BFD S. Pallagatti, Ed.
Internet-Draft Rtbrick
Intended status: Standards Track S. Paragiri
Expires: November 18, 2019 Individual Contributor
V. Govindan
M. Mudigonda
G. Mirsky
ZTE Corp.
May 17, 2019



This document describes the use of the Bidirectional Forwarding Detection (BFD) protocol in point-to-point Virtual eXtensible Local Area Network (VXLAN) tunnels forming up an overlay network.

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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.

One use of VXLAN is in data centers interconnecting virtual machines (VMs) 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 by providing a Layer 2 overlay scheme on a Layer 3 network [RFC7348]. Another use is as an encapsulation for Ethernet VPN [RFC8365].

This document is written assuming the use of VXLAN for virtualized hosts and refers to VMs and VXLAN Tunnel End Points (VTEPs) in hypervisors. However, the concepts are equally applicable to non-virtualized hosts attached to VTEPs in switches.

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 VTEP. VTEPs are responsible for encapsulating and decapsulating frames exchanged among VMs.

Ability to monitor path continuity, i.e., perform proactive continuity check (CC) for point-to-point (p2p) VXLAN tunnels, is important. The asynchronous mode of BFD, as defined in [RFC5880], can be used to monitor a p2p VXLAN tunnel.

In the case where a Multicast Service Node (MSN) (as described in Section 3.3 of [RFC8293]) resides behind an NVE, the mechanisms described in this document apply and can, therefore, be used to test the connectivity from the source NVE to the MSN.

This document describes the use of Bidirectional Forwarding Detection (BFD) protocol to enable monitoring continuity of the path between VXLAN VTEPs, performing as Network Virtualization Endpoints, and/or availability of a replicator multicast service node.

2. Conventions used in this document

2.1. Terminology

BFD Bidirectional Forwarding Detection

CC Continuity Check

p2p Point-to-point

MSN Multicast Service Node

VFI Virtual Forwarding Instance

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. 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). An implementation that supports this specification MUST be able to control the number of BFD sessions that can be created between the same pair of VTEPs. BFD packets intended for a Hypervisor VTEP MUST NOT 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

4. BFD Packet Transmission over VXLAN Tunnel

BFD packet MUST be encapsulated and sent to a remote VTEP as explained in Section 4.1. Implementations SHOULD ensure that the BFD packets follow the same lookup path as VXLAN data packets within the sender system.

4.1. 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:

5. Reception of BFD Packet from VXLAN Tunnel

Once a packet is received, VTEP MUST validate the packet. 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.

5.1. 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.

6. Use of the Specific 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 the reachability of the remote VTEP, an implementation SHOULD choose any of the VNIs but MAY choose VNI = 0.

7. Echo BFD

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

8. IANA Considerations

IANA has assigned TBA as a dedicated MAC address from the IANA 48-bit unicast MAC address registry to be used as the Destination MAC address of the inner Ethernet of VXLAN when carrying BFD control packets.

9. 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.

10. Contributors

Reshad Rahman

11. Acknowledgments

Authors would like to thank Jeff Haas 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.

12. References

12.1. Normative References

[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.

12.2. 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.
[RFC8365] Sajassi, A., Drake, J., Bitar, N., Shekhar, R., Uttaro, J. and W. Henderickx, "A Network Virtualization Overlay Solution Using Ethernet VPN (EVPN)", RFC 8365, DOI 10.17487/RFC8365, March 2018.

Authors' Addresses

Santosh Pallagatti (editor) Rtbrick EMail:
Sudarsan Paragiri Individual Contributor EMail:
Vengada Prasad Govindan Cisco EMail:
Mallik Mudigonda Cisco EMail:
Greg Mirsky ZTE Corp. EMail: