Internet Engineering Task Force (IETF) M. Lasserre

Internet Engineering Task Force (IETF) M. Lasserre
Request for Comments: 7365 F. Balus
Category: Informational Alcatel-Lucent
ISSN: 2070-1721 T. Morin
Orange
N. Bitar
Verizon
Y. Rekhter
Juniper
October 2014

Framework for Data Center (DC) Network Virtualization

Abstract

This document provides a framework for Data Center (DC) Network
Virtualization over Layer 3 (NVO3) and defines a reference model
along with logical components required to design a solution.

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 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/rfc7365.

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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. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

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Table of Contents

1. Introduction ....................................................4
1.1. General Terminology ........................................4
1.2. DC Network Architecture ....................................7
2. Reference Models ................................................8
2.1. Generic Reference Model ....................................8
2.2. NVE Reference Model .......................................10
2.3. NVE Service Types .........................................11
2.3.1. L2 NVE Providing Ethernet LAN-Like Service .........11
2.3.2. L3 NVE Providing IP/VRF-Like Service ...............11
2.4. Operational Management Considerations .....................12
3. Functional Components ..........................................12
3.1. Service Virtualization Components .........................12
3.1.1. Virtual Access Points (VAPs) .......................12
3.1.2. Virtual Network Instance (VNI) .....................12
3.1.3. Overlay Modules and VN Context .....................14
3.1.4. Tunnel Overlays and Encapsulation Options ..........14
3.1.5. Control-Plane Components ...........................14
3.1.5.1. Distributed vs. Centralized
Control Plane .............................14
3.1.5.2. Auto-provisioning and Service Discovery ...15
3.1.5.3. Address Advertisement and Tunnel Mapping ..15
3.1.5.4. Overlay Tunneling .........................16
3.2. Multihoming ...............................................16
3.3. VM Mobility ...............................................17
4. Key Aspects of Overlay Networks ................................17
4.1. Pros and Cons .............................................18
4.2. Overlay Issues to Consider ................................19
4.2.1. Data Plane vs. Control Plane Driven ................19
4.2.2. Coordination between Data Plane and Control Plane ..19
4.2.3. Handling Broadcast, Unknown Unicast, and
Multicast (BUM) Traffic ............................20
4.2.4. Path MTU ...........................................20
4.2.5. NVE Location Trade-Offs ............................21
4.2.6. Interaction between Network Overlays and
Underlays ..........................................22
5. Security Considerations ........................................22
6. Informative References .........................................24
Acknowledgments ...................................................26
Authors' Addresses ................................................26

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1. Introduction

This document provides a framework for Data Center (DC) Network
Virtualization over Layer 3 (NVO3) tunnels. This framework is
intended to aid in standardizing protocols and mechanisms to support
large-scale network virtualization for data centers.

[RFC7364] defines the rationale for using overlay networks in order
to build large multi-tenant data center networks. Compute, storage
and network virtualization are often used in these large data centers
to support a large number of communication domains and end systems.

This document provides reference models and functional components of
data center overlay networks as well as a discussion of technical
issues that have to be addressed.

1.1. General Terminology

This document uses the following terminology:

NVO3 Network: An overlay network that provides a Layer 2 (L2) or
Layer 3 (L3) service to Tenant Systems over an L3 underlay network
using the architecture and protocols as defined by the NVO3 Working
Group.

Network Virtualization Edge (NVE): An NVE is the network entity that
sits at the edge of an underlay network and implements L2 and/or L3
network virtualization functions. The network-facing side of the NVE
uses the underlying L3 network to tunnel tenant frames to and from
other NVEs. The tenant-facing side of the NVE sends and receives
Ethernet frames to and from individual Tenant Systems. An NVE could
be implemented as part of a virtual switch within a hypervisor, a
physical switch or router, or a Network Service Appliance, or it
could be split across multiple devices.

Virtual Network (VN): A VN is a logical abstraction of a physical
network that provides L2 or L3 network services to a set of Tenant
Systems. A VN is also known as a Closed User Group (CUG).

Virtual Network Instance (VNI): A specific instance of a VN from the
perspective of an NVE.

Virtual Network Context (VN Context) Identifier: Field in an overlay
encapsulation header that identifies the specific VN the packet
belongs to. The egress NVE uses the VN Context identifier to deliver
the packet to the correct Tenant System. The VN Context identifier
can be a locally significant identifier or a globally unique
identifier.

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Underlay or Underlying Network: The network that provides the
connectivity among NVEs and that NVO3 packets are tunneled over,
where an NVO3 packet carries an NVO3 overlay header followed by a
tenant packet. The underlay network does not need to be aware that
it is carrying NVO3 packets. Addresses on the underlay network
appear as "outer addresses" in encapsulated NVO3 packets. In
general, the underlay network can use a completely different protocol
(and address family) from that of the overlay. In the case of NVO3,
the underlay network is IP.

Data Center (DC): A physical complex housing physical servers,
network switches and routers, network service appliances, and
networked storage. The purpose of a data center is to provide
application, compute, and/or storage services. One such service is
virtualized infrastructure data center services, also known as
"Infrastructure as a Service".

Virtual Data Center (Virtual DC): A container for virtualized
compute, storage, and network services. A virtual DC is associated
with a single tenant and can contain multiple VNs and Tenant Systems
connected to one or more of these VNs.

Virtual Machine (VM): A software implementation of a physical machine
that runs programs as if they were executing on a physical, non-
virtualized machine. Applications (generally) do not know they are
running on a VM as opposed to running on a "bare metal" host or
server, though some systems provide a para-virtualization environment
that allows an operating system or application to be aware of the
presence of virtualization for optimization purposes.

Hypervisor: Software running on a server that allows multiple VMs to
run on the same physical server. The hypervisor manages and provides
shared computation, memory, and storage services and network
connectivity to the VMs that it hosts. Hypervisors often embed a
virtual switch (see below).

Server: A physical end-host machine that runs user applications. A
standalone (or "bare metal") server runs a conventional operating
system hosting a single-tenant application. A virtualized server
runs a hypervisor supporting one or more VMs.

Virtual Switch (vSwitch): A function within a hypervisor (typically
implemented in software) that provides similar forwarding services to
a physical Ethernet switch. A vSwitch forwards Ethernet frames
between VMs running on the same server or between a VM and a physical
Network Interface Card (NIC) connecting the server to a physical
Ethernet switch or router. A vSwitch also enforces network isolation
between VMs that by policy are not permitted to communicate with each

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other (e.g., by honoring VLANs). A vSwitch may be bypassed when an
NVE is enabled on the host server.

Tenant: The customer using a virtual network and any associated
resources (e.g., compute, storage, and network). A tenant could be
an enterprise or a department/organization within an enterprise.

Tenant System: A physical or virtual system that can play the role of
a host or a forwarding element such as a router, switch, firewall,
etc. It belongs to a single tenant and connects to one or more VNs
of that tenant.

Tenant Separation: Refers to isolating traffic of different tenants
such that traffic from one tenant is not visible to or delivered to
another tenant, except when allowed by policy. Tenant separation
also refers to address space separation, whereby different tenants
can use the same address space without conflict.

Virtual Access Points (VAPs): A logical connection point on the NVE
for connecting a Tenant System to a virtual network. Tenant Systems
connect to VNIs at an NVE through VAPs. VAPs can be physical ports
or virtual ports identified through logical interface identifiers
(e.g., VLAN ID or internal vSwitch Interface ID connected to a VM).

End Device: A physical device that connects directly to the DC
underlay network. This is in contrast to a Tenant System, which
connects to a corresponding tenant VN. An End Device is administered
by the DC operator rather than a tenant and is part of the DC
infrastructure. An End Device may implement NVO3 technology in
support of NVO3 functions. Examples of an End Device include hosts
(e.g., server or server blade), storage systems (e.g., file servers
and iSCSI storage systems), and network devices (e.g., firewall,
load-balancer, and IPsec gateway).

Network Virtualization Authority (NVA): Entity that provides
reachability and forwarding information to NVEs.

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1.2. DC Network Architecture

A generic architecture for data centers is depicted in Figure 1:

,---------.
,' `.
( IP/MPLS WAN )
`. ,'
`-+------+'
\ /
+--------+ +--------+
| DC |+-+| DC |
|gateway |+-+|gateway |
+--------+ +--------+
| /
.--. .--.
( ' '.--.
.-.' Intra-DC '
( network )
( .'-'
'--'._.'. )\ \
/ / '--' \ \
/ / | | \ \
+--------+ +--------+ +--------+
| access | | access | | access |
| switch | | switch | | switch |
+--------+ +--------+ +--------+
/ \ / \ / \
__/_ \ / \ /_ _\__
'--------' '--------' '--------' '--------'
: End : : End : : End : : End :
: Device : : Device : : Device : : Device :
'--------' '--------' '--------' '--------'

Figure 1: A Generic Architecture for Data Centers

An example of multi-tier DC network architecture is presented in
Figure 1. It provides a view of the physical components inside a DC.

A DC network is usually composed of intra-DC networks and network
services, and inter-DC network and network connectivity services.

DC networking elements can act as strict L2 switches and/or provide
IP routing capabilities, including network service virtualization.

In some DC architectures, some tier layers could provide L2 and/or L3
services. In addition, some tier layers may be collapsed, and
Internet connectivity, inter-DC connectivity, and VPN support may be

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handled by a smaller number of nodes. Nevertheless, one can assume
that the network functional blocks in a DC fit in the architecture
depicted in Figure 1.

The following components can be present in a DC:

- Access switch: Hardware-based Ethernet switch aggregating all
Ethernet links from the End Devices in a rack representing the
entry point in the physical DC network for the hosts. It may also
provide routing functionality, virtual IP network connectivity, or
Layer 2 tunneling over IP, for instance. Access switches are
usually multihomed to aggregation switches in the Intra-DC
network. A typical example of an access switch is a Top-of-Rack
(ToR) switch. Other deployment scenarios may use an intermediate
Blade Switch before the ToR, or an End-of-Row (EoR) switch, to
provide similar functions to a ToR.

