| Internet-Draft | EVPN Network Layer Fault Management | September 2023 |
| Govindan, et al. | Expires 17 March 2024 | [Page] |
This document specifies proactive, in-band network layer OAM mechanisms to detect loss of continuity faults that affect unicast and multi-destination paths (used by Broadcast, Unknown Unicast, and Multicast traffic) in an Ethernet VPN (RFC 7432bis) network. The mechanisms specified in the draft use the widely adopted Bidirectional Forwarding Detection (RFC 5880) protocol and other protocols as necessary.¶
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[RFC9062] outlines the OAM requirements of Ethernet VPN networks (EVPN) [rfc7432bis]. This document specifies mechanisms for proactive fault detection at the network (overlay) layer of EVPN, that is to say between PEs (Provider Edge nodes). The mechanisms proposed in this document use the widely adopted Bidirectional Forwarding Detection (BFD) [RFC5880] protocol, which is a lightweight protocol using fixed length messages suitable for implementation in hardware, and other protocols as necessary.¶
EVPN fault detection mechanisms need to consider unicast traffic separately from Broadcast, Unknown Unicast, and Multicast (BUM) traffic since they map to different Forwarding Equivalency Classes (FECs) in EVPN. Hence this document proposes somewhat different continuity fault detection mechanisms depending on the type of traffic and the type of tunnel used as follows:¶
Packet loss and packet delay measurement are out of scope for this document. See [ietf-bmwg-evpntest] for EVPN benchmarking guidance.¶
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.¶
The following acronyms are used in this document.¶
This document specifies BFD based mechanisms for proactive fault detection for EVPN as specified in [rfc7432bis] and also for EVPN using VXLAN encapsulation [RFC8365]. It covers the following:¶
This document does not discuss BFD mechanisms for:¶
This document specifies procedures for BFD asynchronous mode. BFD demand mode is outside the scope of this specification except as it is used in [RFC8563]. The use of the BFD Echo function is outside the scope of this specification.¶
The choice of running BFD at the network layer of the OAM model for EVPN [RFC9062] was made after considering the following:¶
BFD testing between EVPN PE nodes does not guarantee that the EVPN service is functioning. (This can be monitored at the service level, that is CE (Customer Edge) to CE.) For example, an egress EVPN-PE could understand EVPN labeling received but could switch data to an incorrect interface. However, BFD testing in the EVPN Network Layer does provide additional confidence that data transported using those tunnels will reach the expected egress node. When BFD testing in the EVPN overlay fails, that can be used as an indication of a Loss-of-Connectivity defect in the EVPN underlay that would cause EVPN service failure.¶
The mechanisms specified in BFD for MPLS LSPs [RFC5884] [RFC7726] and BFD for VXLAN [RFC8971] are, except as otherwise provided herein, applied to test loss of continuity for unicast EVPN traffic. The MPLS control plane can be verified against the data plane as specified in [RFC8029]. When the discriminators required for de-multiplexing the BFD sessions are not otherwise available, they can be advertised through BGP using the BFD Discriminator Attribute [RFC9026]. Discriminators are needed for MPLS since the label stack does not contain enough information to identify the sender of the packet.¶
The usage of MPLS entropy labels [RFC6790] or various VXLAN source ports takes care of the requirement to monitor various paths of the multi-path server layer network. Each unique realizable path between the participating PE routers MAY be monitored separately when such entropy is used. At least one path of multi-path connectivity between two PE routers MUST be tracked with BFD, but in that case the granularity of fault-detection will be coarser.¶
To support unicast fault management to a PE node, that PE MUST allocate or be configured with a BFD discriminator to be used as Your Discriminator in the BFD messages to it. By default, it advertises this discriminator with BGP using the BFD Discriminator Attribute [RFC9026] with BFD Mode TBD4 in an EVPN MAC/IP Advertisement Route [rfc7432bis] and extracts its peer's discriminator from such an attribute; however, these discriminators MAY be exchanged out-of-band or through some other mechanism outside the scope of this document. If it is desired to establish and maintain a BFD session to a PE when it has not learned a local MAC address and would not otherwise be advertising a MAC/IP route, the PE can advertise a MAC/IP route to the MAC address TBD3.¶
Once a PE knows a unicast route and discriminator for another PE and if it is the higher priority of the two PEs to initiate BFD and is configured to do so, it endeavors to bring UP and maintain a BFD session to that other PE.¶
