RFC 2764
A Framework for IP Based Virtual Private Networks
4.0 VPN Types: Virtual Leased Lines
The simplest form of a VPN is a 'Virtual Leased Line' (VLL) service. In this case a point-to-point link is provided to a customer, connecting two CPE devices, as illustrated below. The link layer type used to connect the CPE devices to the ISP nodes can be any link layer type, for example an ATM VCC or a Frame Relay circuit. The CPE devices can be either routers bridges or hosts.
The two ISP nodes are both connected to an IP network, and an IP
tunnel is set up between them. Each ISP node is configured to bind
the stub link and the IP tunnel together at layer 2 (e.g., an ATM VCC
and the IP tunnel). Frames are relayed between the two links. For
example the ATM Adaptation Layer 5 (AAL5) payload is taken and
encapsulated in an IPSec tunnel, and vice versa. The contents of the
AAL5 payload are opaque to the ISP node, and are not examined there.
+--------+ ----------- +--------+
+---+ | ISP | ( IP ) | ISP | +---+
|CPE|-------| edge |-----( backbone ) -----| edge |------|CPE|
+---+ ATM | node | ( ) | node | ATM +---+
VCC +--------+ ----------- +--------+ VCC
<--------- IP Tunnel -------->
10.1.1.5 subnet = 10.1.1.4/30 10.1.1.6
Addressing used by customer (transparent to provider)
Figure 4.1: VLL Example
To a customer it looks the same as if a single ATM VCC or Frame Relay
circuit were used to interconnect the CPE devices, and the customer
could be unaware that part of the circuit was in fact implemented
over an IP backbone. This may be useful, for example, if a provider
wishes to provide a LAN interconnect service using ATM as the network
interface, but does not have an ATM network that directly
interconnects all possible customer sites.
It is not necessary that the two links used to connect the CPE
devices to the ISP nodes be of the same media type, but in this case
the ISP nodes cannot treat the traffic in an opaque manner, as
described above. Instead the ISP nodes must perform the functions of
an interworking device between the two media types (e.g., ATM and
Frame Relay), and perform functions such as LLC/SNAP to NLPID
conversion, mapping between ARP protocol variants and performing any
media specific processing that may be expected by the CPE devices
(e.g., ATM OAM cell handling or Frame Relay XID exchanges).
The IP tunneling protocol used must support multiprotocol operation
and may need to support sequencing, if that characteristic is
important to the customer traffic. If the tunnels are established
using a signalling protocol, they may be set up in a data driven
manner, when a frame is received from a customer link and no tunnel
exists, or the tunnels may be established at provisioning time and
kept up permanently.
Note that the use of the term 'VLL' in this document is different to that used in the definition of the Diffserv Expedited Forwarding Per Hop Behaviour (EF-PHB) [30]. In that document a VLL is used to mean a low latency, low jitter, assured bandwidth path, which can be provided using the described PHB. Thus the focus there is primarily on link characteristics that are temporal in nature. In this document the term VLL does not imply the use of any specific QoS mechanism, Diffserv or otherwise. Instead the focus is primarily on link characteristics that are more topological in nature, (e.g., such as constructing a link which includes an IP tunnel as one segment of the link). For a truly complete emulation of a link layer both the temporal and topological aspects need to be taken into account.5.0 VPN Types: Virtual Private Routed Networks
5.1 VPRN Characteristics
A Virtual Private Routed Network (VPRN) is defined to be the emulation of a multi-site wide area routed network using IP facilities. This section looks at how a network-based VPRN service can be provided. CPE-based VPRNs are also possible, but are not specifically discussed here. With network-based VPRNs many of the issues that need to be addressed are concerned with configuration and operational issues, which must take into account the split in administrative responsibility between the service provider and the service user. The distinguishing characteristic of a VPRN, in comparison to other types of VPNs, is that packet forwarding is carried out at the network layer. A VPRN consists of a mesh of IP tunnels between ISP routers, together with the routing capabilities needed to forward traffic received at each VPRN node to the appropriate destination site. Attached to the ISP routers are CPE routers connected via one or more links, termed 'stub' links. There is a VPRN specific forwarding table at each ISP router to which members of the VPRN are connected. Traffic is forwarded between ISP routers, and between ISP routers and customer sites, using these forwarding tables, which contain network layer reachability information (in contrast to a Virtual Private LAN Segment type of VPN (VPLS) where the forwarding tables contain MAC layer reachability information - see section 7.0). An example VPRN is illustrated in the following diagram, which shows 3 ISP edge routers connected via a full mesh of IP tunnels, used to interconnect 4 CPE routers. One of the CPE routers is multihomed to the ISP network. In the multihomed case, all stub links may be active, or, as shown, there may be one primary and one or more backup links to be used in case of failure of the primary. The term ' backdoor' link is used to refer to a link between two customer sites
that does not traverse the ISP network.