- Intra-DC Network: Network composed of high-capacity core nodes
(Ethernet switches/routers). Core nodes may provide virtual
Ethernet bridging and/or IP routing services.

- DC Gateway (DC GW): Gateway to the outside world providing DC
interconnect and connectivity to Internet and VPN customers. In
the current DC network model, this may be simply a router
connected to the Internet and/or an IP VPN/L2VPN PE. Some network
implementations may dedicate DC GWs for different connectivity
types (e.g., a DC GW for Internet and another for VPN).

Note that End Devices may be single-homed or multihomed to access
switches.

2. Reference Models

2.1. Generic Reference Model

Figure 2 depicts a DC reference model for network virtualization
overlays where NVEs provide a logical interconnect between Tenant
Systems that belong to a specific VN.

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+--------+ +--------+
| Tenant +--+ +----| Tenant |
| System | | (') | System |
+--------+ | ................. ( ) +--------+
| +---+ +---+ (_)
+--|NVE|---+ +---|NVE|-----+
+---+ | | +---+
/ . +-----+ .
/ . +--| NVA |--+ .
/ . | +-----+ \ .
| . | \ .
| . | Overlay +--+--++--------+
+--------+ | . | Network | NVE || Tenant |
| Tenant +--+ . | | || System |
| System | . \ +---+ +--+--++--------+
+--------+ .....|NVE|.........
+---+
|
|
=====================
| |
+--------+ +--------+
| Tenant | | Tenant |
| System | | System |
+--------+ +--------+

Figure 2: Generic Reference Model for DC Network Virtualization
Overlays

In order to obtain reachability information, NVEs may exchange
information directly between themselves via a control-plane protocol.
In this case, a control-plane module resides in every NVE.

It is also possible for NVEs to communicate with an external Network
Virtualization Authority (NVA) to obtain reachability and forwarding
information. In this case, a protocol is used between NVEs and
NVA(s) to exchange information.

It should be noted that NVAs may be organized in clusters for
redundancy and scalability and can appear as one logically
centralized controller. In this case, inter-NVA communication is
necessary to synchronize state among nodes within a cluster or share
information across clusters. The information exchanged between NVAs
of the same cluster could be different from the information exchanged
across clusters.

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A Tenant System can be attached to an NVE in several ways:

- locally, by being co-located in the same End Device

- remotely, via a point-to-point connection or a switched network

When an NVE is co-located with a Tenant System, the state of the
Tenant System can be determined without protocol assistance. For
instance, the operational status of a VM can be communicated via a
local API. When an NVE is remotely connected to a Tenant System, the
state of the Tenant System or NVE needs to be exchanged directly or
via a management entity, using a control-plane protocol or API, or
directly via a data-plane protocol.

The functional components in Figure 2 do not necessarily map directly
to the physical components described in Figure 1. For example, an
End Device can be a server blade with VMs and a virtual switch. A VM
can be a Tenant System, and the NVE functions may be performed by the
host server. In this case, the Tenant System and NVE function are
co-located. Another example is the case where the End Device is the
Tenant System and the NVE function can be implemented by the
connected ToR. In this case, the Tenant System and NVE function are
not co-located.

Underlay nodes utilize L3 technologies to interconnect NVE nodes.
These nodes perform forwarding based on outer L3 header information,
and generally do not maintain state for each tenant service, albeit
some applications (e.g., multicast) may require control-plane or
forwarding-plane information that pertains to a tenant, group of
tenants, tenant service, or a set of services that belong to one or
more tenants. Mechanisms to control the amount of state maintained
in the underlay may be needed.

2.2. NVE Reference Model

Figure 3 depicts the NVE reference model. One or more VNIs can be
instantiated on an NVE. A Tenant System interfaces with a
corresponding VNI via a VAP. An overlay module provides tunneling
overlay functions (e.g., encapsulation and decapsulation of tenant
traffic, tenant identification, and mapping, etc.).

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+-------- L3 Network -------+
| |
| Tunnel Overlay |
+------------+---------+ +---------+------------+
| +----------+-------+ | | +---------+--------+ |
| | Overlay Module | | | | Overlay Module | |
| +---------+--------+ | | +---------+--------+ |
| |VN Context| | VN Context| |
| | | | | |
| +--------+-------+ | | +--------+-------+ |
| | |VNI| . |VNI| | | | |VNI| . |VNI| |
NVE1 | +-+------------+-+ | | +-+-----------+--+ | NVE2
| | VAPs | | | | VAPs | |
+----+------------+----+ +----+-----------+-----+
| | | |
| | | |
Tenant Systems Tenant Systems

Figure 3: Generic NVE Reference Model

Note that some NVE functions (e.g., data-plane and control-plane
functions) may reside in one device or may be implemented separately
in different devices.

2.3. NVE Service Types

An NVE provides different types of virtualized network services to
multiple tenants, i.e., an L2 service or an L3 service. Note that an
NVE may be capable of providing both L2 and L3 services for a tenant.
This section defines the service types and associated attributes.

