Internet Engineering Task Force Marc Lasserre
Internet Draft Florin Balus
Intended status: Informational Alcatel-Lucent
Expires: May 2014
Thomas Morin
France Telecom Orange
Nabil Bitar
Verizon
Yakov Rekhter
Juniper
November 12, 2013
Framework for DC Network Virtualization
draft-ietf-nvo3-framework-04.txt
Abstract
This document provides a framework for Network Virtualization over
L3 (NVO3) and it defines a reference model along with logical
components required to design a solution.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 12, 2014.
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Copyright Notice
Copyright (c) 2013 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
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warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction..................................................3
1.1. General terminology......................................3
1.2. DC network architecture..................................6
2. Reference Models..............................................8
2.1. Generic Reference Model..................................8
2.2. NVE Reference Model.....................................11
2.3. NVE Service Types.......................................12
2.3.1. L2 NVE providing Ethernet LAN-like service.........12
2.3.2. L3 NVE providing IP/VRF-like service...............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).....................13
3.1.3. Overlay Modules and VN Context.....................13
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/Service discovery..............15
3.1.5.3. Address advertisement and tunnel mapping.........15
3.1.5.4. Overlay Tunneling................................16
3.2. Multi-homing............................................16
3.3. VM Mobility.............................................17
4. Key aspects of overlay networks..............................18
4.1. Pros & Cons.............................................18
4.2. Overlay issues to consider..............................20
4.2.1. Data plane vs Control plane driven.................20
4.2.2. Coordination between data plane and control plane..20
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4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM)
traffic...................................................20
4.2.4. Path MTU...........................................21
4.2.5. NVE location trade-offs............................22
4.2.6. Interaction between network overlays and underlays.23
5. Security Considerations......................................23
6. IANA Considerations..........................................24
7. References...................................................24
7.1. Normative References....................................24
7.2. Informative References..................................24
8. Acknowledgments..............................................24
1. Introduction
This document provides a framework for Data Center Network
Virtualization over Layer3 (L3) tunnels. This framework is intended
to aid in standardizing protocols and mechanisms to support large-
scale network virtualization for data centers.
[NVOPS] 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 an Layer2 (L2) or
Layer3 (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
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physical switch or router, a Network Service Appliance, or 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 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.
Underlay or Underlying Network: The network that provides the
connectivity among NVEs and over which NVO3 packets are tunneled,
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
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environment that allows an operating system or application to be
aware of the presences 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 compute/memory/storage 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 NIC card 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 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: 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, internal vSwitch Interface ID connected to a VM).
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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,
iSCSI storage systems), and network devices (e.g., firewall, load-
balancer, IPSec gateway).
Network Virtualization Authority (NVA): Entity that provides
reachability and forwarding information to NVEs.
1.2. DC network architecture
A generic architecture for Data Centers is depicted in Figure 1:
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,---------.
,' `.
( 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 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
handled by a smaller number of nodes. Nevertheless, one can assume
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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:
o 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 Layer2 tunneling over IP for instance. Access
switches are usually multi-homed 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 EoR (End of
Row) switch, to provide similar function as a ToR.
o 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.
o 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 Virtual Private Network
(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 or multi-homed to access
switches.
2. Reference Models
2.1. Generic Reference Model
Figure 2 depicts a DC reference model for network virtualization
using L3 (IP/MPLS) 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 | .
/ . | +-----+ .
| . | .
| . | L3 Overlay +--+--++--------+
+--------+ | . | Network | NVE || Tenant |
| Tenant +--+ . | | || System |
| System | . \ +---+ +--+--++--------+
+--------+ .....|NVE|.........
+---+
|
|
=====================
| |
+--------+ +--------+
| Tenant | | Tenant |
| System | | System |
+--------+ +--------+
Figure 2 : Generic reference model for DC network virtualization
over a Layer3 (IP) infrastructure
In order to get reachability information, NVEs may exchange
information directly between themselves via a protocol. In this
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case, a control plane module resides in every NVE. This is how
routing control plane modules are implemented in routers for
instance.
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. OpenFlow [OF] is one example of such
a protocol.
