Independent Submission M. Mahalingam
Request for Comments: 7348 Storvisor
Category: Informational D. Dutt
ISSN: 2070-1721 Cumulus Networks
K. Duda
Arista
P. Agarwal
Broadcom
L. Kreeger
Cisco
T. Sridhar
VMware
M. Bursell
Intel
C. Wright
Red Hat
August 2014
Virtual eXtensible Local Area Network (VXLAN): A Framework
for Overlaying Virtualized Layer 2 Networks over Layer 3 Networks
Abstract
This document describes Virtual eXtensible Local Area Network
(VXLAN), which is used to address the need for overlay networks
within virtualized data centers accommodating multiple tenants. The
scheme and the related protocols can be used in networks for cloud
service providers and enterprise data centers. This memo documents
the deployed VXLAN protocol for the benefit of the IETF Internet
community.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7348.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction ....................................................3
1.1. Acronyms and Definitions ...................................4
2. Conventions Used in This Document ...............................4
3. VXLAN Problem Statement .........................................5
3.1. Limitations Imposed by Spanning Tree and VLAN Ranges .......5
3.2. Multi-tenant Environments ..................................5
3.3. Inadequate Table Sizes at ToR Switch .......................6
4. VXLAN ...........................................................6
4.1. Unicast VM-to-VM Communication .............................7
4.2. Broadcast Communication and Mapping to Multicast ...........8
4.3. Physical Infrastructure Requirements .......................9
5. VXLAN Frame Format .............................................10
6. VXLAN Deployment Scenarios .....................................14
6.1. Inner VLAN Tag Handling ...................................18
7. Security Considerations ........................................18
8. IANA Considerations ............................................19
9. References .....................................................19
9.1. Normative References ......................................19
9.2. Informative References ....................................20
10. Acknowledgments ...............................................21
1. Introduction
Server virtualization has placed increased demands on the physical
network infrastructure. A physical server now has multiple Virtual
Machines (VMs) each with its own Media Access Control (MAC) address.
This requires larger MAC address tables in the switched Ethernet
network due to potential attachment of and communication among
hundreds of thousands of VMs.
In the case when the VMs in a data center are grouped according to
their Virtual LAN (VLAN), one might need thousands of VLANs to
partition the traffic according to the specific group to which the VM
may belong. The current VLAN limit of 4094 is inadequate in such
situations.
Data centers are often required to host multiple tenants, each with
their own isolated network domain. Since it is not economical to
realize this with dedicated infrastructure, network administrators
opt to implement isolation over a shared network. In such scenarios,
a common problem is that each tenant may independently assign MAC
addresses and VLAN IDs leading to potential duplication of these on
the physical network.
An important requirement for virtualized environments using a Layer 2
physical infrastructure is having the Layer 2 network scale across
the entire data center or even between data centers for efficient
allocation of compute, network, and storage resources. In such
networks, using traditional approaches like the Spanning Tree
Protocol (STP) for a loop-free topology can result in a large number
of disabled links.
The last scenario is the case where the network operator prefers to
use IP for interconnection of the physical infrastructure (e.g., to
achieve multipath scalability through Equal-Cost Multipath (ECMP),
thus avoiding disabled links). Even in such environments, there is a
need to preserve the Layer 2 model for inter-VM communication.
The scenarios described above lead to a requirement for an overlay
network. This overlay is used to carry the MAC traffic from the
individual VMs in an encapsulated format over a logical "tunnel".
This document details a framework termed "Virtual eXtensible Local
Area Network (VXLAN)" that provides such an encapsulation scheme to
address the various requirements specified above. This memo
documents the deployed VXLAN protocol for the benefit of the IETF Internet
community.
1.1. Acronyms and Definitions
ACL Access Control List
ECMP Equal-Cost Multipath
IGMP Internet Group Management Protocol
IHL Internet Header Length
MTU Maximum Transmission Unit
PIM Protocol Independent Multicast
SPB Shortest Path Bridging
STP Spanning Tree Protocol
ToR Top of Rack
TRILL Transparent Interconnection of Lots of Links
VLAN Virtual Local Area Network
VM Virtual Machine
VNI VXLAN Network Identifier (or VXLAN Segment ID)
VTEP VXLAN Tunnel End Point. An entity that originates and/or
terminates VXLAN tunnels
VXLAN Virtual eXtensible Local Area Network
VXLAN Segment
VXLAN Layer 2 overlay network over which VMs communicate
VXLAN Gateway
an entity that forwards traffic between VXLANs
2. Conventions Used in This Document
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].
