Abstract:
The present invention relates to providing virtual private network (VPN) services between two or more Autonomic Systems (AS). An aggregation edge router (ASBR) is provided in two or more autonomous systems (Asx,Asy). The aggregation edge routers are configured such that routing peering between the two or more autonomous systems is done between the aggregation edge routers in these autonomous systems. Each aggregation edge router is a routing peer of other edge routers (PE) in its own autonomous system. The Multi-Protocol Label Switching (MPLS) network is used between the different autonomous systems.

Description:
PRIORITY CLAIM 
     This is a U.S. national stage of application No. PCT/FI2006/050060, filed on 13 Feb. 2006. Priority is claimed on the following application(s): Country: Finland, Application No.: 20055066, Filed: 14 Feb. 2005, the content of which is/are incorporated here by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to virtual private network (VPN) services, and particularly to providing VPN services between two or more Autonomous Systems (AS). 
     Virtual Private Networks (VPN) are commonly used for connecting trusted parties or “sites” to each other over an untrusted (public) backbone network through a secure tunnel. Two sites have IP connectivity over the common backbone only if there is some VPN which contains them both. Two sites which have no VPN in common have no connectivity over that backbone. If all the sites in a VPN are owned by the same enterprise, the VPN may be thought of as a corporate “intranet”. If the various sites in a VPN are owned by different enterprises, the VPN may be thought of as an “extranet”. A site can be in more than one VPN; e.g., in an intranet and in several extranets. In general, when we use the term VPN, we will not be distinguishing between intranets and extranets. The owners of the sites are often referred to as the “customers”. The owners/operators of the backbone are often referred to as the “Service Providers” (SPs). The customers obtain “VPN service” from the SPs. A customer may be a single enterprise, a set of enterprises, an Internet Service Provider, an Application Service Provider, another SP which offers the same kind of VPN service to its own customers, etc. 
     Security and management considerations may render it advantageous to subdivide a large network into several network parts that need to know as little as possible of each other. This is especially important in the case where these network parts are managed by different service providers. These network parts are often referred to as Autonomous Systems (AS). The Autonomous System corresponds to an administrative domain, such as university, company, backbone network, etc. 
     The Internet Engineering Task Force (IETF) has defined a concept which allows Service Providers to offer Virtual Private Network (“VPN”) services to their customers. Each VPN site must contain one or more Customer Edge (CE) routers. Each CE router is attached, via some sort of attachment circuit, to one or more Provider Edge (PE) routers. CE routers are logically part of the customer&#39;s VPN, and PE and P routers are logically part of the SP&#39;s network. The attachment circuit over which a packet travels when going from CE to PE is known as that packet&#39;s “ingress attachment circuit”, and the PE as the packet&#39;s “ingress PE”. The attachment circuit over which a packet travels when going from PE to CE is known as that packet&#39;s “egress attachment circuit”, and the PE as the packet&#39;s “egress PE”. We will say that a PE router is attached to a particular VPN if it is attached to a CE router which is in a site of that VPN. Similarly, we will say that a PE router is attached to a particular site if it is attached to a CE router which is in that site. When the CE router is a router, it is a routing peer of the PE(s) to which it is attached, but it is not a routing peer of CE routers at other sites. Routers at different sites do not directly exchange routing information with each other; in fact, they do not even need to know of each other at all. As a consequence, the customer has no backbone or “virtual backbone” to manage, and does not have to deal with any inter-site routing issues. Each PE router maintains a number of separate forwarding tables, including VPN Routing and Forwarding tables (VRFs). Every PE-CE attachment circuit is associated, by configuration, with one or more VRFs. An attachment circuit which is associated with a VRF is known as a “VRF attachment circuit”. In the simplest and most typical case, a PE-CE attachment circuit is associated with exactly one VRF. When an IP packet is received over a particular attachment circuit, its destination IP address is looked up in the associated VRF. The result of that lookup determines how to route the packet. The VRF used by the packet&#39;s ingress PE for routing a particular packet is known as the packet&#39;s “ingress VRF”. The packet&#39;s “egress VRF”, is located at the packet&#39;s egress PE. 