2.3.1. L2 NVE Providing Ethernet LAN-Like Service

An L2 NVE implements Ethernet LAN emulation, an Ethernet-based
multipoint service similar to an IETF Virtual Private LAN Service
(VPLS) [RFC4761][RFC4762] or Ethernet VPN [EVPN] service, where the
Tenant Systems appear to be interconnected by a LAN environment over
an L3 overlay. As such, an L2 NVE provides per-tenant virtual
switching instance (L2 VNI) and L3 (IP/MPLS) tunneling encapsulation
of tenant Media Access Control (MAC) frames across the underlay.
Note that the control plane for an L2 NVE could be implemented
locally on the NVE or in a separate control entity.

2.3.2. L3 NVE Providing IP/VRF-Like Service

An L3 NVE provides virtualized IP forwarding service, similar to IETF
IP VPN (e.g., BGP/MPLS IP VPN [RFC4364]) from a service definition
perspective. That is, an L3 NVE provides per-tenant forwarding and

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routing instance (L3 VNI) and L3 (IP/MPLS) tunneling encapsulation of
tenant IP packets across the underlay. Note that routing could be
performed locally on the NVE or in a separate control entity.

2.4. Operational Management Considerations

NVO3 services are overlay services over an IP underlay.

As far as the IP underlay is concerned, existing IP Operations,
Administration, and Maintenance (OAM) facilities are used.

With regard to the NVO3 overlay, both L2 and L3 services can be
offered. It is expected that existing fault and performance OAM
facilities will be used. Sections 4.1 and 4.2.6 provide further
discussion of additional fault and performance management issues to
consider.

As far as configuration is concerned, the DC environment is driven by
the need to bring new services up rapidly and is typically very
dynamic, specifically in the context of virtualized services. It is
therefore critical to automate the configuration of NVO3 services.

3. Functional Components

This section decomposes the network virtualization architecture into
the functional components described in Figure 3 to make it easier to
discuss solution options for these components.

3.1. Service Virtualization Components

3.1.1. Virtual Access Points (VAPs)

Tenant Systems are connected to VNIs through Virtual Access Points
(VAPs).

VAPs can be physical ports or virtual ports identified through
logical interface identifiers (e.g., VLAN ID and internal vSwitch
Interface ID connected to a VM).

3.1.2. Virtual Network Instance (VNI)

A VNI is a specific VN instance on an NVE. Each VNI defines a
forwarding context that contains reachability information and
policies.

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3.1.3. Overlay Modules and VN Context

Mechanisms for identifying each tenant service are required to allow
the simultaneous overlay of multiple tenant services over the same
underlay L3 network topology. In the data plane, each NVE, upon
sending a tenant packet, must be able to encode the VN Context for
the destination NVE in addition to the L3 tunneling information
(e.g., source IP address identifying the source NVE and the
destination IP address identifying the destination NVE, or MPLS
label). This allows the destination NVE to identify the tenant
service instance and therefore appropriately process and forward the
tenant packet.

The overlay module provides tunneling overlay functions: tunnel
initiation/termination as in the case of stateful tunnels (see
Section 3.1.4) and/or encapsulation/decapsulation of frames from the
VAPs/L3 underlay.

In a multi-tenant context, tunneling aggregates frames from/to
different VNIs. Tenant identification and traffic demultiplexing are
based on the VN Context identifier.

The following approaches can be considered:

- VN Context identifier per Tenant: This is a globally unique (on a
per-DC administrative domain) VN identifier used to identify the
corresponding VNI. Examples of such identifiers in existing
technologies are IEEE VLAN IDs and Service Instance IDs (I-SIDs)
that identify virtual L2 domains when using IEEE 802.1Q and IEEE
802.1ah, respectively. Note that multiple VN identifiers can
belong to a tenant.

- One VN Context identifier per VNI: Each VNI value is automatically
generated by the egress NVE, or a control plane associated with
that NVE, and usually distributed by a control-plane protocol to
all the related NVEs. An example of this approach is the use of
per-VRF MPLS labels in IP VPN [RFC4364]. The VNI value is
therefore locally significant to the egress NVE.

- One VN Context identifier per VAP: A value locally significant to
an NVE is assigned and usually distributed by a control-plane
protocol to identify a VAP. An example of this approach is the
use of per-CE MPLS labels in IP VPN [RFC4364].

Note that when using one VN Context per VNI or per VAP, an additional
global identifier (e.g., a VN identifier or name) may be used by the
control plane to identify the tenant context.

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3.1.4. Tunnel Overlays and Encapsulation Options

Once the VN Context identifier is added to the frame, an L3 tunnel
encapsulation is used to transport the frame to the destination NVE.