3. VXLAN Problem Statement
This section provides further details on the areas that VXLAN is
intended to address. The focus is on the networking infrastructure
within the data center and the issues related to them.
3.1. Limitations Imposed by Spanning Tree and VLAN Ranges
Current Layer 2 networks use the IEEE 802.1D Spanning Tree Protocol
(STP) [802.1D] to avoid loops in the network due to duplicate paths.
STP blocks the use of links to avoid the replication and looping of
frames. Some data center operators see this as a problem with Layer
2 networks in general, since with STP they are effectively paying for
more ports and links than they can really use. In addition,
resiliency due to multipathing is not available with the STP model.
Newer initiatives, such as TRILL [RFC6325] and SPB [802.1aq], have
been proposed to help with multipathing and surmount some of the
problems with STP. STP limitations may also be avoided by
configuring servers within a rack to be on the same Layer 3 network,
with switching happening at Layer 3 both within the rack and between
racks. However, this is incompatible with a Layer 2 model for inter-
VM communication.
A key characteristic of Layer 2 data center networks is their use of
Virtual LANs (VLANs) to provide broadcast isolation. A 12-bit VLAN
ID is used in the Ethernet data frames to divide the larger Layer 2
network into multiple broadcast domains. This has served well for
many data centers that require fewer than 4094 VLANs. With the
growing adoption of virtualization, this upper limit is seeing
pressure. Moreover, due to STP, several data centers limit the
number of VLANs that could be used. In addition, requirements for
multi-tenant environments accelerate the need for larger VLAN limits,
as discussed in Section 3.3.
3.2. Multi-tenant Environments
Cloud computing involves on-demand elastic provisioning of resources
for multi-tenant environments. The most common example of cloud
computing is the public cloud, where a cloud service provider offers
these elastic services to multiple customers/tenants over the same
physical infrastructure.
Isolation of network traffic by a tenant could be done via Layer 2 or
Layer 3 networks. For Layer 2 networks, VLANs are often used to
segregate traffic -- so a tenant could be identified by its own VLAN,
for example. Due to the large number of tenants that a cloud
provider might service, the 4094 VLAN limit is often inadequate. In
addition, there is often a need for multiple VLANs per tenant, which
exacerbates the issue.
A related use case is cross-pod expansion. A pod typically consists
of one or more racks of servers with associated network and storage
connectivity. Tenants may start off on a pod and, due to expansion,
require servers/VMs on other pods, especially in the case when
tenants on the other pods are not fully utilizing all their
resources. This use case requires a "stretched" Layer 2 environment
connecting the individual servers/VMs.
Layer 3 networks are not a comprehensive solution for multi-tenancy
either. Two tenants might use the same set of Layer 3 addresses
within their networks, which requires the cloud provider to provide
isolation in some other form. Further, requiring all tenants to use
IP excludes customers relying on direct Layer 2 or non-IP Layer 3
protocols for inter VM communication.
3.3. Inadequate Table Sizes at ToR Switch
Today's virtualized environments place additional demands on the MAC
address tables of Top-of-Rack (ToR) switches that connect to the
servers. Instead of just one MAC address per server link, the ToR
now has to learn the MAC addresses of the individual VMs (which could
range in the hundreds per server). This is needed because traffic
to/from the VMs to the rest of the physical network will traverse the
link between the server and the switch. A typical ToR switch could
connect to 24 or 48 servers depending upon the number of its server-
facing ports. A data center might consist of several racks, so each
ToR switch would need to maintain an address table for the
communicating VMs across the various physical servers. This places a
much larger demand on the table capacity compared to non-virtualized
environments.
If the table overflows, the switch may stop learning new addresses
until idle entries age out, leading to significant flooding of
subsequent unknown destination frames.
4. VXLAN
VXLAN (Virtual eXtensible Local Area Network) addresses the above
requirements of the Layer 2 and Layer 3 data center network
infrastructure in the presence of VMs in a multi-tenant environment.