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.
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 provided 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 dataplane 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 per tenant-service state albeit some
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applications (e.g., multicast) may require control plane or
forwarding plane information that pertain 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.).
+-------- 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
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in different devices. In addition, NVE functions can be implemented
in a hierarchical fashion. For instance, an End Device can act as an
NVE Spoke, while an access switch can act as an NVE hub.
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 VPLS or 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 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 IPVPN [RFC4364]) from a service
definition perspective. That is, an L3 NVE provides per-tenant
forwarding and 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.
3. Functional components
This section decomposes the Network Virtualization architecture into
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).
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VAPs can be physical ports or virtual ports identified through
logical interface identifiers (e.g., VLAN ID, 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.
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 simply encapsulation/decapsulation of frames
from 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:
o One VN Context identifier per Tenant: A globally unique (on a
per-DC administrative domain) VN identifier is used to identify
the corresponding VNI. Examples of such identifiers in existing
technologies are IEEE VLAN IDs and ISID tags that identify
virtual L2 domains when using IEEE 802.1aq and IEEE 802.1ah,
respectively.
o One VN Context identifier per VNI: A per-VNI local value is
automatically generated by the egress NVE, or a control plane
associated with that NVE, and usually distributed by a control
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plane protocol to all the related NVEs. An example of this
approach is the use of per VRF MPLS labels in IP VPN [RFC4364].
o One VN Context identifier per VAP: A per-VAP local value is
assigned and usually distributed by a control plane protocol.
An example of this approach is the use of per CE-PE 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.
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., GRE, L2TP, 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, 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. Thus, stateless tunneling is preferred when
feasible.
3.1.5. Control Plane Components
3.1.5.1. Distributed vs Centralized Control Plane
A control/management plane entity 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.
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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.
3.1.5.2. Auto-provisioning/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 to VNIs on other NVEs via a
control protocol exchanging 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.
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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/Service discovery could be accomplished by the same
protocol. In this scenario, the auto-provisioning/Service discovery
could be combined with (be inferred from) the address advertisement
and associated tunnel mapping. Furthermore, a control plane protocol
that carries both MAC and IP addresses eliminates the need for ARP,
and hence addresses one of the issues with explosive ARP handling.
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 encapsulating
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 may need to be established, interconnecting
NVEs. 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 setup the tunnel
path for some tunneling technologies. This applies to both unicast
and multicast tunneling.
3.2. Multi-homing
Multi-homing techniques can be used to increase the reliability of
an NVO3 network. It is also important to ensure that physical
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diversity in an NVO3 network is taken into account to avoid single
points of failure.
Multi-homing 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 as the means to re-route traffic upon failures techniques or
on MPLS re-rerouting capabilities.
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 Layer2 (L2) construct such as a
VLAN and it can be multi-homed to various NVEs. An NVE may provide
an L2 service to the end system or an l3 service. An NVE may be
multi-homed to a next layer in the DC at Layer2 (L2) or Layer3
(L3). When an NVE provides an L2 service and is not co-located with
the end system, techniques such as Ethernet Link Aggregation Group
(LAG) or Spanning Tree Protocol (STP) can be used to switch traffic
between an end system and connected NVEs without creating
loops. Similarly, when the NVE provides L3 service, similar dual-
homing techniques can be used. When the NVE provides a 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
multi-homed 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 out of 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.
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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 performance to VM applications.
4. Key aspects of overlay networks
The intent of this section is to highlight specific issues that
proposed overlay solutions need to address.
4.1. Pros & Cons
An overlay network is a layer of virtual network topology on top of
the physical network.
Overlay networks offer the following key advantages:
o 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.
o Tunneling is used to aggregate traffic and hide tenant
addresses from the underlay network, and hence offer the
advantage of minimizing the amount of forwarding state required
within the underlay network
o Decoupling of the overlay addresses (MAC and IP) used by VMs
from the underlay network for tenant separation and separation
of the tenant address spaces from the underlay address space.