10.1.1.0/30 +--------+ +--------+ 10.2.2.0/30
+---+ | ISP | IP tunnel | ISP | +---+
|CPE|-------| edge |<--------------------->| edge |-------|CPE|
+---+ stub | router | 10.9.9.4/30 | router | stub +---+
link +--------+ +--------+ link :
| ^ | | ^ :
| | | --------------- | | :
| | +----( )----+ | :
| | ( IP BACKBONE ) | :
| | ( ) | :
| | --------------- | :
| | | | :
| |IP tunnel +--------+ IP tunnel| :
| | | ISP | | :
| +---------->| edge |<----------+ :
| 10.9.9.8/30 | router | 10.9.9.12/30 :
backup| +--------+ backdoor:
link | | | link :
| stub link | | stub link :
| | | :
| +---+ +---+ :
+-------------|CPE| |CPE|.......................:
10.3.3.0/30 +---+ +---+ 10.4.4.0/30
Figure 5.1: VPRN Example
The principal benefit of a VPRN is that the complexity and the
configuration of the CPE routers is minimized. To a CPE router, the
ISP edge router appears as a neighbor router in the customer's
network, to which it sends all traffic, using a default route. The
tunnel mesh that is set up to transfer traffic extends between the
ISP edge routers, not the CPE routers. In effect the burden of
tunnel establishment and maintenance and routing configuration is
outsourced to the ISP. In addition other services needed for the
operation of a VPN such as the provision of a firewall and QoS
processing can be handled by a small number of ISP edge routers,
rather than a large number of potentially heterogeneous CPE devices.
The introduction and management of new services can also be more
easily handled, as this can be achieved without the need to upgrade
any CPE equipment. This latter benefit is particularly important
when there may be large numbers of residential subscribers using VPN
services to access private corporate networks. In this respect the
model is somewhat akin to that used for telephony services, whereby
new services (e.g., call waiting) can be introduced with no change in
subscriber equipment.
The VPRN type of VPN is in contrast to one where the tunnel mesh extends to the CPE routers, and where the ISP network provides layer 2 connectivity alone. The latter case can be implemented either as a set of VLLs between CPE routers (see section 4.0), in which case the ISP network provides a set of layer 2 point-to-point links, or as a VPLS (see section 7.0), in which case the ISP network is used to emulate a multiaccess LAN segment. With these scenarios a customer may have more flexibility (e.g., any IGP or any protocol can be run across all customer sites) but this usually comes at the expense of a more complex configuration for the customer. Thus, depending on customer requirements, a VPRN or a VPLS may be the more appropriate solution. Because a VPRN carries out forwarding at the network layer, a single VPRN only directly supports a single network layer protocol. For multiprotocol support, a separate VPRN for each network layer protocol could be used, or one protocol could be tunneled over another (e.g., non-IP protocols tunneled over an IP VPRN) or alternatively the ISP network could be used to provide layer 2 connectivity only, such as with a VPLS as mentioned above. The issues to be addressed for VPRNs include initial configuration, determination by an ISP edge router of the set of links that are in each VPRN, the set of other routers that have members in the VPRN, and the set of IP address prefixes reachable via each stub link, determination by a CPE router of the set of IP address prefixes to be forwarded to an ISP edge router, the mechanism used to disseminate stub reachability information to the correct set of ISP routers, and the establishment and use of the tunnels used to carry the data traffic. Note also that, although discussed first for VPRNs, many of these issues also apply to the VPLS scenario described later, with the network layer addresses being replaced by link layer addresses. Note that VPRN operation is decoupled from the mechanisms used by the customer sites to access the Internet. A typical scenario would be for the ISP edge router to be used to provide both VPRN and Internet connectivity to a customer site. In this case the CPE router just has a default route pointing to the ISP edge router, with the latter being responsible for steering private traffic to the VPRN and other traffic to the Internet, and providing firewall functionality between the two domains. Alternatively a customer site could have Internet connectivity via an ISP router not involved in the VPRN, or even via a different ISP. In this case the CPE device is responsible for splitting the traffic into the two domains and providing firewall functionality.
5.1.1 Topology
The topology of a VPRN may consist of a full mesh of tunnels between each VPRN node, or may be an arbitrary topology, such as a set of remote offices connected to the nearest regional site, with these regional sites connected together via a full or partial mesh. With VPRNs using IP tunnels there is much less cost assumed with full meshing than in cases where physical resources (e.g., a leased line) must be allocated for each connected pair of sites, or where the tunneling method requires resources to be allocated in the devices used to interconnect the edge routers (e.g., Frame Relay DLCIs). A full mesh topology yields optimal routing, since it precludes the need for traffic between two sites to traverse a third. Another attraction of a full mesh is that there is no need to configure topology information for the VPRN. Instead, given the member routers of a VPRN, the topology is implicit. If the number of ISP edge routers in a VPRN is very large, however, a full mesh topology may not be appropriate, due to the scaling issues involved, for example, the growth in the number of tunnels needed between sites, (which for n sites is n(n-1)/2), or the number of routing peers per router. Network policy may also lead to non full mesh topologies, for example an administrator may wish to set up the topology so that traffic between two remote sites passes through a central site, rather than go directly between the remote sites. It is also necessary to deal with the scenario where there is only partial connectivity across the IP backbone under certain error conditions (e.g. A can reach B, and B can reach C, but A cannot reach C directly), which can occur if policy routing is being used. For a network-based VPRN, it is assumed that each customer site CPE router connects to an ISP edge router through one or more point-to- point stub links (e.g. leased lines, ATM or Frame Relay connections). The ISP routers are responsible for learning and disseminating reachability information amongst themselves. The CPE routers must learn the set of destinations reachable via each stub link, though this may be as simple as a default route. The stub links may either be dedicated links, set up via provisioning, or may be dynamic links set up on demand, for example using PPP, voluntary tunneling (see section 6.3), or ATM signalling. With dynamic links it is necessary to authenticate the subscriber, and determine the authorized resources that the subscriber can access (e.g. which VPRNs the subscriber may join). Other than the way the subscriber is initially bound to the VPRN, (and this process may involve extra considerations such as dynamic IP address assignment), the subsequent VPRN mechanisms and services can be used for both types of subscribers in the same way.