It runs over the existing networking infrastructure and provides a
means to "stretch" a Layer 2 network. In short, VXLAN is a Layer 2
overlay scheme on a Layer 3 network. Each overlay is termed a VXLAN
segment. Only VMs within the same VXLAN segment can communicate with
each other. Each VXLAN segment is identified through a 24-bit
segment ID, termed the "VXLAN Network Identifier (VNI)". This allows
up to 16 M VXLAN segments to coexist within the same administrative
domain.
The VNI identifies the scope of the inner MAC frame originated by the
individual VM. Thus, you could have overlapping MAC addresses across
segments but never have traffic "cross over" since the traffic is
isolated using the VNI. The VNI is in an outer header that
encapsulates the inner MAC frame originated by the VM. In the
following sections, the term "VXLAN segment" is used interchangeably
with the term "VXLAN overlay network".
Due to this encapsulation, VXLAN could also be called a tunneling
scheme to overlay Layer 2 networks on top of Layer 3 networks. The
tunnels are stateless, so each frame is encapsulated according to a
set of rules. The end point of the tunnel (VXLAN Tunnel End Point or
VTEP) discussed in the following sections is located within the
hypervisor on the server that hosts the VM. Thus, the VNI- and
VXLAN-related tunnel / outer header encapsulation are known only to
the VTEP -- the VM never sees it (see Figure 1). Note that it is
possible that VTEPs could also be on a physical switch or physical
server and could be implemented in software or hardware. One use
case where the VTEP is a physical switch is discussed in Section 6 on
VXLAN deployment scenarios.
The following sections discuss typical traffic flow scenarios in a
VXLAN environment using one type of control scheme -- data plane
learning. Here, the association of VM's MAC to VTEP's IP address is
discovered via source-address learning. Multicast is used for
carrying unknown destination, broadcast, and multicast frames.
In addition to a learning-based control plane, there are other
schemes possible for the distribution of the VTEP IP to VM MAC
mapping information. Options could include a central
authority-/directory-based lookup by the individual VTEPs,
distribution of this mapping information to the VTEPs by the central
authority, and so on. These are sometimes characterized as push and
pull models, respectively. This document will focus on the data
plane learning scheme as the control plane for VXLAN.
4.1. Unicast VM-to-VM Communication
Consider a VM within a VXLAN overlay network. This VM is unaware of
VXLAN. To communicate with a VM on a different host, it sends a MAC
frame destined to the target as normal. The VTEP on the physical
host looks up the VNI to which this VM is associated. It then
determines if the destination MAC is on the same segment and if there
is a mapping of the destination MAC address to the remote VTEP. If
so, an outer header comprising an outer MAC, outer IP header, and
VXLAN header (see Figure 1 in Section 5 for frame format) are
prepended to the original MAC frame. The encapsulated packet is
forwarded towards the remote VTEP. Upon reception, the remote VTEP
verifies the validity of the VNI and whether or not there is a VM on
that VNI using a MAC address that matches the inner destination MAC
address. If so, the packet is stripped of its encapsulating headers
and passed on to the destination VM. The destination VM never knows
about the VNI or that the frame was transported with a VXLAN
encapsulation.
In addition to forwarding the packet to the destination VM, the
remote VTEP learns the mapping from inner source MAC to outer source
IP address. It stores this mapping in a table so that when the
destination VM sends a response packet, there is no need for an
"unknown destination" flooding of the response packet.
Determining the MAC address of the destination VM prior to the
transmission by the source VM is performed as with non-VXLAN
environments except as described in Section 4.2. Broadcast frames
are used but are encapsulated within a multicast packet, as detailed
in the Section 4.2.
4.2. Broadcast Communication and Mapping to Multicast
Consider the VM on the source host attempting to communicate with the
destination VM using IP. Assuming that they are both on the same
subnet, the VM sends out an Address Resolution Protocol (ARP)
broadcast frame. In the non-VXLAN environment, this frame would be
sent out using MAC broadcast across all switches carrying that VLAN.
With VXLAN, a header including the VXLAN VNI is inserted at the
beginning of the packet along with the IP header and UDP header.
However, this broadcast packet is sent out to the IP multicast group
on which that VXLAN overlay network is realized.