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o Support of a large number of virtual network identifiers
Overlay networks also create several challenges:
o Overlay networks have typically 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.
o Traffic carried over an overlay may not traverse firewalls and
NAT devices.
o 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.
o Hash-based load balancing may not be optimal as the hash
algorithm may not work well due to the limited number of
combinations of tunnel source and destination addresses. Other
NVO3 mechanisms may use additional entropy information than
source and destination addresses.
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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
dataplane learning or a control plane. For an L3 NVE, the NVE needs
to be able to determine IP addresses of the Tenant Systems connected
via a VAP.
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:
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o Ingress replication
o 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, but 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, and 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. TCP will
adjust its maximum segment size accordingly.
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:
o Classical ICMP-based MTU Path Discovery [RFC1191] [RFC1981]
o
Tenant Systems rely on ICMP messages to discover the MTU
of the end-to-end path to its destination. This method is
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not always possible, such as when traversing middle boxes
(e.g. firewalls) which disable ICMP for security reasons
o Extended MTU Path Discovery techniques such as defined in
[RFC4821]
It is also 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. The assumption is that some hardware
assist is available on the NVE node to perform such SAR operations.
However, fragmentation by the NVE can lead to performance and
congestion issues due to TCP dynamics and might require new
congestion avoidance mechanisms from the underlay network [FLOYD].
Finally, the underlay network may 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:
o Processing and memory requirements
o Datapath (e.g. lookups, filtering,
encapsulation/decapsulation)
o Control plane processing (e.g. routing, signaling, OAM) and
where specific control plane functions should be enabled
o FIB/RIB size
o Multicast support
o Routing/signaling protocols
o Packet replication capability
o Multicast FIB
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o Fragmentation support
o QoS support (e.g. marking, policing, queuing)
o 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 generally
coordination in placing overlay demand on an underlay network, may
be achieved by providing mechanisms to exchange performance and
liveliness information between the underlay and overlay(s) or the
use of such information by a coordination system. Such information
may include:
o Performance metrics (throughput, delay, loss, jitter)
o Cost metrics
5. Security Considerations
NVO3 solutions must at least consider and address the following:
. Secure and authenticated communication between an NVE and an
NVE management system and/or control system.
. Isolation between tenant overlay networks. The use of per-
tenant FIB tables (VNIs) on an NVE is essential.
. Security of any protocol used to carry overlay network
information.
. Preventing packets from reaching the wrong NVI, especially
during VM moves.
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. It may desirable to restrict the types of information that can
be exchanged between overlays and underlays (e.g. topology
information)
6. IANA Considerations
IANA does not need to take any action for this draft.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative References
[NVOPS] Narten, T. et al, "Problem Statement : Overlays for Network
Virtualization", draft-narten-nvo3-overlay-problem-
statement (work in progress)
[OVCPREQ] Kreeger, L. et al, "Network Virtualization Overlay Control
Protocol Requirements", draft-kreeger-nvo3-overlay-cp
(work in progress)
[FLOYD] Sally Floyd, Allyn Romanow, "Dynamics of TCP Traffic over
ATM Networks", IEEE JSAC, V. 13 N. 4, May 1995
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006.
[RFC1191] Mogul, J. "Path MTU Discovery", RFC1191, November 1990
[RFC1981] McCann, J. et al, "Path MTU Discovery for IPv6", RFC1981,
August 1996
[RFC4821] Mathis, M. et al, "Packetization Layer Path MTU
Discovery", RFC4821, March 2007
8. Acknowledgments
In addition to the authors the following people have contributed to
this document:
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Dimitrios Stiliadis, Rotem Salomonovitch, Lucy Yong, Thomas Narten,
Larry Kreeger.
This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Marc Lasserre
Alcatel-Lucent
Email: marc.lasserre@alcatel-lucent.com
Florin Balus
Alcatel-Lucent
777 E. Middlefield Road
Mountain View, CA, USA 94043
Email: florin.balus@alcatel-lucent.com
Thomas Morin
France Telecom Orange
Email: thomas.morin@orange.com
Nabil Bitar
Verizon
40 Sylvan Road
Waltham, MA 02145
Email: nabil.bitar@verizon.com
Yakov Rekhter
Juniper
Email: yakov@juniper.net
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