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Internet Engineering Task Force (IETF) T. Narten, Ed.
Request for Comments: 7364 IBM
Category: Informational E. Gray, Ed.
ISSN: 2070-1721 Ericsson
D. Black
EMC
L. Fang
Microsoft
L. Kreeger
Cisco
M. Napierala
AT&T
October 2014
Problem Statement: Overlays for Network Virtualization
Abstract
This document describes issues associated with providing multi-
tenancy in large data center networks and how these issues may be
addressed using an overlay-based network virtualization approach. A
key multi-tenancy requirement is traffic isolation so that one
tenant's traffic is not visible to any other tenant. Another
requirement is address space isolation so that different tenants can
use the same address space within different virtual networks.
Traffic and address space isolation is achieved by assigning one or
more virtual networks to each tenant, where traffic within a virtual
network can only cross into another virtual network in a controlled
fashion (e.g., via a configured router and/or a security gateway).
Additional functionality is required to provision virtual networks,
associating a virtual machine's network interface(s) with the
appropriate virtual network and maintaining that association as the
virtual machine is activated, migrated, and/or deactivated. Use of
an overlay-based approach enables scalable deployment on large
network infrastructures.
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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/rfc7364.
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
2. Terminology .....................................................6
3. Problem Areas ...................................................6
3.1. Need for Dynamic Provisioning ..............................6
3.2. Virtual Machine Mobility Limitations .......................7
3.3. Inadequate Forwarding Table Sizes ..........................7
3.4. Need to Decouple Logical and Physical Configuration ........7
3.5. Need for Address Separation between Virtual Networks .......8
3.6. Need for Address Separation between Virtual Networks and ...8
3.7. Optimal Forwarding .........................................9
4. Using Network Overlays to Provide Virtual Networks .............10
4.1. Overview of Network Overlays ..............................10
4.2. Communication between Virtual and Non-virtualized
Networks ..................................................12
4.3. Communication between Virtual Networks ....................12
4.4. Overlay Design Characteristics ............................13
4.5. Control-Plane Overlay Networking Work Areas ...............14
4.6. Data-Plane Work Areas .....................................15
5. Related IETF and IEEE Work .....................................15
5.1. BGP/MPLS IP VPNs ..........................................16
5.2. BGP/MPLS Ethernet VPNs ....................................16
5.3. 802.1 VLANs ...............................................17
5.4. IEEE 802.1aq -- Shortest Path Bridging ....................17
5.5. VDP .......................................................17
5.6. ARMD ......................................................18
5.7. TRILL .....................................................18
5.8. L2VPNs ....................................................18
5.9. Proxy Mobile IP ...........................................19
5.10. LISP .....................................................19
6. Summary ........................................................19
7. Security Considerations ........................................19
8. References .....................................................20
8.1. Normative Reference .......................................20
8.2. Informative References ....................................20
Acknowledgments ...................................................22
Contributors ......................................................22
Authors' Addresses ................................................23
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1. Introduction
Data centers are increasingly being consolidated and outsourced in an
effort to improve the deployment time of applications and reduce
operational costs. This coincides with an increasing demand for
compute, storage, and network resources from applications. In order
to scale compute, storage, and network resources, physical resources
are being abstracted from their logical representation, in what is
referred to as server, storage, and network virtualization.
Virtualization can be implemented in various layers of computer
systems or networks.
The demand for server virtualization is increasing in data centers.
With server virtualization, each physical server supports multiple
virtual machines (VMs), each running its own operating system,
middleware, and applications. Virtualization is a key enabler of
workload agility, i.e., allowing any server to host any application
and providing the flexibility of adding, shrinking, or moving
services within the physical infrastructure. Server virtualization
provides numerous benefits, including higher utilization, increased
security, reduced user downtime, reduced power usage, etc.
Multi-tenant data centers are taking advantage of the benefits of
server virtualization to provide a new kind of hosting, a virtual
hosted data center. Multi-tenant data centers are ones where
individual tenants could belong to a different company (in the case
of a public provider) or a different department (in the case of an
internal company data center). Each tenant has the expectation of a
level of security and privacy separating their resources from those
of other tenants. For example, one tenant's traffic must never be
exposed to another tenant, except through carefully controlled
interfaces, such as a security gateway (e.g., a firewall).
To a tenant, virtual data centers are similar to their physical
counterparts, consisting of end stations attached to a network,
complete with services such as load balancers and firewalls. But
unlike a physical data center, Tenant Systems connect to a virtual
network (VN). To Tenant Systems, a virtual network looks like a
normal network (e.g., providing an Ethernet or L3 service), except
that the only end stations connected to the virtual network are those
belonging to a tenant's specific virtual network.
A tenant is the administrative entity on whose behalf one or more
specific virtual network instances and their associated services
(whether virtual or physical) are managed. In a cloud environment, a
tenant would correspond to the customer that is using a particular
virtual network. However, a tenant may also find it useful to create
multiple different virtual network instances. Hence, there is a one-
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to-many mapping between tenants and virtual network instances. A
single tenant may operate multiple individual virtual network
instances, each associated with a different service.
How a virtual network is implemented does not generally matter to the
tenant; what matters is that the service provided (Layer 2 (L2) or
Layer 3 (L3)) has the right semantics, performance, etc. It could be
implemented via a pure routed network, a pure bridged network, or a
combination of bridged and routed networks. A key requirement is
that each individual virtual network instance be isolated from other
virtual network instances, with traffic crossing from one virtual
network to another only when allowed by policy.
For data center virtualization, two key issues must be addressed.
First, address space separation between tenants must be supported.
Second, it must be possible to place (and migrate) VMs anywhere in
the data center, without restricting VM addressing to match the
subnet boundaries of the underlying data center network.
This document outlines problems encountered in scaling the number of
isolated virtual networks in a data center. Furthermore, the
document presents issues associated with managing those virtual
networks in relation to operations, such as virtual network creation/
deletion and end-node membership change. Finally, this document
makes the case that an overlay-based approach has a number of
advantages over traditional, non-overlay approaches. The purpose of
this document is to identify the set of issues that any solution has
to address in building multi-tenant data centers. With this
approach, the goal is to allow the construction of standardized,
interoperable implementations to allow the construction of multi-
tenant data centers.
This document is the problem statement for the "Network
Virtualization over Layer 3" (NVO3) Working Group. NVO3 is focused
on the construction of overlay networks that operate over an IP (L3)
underlay transport network. NVO3 expects to provide both L2 service
and IP service to Tenant Systems (though perhaps as two different
solutions). Some deployments require an L2 service, others an L3
service, and some may require both.
Section 2 gives terminology. Section 3 describes the problem space
details. Section 4 describes overlay networks in more detail.
Section 5 reviews related and further work, and Section 6 closes with
a summary.
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2. Terminology
This document uses the same terminology as [RFC7365]. In addition,
this document use the following terms.
Overlay Network: A virtual network in which the separation of
tenants is hidden from the underlying physical infrastructure.
That is, the underlying transport network does not need to know
about tenancy separation to correctly forward traffic. IEEE 802.1
Provider Backbone Bridging (PBB) [IEEE-802.1Q] is an example of an
L2 overlay network. PBB uses MAC-in-MAC encapsulation (where
"MAC" refers to "Media Access Control"), and the underlying
transport network forwards traffic using only the Backbone MAC
(B-MAC) and Backbone VLAN Identifier (B-VID) in the outer header.
The underlay transport network is unaware of the tenancy
separation provided by, for example, a 24-bit Backbone Service
Instance Identifier (I-SID).
C-VLAN: This document refers to Customer VLANs (C-VLANs) as
implemented by many routers, i.e., an L2 virtual network
identified by a Customer VLAN Identifier (C-VID). An end station
(e.g., a VM) in this context that is part of an L2 virtual network
will effectively belong to a C-VLAN. Within an IEEE 802.1Q-2011
network, other tags may be used as well, but such usage is
generally not visible to the end station. Section 5.3 provides
more details on VLANs defined by [IEEE-802.1Q].
This document uses the phrase "virtual network instance" with its
ordinary meaning to represent an instance of a virtual network. Its
usage may differ from the "VNI" acronym defined in the framework
document [RFC7365]. The "VNI" acronym is not used in this document.
3. Problem Areas
The following subsections describe aspects of multi-tenant data
center networking that pose problems for network infrastructure.
Different problem aspects may arise based on the network architecture
and scale.
3.1. Need for Dynamic Provisioning
Some service providers offer services to multiple customers whereby
services are dynamic and the resources assigned to support them must
be able to change quickly as demand changes. In current systems, it
can be difficult to provision resources for individual tenants (e.g.,
QoS) in such a way that provisioned properties migrate automatically
when services are dynamically moved around within the data center to
optimize workloads.
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3.2. Virtual Machine Mobility Limitations
A key benefit of server virtualization is virtual machine (VM)
mobility. A VM can be migrated from one server to another live,
i.e., while continuing to run and without needing to shut down and
restart at the new location. A key requirement for live migration is
that a VM retain critical network state at its new location,
including its IP and MAC address(es). Preservation of MAC addresses
may be necessary, for example, when software licenses are bound to
MAC addresses. More generally, any change in the VM's MAC addresses
resulting from a move would be visible to the VM and thus potentially
result in unexpected disruptions. Retaining IP addresses after a
move is necessary to prevent existing transport connections (e.g.,
TCP) from breaking and needing to be restarted.
In data center networks, servers are typically assigned IP addresses
based on their physical location, for example, based on the Top-of-
Rack (ToR) switch for the server rack or the C-VLAN configured to the
server. Servers can only move to other locations within the same IP
subnet. This constraint is not problematic for physical servers,
which move infrequently, but it restricts the placement and movement
of VMs within the data center. Any solution for a scalable multi-
tenant data center must allow a VM to be placed (or moved) anywhere
within the data center without being constrained by the subnet
boundary concerns of the host servers.
3.3. Inadequate Forwarding Table Sizes
Today's virtualized environments place additional demands on the
forwarding tables of forwarding nodes in the physical infrastructure.