To effect this, we need to have a mapping between the VXLAN VNI and
the IP multicast group that it will use. This mapping is done at the
management layer and provided to the individual VTEPs through a
management channel. Using this mapping, the VTEP can provide IGMP
membership reports to the upstream switch/router to join/leave the
VXLAN-related IP multicast groups as needed. This will enable
pruning of the leaf nodes for specific multicast traffic addresses
based on whether a member is available on this host using the
specific multicast address (see [RFC4541]). In addition, use of
multicast routing protocols like Protocol Independent Multicast -
Sparse Mode (PIM-SM see [RFC4601]) will provide efficient multicast
trees within the Layer 3 network.
The VTEP will use (*,G) joins. This is needed as the set of VXLAN
tunnel sources is unknown and may change often, as the VMs come up /
go down across different hosts. A side note here is that since each
VTEP can act as both the source and destination for multicast
packets, a protocol like bidirectional PIM (BIDIR-PIM -- see
[RFC5015]) would be more efficient.
The destination VM sends a standard ARP response using IP unicast.
This frame will be encapsulated back to the VTEP connecting the
originating VM using IP unicast VXLAN encapsulation. This is
possible since the mapping of the ARP response's destination MAC to
the VXLAN tunnel end point IP was learned earlier through the ARP
request.
Note that multicast frames and "unknown MAC destination" frames are
also sent using the multicast tree, similar to the broadcast frames.
4.3. Physical Infrastructure Requirements
When IP multicast is used within the network infrastructure, a
multicast routing protocol like PIM-SM can be used by the individual
Layer 3 IP routers/switches within the network. This is used to
build efficient multicast forwarding trees so that multicast frames
are only sent to those hosts that have requested to receive them.
Similarly, there is no requirement that the actual network connecting
the source VM and destination VM should be a Layer 3 network: VXLAN
can also work over Layer 2 networks. In either case, efficient
multicast replication within the Layer 2 network can be achieved
using IGMP snooping.
VTEPs MUST NOT fragment VXLAN packets. Intermediate routers may
fragment encapsulated VXLAN packets due to the larger frame size.
The destination VTEP MAY silently discard such VXLAN fragments. To
ensure end-to-end traffic delivery without fragmentation, it is
RECOMMENDED that the MTUs (Maximum Transmission Units) across the
physical network infrastructure be set to a value that accommodates
the larger frame size due to the encapsulation. Other techniques
like Path MTU discovery (see [RFC1191] and [RFC1981]) MAY be used to
address this requirement as well.
5. VXLAN Frame Format
The VXLAN frame format is shown below. Parsing this from the bottom
of the frame -- above the outer Frame Check Sequence (FCS), there is
an inner MAC frame with its own Ethernet header with source,
destination MAC addresses along with the Ethernet type, plus an
optional VLAN. See Section 6 for further details of inner VLAN tag
handling.
The inner MAC frame is encapsulated with the following four headers
(starting from the innermost header):
VXLAN Header: This is an 8-byte field that has:
- Flags (8 bits): where the I flag MUST be set to 1 for a valid
VXLAN Network ID (VNI). The other 7 bits (designated "R") are
reserved fields and MUST be set to zero on transmission and
ignored on receipt.
- VXLAN Segment ID/VXLAN Network Identifier (VNI): this is a
24-bit value used to designate the individual VXLAN overlay
network on which the communicating VMs are situated. VMs in
different VXLAN overlay networks cannot communicate with each
other.
- Reserved fields (24 bits and 8 bits): MUST be set to zero on
transmission and ignored on receipt.
Outer UDP Header: This is the outer UDP header with a source port
provided by the VTEP and the destination port being a well-known
UDP port.
- Destination Port: IANA has assigned the value 4789 for the
VXLAN UDP port, and this value SHOULD be used by default as the
destination UDP port. Some early implementations of VXLAN have
used other values for the destination port. To enable
interoperability with these implementations, the destination
port SHOULD be configurable.
- Source Port: It is recommended that the UDP source port number
be calculated using a hash of fields from the inner packet --
one example being a hash of the inner Ethernet frame's headers.
This is to enable a level of entropy for the ECMP/load-balancing ECMP/load-
balancing of the
VM to VM VM-to-VM traffic across the VXLAN overlay.
When calculating the UDP source port number in this manner, it
is RECOMMENDED that the value be in the dynamic/private port
range 49152-65535 [RFC6335].
