Patent Publication Number: US-7225270-B2

Title: Selective diversion and injection of communication traffic

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application 60/461,390, filed Apr. 9, 2003. It is a continuation-in-part of U.S. patent application Ser. No. 09/929,877, filed Aug. 14, 2001 (and published as US 2002/0083175 A1), and of PCT Patent Application PCT/IL02/00996, filed Dec. 10, 2002. All these related applications are assigned to the assignee of the present patent application, and their disclosures are incorporated herein by reference. 

   COPYRIGHT NOTICE 
   Program listings in the disclosure of this patent document contain material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
   FIELD OF THE INVENTION 
   The present invention relates generally to computer networks, and specifically to methods and systems for diverting and processing selected traffic in a computer network. 
   BACKGROUND OF THE INVENTION 
   The above-mentioned U.S. patent application publication US 2002/0083175 describes a method for protecting against overload conditions in a set of one or more potential “victims” on a network, based on diverting traffic that is destined for the victims. To carry out this protection, a first set of network elements, such as routers, redirect the traffic to a second set of network elements, referred to as “guard machines.” The diversion is actuated when a potential victim comes under an anomalous traffic condition, such as might be caused by a Distributed Denial of Service (DDoS) attack. The guard machines filter the diverted traffic to remove malicious (or excessive) traffic, and forward the legitimate traffic on to the victim. The publication describes a number of methods by which traffic diversion may be effected. 
   SUMMARY OF THE INVENTION 
   In embodiments of the present invention, network communication traffic that is destined for a particular target address (or set of addresses) is diverted to a traffic processor. After processing the diverted traffic, the traffic processor passes at least some of the processed traffic on to the original target address. This sort of traffic diversion and processing is particularly useful in the context of filtering out malicious traffic, as described in the above-mentioned U.S. and PCT patent applications. It may also be used in other sorts of network applications, such as in analysis of traffic flows for purposes of billing, forensics and traffic engineering. 
   In some embodiments of the present invention, a Layer-3 packet router, such as an Internet Protocol (IP) router, receives communication traffic from a network via a first port that is connected to the network. The traffic comprises packets that are destined for a target address, which is accessible via a second port of the router. The routing tables of the router are specially programmed so that when diversion is desired (due to a suspected attack, for example), the router diverts the packets that are destined for the target address to a traffic processor via a third port of the router. The processor returns the processed packets to the router via the third port. The special programming of the routing tables causes the router to convey these packets from the third port to the second port for delivery to the target address. 
   In some embodiments, multiple routers in a selected area of a network cooperate to divert traffic to the traffic processor. The selected area may comprise, for example, a private network or other autonomous system, which is coupled by peering routers to a Wide Area Network (WAN), such as the Internet. The peering routers are coupled by tunnels through the network to the diverting router that is connected to the traffic processor. When diversion is desired, the peering routers are instructed to pass traffic that is destined for the target address via the tunnels to the diverting router. In this manner, diversion can be accomplished by issuing the appropriate routing instructions to the peering routers, without active involvement of the remaining routers in the area of the network. 
   In further embodiments of the present invention, the traffic processor and the router from which the traffic is to be diverted to the traffic processor are coupled together in a common subnet, typically via a Layer-2 switch, such as an Ethernet switch. In such embodiments, the processed traffic is conveyed by the traffic processor back to the Layer-2 switch, which then forwards the traffic to the target address either directly (if the target address is also in the same subnet) or via another router in the subnet. 
   There is therefore provided, in accordance with an embodiment of the present invention, a method for communication, including: 
   coupling a first port of a Layer-3 packet router to receive communication traffic from a network, the traffic including packets destined for a target address, which is accessible via a second port of the router; 
   at the router, diverting the packets that are destined for the target address to a traffic processor via a third port of the router; 
   processing the diverted packets at the traffic processor, and returning the processed packets to the router via the third port; and 
   at the router, conveying the processed packets from the third port to the second port for delivery to the target address. 
   In disclosed embodiments, diverting the packets includes detecting an indication that at least some of the traffic destined for the target address is of malicious origin, and diverting the packets responsively to the indication. Typically, processing the diverted packets includes filtering the diverted packets in order to identify the packets of the malicious origin, and inhibiting delivery of the identified packets. 
   In some embodiments, diverting the packets includes sending a Border Gateway Protocol (BGP) announcement from the traffic processor to the router, instructing the router to divert the packets. Typically, sending the BGP announcement includes inserting at least one of a “no-advertise” and a “no-export” string in the BGP announcement. 
   In some embodiments, diverting the packets includes establishing a tunnel through the network from a peering router to the first port of the Layer-3 packet router, configuring the Layer-3 packet router to forward the packets that it receives through the tunnel to the third port, and instructing the peering router to forward the packets that are destined for the target address through the tunnel. Establishing the tunnel may include establishing a plurality of tunnels from peering routers at an edge of an area of a network to the Layer-3 packet router within the area. 
   Typically, returning the processed packets includes selecting, at the traffic processor, a path via the Layer-3 packet router to the target address, and directing the Layer-3 packet router to convey the processed packets to a next-hop router along the selected path. Selecting the path may include identifying a plurality of paths, passing through respective next-hop routers to the target address, and selecting one of the next-hop routers through which the processed packets are to be conveyed. In some embodiments, selecting the one of the next-hop routers includes detecting, at the traffic processor, a change in the network between the second port of the router and the target address, and responsively to the change, selecting a different one of the next-hop routers through which to convey the processed packets to the target address. 
   Additionally or alternatively, selecting the path includes receiving routing information at the traffic processor from the Layer-3 packet router, and identifying the path based on the routing information. Typically, receiving the routing information includes receiving announcements generated by routers in the network in accordance with an automatic routing protocol. In one embodiment, receiving the announcements includes establishing at least one tunnel through the network between the traffic processor and the next-hop router, and receiving the announcements responsively to the at least one tunnel. In an alternative embodiment, receiving the routing information includes querying the Layer-3 packet router using a management protocol. 