The core problem is that location independence results in specific
end state information being propagated into the forwarding system
(e.g., /32 host routes in IPv4 networks or MAC addresses in IEEE
802.3 Ethernet networks). In L2 networks, for instance, instead of
just one address per server, the network infrastructure may have to
learn addresses of the individual VMs (which could range in the
hundreds per server). This increases the demand on a forwarding
node's table capacity compared to non-virtualized environments.
3.4. Need to Decouple Logical and Physical Configuration
Data center operators must be able to achieve high utilization of
server and network capacity. For efficient and flexible allocation,
operators should be able to spread a virtual network instance across
servers in any rack in the data center. It should also be possible
to migrate compute workloads to any server anywhere in the network
while retaining the workload's addresses.
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In networks of many types (e.g., IP subnets, MPLS VPNs, VLANs, etc.),
moving servers elsewhere in the network may require expanding the
scope of a portion of the network (e.g., subnet, VPN, VLAN, etc.)
beyond its original boundaries. While this can be done, it requires
potentially complex network configuration changes and may, in some
cases (e.g., a VLAN or L2VPN), conflict with the desire to bound the
size of broadcast domains. In addition, when VMs migrate, the
physical network (e.g., access lists) may need to be reconfigured,
which can be time consuming and error prone.
An important use case is cross-pod expansion. A pod typically
consists of one or more racks of servers with associated network and
storage connectivity. A tenant's virtual network may start off on a
pod and, due to expansion, require servers/VMs on other pods,
especially the case when other pods are not fully utilizing all their
resources. This use case requires that virtual networks span
multiple pods in order to provide connectivity to all of the tenants'
servers/VMs. Such expansion can be difficult to achieve when tenant
addressing is tied to the addressing used by the underlay network or
when the expansion requires that the scope of the underlying C-VLAN
expand beyond its original pod boundary.
3.5. Need for Address Separation between Virtual Networks
Individual tenants need control over the addresses they use within a
virtual network. But it can be problematic when different tenants
want to use the same addresses or even if the same tenant wants to
reuse the same addresses in different virtual networks.
Consequently, virtual networks must allow tenants to use whatever
addresses they want without concern for what addresses are being used
by other tenants or other virtual networks.
3.6. Need for Address Separation between Virtual Networks and
Infrastructure
As in the previous case, a tenant needs to be able to use whatever
addresses it wants in a virtual network independent of what addresses
the underlying data center network is using. Tenants (and the
underlay infrastructure provider) should be able use whatever
addresses make sense for them without having to worry about address
collisions between addresses used by tenants and those used by the
underlay data center network.
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3.7. Optimal Forwarding
Another problem area relates to the optimal forwarding of traffic
between peers that are not connected to the same virtual network.
Such forwarding happens when a host on a virtual network communicates
with a host not on any virtual network (e.g., an Internet host) as
well as when a host on a virtual network communicates with a host on
a different virtual network. A virtual network may have two (or
more) gateways for forwarding traffic onto and off of the virtual
network, and the optimal choice of which gateway to use may depend on
the set of available paths between the communicating peers. The set
of available gateways may not be equally "close" to a given
destination. The issue appears both when a VM is initially
instantiated on a virtual network or when a VM migrates or is moved
to a different location. After a migration, for instance, a VM's
best-choice gateway for such traffic may change, i.e., the VM may get
better service by switching to the "closer" gateway, and this may
improve the utilization of network resources.
IP implementations in network endpoints typically do not distinguish
between multiple routers on the same subnet -- there may only be a
single default gateway in use, and any use of multiple routers
usually considers all of them to be one hop away. Routing protocol
functionality is constrained by the requirement to cope with these
endpoint limitations -- for example, the Virtual Router Redundancy
Protocol (VRRP) has one router serve as the master to handle all
outbound traffic. This problem can be particularly acute when the
virtual network spans multiple data centers, as a VM is likely to
receive significantly better service when forwarding external traffic
through a local router compared to using a router at a remote data
center.
The optimal forwarding problem applies to both outbound and inbound
traffic. For outbound traffic, the choice of outbound router
determines the path of outgoing traffic from the VM, which may be
sub-optimal after a VM move. For inbound traffic, the location of
the VM within the IP subnet for the VM is not visible to the routers
beyond the virtual network. Thus, the routing infrastructure will
have no information as to which of the two externally visible
gateways leading into the virtual network would be the better choice
for reaching a particular VM.
The issue is further complicated when middleboxes (e.g., load
balancers, firewalls, etc.) must be traversed. Middleboxes may have
session state that must be preserved for ongoing communication, and
traffic must continue to flow through the middlebox, regardless of
which router is "closest".
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4. Using Network Overlays to Provide Virtual Networks
Virtual networks are used to isolate a tenant's traffic from that of
other tenants (or even traffic within the same tenant network that
requires isolation). There are two main characteristics of virtual
networks:
1. Virtual networks isolate the address space used in one virtual
network from the address space used by another virtual network.
The same network addresses may be used in different virtual
networks at the same time. In addition, the address space used
by a virtual network is independent from that used by the
underlying physical network.
2. Virtual networks limit the scope of packets sent on the virtual
network. Packets sent by Tenant Systems attached to a virtual
network are delivered as expected to other Tenant Systems on that
virtual network and may exit a virtual network only through
controlled exit points, such as a security gateway. Likewise,
packets sourced from outside of the virtual network may enter the
virtual network only through controlled entry points, such as a
security gateway.
4.1. Overview of Network Overlays
To address the problems described in Section 3, a network overlay
approach can be used.
The idea behind an overlay is quite straightforward. Each virtual
network instance is implemented as an overlay. The original packet
is encapsulated by the first-hop network device, called a Network
Virtualization Edge (NVE), and tunneled to a remote NVE. The
encapsulation identifies the destination of the device that will
perform the decapsulation (i.e., the egress NVE for the tunneled
packet) before delivering the original packet to the endpoint. The
rest of the network forwards the packet based on the encapsulation
header and can be oblivious to the payload that is carried inside.
Overlays are based on what is commonly known as a "map-and-encap"
architecture. When processing and forwarding packets, three distinct
and logically separable steps take place:
1. The first-hop overlay device implements a mapping operation that
determines where the encapsulated packet should be sent to reach
its intended destination VM. Specifically, the mapping function
maps the destination address (either L2 or L3) of a packet
received from a VM into the corresponding destination address of
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the egress NVE device. The destination address will be the
underlay address of the NVE device doing the decapsulation and is
an IP address.
2. Once the mapping has been determined, the ingress overlay NVE
device encapsulates the received packet within an overlay header.
3. The final step is to actually forward the (now encapsulated)
packet to its destination. The packet is forwarded by the
underlay (i.e., the IP network) based entirely on its outer
address. Upon receipt at the destination, the egress overlay NVE
device decapsulates the original packet and delivers it to the
intended recipient VM.
Each of the above steps is logically distinct, though an
implementation might combine them for efficiency or other reasons.
It should be noted that in L3 BGP/VPN terminology, the above steps
are commonly known as "forwarding" or "virtual forwarding".
The first-hop NVE device can be a traditional switch or router or the
virtual switch residing inside a hypervisor. Furthermore, the
endpoint can be a VM, or it can be a physical server. Examples of
architectures based on network overlays include BGP/MPLS IP VPNs
[RFC4364], Transparent Interconnection of Lots of Links (TRILL)
[RFC6325], the Locator/ID Separation Protocol (LISP) [RFC6830], and
Shortest Path Bridging (SPB) [IEEE-802.1aq].
In the data plane, an overlay header provides a place to carry either
the virtual network identifier or an identifier that is locally
significant to the edge device. In both cases, the identifier in the
overlay header specifies which specific virtual network the data
packet belongs to. Since both routed and bridged semantics can be
supported by a virtual data center, the original packet carried
within the overlay header can be an Ethernet frame or just the IP
packet.
A key aspect of overlays is the decoupling of the "virtual" MAC and/
or IP addresses used by VMs from the physical network infrastructure
and the infrastructure IP addresses used by the data center. If a VM
changes location, the overlay edge devices simply update their
mapping tables to reflect the new location of the VM within the data
center's infrastructure space. Because an overlay network is used, a
VM can now be located anywhere in the data center that the overlay
reaches without regard to traditional constraints imposed by the
underlay network, such as the C-VLAN scope or the IP subnet scope.
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Multi-tenancy is supported by isolating the traffic of one virtual
network instance from traffic of another. Traffic from one virtual
network instance cannot be delivered to another instance without
(conceptually) exiting the instance and entering the other instance
via an entity (e.g., a gateway) that has connectivity to both virtual
network instances. Without the existence of a gateway entity, tenant
traffic remains isolated within each individual virtual network
instance.
Overlays are designed to allow a set of VMs to be placed within a
single virtual network instance, whether that virtual network
provides a bridged network or a routed network.
4.2. Communication between Virtual and Non-virtualized Networks
Not all communication will be between devices connected to
virtualized networks. Devices using overlays will continue to access
devices and make use of services on non-virtualized networks, whether
in the data center, the public Internet, or at remote/branch
campuses. Any virtual network solution must be capable of
interoperating with existing routers, VPN services, load balancers,
intrusion-detection services, firewalls, etc., on external networks.
Communication between devices attached to a virtual network and
devices connected to non-virtualized networks is handled
architecturally by having specialized gateway devices that receive
packets from a virtualized network, decapsulate them, process them as
regular (i.e., non-virtualized) traffic, and finally forward them on
to their appropriate destination (and vice versa).
A wide range of implementation approaches are possible. Overlay
gateway functionality could be combined with other network
functionality into a network device that implements the overlay
functionality and then forwards traffic between other internal
components that implement functionality such as full router service,
load balancing, firewall support, VPN gateway, etc.
4.3. Communication between Virtual Networks
Communication between devices on different virtual networks is
handled architecturally by adding specialized interconnect
functionality among the otherwise isolated virtual networks. For a
virtual network providing an L2 service, such interconnect
functionality could be IP forwarding configured as part of the
"default gateway" for each virtual network. For a virtual network
providing L3 service, the interconnect functionality could be IP
forwarding configured as part of routing between IP subnets, or it
could be based on configured inter-virtual-network traffic policies.
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In both cases, the implementation of the interconnect functionality
could be distributed across the NVEs and could be combined with other
network functionality (e.g., load balancing and firewall support)
that is applied to traffic forwarded between virtual networks.
4.4. Overlay Design Characteristics
Below are some of the characteristics of environments that must be
taken into account by the overlay technology.
1. Highly distributed systems: The overlay should work in an
environment where there could be many thousands of access
switches (e.g., residing within the hypervisors) and many more
Tenant Systems (e.g., VMs) connected to them. This leads to a
distributed mapping system that puts a low overhead on the
overlay tunnel endpoints.
2. Many highly distributed virtual networks with sparse membership:
Each virtual network could be highly dispersed inside the data
center. Also, along with expectation of many virtual networks,
the number of Tenant Systems connected to any one virtual network
is expected to be relatively low; therefore, the percentage of
NVEs participating in any given virtual network would also be
expected to be low. For this reason, efficient delivery of
multi-destination traffic within a virtual network instance
should be taken into consideration.
3. Highly dynamic Tenant Systems: Tenant Systems connected to
virtual networks can be very dynamic, both in terms of
creation/deletion/power-on/power-off and in terms of mobility
from one access device to another.
4. Be incrementally deployable, without necessarily requiring major
upgrade of the entire network: The first-hop device (or end
system) that adds and removes the overlay header may require new
software and may require new hardware (e.g., for improved
performance). The rest of the network should not need to change
just to enable the use of overlays.
5. Work with existing data center network deployments without
requiring major changes in operational or other practices: For
example, some data centers have not enabled multicast beyond
link-local scope. Overlays should be capable of leveraging
underlay multicast support where appropriate, but not require its
enablement in order to use an overlay solution.
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6. Network infrastructure administered by a single administrative
domain: This is consistent with operation within a data center,
and not across the Internet.
4.5. Control-Plane Overlay Networking Work Areas
There are three specific and separate potential work areas in the
area of control-plane protocols needed to realize an overlay
solution. The areas correspond to different possible "on-the-wire"
protocols, where distinct entities interact with each other.
One area of work concerns the address dissemination protocol an NVE
uses to build and maintain the mapping tables it uses to deliver
encapsulated packets to their proper destination. One approach is to
build mapping tables entirely via learning (as is done in 802.1
networks). Another approach is to use a specialized control-plane
protocol. While there are some advantages to using or leveraging an
existing protocol for maintaining mapping tables, the fact that large
numbers of NVEs will likely reside in hypervisors places constraints
on the resources (CPU and memory) that can be dedicated to such
functions.
From an architectural perspective, one can view the address-mapping
dissemination problem as having two distinct and separable
components. The first component consists of a back-end Network
Virtualization Authority (NVA) that is responsible for distributing
and maintaining the mapping information for the entire overlay
system. For this document, we use the term "NVA" to refer to an
entity that supplies answers, without regard to how it knows the
answers it is providing. The second component consists of the on-
the-wire protocols an NVE uses when interacting with the NVA.
The first two areas of work are thus: describing the NVA function and
defining NVA-NVE interactions.
The back-end NVA could provide high performance, high resiliency,
failover, etc., and could be implemented in significantly different
ways. For example, one model uses a traditional, centralized
"directory-based" database, using replicated instances for
reliability and failover. A second model involves using and possibly
extending an existing routing protocol (e.g., BGP, IS-IS, etc.). To
support different architectural models, it is useful to have one
standard protocol for the NVE-NVA interaction while allowing
different protocols and architectural approaches for the NVA itself.
Separating the two allows NVEs to transparently interact with
different types of NVAs, i.e., either of the two architectural models
described above. Having separate protocols could also allow for a
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simplified NVE that only interacts with the NVA for the mapping table
entries it needs and allows the NVA (and its associated protocols) to
evolve independently over time with minimal impact to the NVEs.
A third work area considers the attachment and detachment of VMs (or
Tenant Systems [RFC7365], more generally) from a specific virtual
network instance. When a VM attaches, the NVE associates the VM with
a specific overlay for the purposes of tunneling traffic sourced from
or destined to the VM. When a VM disconnects, the NVE should notify
the NVA that the Tenant System to NVE address mapping is no longer
valid. In addition, if this VM was the last remaining member of the
virtual network, then the NVE can also terminate any tunnels used to
deliver tenant multi-destination packets within the VN to the NVE.
In the case where an NVE and hypervisor are on separate physical
devices separated by an access network, a standardized protocol may
be needed.
In summary, there are three areas of potential work. The first area
concerns the implementation of the NVA function itself and any
protocols it needs (e.g., if implemented in a distributed fashion).
A second area concerns the interaction between the NVA and NVEs. The
third work area concerns protocols associated with attaching and
detaching a VM from a particular virtual network instance. All three
work areas are important to the development of scalable,
interoperable solutions.
4.6. Data-Plane Work Areas
The data plane carries encapsulated packets for Tenant Systems. The
data-plane encapsulation header carries a VN Context identifier
[RFC7365] for the virtual network to which the data packet belongs.
Numerous encapsulation or tunneling protocols already exist that can
be leveraged. In the absence of strong and compelling justification,
it would not seem necessary or helpful to develop yet another
encapsulation format just for NVO3.
5. Related IETF and IEEE Work
The following subsections discuss related IETF and IEEE work. These
subsections are not meant to provide complete coverage of all IETF
and IEEE work related to data centers, and the descriptions should
not be considered comprehensive. Each area aims to address
particular limitations of today's data center networks. In all
areas, scaling is a common theme as are multi-tenancy and VM
mobility. Comparing and evaluating the work result and progress of
each work area listed is out of the scope of this document. The
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intent of this section is to provide a reference to the interested
readers. Note that NVO3 is scoped to running over an IP/L3 underlay
network.
5.1. BGP/MPLS IP VPNs
BGP/MPLS IP VPNs [RFC4364] support multi-tenancy, VPN traffic
isolation, address overlapping, and address separation between
tenants and network infrastructure. The BGP/MPLS control plane is
used to distribute the VPN labels and the tenant IP addresses that
identify the tenants (or to be more specific, the particular VPN/
virtual network) and tenant IP addresses. Deployment of enterprise
L3 VPNs has been shown to scale to thousands of VPNs and millions of
VPN prefixes. BGP/MPLS IP VPNs are currently deployed in some large
enterprise data centers. The potential limitation for deploying BGP/
MPLS IP VPNs in data center environments is the practicality of using
BGP in the data center, especially reaching into the servers or
hypervisors. There may be computing workforce skill set issues,
equipment support issues, and potential new scaling challenges. A
combination of BGP and lighter-weight IP signaling protocols, e.g.,
the Extensible Messaging and Presence Protocol (XMPP), has been
proposed to extend the solutions into the data center environment
[END-SYSTEM] while taking advantage of built-in VPN features with its
rich policy support; it is especially useful for inter-tenant
connectivity.
5.2. BGP/MPLS Ethernet VPNs
Ethernet Virtual Private Networks (E-VPNs) [EVPN] provide an emulated
L2 service in which each tenant has its own Ethernet network over a
common IP or MPLS infrastructure. A BGP/MPLS control plane is used
to distribute the tenant MAC addresses and the MPLS labels that
identify the tenants and tenant MAC addresses. Within the BGP/MPLS
control plane, a 32-bit Ethernet tag is used to identify the
broadcast domains (VLANs) associated with a given L2 VLAN service
instance, and these Ethernet tags are mapped to VLAN IDs understood
by the tenant at the service edges. This means that any VLAN-based
limitation on the customer site is associated with an individual
tenant service edge, enabling a much higher level of scalability.
Interconnection between tenants is also allowed in a controlled
fashion.
VM mobility [MOBILITY] introduces the concept of a combined L2/L3 VPN
service in order to support the mobility of individual virtual
machines (VMs) between data centers connected over a common IP or
MPLS infrastructure.
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5.3. 802.1 VLANs
VLANs are a well-understood construct in the networking industry,
providing an L2 service via a physical network in which tenant
forwarding information is part of the physical network
infrastructure. A VLAN is an L2 bridging construct that provides the
semantics of virtual networks mentioned above: a MAC address can be
kept unique within a VLAN, but it is not necessarily unique across
VLANs. Traffic scoped within a VLAN (including broadcast and
multicast traffic) can be kept within the VLAN it originates from.
Traffic forwarded from one VLAN to another typically involves router
(L3) processing. The forwarding table lookup operation may be keyed
on {VLAN, MAC address} tuples.
VLANs are a pure L2 bridging construct, and VLAN identifiers are
carried along with data frames to allow each forwarding point to know
what VLAN the frame belongs to. Various types of VLANs are available
today and can be used for network virtualization, even together. The
C-VLAN, Service VLAN (S-VLAN), and Backbone VLAN (B-VLAN) IDs
[IEEE-802.1Q] are 12 bits. The 24-bit I-SID [IEEE-802.1aq] allows
the support of more than 16 million virtual networks.
5.4. IEEE 802.1aq -- Shortest Path Bridging
Shortest Path Bridging (SPB) [IEEE-802.1aq] is an overlay based on
IS-IS that operates over L2 Ethernets. SPB supports multipathing and
addresses a number of shortcomings in the original Ethernet Spanning
Tree Protocol. Shortest Path Bridging Mac (SPBM) uses IEEE 802.1ah
PBB (MAC-in-MAC) encapsulation and supports a 24-bit I-SID, which can
be used to identify virtual network instances. SPBM provides multi-
pathing and supports easy virtual network creation or update.
SPBM extends IS-IS in order to perform link-state routing among core
SPBM nodes, obviating the need for bridge learning for communication
among core SPBM nodes. Learning is still used to build and maintain
the mapping tables of edge nodes to encapsulate Tenant System traffic
for transport across the SPBM core.