     An Autonomous System AS has an Autonomous System Border Router for connections to other Autonomous System(s). The Border Gateway Protocol (“BGP”) is used to distribute the customers routes across the provider&#39;s IP (Internet Protocol) backbone network, and Multiprotocol Label Switching (“MPLS”) is used to tunnel customer packets across the provider&#39;s backbone. This is known as a “BGP/MPLS IP VPN”. More specifically, if two sites of a VPN attach to PEs which are in the same Autonomous System, the PEs can distribute VPN-IPv4 routes to each other by means of an IBGP connection between them. The term “IBGP” refers to the set of protocols and procedures used when there is a BGP connection between two BGP speakers in the same Autonomous System. This is distinguished from “EBGP”, the set of procedures used between two BGP speakers in different Autonomous. Systems. Alternatively, each can have an IBGP connection to a route reflector [BGP-RR]. MP-iBGP is employed between two routers within the same AS, and MP-eBGP is used between routers in different AS systems, as will be described below with reference to  FIG. 3 . 
     RFC2547bis (draft-ietf-I3vpn-rfc2547bis-03.txt, Internet Engineering Task Force (IETF): BGP/MPLS IP VPNS), especially Chapter 10, defines different ways for providing IP VPN service in a situation wherein the customer sites are attached to provider edge routers PE belonging to different Autonomic Systems (AS) managed by different service providers. 
       FIG. 1  illustrates the first inter-AS option (a) defined by RFC2547bis. In the example shown, provider edge routers PE 1  . . .  4  are the ingress/egress PEs of a packet. CE routers are typically located at the customer&#39;s facilities. PE has a logical attachment circuit leading to a CE router and associated with the VPN Routing and Forwarding table (VRF) which implements the virtual network of the respective customer. In the inter-AS (a), the Autonomous System Border Routers ASBR are directly interconnected (i.e., there is no Label Switched Path LSP or Multiprotocol Label Switching MPLS network between thereof). VRFs are distinguished from each other by means of a physical or logical connection between the ASBR routers so that the VRFs implementing the same VPN are associated with each other using a different physical connection or subconnection (logical connection). In the example shown  FIG. 1 , VRF 1  in the ASBR 1  and VRF 1  in the ASBR 2  are associated with each other. The routing of the VPN routes for this option (a) is as follows. Let us consider the case where a router CE 1  at the customer&#39;s site advertises the IP network ‘10.0.0.0/24’ to the edge router PE 1  by means of an eBGP session. The router PE 1  installs the network ‘10.0.0.0/24’ to an appropriate VRF (e.g. VRF 1 ) and thereafter advertises the network ‘10.0.0.0/24’ further to other PE routers (such as PE 2 , ASBR 1 ) within the MPLS network of the same Autonomic System AS by means of the MP-iBGB protocol. As a result, also ASBR 1  receives the advertisement and installs the route to a VRF. The ASBR 1  then advertises the route as a normal IPv4 route to the ASBR 2  by means of the eBGP protocol. The ASBR 2  advertises the route to other PE routers (e.g. PE 3 ,PE 4 ) within its own Autonomic System AS by means of the MP-IBGP protocol, the other PE routers then advertising the route as a normal IPv4 route to the CE routers. 
     A problem in this option (a) is that the MPLS technique cannot be used between the ASBR routers but the use of logical or physical connection is required in the manner described above. Routing of the VPN routes requires one session per each VPN, which can also prevent scalability to a large number of VPNs. 
       FIG. 2  illustrates the second inter-AS option (b) defined by RFC2547bis. In this option, no actual VRFs are maintained in the ASBR routers. The ASBR routers have to maintain the VPN routes but they are not installed to VRFs. VPN route information, which also contains a label (VPN label) used for the respective route, is distributed using the MP-eGBP protocol. Typically the ASBR router rewrites the next hop attribute in the BGP message to address the ASBR router itself. Thus, the packet transferred between ASBR routers is labelled with a VPN label but contains no Packet Switched Network (PSN) label, due to which there can be no MPLS network with a PSN tunnel between the ASBR routers. This would be needed if more than two ASBR routers were interconnected to each other, or if the ASBR routers were remote from each other and MPLS technique were employed in the switching layer. Otherwise, the routing of the VPN route is similar to that of the option (a), expect that the a single MP-eBGP session is used between the ASBR routers to distribute the VPN-IPv4 addresses, and several eBGP sessions are not required for that purpose. 
