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