SPB is compatible with all other 802.1 standards and thus allows
leveraging of other features, e.g., VSI Discovery Protocol (VDP),
Operations, Administration, and Maintenance (OAM), or scalability
solutions.
5.5. VDP
VDP is the Virtual Station Interface (VSI) Discovery and
Configuration Protocol specified by IEEE P802.1Qbg [IEEE-802.1Qbg].
VDP is a protocol that supports the association of a VSI with a port.
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VDP is run between the end station (e.g., a server running a
hypervisor) and its adjacent switch (i.e., the device on the edge of
the network). VDP is used, for example, to communicate to the switch
that a virtual machine (virtual station) is moving, i.e., designed
for VM migration.
5.6. ARMD
The Address Resolution for Massive numbers of hosts in the Data
center (ARMD) WG examined data center scaling issues with a focus on
address resolution and developed a problem statement document
[RFC6820]. While an overlay-based approach may address some of the
"pain points" that were raised in ARMD (e.g., better support for
multi-tenancy), analysis will be needed to understand the scaling
trade-offs of an overlay-based approach compared with existing
approaches. On the other hand, existing IP-based approaches such as
proxy ARP may help mitigate some concerns.
5.7. TRILL
TRILL is a network protocol that provides an Ethernet L2 service to
end systems and is designed to operate over any L2 link type. TRILL
establishes forwarding paths using IS-IS routing and encapsulates
traffic within its own TRILL header. TRILL, as originally defined,
supports only the standard (and limited) 12-bit C-VID identifier.
Work to extend TRILL to support more than 4094 VLANs has recently
completed and is defined in [RFC7172]
5.8. L2VPNs
The IETF has specified a number of approaches for connecting L2
domains together as part of the L2VPN Working Group. That group,
however, has historically been focused on provider-provisioned L2
VPNs, where the service provider participates in management and
provisioning of the VPN. In addition, much of the target environment
for such deployments involves carrying L2 traffic over WANs. Overlay
approaches as discussed in this document are intended be used within
data centers where the overlay network is managed by the data center
operator rather than by an outside party. While overlays can run
across the Internet as well, they will extend well into the data
center itself (e.g., up to and including hypervisors) and include
large numbers of machines within the data center itself.
Other L2VPN approaches, such as the Layer 2 Tunneling Protocol (L2TP)
[RFC3931] require significant tunnel state at the encapsulating and
decapsulating endpoints. Overlays require less tunnel state than
other approaches, which is important to allow overlays to scale to
hundreds of thousands of endpoints. It is assumed that smaller
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switches (i.e., virtual switches in hypervisors or the adjacent
devices to which VMs connect) will be part of the overlay network and
be responsible for encapsulating and decapsulating packets.
5.9. Proxy Mobile IP
Proxy Mobile IP [RFC5213] [RFC5844] makes use of the Generic Routing
Encapsulation (GRE) Key Field [RFC5845] [RFC6245], but not in a way
that supports multi-tenancy.
5.10. LISP
LISP [RFC6830] essentially provides an IP-over-IP overlay where the
internal addresses are end station identifiers and the outer IP
addresses represent the location of the end station within the core
IP network topology. The LISP overlay header uses a 24-bit Instance
ID used to support overlapping inner IP addresses.
6. Summary
This document has argued that network virtualization using overlays
addresses a number of issues being faced as data centers scale in
size. In addition, careful study of current data center problems is
needed for development of proper requirements and standard solutions.
This document identifies three potential control protocol work areas.
The first involves a back-end NVA and how it learns and distributes
the mapping information NVEs use when processing tenant traffic. A
second involves the protocol an NVE would use to communicate with the
back-end NVA to obtain the mapping information. The third potential
work concerns the interactions that take place when a VM attaches or
detaches from a specific virtual network instance.
There are a number of approaches that provide some, if not all, of
the desired semantics of virtual networks. Each approach needs to be
analyzed in detail to assess how well it satisfies the requirements.
7. Security Considerations
Because this document describes the problem space associated with the
need for virtualization of networks in complex, large-scale, data-
center networks, it does not itself introduce any security risks.
However, it is clear that security concerns need to be a
consideration of any solutions proposed to address this problem
space.
Solutions will need to address both data-plane and control-plane
security concerns.
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In the data plane, isolation of virtual network traffic from other
virtual networks is a primary concern -- for NVO3, this isolation may
be based on VN identifiers that are not involved in underlay network
packet forwarding between overlay edges (NVEs). Use of a VN
identifier in the overlay reduces the underlay network's role in
isolating virtual networks by comparison to approaches where VN
identifiers are involved in packet forwarding (e.g., 802.1 VLANs as
described in Section 5.3).
In addition to isolation, assurances against spoofing, snooping,
transit modification and denial of service are examples of other
important data-plane considerations. Some limited environments may
even require confidentiality.
In the control plane, the primary security concern is ensuring that
an unauthorized party does not compromise the control-plane protocol
in ways that improperly impact the data plane. Some environments may
also be concerned about confidentiality of the control plane.
