Internet Engineering Task Force Marc Lasserre
Internet Draft Florin Balus
Intended status: Informational Alcatel-Lucent
Expires: March 2013
Thomas Morin
France Telecom Orange
Nabil Bitar
Verizon
Yakov Rekhter
Juniper
October 19, 2012
Framework for DC Network Virtualization
draft-ietf-nvo3-framework-01.txt
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Abstract
Several IETF drafts relate to the use of overlay networks to support
large scale virtual data centers. This draft provides a framework
for Network Virtualization over L3 (NVO3) and is intended to help
plan a set of work items in order to provide a complete solution
set. It defines a logical view of the main components with the
intention of streamlining the terminology and focusing the solution
set.
Table of Contents
1. Introduction................................................3
1.1. Conventions used in this document.......................4
1.2. General terminology.....................................4
1.3. DC network architecture.................................6
1.4. Tenant networking view..................................7
2. Reference Models............................................8
2.1. Generic Reference Model.................................8
2.2. NVE Reference Model....................................10
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. Generic service virtualization components..............12
3.1.1. Virtual Access Points (VAPs)......................13
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.........15
3.1.5.2. Auto-provisioning/Service discovery.............15
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3.1.5.3. Address advertisement and tunnel mapping.........16
3.1.5.4. Tunnel management...............................17
3.2. Multi-homing..........................................17
3.3. Service Overlay Topologies.............................18
4. Key aspects of overlay networks.............................18
4.1. Pros & Cons...........................................18
4.2. Overlay issues to consider.............................19
4.2.1. Data plane vs Control plane driven................19
4.2.2. Coordination between data plane and control plane..20
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...........................21
4.2.6. Interaction between network overlays and underlays.22
5. Security Considerations.....................................23
6. IANA Considerations........................................23
7. References.................................................23
7.1. Normative References...................................23
7.2. Informative References.................................23
8. Acknowledgments............................................24
1. Introduction
This document provides a framework for Data Center Network
Virtualization over L3 tunnels. This framework is intended to aid in
standardizing protocols and mechanisms to support large scale
network virtualization for data centers.
Several IETF drafts relate to the use of overlay networks for data
centers.
[NVOPS] defines the rationale for using overlay networks in order to
build large data center networks. The use of virtualization leads to
a very large number of communication domains and end systems to cope
with.
[OVCPREQ] describes the requirements for a control plane protocol
required by overlay border nodes to exchange overlay mappings.
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 in the design of standards and
mechanisms for large scale data centers.
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1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
1.2. General terminology
This document uses the following terminology:
NVE: Network Virtualization Edge. It is a network entity that sits
on the edge of the NVO3 network. It implements network
virtualization functions that allow for L2 and/or L3 tenant
separation and for hiding tenant addressing information (MAC and IP
addresses). An NVE could be implemented as part of a virtual switch
within a hypervisor, a physical switch or router, a Network Service
Appliance.
VN: Virtual Network. This is a virtual L2 or L3 domain that belongs
to a tenant.
VNI: Virtual Network Instance. This is one instance of a virtual
overlay network. Two Virtual Networks are isolated from one another
and may use overlapping addresses.
Virtual Network Context or VN Context: Field that is part of the
overlay encapsulation header which allows the encapsulated frame to
be delivered to the appropriate virtual network endpoint by the
egress NVE. The egress NVE uses this field to determine the
appropriate virtual network context in which to process the packet.
This field MAY be an explicit, unique (to the administrative domain)
virtual network identifier (VNID) or MAY express the necessary
context information in other ways (e.g. a locally significant
identifier).
VNID: Virtual Network Identifier. In the case where the VN context
has global significance, this is the ID value that is carried in
each data packet in the overlay encapsulation that identifies the
Virtual Network the packet belongs to.
Underlay or Underlying Network: This is the network that provides
the connectivity between NVEs. The Underlying Network can be
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completely unaware of the overlay packets. Addresses within the
Underlying Network are also referred to as "outer addresses" because
they exist in the outer encapsulation. The Underlying Network can
use a completely different protocol (and address family) from that
of the overlay.