Different IP tunneling options (e.g., Generic Routing Encapsulation
(GRE), the Layer 2 Tunneling Protocol (L2TP), and IPsec) and MPLS
tunneling can be used. Tunneling could be stateless or stateful.
Stateless tunneling simply entails the encapsulation of a tenant
packet with another header necessary for forwarding the packet across
the underlay (e.g., IP tunneling over an IP underlay). Stateful
tunneling, on the other hand, entails maintaining tunneling state at
the tunnel endpoints (i.e., NVEs). Tenant packets on an ingress NVE
can then be transmitted over such tunnels to a destination (egress)
NVE by encapsulating the packets with a corresponding tunneling
header. The tunneling state at the endpoints may be configured or
dynamically established. Solutions should specify the tunneling
technology used and whether it is stateful or stateless. In this
document, however, tunneling and tunneling encapsulation are used
interchangeably to simply mean the encapsulation of a tenant packet
with a tunneling header necessary to carry the packet between an
ingress NVE and an egress NVE across the underlay. It should be
noted that stateful tunneling, especially when configuration is
involved, does impose management overhead and scale constraints.
When confidentiality is required, the use of opportunistic security
[OPPSEC] can be used as a stateless tunneling solution.

3.1.5. Control-Plane Components

3.1.5.1. Distributed vs. Centralized Control Plane

Control- and management-plane entities can be centralized or
distributed. Both approaches have been used extensively in the past.
The routing model of the Internet is a good example of a distributed
approach. Transport networks have usually used a centralized
approach to manage transport paths.

It is also possible to combine the two approaches, i.e., using a
hybrid model. A global view of network state can have many benefits,
but it does not preclude the use of distributed protocols within the
network. Centralized models provide a facility to maintain global
state and distribute that state to the network. When used in
combination with distributed protocols, greater network efficiencies,
improved reliability, and robustness can be achieved. Domain- and/or
deployment-specific constraints define the balance between
centralized and distributed approaches.

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3.1.5.2. Auto-provisioning and Service Discovery

NVEs must be able to identify the appropriate VNI for each Tenant
System. This is based on state information that is often provided by
external entities. For example, in an environment where a VM is a
Tenant System, this information is provided by VM orchestration
systems, since these are the only entities that have visibility of
which VM belongs to which tenant.

A mechanism for communicating this information to the NVE is
required. VAPs have to be created and mapped to the appropriate VNI.
Depending upon the implementation, this control interface can be
implemented using an auto-discovery protocol between Tenant Systems
and their local NVE or through management entities. In either case,
appropriate security and authentication mechanisms to verify that
Tenant System information is not spoofed or altered are required.
This is one critical aspect for providing integrity and tenant
isolation in the system.

NVEs may learn reachability information for VNIs on other NVEs via a
control protocol that exchanges such information among NVEs or via a
management-control entity.

3.1.5.3. Address Advertisement and Tunnel Mapping

As traffic reaches an ingress NVE on a VAP, a lookup is performed to
determine which NVE or local VAP the packet needs to be sent to. If
the packet is to be sent to another NVE, the packet is encapsulated
with a tunnel header containing the destination information
(destination IP address or MPLS label) of the egress NVE.
Intermediate nodes (between the ingress and egress NVEs) switch or
route traffic based upon the tunnel destination information.

A key step in the above process consists of identifying the
destination NVE the packet is to be tunneled to. NVEs are
responsible for maintaining a set of forwarding or mapping tables
that hold the bindings between destination VM and egress NVE
addresses. Several ways of populating these tables are possible:
control plane driven, management plane driven, or data plane driven.

When a control-plane protocol is used to distribute address
reachability and tunneling information, the auto-provisioning and
service discovery could be accomplished by the same protocol. In
this scenario, the auto-provisioning and service discovery could be
combined with (be inferred from) the address advertisement and
associated tunnel mapping. Furthermore, a control-plane protocol

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that carries both MAC and IP addresses eliminates the need for the
Address Resolution Protocol (ARP) and hence addresses one of the
issues with explosive ARP handling as discussed in [RFC6820].

3.1.5.4. Overlay Tunneling

For overlay tunneling, and dependent upon the tunneling technology
used for encapsulating the Tenant System packets, it may be
sufficient to have one or more local NVE addresses assigned and used
in the source and destination fields of a tunneling encapsulation
header. Other information that is part of the tunneling
encapsulation header may also need to be configured. In certain
cases, local NVE configuration may be sufficient while in other
cases, some tunneling-related information may need to be shared among
NVEs. The information that needs to be shared will be technology
dependent. For instance, potential information could include tunnel
identity, encapsulation type, and/or tunnel resources. In certain
cases, such as when using IP multicast in the underlay, tunnels that
interconnect NVEs may need to be established. When tunneling
information needs to be exchanged or shared among NVEs, a control-
plane protocol may be required. For instance, it may be necessary to
provide active/standby status information between NVEs, up/down
status information, pruning/grafting information for multicast
tunnels, etc.

In addition, a control plane may be required to set up the tunnel
path for some tunneling technologies. This applies to both unicast
and multicast tunneling.

3.2. Multihoming

Multihoming techniques can be used to increase the reliability of an
NVO3 network. It is also important to ensure that the physical
diversity in an NVO3 network is taken into account to avoid single
points of failure.

Multihoming can be enabled in various nodes, from Tenant Systems into
ToRs, ToRs into core switches/routers, and core nodes into DC GWs.

The NVO3 underlay nodes (i.e., from NVEs to DC GWs) rely on IP
routing techniques or MPLS re-rerouting capabilities as the means to
re-route traffic upon failures.