5.1.2 Addressing
The addressing used within a VPRN may have no relation to the addressing used on the IP backbone over which the VPRN is instantiated. In particular non-unique private IP addressing may be used [4]. Multiple VPRNs may be instantiated over the same set of physical devices, and they may use the same or overlapping address spaces.5.1.3 Forwarding
For a VPRN the tunnel mesh forms an overlay network operating over an IP backbone. Within each of the ISP edge routers there must be VPN specific forwarding state to forward packets received from stub links ('ingress traffic') to the appropriate next hop router, and to forward packets received from the core ('egress traffic') to the appropriate stub link. For cases where an ISP edge router supports multiple stub links belonging to the same VPRN, the tunnels can, as a local matter, either terminate on the edge router, or on a stub link. In the former case a VPN specific forwarding table is needed for egress traffic, in the latter case it is not. A VPN specific forwarding table is generally needed in the ingress direction, in order to direct traffic received on a stub link onto the correct IP tunnel towards the core. Also since a VPRN operates at the internetwork layer, the IP packets sent over a tunnel will have their Time to Live (TTL) field decremented in the normal manner, preventing packets circulating indefinitely in the event of a routing loop within the VPRN.5.1.4 Multiple concurrent VPRN connectivity
Note also that a single customer site may belong concurrently to multiple VPRNs and may want to transmit traffic both onto one or more VPRNs and to the default Internet, over the same stub link. There are a number of possible approaches to this problem, but these are outside the scope of this document.5.2 VPRN Related Work
VPRN requirements and mechanisms have been discussed previously in a number of different documents. One of the first was [10], which showed how the same VPN functionality can be implemented over both MPLS and non-MPLS networks. Some others are briefly discussed below. There are two main variants as regards the mechanisms used to provide VPRN membership and reachability functionality, - overlay and piggybacking. These are discussed in greater detail in sections
5.3.2, 5.3.3 and 5.3.4 below. An example of the overlay model is described in [14], which discusses the provision of VPRN functionality by means of a separate per-VPN routing protocol instance and route and forwarding table instantiation, otherwise known as virtual routing. Each VPN routing instance is isolated from any other VPN routing instance, and from the routing used across the backbone. As a result any routing protocol (e.g. OSPF, RIP2, IS-IS) can be run with any VPRN, independently of the routing protocols used in other VPRNs, or in the backbone itself. The VPN model described in [12] is also an overlay VPRN model using virtual routing. That document is specifically geared towards the provision of VPRN functionality over MPLS backbones, and it describes how VPRN membership dissemination can be automated over an MPLS backbone, by performing VPN neighbor discovery over the base MPLS tunnel mesh. [31] extends the virtual routing model to include VPN areas, and VPN border routers which route between VPN areas. VPN areas may be defined for administrative or technical reasons, such as different underlying network infrastructures (e.g. ATM, MPLS, IP). In contrast [15] describes the provision of VPN functionality using a piggybacking approach for membership and reachability dissemination, with this information being piggybacked in Border Gateway Protocol 4 (BGP) [32] packets. VPNs are constructed using BGP policies, which are used to control which sites can communicate with each other. [13] also uses BGP for piggybacking membership information, and piggybacks reachability information on the protocol used to establish MPLS LSPs (CR-LDP or extended RSVP). Unlike the other proposals, however, this proposal requires the participation on the CPE router to implement the VPN functionality.5.3 VPRN Generic Requirements
There are a number of common requirements which any network-based VPRN solution must address, and there are a number of different mechanisms that can be used to meet these requirements. These generic issues are 1) The use of a globally unique VPN identifier in order to be able to refer to a particular VPN. 2) VPRN membership determination. An edge router must learn of the local stub links that are in each VPRN, and must learn of the set of other routers that have members in that VPRN. 3) Stub link reachability information. An edge router must learn the set of addresses and address prefixes reachable via each stub link.
4) Intra-VPRN reachability information. Once an edge router has
determined the set of address prefixes associated with each of its
stub links, then this information must be disseminated to each
other edge router in the VPRN.
5) Tunneling mechanism. An edge router must construct the necessary
tunnels to other routers that have members in the VPRN, and must
perform the encapsulation and decapsulation necessary to send and
receive packets over the tunnels.
5.3.1 VPN Identifier
The IETF [16] and the ATM Forum [17] have standardized on a single
format for a globally unique identifier used to identify a VPN - a
VPN-ID. Only the format of the VPN-ID has been defined, not its
semantics or usage. The aim is to allow its use for a wide variety
of purposes, and to allow the same identifier to used with different
technologies and mechanisms. For example a VPN-ID can be included in
a MIB to identify a VPN for management purposes. A VPN-ID can be
used in a control plane protocol, for example to bind a tunnel to a
VPN at tunnel establishment time. All packets that traverse the
tunnel are then implicitly associated with the identified VPN. A
VPN-ID can be used in a data plane encapsulation, to allow for an
explicit per-packet identification of the VPN associated with the
packet. If a VPN is implemented using different technologies (e.g.,
IP and ATM) in a network, the same identifier can be used to identify
the VPN across the different technologies. Also if a VPN spans
multiple administrative domains the same identifier can be used
everywhere.
Most of the VPN schemes developed (e.g. [11], [12], [13], [14])
require the use of a VPN-ID that is carried in control and/or data
packets, which is used to associate the packet with a particular VPN.
Although the use of a VPN-ID in this manner is very common, it is not
universal. [15] describes a scheme where there is no protocol field
used to identify a VPN in this manner. In this scheme the VPNs as
understood by a user, are administrative constructs, built using BGP
policies. There are a number of attributes associated with VPN
routes, such as a route distinguisher, and origin and target "VPN",
that are used by the underlying protocol mechanisms for
disambiguation and scoping, and these are also used by the BGP policy
mechanism in the construction of VPNs, but there is nothing
corresponding with the VPN-ID as used in the other documents.
Note also that [33] defines a multiprotocol encapsulation for use
over ATM AAL5 that uses the standard VPN-ID format.
5.3.2 VPN Membership Information Configuration and Dissemination
In order to establish a VPRN, or to insert new customer sites into an established VPRN, an ISP edge router must determine which stub links are associated with which VPRN. For static links (e.g. an ATM VCC) this information must be configured into the edge router, since the edge router cannot infer such bindings by itself. An SNMP MIB allowing for bindings between local stub links and VPN identities is one solution. For subscribers that attach to the network dynamically (e.g. using PPP or voluntary tunneling) it is possible to make the association between stub link and VPRN as part of the end user authentication processing that must occur with such dynamic links. For example the VPRN to which a user is to be bound may be derived from the domain name the used as part of PPP authentication. If the user is successfully authenticated (e.g. using a Radius server), then the newly created dynamic link can be bound to the correct VPRN. Note that static configuration information is still needed, for example to maintain the list of authorized subscribers for each VPRN, but the location of this static information could be an external authentication server rather than on an ISP edge router. Whether the link was statically or dynamically created, a VPN-ID can be associated with that link to signify to which VPRN it is bound. After learning which stub links are bound to which VPRN, each edge router must learn either the identity of, or, at least, the route to, each other edge router supporting other stub links in that particular VPRN. Implicit in the latter is the notion that there exists some mechanism by which the configured edge routers can then use this edge router and/or stub link identity information to subsequently set up the appropriate tunnels between them. The problem of VPRN member dissemination between participating edge routers, can be solved in a variety of ways, discussed below.5.3.2.1 Directory Lookup
The members of a particular VPRN, that is, the identity of the edge routers supporting stub links in the VPRN, and the set of static stub links bound to the VPRN per edge router, could be configured into a directory, which edge routers could query, using some defined mechanism (e.g. Lightweight Directory Access Protocol (LDAP) [34]), upon startup. Using a directory allows either a full mesh topology or an arbitrary topology to be configured. For a full mesh, the full list of member routers in a VPRN is distributed everywhere. For an arbitrary topology, different routers may receive different member lists.
Using a directory allows for authorization checking prior to disseminating VPRN membership information, which may be desirable where VPRNs span multiple administrative domains. In such a case, directory to directory protocol mechanisms could also be used to propagate authorized VPRN membership information between the directory systems of the multiple administrative domains. There also needs to be some form of database synchronization mechanism (e.g. triggered or regular polling of the directory by edge routers, or active pushing of update information to the edge routers by the directory) in order for all edge routers to learn the identity of newly configured sites inserted into an active VPRN, and also to learn of sites removed from a VPRN.5.3.2.2 Explicit Management Configuration
A VPRN MIB could be defined which would allow a central management system to configure each edge router with the identities of each other participating edge router and the identity of each of the static stub links bound to the VPRN. Like the use of a directory, this mechanism allows both full mesh and arbitrary topologies to be configured. Another mechanism using a centralized management system is to use a policy server and use the Common Open Policy Service (COPS) protocol [35] to distribute VPRN membership and policy information, such as the tunnel attributes to use when establishing a tunnel, as described in [36]. Note that this mechanism allows the management station to impose strict authorization control; on the other hand, it may be more difficult to configure edge routers outside the scope of the management system. The management configuration model can also be considered a subset of the directory method, in that the management directories could use MIBs to push VPRN membership information to the participating edge routers, either subsequent to, or as part of, the local stub link configuration process.5.3.2.3 Piggybacking in Routing Protocols
VPRN membership information could be piggybacked into the routing protocols run by each edge router across the IP backbone, since this is an efficient means of automatically propagating information throughout the network to other participating edge routers. Specifically, each route advertisement by each edge router could include, at a minimum, the set of VPN identifiers associated with each edge router, and adequate information to allow other edge routers to determine the identity of, and/or, the route to, the particular edge router. Other edge routers would examine received route advertisements to determine if any contained information was
relevant to a supported (i.e., configured) VPRN; this determination could be done by looking for a VPN identifier matching a locally configured VPN. The nature of the piggybacked information, and related issues, such as scoping, and the means by which the nodes advertising particular VPN memberships will be identified, will generally be a function both of the routing protocol and of the nature of the underlying transport. Using this method all the routers in the network will have the same view of the VPRN membership information, and so a full mesh topology is easily supported. Supporting an arbitrary topology is more difficult, however, since some form of pruning would seem to be needed. The advantage of the piggybacking scheme is that it allows for efficient information dissemination, but it does require that all nodes in the path, and not just the participating edge routers, be able to accept such modified route advertisements. A disadvantage is that significant administrative complexity may be required to configure scoping mechanisms so as to both permit and constrain the dissemination of the piggybacked advertisements, and in itself this may be quite a configuration burden, particularly if the VPRN spans multiple routing domains (e.g. different autonomous systems / ISPs). Furthermore, unless some security mechanism is used for routing updates so as to permit only all relevant edge routers to read the piggybacked advertisements, this scheme generally implies a trust model where all routers in the path must perforce be authorized to know this information. Depending upon the nature of the routing protocol, piggybacking may also require intermediate routers, particularly autonomous system (AS) border routers, to cache such advertisements and potentially also re-distribute them between multiple routing protocols. Each of the schemes described above have merit in particular situations. Note that, in practice, there will almost always be some centralized directory or management system which will maintain VPRN membership information, such as the set of edge routers that are allowed to support a certain VPRN, the bindings of static stub links to VPRNs, or authentication and authorization information for users that access the network via dynamics links. This information needs to be configured and stored in some form of database, so that the additional steps needed to facilitate the configuration of such information into edge routers, and/or, facilitate edge router access to such information, may not be excessively onerous.
5.3.3 Stub Link Reachability Information
There are two aspects to stub site reachability - the means by which VPRN edge routers determine the set of VPRN addresses and address prefixes reachable at each stub site, and the means by which the CPE routers learn the destinations reachable via each stub link. A number of common scenarios are outlined below. In each case the information needed by the ISP edge router is the same - the set of VPRN addresses reachable at the customer site, but the information needed by the CPE router differs.5.3.3.1 Stub Link Connectivity Scenarios
5.3.3.1.1 Dual VPRN and Internet Connectivity
The CPE router is connected via one link to an ISP edge router, which provides both VPRN and Internet connectivity. This is the simplest case for the CPE router, as it just needs a default route pointing to the ISP edge router.5.3.3.1.2 VPRN Connectivity Only
The CPE router is connected via one link to an ISP edge router, which provides VPRN, but not Internet, connectivity. The CPE router must know the set of non-local VPRN destinations reachable via that link. This may be a single prefix, or may be a number of disjoint prefixes. The CPE router may be either statically configured with this information, or may learn it dynamically by running an instance of an Interior Gateway Protocol (IGP). For simplicity it is assumed that the IGP used for this purpose is RIP, though it could be any IGP. The ISP edge router will inject into this instance of RIP the VRPN routes which it learns by means of one of the intra-VPRN reachability mechanisms described in section 5.3.4. Note that the instance of RIP run to the CPE, and any instance of a routing protocol used to learn intra-VPRN reachability (even if also RIP) are separate, with the ISP edge router redistributing the routes from one instance to another.
5.3.3.1.3 Multihomed Connectivity
The CPE router is multihomed to the ISP network, which provides VPRN connectivity. In this case all the ISP edge routers could advertise the same VPRN routes to the CPE router, which then sees all VPRN prefixes equally reachable via all links. More specific route redistribution is also possible, whereby each ISP edge router advertises a different set of prefixes to the CPE router.5.3.3.1.4 Backdoor Links
The CPE router is connected to the ISP network, which provides VPRN connectivity, but also has a backdoor link to another customer site In this case the ISP edge router will advertise VPRN routes as in case 2 to the CPE device. However now the same destination is reachable via both the ISP edge router and via the backdoor link. If the CPE routers connected to the backdoor link are running the customer's IGP, then the backdoor link may always be the favored link as it will appear an an 'internal' path, whereas the destination as injected via the ISP edge router will appear as an 'external' path (to the customer's IGP). To avoid this problem, assuming that the customer wants the traffic to traverse the ISP network, then a separate instance of RIP should be run between the CPE routers at both ends of the backdoor link, in the same manner as an instance of RIP is run on a stub or backup link between a CPE router and an ISP edge router. This will then also make the backdoor link appear as an external path, and by adjusting the link costs appropriately, the ISP path can always be favored, unless it goes down, when the backdoor link is then used. The description of the above scenarios covers what reachability information is needed by the ISP edge routers and the CPE routers, and discusses some of the mechanisms used to convey this information. The sections below look at these mechanisms in more detail.5.3.3.1 Routing Protocol Instance
A routing protocol can be run between the CPE edge router and the ISP edge router to exchange reachability information. This allows an ISP edge router to learn the VPRN prefixes reachable at a customer site, and also allows a CPE router to learn the destinations reachable via the provider network.
The extent of the routing domain for this protocol instance is generally just the ISP edge router and the CPE router although if the customer site is also running the same protocol as its IGP, then the domain may extend into customer site. If the customer site is running a different routing protocol then the CPE router redistributes the routes between the instance running to the ISP edge router, and the instance running into the customer site. Given the typically restricted scope of this routing instance, a simple protocol will generally suffice. RIP is likely to be the most common protocol used, though any routing protocol, such as OSPF, or BGP run in internal mode (IBGP), could also be used. Note that the instance of the stub link routing protocol is different from any instance of a routing protocol used for intra-VPRN reachability. For example, if the ISP edge router uses routing protocol piggybacking to disseminate VPRN membership and reachability information across the core, then it may redistribute suitably labeled routes from the CPE routing instance to the core routing instance. The routing protocols used for each instance are decoupled, and any suitable protocol can be used in each case. There is no requirement that the same protocol, or even the same stub link reachability information gathering mechanism, be run between each CPE router and associated ISP edge router in a particular VPRN, since this is a purely local matter. This decoupling allows ISPs to deploy a common (across all VPRNs) intra-VPRN reachability mechanism, and a common stub link reachability mechanism, with these mechanisms isolated both from each other, and from the particular IGP used in a customer network. In the first case, due to the IGP-IGP boundary implemented on the ISP edge router, the ISP can insulate the intra-VPRN reachability mechanism from misbehaving stub link protocol instances. In the second case the ISP is not required to be aware of the particular IGP running in a customer site. Other scenarios are possible, where the ISP edge routers are running a routing protocol in the same instance as the customer's IGP, but are unlikely to be practical, since it defeats the purpose of a VPRN simplifying CPE router configuration. In cases where a customer wishes to run an IGP across multiple sites, a VPLS solution is more suitable. Note that if a particular customer site concurrently belongs to multiple VPRNs (or wishes to concurrently communicate with both a VPRN and the Internet), then the ISP edge router must have some means of unambiguously mapping stub link address prefixes to particular VPRNs. A simple way is to have multiple stub links, one per VPRN. It is also possible to run multiple VPRNs over one stub link. This could be done either by ensuring (and appropriately configuring the
ISP edge router to know) that particular disjoint address prefixes are mapped into separate VPRNs, or by tagging the routing advertisements from the CPE router with the appropriate VPN identifier. For example if MPLS was being used to convey stub link reachability information, different MPLS labels would be used to differentiate the disjoint prefixes assigned to particular VPRNs. In any case, some administrative procedure would be required for this coordination.5.3.3.2 Configuration
The reachability information across each stub link could be manually configured, which may be appropriate if the set of addresses or prefixes is small and static.5.3.3.3 ISP Administered Addresses
The set of addresses used by each stub site could be administered and allocated via the VPRN edge router, which may be appropriate for small customer sites, typically containing either a single host, or a single subnet. Address allocation can be carried out using protocols such as PPP or DHCP [37], with, for example, the edge router acting as a Radius client and retrieving the customer's IP address to use from a Radius server, or acting as a DHCP relay and examining the DHCP reply message as it is relayed to the customer site. In this manner the edge router can build up a table of stub link reachability information. Although these address assignment mechanisms are typically used to assign an address to a single host, some vendors have added extensions whereby an address prefix can be assigned, with, in some cases, the CPE device acting as a "mini-DHCP" server and assigning addresses for the hosts in the customer site. Note that with these schemes it is the responsibility of the address allocation server to ensure that each site in the VPN received a disjoint address space. Note also that an ISP would typically only use this mechanism for small stub sites, which are unlikely to have backdoor links.5.3.3.4 MPLS Label Distribution Protocol
In cases where the CPE router runs MPLS, LDP can be used to convey the set of prefixes at a stub site to a VPRN edge router. Using the downstream unsolicited mode of label distribution the CPE router can distribute a label for each route in the stub site. Note however that the processing carried out by the edge router in this case is more than just the normal LDP processing, since it is learning new routes via LDP, rather than the usual case of learning labels for existing routes that it has learned via standard routing mechanisms.
5.3.4 Intra-VPN Reachability Information
Once an edge router has determined the set of prefixes associated with each of its stub links, then this information must be disseminated to each other edge router in the VPRN. Note also that there is an implicit requirement that the set of reachable addresses within the VPRN be locally unique that is, each VPRN stub link (not performing load sharing) maintain an address space disjoint from any other, so as to permit unambiguous routing. In practical terms, it is also generally desirable, though not required, that this address space be well partitioned i.e., specific, disjoint address prefixes per edge router, so as to preclude the need to maintain and disseminate large numbers of host routes. The problem of intra-VPN reachability information dissemination can be solved in a number of ways, some of which include the following:5.3.4.1 Directory Lookup
Along with VPRN membership information, a central directory could maintain a listing of the address prefixes associated with each customer site. Such information could be obtained by the server through protocol interactions with each edge router. Note that the same directory synchronization issues discussed above in section 5.3.2 also apply in this case.5.3.4.2 Explicit Configuration
The address spaces associated with each edge router could be explicitly configured into each other router. This is clearly a non-scalable solution, particularly when arbitrary topologies are used, and also raises the question of how the management system learns such information in the first place.5.3.4.3 Local Intra-VPRN Routing Instantiations
In this approach, each edge router runs an instance of a routing protocol (a 'virtual router') per VPRN, running across the VPRN tunnels to each peer edge router, to disseminate intra-VPRN reachability information. Both full-mesh and arbitrary VPRN topologies can be easily supported, since the routing protocol itself can run over any topology. The intra-VPRN routing advertisements could be distinguished from normal tunnel data packets either by being addressed directly to the peer edge router, or by a tunnel specific mechanism.
Note that this intra-VPRN routing protocol need have no relationship either with the IGP of any customer site or with the routing protocols operated by the ISPs in the IP backbone. Depending on the size and scale of the VPRNs to be supported either a simple protocol like RIP or a more sophisticated protocol like OSPF could be used. Because the intra-VPRN routing protocol operates as an overlay over the IP backbone it is wholly transparent to any intermediate routers, and to any edge routers not within the VPRN. This also implies that such routing information can remain opaque to such routers, which may be a necessary security requirements in some cases. Also note that if the routing protocol runs directly over the same tunnels as the data traffic, then it will inherit the same level of security as that afforded the data traffic, for example strong encryption and authentication. If the tunnels over which an intra-VPRN routing protocol runs are dedicated to a specific VPN (e.g. a different multiplexing field is used for each VPN) then no changes are needed to the routing protocol itself. On the other hand if shared tunnels are used, then it is necessary to extend the routing protocol to allow a VPN-ID field to be included in routing update packets, to allow sets of prefixes to be associated with a particular VPN.5.3.4.4 Link Reachability Protocol
By link reachability protocol is meant a protocol that allows two nodes, connected via a point-to-point link, to exchange reachability information. Given a full mesh topology, each edge router could run a link reachability protocol, for instance some variation of MPLS CR-LDP, across the tunnel to each peer edge router in the VPRN, carrying the VPN-ID and the reachability information of each VPRN running across the tunnel between the two edge routers. If VPRN membership information has already been distributed to an edge router, then the neighbor discovery aspects of a traditional routing protocol are not needed, as the set of neighbors is already known. TCP connections can be used to interconnect the neighbors, to provide reliability. This approach may reduce the processing burden of running routing protocol instances per VPRN, and may be of particular benefit where a shared tunnel mechanism is used to connect a set of edge routers supporting multiple VPRNs. Another approach to developing a link reachability protocol would be to base it on IBGP. The problem that needs to be solved by a link reachability protocol is very similar to that solved by IBGP - conveying address prefixes reliably between edge routers.
Using a link reachability protocol it is straightforward to support a full mesh topology - each edge router conveys its own local reachability information to all other routers, but does not redistribute information received from any other router. However once an arbitrary topology needs to be supported, the link reachability protocol needs to develop into a full routing protocol, due to the need to implement mechanisms to avoid loops, and there would seem little benefit in reinventing another routing protocol to deal with this. Some reasons why partially connected meshes may be needed even in a tunneled environment are discussed in section 5.1.1.5.3.4.5 Piggybacking in IP Backbone Routing Protocols
As with VPRN membership, the set of address prefixes associated with each stub interface could also be piggybacked into the routing advertisements from each edge router and propagated through the network. Other edge routers extract this information from received route advertisements in the same way as they obtain the VPRN membership information (which, in this case, is implicit in the identification of the source of each route advertisement). Note that this scheme may require, depending upon the nature of the routing protocols involved, that intermediate routers, e.g. border routers, cache intra-VPRN routing information in order to propagate it further. This also has implications for the trust model, and for the level of security possible for intra-VPRN routing information. Note that in any of the cases discussed above, an edge router has the option of disseminating its stub link prefixes in a manner so as to permit tunneling from remote edge routers directly to the egress stub links. Alternatively, it could disseminate the information so as to associate all such prefixes with the edge router, rather than with specific stub links. In this case, the edge router would need to implement a VPN specific forwarding mechanism for egress traffic, to determine the correct egress stub link. The advantage of this is that it may significantly reduce the number of distinct tunnels or tunnel label information which need to be constructed and maintained. Note that this choice is purely a local manner and is not visible to remote edge routers.5.3.5 Tunneling Mechanisms
Once VPRN membership information has been disseminated, the tunnels comprising the VPRN core can be constructed. One approach to setting up the tunnel mesh is to use point-to-point IP tunnels, and the requirements and issues for such tunnels have been discussed in section 3.0. For example while tunnel establishment can be done through manual configuration, this is
clearly not likely to be a scalable solution, given the O(n^2) problem of meshed links. As such, tunnel set up should use some form of signalling protocol to allow two nodes to construct a tunnel to each other knowing only each other's identity. Another approach is to use the multipoint to point 'tunnels' provided by MPLS. As noted in [38], MPLS can be considered to be a form of IP tunneling, since the labels of MPLS packets allow for routing decisions to be decoupled from the addressing information of the packets themselves. MPLS label distribution mechanisms can be used to associate specific sets of MPLS labels with particular VPRN address prefixes supported on particular egress points (i.e., stub links of edge routers) and hence allow other edge routers to explicitly label and route traffic to particular VPRN stub links. One attraction of MPLS as a tunneling mechanism is that it may require less processing within each edge router than alternative tunneling mechanisms. This is a function of the fact that data security within a MPLS network is implicit in the explicit label binding, much as with a connection oriented network, such as Frame Relay. This may hence lessen customer concerns about data security and hence require less processor intensive security mechanisms (e.g., IPSec). However there are other potential security concerns with MPLS. There is no direct support for security features such as authentication, confidentiality, and non-repudiation and the trust model for MPLS means that intermediate routers, (which may belong to different administrative domains), through which membership and prefix reachability information is conveyed, must be trusted, not just the edge routers themselves.5.4 Multihomed Stub Routers
The discussion thus far has implicitly assumed that stub routers are connected to one and only one VPRN edge router. In general, this restriction should be capable of being relaxed without any change to VPRN operation, given general market interest in multihoming for reliability and other reasons. In particular, in cases where the stub router supports multiple redundant links, with only one operational at any given time, with the links connected either to the same VPRN edge router, or to two or more different VPRN edge routers, then the stub link reachability mechanisms will both discover the loss of an active link, and the activation of a backup link. In the former situation, the previously connected VPRN edge router will cease advertising reachability to the stub node, while the VPRN edge router with the now active link will begin advertising reachability, hence restoring connectivity.
An alternative scenario is where the stub node supports multiple active links, using some form of load sharing algorithm. In such a case, multiple VPRN edge routers may have active paths to the stub node, and may so advertise across the VPRN. This scenario should not cause any problem with reachability across the VPRN providing that the intra-VPRN reachability mechanism can accommodate multiple paths to the same prefix, and has the appropriate mechanisms to preclude looping - for instance, distance vector metrics associated with each advertised prefix.5.5 Multicast Support
Multicast and broadcast traffic can be supported across VPRNs either by edge replication or by native multicast support in the backbone. These two cases are discussed below.5.5.1 Edge Replication
This is where each VPRN edge router replicates multicast traffic for transmission across each link in the VPRN. Note that this is the same operation that would be performed by CPE routers terminating actual physical links or dedicated connections. As with CPE routers, multicast routing protocols could also be run on each VPRN edge router to determine the distribution tree for multicast traffic and hence reduce unnecessary flood traffic. This could be done by running instances of standard multicast routing protocols, e.g. Protocol Independent Multicast (PIM) [39] or Distance Vector Multicast Routing Protocol (DVMRP) [40], on and between each VPRN edge router, through the VPRN tunnels, in the same way that unicast routing protocols might be run at each VPRN edge router to determine intra-VPN unicast reachability, as discussed in section 5.3.4. Alternatively, if a link reachability protocol was run across the VPRN tunnels for intra-VPRN reachability, then this could also be augmented to allow VPRN edge routers to indicate both the particular multicast groups requested for reception at each edge node, and also the multicast sources at each edge site. In either case, there would need to be some mechanism to allow for the VPRN edge routers to determine which particular multicast groups were requested at each site and which sources were present at each site. How this could be done would, in general, be a function of the capabilities of the CPE stub routers at each site. If these run multicast routing protocols, then they can interact directly with the equivalent protocols at each VPRN edge router. If the CPE device does not run a multicast routing protocol, then in the absence of Internet Group Management Protocol (IGMP) proxying [41] the customer site would be limited to a single subnet connected to the VPRN edge router via a bridging device, as the scope of an IGMP message is
limited to a single subnet. However using IGMP-proxying the CPE router can engage in multicast forwarding without running a multicast routing protocol, in constrained topologies. On its interfaces into the customer site the CPE router performs the router functions of IGMP, and on its interface to the VPRN edge router it performs the host functions of IGMP.5.5.2 Native Multicast Support
This is where VPRN edge routers map intra-VPRN multicast traffic onto a native IP multicast distribution mechanism across the backbone. Note that intra-VPRN multicast has the same requirements for isolation from general backbone traffic as intra-VPRN unicast traffic. Currently the only IP tunneling mechanism that has native support for multicast is MPLS. On the other hand, while MPLS supports native transport of IP multicast packets, additional mechanisms would be needed to leverage these mechanisms for the support of intra-VPRN multicast. For instance, each VPRN router could prefix multicast group addresses within each VPRN with the VPN-ID of that VPRN and then redistribute these, essentially treating this VPN-ID/intra-VPRN multicast address tuple as a normal multicast address, within the backbone multicast routing protocols, as with the case of unicast reachability, as discussed previously. The MPLS multicast label distribution mechanisms could then be used to set up the appropriate multicast LSPs to interconnect those sites within each VPRN supporting particular multicast group addresses. Note, however, that this would require each of the intermediate LSRs to not only be aware of each intra-VPRN multicast group, but also to have the capability of interpreting these modified advertisements. Alternatively, mechanisms could be defined to map intra-VPRN multicast groups into backbone multicast groups. Other IP tunneling mechanisms do not have native multicast support. It may prove feasible to extend such tunneling mechanisms by allocating IP multicast group addresses to the VPRN as a whole and hence distributing intra-VPRN multicast traffic encapsulated within backbone multicast packets. Edge VPRN routers could filter out unwanted multicast groups. Alternatively, mechanisms could also be defined to allow for allocation of backbone multicast group addresses for particular intra-VPRN multicast groups, and to then utilize these, through backbone multicast protocols, as discussed above, to limit forwarding of intra-VPRN multicast traffic only to those nodes within the group.
A particular issue with the use of native multicast support is the provision of security for such multicast traffic. Unlike the case of edge replication, which inherits the security characteristics of the underlying tunnel, native multicast mechanisms will need to use some form of secure multicast mechanism. The development of architectures and solutions for secure multicast is an active research area, for example see [42] and [43]. The Secure Multicast Group (SMuG) of the IRTF has been set up to develop prototype solutions, which would then be passed to the IETF IPSec working group for standardization. However considerably more development is needed before scalable secure native multicast mechanisms can be generally deployed.5.6 Recommendations
The various proposals that have been developed to support some form of VPRN functionality can be broadly classified into two groups - those that utilize the router piggybacking approach for distributing VPN membership and/or reachability information ([13],[15]) and those that use the virtual routing approach ([12],[14]). In some cases the mechanisms described rely on the characteristics of a particular infrastructure (e.g. MPLS) rather than just IP. Within the context of the virtual routing approach it may be useful to develop a membership distribution protocol based on a directory or MIB. When combined with the protocol extensions for IP tunneling protocols outlined in section 3.2, this would then provide the basis for a complete set of protocols and mechanisms that support interoperable VPRNs that span multiple administrations over an IP backbone. Note that the other major pieces of functionality needed - the learning and distribution of customer reachability information, can be performed by instances of standard routing protocols, without the need for any protocol extensions. Also for the constrained case of a full mesh topology, the usefulness of developing a link reachability protocol could be examined, however the limitations and scalability issues associated with this topology may not make it worthwhile to develop something specific for this case, as standard routing will just work. Extending routing protocols to allow a VPN-ID to carried in routing update packets could also be examined, but is not necessary if VPN specific tunnels are used.
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