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 and/or compute and/or storage services. One such service
is virtualized data center services, also known as Infrastructure as
a Service.
Virtual Data Center or Virtual DC: A container for virtualized
compute, storage and network services. Managed by a single tenant, a
Virtual DC can contain multiple VNs and multiple Tenant Systems that
are connected to one or more of these VNs.
VM: Virtual Machine. Several Virtual Machines can share the
resources of a single physical computer server using the services of
a Hypervisor (see below definition).
Hypervisor: Server virtualization software running on a physical
compute server that hosts Virtual Machines. The hypervisor provides
shared compute/memory/storage and network connectivity to the VMs
that it hosts. Hypervisors often embed a Virtual Switch (see below).
Virtual Switch: A function within a Hypervisor (typically
implemented in software) that provides similar services to a
physical Ethernet switch. It switches Ethernet frames between VMs'
virtual NICs within the same physical server, or between a VM and a
physical NIC card connecting the server to a physical Ethernet
switch. It also enforces network isolation between VMs that should
not communicate with each other.
Tenant: In a DC, a tenant refers to a customer that could an
organization within an enterprise, or an enterprise with a set of DC
compute, storage and network resources associated with it.
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.
End device: A physical system to which networking service is
provided. Examples include hosts (e.g. server or server blade),
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storage systems (e.g. file servers, iSCSI storage systems) and
network devices (e.g. firewall, load-balancer, IPSec gateway). An
end device may include internal networking functionality that
interconnects the device's components (e.g. virtual switches that
interconnects VMs running on the same server). NVE functionality may
be implemented as part of that internal networking.
ELAN: MEF ELAN, multipoint to multipoint Ethernet service
EVPN: Ethernet VPN as defined in [EVPN]
1.3. DC network architecture
A generic architecture for Data Centers is depicted in Figure 1:
,---------.
,' `.
( IP/MPLS WAN )
`. ,'
`-+------+'
+--+--+ +-+---+
|DC GW|+-+|DC GW|
+-+---+ +-----+
| /
.--. .--.
( ' '.--.
.-.' Intra-DC '
( network )
( .'-'
'--'._.'. )\ \
/ / '--' \ \
/ / | | \ \
+---+--+ +-`.+--+ +--+----+
| ToR | | ToR | | ToR |
+-+--`.+ +-+-`.-+ +-+--+--+
/ \ / \ / \
__/_ \ / \ /_ _\__
'--------' '--------' '--------' '--------'
: 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
this figure. It provides a view of physical components inside a DC.
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A cloud network is composed of intra-Data Center (DC) networks and
network services, and, inter-DC network and network connectivity
services. Depending upon the scale, DC distribution, operations
model, Capex and Opex aspects, DC networking elements can act as
strict L2 switches and/or provide IP routing capabilities, including
also service virtualization.
In some DC architectures, it is possible that some tier layers
provide L2 and/or L3 services, are collapsed, and that Internet
connectivity, inter-DC connectivity and VPN support are handled by a
smaller number of nodes. Nevertheless, one can assume that the
functional blocks fit with the architecture above.
The following components can be present in a DC:
o Top of Rack (ToR): 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. ToRs
may also provide routing functionality, virtual IP network
connectivity, or Layer2 tunneling over IP for instance. ToRs
are usually multi-homed to switches in the Intra-DC network.
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: High capacity network composed of core
switches aggregating multiple ToRs. Core switches are usually
Ethernet switches but can also support routing capabilities.
o 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 IPVPN/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 ToRs.
1.4. Tenant networking view
The DC network architecture is used to provide L2 and/or L3 service
connectivity to each tenant. An example is depicted in Figure 2:
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+----- L3 Infrastructure ----+
| |
,--+--. ,--+--.
.....( Rtr1 )...... ( Rtr2 )
| `-----' | `-----'
| Tenant1 |LAN12 Tenant1|
|LAN11 ....|........ |LAN13
.............. | | ..............
| | | | | |
,-. ,-. ,-. ,-. ,-. ,-.