Once the BFD session is UP, the ends of the BFD session MUST NOT change the local discriminator values of the BFD Control packets they generate, unless they first bring down the session as specified in [RFC5884]. The BFD session is brought down if a PE is no longer configured to maintain it or if a route and discriminator are no longer available.¶
Section 5.1 below discusses BUM traffic fault detection for P2P and MP2P tunnels using ingress replication and Section 5.2 discusses such fault detection for P2MP tunnels.¶
Ingress replication uses separate P2P or 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 specified by this document takes advantage of the fact that the head makes a unique copy for each tail.¶
Another key aspect to be considered in EVPN is the advertisement of the Inclusive Multicast Ethernet Tag Route [rfc7432bis]. The BUM traffic flows from a head node to a particular tail only after the head receives such an inclusive multicast route from the tail. This route contains the BUM EVPN MPLS label (downstream allocated) corresponding to the MP2P tunnel for MPLS encapsulation and contains the IP address of the PE originating the inclusive multicast route for use in VXLAN encapsulation. It also contains a BFD Discriminator Attribute [RFC9026] with BFD Mode TDB4 giving the BFD discriminator that will be used by the tail. This is the P2P mode since a P2P BFD session is used in both the P2P and MP2P cases with ingress replication.¶
There MAY exist multiple BFD sessions between a head PE and an individual tail due to (1) the usage of MPLS entropy labels [RFC6790] or VXLAN source ports for an inclusive multicast FEC and (2) due to multiple MP2P tunnels indicated by different tail labels for MPLS or different IP addresses for VXLAN. If configured to do so, once a PE knows a multicast route and discriminator for another PE it endeavors to bring UP and maintain a BFD session to that other PE. Once a BFD session for a path is UP, the ends of the BFD session MUST NOT change the local discriminator values of the BFD Control packets they generate, unless they first bring down the session as specified in [RFC5884]. The BFD session is brought down if a PE is no longer configured to maintain it or if a route and discriminator are no longer available.¶
Fault detection for BUM traffic distributed using a P2MP tunnel uses BFD Multipoint Active Tails [RFC8563] in one of the three methods providing head notification. Which method is used depends on the configuration. Sections 5.2.2 and 5.2.3 of [RFC8563] describe two of these methods ("Head Notification and Tail Solicitation with Multipoint Polling" and "Head Notification with Composite Polling"). The third method ("Head Notification without Polling") is touched on in Section 5.2.1 of [RFC8563] and fully specified in [ietf-mpls-p2mp-bfd]. All three of these modes assume the existence of a unicast path from each tail to the head. In addition, Head Notification with Composite Polling assumes a head to tail unicast path disjoint from the path used by the P2MP tunnel.¶
The BUM traffic flows from a head node to the tails after the head transmits an Inclusive Multicast Tag Route [rfc7432bis]. It contains the BUM EVPN MPLS label (upstream allocated) corresponding to the P2MP tunnel for MPLS encapsulation. It also includes a BFD Discriminator Attribute [RFC9026] with the BFD Mode set to 1 and a Source IP Address TLV, which gives the address associated with the MultiPoint Head of the P2MP session. This BFD discriminator advertised by the head in the inclusive multicast route or otherwise configured at or communicated to a tail MUST be used in any reverse BFD control message as Your Discriminator so the head can determine the tail of which P2MP BFD session is responding. If configured to do so, once a PE knows a P2MP multicast route and the needed discriminators, it brings UP and maintains a P2MP BFD active tails session to the tails. The BFD session is brought down if a PE is no longer configured to maintain it or the multicast route and discriminators are no longer available.¶
For MPLS encapsulation of the head to tails BFD, Label Switched Multicast is used. For VXLAN encapsulation, BFD is delivered to the tails through underlay multicast using an outer multicast IP address.¶
The sections below discuss the MPLS and VXLAN encapsulations of BFD for EVPN network layer fault management.¶
This section describes use of the Generic Associated Channel Label (GAL) for BFD encapsulation in MPLS based EVPN network layer fault management.¶
As shown in Figure 1, the packet initially contains the following labels: LSP label (transport), the optional entropy label, the EVPN Unicast label, and then the Generic Associated Channel label with the G-ACh type set to TBD1. The G-ACh payload of the packet MUST contain the destination L2 header (in overlay space) followed by the IP header that encapsulates the BFD packet. The MAC address of the inner packet is used to validate the <EVI, MAC> in the receiving node.¶