The
- UDP checksum field Checksum: It SHOULD be transmitted as zero. When a packet
is received with a UDP checksum of zero, it MUST be accepted
for decapsulation. Optionally, if the encapsulating end point
includes a non-zero UDP checksum, it MUST be correctly
calculated across the entire packet including the IP header,
UDP header, VXLAN header, and encapsulated MAC frame. When a
decapsulating end point receives a packet with a non-zero
checksum, it MAY choose to verify the checksum value. If it
chooses to perform such verification, and the verification
fails, the packet MUST be dropped. If the decapsulating
destination chooses not to perform the verification, or
performs it successfully, the packet MUST be accepted for
decapsulation.
Outer IP Header: This is the outer IP header with the source IP
address indicating the IP address of the VTEP over which the
communicating VM (as represented by the inner source MAC address)
is running. The destination IP address can be a unicast or
multicast IP address (see Sections 4.1 and 4.2). When it is a
unicast IP address, it represents the IP address of the VTEP
connecting the communicating VM as represented by the inner
destination MAC address. For multicast destination IP addresses,
please refer to the scenarios detailed in Section 4.2.
Outer Ethernet Header (example): Figure 1 is an example of an inner
Ethernet frame encapsulated within an outer Ethernet + IP + UDP +
VXLAN header. The outer destination MAC address in this frame may
be the address of the target VTEP or of an intermediate Layer 3
router. The outer VLAN tag is optional. If present, it may be
used for delineating VXLAN traffic on the LAN.
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 Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination MAC Address | Outer Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q | Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x0800 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer IPv4 Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live |Protocl=17(UDP)| Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Source IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination IPv4 Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = See Section 5 | Dest Port = VXLAN Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VXLAN Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|R|R|I|R|R|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VXLAN Network Identifier (VNI) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inner Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address | Inner Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q | Inner.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype of Original Payload | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Original Ethernet Payload |
| |
|(Note that the original Ethernet Frame's FCS is not included) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Frame Check Sequence:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New FCS (Frame Check Sequence) for Outer Ethernet Frame |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: VXLAN Frame Format with IPv4 Outer Header
The frame format above shows tunneling of Ethernet frames using IPv4
for transport. Use of VXLAN with IPv6 transport is detailed below.
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 Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Destination MAC Address | Outer Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Outer Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q | Outer.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype = 0x86DD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer IPv6 Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Traffic Class | Flow Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Length | NxtHdr=17(UDP)| Hop Limit |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Outer Source IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Outer Destination IPv6 Address +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Outer UDP Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port = See Section 5 | Dest Port = VXLAN Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP Length | UDP Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
VXLAN Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R|R|R|R|I|R|R|R| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| VXLAN Network Identifier (VNI) | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Inner Ethernet Header:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Destination MAC Address | Inner Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Inner Source MAC Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|OptnlEthtype = C-Tag 802.1Q | Inner.VLAN Tag Information |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Payload:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethertype of Original Payload | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Original Ethernet Payload |
| |
|(Note that the original Ethernet Frame's FCS is not included) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Frame Check Sequence:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| New FCS (Frame Check Sequence) for Outer Ethernet Frame |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: VXLAN Frame Format with IPv6 Outer Header
6. VXLAN Deployment Scenarios
VXLAN is typically deployed in data centers on virtualized hosts,
which may be spread across multiple racks. The individual racks may
be parts of a different Layer 3 network or they could be in a single
Layer 2 network. The VXLAN segments/overlay networks are overlaid on
top of these Layer 2 or Layer 3 networks.
Consider Figure 3, which depicts two virtualized servers attached to
a Layer 3 infrastructure. The servers could be on the same rack, on
different racks, or potentially across data centers within the same
administrative domain. There are four VXLAN overlay networks
identified by the VNIs 22, 34, 74, and 98. Consider the case of
VM1-1 in Server 1 and VM2-4 on Server 2, which are on the same VXLAN
overlay network identified by VNI 22. The VMs do not know about the
overlay networks and transport method since the encapsulation and
decapsulation happen transparently at the VTEPs on Servers 1 and 2.
The other overlay networks and the corresponding VMs are VM1-2 on
Server 1 and VM2-1 on Server 2, both on VNI 34; VM1-3 on Server 1 and
VM2-2 on Server 2 on VNI 74; and finally VM1-4 on Server 1 and VM2-3
on Server 2 on VNI 98.