More generally, denial-of-service concerns may also be a
consideration. For example, a tenant on one virtual network could
consume excessive network resources in a way that degrades services
for other tenants on other virtual networks.
8. References
8.1. Normative Reference
[RFC7365] Lasserre, M., Balus, F., Morin, T., Bitar, N., and Y.
Rekhter, "Framework for Data Center (DC) Network
Virtualization", RFC 7365, October 2014,
<http://www.rfc-editor.org/info/rfc7365>.
8.2. Informative References
[END-SYSTEM]
Marques, P., Fang, L., Sheth, N., Napierala, M., and N.
Bitar, "BGP-signaled end-system IP/VPNs", Work in
Progress, draft-ietf-l3vpn-end-system-04, October 2014.
[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.
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[IEEE-802.1Q]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges and Virtual
Bridged Local Area Networks", IEEE 802.1Q-2011, August
2011, <http://standards.ieee.org/getieee802/
download/802.1Q-2011.pdf>.
[IEEE-802.1Qbg]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges and Virtual
Bridged Local Area Networks -- Amendment 21: Edge Virtual
Bridging", IEEE 802.1Qbg-2012, July 2012,
<http://standards.ieee.org/getieee802/
download/802.1Qbg-2012.pdf>.
[IEEE-802.1aq]
IEEE, "IEEE Standard for Local and metropolitan area
networks -- Media Access Control (MAC) Bridges and Virtual
Bridged Local Area Networks -- Amendment 20: Shortest Path
Bridging", IEEE 802.1aq, June 2012,
<http://standards.ieee.org/getieee802/
download/802.1aq-2012.pdf>.
[MOBILITY] Aggarwal, R., Rekhter, Y., Henderickx, W., Shekhar, R.,
Fang, L., and A. Sajassi, "Data Center Mobility based on
E-VPN, BGP/MPLS IP VPN, IP Routing and NHRP", Work in
Progress, draft-raggarwa-data-center-mobility-07, June
2014.
[RFC3931] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005,
<http://www.rfc-editor.org/info/rfc3931>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, February 2006,
<http://www.rfc-editor.org/info/rfc4364>.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008,
<http://www.rfc-editor.org/info/rfc5213>.
[RFC5844] Wakikawa, R. and S. Gundavelli, "IPv4 Support for Proxy
Mobile IPv6", RFC 5844, May 2010,
<http://www.rfc-editor.org/info/rfc5844>.
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[RFC5845] Muhanna, A., Khalil, M., Gundavelli, S., and K. Leung,
"Generic Routing Encapsulation (GRE) Key Option for Proxy
Mobile IPv6", RFC 5845, June 2010,
<http://www.rfc-editor.org/info/rfc5845>.
[RFC6245] Yegani, P., Leung, K., Lior, A., Chowdhury, K., and J.
Navali, "Generic Routing Encapsulation (GRE) Key Extension
for Mobile IPv4", RFC 6245, May 2011,
<http://www.rfc-editor.org/info/rfc6245>.
[RFC6325] Perlman, R., Eastlake, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011,
<http://www.rfc-editor.org/info/6325>.
[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>.
[RFC6830] Farinacci, D., Fuller, V., Meyer, D., and D. Lewis, "The
Locator/ID Separation Protocol (LISP)", RFC 6830, January
2013, <http://www.rfc-editor.org/info/rfc6830>.
[RFC7172] Eastlake, D., Zhang, M., Agarwal, P., Perlman, R., and D.
Dutt, "Transparent Interconnection of Lots of Links
(TRILL): Fine-Grained Labeling", RFC 7172, May 2014,
<http://www.rfc-editor.org/info/rfc7172>.
Acknowledgments
Helpful comments and improvements to this document have come from Lou
Berger, John Drake, Ilango Ganga, Ariel Hendel, Vinit Jain, Petr
Lapukhov, Thomas Morin, Benson Schliesser, Qin Wu, Xiaohu Xu, Lucy
Yong, and many others on the NVO3 mailing list.
Special thanks to Janos Farkas for his persistence and numerous
detailed comments related to the lack of precision in the text
relating to IEEE 802.1 technologies.
Contributors
Dinesh Dutt and Murari Sridharin were original co-authors of the
Internet-Draft that led to the BoF that formed the NVO3 WG. That
original draft eventually became the basis for this document.
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Authors' Addresses
Thomas Narten (editor)
IBM
Research Triangle Park, NC
United States
EMail: narten@us.ibm.com
Eric Gray (editor)
Ericsson
EMail: eric.gray@ericsson.com
David Black
EMC Corporation
176 South Street
Hopkinton, MA 01748
United States
EMail: david.black@emc.com
Luyuan Fang
Microsoft
5600 148th Ave NE
Redmond, WA 98052
United States
EMail: lufang@microsoft.com
Lawrence Kreeger
Cisco
170 W. Tasman Avenue
San Jose, CA 95134
United States
EMail: kreeger@cisco.com
Maria Napierala
AT&T
200 S. Laurel Avenue
Middletown, NJ 07748
United States
EMail: mnapierala@att.com
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