<---------- 4 bytes ----------> +-------------------------------+ ----- | LSP Label | | +-------------------------------+ | : entropy label indicator : | + (optional) + MPLS Label Stack : entropy label : | +-------------------------------+ | | EVPN Unicast label | +-------------------------------+ | | Generic Assoc. Channel Label | | +-------------------------------+ ----- | ACH word, Type TBD1 no TLVs | +-------------------------------+ --- ------- | Destination MAC Address | | | + +---------------+ | | | TBD2 | | | | +---------------+ + L2 Header | | Source MAC Address | | | +---------------+---------------+ | | | VLAN Ethertype| VLAN-ID | | | +---------------+---------------+ | | |IP4/6 Ethertype| | | +---------------+---------------+ --- | / / G-ACh Payload /... IPv4/6 Header .../ | / / | +-------------------------------+ | | | | + UDP Header + | | | | +-------------------------------+ | | | | + BFD Control Packet + | / / | /... .../ ---------------
The packet initially contains the following labels: LSP label (transport), optionally the entropy label, the BUM label, and the split horizon label [rfc7432bis] (where applicable). The G-ACh type is set to TBD1. The G-ACh payload of the packet is as described in Section 6.1.1 except that the destination MAC address is the dedicated multicast MAC TBD2.¶
The encapsulation is the same as in Section 6.1.2 for ingress replication except that the transport label identifies the P2MP tunnel, in effect the set of tail PEs, rather than identifying a single destination PE at the end of an MP2P tunnel.¶
This section describes the use of the VXLAN [RFC7348] [RFC8365] for BFD encapsulation in VXLAN based EVPN fault management.¶
Figure 2 below shows the unicast VXLAN encapsulation on the wire on an Ethernet link. The outer and inner IP headers have a unicast source and destination IP address that are the addresses of the PEs that are the BFD message source and destination. If the BFD source has multiple IP addresses, entropy MAY be further obtained by using any of those addresses assuming the source is prepared for responses directed to the IP address used.¶
<---------- 4 bytes ----------> +-------------------------------+ --- | Destination MAC Address | | + +---------------+ | | | | Outer +---------------+ + L2 Header | Source MAC Address | | +-------------------------------+ | | VLAN Ethertype| VLAN-ID | | +---------------+---------------+ | |IP4/6 Ethertype| | +---------------+---------------+ --- / / /... IP4/6 Header .../ / / +-------------------------------+ | | + UDP Header + | | +-------------------------------+ | | + VXLAN Header + | | +-------------------------------+ --- | Destination MAC Address | | + +---------------+ | | | | Inner +---------------+ + L2 Header | Source MAC Address | | +---------------+---------------+ | | IP4 Ethertype | | +---------------+---------------+ --- / / /... IP4 Header .../ / / +-------------------------------+ | | + UDP Header + | | +---------------+---------------+ | | + BFD Control Packet + | | /... .../
The BFD packet construction is as given in Section 6.2.1 except as follows:¶
The VXLAN encapsulation for the head-to-tails BFD packets uses the multicast destination IP corresponding to the VXLAN VNI.¶
The destination UDP port MUST be 3784. For entropy purposes, the source UDP port can vary but MUST be in the range 49152 through 65535 [RFC5881]. If the head PE has multiple IP addresses, entropy MAY be further obtained by using any of those addresses.¶
The Your BFD discriminator is the value distributed for this multicast fault management purpose as discussed in Section 5.2.¶
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.¶
The following IANA Actions are requested.¶
IANA is requested to assign a channel type from the "Pseudowire Associated Channel Types" registry in [RFC4385] as follows.¶
Value Description Reference ----- ------------ ------------ TBD1 BFD-EVPN OAM [this document]¶
IANA is requested to assign parallel multicast and unicast MAC addresses under the IANA OUI [0x01005E900101 and 0x00005E900101 suggested] as follows:¶
IANA Multicast 48-bit MAC Addresses Address Usage Reference ------- --------------------- --------------- TBD2 EVPN Network Layer OAM [this document]¶
IANA Unicast 48-bit MAC Addresses Address Usage Reference ------- --------------------- --------------- TBD3 EVPN Network Layer OAM [this document]¶
IANA is requested to assign a value from the IETF Review range in the BFD Mode sub-registry on the Border Gateway Protocol Parameters Registry web page as follows:¶
Value Description Reference ----- --------------- --------------- TBD4 P2P BFD Session [this document]¶
Security considerations discussed in [RFC5880], [RFC5883], and [RFC8029] apply.¶
MPLS security considerations [RFC5920] apply to BFD Control packets encapsulated in a MPLS label stack. When BPD Control packets are routed, the authentication considerations discussed in [RFC5883] should be followed.¶
VXLAN BFD security considerations in [RFC8971] apply to BFD packets encapsulate in VXLAN.¶
The authors wish to thank the following for their comments and suggestions:¶
Mach Chen, Jorge Rabadan¶