Internet Engineering Task Force (IETF) S. Pallagatti, Ed.

Internet Engineering Task Force (IETF) S. Pallagatti, Ed.
Request for Comments: 8971 VMware
Category: Informational G. Mirsky, Ed.
ISSN: 2070-1721 ZTE Corp.
S. Paragiri
Individual Contributor
V. Govindan
M. Mudigonda
Cisco
December 2020

Bidirectional Forwarding Detection (BFD) for Virtual eXtensible Local
Area Network (VXLAN)

Abstract

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

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8971.

Copyright Notice

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Table of Contents

1. Introduction
2. Conventions Used in This Document
2.1. Abbreviations
2.2. Requirements Language
3. Deployment
4. Use of the Management VNI
5. BFD Packet Transmission over VXLAN Tunnel
6. Reception of BFD Packet from VXLAN Tunnel
7. Echo BFD
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

"Virtual eXtensible Local Area Network (VXLAN)" [RFC7348] provides an
encapsulation scheme that allows the building of 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 the 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, because a VXLAN tunnel is terminated on a
VTEP. VTEPs are responsible for encapsulating and decapsulating
frames exchanged among VMs.

The 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], is
used to monitor a p2p VXLAN tunnel.

In the case where a Multicast Service Node (MSN) (as described in
Section 3.3 of [RFC8293]) participates in VXLAN, the mechanisms
described in this document apply and can, therefore, be used to test
the continuity of the path between the source Network Virtualization
Endpoint (NVE) and the MSN.

This document describes the use of the Bidirectional Forwarding
Detection (BFD) protocol to enable monitoring continuity of the path
between VXLAN VTEPs that are performing as VNEs, and/or between the
source NVE and a replicator MSN using a Management VXLAN Network
Identifier (VNI) (Section 4). All other uses of the specification to
test toward other VXLAN endpoints are out of scope.

2. Conventions Used in This Document

2.1. Abbreviations

BFD: Bidirectional Forwarding Detection

CC: Continuity Check

FCS: Frame Check Sequence

MSN: Multicast Service Node

NVE: Network Virtualization Endpoint

p2p: Point-to-point

VFI: Virtual Forwarding Instance

VM: Virtual Machine

VNI: VXLAN Network Identifier (or VXLAN Segment ID)

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 a scenario with two servers: each 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). Using a BFD session to monitor
a set of VXLAN VNIs between the same pair of VTEPs might help to
detect and localize problems caused by misconfiguration. 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. This method is applicable whether the VTEP is a
virtual or physical device.

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

Figure 1: Reference VXLAN Domain

At the same time, a service-layer BFD session may be used between the
tenants of VTEPs IP1 and IP2 to provide end-to-end fault management;
this use case is outside the scope of this document. In such a case,
for VTEPs, the BFD Control packets of that session are
indistinguishable from data packets.

For BFD Control packets encapsulated in VXLAN (Figure 2), the inner
destination IP address SHOULD be set to one of the loopback addresses
from 127/8 range for IPv4 or to one of IPv4-mapped IPv6 loopback
addresses from ::ffff:127.0.0.0/104 range for IPv6.

4. Use of the Management 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. BFD control messages MUST be sent using the
Management VNI, which acts as the control and management channel
between VTEPs. An implementation MAY support operating BFD on
another (non-Management) VNI, although the implications of this are
outside the scope of this document. The selection of the VNI number
of the Management VNI MUST be controlled through a management plane.
An implementation MAY use VNI number 1 as the default value for the
Management VNI. All VXLAN packets received on the Management VNI
MUST be processed locally and MUST NOT be forwarded to a tenant.

5. BFD Packet Transmission over VXLAN Tunnel

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

BFD packets are encapsulated in VXLAN as described below. The VXLAN
packet format is defined in Section 5 of [RFC7348]. The value in the
VNI field of the VXLAN header MUST be set to the value selected as
the Management VNI. 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 Packet ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Ethernet FCS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: VXLAN Encapsulation of BFD Control Packet

The BFD packet MUST be carried inside the inner Ethernet frame of the
VXLAN packet. The choice of destination Media Access Control (MAC)
and destination IP addresses for the inner Ethernet frame MUST ensure
that the BFD Control packet is not forwarded to a tenant but is
processed locally at the remote VTEP. The inner Ethernet frame
carrying the BFD Control packet has the following format:

Ethernet Header:
Destination MAC: A Management VNI, which does not have any
tenants, will have no dedicated MAC address for decapsulated
traffic. The value 00-52-02 SHOULD be used in this field.