     A problem in this option (b) is that a label switched path is needed between the ingress and the egress PEs, because the ASBR routers will not terminate the label switched path to a VRF instance. In this case, a label switched path is formed with VPN labels which are distributed by means of the MP-BGP protocol. As routes are not installed in a VRF, the service providers must take care of filtering of RT attributes for security reasons. The option (b) does not require use of subconnections to associate VRFs between the ASBR routers, which is an advantage over the option (a). 
       FIG. 3  illustrates the third inter-AS option (c) defined by RFC2547bis. In this option, the eBGP protocol is used between the ASBR routers to advertise an IPv4 address of the ingress edge routers (PE 1  . . . PE 4  in the example shown in  FIG. 3 ) and a corresponding label to other Autonomous Systems AS. This way, a label switched path is provided between the ingress and the egress PEs of the packet. In this case, there is a PSN tunnel within which the VPN labels are transferred. VPN-IPv4 routes and VPN labels are typically distributed by means of a VPN Route Reflector. Each PE router (e.g. PE 1 ,PE 2 ,ASBR 1 , or PE 3 ,PE 4 , ASBR 2 ) in the same Autonomic System AS has an MP-iBGP session to a VPN Route Reflector of the specific AS. VPN Route Reflectors of different ASs exchange the VPN-IPv4 using the MP-eBGP protocol. However, label switched paths between several edge routers of different service providers pose a security risk. An advantage of the option (c) is that the ASBR router does not need to maintain VPN routes. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a new method for providing VPN services between autonomic systems. 
     According to the present invention, an aggregation edge router is provided in two or more different autonomously managed network parts, referred to as autonomous systems herein. The aggregation edge routers are configured such that routing peering between the two or more autonomous systems is done between the aggregation edge routers in these autonomous systems. Each aggregation edge router is a routing peer of other edge routers in its own autonomous system. Thus, the other edge routers in the autonomous system must know reachability information, such as a label switched route, only to the aggregation edge router in the same autonomic system. An ingress edge router in one autonomous system does not need to know reachability information on the egress edge router in another automatic system. Also an aggregation edge router in one autonomous system must only be able to make contact with a peer aggregation edge router in another autonomous system. By the merits of the present invention, the scalability and security of the network are improved, The invention also allows the use of a Multiprotocol Label Switching MPLS network between the different autonomous systems. Thus, a separate logical or physical connection is not needed for each VRF in the ASBR as in one of the prior art approaches. 
     In an embodiment of the invention, the aggregation edge router acts as an Autonomous System Border Router (ASBR). 
     In an embodiment of the invention, a Multiprotocol internal Border Gateway Protocol (MP-iBGP) is employed between the aggregation edge router and other edge routers in the same autonomous system for distributing Internet Protocol-Virtual Private Network (IP-VPN) routes. 
     In an embodiment of the invention, a Multiprotocol external Border Gateway Protocol (MP-eBGP) is employed between aggregation edge routers in different autonomous systems for distributing IP-VPN routes. 
     In an embodiment of the invention, when receiving IP-VPN routes from other edge routers in the same autonomous system, the aggregation edge router may import the IP-VPN routes into one or more appropriate VPN Routing and Forwarding tables (VRF). Before the IP-VPN routes are redistributed to a peer aggregation edge router in another autonomous system, the aggregation edge router may further replace associated route distinguisher (RD) and route target (RT) attributes of the route by those configured to the corresponding VRF or VRFs in the aggregation edge router and set the next hop to itself, e.g. by replacing the next hop attribute of the route by an IP address of the aggregation edge router. 
     In an embodiment of the invention, when receiving IP-VPN routes from an aggregation edge router in another autonomous system, the aggregation edge router may import the IP-VPN routes into one or more appropriate VPN Routing and Forwarding tables (VRF). Before re-advertising the IP-VPN routes to other edge routers in its own autonomous system, the aggregation edge router may further replace associated route distinguisher (RD) and export route target (RT) attributes of the route by those configured to the corresponding VRF or VRFs in the aggregation edge router and set the next hop to itself, e.g. by replacing the next hop attribute of the route by an IP address of the aggregation edge router. 
     The advantages of replacing the route distinguisher (RD) and route target (RT) attributes include the following ones. When the same route is imported in re-advertising aggregation edge router into multiple VRFs, it can be exported to MP-BGP neighbors from those VRFs with different sets of the export route targets. If the route advertised to MP-BGP neighbors had the same RD, then only those RTs that were along with the best route would take effect in the receiving MP-BGP neighbour. If multiple aggregation edge routers use different route distinguishers, other edge routers can use load balancing if the packet&#39;s egress edge router is connected to multiple aggregation edge routers (multi-homing of PE router). 
     The present invention eliminates the need of reachability information between the packet&#39;s ingress and egress routers. The present invention does not require a label switched path between the packet&#39;s ingress and egress edge routers. Neither does the present invention require a BGP session between ingress and egress edge routers; not even via a Route Reflector. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following description, reference is made to the accompanying drawings which form a part hereof, and which illustrate several embodiments of the present invention. In the drawings: 
         FIG. 1  shows a system diagram illustrating the first inter-AS option (a) defined by RFC2547bis; 
         FIG. 2  shows a system diagram illustrating the second inter-AS option (b) defined by RFC2547bis; 
         FIG. 3  shows a system diagram illustrating the third inter-AS option (c) defined by RFC2547bis; and 
         FIG. 4  shows a system diagram illustrating an example arrangement embodying basic principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The principles of the invention will be described in more detail by means of illustrating examples. In these examples, the aggregation edge router according to the invention is implemented at an Autonomous System Border Router (ASBR) and in a “BGP/MPLS IP VPN” environment substantially meeting the RFC2547bis. Details of the “BGP/MPLS IP VPN” can be obtained from relevant Requests for Comment (RFCs) and Internet-Drafts available at http://www.ietf.org and incorporated herein by reference. 
     However, before describing the example embodiments of the invention, RT and RD attributes of the “BGP/MPLS IP VPN” are described briefly in order to facilitate description of the examples. 
     A VPN-IPv4 address is a 12-byte quantity, beginning with an 8-byte “Route Distinguisher (RD)” and ending with a 4-byte IPv4 address. If several VPNs use the same IPv4 address prefix, the Provider Edge routers (PE) trans-late these into unique VPN-IPv4 address prefixes. This ensures that if the same address is used in several different VPNs, it is possible for BGP to carry several completely different routes to that address, one for each VPN. Since VPN-IPv4 addresses and IPv4 addresses are different address families, BGP never treats them as comparable addresses. An RD is simply a number, and it does not contain any inherent information; it does not identify the origin of the route or the set of VPNs to which the route is to be distributed. The purpose of the RD is solely to allow one to create distinct routes to a common IPv4 address prefix. Other means are used to determine where to redistribute the route. The RD can also be used to create multiple different routes to the very same system. The RDs are structured so that every service provider can administer its own “numbering space” (i.e., can make its own assignments of RDs), without conflicting with the RD assignments made by any other service provider. A PE needs to be configured such that routes which lead to a particular Customer Edge (CE) router will become associated with a particular RD. The configuration may cause all routes leading to the same CE to be associated with the same RD, or it may be cause different routes to be associated with different RDs, even if they lead to the same CE. 
     When a VPN-IPv4 route is created (from an IPv4 route which the PE has learned from a CE) by the PE router, it is associated with one or more Route Target (RT) attributes. These are carried in BGP as attributes of the route. Any route associated with the Route Target RT must be distributed to every PE router that has a VRF associated with the Route Target T. When such a route is received by a PE router, it is eligible to be installed in those of the PE&#39;s VRFs which are associated with the Route Target. A Route Target attribute can be thought of as identifying a set of sites, or a set of VRFs. Associating a particular Route Target attribute with a route allows that route to be placed in the VRFs that are used for routing traffic which is received from the corresponding sites. There is a set of Route Targets that a PE router attaches to a route received from site S; these may be called the “Export Targets”. There is also a set of Route Targets that a PE router uses to determine whether a route received from another PE router could be placed in the VRF associated with site S; these may be called the “Import Targets”. The two sets are distinct, and need not be the same. 
       FIG. 4  shows an example arrangement embodying basic principles of the present invention. In the embodiment illustrated in  FIG. 4 , Provider Edge routers PE 1  . . .  4  are the packet&#39;s ingress/egress edge routers. Customer Edge (CE) routers are typically located at the customer&#39;s facilities. PE has a logical attachment circuit leading to a CE router and associated with the VPN Routing and Forwarding table (VRF) which implements the virtual network of the respective customer. In this embodiment of the invention, the VPN between VPN sites is implemented substantially in accordance with the RFC2547 specifications but applying the principles of the present invention. In the embodiment of the invention, the ASBR routers (ABRS 1  for the Autonomic System ASx, ABRS 2  for the Autonomic System ASy) also include VPN Routing and Forwarding tables (VRFs). Packet Switched Network (PSN) tunnels are provided between the packet&#39;s ingress/egress edge routers (In the illustrated example PE 1 , PE 2  for the Autonomic System ASx, and PE 3 , PE 4  for the Autonomic System ASy). The PSN tunnels may be implemented in the form of a Label Switched Path (LSP), for example, or by means of GRE, IPinIP or any other suitable technique. The type of the PSN tunnel used is not relevant to the basic invention. 
     The PSN tunnels PSN 1 , PSN 2 , and PSN 4 , PSN 5  (and respective Label switched paths LSP 1 , LSP 2 , LSP 4 , and LSP 5 ) from PE routers at the ABSR of the own system, i.e. routers PE 1 , PE 2  and PE 3 , PE 4  terminate at ABSR 1  and ABSR 2 , respectively. The PSN tunnel PSN 3  (and respective Label switched path LSP 3 ) between the Autonomous Systems ASx and ASy terminate at ABSR 1  and ABSR 2 . As a consequence, no label switched PSN end-to-end tunnel is established between the packet&#39;s ingress/egress edge routers located in different Autonomic Systems ASx and ASy. 
     Within the Autonomic System ASx, the only router that must have information on edge routers located within the other Autonomic System ASy is the router ASBR 1 , and even ASBR 1  must only be able to make contact with ASBR 2  in the ASy. Further, as the ASBR routers contain VPN Routing and Forwarding tables (VRFs) into which the VPN routes are installed, no VPN labeled path is established either. This is due to that the switching is performed on IP level in the ABSR routers, instead of the MPLS level. When an ABSR router receives an IP-VPN route through the MP-iBGP protocol from the ABRS router&#39;s own Autonomic System AS, the ABRS router installs the IP-VPN route to an appropriate VRF. The appropriate VRF can be determined based on a RT attribute in the route advertising by comparing the RT attribute with a RT attribute set to the VRF. This mechanism may be in accordance with the pre-sent RFC2547bis procedure. 
     In accordance with principles of the present invention, the VPN route installed in the VRF is re-advertised to the other ASBR router by means of MP-eBGP, while the route attributes are rewritten to be in accordance with settings of the advertising ASBR router. In the case a VPN-IPv4 route is installed to more than one VRF, the route is advertised to the other ASBR router from each VRF. 
     In the illustrated example, the following modifications or changes are made in re-advertised route by the ASBR 1  according to the present invention:
         RT attributes of the route are replaced by export RT attributes set for the VRF in the advertising ASBR 1  router.   Next Hop attribute of the mute is replaced by an IP address of the advertising ASBR 1  router.   The RD of the route is replaced by an RD set for the VRF in the advertising ASBR 1  router.       

     When the ASBR 2  router receives a BGP message modified as described above, the ASBR 2  installs the advertised route (assuming that the RT attributes match and the route is the best route, if there are more than one available) to the ASBR 2 &#39;s own VRF and makes corresponding changes to the route before advertising the mute in its own Autonomic System ASy. More specifically, the ASBR 2  makes the following modifications or changes in re-advertised route according to the present invention:
         RT attributes of the route are replaced by export RT attributes set for the VRF in the ASBR 2  router.   Next Hop attribute of the route is replaced by an IP address of the ASBR 2  router.   RD of the route is replaced by an RD set to the VRF in the ASBR 2  router.       

     As a result, the other edge routers in the Autonomic System ASx or ASy must know a label switched route only to another router in the same Autonomic System AS, i.e. to the ASBR 1  or ASBR 2 , respectively. No label switched path is needed between the packet&#39;s ingress and egress edge routers, which improves the security and scalability of the network. 
     The above specification, examples and data provide an enabling description of the making and use of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention can vary within the scope and spirit of claims hereinafter appended.