   In a disclosed embodiment, directing the router includes establishing a tunnel through the network from the traffic processor via the router to the next-hop router, and passing the packets through the tunnel. 
   In some embodiments, conveying the processed packets includes programming the router with a forwarding rule with respect to the packets received by the router on the third port, so as to override a main routing table of the router, and forwarding the processed packets responsively to the forwarding rule. Programming the router may include invoking policy-based routing (PBR) or filter-based forwarding (FBF). Alternatively or additionally, programming the router includes configuring the router to apply the forwarding rule responsively to a type of service (ToS) field in the processed packets, and conveying the processed packets includes setting a value of the ToS field in the packets so as to cause the forwarding rule to be invoked. 
   In other embodiments, conveying the processed packets includes adding a virtual private network (VPN) designation to the processed packets, and conveying the processed packets includes programming the router with a VPN routing table, and forwarding the processed packets responsively to the VPN routing table. Typically, adding the VPN designation comprises adding a virtual local area network (VLAN) tag to the processed packets. In one of these embodiments, adding the VLAN tag includes defining a plurality of VLANs corresponding to different routes to the target address, and wherein adding the VLAN tag includes selecting one of the routes, and setting a value of the VLAN tag to designate the selected one of the routes. 
   There is also provided, in accordance with an embodiment of the present invention, a method for communication, including: 
   coupling one or more peering routers in an area of a network to receive communication traffic from outside the area, the traffic including packets destined for a target address; 
   forwarding the packets to the target address over one or more first routes via one or more internal routers within the area of the network; 
   establishing one or more tunnels through the network from the peering routers via one or more first ports of a diverting router within the area of the network to a second port of the diverting router; 
   coupling a traffic processor to the second port of the diverting router; 
   in response to a characteristic of the traffic, instructing the one or more peering routers to forward the packets that are destined for the target address through the one or more tunnels instead of over the first routes; 
   at the traffic processor, processing the packets that were forwarded through the tunnels via the diverting router; and 
   conveying at least some of the processed packets from the traffic processor to the target address. 
   Typically, instructing the one or more peering routers includes sending a Border Gateway Protocol (BGP) announcement from the traffic processor to the one or more peering routers. Additionally or alternatively, instructing the one or more peering routers includes sending an instruction to the one or more peering routers, without modifying routing tables of the internal routers. The diverting router may serve as one of the internal routers on at least one of the first routes. 
   Typically, conveying the at least some of the processed packets includes conveying the at least some of the processed packets via the diverting router to the target address. Additionally or alternatively, conveying the at least some of the processed packets includes establishing a further tunnel between the traffic processor and an edge router on a path to the target address, and conveying the at least some of the processed packets through the further tunnel. 
   There is additionally provided, in accordance with an embodiment of the present invention, a method for communication, including: 
   coupling a first port of a Layer-3 packet router to receive communication traffic from a network, the traffic including packets destined for a target address; 
   coupling a second port of the Layer-3 packet router to a subnet, through which the target address is accessible; 
   diverting the packets that are destined for the target address to a traffic processor on the subnet via a Layer-2 switch in the subnet; 
   processing the diverted packets at the traffic processor, and returning the processed packets to the Layer-2 switch; and 
   conveying the processed packets from the Layer-2 switch to the target address. 
   In some embodiments, the target address is in the subnet. In other embodiments, the target address is outside the subnet, and conveying the processed packets includes passing the processed packets from the Layer-2 switch to a further router in the subnet, and routing the processed packets from the further router to the target address. 
   There is further provided, in accordance with an embodiment of the present invention, apparatus for communication, including: 
   a Layer-3 packet router, including at least first, second and third ports, wherein the first port is coupled to receive communication traffic from a network, the traffic including packets destined for a target address, which is accessible via a second port of the router; and 
   a traffic processor, which is coupled to the third port of the router, and is adapted to cause the router to divert the packets that are destined for the target address to the third port, and is further adapted to process the diverted packets and to return the processed packets to the router via the third port so as to cause the router to convey the processed packets from the third port to the second port for delivery to the target address. 
   There is moreover provided, in accordance with an embodiment of the present invention, apparatus for communication, including: 
   one or more peering routers in an area of a network, which are coupled to receive communication traffic from outside the area, the traffic including packets destined for a target address; 
   one or more internal routers within the area of the network, which are coupled to receive the packets from the one or more peering routers, and to forward the packets to the target address over one or more first routes; 
   a diverting router within the area of the network, the diverting router including one or more first ports and a second port, wherein the first ports are coupled to the peering routers by one or more tunnels through the network, and the diverting router is configured to forward the packets that it receives through the one or more tunnels to the second port; 
   a traffic processor, which is coupled to the second port of the diverting router, and which is adapted, in response to a characteristic of the traffic, to instruct the one or more peering routers to forward the packets that are destined for the target address through the one or more tunnels instead of over the first routes, to process the packets that were forwarded through the tunnels via the diverting router, and to convey at least some of the processed packets to the target address. 
   There is furthermore provided, in accordance with an embodiment of the present invention, apparatus for communication, including: 
   a Layer-2 switch, located in a subnet through which a target address is accessible; 
   a Layer-3 packet router, including first and second ports, wherein the first port is coupled to receive communication traffic from a network, the traffic including packets destined for the target address, and the second port is coupled to the Layer-2 switch; and 
   a traffic processor, which is adapted to cause the Layer-3 packet router to divert the packets that are destined for the target address via the Layer-2 switch to the traffic processor, and which is further adapted to process the diverted packets and to return the processed packets to the Layer-2 switch so as to cause the Layer-2 switch to convey the processed packets to the target address. 
