RFC2917 - A Core MPLS IP VPN Architecture

Network Working Group K. Muthukrishnan

Request for Comments: 2917 LUCent Technologies

Category: Informational A. Malis

Vivace Networks, Inc.

September 2000

A Core MPLS IP VPN Architecture

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

This memo presents an approach for building core Virtual Private

Network (VPN) services in a service provider's MPLS backbone. This

approach uses Multiprotocol Label Switching (MPLS) running in the

backbone to provide premium services in addition to best effort

services. The central vision is for the service provider to provide a

virtual router service to their customers. The keystones of this

architecture are ease of configuration, user security, network

security, dynamic neighbor discovery, scaling and the use of existing

routing protocols as they exist today without any modifications.

1. Acronyms

ARP Address Resolution Protocol

CE Customer Edge router

LSP Label Switched Path

PNA Private Network Administrator

SLA Service Level Agreement

SP Service Provider

SPED Service Provider Edge Device

SPNA SP Network Administrator

VMA VPN Multicast Address

VPNID VPN Identifier

VR Virtual Router

VRC Virtual Router Console

2. Introduction

This memo describes an approach for building IP VPN services out of

the backbone of the SP's network. Broadly speaking, two possible

approaches present themselves: the overlay model and the virtual

router approach. The overlay model is based on overloading some

semantic(s) of existing routing protocols to carry reachability

information. In this document, we focus on the virtual router

service.

The approach presented here does not depend on any modifications of

any existing routing protocols. Neighbor discovery is aided by the

use of an emulated LAN and is achieved by the use of ARP. This memo

makes a concerted effort to draw the line between the SP and the PNA:

the SP owns and manages layer 1 and layer 2 services while layer 3

services belong to and are manageable by the PNA. By the provisioning

of fully logically independent routing domains, the PNA has been

given the flexibility to use private and unregistered addresses. Due

to the use of private LSPs and the use of VPNID encapsulation using

label stacks over shared LSPs, data security is not an issue.

The approach espoused in this memo differs from that described in RFC

2547 [Rosen1] in that no specific routing protocol has been

overloaded to carry VPN routes. RFC2547 specifies a way to modify

BGP to carry VPN unicast routes across the SP's backbone. To carry

multicast routes, further architectural work will be necessary.

3. Virtual Routers

A virtual router is a collection of threads, either static or

dynamic, in a routing device, that provides routing and forwarding

services much like physical routers. A virtual router need not be a

separate operating system process (although it could be); it simply

has to provide the illusion that a dedicated router is available to

satisfy the needs of the network(s) to which it is connected. A

virtual router, like its physical counterpart, is an element in a

routing domain. The other routers in this domain could be physical or

virtual routers themselves. Given that the virtual router connects to

a specific (logically discrete) routing domain and that a physical

router can support multiple virtual routers, it follows that a

physical router supports multiple (logically discreet) routing

domains.

From the user (VPN customer) standpoint, it is imperative that the

virtual router be as equivalent to a physical router as possible. In

other Words, with very minor and very few exceptions, the virtual

router should appear for all purposes (configuration, management,

monitoring and troubleshooting) like a dedicated physical router. The

main motivation behind this requirement is to avoid upgrading or re-

configuring the large installed base of routers and to avoid

retraining of network administrators.

The ASPects of a router that a virtual router needs to emulate are:

1. Configuration of any combination of routing protocols

2. Monitoring of the network

3. Troubleshooting.

Every VPN has a logically independent routing domain. This enhances

the SP's ability to offer a fully flexible virtual router service

that can fully serve the SP's customer without requiring physical

per-VPN routers. This means that the SP's "hardware" investments,

namely routers and links between them, can be re-used by multiple

customers.

4. Objectives

1. Easy, scalable configuration of VPN endpoints in the service

provider network. At most, one piece of configuration should be

necessary when a CE is added.

2. No use of SP resources that are globally unique and hard to get

such as IP addresses and subnets.

3. Dynamic discovery of VRs (Virtual Routers) in the SP's cloud. This

is an optional, but extremely valuable "keep it simple" goal.

4. Virtual Routers should be fully configurable and monitorable by

the VPN network administrator. This provides the PNA with the

flexibility to either configure the VPN themselves or outsource

configuration tasks to the SP.

5. Quality of data forwarding should be configurable on a VPN-by-VPN

basis. This should translate to continuous (but perhaps discrete)

grades of service. Some examples include best effort, dedicated

bandwidth, QOS, and policy based forwarding services.

6. Differentiated services should be configurable on a VPN-by-VPN

basis, perhaps based on LSPs set up for exclusive use for

forwarding data traffic in the VPN.

7. Security of internet routers extended to virtual routers. This

means that the virtual router's data forwarding and routing

functions should be as secure as a dedicated, private physical

router. There should be no unintended leak of information (user

data and reachability information) from one routing domain to

another.

8. Specific routing protocols should not be mandated between virtual

routers. This is critical to ensuring the VPN customer can setup

the network and policies as the customer sees fit. For example,

some protocols are strong in filtering, while others are strong in

traffic engineering. The VPN customer might want to eXPloit both

to achieve "best of breed" network quality.

9. No special extensions to existing routing protocols such as BGP,

RIP, OSPF, ISIS etc. This is critical to allowing the future

addition of other services such as NHRP and multicast. In

addition, as advances and addenda are made to existing protocols

(such as traffic engineering extensions to ISIS and OSPF), they

can be easily incorporated into the VPN implementation.

5. Architectural Requirements

The service provider network must run some form of multicast routing

to all nodes that will have VPN connections and to nodes that must

forward multicast datagrams for virtual router discovery. A specific

multicast routing protocol is not mandated. An SP may run MOSPF or

DVMRP or any other protocol.

6. Architectural Outline

1. Every VPN is assigned a VPNID which is unique within the SP's

network. This identifier unambiguously identifies the VPN with

which a packet or connection is associated. The VPNID of zero is

reserved; it is associated with and represents the public

internet. It is recommended, but not required that these VPN

identifiers will be compliant with RFC2685 [Fox].

2. The VPN service is offered in the form of a Virtual Router

service. These VRs reside in the SPED and are as such confined

to the edge of the SP's cloud. The VRs will use the SP's network

for data and control packet forwarding but are otherwise

invisible outside the SPEDs.

3. The "size" of the VR contracted to the VPN in a given SPED is

expressed by the quantity of IP resources such as routing

interfaces, route filters, routing entries etc. This is entirely

under the control of the SP and provides the fine granularity

that the SP requires to offer virtually infinite grades of VR

service on a per-SPED level. [Example: one SPED may be the

aggregating point (say headquarters of the corporation) for a

given VPN and a number of other SPEDs may be Access points

(branch Offices). In this case, the SPED connected to the

headquarters may be contracted to provide a large VR while the

SPEDs connected to the branch offices may house small, perhaps

stub VRs]. This provision also allows the SP to design the

network with an end goal of distributing the load among the

routers in the network.

4. One indicator of the VPN size is the number of SPEDs in the SP's

network that have connections to CPE routers in that VPN. In

this respect, a VPN with many sites that need to be connected is

a "large" VPN whereas one with a few sites is a "small" VPN.

Also, it is conceivable that a VPN grows or shrinks in size over

time. VPNs may even merge due to corporate mergers, acquisitions

and partnering agreements. These changes are easy to accommodate

in this architecture, as globally unique IP resources do not have

to be dedicated or assigned to VPNs. The number of SPEDs is not

limited by any artificial configuration limits.

5. The SP owns and manages Layer 1 and Layer 2 entities. To be

specific, the SP controls physical switches or routers, physical

links, logical layer 2 connections (such as DLCI in Frame Relay

and VPI/VCI in ATM) and LSPs (and their assignment to specific

VPNs). In the context of VPNs, it is the SP's responsibility to

contract and assign layer 2 entities to specific VPNs.

6. Layer 3 entities belong to and are manageable by the PNA.

Examples of these entities include IP interfaces, choice of

dynamic routing protocols or static routes, and routing

interfaces. Note that although Layer 3 configuration logically

falls under the PNA's area of responsibility, it is not necessary

for the PNA to execute it. It is quite viable for the PNA to

outsource the IP administration of the virtual routers to the

Service Provider. Regardless of who assumes responsibility for

configuration and monitoring, this approach provides a full

routing domain view to the PNA and empowers the PNA to design the

network to achieve intranet, extranet and traffic engineering

goals.

7. The VPNs can be managed as if physical routers rather than VRs

were deployed. Therefore, management may be performed using SNMP

or other similar methods or directly at the VR console (VRC).

8. Industry-standard troubleshooting tools such as 'ping,'

'traceroute,' in a routing domain domain comprised exclusively of

dedicated physical routers. Therefore, monitoring and .bp

troubleshooting may be performed using SNMP or similar methods,

but may also include the use of these standard tools. Again, the

VRC may be used for these purposes just like any physical router.

9. Since the VRC is visible to the user, router specific security

checks need to be put in place to make sure the VPN user is

allowed access to Layer 3 resources in that VPN only and is

disallowed from accessing physical resources in the router. Most

routers achieve this through the use of database views.

10. The VRC is available to the SP as well. If configuration and

monitoring has been outsourced to the SP, the SP may use the VRC

to accomplish these tasks as if it were the PNA.

11. The VRs in the SPEDs form the VPN in the SP's network. Together,

they represent a virtual routing domain. They dynamically

discover each other by utilizing an emulated LAN resident in the

SP's network.

Each VPN in the SP's network is assigned one and only one multicast

address. This address is chosen from the administratively scoped

range (239.192/14) [Meyer] and the only requirement is that the

multicast address can be uniquely mapped to a specific VPN. This is

easily automated by routers by the use of a simple function to

unambiguously map a VPNid to the multicast address. Subscription to

this multicast address allows a VR to discover and be discovered by

other VRs. It is important to note that the multicast address does

not have to be configured.

12. Data forwarding may be done in one of several ways:

1. An LSP with best-effort characteristics that all VPNS can use.

2. An LSP dedicated to a VPN and traffic engineered by the VPN

customer.

3. A private LSP with differentiated characteristics.

4. Policy based forwarding on a dedicated L2 Virtual Circuit

The choice of the preferred method is negotiable between the SP and

the VPN customer, perhaps constituting part of the SLA between them.

This allows the SP to offer different grades of service to different

VPN customers.

Of course, hop-by-hop forwarding is also available to forward routing

packets and to forward user data packets during periods of LSP

establishment and failure.

13. This approach does not mandate that separate operating system

tasks for each of the routing protocols be run for each VR that

the SPED houses. Specific implementations may be tailored to the

particular SPED in use. Maintaining separate routing databases

and forwarding tables, one per VR, is one way to get the highest

performance for a given SPED.

7. Scalable Configuration

A typical VPN is expected to have 100s to 1000s of endpoints within

the SP cloud. Therefore, configuration should scale (at most)

linearly with the number of end points. To be specific, the

administrator should have to add a couple of configuration items when

a new customer site joins the set of VRs constituting a specific VPN.

Anything worse will make this task too daunting for the service

provider. In this architecture, all that the service provider needs

to allocate and configure is the ingress/egress physical link (e.g.

Frame Relay DLCI or ATM VPI/VCI) and the virtual connection between

the VR and the emulated LAN.

8. Dynamic Neighbor Discovery

The VRs in a given VPN reside in a number of SPEDs in the network.

These VRs need to learn about each other and be connected.

One way to do this is to require the manual configuration of

neighbors. As an example, when a new site is added to a VPN, this

would require the configuration of all the other VRs as neighbors.

This is obviously not scalable from a configuration and network

resource standpoint.

The need then arises to allow these VRs to dynamically discover each

other. Neighbor discovery is facilitated by providing each VPN with

a limited emulated LAN. This emulated LAN is used in several ways:

1. Address resolution uses this LAN to resolve next-hop (private) IP

addresses associated with the other VRs.

2. Routing protocols such as RIP and OSPF use this limited emulated

LAN for neighbor discovery and to send routing updates.

The per-VPN LAN is emulated using an IP multicast address. In the

interest of conserving public address space and because this

multicast address needs to be visible only in the SP network space,

we would use an address from the Organizationally scoped multicast

addresses (239.192/14) as described in [Meyer]. Each VPN is allocated

an address from this range. To completely eliminate configuration in

this regard, this address is computed from the VPNID.

9. VPN IP Domain Configuration

151.0.0.1

################

# #

# ROUTER 'A' #

# #

################

# #

############# ###############

# # # #

# ROUTER 'B'# # ROUTER 'C' #

# # # #

############# ###############

152.0.0.2 153.0.0.3

Figure 1 'Physical Routing Domain'

The physical domain in the SP's network is shown in the above figure.

In this network, physical routers A, B and C are connected together.

Each of the routers has a 'public' IP address assigned to it. These

addresses uniquely identify each of the routers in the SP's network.

172.150.0/18 172.150.128/18

----------------------- ---------------------------

172.150.128.1

ROUTER 'A' (151.0.0.1) ---------

############# Parts DB

---#-----------# /---------/

OSPF # # ISIS /----------/

------------# VR - A #--------------

#----------#-

#############10.0.1/24

----------------#-#--------------------

10.0.0.2/24# # 10.0.0.3/24

------------- # # ----------------

############### # ###############

# VR - B # # # VR - C #

#-------------# ROUTER 'B'##------------#----

(152.0.0.2)############### ############### (153.0.0.3)

------------------------- ROUTER 'C' Extranet

172.150.64/18 V

Vendors

Figure 2 'Virtual Routing Domain'

Each Virtual Router is configurable by the PNA as though it were a

private physical router. Of course, the SP limits the resources that

this Virtual Router may consume on a SPED-by-SPED basis. Each VPN has

a number of physical connections (to CPE routers) and a number of

logical connections (to the emulated LAN). Each connection is IP-

capable and can be configured to utilize any combination of the

standard routing protocols and routing policies to achieve specific

corporate network goals.

To illustrate, in Figure 1, 3 VRs reside on 3 SPEDs in VPN 1. Router

'A' houses VR-A, router 'B' houses VR-B and router 'C' houses VR-C.

VR-C and VR-B have a physical connection to CPE equipment, while VR-A

has 2 physical connections. Each of the VRs has a fully IP-capable

logical connection to the emulated LAN. VR-A has the (physical)

connections to the headquarters of the company and runs OSPF over

those connections. Therefore, it can route packets to 172.150.0/18

and 172.150.128/18. VR-B runs RIP in the branch office (over the

physical connection) and uses RIP (over the logical connection) to

export 172.150.64/18 to VR-A. VR-A advertises a default route to VR-B

over the logical connection. Vendors use VR-C as the extranet

connection to connect to the parts database at 172.150.128.1. Hence,

VR-C advertises a default route to VR-A over the logical connection.

VR-A exports only 175.150.128.1 to VR-C. This keeps the rest of the

corporate network from a security problem.

The network administrator will configure the following:

1. OSPF connections to the 172.150.0/18 and 172.150.128/18 network

in VR-A.

2. RIP connections to VR-B and VR-C on VR-A.

3. Route policies on VR-A to advertise only the default route to

VR-B.

4. Route policies on VR-A to advertise only 172.159.128.1 to VR-C.

5. RIP on VR-B to VR-A.

6. RIP on VR-C to advertise a default route to VR-A.

10. Neighbor Discovery Example

In Figure #1, the SPED that houses VR-A (SPED-A) uses a public

address of 150.0.0.1/24, SPED-B uses 150.0.0.2/24 and SPED-C uses

150.0.0.3/24. As noted, the connection between the VRs is via an

emulated LAN. For interface addresses on the emulated LAN

connection, VR-A uses 10.0.0.1/24, VR-B uses 10.0.0.2/24 and VR-C

uses 10.0.0.3/24.

Let's take the case of VR-A sending a packet to VR-B. To get VR-B's

address (SPED-B's address), VR-A sends an ARP request packet with the

address of VR-B (10.0.0.2) as the logical address. The source logical

address is 10.0.0.1 and the hardware address is 151.0.0.1. This ARP

request is encapsulated in this VPN's multicast address and sent out.

SPED B and SPED-C receive a copy of the packet. SPED-B recognizes

10.0.0.2 in the context of VPN 1 and responds with 152.0.0.2 as the

"hardware" address. This response is sent to the VPN multicast

address to promote the use of promiscuous ARP and the resulting

decrease in network traffic.

Manual configuration would be necessary if neighbor discovery were

not used. In this example, VR-A would be configured with a static ARP

entry for VR-B's logical address (10.0.0.1) with the "hardware"

address set to 152.0.0.2.

11. Forwarding

As mentioned in the architectural outline, data forwarding may be

done in one of several ways. In all techniques except the Hop-by-Hop

technique for forwarding routing/control packets, the actual method

is configurable. At the high end, policy based forwarding for quick

service and at the other end best effort forwarding using public LSP

is used. The order of forwarding preference is as follows:

1. Policy based forwarding.

2. Optionally configured private LSP.

3. Best-effort public LSP.

11.1 Private LSP

This LSP is optionally configured on a per-VPN basis. This LSP is

usually associated with non-zero bandwidth reservation and/or a

specific differentiated service or QOS class. If this LSP is

available, it is used for user data and for VPN private control data

forwarding.

11.2 Best Effort Public LSP

VPN data packets are forwarded using this LSP if a private LSP with

specified bandwidth and/or QOS characteristics is either not

configured or not presently available. The LSP used is the one

destined for the egress router in VPN 0. The VPNID in the shim header

is used to de-multiplex data packets from various VPNs at the egress

router.

12. Differentiated Services

Configuring private LSPs for VPNs allows the SP to offer

differentiated services to paying customers. These private LSPs could

be associated with any available L2 QOS class or Diff-Serv

codepoints. In a VPN, multiple private LSPs with different service

classes could be configured with flow profiles for sorting the

packets among the LSPs. This feature, together with the ability to

size the virtual routers, allows the SP to offer truly differentiated

services to the VPN customer.

13. Security Considerations

13.1 Routing Security

The use of standard routing protocols such as OSPF and BGP in their

unmodified form means that all the encryption and security methods

(such as MD5 authentication of neighbors) are fully available in VRs.

Making sure that routes are not accidentally leaked from one VPN to

another is an implementation issue. One way to achieve this is to

maintain separate routing and forwarding databases.

13.2 Data Security

This allows the SP to assure the VPN customer that data packets in

one VPN never have the opportunity to wander into another. From a

routing standpoint, this could be achieved by maintaining separate

routing databases for each virtual router. From a data forwarding

standpoint, the use of label stacks in the case of shared LSPs

[Rosen2] [Callon] or the use of private LSPs guarantees data privacy.

Packet filters may also be configured to help ease the problem.

13.3 Configuration Security

Virtual routers appear as physical routers to the PNA. This means

that they may be configured by the PNA to achieve connectivity

between offices of a corporation. Obviously, the SP has to guarantee

that the PNA and the PNA's designees are the only ones who have

access to the VRs on the SPEDs the private network has connections

to. Since the virtual router console is functionally equivalent to a

physical router, all of the authentication methods available on a

physical console such as password, RADIUS, etc. are available to the

PNA.

13.4 Physical Network Security

When a PNA logs in to a SPED to configure or monitor the VPN, the PNA

is logged into the VR for the VPN. The PNA has only layer 3

configuration and monitoring privileges for the VR. Specifically, the

PNA has no configuration privileges for the physical network. This

provides the guarantee to the SP that a VPN administrator will not be

able to inadvertently or otherwise adversely affect the SP's network.

14. Virtual Router Monitoring

All of the router monitoring features available on a physical router

are available on the virtual router. This includes utilities such as

"ping" and "traceroute". In addition, the ability to display private

routing tables, link state databases, etc. are available.

15. Performance Considerations

For the purposes of discussing performance and scaling issues,

today's routers can be split into two planes: the routing (control)

plane and the forwarding plane.

In looking at the routing plane, most modern-day routing protocols

use some form of optimized calculation methodologies to calculate the

shortest path(s) to end stations. For instance, OSPF and ISIS use the

Djikstra algorithm while BGP uses the "Decision Process". These

algorithms are based on parsing the routing database and computing

the best paths to end stations. The performance characteristics of

any of these algorithms is based on either topological

characteristics (ISIS and OSPF) or the number of ASs in the path to

the destinations (BGP). But it is important to note that the overhead

in setting up and beginning these calculations is very little for

most any modern day router. This is because, although we refer to

routing calculation input as "databases", these are memory resident

data structures.

Therefore, the following conclusions can be drawn:

1. Beginning a routing calculation for a routing domain is nothing

more than setting up some registers to point to the right database

objects.

2. Based on 1, the performance of a given algorithm is not

significantly worsened by the overhead required to set it up.

3. Based on 2, it follows that, when a number of routing calculations

for a number of virtual routers has to be performed by a physical

router, the complexity of the resulting routing calculation is

nothing more than the sum of the complexities of the routing

calculations of the individual virtual routers.

4. Based on 3, it follows that whether an overlay model is used or a

virtual routing model is employed, the performance characteristics

of a router are dependent purely on its hardware capabilities and

the choice of data structures and algorithms.

To illustrate, let's say a physical router houses N VPNs, all running

some routing protocol say RP. Let's also suppose that the average

performance of RP's routing calculation algorithm is f(X,Y) where x

and y are parameters that determine performance of the algorithm for

that routing protocol. As an example, for Djikstra algorithm users

such as OSPF, X could be the number of nodes in the area while Y

could be the number of links. The performance of an arbitrary VPN n

is f (Xn, Yn). The performance of the (physical) router is the sum of

f(Xi, Yi) for all values of i in 0 <= i <= N. This conclusion is

independent of the chosen VPN approach (virtual router or overlay

model).

In the usual case, the forwarding plane has two inputs: the

forwarding table and the packet header. The main performance

parameter is the lookup algorithm. The very best performance one can

get for a IP routing table lookup is by organizing the table as some

form of a tree and use binary search methods to do the actual lookup.

The performance of this algorithm is O(log n).

Hence, as long as the virtual routers' routing tables are distinct

from each other, the lookup cost is constant for finding the routing

table and O(log n) to find the entry. This is no worse or different

from any router and no different from a router that employs overlay

techniques to deliver VPN services. However, when the overlay router

utilizes integration of multiple VPNs' routing tables, the

performance is O(log m*n) where 'm' is the number of VPNs that the

routing table holds routes for.

16. Acknowledgements

The authors wish to thank Dave Ryan, Lucent Technologies for his

invaluable in-depth review of this version of this memo.

17. References

[Callon] Callon R., et al., "A Framework for Multiprotocol Label

Switching", Work in Progress.

[Fox] Fox, B. and B. Gleeson,"Virtual Private Networks

Identifier", RFC2685, September 1999.

[Meyer] Meyer, D., "Administratively Scoped IP Multicast", RFC2365,

July 1998.

[Rosen1] Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs", RFC2547, March

1999.

[Rosen2] Rosen E., Viswanathan, A. and R. Callon, "Multiprotocol

Label Switching Architecture", Work in Progress.

18. Authors' Addresses

Karthik Muthukrishnan

Lucent Technologies

1 Robbins Road

Westford, MA 01886

Phone: (978) 952-1368

EMail: mkarthik@lucent.com

Andrew Malis

Vivace Networks, Inc.

2730 Orchard Parkway

San Jose, CA 95134

Phone: (408) 383-7223

EMail: Andy.Malis@vivacenetworks.com

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