BFD S. Pallagatti, Ed.
Internet-Draft Rtbrick
Intended status: Standards Track S. Paragiri
Expires: May 27, 2019 Juniper Networks
V. Govindan
M. Mudigonda
G. Mirsky
ZTE Corp.
November 23, 2018



This document describes the use of the Bidirectional Forwarding Detection (BFD) protocol in Virtual eXtensible Local Area Network (VXLAN) overlay networks.

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Table of Contents

1. Introduction

"Virtual eXtensible Local Area Network" (VXLAN) [RFC7348]. provides an encapsulation scheme that allows building an overlay network by decoupling the address space of the attached virtual hosts from that of the network.

VXLAN is typically deployed in data centers interconnecting virtualized hosts of a tenant. VXLAN addresses requirements of the Layer 2 and Layer 3 data center network infrastructure in the presence of VMs in a multi-tenant environment, discussed in section 3 [RFC7348], by providing Layer 2 overlay scheme on a Layer 3 network.

In the absence of a router in the overlay, a VM can communicate with another VM only if they are on the same VXLAN segment. VMs are unaware of VXLAN tunnels as a VXLAN tunnel is terminated on a VXLAN Tunnel End Point (VTEP) (hypervisor/TOR). VTEPs (hypervisor/TOR) are responsible for encapsulating and decapsulating frames exchanged among VMs.

Ability to monitor path continuity, i.e., perform proactive continuity check (CC) for these tunnels, is important. The asynchronous mode of BFD, as defined in [RFC5880], can be used to monitor a VXLAN tunnel. Use of [I-D.ietf-bfd-multipoint] is for future study.

Also, BFD in VXLAN can be used to monitor the particular service nodes that are designated to handle Layer 2 broadcast properly, unknown unicast, and multicast traffic. Such nodes, discussed in details in [RFC8293], are often referred to as "replicators", are usually virtual VTEPs and can be monitored by physical VTEPs to minimize BUM traffic directed to the unavailable replicator.

This document describes the use of Bidirectional Forwarding Detection (BFD) protocol VXLAN to enable monitoring continuity of the path between Network Virtualization Edges (NVEs) and/or availability of a replicator service node using BFD.

In this document, the terms NVE and VTEP are used interchangeably.

2. Conventions used in this document

2.1. Terminology

BFD - Bidirectional Forwarding Detection

CC - Continuity Check

NVE - Network Virtualization Edge

TOR - Top of Rack

VM - Virtual Machine

VTEP - VXLAN Tunnel End Point

VXLAN - Virtual eXtensible Local Area Network

2.2. Requirements Language

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.

3. Use cases

The primary use case of BFD for VXLAN is for continuity check of a tunnel. By exchanging BFD control packets between VTEPs, an operator exercises the VXLAN path in both the underlay and overlay thus ensuring the VXLAN path availability and VTEPs reachability. BFD failure detection can be used for maintenance. There are other use cases such as the following:

4. Deployment

Figure 1 illustrates the scenario with two servers, each of them hosting two VMs. The servers host VTEPs that terminate two VXLAN tunnels with VNI number 100 and 200 respectively. Separate BFD sessions can be established between the VTEPs (IP1 and IP2) for monitoring each of the VXLAN tunnels (VNI 100 and 200). The implementation SHOULD have a reasonable upper bound on the number of BFD sessions that can be created between the same pair of VTEPs. No BFD packets intended for a Hypervisor VTEP should be forwarded to a VM as a VM may drop BFD packets leading to a false negative. This method is applicable whether the VTEP is a virtual or physical device.

   |        Server 1          |
   |                          |
   | +----+----+  +----+----+ |
   | |VM1-1    |  |VM1-2    | |
   | |VNI 100  |  |VNI 200  | |
   | |         |  |         | |
   | +---------+  +---------+ |
   | Hypervisor VTEP (IP1)    |
                         |   +-------------+
                         |   |   Layer 3   |
                         |---|   Network   |
                             |             |
                                      |    Hypervisor VTEP (IP2) |
                                      | +----+----+  +----+----+ |
                                      | |VM2-1    |  |VM2-2    | |
                                      | |VNI 100  |  |VNI 200  | |
                                      | |         |  |         | |
                                      | +---------+  +---------+ |
                                      |      Server 2            |

Figure 1: Reference VXLAN domain

5. BFD Packet Transmission over VXLAN Tunnel

BFD packet MUST be encapsulated and sent to a remote VTEP as explained in Section 5.1. Implementations SHOULD ensure that the BFD packets follow the same lookup path as VXLAN data packets within the sender system.

5.1. BFD Packet Encapsulation in VXLAN

BFD packets are encapsulated in VXLAN as described below. The VXLAN packet format is defined in Section 5 of [RFC7348]. The Outer IP/UDP and VXLAN headers MUST be encoded by the sender as defined in [RFC7348].