   There is also provided, in accordance with an embodiment of the present invention, a computer software product, for use by a computer in conjunction with a Layer-3 packet router that includes at least first, second and third ports, wherein the first port is coupled to receive communication traffic from a network, the traffic including packets destined for a target address, which is accessible via a second port of the router, and the computer is coupled to the third port, the product including: 
   a computer-readable medium in which program instructions are stored, which instructions, when read by the computer, cause the computer to instruct the router to divert the packets that are destined for the target address to the third port, and further cause the computer to process the diverted packets and to return the processed packets to the router via the third port so as to cause the router to convey the processed packets from the third port to the second port for delivery to the target address. 
   The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram that schematically illustrates a protected area of a computer network, in accordance with an embodiment of the present invention; 
       FIG. 2  is a flow chart that schematically illustrates a method for packet diversion, in accordance with an embodiment of the present invention; 
       FIG. 3  is a flow chart that schematically illustrates a method for configuring a router, in accordance with an embodiment of the present invention; 
       FIG. 4  is a block diagram that schematically illustrates tunnels within a network, which are used for packet injection in accordance with an embodiment of the present invention; 
       FIG. 5  is a block diagram that schematically illustrates a protected area of a network, in accordance with another embodiment of the present invention; and 
       FIG. 6  is a block diagram that schematically illustrates a protected area of a network, in accordance with yet another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   Overview of Traffic Diversion in Layer-3 Topologies 
     FIG. 1  is a block diagram that schematically illustrates a communication network  20 , in accordance with an embodiment of the present invention. A guard device  22  serves as a traffic processor, which filters out malicious traffic that is directed to target addresses in a protected area  24  of network  20 . Protected area  24  communicates with a wide-area network (WAN)  26 , typically the Internet, through a Layer-3 router  28 , typically an Internet Protocol (IP) router. The protected area comprises various other network elements, such as a server  30  and internal routers  32 ,  34 , as well as other computers, switches and local-area networks (LANs), which are omitted from this figure for the sake of simplicity. Typically, although not necessarily, protected area  24  comprises a private network, such as an enterprise or campus network, or a network operated by an Internet Service Provider (ISP). 
   In the context of the present patent application and in the claims, the terms “Layer-2” and “Layer-3” are used in accordance with their conventional meanings, as given by the well-known Open Systems Interface (OSI) model. Thus, Layer-2 refers to the data link layer, and includes medium access control (MAC) functionality as provided, for example, by Ethernet local area networks (LANs), switches and other components. Layer-3 refers to the network layer, and includes functions such as IP packet routing, for example. Although the embodiments described hereinbelow make reference particularly to. Ethernet and IP and to certain common Ethernet and IP device types and routing protocols, the principles of the present invention may similarly be applied, mutatis mutandis, in networks using other protocols and devices. 
   For simplicity and clarity, certain aspects of the present invention will be described hereinbelow with reference to protection of server  30  (which may also be referred to as the “victim” or “target”) from Denial of Service (DoS) attacks. It will be understood, however, that guard device  22  may typically be configured to protect not only server  30 , but also other computers in protected area  24 . Furthermore, multiple guard devices may be deployed in connection with router  28  or with other routers or switches of other types in area  24 . Typically, guard device  22  comprises a general-purpose computer, which is programmed in software to carry out the functions described herein. Alternatively, guard device  22  may be implemented in dedicated hardware logic, or using a combination of hardware and software elements. The guard device may be a standalone unit, or it may alternatively be integrated with other communication or computing equipment, such as router  28 , a firewall, or another intrusion detection system (not shown). Although the guard device is shown and described herein, for the sake of simplicity, as a single, discrete unit, in practice the functionality of the guard device may be distributed among multiple physical units, which may be collocated or distributed among a number of different locations. 
   Router  28  (marked R 1 , and referred to alternatively as the “divert-from router”) receives packets from WAN  26  on a first port  35 . Under normal network conditions, when router  28  receives packets that are addressed to the IP address of server  30  (referred to hereinbelow as the “target address”), it simply passes the packets on to server  30  via a port  39  or  41 . As shown in the figure, either of routers  32  and  34  (marked R 2  and R 3 ) is capable of passing the packets on to server  30 . The choice of which of ports  39 ,  41  to use for forwarding traffic to server  30  is normally determined by routing tables held by router  28 . 
     FIG. 2  is a flow chart that schematically illustrates a method for diversion of traffic to guard device  22 , in accordance with an embodiment of the present invention. Upon detection of anomalous traffic conditions, which may be indicative of a DoS attack on server  30 , router  28  is instructed to divert traffic that is destined for the server to guard device  22 , at a diversion initiation step  50 . Methods and criteria for detection of anomalous traffic conditions are described, for example, in the above-mentioned U.S. and PCT patent applications. Anomalous traffic characteristics may be detected by the guard device, by server  30 , or by other elements in area  24 , and the instructions to router  28  and to other network elements to begin traffic diversion may come the guard device itself for from another element in area  24 . Diversion is effected by instructing router  28  to change its routing table so that traffic addressed to server  30  is routed over a diversion path  36  via a port  37  to guard device. Methods for carrying out this step are described hereinbelow. 
   After receiving the instruction to change its routing table, router  28  diverts all traffic destined for server  30  to guard device  22 , at a diversion step  52 . The guard device processes each packet, at a packet processing stage  54 , in order to prevent malicious traffic from reaching server  30 . For example, guard device  22  may filter the packets, at a packet filtering step  56 , in order to detect certain features that are characteristic of illegitimate, malicious traffic. Additionally or alternatively, the guard device may attempt to determine whether the source address of each packet is authentic or spoofed. Methods for packet filtering and for distinguishing between authentic and illegitimate source addresses are described in the above-mentioned U.S. and PCT patent applications, as well as in other publications and patent applications, such as U.S. patent application Ser. No. 10/232,993, filed Aug. 29, 2002, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference. Guard device  22  inhibits delivery of packets that appear to be malicious, at an inhibition step  58 . Typically, the guard device either discards the suspect packets or delivers them to server  30  with low priority, so that they do not prevent the server from handling normal traffic that has been authenticated by the guard device. 
   Guard device  22  returns filtered traffic to port  37  of router  28  via a return path  38 , at a packet return step  60 . Router  28  then forwards the traffic over an onward path  40  or  42 , via either router  32  (R 2 ) or router  34  (R 3 ), at a forwarding step  62 . This next router along the path from the guard device to the target address is referred to hereinbelow as the “next-hop router.” Note that the regular routing table of router  28  was changed at step  50  to list port  37  as the route to server  30 . Based on this routing table, upon receiving packets from guard device  22  that are addressed to server  30 , at step  60 , the router would normally just send the packets back to port  37 , in an endless loop. Techniques for circumventing this normal router behavior, so as to cause the packets to be forwarded to the next-hop router, are described hereinbelow. Alternatively, guard device  22  may be connected directly to a port of the next-hop router, but this arrangement is costly in terms of the extra wiring and port infrastructure that are required to support it. 
     FIG. 3  is a flow chart that schematically shows details of diversion initiation step  50 , in accordance with an embodiment of the present invention. As noted above, two changes are typically made in the routing tables of router  28 : Router  28  is configured to divert traffic destined for the address of server  30  to guard device  22  via port  37 , at a diversion configuration step  66 . The router is configured to forward traffic from guard device  22  to server  30  via the appropriate next-hop router (such as router  32  or  34 ), at a forwarding configuration step  68 . 
   Traffic Diversion Methods 
   Typically, for the purpose of traffic diversion at step  66 , guard device  22  issues a routing announcement to router  28 , such as a BGP announcement using the well-known Border Gateway Protocol (BGP). BGP is described in detail in by Rekhter et al., in Request for Comments (RFC) 1771 of the Internet Engineering Task Force (IETF), entitled “A Border Gateway Protocol 4 (BGP-4)” (1995), which is incorporated herein by reference. This RFC and other IETF RFC documents cited hereinbelow are available at www.ietf.org/rfc.html. The BGP announcement causes router  28  to modify its routing table so that path  36  is listed as the best path to the target address. When the area of the network protected by the guard device is connected to other areas of WAN  26  by one or more peering routers (as shown below in  FIG. 4 , for example), BGP announcements may be issued to the peering routers, as well. BGP has an inherent added advantage that the BGP routing algorithm will automatically cancel diversion to guard device  22  in case of a failure of the guard device. 
   Typically, protected area  24  constitutes an autonomous system (AS) within network  20 , which communicates with other autonomous systems in WAN  26 . Routers within a given AS communicate with one another using Internal BGP (iBGP) announcements, and this is the type of announcement that guard device  22  sends to router  28  at step  66 . Alternatively, External BGP (eBGP) announcements may be used, depending on the configuration of the guard device and routers in protected area  24 . In order to ensure that the BGP announcement by the guard device overrides any previous routing decisions, the guard device typically uses a longer (more specific) prefix to represent the target address in its BGP announcement than was used in previous announcements. The guard device may even use the full IP address of server  30  in the BGP announcement. Alternatively, guard device  22  may assign a high value to the “weight” field provided in its BGP announcements, which will cause the announcements to override previous routing decisions based on announcements of lower weight. 
   The BGP announcement issued by guard device  22  at step  66  causes router  28  to change its routing table, but preferably has no effect on the routing tables of routers  32  and  34 . For this purpose, the guard device inserts “no-advertise” and “no-export” community strings in the BGP announcement. Inclusion of these strings prevents router  28  from propagating the BGP announcement and thus limits the effect of the announcement to the intended router. The guard device may also add other community strings, such as an identifying string that permits a system operator studying the router logs to identify routing decisions that were initiated by the guard device. 
   Traffic Forwarding 
   A number of alternative techniques for carrying out forwarding configuration step  68  are described hereinbelow. Typically, guard device  22  sends a control message (using BGP, for example) to router  28  indicating the next-hop router for forwarding. For this purpose, the guard device may be pre-programmed with the identity of the next-hop router for each target address in area  24 . Alternatively, the guard device may use an autonomous next-hop discovery mechanism to determine the next-hop router that should be used for each target address. Methods for next-hop discovery are described hereinbelow. These methods enable the guard device to respond dynamically to changes in the network configuration. 
   A number of exemplary techniques for configuring router  28  at step  68  will now be described: 
   1) Destination-based Policy-Based Routing (PBR-DST) 
   This is a static method, which uses special policy-based routing (PBR) facilities provided by many routers. These facilities permit the system operator to create special routing rules for certain traffic, which override the normal routing table of the router. In the example shown in  FIG. 1 , PBR is used to program router  28  to forward to the next-hop router (router  32  or  34 ) those packets that it receives on port  37  and which have a destination address equal to the IP address of server  30 . This rule overrides the entry with respect to this IP address in the main routing table of router  28 , which was programmed by BGP announcement at step  66  to forward such traffic to guard device  22  on port  37 . Router  28  applies this PBR rule only to packets that it receives on port  37 . Guard device  22  may configure the PBR rule in advance, so that when it activates diversion of traffic for server  30  at step  50  ( FIG. 2 ), PBR forwarding goes into effect at the same time, without the need to reconfigure router  28  in real time. 
   The term “PBR” is known in the art mainly in reference to routers distributed by Cisco Systems (San Jose, Calif.). Cisco PBR is described, for instance, in a document entitled “Configuring Policy-Based Routing” (2004), which is available at www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800c60d2.html, and is incorporated herein by reference. Similar facilities are offered by other router manufacturers, in some cases under different names. For example, routers distributed by Juniper Networks (Sunnyvale, Calif.) provide “Filter-Based Forwarding” (FBF) with similar capabilities to PBR. FBF is described by Semeria in “Filter Based Forwarding” (2001), which is available at www.juniper.net/solutions/literature/white_papers/552004. pdf, and which is also incorporated herein by reference. In the case of FBF, the filter configuration may be used to specify the IP destination address of server  30 ; a routing instance configuration specifies the routing table or tables that are applied in forwarding packets matching the configured filter; and an interface routes configuration specifies the routing instance that is to be used for packets matching the filter that are received at each port of the router. A forwarding technique based specifically on Juniper FBF is described in detail hereinbelow. 
   Listing I below gives a sequence of commands that may be used to program router  28  for PBR-DST forwarding of traffic from guard  22  to next-hop router  32 , as illustrated in  FIG. 1 . The listing uses Cisco router programming conventions. Terms in brackets &lt; . . . &gt; are to be replaced by appropriate parameter values. Each line represents a command, which should be typed into the router interface followed by &lt;ENTER&gt;. 
   
     
       
         
             
           
             
                 
             
             
               LISTING I - ROUTER PROGRAMMING FOR PBR-DST 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
                 
               R7200(config)# interface FastEthernet 0/2 
             
             
                 
               R7200(config-if)# description &lt;Port 37&gt; 
             
             
                 
               R7200(config-if)# ip address &lt;IP address of port 37&gt; &lt; IP 
             
             
                 
                 mask of port 37&gt; 
             
             
                 
               R7200(config-if)# no ip directed-broadcast 
             
             
                 
               R7200(config-if)# ip policy route-map &lt;PBR-name of guard 
             
             
                 
                 device 22&gt; 
             
             
                 
               R7200(config-if)# exit 
             
             
                 
               R7200(config)# ip access-list extended &lt;Name of server 
             
             
                 
                 30&gt; 
             
             
                 
               R7200(config-ext-nacl)# permit ip any host &lt;IP address of 
             
             
                 
                 server 30&gt; 
             
             
                 
               R7200(config-ext-nacl)# exit 
             
             
                 
               R7200(config)# route-map &lt;PBR-name of guard device 22&gt; 
             
             
                 
                 permit 10 
             
             
                 
               R7200(config-route-map)# match ip address &lt;Name of server 
             
             
                 
                 30&gt; 
             
             
                 
               R7200(config-route-map)# set ip next-hop &lt;IP address of 
             
             
                 
                 router 32&gt; 
             
             
                 
               R7200(config-route-map)# exit 
             
             
                 
               R7200(config)# route-map &lt;PBR-name of guard device 22&gt; 
             
             
                 
                 permit 100 
             
             
                 
               R7200(config-route-map)# description let thru all other 
             
             
                 
                 packets without modifying next-hop 
             
             
                 
                 
             
          
         
       
     
   
   As noted above, PBR-DST is a static forwarding method. In other words, once the system operator has programmed the PBR rules indicating the next-hop router in case of traffic diversion, all diverted packets will pass through the next-hop router unless and until the PBR rules are reprogrammed. Dynamic forwarding methods using PBR are described hereinbelow. 
   2) VPN Routing and Forwarding (VRF) 
   Some embodiments of the present invention make use of virtual private network (VPN) routing capabilities that are offered by many router manufacturers. System operators normally use these facilities to define multiple VPNs that share the same physical infrastructure, often by means of network tunneling protocols. For example, Cisco routers have a feature known as VPN routing and forwarding (VRF), as described in “Designing MPLS Extensions for Customer Edge Routers” (Product Bulletin No. 1575, 2003), which is available at www.cisco.com/warp/public/cc/pd/rt/2600/prodlit/1575_pp. htm, and which is incorporated herein by reference. MPLS is described in detail by Rosen et al., in IETF RFC 3031, entitled “Multiprotocol Label Switching Architecture” (January, 2001). VRF may similarly be applied, mutatis mutandis, in other types of VPNs, such as VPN services over physical interfaces or over other sorts of tunnel interfaces, such as Generic Routing Encapsulation (GRE) or Layer Two Tunneling Protocol (L2TP) tunnels. 
   Many VPN implementations use virtual bridged local area networks, as specified in the IEEE 802.1Q standard. In accordance with this standard, a VLAN tag is added to each Ethernet frame, thus defining the virtual local area network (VLAN) on which the frame is to be transmitted. Each VLAN on a given port of router  28  is treated by the router as a separate virtual port and can have its own routing table, which is separate from the main (native VLAN) routing table. This feature is used in the present embodiment. 
   In one embodiment of the present invention, a unique VLAN is configured on port  37  of router  28  for each destination (such as server  30 ) or group of destinations that are protected by guard device  22 . This technique is referred to herein as destination-based VRF, or VRF-DST. To implement VRF-DST in the system shown in  FIG. 1 , two interfaces are configured on port  37  of router  28 :
     The native VLAN interface is used for traffic that is received via port  35  and is diverted onto path  36  to guard device  22 , as specified in the main routing table of router  28 . (At step  66 , guard device  22  uses the native VLAN interface to send the BGP announcement to router  28  initiating traffic diversion.)   A second VLAN interface is used to inject traffic received from guard device  22  over path  38  to next-hop router  32 , in accordance with a VRF table that is configured for this VLAN. Note that this is a static forwarding method, like PBR-DST. A dynamic variant is described below.   

   Listing II provides a sequence of commands that can be used to program routers  28  and  32  to carry out VRF-DST forwarding from guard device  22  to server  30 . This listing uses Cisco router programming conventions, as in Listing I above. In this example, the native VLAN interface of port  37  is assumed to have IP address 192.168.8.1, while the VLAN interface used for traffic on path  38  has IP address 192.168.5.1. Guard device  22  is similarly configured with separate VLAN interfaces for paths  36  and  38 , on the same subnets as the corresponding interfaces of port  37 . Port  39  of router  28 , which connects to path  40 , is assumed to have IP address 192.168.250.1/24, while server  30  has IP address 192.168.240.2/24. The native VLAN is configured as VLAN  1 , while VLAN  5  is used for traffic injection from guard device  22  to router  32 . Instructions for the system operator are given in italics. 
   