When a Tenant System is co-located with the NVE, the Tenant System is
effectively single-homed to the NVE via a virtual port. When the
Tenant System and the NVE are separated, the Tenant System is
connected to the NVE via a logical L2 construct such as a VLAN, and
it can be multihomed to various NVEs. An NVE may provide an L2

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service to the end system or an l3 service. An NVE may be multihomed
to a next layer in the DC at L2 or L3. When an NVE provides an L2
service and is not co-located with the end system, loop-avoidance
techniques must be used. Similarly, when the NVE provides L3
service, similar dual-homing techniques can be used. When the NVE
provides an L3 service to the end system, it is possible that no
dynamic routing protocol is enabled between the end system and the
NVE. The end system can be multihomed to multiple physically
separated L3 NVEs over multiple interfaces. When one of the links
connected to an NVE fails, the other interfaces can be used to reach
the end system.

External connectivity from a DC can be handled by two or more DC
gateways. Each gateway provides access to external networks such as
VPNs or the Internet. A gateway may be connected to two or more edge
nodes in the external network for redundancy. When a connection to
an upstream node is lost, the alternative connection is used, and the
failed route withdrawn.

3.3. VM Mobility

In DC environments utilizing VM technologies, an important feature is
that VMs can move from one server to another server in the same or
different L2 physical domains (within or across DCs) in a seamless
manner.

A VM can be moved from one server to another in stopped or suspended
state ("cold" VM mobility) or in running/active state ("hot" VM
mobility). With "hot" mobility, VM L2 and L3 addresses need to be
preserved. With "cold" mobility, it may be desired to preserve at
least VM L3 addresses.

Solutions to maintain connectivity while a VM is moved are necessary
in the case of "hot" mobility. This implies that connectivity among
VMs is preserved. For instance, for L2 VNs, ARP caches are updated
accordingly.

Upon VM mobility, NVE policies that define connectivity among VMs
must be maintained.

During VM mobility, it is expected that the path to the VM's default
gateway assures adequate QoS to VM applications, i.e., QoS that
matches the expected service-level agreement for these applications.

4. Key Aspects of Overlay Networks

The intent of this section is to highlight specific issues that
proposed overlay solutions need to address.

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4.1. Pros and Cons

An overlay network is a layer of virtual network topology on top of
the physical network.

Overlay networks offer the following key advantages:

- Unicast tunneling state management and association of Tenant
Systems reachability are handled at the edge of the network (at
the NVE). Intermediate transport nodes are unaware of such state.
Note that when multicast is enabled in the underlay network to
build multicast trees for tenant VNs, there would be more state
related to tenants in the underlay core network.

- Tunneling is used to aggregate traffic and hide tenant addresses
from the underlay network and hence offers the advantage of
minimizing the amount of forwarding state required within the
underlay network.

- Decoupling of the overlay addresses (MAC and IP) used by VMs from
the underlay network provides tenant separation and separation of
the tenant address spaces from the underlay address space.

- Overlay networks support of a large number of virtual network
identifiers.

Overlay networks also create several challenges:

- Overlay networks typically have no control of underlay networks
and lack underlay network information (e.g., underlay
utilization):

o Overlay networks and/or their associated management entities
typically probe the network to measure link or path properties,
such as available bandwidth or packet loss rate. It is
difficult to accurately evaluate network properties. It might
be preferable for the underlay network to expose usage and
performance information.

o Miscommunication or lack of coordination between overlay and
underlay networks can lead to an inefficient usage of network
resources.

o When multiple overlays co-exist on top of a common underlay
network, the lack of coordination between overlays can lead to
performance issues and/or resource usage inefficiencies.

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- Traffic carried over an overlay might fail to traverse firewalls
and NAT devices.

- Multicast service scalability: Multicast support may be required
in the underlay network to address tenant flood containment or
efficient multicast handling. The underlay may also be required
to maintain multicast state on a per-tenant basis or even on a
per-individual multicast flow of a given tenant. Ingress
replication at the NVE eliminates that additional multicast state
in the underlay core, but depending on the multicast traffic
volume, it may cause inefficient use of bandwidth.

4.2. Overlay Issues to Consider

4.2.1. Data Plane vs. Control Plane Driven

In the case of an L2 NVE, it is possible to dynamically learn MAC
addresses against VAPs. It is also possible that such addresses be
known and controlled via management or a control protocol for both L2
NVEs and L3 NVEs. Dynamic data-plane learning implies that flooding
of unknown destinations be supported and hence implies that broadcast
and/or multicast be supported or that ingress replication be used as
described in Section 4.2.3. Multicasting in the underlay network for
dynamic learning may lead to significant scalability limitations.
Specific forwarding rules must be enforced to prevent loops from
happening. This can be achieved using a spanning tree, a shortest
path tree, or a split-horizon mesh.

It should be noted that the amount of state to be distributed is
dependent upon network topology and the number of virtual machines.
Different forms of caching can also be utilized to minimize state
distribution between the various elements. The control plane should
not require an NVE to maintain the locations of all the Tenant
Systems whose VNs are not present on the NVE. The use of a control
plane does not imply that the data plane on NVEs has to maintain all
the forwarding state in the control plane.