+------------+-------------+
| Server 1 |
| +----+----+ +----+----+ |
| |VM1-1 | |VM1-2 | |
| |VNI 22 | |VNI 34 | |
| | | | | |
| +---------+ +---------+ |
| |
| +----+----+ +----+----+ |
| |VM1-3 | |VM1-4 | |
| |VNI 74 | |VNI 98 | |
| | | | | |
| +---------+ +---------+ |
| Hypervisor VTEP (IP1) |
+--------------------------+
|
|
|
| +-------------+
| | Layer 3 |
|---| Network |
| |
+-------------+
|
|
+-----------+
|
|
+------------+-------------+
| Server 2 |
| +----+----+ +----+----+ |
| |VM2-1 | |VM2-2 | |
| |VNI 34 | |VNI 74 | |
| | | | | |
| +---------+ +---------+ |
| |
| +----+----+ +----+----+ |
| |VM2-3 | |VM2-4 | |
| |VNI 98 | |VNI 22 | |
| | | | | |
| +---------+ +---------+ |
| Hypervisor VTEP (IP2) |
+--------------------------+
Figure 3: VXLAN Deployment - VTEPs across a Layer 3 Network
One deployment scenario is where the tunnel termination point is a
physical server that understands VXLAN. An alternate scenario is
where nodes on a VXLAN overlay network need to communicate with nodes
on legacy networks that could be VLAN based. These nodes may be
physical nodes or virtual machines. To enable this communication, a
network can include VXLAN gateways (see Figure 4 below with a switch
acting as a VXLAN gateway) that forward traffic between VXLAN and
non-VXLAN environments.
Consider Figure 4 for the following discussion. For incoming frames
on the VXLAN connected interface, the gateway strips out the VXLAN
header and forwards it to a physical port based on the destination
MAC address of the inner Ethernet frame. Decapsulated frames with
the inner VLAN ID SHOULD be discarded unless configured explicitly to
be passed on to the non-VXLAN interface. In the reverse direction,
incoming frames for the non-VXLAN interfaces are mapped to a specific
VXLAN overlay network based on the VLAN ID in the frame. Unless
configured explicitly to be passed on in the encapsulated VXLAN
frame, this VLAN ID is removed before the frame is encapsulated for
VXLAN.
These gateways that provide VXLAN tunnel termination functions could
be ToR/access switches or switches higher up in the data center
network topology -- e.g., core or even WAN edge devices. The last
case (WAN edge) could involve a Provider Edge (PE) router that
terminates VXLAN tunnels in a hybrid cloud environment. In all these
instances, note that the gateway functionality could be implemented
in software or hardware.
+---+-----+---+ +---+-----+---+
| Server 1 | | Non-VXLAN |
(VXLAN enabled)<-----+ +---->| server |
+-------------+ | | +-------------+
| |
+---+-----+---+ | | +---+-----+---+
|Server 2 | | | | Non-VXLAN |
(VXLAN enabled)<-----+ +---+-----+---+ +---->| server |
+-------------+ | |Switch acting| | +-------------+
|---| as VXLAN |-----|
+---+-----+---+ | | Gateway |
| Server 3 | | +-------------+
(VXLAN enabled)<-----+
+-------------+ |
|
+---+-----+---+ |
| Server 4 | |
(VXLAN enabled)<-----+
+-------------+
Figure 4: VXLAN Deployment - VXLAN Gateway
6.1. Inner VLAN Tag Handling
Inner VLAN Tag Handling in VTEP and VXLAN gateway should conform to
the following:
Decapsulated VXLAN frames with the inner VLAN tag SHOULD be discarded
unless configured otherwise. On the encapsulation side, a VTEP
SHOULD NOT include an inner VLAN tag on tunnel packets unless
configured otherwise. When a VLAN-tagged packet is a candidate for
VXLAN tunneling, the encapsulating VTEP SHOULD strip the VLAN tag
unless configured otherwise.
7. Security Considerations
Traditionally, Layer 2 networks can only be attacked from 'within' by
rogue end points -- either by having inappropriate access to a LAN
and snooping on traffic, by injecting spoofed packets to 'take over'
another MAC address, or by flooding and causing denial of service. A
MAC-over-IP mechanism for delivering Layer 2 traffic significantly
extends this attack surface. This can happen by rogues injecting
themselves into the network by subscribing to one or more multicast
groups that carry broadcast traffic for VXLAN segments and also by
sourcing MAC-over-UDP frames into the transport network to inject
spurious traffic, possibly to hijack MAC addresses.