(VM )....(VM ) (VM )... (VM ) (VM )....(VM )
`-' `-' `-' `-' `-' `-'
Figure 2 : Logical Service connectivity for a single tenant
In this example one or more L3 contexts and one or more LANs (e.g.,
one per application type) running on DC switches are assigned for DC
tenant 1.
For a multi-tenant DC, a virtualized version of this type of service
connectivity needs to be provided for each tenant by the Network
Virtualization solution.
2. Reference Models
2.1. Generic Reference Model
The following diagram shows a DC reference model for network
virtualization using Layer3 overlays where NVEs provide a logical
interconnect between Tenant Systems that belong to specific tenant
network.
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+--------+ +--------+
| Tenant +--+ +----| Tenant |
| System | | (') | System |
+--------+ | ................... ( ) +--------+
| +-+--+ +--+-+ (_)
| | NV | | NV | |
+--|Edge| |Edge|---+
+-+--+ +--+-+
/ . .
/ . L3 Overlay +--+-++--------+
+--------+ / . Network | NV || Tenant |
| Tenant +--+ . |Edge|| System |
| System | . +----+ +--+-++--------+
+--------+ .....| NV |........
|Edge|
+----+
|
|
=====================
| |
+--------+ +--------+
| Tenant | | Tenant |
| System | | System |
+--------+ +--------+
Figure 3 : Generic reference model for DC network virtualization
over a Layer3 infrastructure
A Tenant System can be attached to a Network Virtualization Edge
(NVE) node in several ways:
- locally, by being co-located i.e. resident in the same device
- remotely, via a point-to-point connection or a switched network
(e.g. Ethernet)
When an NVE is local, the state of Tenant Systems 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 remote, the
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state of Tenant Systems needs to be exchanged via a data or control
plane protocol, or via a management entity.
The functional components in this picture do not necessarily map
directly with the physical components described in Figure 1.
For example, an End Device can be a server blade with VMs and
virtual switch, i.e. the VM is the Tenant System and the NVE
functions may be performed by the virtual switch and/or the
hypervisor. In this case, the Tenant System and NVE function are co-
located.
Another example is the case where an End Device can be a traditional
physical server (no VMs, no virtual switch), i.e. the server is the
Tenant System and the NVE function may be performed by the ToR.
Other End Devices in this category are Physical Network Appliances
or Storage Systems.
The NVE implements network virtualization functions that allow for
L2 and/or L3 tenant separation and for hiding tenant addressing
information (MAC and IP addresses), tenant-related control plane
activity and service contexts from the Routed Backbone nodes.
Core nodes utilize L3 techniques to interconnect NVE nodes in
support of the overlay network. These devices perform forwarding
based on outer L3 tunnel header, and generally do not maintain per
tenant-service state albeit some 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 tunnels. When such tenant or tenant-
service related information is maintained in the core, overlay
virtualization provides knobs to control that information.
2.2. NVE Reference Model
The NVE is composed of a Virtual Network instance that Tenant
Systems interface with and an overlay module that provides tunneling
overlay functions (e.g. encapsulation/decapsulation of tenant
traffic from/to the tenant forwarding instance, tenant
identification and mapping, etc), as described in figure 4:
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+------- L3 Network ------+
| |
| Tunnel Overlay |
+------------+---------+ +---------+------------+
| +----------+-------+ | | +---------+--------+ |
| | Overlay Module | | | | Overlay Module | |
| +---------+--------+ | | +---------+--------+ |
| |VN context| | VN context| |
| | | | | |
| +--------+-------+ | | +--------+-------+ |
| | |VNI| . |VNI| | | | |VNI| . |VNI| |
NVE1 | +-+------------+-+ | | +-+-----------+--+ | NVE2
| | VAPs | | | | VAPs | |
+----+------------+----+ +----+-----------+-----+
| | | |
-------+------------+-----------------+-----------+-------
| | Tenant | |
| | Service IF | |
Tenant Systems Tenant Systems
Figure 4 : Generic reference model for NV Edge
Note that some NVE functions (e.g. data plane and control plane
functions) may reside in one device or may be implemented separately
in different devices.