4.2.2. Coordination between Data Plane and Control Plane

For an L2 NVE, the NVE needs to be able to determine MAC addresses of
the Tenant Systems connected via a VAP. This can be achieved via
data-plane learning or a control plane. For an L3 NVE, the NVE needs
to be able to determine the IP addresses of the Tenant Systems
connected via a VAP.

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In both cases, coordination with the NVE control protocol is needed
such that when the NVE determines that the set of addresses behind a
VAP has changed, it triggers the NVE control plane to distribute this
information to its peers.

4.2.3. Handling Broadcast, Unknown Unicast, and Multicast (BUM) Traffic

There are several options to support packet replication needed for
broadcast, unknown unicast, and multicast. Typical methods include:

- Ingress replication

- Use of underlay multicast trees

There is a bandwidth vs. state trade-off between the two approaches.
Depending upon the degree of replication required (i.e., the number
of hosts per group) and the amount of multicast state to maintain,
trading bandwidth for state should be considered.

When the number of hosts per group is large, the use of underlay
multicast trees may be more appropriate. When the number of hosts is
small (e.g., 2-3) and/or the amount of multicast traffic is small,
ingress replication may not be an issue.

Depending upon the size of the data center network and hence the
number of (S,G) entries, and also the duration of multicast flows,
the use of underlay multicast trees can be a challenge.

When flows are well known, it is possible to pre-provision such
multicast trees. However, it is often difficult to predict
application flows ahead of time; hence, programming of (S,G) entries
for short-lived flows could be impractical.

A possible trade-off is to use in the underlay shared multicast trees
as opposed to dedicated multicast trees.

4.2.4. Path MTU

When using overlay tunneling, an outer header is added to the
original frame. This can cause the MTU of the path to the egress
tunnel endpoint to be exceeded.

It is usually not desirable to rely on IP fragmentation for
performance reasons. Ideally, the interface MTU as seen by a Tenant
System is adjusted such that no fragmentation is needed.

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It is possible for the MTU to be configured manually or to be
discovered dynamically. Various Path MTU discovery techniques exist
in order to determine the proper MTU size to use:

- Classical ICMP-based Path MTU Discovery [RFC1191] [RFC1981]

Tenant Systems rely on ICMP messages to discover the MTU of the
end-to-end path to its destination. This method is not always
possible, such as when traversing middleboxes (e.g., firewalls)
that disable ICMP for security reasons.

- Extended Path MTU Discovery techniques such as those defined in
[RFC4821]

Tenant Systems send probe packets of different sizes and rely on
confirmation of receipt or lack thereof from receivers to allow a
sender to discover the MTU of the end-to-end paths.

While it could also be possible to rely on the NVE to perform
segmentation and reassembly operations without relying on the Tenant
Systems to know about the end-to-end MTU, this would lead to
undesired performance and congestion issues as well as significantly
increase the complexity of hardware NVEs required for buffering and
reassembly logic.

Preferably, the underlay network should be designed in such a way
that the MTU can accommodate the extra tunneling and possibly
additional NVO3 header encapsulation overhead.

4.2.5. NVE Location Trade-Offs

In the case of DC traffic, traffic originated from a VM is native
Ethernet traffic. This traffic can be switched by a local virtual
switch or ToR switch and then by a DC gateway. The NVE function can
be embedded within any of these elements.

There are several criteria to consider when deciding where the NVE
function should happen:

- Processing and memory requirements

o Datapath (e.g., lookups, filtering, and
encapsulation/decapsulation)

o Control-plane processing (e.g., routing, signaling, and OAM)
and where specific control-plane functions should be enabled

- FIB/RIB size

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- Multicast support

o Routing/signaling protocols

o Packet replication capability

o Multicast FIB

- Fragmentation support

- QoS support (e.g., marking, policing, and queuing)

- Resiliency

4.2.6. Interaction between Network Overlays and Underlays

When multiple overlays co-exist on top of a common underlay network,
resources (e.g., bandwidth) should be provisioned to ensure that
traffic from overlays can be accommodated and QoS objectives can be
met. Overlays can have partially overlapping paths (nodes and
links).

Each overlay is selfish by nature. It sends traffic so as to
optimize its own performance without considering the impact on other
overlays, unless the underlay paths are traffic engineered on a per-
overlay basis to avoid congestion of underlay resources.

Better visibility between overlays and underlays, or general
coordination in placing overlay demands on an underlay network, may
be achieved by providing mechanisms to exchange performance and
liveliness information between the underlay and overlay(s) or by the
use of such information by a coordination system. Such information
may include:

- Performance metrics (throughput, delay, loss, jitter) such as
defined in [RFC3148], [RFC2679], [RFC2680], and [RFC3393].

- Cost metrics

5. Security Considerations

There are three points of view when considering security for NVO3.
First, the service offered by a service provider via NVO3 technology
to a tenant must meet the mutually agreed security requirements.
Second, a network implementing NVO3 must be able to trust the virtual
network identity associated with packets received from a tenant.
Third, an NVO3 network must consider the security associated with
running as an overlay across the underlay network.

