for EVPN, as using different values in the destination IP address for data and OAM (BFD) packets could cause the BFD packets to follow a different path than that of data packets. Hence this option MUST NOT be used for EVPN.

8. Scalability Considerations

The mechanisms proposed by this draft could affect the packet load on the network and its elements especially when supporting configurations involving a large number of EVIs. The option of slowing down or speeding up BFD timer values can be used by an administrator or a network management entity to maintain the overhead incurred due to fault monitoring at an acceptable level.

9. IANA Considerations

IANA is requested for two channel types from the "Pseudowire Associated Channel Types" registry in [RFC4385].

• TBD1 BFD-EVPN CC message
• TBD2 BFD-EVPN CV message

Ed Note: Do we need a CC code point? TBD

IANA is requested to allocate the following code-points from the "CC/CV MEP-ID TLV" registry [RFC6428]. The parent registry is the "Pseudowire Associated Channel Types" registry of [RFC4385] . All code points within this registry shall be allocated according to the "Standards Action" procedures as specified in [RFC5226]. The items tracked in the registry will be the type, associated name, and reference. The requested values are:

• TBD3 - CV code-point for BFD EVPN Inclusive multicast.
• TBD4 - CV code-point for BFD EVPN Unicast.

10. Security Considerations

TBD.

11. References

11.1. Normative References

11.2. Informative References

Authors' Addresses