For example, the NVE functionality could reside solely on the End
Devices, on the ToRs or on both the End Devices and the ToRs. In the
latter case we say that the End Device NVE component acts as the NVE
Spoke, and ToRs act as NVE hubs. Tenant Systems will interface with
VNIs maintained on the NVE spokes, and VNIs maintained on the NVE
spokes will interface with VNIs maintained on the NVE hubs.
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2.3. NVE Service Types
NVE components may be used to provide different types of virtualized
service connectivity. This section defines the service types and
associated attributes
2.3.1. L2 NVE providing Ethernet LAN-like service
L2 NVE implements Ethernet LAN emulation (ELAN), an Ethernet based
multipoint service where the Tenant Systems appear to be
interconnected by a LAN environment over a set of L3 tunnels. It
provides per tenant virtual switching instance with MAC addressing
isolation and L3 tunnel encapsulation across the core.
2.3.2. L3 NVE providing IP/VRF-like service
Virtualized IP routing and forwarding is similar from a service
definition perspective with IETF IP VPN (e.g., BGP/MPLS IPVPN and
IPsec VPNs). It provides per tenant routing instance with addressing
isolation and L3 tunnel encapsulation across the core.
3. Functional components
This section breaks down the Network Virtualization architecture
into functional components to make it easier to discuss solution
options for different modules.
This version of the document gives an overview of generic functional
components that are shared between L2 and L3 service types. Details
specific for each service type will be added in future revisions.
3.1. Generic service virtualization components
A Network Virtualization solution is built around a number of
functional components as depicted in Figure 5:
+------- L3 Network ------+
| |
| Tunnel Overlay |
+------------+--------+ +--------+------------+
| +----------+------+ | | +------+----------+ |
| | Overlay Module | | | | Overlay Module | |
| +--------+--------+ | | +--------+--------+ |
| |VN Context| | |VN Context|
| | | | | |
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| +-------+-------+ | | +-------+-------+ |
| ||VNI| ... |VNI|| | | ||VNI| ... |VNI|| |
NVE1 | +-+-----------+-+ | | +-+-----------+-+ | NVE2
| | VAPs | | | | VAPs | |
+----+-----------+----+ +----+-----------+----+
| | | |
-----+-----------+-----------------+-----------+-----
| | Tenant | |
| | Service IF | |
Tenant Systems Tenant Systems
Figure 5 : Generic reference model for NV Edge
3.1.1. Virtual Access Points (VAPs)
Tenant Systems are connected to the VNI Instance through Virtual
Access Points (VAPs).
The VAPs can be physical ports or virtual ports identified through
logical interface identifiers (VLANs, internal VSwitch Interface ID
leading to a VM).
3.1.2. Virtual Network Instance (VNI)
The VNI represents a set of configuration attributes defining access
and tunnel policies and (L2 and/or L3) forwarding functions.
Per tenant FIB tables and control plane protocol instances are used
to maintain separate private contexts between tenants. Hence tenants
are free to use their own addressing schemes without concerns about
address overlapping with other tenants.
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 tunnel source address
identifying the source NVE and the tunnel destination L3 address
identifying the destination NVE. This allows the destination NVE to
identify the tenant service instance and therefore appropriately
process and forward the tenant packet.
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The Overlay module provides tunneling overlay functions: tunnel
initiation/termination, encapsulation/decapsulation of frames from
VAPs/L3 Backbone and may provide for transit forwarding of IP
traffic (e.g., transparent tunnel forwarding).
In a multi-tenant context, the tunnel aggregates frames from/to
different VNIs. Tenant identification and traffic demultiplexing are
based on the VN Context (e.g. VNID).
The following approaches can been considered:
o One VN Context per Tenant: A globally unique (on a per-DC
administrative domain) VNID is used to identify the related
Tenant instances. An example of this approach is the use of
IEEE VLAN or ISID tags to provide virtual L2 domains.
o One VN Context per VNI: A per-tenant local value is
automatically generated by the egress NVE and usually
distributed by a control plane protocol to all the related
NVEs. An example of this approach is the use of per VRF MPLS
labels in IP VPN [RFC4364].
o One VN Context 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 may be used by the control plane to
identify the Tenant context.