This document does not incorporate specific measures against such
attacks, relying instead on other traditional mechanisms layered on
top of IP. This section, instead, sketches out some possible
approaches to security in the VXLAN environment.
Traditional Layer 2 attacks by rogue end points can be mitigated by
limiting the management and administrative scope of who deploys and
manages VMs/gateways in a VXLAN environment. In addition, such
administrative measures may be augmented by schemes like 802.1X
[802.1X] for admission control of individual end points. Also, the
use of the UDP-based encapsulation of VXLAN enables configuration and
use of the 5-tuple-based ACL (Access Control List) functionality in
physical switches.
Tunneled traffic over the IP network can be secured with traditional
security mechanisms like IPsec that authenticate and optionally
encrypt VXLAN traffic. This will, of course, need to be coupled with
an authentication infrastructure for authorized end points to obtain
and distribute credentials.
VXLAN overlay networks are designated and operated over the existing
LAN infrastructure. To ensure that VXLAN end points and their VTEPs
are authorized on the LAN, it is recommended that a VLAN be
designated for VXLAN traffic and the servers/VTEPs send VXLAN traffic
over this VLAN to provide a measure of security.
In addition, VXLAN requires proper mapping of VNIs and VM membership
in these overlay networks. It is expected that this mapping be done
and communicated to the management entity on the VTEP and the
gateways using existing secure methods.
8. IANA Considerations
A well-known UDP port (4789) has been assigned by the IANA in the
Service Name and Transport Protocol Port Number Registry for VXLAN.
See Section 5 for discussion of the port number.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[802.1aq] IEEE, "Standard for Local and metropolitan area networks --
Media Access Control (MAC) Bridges and Virtual Bridged
Local Area Networks -- Amendment 20: Shortest Path
Bridging", IEEE P802.1aq-2012, 2012.
[802.1D] IEEE, "Draft Standard for Local and Metropolitan Area
Networks/ Media Access Control (MAC) Bridges", IEEE
P802.1D-2004, 2004.
[802.1X] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Port-Based Network Acces Control", IEEE Std
802.1X-2010, February 2010.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, May 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC5015] Handley, M., Kouvelas, I., Speakman, T., and L. Vicisano,
"Bidirectional Protocol Independent Multicast (BIDIR-PIM)",
RFC 5015, October 2007.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165, RFC
6335, August 2011.
10. Acknowledgments
The authors wish to thank: Ajit Sanzgiri for contributions to the
Security Considerations section and editorial inputs; Joseph Cheng,
Margaret Petrus, Milin Desai, Nial de Barra, Jeff Mandin, and Siva
Kollipara for their editorial reviews, inputs, and comments.
Authors' Addresses
Mallik Mahalingam
Storvisor, Inc.
640 W. California Ave, Suite #110
Sunnyvale, CA 94086.
USA
EMail: mallik_mahalingam@yahoo.com
Dinesh G. Dutt
Cumulus Networks
140C S. Whisman Road
Mountain View, CA 94041
USA
EMail: ddutt.ietf@hobbesdutt.com
Kenneth Duda
Arista Networks
5453 Great America Parkway
Santa Clara, CA 95054
USA
EMail: kduda@arista.com
Puneet Agarwal
Broadcom Corporation
3151 Zanker Road
San Jose, CA 95134
USA
EMail: pagarwal@broadcom.com
Lawrence Kreeger
Cisco Systems, Inc.
170 W. Tasman Avenue
San Jose, CA 95134
USA
EMail: kreeger@cisco.com
T. Sridhar
VMware, Inc.
3401 Hillview
Palo Alto, CA 94304
USA
EMail: tsridhar@vmware.com
Mike Bursell
Intel
Bowyer's, North Road
Great Yeldham
Halstead
Essex. C09 4QD
UK
EMail: mike.bursell@intel.com
Chris Wright
Red Hat, Inc.
1801 Varsity Drive
100 East Davie Street
Raleigh, NC 27606 27601
USA
EMail: chrisw@redhat.com