     
       
         
             
           
             
                 
             
             
               LISTING II - ROUTER PROGRAMMING FOR VRF-DST 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
          
             
                 
               Configure route on router 28 from port 39 to server 30 
             
             
                 
               via router 32, by BGP announcement from guard device 22 
             
             
                 
               on VLAN 5: 
             
             
                 
               ip route 192.168.240.0 255.255.255.0 192.168.5.1 
             
             
                 
               Create a VRF table on router 28: 
             
             
                 
               ip vrf Guard-vrf 
             
             
                 
                rd 1680:1 
             
             
                 
                route-target export 1680:1 
             
             
                 
                route-target import 1680:1 
             
             
                 
               Configure the native VLAN on router 28: 
             
             
                 
               interface fastEthernet1/0.1 
             
             
                 
                encapsulation dot1Q 1 native 
             
             
                 
                description &lt;&lt; VLAN FOR GUARD-DIVERSION &gt;&gt; 
             
             
                 
                ip address 192.168.8.1 255.255.255.0 
             
             
                 
                no ip directed-broadcast 
             
             
                 
               Configure the VLAN interface on router 32: 
             
             
                 
               Interface fastEthernet 1/0.5 
             
             
                 
                encapsulation dot1Q 5 
             
             
                 
                description &lt;&lt; VLAN FOR GUARD-INJECTION &gt;&gt; 
             
             
                 
                ip vrf forwarding Guard-vrf 
             
             
                 
                ip address 192.168.5.1 255.255.255.0 
             
             
                 
               Configure the interface from router 32 to server 30: 
             
             
                 
               interface fastEthernet 2/0 
             
             
                 
                description &lt;&lt; LINK TO SERVER &gt;&gt; 
             
             
                 
                ip address 192.168.250.1 255.255.255.0 
             
             
                 
                 
             
          
         
       
     
   
   In an alternative embodiment, multiple VPNs may be established in order to permit guard device  22  to select the route to use for packets destined for server  30 . For this purpose, in the system shown in  FIG. 1 , multiple VLANs are configured on port  37  of router  28 , each corresponding to a different route to server  30 . For example, VLAN  5  may correspond to path  40  via router  32  to server  30 , while VLAN  6  corresponds to path  42  via router  34 . Router  28  in this case has (at least) three virtual ports on port  37 , each of which has its own routing table: Packets on the native VLAN use the main routing table, as described above, while packets on VLAN  5  and VLAN  6  use their own, respective VRF tables for routing of packets from guard device  22  to server  30 . Configuration of the routing tables and interfaces is performed using commands similar to those given above in Listing II. 
   Guard  22  may use this multi-VLAN facility in dynamically selecting the route to use for each packet that is destined for server  30 . Thus, at step  60  ( FIG. 2 ), guard device  22  inserts a VLAN tag into the packets that it sends over path  38  to router  28 , and sets the VLAN value to either 5 or 6 in order to cause the router to forward the packets via router  32  or router  34  as desired. Guard device  22  may select the VLAN tag value (and hence the route) depending on changing network conditions or other factors of which the guard device may become aware, typically by monitoring routing protocol messages exchanged among the routers in system 20. 
   3) Type-of-Service Policy-Based Routing (ToS-PBR) 
   IP packets conventionally include a ToS field in their header. This field was initially intended to permit different service levels to be defined for different IP packets in a network. In the present embodiment, however, guard device  22  inserts a ToS field value in the IP header of packets that it sends over path  38  to router  28  at step  60  ( FIG. 2 ) in order to control the onward routing of these packets. PBR is configured on port  37  of router  28  so as to select the onward packet route depending on the packet ToS value. Thus, for example, PBR on router  28  may be programmed so that packets received on port  37  with the destination address of server  30  and ToS=2 are forwarded to router  32 . 
   Furthermore, guard  22  may use the ToS field to dynamically select the route to use for packets destined for server  30 . For example, the PBR rules on router  28  may specify that packets addressed to server  30  with ToS=2 are to be forwarded to router  32 , while those with ToS=3 are to be forwarded to router  34 . As in the preceding embodiment, guard device  22  may select the ToS (and hence the route) depending on changing network conditions or other factors of which the guard device may become aware. 
   4) VLAN Policy-Based Routing (VLAN PBR) 
   This technique combines aspects of techniques (1) and (2) that are described above. Like the multi-VPN variant of technique (2), VLAN PBR permits guard device  22  to dynamically select the packet injection route. In the present embodiment, router  28  uses the VLAN tag of packets received on port  37  as a filtering criterion for selection of the appropriate PBR rule. Thus, for example, the router may be configured to forward packets destined for server  30  on VLAN  5  via path  40  to router  32 , while forwarding packets destined for server  30  on VLAN  6  via path  42  to router  34 . The routing tables and interfaces are configured using commands similar to those given above in Listings I and II. At step  60 , guard device  22  selects the value of the VLAN tag in packets that it sends to router  28  depending on the desired route. 
   5) FBF Instance-based Routing 
   Juniper FBF, as noted above, permits a separate routing table to be created in router  28  for purposes of routing traffic sent to the router by guard device  22  over path  38 . Router  28  is programmed with a FBF rule, instructing the router to use this separate routing table (referred to hereinbelow as the Guard-Interface-Routing-Table) in routing all traffic arriving on port  37  from guard device  22 . The Guard-Interface-Routing-Table is populated with all the routes from the main routing table of router  28 , except those routes that are specified explicitly by guard device  22 . The route entries specified by the guard device are identified by a special community string inserted by the guard device. 
   Thus, in the example shown in  FIG. 1 , guard device  22  will create an entry in the Guard-Interface-Routing-Table indicating that traffic destined for server  30  should be forwarded to router  32 . This entry will be preserved even when the guard device modifies the main routing table of router  28 , at step  50 , to divert all other traffic destined for server  30  from router  28  to the guard device. The guard device may modify the entries in the Guard-Interface-Routing-Table using appropriate BGP announcements. Thus, the present FBF-based technique permits the guard device to modify packet routing dynamically in response to network conditions or other requirements. 
   Listing III below provides a sequence of commands that can be used to program router  28  to perform packet injection from guard device  22  to server  30 , using Juniper FBF. For the purposes of this example, it is assumed that port  37  of router  28  has IP address 192.168.8.16/24; the ports of routers  32  and  34  facing router  28  have IP addresses 192.168.240.2/24 and 192.168.230.2/24, respectively; and server  30  has IP address 192.168.240.2/24. Instructions for the system operator are given in italics. 
                           LISTING III - ROUTER PROGRAMMING FOR FBF                                            Configure port 37:           interfaces {            ge-0/0/0 {             unit 0 {              family inet {               filter {                input guard-filter;               }               address 192.168.8.16/24;              }             }            }           }           Configure port 39 interface to router 32:            interfaces {            fe-0/0/0 {             unit 0 {              family inet {               address 192.168.250.1/24:           Configure port 41 interface to router 34:            interfaces {            fe-0/0/0 {             unit 0 {              family inet {               address 192.168.230.1/24:           Configure filter for port 37 so that packets destined for           port 35 are referred to the main routing table, while all           other packets are handled by the Guard-interface-routing-           table:           filter guard-filter {            term 10 {               from {                destination-port 179;               }               then accept;              }              term 20 {               then routing-instance guard-interface-                routing-table;             }           }           Configure routing instance (add Guard-interface-routing-           table to router 28):           routing-instances {             Guard-interface-routing-table              instance-type forwarding;              routing-options {               instance-import without-guard-announcement;               auto-export;   /* no export of routes              }           }           Define instance-import policy (“without-guard-           Announcement”):           policy-options {            policy-statement without-guard-announcement {             term 10 {              from {               instance master;    /* main routing table               protocol bgp;               community riverhead;  /* special community                           /* string              }              then reject;             }             term 20 {              then accept;             }            }           }                        
6) Tunnel-based Forwarding
 