3.1.4. Tunnel Overlays and Encapsulation options
Once the VN context is added to the frame, a L3 Tunnel encapsulation
is used to transport the frame to the destination NVE. The backbone
devices do not usually keep any per service state, simply forwarding
the frames based on the outer tunnel header.
Different IP tunneling options (GRE/L2TP/IPSec) and tunneling
options (BGP VPN, PW, VPLS) are available for both Ethernet and IP
formats.
3.1.5. Control Plane Components
Control plane components may be used to provide the following
capabilities:
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. Auto-provisioning/Service discovery
. Address advertisement and tunnel mapping
. Tunnel management
A control plane component can be an on-net control protocol or a
management control entity.
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.
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 controllers provide a facility to maintain
global and distribute that state to the network which in combination
with distributed protocols can aid in achieving greater network
efficiencies, improve reliability and robustness. Domain and/or
deployment specific constraints define the balance between
centralized and distributed approaches.
On one hand, a control plane module can reside in every NVE. This is
how routing control plane modules are implemented in routers. At the
same time, an external controller can manage a group of NVEs via an
agent sitting in each NVE. This is how an SDN controller could
communicate with the nodes it controls, via OpenFlow for instance.
In the case where a centralized control plane is preferred, the
controller will need to be distributed to more than one node for
redundancy. Depending upon the size of the DC domain, hence the
number of NVEs to manage, it should be possible to use several
external controllers. Inter-controller communication will thus be
necessary for scalability and redundancy.
3.1.5.2. Auto-provisioning/Service discovery
NVEs must be able to select the appropriate VNI for each Tenant
System. This is based on state information that is often provided by
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external entities. For example, in a VM environment, this
information is provided by compute management systems, since these
are the only entities that have visibility on which VM belongs to
which tenant.
A mechanism for communicating this information between Tenant
Systems and the local NVE is required. As a result the VAPs are
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.
When a protocol is used, 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.
Another control plane protocol can also be used to advertize
supported VNs to other NVEs. Alternatively, management control
entities can also be used to perform these functions.
3.1.5.3. Address advertisement and tunnel mapping
As traffic reaches an ingress NVE, a lookup is performed to
determine which tunnel the packet needs to be sent to. It is then
encapsulated with a tunnel header containing the destination address
of the egress overlay node. Intermediate nodes (between the ingress
and egress NVEs) switch or route traffic based upon the outer
destination address.
One key step in this process consists of mapping a final destination
address to the proper tunnel. NVEs are responsible for maintaining
such mappings in their lookup tables. Several ways of populating
these lookup tables are possible: control plane driven, management
plane driven, or data plane driven.
When a control plane protocol is used to distribute address
advertisement 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 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.
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3.1.5.4. Tunnel management
A control plane protocol may be required to exchange tunnel state
information. This may include setting up tunnels and/or providing
tunnel state information.
This applies to both unicast and multicast tunnels.
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.
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
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 and/or ECMP
techniques.
Tenant systems can either be L2 or L3 nodes. In the former case
(L2), techniques such as LAG or STP for instance can be used. In the
latter case (L3), it is possible that no dynamic routing protocol is
enabled. Tenant systems can be multi-homed into remote NVE using
several interfaces (physical NICS or vNICS) with an IP address per
interface either to the same nvo3 network or into different nvo3
networks. When one of the links fails, the corresponding IP is not
reachable but the other interfaces can still be used. When a tenant
system is co-located with an NVE, IP routing can be relied upon to
handle routing over diverse links to TORs.
External connectivity is handled by to or more nvo3 gateways. Each
gateway is connected to a different domain (e.g. ISP) and runs BGP
multi-homing. They serve as an access point to external networks
such as VPNs or the Internet. When a connection to an upstream
router is lost, the alternative connection is used and the failed
route withdrawn.
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3.3. Service Overlay Topologies
A number of service topologies may be used to optimize the service
connectivity and to address NVE performance limitations.