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To meet a tenant's security requirements, the NVO3 service must
deliver packets from the tenant to the indicated destination(s) in
the overlay network and external networks. The NVO3 service provides
data confidentiality through data separation. The use of both VNIs
and tunneling of tenant traffic by NVEs ensures that NVO3 data is
kept in a separate context and thus separated from other tenant
traffic. The infrastructure supporting an NVO3 service (e.g.,
management systems, NVEs, NVAs, and intermediate underlay networks)
should be limited to authorized access so that data integrity can be
expected. If a tenant requires that its data be confidential, then
the Tenant System may choose to encrypt its data before transmission
into the NVO3 service.

An NVO3 service must be able to verify the VNI received on a packet
from the tenant. To ensure this, not only tenant data but also NVO3
control data must be secured (e.g., control traffic between NVAs and
NVEs, between NVAs, and between NVEs). Since NVEs and NVAs play a
central role in NVO3, it is critical that secure access to NVEs and
NVAs be ensured such that no unauthorized access is possible. As
discussed in Section 3.1.5.2, identification of Tenant Systems is
based upon state that is often provided by management systems (e.g.,
a VM orchestration system in a virtualized environment). Secure
access to such management systems must also be ensured. When an NVE
receives data from a Tenant System, the tenant identity needs to be
verified in order to guarantee that it is authorized to access the
corresponding VN. This can be achieved by identifying incoming
packets against specific VAPs in some cases. In other circumstances,
authentication may be necessary. Once this verification is done, the
packet is allowed into the NVO3 overlay, and no integrity protection
is provided on the overlay packet encapsulation (e.g., the VNI,
destination NVE, etc.).

Since an NVO3 service can run across diverse underlay networks, when
the underlay network is not trusted to provide at least data
integrity, data encryption is needed to assure correct packet
delivery.

It is also desirable to restrict the types of information (e.g.,
topology information as discussed in Section 4.2.6) that can be
exchanged between an NVO3 service and underlay networks based upon
their agreed security requirements.

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6. Informative References

[EVPN] Sajassi, A., Aggarwal, R., Bitar, N., Isaac, A., and J.
Uttaro, "BGP MPLS Based Ethernet VPN", Work in Progress,
draft-ietf-l2vpn-evpn-10, October 2014.

[OPPSEC] Dukhovni, V. "Opportunistic Security: Some Protection Most
of the Time", Work in Progress, draft-dukhovni-
opportunistic-security-04, August 2014.

[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990, <http://www.rfc-editor.org/info/rfc1191>.

[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996,
<http://www.rfc-editor.org/info/rfc1981>.

[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999,
<http://www.rfc-editor.org/info/rfc2679>.

[RFC2680] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Packet Loss Metric for IPPM", RFC 2680, September 1999,
<http://www.rfc-editor.org/info/rfc2680>.

[RFC3148] Mathis, M. and M. Allman, "A Framework for Defining
Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
2001, <http://www.rfc-editor.org/info/rfc3148>.

[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002, <http://www.rfc-editor.org/info/rfc3393>.

[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006,
<http://www.rfc-editor.org/info/rfc4364>.

[RFC4761] Kompella, K., Ed., and Y. Rekhter, Ed., "Virtual Private
LAN Service (VPLS) Using BGP for Auto-Discovery and
Signaling", RFC 4761, January 2007,
<http://www.rfc-editor.org/info/rfc4761>.

[RFC4762] Lasserre, M., Ed., and V. Kompella, Ed., "Virtual Private
LAN Service (VPLS) Using Label Distribution Protocol (LDP)
Signaling", RFC 4762, January 2007,
<http://www.rfc-editor.org/info/rfc4762>.

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RFC 7365 Framework for DC Network Virtualization October 2014

[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.

[RFC6820] Narten, T., Karir, M., and I. Foo, "Address Resolution
Problems in Large Data Center Networks", RFC 6820, January
2013, <http://www.rfc-editor.org/info/rfc6820>.

[RFC7364] Narten, T., Ed., Gray, E., Ed., Black, D., Fang, L.,
Kreeger, L., and M. Napierala, "Problem Statement:
Overlays for Network Virtualization", RFC 7364, October
2014, <http://www.rfc-editor.org/info/rfc7364>.

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Acknowledgments

In addition to the authors, the following people contributed to this
document: Dimitrios Stiliadis, Rotem Salomonovitch, Lucy Yong, Thomas
Narten, Larry Kreeger, and David Black.

Authors' Addresses

Marc Lasserre
Alcatel-Lucent
EMail: [email protected]

Florin Balus
Alcatel-Lucent
777 E. Middlefield Road
Mountain View, CA 94043
United States
EMail: [email protected]

Thomas Morin
Orange
EMail: [email protected]

Nabil Bitar
Verizon
50 Sylvan Road
Waltham, MA 02145
United States
EMail: [email protected]

Yakov Rekhter
Juniper
EMail: [email protected]

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