     FIG. 4  is a block diagram that schematically illustrates the use of tunnels  71  and  73  for packet injection from guard device  22  to servers  30  and  69 , respectively, in accordance with an embodiment of the present invention. Tunnels  71  and  73  may be defined using any suitable network tunneling protocol known in the art, such as MPLS, GRE, IPIP or L2TP. MPLS is described in RFC 3031, as noted above. GRE is described by Farinacci et al., in IETF RFC 2784, entitled “Generic Routing Encapsulation” (2000). IPIP is described by Perkins in IETF RFC 2003, entitled “IP Encapsulation within IP” (1996). Both of these documents are incorporated herein by reference. 
   As shown in  FIG. 4 , tunnels  71  and  73  connect guard device  22  to routers  32  and  34 , respectively. In order to inject packets via router  28  to server  30  or  69  at step  60 , the guard device encapsulates the packets with a suitable label in accordance with the selected tunneling protocol. The label in each packet identifies the tunnel that the packet is to traverse. Router  28  is configured to support the selected protocol, so that upon receiving an encapsulated packet, router  28  simply forwards the packet along the pre-configured label-switched path (i.e., through the appropriate tunnel), without referring to its own routing table or making any other routing decision. Upon receiving the encapsulated packet, router  32  or  34  de-encapsulates the packet and then passes it on to server  30  or  69  in accordance with the destination IP address of the packet. 
   Note that in many networks, multiple tunnels may be established from guard device  22  leading to server  30 . In the example shown in  FIG. 1 , for instance, one tunnel may be established along path  40  to router  32 , and another along path  42  to router  34 . At step  60 , guard device  22  may then select one of the tunnels dynamically, in response to changes in network conditions or other factors. 
   Centralized Diversion in a Protected Network 
     FIG. 5  is a block diagram that schematically illustrates a protected area  70  of a network, in which peering routers  74  are configured to divert selected traffic to guard device  22  in accordance with an embodiment of the present invention. The protected area is typically configured as an autonomous system (AS), which communicates with other autonomous systems  72  via peering routers  74 . For example, protected area  70  may comprise a campus or enterprise network or an access network maintained by an Internet Service Provider (ISP), while systems  72  are part of a WAN, such as the Internet, or belong to other entities. More generally, however, peering routers  74  may comprise any group of one or more routers that are coupled to receive communication traffic from outside a given area of the network and to convey the traffic to internal routers (and other network elements) inside the area. Under normal traffic conditions, when peering routers  74  receive traffic destined for server  30  from systems  72 , the peering routers forward the traffic through area  70  via internal routers  76 ,  77  in area  70  to an edge router  80 . The edge router then forwards the traffic to server  30 , possibly via an external router  82  on the premises of the customer operating the server. 
   When traffic diversion is required, such as during a suspected DDoS attack on server  30 , a divert-from router  78  inside area  70  diverts the traffic that is destined for server  30  to guard device  22 . (Typically, the divert-from router also serves as one of the intermediate routers for normal traffic.) In this embodiment, however, it is also necessary to ensure that all traffic destined for server  30  that is received by peering routers  74  be forwarded to divert-from router  78 , and not bypass the divert-from router via another path, through router  77 , for example. Although guard device  22  could achieve this result by sending a BGP announcement at step  50  ( FIG. 2 ) to all the routers in area  70 , such an announcement incurs substantial overhead, and it is desirable to limit the BGP announcement to a smaller number of routers. 
   For this purpose, in the present embodiment, tunnels  84  are established between peering routers  74  and guard device  22 , via divert-from router  78 . Any suitable tunneling protocol may be used for this purpose, including the above-mentioned MPLS, GRE, IPIP and L2TP protocols. When diversion to guard device  22  is invoked, the peering routers encapsulate packets destined for server  30  with the appropriate tunneling labels, and forward the encapsulated packets through tunnels  84 . These tunnels pass through intermediate routers  76 , which simply forward tunnel-encapsulated packets along the pre-defined label-switched tunnel paths, without having to refer to their own routing tables. Router  78  may be configured as an egress proxy, as defined by MPLS, meaning that the router de-encapsulates the packets sent over tunnels  84  and delivers the original packets, without encapsulation, to guard device  22  at the end of the tunnel. This proxy feature relieves the guard device of having to support the tunneling protocol itself. 