  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
 |                                                               |
 ~                      Outer Ethernet Header                    ~
 |                                                               |
 |                                                               |
 ~                        Outer IPvX Header                      ~
 |                                                               |
 |                                                               |
 ~                        Outer UDP Header                       ~
 |                                                               |
 |                                                               |
 ~                           VXLAN Header                        ~
 |                                                               |
 |                                                               |
 ~                    Inner Ethernet Header                      ~
 |                                                               |
 |                                                               |
 ~                        Inner IPvX Header                      ~
 |                                                               |
 |                                                               |
 ~                         Inner UDP Header                      ~
 |                                                               |
 |                                                               |
 ~                       BFD Control Message                     ~
 |                                                               |
 |                            FCS                                |

Figure 2: VXLAN Encapsulation of BFD Control Message

The BFD packet MUST be carried inside the inner MAC frame of the VXLAN packet. The inner MAC frame carrying the BFD payload has the following format:

6. Reception of BFD packet from VXLAN Tunnel

Once a packet is received, VTEP MUST validate the packet as described in Section 4.1 of [RFC7348]. If the Destination MAC of the inner MAC frame matches the dedicated MAC or the MAC address of the VTEP the packet MUST be processed further.

The UDP destination port and the TTL of the inner IP packet MUST be validated to determine if the received packet can be processed by BFD. BFD packet with inner MAC set to VTEP or dedicated MAC address MUST NOT be forwarded to VMs.

To ensure BFD detects the proper configuration of VXLAN Network Identifier (VNI) in a remote VTEP, a lookup SHOULD be performed with the MAC-DA and VNI as key in the Virtual Forwarding Instance (VFI) table of the originating/terminating VTEP to exercise the VFI associated with the VNI.

6.1. Demultiplexing of the BFD packet

Demultiplexing of IP BFD packet has been defined in Section 3 of [RFC5881]. Since multiple BFD sessions may be running between two VTEPs, there needs to be a mechanism for demultiplexing received BFD packets to the proper session. The procedure for demultiplexing packets with Your Discriminator equal to 0 is different from [RFC5880]. For such packets, the BFD session MUST be identified using the inner headers, i.e., the source IP, the destination IP, and the source UDP port number present in the IP header carried by the payload of the VXLAN encapsulated packet. The VNI of the packet SHOULD be used to derive interface-related information for demultiplexing the packet. If BFD packet is received with non-zero Your Discriminator, then BFD session MUST be demultiplexed only with Your Discriminator as the key.

7. Use of reserved VNI

In most cases, a single BFD session is sufficient for the given VTEP to monitor the reachability of a remote VTEP, regardless of the number of VNIs in common. When the single BFD session is used to monitor reachability of the remote VTEP, an implementation SHOULD use a VNI of 0.

8. Echo BFD

Support for echo BFD is outside the scope of this document.

9. IANA Considerations

IANA has assigned TBA as a dedicated MAC address from the IANA 8-bit unicast MAC address registry to be used as the Destination MAC address of the inner Ethernet of VXLAN when carrying BFD control packets.

10. Security Considerations

The document requires setting the inner IP TTL to 1 which could be used as a DDoS attack vector. Thus the implementation MUST have throttling in place to control the rate of BFD control packets sent to the control plane. Throttling MAY be relaxed for BFD packets based on port number.

The implementation SHOULD have a reasonable upper bound on the number of BFD sessions that can be created between the same pair of VTEPs.

Other than inner IP TTL set to 1 and limit the number of BFD sessions between the same pair of VTEPs, this specification does not raise any additional security issues beyond those of the specifications referred to in the list of normative references.

11. Contributors

Reshad Rahman

12. Acknowledgments

Authors would like to thank Jeff Hass of Juniper Networks for his reviews and feedback on this material.

Authors would also like to thank Nobo Akiya, Marc Binderberger, Shahram Davari, Donald E. Eastlake 3rd, and Anoop Ghanwani for the extensive reviews and the most detailed and helpful comments.

13. References

13.1. Normative References

[I-D.ietf-bfd-multipoint] Katz, D., Ward, D., Networks, J. and G. Mirsky, "BFD for Multipoint Networks", Internet-Draft draft-ietf-bfd-multipoint-18, June 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010.
[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881, DOI 10.17487/RFC5881, June 2010.
[RFC7348] Mahalingam, M., Dutt, D., Duda, K., Agarwal, P., Kreeger, L., Sridhar, T., Bursell, M. and C. Wright, "Virtual eXtensible Local Area Network (VXLAN): A Framework for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks", RFC 7348, DOI 10.17487/RFC7348, August 2014.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.

13.2. Informational References

[RFC8293] Ghanwani, A., Dunbar, L., McBride, M., Bannai, V. and R. Krishnan, "A Framework for Multicast in Network Virtualization over Layer 3", RFC 8293, DOI 10.17487/RFC8293, January 2018.

Authors' Addresses

Santosh Pallagatti (editor) Rtbrick EMail:
Sudarsan Paragiri Juniper Networks 1194 N. Mathilda Ave. Sunnyvale, California 94089-1206 USA EMail:
Vengada Prasad Govindan Cisco EMail:
Mallik Mudigonda Cisco EMail:
Greg Mirsky ZTE Corp. EMail: