RFC 4110
A Framework for Layer 3 Provider-Provisioned Virtual Private Networks (PPVPNs)
2. Reference Models
This section describes PPVPN reference models. The purpose of discussing reference models is to clarify the common components and pieces that are needed to build and deploy a PPVPN. Two types of VPNs, layer 3 PE-based VPN and layer 3 provider-provisioned CE-based VPN are covered in separated sections below.2.1. Reference Model for Layer 3 PE-based VPN
This subsection describes functional components and their relationship for implementing layer 3 PE-based VPN. Figure 2.1 shows the reference model for layer 3 PE-based VPNs and Figures 2.2 and 2.3 show relationship between entities in the reference model. As shown in Figure 2.1, the customer interface is defined as the interface which exists between CE and PE devices, and the network interface is defined as the interface which exists between a pair of PE devices. Figure 2.2 illustrates a single logical tunnel between each pair of VFIs supporting the same VPN. Other options are possible. For example, a single tunnel might occur between two PEs, with multiple per-VFI tunnels multiplexed over the PE to PE tunnel. Similarly, there may be multiple tunnels between two VFIs, for example to optimize forwarding within the VFI. Other possibilities will be discussed later in this framework document.
+---------+ +------------------------------------+ +---------+ | | | | | | | | | +------+ +------+ : +------+ +------+ : | | | | | | : | CE | | CE | : | | | P | | PE | : |device| |device| : +------+ VPN tunnel : |router| |device| : | of | | of |-:--| |================:===============| |--:-|VPN A| |VPN A| : | | : +------+ +------+ : +------+ +------+ : | PE | : | | : | +------+ : |device| Network interface | | : | | CE | : | | : +------+ : +------+ |device|-:--| |================:===============| |--:-| CE | | of | : +------+ : VPN tunnel | PE | : |device| |VPN B| : | | |device| : | of | +------+ : | | +------------+ +------------+ | | : |VPN B| | : | | | Customer | | Network | +------+ : +------+ |Customer | | | management | | management | | | : | |interface| | | function | | function | | |Customer | | | | +------------+ +------------+ | |interface| | | | | | | +---------+ +------------------------------------+ +---------+ | Access | |<---------- SP network(s) --------->| | Access | | network | | single or multiple SP domains | | network | Figure 2.1: Reference model for layer 3 PE-based VPN. +----------+ +----------+ +-----+ |PE device | |PE device | +-----+ | CE | | | | | | CE | | dev | Access | +------+ | | +------+ | Access | dev | | of | conn. | |VFI of| | VPN tunnel | |VFI of| | conn. | of | |VPN A|----------|VPN A |======================|VPN A |----------|VPN A| +-----+ | +------+ | | +------+ | +-----+ | | | | +-----+ Access | +------+ | | +------+ | Access +-----+ | CE | conn. | |VFI of| | VPN tunnel | |VFI of| | conn. | CE | | dev |----------|VPN B |======================|VPN B |----------| dev | | of | | +------+ | | +------+ | | of | |VPN B| | | | | |VPN B| +-----+ +----------+ +----------+ +-----+ Figure 2.2: Relationship between entities in reference model (1).
+----------+ +----------+ +-----+ |PE device | |PE device | +-----+ | CE | | | | | | CE | | dev | Access | +------+ | | +------+ | Access | dev | | of | conn. | |VFI of| | | |VFI of| | conn. | of | |VPN A|----------|VPN A | | | |VPN A |----------|VPN A| +-----+ | +------+\| Tunnel |/+------+ | +-----+ | >==================< | +-----+ Access | +------+/| |\+------+ | Access +-----+ | CE | conn. | |VFI of| | | |VFI of| | conn. | CE | | dev |----------|VPN B | | | |VPN B |----------| dev | | of | | +------+ | | +------+ | | of | |VPN B| | | | | |VPN B| +-----+ +----------+ +----------+ +-----+ Figure 2.3: Relationship between entities in reference model (2).2.1.1. Entities in the Reference Model
The entities in the reference model are described below. o Customer edge (CE) device In the context of layer 3 provider-provisioned PE-based VPNs, a CE device may be a router, LSR, or host that has no VPN-specific functionality. It is attached via an access connection to a PE device. o P router A router within a provider network which is used to interconnect PE devices, but which does not have any VPN state and does not have any direct attachment to CE devices. o Provider edge (PE) device In the context of layer 3 provider-provisioned PE-based VPNs, a PE device implements one or more VFIs and maintains per-VPN state for the support of one or more VPNs. It may be a router, LSR, or other device that includes VFIs and provider edge VPN functionality such as provisioning, management, and traffic classification and separation. (Note that access connections are terminated by VFIs from the functional point of view). A PE device is attached via an access connection to one or more CE devices.
o Customer site
A customer site is a set of users that have mutual IP reachability
without use of a VPN backbone that goes beyond the site.
o SP networks
An SP network is an IP or MPLS network administered by a single
service provider.
o Access connection
An access connection represents an isolated layer 2 connectivity
between a CE device and a PE device. Access connections can be,
e.g., dedicated physical circuits, logical circuits (such as FR,
ATM, and MAC), or IP tunnels (e.g., using IPsec, L2TP, or MPLS).
o Access network
An access network provides access connections between CE and PE
devices. It may be a TDM network, layer 2 network (e.g., FR, ATM,
and Ethernet), or IP network over which access is tunneled (e.g.,
using L2TP [RFC2661] or MPLS).
o VPN tunnel
A VPN tunnel is a logical link between two VPN edge devices. A VPN
packet is carried on a tunnel by encapsulating it before
transmitting it over the VPN backbone.
Multiple VPN tunnels at one level may be hierarchically multiplexed
into a single tunnel at another level. For example, multiple per-
VPN tunnels may be multiplexed into a single PE to PE tunnel (e.g.,
GRE, IP-in-IP, IPsec, or MPLS tunnel). This is illustrated in
Figure 2.3. See section 4.3 for details.
o VPN forwarding instance (VFI)
A single PE device is likely to be connected to a number of CE
devices. The CE devices are unlikely to all be in the same VPN.
The PE device must therefore maintain a separate forwarding
instances for each VPN to which it is connected. A VFI is a
logical entity, residing in a PE, that contains the router
information base and forwarding information base for a VPN. The
interaction between routing and VFIs is discussed in section 4.4.2.
o Customer management function
The customer management function supports the provisioning of
customer specific attributes, such as customer ID, personal
information (e.g., name, address, phone number, credit card number,
and etc.), subscription services and parameters, access control
policy information, billing and statistical information, and etc.
The customer management function may use a combination of SNMP
manager, directory service (e.g., LDAP [RFC3377]), or proprietary
network management system.
o Network management function
The network management function supports the provisioning and
monitoring of PE or CE device attributes and their relationships.
The network management function may use a combination of SNMP
manager, directory service (e.g., LDAP [RFC3377]), or proprietary
network management system.
2.1.2. Relationship Between CE and PE
For robustness, a CE device may be connected to more than one PE
device, resulting in a multi-homing arrangement. Four distinct types
of multi-homing arrangements, shown in Figure 2.4, may be supported.
+---------------- +--------------- | | +------+ +------+ +---------| PE | +---------| PE | | |device| | |device| SP network | +------+ | +------+ +------+ | +------+ | | CE | | | CE | +--------------- |device| | SP network |device| +--------------- +------+ | +------+ | | +------+ | +------+ | | PE | | | PE | +---------|device| +---------|device| SP network +------+ +------+ | | +---------------- +--------------- This type includes a CE device connected to a PE device via two access connections. (a) (b) +---------------- +--------------- | | +------+ +------+ +------+ +------+ | CE |-----| PE | | CE |-----| PE | |device| |device| |device| |device| SP network +------+ +------+ +------+ +------+ | | | | | Backdoor | | Backdoor +--------------- | link | SP network | link +--------------- | | | | +------+ +------+ +------+ +------+ | CE | | PE | | CE | | PE | |device|-----|device| |device|-----|device| SP network +------+ +------+ +------+ +------+ | | +---------------- +--------------- (c) (d) Figure 2.4: Four types of double-homing arrangements.2.1.3. Interworking Model
It is quite natural to assume that multiple different layer 3 VPN approaches may be implemented, particularly if the VPN backbone includes more than one SP network. For example, (1) each SP chooses one or more layer 3 PE-based VPN approaches out of multiple vendor's implementations, implying that different SPs may choose different
approaches; and (2) an SP may deploy multiple networks of layer 3
PE-based VPNs (e.g., an old network and a new network). Thus it is
important to allow interworking of layer 3 PE-based VPNs making use
of multiple different layer 3 VPN approaches.
There are three scenarios that enable layer 3 PE-based VPN
interworking among different approaches.
o Interworking function
This scenario enables interworking using a PE that is located at
one or more points which are logically located between VPNs based
on different layer 3 VPN approaches. For example, this PE may be
located on the boundary between SP networks which make use of
different layer 3 VPN approaches [VPN-DISC]. A PE at one of these
points is called an interworking function (IWF), and an example
configuration is shown in Figure 2.5.
+------------------+ +------------------+
| | | |
+------+ VPN tunnel +------+ VPN tunnel +------+
| |==============| |==============| |
| | | | | |
| PE | | PE | | PE |
| | |device| | |
|device| |(IWF) | |device|
| | VPN tunnel | | VPN tunnel | |
| |==============| |==============| |
+------+ +------+ +------+
| | | |
+------------------+ +------------------+
|<-VPN approach 1->| |<-VPN approach 2->|
Figure 2.5: Interworking function.
o Interworking interface
This scenario enables interworking using tunnels between PEs
supporting by different layer 3 VPN approaches. As shown in Figure
2.6, interworking interface is defined as the interface which
exists between a pair of PEs and connects two SP networks
implemented with different approaches. This interface is similar
to the customer interface located between PE and CE, but the
interface is supported by tunnels to identify VPNs, while the
customer interface is supported by access connections.
+------------------+ +------------------+ | | : | | +------+ VPN tunnel +------+Tunnel: +------+ VPN tunnel +------+ | |============| |======:======| |============| | | | | | : | | | | | PE | | PE | : | PE | | PE | | | | | : | | | | |device| |device| : |device| |device| | | VPN tunnel | |Tunnel: | | VPN tunnel | | | |============| |======:======| |============| | +------+ +------+ : +------+ +------+ | | : | | +------------------+ Interworking +------------------+ |<-VPN approach 1->| interface |<-VPN approach 2->| Figure 2.6: Interworking interface. o Customer-based interworking If some customer site has a CE attached to one kind of VPN, and a CE attached to another kind, communication between the two kinds of VPN occurs automatically.2.2. Reference Model for Layer 3 Provider-Provisioned CE-based VPN
This subsection describes functional components and their relationship for implementing layer 3 provider-provisioned CE-based VPN. Figure 2.7 shows the reference model for layer 3 provider-provisioned CE-based VPN. As shown in Figure 2.7, the customer interface is defined as the interface which exists between CE and PE devices. In this model, a CE device maintains one or more VPN tunnel endpoints, and a PE device has no VPN-specific functionality. As a result, the interworking issues of section 2.1.3 do not arise.
+---------+ +------------------------------------+ +---------+ | | | | | | | | | +------+ +------+ : +------+ +------+ : | | | | | | : | CE | | CE | : | | | P | | PE | : |device| |device| : +------+ VPN tunnel |router| |device| : | of | | of |=:====================================================:=|VPN A| |VPN A| : | | +------+ +------+ : +------+ +------+ : | PE | | | : | +------+ : |device| | | : | | CE | : | | VPN tunnel +------+ : +------+ |device|=:====================================================:=| CE | | of | : +------+ | PE | : |device| |VPN B| : | | |device| : | of | +------+ : | | +------------+ +------------+ | | : |VPN B| | : | | | Customer | | Network | +------+ : +------+ |Customer | | | management | | management | | | : | |interface| | | function | | function | | |Customer | | | | +------------+ +------------+ | |interface| | | | | | | +---------+ +------------------------------------+ +---------+ | Access | |<---------- SP network(s) --------->| | Access | | network | | | | network | Figure 2.7: Reference model for layer 3 provider-provisioned CE-based VPN.2.2.1. Entities in the Reference Model
The entities in the reference model are described below. o Customer edge (CE) device In the context of layer 3 provider-provisioned CE-based VPNs, a CE device provides layer 3 connectivity to the customer site. It may be a router, LSR, or host that maintains one or more VPN tunnel endpoints. A CE device is attached via an access connection to a PE device and usually located at the edge of a customer site or co-located on an SP premises. o P router (see section 2.1.1) o Provider edge (PE) device In the context of layer 3 provider-provisioned CE-based VPNs, a PE device may be a router, LSR, or other device that has no VPN-specific functionality. It is attached via an access connection to one or more CE devices.
o Customer Site (see section 2.1.1) o SP networks An SP network is a network administrated by a single service provider. It is an IP or MPLS network. In the context of layer 3 provider-provisioned CE-based VPNs, the SP network consists of the SP's network and the SP's management functions that manage both its own network and the customer's VPN functions on the CE device. o Access connection (see section 2.1.1) o Access network (see section 2.1.1) o VPN tunnel A VPN tunnel is a logical link between two entities which is created by encapsulating packets within an encapsulating header for purpose of transmission between those two entities for support of VPNs. In the context of layer 3 provider-provisioned CE-based VPNs, a VPN tunnel is an IP tunnel (e.g., using GRE, IP-in-IP, IPsec, or L2TP) or an MPLS tunnel between two CE devices over the SP's network. o Customer management function (see section 2.1.1) o Network management function The network management function supports the provisioning and monitoring of PE or CE device attributes and their relationships, covering PE and CE devices that define the VPN connectivity of the customer VPNs. The network management function may use a combination of SNMP manager, directory service (e.g., LDAP [RFC3377]), or proprietary network management system.3. Customer Interface
3.1. VPN Establishment at the Customer Interface
3.1.1. Layer 3 PE-based VPN
It is necessary for each PE device to know which CEs it is attached to, and what VPNs each CE is associated with. VPN membership refers to the association of VPNs, CEs, and PEs. A given CE belongs to one or more VPNs. Each PE is therefore
associated with a set of VPNs, and a given VPN has a set of associated PEs which are supporting that VPN. If a PE has at least one attached CE belonging to a given VPN, then state information for that VPN (e.g., the VPN routes) must exist on that PE. The set of VPNs that exist on a PE may change over time as customer sites are added to or removed from the VPNs. In some layer 3 PE-based PPVPN schemes, VPN membership information (i.e., information about which PEs are attached to which VPNs) is explicitly distributed. In others, the membership information is inferred from other information that is distributed. Different schemes use the membership information in different ways, e.g., some to determine what set of tunnels to set up, some to constrain the distribution of VPN routing information. A VPN site may be added or deleted as a result of a provisioning operation carried out by the network administrator, or may be dynamically added or deleted as a result of a subscriber initiated operation; thus VPN membership information may be either static or dynamic, as discussed below.3.1.1.1. Static Binding
Static binding occurs when a provisioning action binds a particular PE-CE access link to a particular VPN. For example, a network administrator may set up a dedicated link layer connection, such as an ATM VCC or a FR DLCI, between a PE device and a CE device. In this case the binding between a PE-CE access connection and a particular VPN to fixed at provisioning time, and remains the same until another provisioning action changes the binding.3.1.1.2. Dynamic Binding
Dynamic binding occurs when some real-time protocol interaction causes a particular PE-CE access link to be temporarily bound to a particular VPN. For example, a mobile user may dial up the provider network and carry out user authentication and VPN selection procedures. Then the PE to which the user is attached is not one permanently associated with the user, but rather one that is typically geographically close to where the mobile user happens to be. Another example of dynamic binding is that of a permanent access connection between a PE and a CE at a public facility such as a hotel or conference center, where the link may be accessed by multiple users in turn, each of which may wish to connect to a different VPN. To support dynamically connected users, PPP and RADIUS are commonly used, as these protocols provide for user identification, authentication and VPN selection. Other mechanisms are also
possible. For example a user's HTTP traffic may be initially intercepted by a PE and diverted to a provider hosted web server. After a dialogue that includes user authentication and VPN selection, the user can then be connected to the required VPN. This is sometimes referred to as a "captive portal". Independent of the particular mechanisms used for user authentication and VPN selection, an implication of dynamic binding is that a user for a given VPN may appear at any PE at any time. Thus VPN membership may change at any time as a result of user initiated actions, rather than as a result of network provisioning actions. This suggests that there needs to be a way to distribute membership information rapidly and reliably when these user-initiated actions take place.3.1.2. Layer 3 Provider-Provisioned CE-based VPN
In layer 3 provider-provisioned CE-based VPNs, the PE devices have no knowledge of the VPNs. A PE device attached to a particular VPN has no knowledge of the addressing or routing information of that specific VPN. CE devices have IP or MPLS connectivity via a connection to a PE device, which just provides ordinary connectivity to the global IP address space or to an address space which is unique in a particular SPs network. The IP connectivity may be via a static binding, or via some kind of dynamic binding. The establishment of the VPNs is done at each CE device, making use of the IP or MPLS connectivity to the others. Therefore, it is necessary for a given CE device to know which other CE devices belong to the same VPN. In this context, VPN membership refers to the association of VPNs and CE devices.3.2. Data Exchange at the Customer Interface
3.2.1. Layer 3 PE-based VPN
For layer 3 PE-based VPNs, the exchange is normal IP packets, transmitted in the same form which is available for interconnecting routers in general. For example, IP packets may be exchanged over Ethernet, SONET, T1, T3, dial-up lines, and any other link layer available to the router. It is important to note that those link layers are strictly local to the interface for the purpose of carrying IP packets, and are terminated at each end of the customer interface. The IP packets may contain addresses which, while unique within the VPN, are not unique on the VPN backbone. Optionally, the data exchange may use MPLS to carry the IP packets.
3.2.2. Layer 3 Provider-Provisioned CE-based VPN
The data exchanged at the customer interface are always normal IP packets that are routable on the VPN backbone, and whose addresses are unique on the VPN backbone. Optionally, MPLS frames can be used, if the appropriate label-switched paths exist across the VPN backbone. The PE device does not know whether these packets are VPN packets or not. At the current time, MPLS is not commonly offered as a customer-visible service, so that CE-based VPNs most commonly make use of IP services.3.3. Customer Visible Routing
Once VPN tunnels are set up between pairs of VPN edge devices, it is necessary to set up mechanisms which ensure that packets from the customer network get sent through the proper tunnels. This routing function must be performed by the VPN edge device.3.3.1. Customer View of Routing for Layer 3 PE-based VPNs
There is a PE-CE routing interaction which enables a PE to obtain those addresses, from the customer network, that are reachable via the CE. The PE-CE routing interaction also enables a CE device to obtain those addresses, from the customer network, which are reachable via the PE; these will generally be addresses that are at other sites in the customer network. The PE-CE routing interaction can make use of static routing, an IGP (such as RIP, OSPF, IS-IS, etc.), or BGP. If the PE-CE interaction is done via an IGP, the PE will generally maintain at least several independent IGP instances; one for the backbone routing, and one for each VPN. Thus the PE participates in the IGP of the customer VPNs, but the CE does not participate in the backbone's IGP. If the PE-CE interaction is done via BGP, the PE MAY support one instance of BGP for each VPN, as well as an additional instance of BGP for the public Internet routes. Alternatively, the PE might support a single instance of BGP, using, e.g., different BGP Address Families to distinguish the public Internet routes from the VPN routes. Routing information which a PE learns from a CE in a particular VPN must be forwarded to the other PEs that are attached to the same VPN. Those other PEs must then forward the information in turn to the other CEs of that VPN.
The PE-PE routing distribution can be done as part of the same routing instance to which the PE-CE interface belongs. Alternatively, it can be done via a different routing instance, possibly using a different routing algorithm. In this case, the PE must redistribute VPN routes from one routing instance to another. Note that VPN routing information is never distributed to the P routers. VPN routing information is known at the edge of the VPN backbone, but not in the core. If the VPN's IGP is different than the routing algorithm running on the CE-PE link, then the CE must support two routing instances, and must redistribute the VPN's routes from one instance to the other (e.g., [VPN-BGP-OSPF]). In the case of layer 3 PE-based VPNs a single PE device is likely to provide service for several different VPNs. Since different VPNs may have address spaces which are not mutually unique, a PE device must have several forwarding tables, in general one for each VPN to which it is attached. These will be referred to as VPN Forwarding Instances (VFIs). Each VFI is a logical entity internal to the PE device. VFIs are defined in section 2.1.1, and discussed in more detail in section 4.4.2. The scaling and management of the customer network (as well as the operation of the VPN) will depend upon the implementation approach and the manner in which routing is done.3.3.1.1. Routing for Intranets
In the intranet case all of the sites to be interconnected belong to the same administration (for example, the same company). The options for routing within a single customer network include: o A single IGP area (using OSPF, IS-IS, or RIP) o Multiple areas within a single IGP o A separate IGP within each site, with routes redistributed from each site to backbone routing (i.e., to a backbone as seen by the customer network). Note that these options look at routing from the perspective of the overall routing in the customer network. This list does not specify whether PE device is considered to be in a site or not. This issue is discussed below.
A single IGP area (such as a single OSPF area, a single IS-IS area, or a single instance of RIP between routers) may be used. One could have, all routers within the customer network (including the PEs, or more precisely, including a VFI within each PE) appear within a single area. Tunnels between the PEs could also appear as normal links. In some cases the multi-level hierarchy of OSPF or IS-IS may be used. One way to apply this to VPNs would be to have each site be a single OSPF or IS-IS area. The VFIs will participate in routing within each site as part of that area. The VFIs may then be interconnected as the backbone (OSPF area 0 or IS-IS level 2). If OSPF is used, the VFIs therefore appear to the customer network as area border routers. If IS-IS is used, the VFIs therefore participate in level 1 routing within the local area, and appear to the customer network as if they are level 2 routers in the backbone. Where an IGP is used across the entire network, it is straightforward for VPN tunnels, access connections, and backdoor links to be mixed in a network. Given that OSPF or IS-IS metrics will be assigned to all links, paths via alternate links can be compared and the shortest cost path will be used regardless of whether it is via VPN tunnels, access connections, or backdoor links. If multiple sites of a VPN do not use a common IGP, or if the backbone does not use the same common IGP as the sites, then special procedures may be needed to ensure that routes to/from other sites are treated as intra-area routes, rather than as external routes (depending upon the VPN approach taken). Another option is to operate each site as a separate routing domain. For example each site could operate as a single OSPF area, a single IS-IS area, or a RIP domain. In this case the per-site routing domains will need to redistribute routes into a backbone routing domain (Note: in this context the "backbone routing domain" refers to a backbone as viewed by the customer network). In this case it is optional whether or not the VFIs participate in the routing within each site.3.3.1.2. Routing for Extranets
In the extranet case the sites to be interconnected belong to multiple different administrations. In this case IGPs (such as OSPF, IS-IS, or RIP) are normally not used across the interface between organizations. Either static routes or BGP may be used between sites. If the customer network administration wishes to maintain control of routing between its site and other networks, then either
static routing or BGP may be used across the customer interface. If the customer wants to outsource all such control to the provider, then an IGP or static routes may be used at this interface. The use of BGP between sites allows for policy based routing between sites. This is particularly useful in the extranet case. Note that private IP addresses or non-unique IP address (e.g., unregistered addresses) should not be used for extranet communication.3.3.1.3. CE and PE Devices for Layer 3 PE-based VPNs
When using a single IGP area across an intranet, the entire customer network participates in a single area of an IGP. In this case, for layer 3 PE-based VPNs both CE and PE devices participate as normal routers within the area. The other options make a distinction between routing within a site, and routing between sites. In this case, a CE device would normally be considered as part of the site where it is located. However, there is an option regarding how the PE devices should be considered. In some cases, from the perspective of routing within the customer network, a PE device (or more precisely a VFI within a PE device) may be considered to be internal to the same area or routing domain as the site to which it is attached. This simplifies the management responsibilities of the customer network administration, since inter-area routing would be handled by the provider. For example, from the perspective of routing within the customer network, the CE devices may be the area border or AS boundary routers of the IGP area. In this case, static routing, BGP, or whatever routing is used in the backbone, may be used across the customer interface.3.3.2. Customer View of Routing for Layer 3 Provider-Provisioned CE-based VPNs
For layer 3 provider-provisioned CE-based VPNs, the PE devices are not aware of the set of addresses which are reachable at particular customer sites. The CE and PE devices do not exchange the customer's routing information. Customer sites that belong to the same VPN may exchange routing information through the CE-CE VPN tunnels that appear, to the customers IGP, as router adjacencies. Alternatively, instead of
exchanging routing information through the VPN tunnels, the SP's management system may take care of the configuration of the static route information of one site towards the other sites in the VPN. Routing within the customer site may be done in any possible way, using any kind of routing protocols (see section 3.3.3). As the CE device receives an IP or MPLS service from the SP, the CE and PE devices may exchange routing information that is meaningful within the SP routing realm. Moreover, as the forwarding of tunneled customer packets in the SP network will be based on global IP forwarding, the routes to the various CE devices must be known in the entire SP's network. This means that a CE device may need to participate in two different routing processes: o routing in its own private network (VPN routing), within its own site and with the other VPN sites through the VPN tunnels, possibly using private addresses. o routing in the SP network (global routing), as such peering with its PE. However, in many scenarios, the use of static/default routes at the CE-PE interface might be all the global routing that is required.3.3.3. Options for Customer Visible Routing
The following technologies are available for the exchange of routing information. o Static routing Routing tables may be configured through a management system. o RIP (Routing Information Protocol) [RFC2453] RIP is an interior gateway protocol and is used within an autonomous system. It sends out routing updates at regular intervals and whenever the network topology changes. Routing information is then propagated by the adjacent routers to their neighbors and thus to the entire network. A route from a source to a destination is the path with the least number of routers. This number is called the "hop count" and its maximum value is 15. This implies that RIP is suitable for a small- or medium-sized networks.
o OSPF (Open Shortest Path First) [RFC2328] OSPF is an interior gateway protocol and is applied to a single autonomous system. Each router distributes the state of its interfaces and neighboring routers as a link state advertisement, and maintains a database describing the autonomous system's topology. A link state is advertised every 30 minutes or when the topology is reconfigured. Each router maintains an identical topological database, from which it constructs a tree of shortest paths with itself as the root. The algorithm is known as the Shortest Path First or SPF. The router generates a routing table from the tree of shortest paths. OSPF supports a variable length subnet mask, which enables effective use of the IP address space. OSPF allows sets of networks to be grouped together into an area. Each area has its own topological database. The topology of the area is invisible from outside its area. The areas are interconnected via a "backbone" network. The backbone network distributes routing information between the areas. The area routing scheme can reduce the routing traffic and compute the shortest path trees and is indispensable for larger scale networks. Each multi-access network with multiple routers attached has a designated router. The designated router generates a link state advertisement for the multi-access network and synchronizes the topological database with other adjacent routers in the area. The concept of designated router can thus reduce the routing traffic and compute shortest path trees. To achieve high availability, a backup designated router is used. o IS-IS (intermediate system to intermediate system) [RFC1195] IS-IS is a routing protocol designed for the OSI (Open Systems Interconnection) protocol suites. Integrated IS-IS is derived from IS-IS in order to support the IP protocol. In the Internet community, IS-IS means integrated IS-IS. In this, a link state is advertised over a connectionless network service. IS-IS has the same basic features as OSPF. They include: link state advertisement and maintenance of a topological database within an area, calculation of a tree of shortest paths, generation of a routing table from a tree of shortest paths, the area routing scheme, a designated router, and a variable length subnet mask.
o BGP-4 (Border Gateway Protocol version 4) [RFC1771] BGP-4 is an exterior gateway protocol and is applied to the routing of inter-autonomous systems. A BGP speaker establishes a session with other BGP speakers and advertises routing information to them. A session may be an External BGP (EBGP) that connects two BGP speakers within different autonomous systems, or an internal BGP (IBGP) that connects two BGP speakers within a single autonomous system. Routing information is qualified with path attributes, which differentiate routes for the purpose of selecting an appropriate one from possible routes. Also, routes are grouped by the community attribute [RFC1997] [BGP-COM]. The IBGP mesh size tends to increase dramatically with the number of BGP speakers in an autonomous system. BGP can reduce the number of IBGP sessions by dividing the autonomous system into smaller autonomous systems and grouping them into a single confederation [RFC3065]. Route reflection is another way to reduce the number of IBGP sessions [RFC1966]. BGP divides the autonomous system into clusters. Each cluster establishes the IBGP full mesh within itself, and designates one or more BGP speakers as "route reflectors," which communicate with other clusters via their route reflectors. Route reflectors in each cluster maintain path and attribute information across the autonomous system. The autonomous system still functions like a fully meshed autonomous system. On the other hand, confederations provide finer control of routing within the autonomous system by allowing for policy changes across confederation boundaries, while route reflection requires the use of identical policies. BGP-4 has been extended to support IPv6, IPX, and others as well as IPv4 [RFC2858]. Multiprotocol BGP-4 carries routes from multiple "address families".4. Network Interface and SP Support of VPNs
4.1. Functional Components of a VPN
The basic functional components of an implementation of a VPN are: o A mechanism to acquire VPN membership/capability information o A mechanism to tunnel traffic between VPN sites o For layer 3 PE-based VPNs, a means to learn customer routes, distribute them between the PEs, and to advertise reachable destinations to customer sites.
Based on the actual implementation, these functions could be implemented on a per-VPN basis or could be accomplished via a common mechanism shared by all VPNs. For instance, a single process could handle the routing information for all the VPNs or a separate process may be created for each VPN. Logically, the establishment of a VPN can be thought of as composed of the following three stages. In the first stage, the VPN edge devices learn of each other. In the second stage, they establish tunnels to each other. In the third stage, they exchange routing information with each other. However, not all VPN solutions need be decomposed into these three stages. For example, in some VPN solutions, tunnels are not established after learning membership information; rather, pre-existing tunnels are selected and used. Also, in some VPN solutions, the membership information and the routing information are combined. In the membership/capability discovery stage, membership and capability information needs to be acquired to determine whether two particular VPN edge devices support any VPNs in common. This can be accomplished, for instance, by exchanging VPN identifiers of the configured VPNs at each VPN edge device. The capabilities of the VPN edge devices need to be determined, in order to be able to agree on a common mechanism for tunneling and/or routing. For instance, if site A supports both IPsec and MPLS as tunneling mechanisms and site B supports only MPLS, they can both agree to use MPLS for tunneling. In some cases the capability information may be determined implicitly, for example some SPs may implement a single VPN solution. Likewise, the routing information for VPNs can be distributed using the methods discussed in section 4.4. In the tunnel establishment stage, mechanisms may need to be invoked to actually set up the tunnels. With IPsec, for instance, this could involve the use of IKE to exchange keys and policies for securing the data traffic. However, if IP tunneling, e.g., is used, there may not be any need to explicitly set up tunnels; if MPLS tunnels are used, they may be pre-established as part of normal MPLS functioning. In the VPN routing stage, routing information for the VPN sites must be exchanged before data transfer between the sites can take place. Based on the VPN model, this could involve the use of static routes, IGPs such as OSPF/ISIS/RIP, or an EGP such as BGP. VPN membership and capability information can be distributed from a central management system, using protocols such as, e.g., LDAP. Alternatively, it can be distributed manually. However, as manual configuration does not scale and is error prone, its use is discouraged. As a third alternative, VPN information can be
distributed via protocols that ensure automatic and consistent distribution of information in a timely manner, much as routing protocols do for routing information. This may suggest that the information be carried in routing protocols themselves, though only if this can be done without negatively impacting the essential routing functions. It can be seen that quite a lot of information needs to be exchanged in order to establish and maintain a VPN. The scaling and stability consequences need to be analyzed for any VPN approach. While every VPN solution must address the functionality of all three components, the combinations of mechanisms used to provide the needed functionality, and the order in which different pieces of functionality are carried out, may differ. For layer 3 provider-provisioned CE-based VPNs, the VPN service is offering tunnels between CE devices. IP routing for the VPN is done by the customer network. With these solutions, the SP is involved in the operation of the membership/capability discovery stage and the tunnel establishment stage. The IP routing functional component may be entirely up to the customer network, or alternatively, the SP's management system may be responsible for the distribution of the reachability information of the VPN sites to the other sites of the same VPN.4.2. VPN Establishment and Maintenance
For a layer 3 provider-provisioned VPN the SP is responsible for the establishment and maintenance of the VPNs. Many different approaches and schemes are possible in order to provide layer 3 PPVPNs, however there are some generic problems that any VPN solution must address, including: o For PE-based VPNs, when a new site is added to a PE, how do the other PEs find out about it? When a PE first gets attached to a given VPN, how does it determine which other PEs are attached to the same VPN. For CE-based VPNs, when a new site is added, how does its CE find out about all the other CEs at other sites of the same VPN? o In order for layer 3 PE-based VPNs to scale, all routes for all VPNs cannot reside on all PEs. How is the distribution of VPN routing information constrained so that it is distributed to only those devices that need it?
o An administrator may wish to provision different topologies for
different VPNs (e.g., a full mesh or a hub & spoke topology). How
is this achieved?
This section looks at some of these generic problems and at some of
the mechanisms that can be used to solve them.
4.2.1. VPN Discovery
Mechanisms are needed to acquire information that allows the
establishment and maintenance of VPNs. This may include, for
example, information on VPN membership, topology, and VPN device
capabilities. This information may be statically configured, or
distributed by an automated protocol. As a result of the operation
of these mechanisms and protocols, a device is able to determine
where to set up tunnels, and where to advertise the VPN routes for
each VPN.
With a physical network, the equivalent problem can by solved by the
control of the physical interconnection of links, and by having a
router run a discovery/hello protocol over its locally connected
links. With VPNs both the routers and the links (tunnels) may be
logical entities, and thus some other mechanisms are needed.
A number of different approaches are possible for VPN discovery. One
scheme uses the network management system to configure and provision
the VPN edge devices. This approach can also be used to distribute
VPN discovery information, either using proprietary protocols or
using standard management protocols and MIBs. Another approach is
where the VPN edge devices act as clients of a centralized directory
or database server that contains VPN discovery information. Another
possibility is where VPN discovery information is piggybacked onto a
routing protocol running between the VPN edge devices [VPN-DISC].
4.2.1.1. Network Management for Membership Information
SPs use network management extensively to configure and monitor the
various devices that are spread throughout their networks. This
approach could be also used for distributing VPN related information.
A network management system (either centralized or distributed) could
be used by the SP to configure and provision VPNs on the VPN edge
devices at various locations. VPN configuration information could be
entered into a network management application and distributed to the
remote sites via the same means used to distribute other network
management information. This approach is most natural when all the
devices that must be provisioned are within a single SP's network,
since the SP has access to all VPN edge devices in its domain. Security and access control are important, and could be achieved for example using SNMPv3, SSH, or IPsec tunnels.4.2.1.2. Directory Servers
An SP typically needs to maintain a database of VPN configuration/membership information, regardless of the mechanisms used to distribute it. LDAPv3 [RFC3377] is a standard directory protocol which makes it possible to use a common mechanism for both storing such information and distributing it. To facilitate interoperability between different implementations, as well as between the management systems of different SPs, a standard schema for representing VPN membership and configuration information would have to be developed. LDAPv3 supports authentication of messages and associated access control, which can be used to limit access to VPN information to authorized entities.4.2.1.3. Augmented Routing for Membership Information
Extensions to the use of existing BGP mechanisms, for distribution of VPN membership information, are proposed in [VPN-2547BIS]. In that scheme, BGP is used to distribute VPN routes, and each route carries a set of attributes which indicate the VPN (or VPNs) to which the route belongs. This allows the VPN discovery information and routing information to be combined in a single protocol. Information needed to establish per-VPN tunnels can also be carried as attributes of the routes. This makes use of the BGP protocol's ability to effectively carry large amounts of routing information. It is also possible to use BGP to distribute just the membership/capability information, while using a different technique to distribute the routing. BGP's update message would be used to indicate that a PE is attached to a particular VPN; BGP's withdraw message would be used to indicate that a PE has ceased to be attached to a particular VPN. This makes use of the BGP protocol's ability to dynamically distribute real-time changes in a reliable and fairly rapid manner. In addition, if a BGP route reflector is used, PEs never have to be provisioned with each other's IP addresses at all. Both cases make use of BGP's mechanisms, such as route filters, for constraining the distribution of information. Augmented routing may be done in combination with aggregated routing, as discussed in section 4.4.4. Of course, when using BGP for distributing any kind of VPN-specific information, one must ensure
that one is not disrupting the classical use of BGP for distributing public Internet routing information. For further discussion of this, see the discussion of aggregated routing, section 4.4.4.4.2.1.4. VPN Discovery for Inter-SP VPNs
When two sites of a VPN are connected to different SP networks, the SPs must support a common mechanism for exchanging membership/capability information. This might make use of manual configuration or automated exchange of information between the SPs. Automated exchange may be facilitated if one or more mechanisms for VPN discovery are standardized and supported across the multiple SPs. Inter-SP trust relationships will need to be established, for example to determine which information and how much information about the VPNs may be exchanged between SPs. In some cases different service providers may deploy different approaches for VPN discovery. Where this occurs, this implies that for multi-SP VPNs, some manual coordination and configuration may be necessary. The amount of information which needs to be shared between SPs may vary greatly depending upon the number of size of the multi-SP VPNs. The SPs will therefore need to determine and agree upon the expected amount of membership information to be exchanged, and the dynamic nature of this information. Mechanisms may also be needed to authenticate the VPN membership information. VPN information should be distributed only to places where it needs to go, whether that is intra-provider or inter-provider. In this way, the distribution of VPN information is unlike the distribution of inter-provider routing information, as the latter needs to be distributed throughout the Internet. In addition, the joint support of a VPN by two SPs should not require any third SP to maintain state for that VPN. Again, notice the difference with respect to inter-provider routing; in inter-provider routing: sending traffic from one SP to another may indeed require routing state in a third SP. As one possible example: Suppose that there are two SPs A and C, which want to support a common VPN. Suppose that A and C are interconnected via SP B. In this case B will need to know how to route traffic between A and C, and therefore will need to know something about A and C (such as enough routing information to forward IP traffic and/or connect MPLS LSPs between PEs or route reflectors in A and C). However, for scaling purposes it is desirable that B not need to know VPN-specific information about the VPNs which are supported by A and C.
4.2.2. Constraining Distribution of VPN Routing Information
In layer 3 provider-provisioned CE-based VPNs, the VPN tunnels connect CE devices. In this case, distribution of IP routing information occurs between CE devices on the customer sites. No additional constraints on the distribution of VPN routing information are necessary. In layer 3 PE-based VPNs, however, the PE devices must be aware of VPN routing information (for the VPNs to which they are attached). For scalability reasons, one does not want a scheme in which all PEs contain all routes for all VPNs. Rather, only the PEs that are attached to sites in a given VPN should contain the routing information for that VPN. This means that the distribution of VPN routing information between PE devices must be constrained. As VPN membership may change dynamically, it is necessary to have a mechanism that allows VPN route information to be distributed to any PE where there is an attached user for that VPN, and allows for the removal of this information when it is no longer needed. In the Virtual Router scheme, per-VPN tunnels must be established before any routes for a VPN are distributed, and the routes are then distributed through those tunnels. Thus by establishing the proper set of tunnels, one implicitly constrains and controls the distribution of per-VPN routing information. In this scheme, the distribution of membership information consists of the set of VPNs that exists on each PE, as well as information about the desired topology. This enables a PE to determine the set of remote PEs to which it must establish tunnels for a particular VPN. In the aggregated routing scheme (see section 4.4.4), the distribution of VPN routing information is constrained by means of route filtering. As VPN membership changes on a PE, the route filters in use between the PE and its peers can be adjusted. Each peer may then adjust the filters in use with each of its peers in turn, and thus the changes propagate across the network. When BGP is used, this filtering may take place at route reflectors as discussed in section 4.4.4.4.2.3. Controlling VPN Topology
The topology for a VPN consists of a set of nodes interconnected via tunnels. The topology may be a full mesh, a hub and spoke topology, or an arbitrary topology. For a VPN the set of nodes will include all VPN edge devices that have attached sites for that VPN. Naturally, whatever the topology, all VPN sites are reachable from each other; the topology simply constrains the way traffic is routed
among the sites. For example, in one topology traffic between site A and site B goes from one to the other directly over the VPN backbone; in another topology, traffic from site A to site B must traverse site C before reaching site B. The simplest topology is a full mesh, where a tunnel exists between every pair of VPN edge devices. If we assume the use of point-to- point tunnels (rather than multipoint-to-point), then with a full mesh topology there are N*(N-1)/2 duplex tunnels or N*(N-1) simplex tunnels for N VPN edge devices. Each tunnel consumes some resources at a VPN edge device, and depending on the type of tunnel, may or may not consume resources in intermediate routers or LSRs. One reason for using a partial mesh topology is to reduce the number of tunnels a VPN edge device, and/or the network, needs to support. Another reason is to support the scenario where an administrator requires all traffic from certain sites to traverse some particular site for policy or control reasons, such as to force traffic through a firewall, or for monitoring or accounting purposes. Note that the topologies used for each VPN are separate, and thus the same VPN edge device may be part of a full mesh topology for one VPN, and of a partial mesh topology for another VPN. An example of where a partial mesh topology could be suitable is for a VPN that supports a large number of telecommuters and a small number of corporate sites. Most traffic will be between telecommuters and the corporate sites, not between pairs of telecommuters. A hub and spoke topology for the VPN would thus map onto the underlying traffic flow, with the telecommuters attached to spoke VPN edge devices and the corporate sites attached to hub VPN edge devices. Traffic between telecommuters is still supported, but this traffic traverses a hub VPN edge device. The selection of a topology for a VPN is an administrative choice, but it is useful to examine protocol mechanisms that can be used to automate the construction of the desired topology, and thus reduce the amount of configuration needed. To this end it is useful for a VPN edge device to be able to advertise per-VPN topology information to other VPN edge devices. It may be simplest to advertise this at the same time as the membership information is advertised, using the same mechanisms. A simple scheme is where a VPN edge device advertises itself either as a hub or as a spoke, for each VPN that it has. When received by other VPN edge devices this information can be used when determining whether to establish a tunnel. A more comprehensive scheme allows a VPN edge device to advertise a set of topology groups, with tunnels established between a pair of VPN edge devices if they have a group in common.