Source MAC: MAC address associated with the originating VTEP.

Ethertype: This is set to 0x0800 if the inner IP header is IPv4
and set to 0x86DD if the inner IP header is IPv6.

IP header:
Destination IP: This IP address MUST NOT be of one of tenant's IP
addresses. The IP address SHOULD be selected from the range
127/8 for IPv4 and from the range ::ffff:127.0.0.0/104 for
IPv6. Alternatively, the destination IP address MAY be set to
VTEP's IP address.

Source IP: IP address of the originating VTEP.

TTL or Hop Limit: MUST be set to 255, in accordance with
[RFC5881].

The destination UDP port is set to 3784 and the fields of the BFD
Control packet are encoded as specified in [RFC5881].

6. Reception of BFD Packet from VXLAN Tunnel

Once a packet is received, the VTEP MUST validate the packet. If the
packet is received on the Management VNI and is identified as a BFD
Control packet addressed to the VTEP, then the packet can be
processed further. Processing of BFD Control packets received on a
non-Management VNI is outside the scope of this specification.

The received packet's inner IP payload is then validated according to
Sections 4 and 5 in [RFC5881].

7. Echo BFD

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

8. IANA Considerations

IANA has assigned a single MAC address of the value 00-52-02 from the
"Unassigned (small allocations)" block of the "IANA Unicast 48-bit
MAC Addresses" registry as follows: the "Usage" field is "BFD for
VXLAN". The "Reference" is this document.

9. Security Considerations

Security issues discussed in [RFC5880], [RFC5881], and [RFC7348]
apply to this document.

This document recommends using an address from the internal host
loopback addresses 127/8 range for IPv4, or an IP4-mapped IPv6
loopback address from the ::ffff:127.0.0.0/104 range for IPv6, as the
destination IP address in the inner IP header. Using such an address
prevents the forwarding of the encapsulated BFD control message by a
transient node, in case the VXLAN tunnel is broken, in accordance
with [RFC1812].

| A router SHOULD NOT forward, except over a loopback interface,
| any packet that has a destination address on network 127. A
| router MAY have a switch that allows the network manager to
| disable these checks. If such a switch is provided, it MUST
| default to performing the checks.

The use of IPv4-mapped IPv6 addresses has the same property as using
the IPv4 network 127/8. Moreover, the IPv4-mapped IPv6 addresses'
prefix is not advertised in any routing protocol.

If the implementation supports establishing multiple BFD sessions
between the same pair of VTEPs, there SHOULD be a mechanism to
control the maximum number of such sessions that can be active at the
same time.

10. References

10.1. Normative References

[RFC1812] Baker, F., Ed., "Requirements for IP Version 4 Routers",
RFC 1812, DOI 10.17487/RFC1812, June 1995,
<https://www.rfc-editor.org/info/rfc1812>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.

[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.

[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,
<https://www.rfc-editor.org/info/rfc7348>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

10.2. Informative 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,
<https://www.rfc-editor.org/info/rfc8293>.

[RFC8365] Sajassi, A., Ed., Drake, J., Ed., 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,
<https://www.rfc-editor.org/info/rfc8365>.

Acknowledgments

The authors would like to thank Jeff Haas of Juniper Networks for his
reviews and feedback on this material.

The authors would also like to thank Nobo Akiya, Marc Binderberger,
Shahram Davari, Donald E. Eastlake 3rd, Anoop Ghanwani, Dinesh Dutt,
Joel Halpern, and Carlos Pignataro for the extensive reviews and the
most detailed and constructive comments.

Contributors

Reshad Rahman
Cisco

Email: [email protected]

Authors' Addresses

Santosh Pallagatti (editor)
VMware

Email: [email protected]

Greg Mirsky (editor)
ZTE Corp.

Email: [email protected]

Sudarsan Paragiri
Individual Contributor

Email: [email protected]

Vengada Prasad Govindan
Cisco

Email: [email protected]

Mallik Mudigonda
Cisco

Email: [email protected]