The topology described in Figure 3 suggests the use of a tunnel mesh
between the NVEs where each tenant instance is one hop away from a
service processing perspective. Partial mesh topologies and an NVE
hierarchy may be used where certain NVEs may act as service transit
points.
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 is handled at the edge of
the network. Intermediate transport nodes are unaware of such
state. Note that this is not the case when multicast is enabled
in the core network.
o Tunnels are used to aggregate traffic 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. This offers a clear separation
between addresses used within the overlay and the underlay
networks and it enables the use of overlapping addresses spaces
by Tenant Systems
o Support of a large number of virtual network identifiers
Overlay networks also create several challenges:
o Overlay networks have no controls of underlay networks and lack
critical network information
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o Overlays typically probe the network to measure link
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 between overlay and underlay networks can lead
to an inefficient usage of network resources.
o Fairness of resource sharing and collaboration among end-nodes
in overlay networks are two critical issues
o When multiple overlays co-exist on top of a common underlay
network, the lack of coordination between overlays can lead to
performance issues.
o Overlaid traffic may not traverse firewalls and NAT devices.
o Multicast service scalability. Multicast support may be
required in the overlay network to address for each tenant
flood containment or efficient multicast handling.
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
4.2. Overlay issues to consider
4.2.1. Data plane vs Control plane driven
In the case of an L2NVE, 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
L2NVEs and L3NVEs.
Dynamic data plane learning implies that flooding of unknown
destinations be supported and hence implies that broadcast and/or
multicast be supported. Multicasting in the core 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
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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.
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 end systems present on 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 end systems present on 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 local NVE control plane to
distribute this information to its peers.
4.2.3. Handling Broadcast, Unknown Unicast and Multicast (BUM) traffic
There are two techniques to support packet replication needed for
broadcast, unknown unicast and multicast:
o Ingress replication
o Use of core 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 is of consideration.
When the number of hosts per group is large, the use of core
multicast trees may be more appropriate. When the number of hosts is
small (e.g. 2-3), 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 core 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 core shared multicast trees as
opposed to dedicated multicast trees.
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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.
In this section, we will only consider the case of an IP overlay.
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 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 overlay layer 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 overlay layer can lead
to performance and congestion issues due to TCP dynamics and might
require new congestion avoidance mechanisms from then underlay
network [FLOYD].
Finally, the underlay network may be designed in such a way that the
MTU can accommodate the extra tunnel 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 VM switch
or ToR switch and then by a DC gateway. The NVE function can be
embedded within any of these elements.
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There are several criteria to consider when deciding where the NVE
processing boundary happens:
o Processing and memory requirements
o Datapath (e.g. lookups, filtering,
encapsulation/decapsulation)
o Control plane processing (e.g. routing, signaling, OAM)
o FIB/RIB size
o Multicast support
o Routing protocols
o Packet replication capability
o Fragmentation support
o QoS transparency
o Resiliency
4.2.6. Interaction between network overlays and underlays
When multiple overlays co-exist on top of a common underlay network,
this can cause some performance issues. These overlays have
partially overlapping paths and nodes.
Each overlay is selfish by nature in that it sends traffic so as to
optimize its own performance without considering the impact on other
overlays, unless the underlay tunnels are traffic engineered on a
per overlay basis so as to avoid sharing underlay resources.
Better visibility between overlays and underlays can be achieved by
providing mechanisms to exchange information about:
o Performance metrics (throughput, delay, loss, jitter)
o Cost metrics
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5. Security Considerations
As a framework document, no protocols are being defined and hence no
specific security consideration are raised.
The following security aspects shall be discussed in respective
solutions documents:
Traffic isolation between NVO3 domains is guaranteed by the use of
per tenant FIB tables (VNIs).
The creation of overlay networks and the tenant to overlay mapping
function can introduce significant security risks. When dynamic
protocols are used, authentication should be supported. When a
centralized controller is used, access to that controller should be
restricted to authorized personnel. This can be achieved via login
authentication.
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.
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[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:
Dimitrios Stiliadis, Rotem Salomonovitch, Alcatel-Lucent
Lucy Yong, Huawei
This document was prepared using 2-Word-v2.0.template.dot.
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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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