   When guard device  22  determines that diversion of packets is necessary, it activates diversion (step  50 ,  FIG. 2 ) by sending an iBGP announcement to peering routers  74 . This announcement includes no-advertise and no-export strings, so that the peering routers do not propagate the routing change to any of the other routers in area  70 . The iBGP announcement instructs routers  74  that all traffic destined for server  30  should be forwarded via tunnels  84 . (It is not necessary to send the iBGP announcement to divert-from router  78 , since the divert-from router, as egress proxy, passes all packets received through tunnels  84  on to guard device  22  without having to make its own routing decision.) As noted earlier, the iBGP announcement sent by guard device  22  at step  50  overrides the original BGP announcement used for normal routing of packets to server  30 , typically by using a longer IP address prefix in the announcement at step  50  than was used in the original announcement. 
   After processing the packets at stage  54 , guard device  22  conveys the packets back to router  78  at step  60  ( FIG. 2 ). In this case (unlike the preceding embodiments), the main routing table of router  78  has not been modified by guard device  22 . Router  78  will therefore pass the packets injected by guard device  22  on to server  30  via the normal route that was previously determined. Alternatively, guard device  22  may be coupled to convey the processed packets to server  30  via another route, such as directly via router  80 , bypassing router  78 . 
   Further alternatively or additionally, it is possible (if less efficient) for guard device  22  to send a BGP announcement at step  50  instructing all routers  74 ,  76  to forward traffic destined for server  30  to guard device  22 . In this case, it is necessary to ensure that routers  78  and  80  will still forward traffic injected by the guard device on to server  30 , notwithstanding the BGP announcement. One way to achieve this result is to use a tunnel from guard device  22  to router  80  or router  82 , as in the embodiment of  FIG. 4 , described above. A tunnel from guard device  22  leading to server  30  may also be used in conjunction with diversion via tunnels  84 . 
   Although server  30  is shown in  FIG. 4  as being outside the AS of area  70 , the methods described hereinabove may similarly be applied in diverting traffic that is destined for nodes inside the AS. 
   Traffic Diversion in Layer-2 Topologies 
     FIG. 6  is a block diagram that schematically illustrates a protected area  100  of a network, in accordance with another embodiment of the present invention. In this topology, divert-from router  28  is connected via a Layer-2 switch  104 , such as an Ethernet switch, to other elements in a LAN  102 . The LAN also contains guard device  22  and next-hop routers  108  and  110 , which are coupled to forward traffic to server  30 . At the Layer-3 level, all the elements of LAN  102  belong to the same IP subnet. 
   When guard device  22  instructs router  28  to activate diversion of packets destined for server  30 , the diverted packets are conveyed to the guard device over path  36  through switch  104 . Path  38 , for injection of processed traffic by guard device  22 , passes back through switch  104  but does not generally pass back through router  28  as in the embodiments described above. Therefore, there is generally no need in the Layer-2 topology to configure special filtering or other routing rules on router  28  to handle the packets that are returned by the guard device for forwarding to server  30 . Rather, guard device  22  resolves the MAC address of router  108  or  110 , and then forwards the packets via switch  104  to the appropriate MAC address. Any suitable method known in the art may be used by the guard device to ascertain the MAC address to which the packets are to be forwarded. For example, the guard device may issue an Address Resolution Protocol (ARP) query on LAN  102  for this purpose. 
   Guard device  22  may also be configured to protect network elements in LAN  102 , such as a server  112 . In this case, the guard device and server  112  belong to the same IP subnet. Thus, the guard device may forward traffic to the server directly through switch  104 , without further IP forwarding. 
   Next-Hop Discovery 
   A number of different methods may be used to inform guard device  22  of the route that is to be used in forwarding traffic to server  30 . One method is simply to pre-program the guard device and/or router  28  with routing information with respect to all of the network elements that the guard device is intended to protect. This information is used in forwarding packets at steps  60  and  62  ( FIG. 2 ). 
   Alternatively, in some embodiments of the present invention, guard device  22  is configured to automatically learn the identity of the next-hop router to be used in forwarding packets to server  30  and to other network elements. Typically, the guard device uses information provided by routing protocols, such as BGP or an Interior Gateway Protocol (IGP) used by the routers in network  20  ( FIG. 1 ) or in protected area  70  ( FIG. 4 ). Common IGPs include the Routing Information Protocol (RIP), Open Shortest Path First (OSPF) and the Intermediate System-to-Intermediate System protocol (IS-IS). The routing information that is provided by the routing protocol to divert-from router  28  ( FIG. 1 ) or router  78  ( FIG. 4 ) indicates the IP addresses of the possible next-hop router or routers to server  30 . Thus, if guard device  22  has the same view of this routing information as does the divert-from router, it will be able to determine the next-hop router to use in each case. In some cases, internal routing information provided by IGP is sufficient, while in other cases BGP information is required, as well, depending on whether guard device  22  and server  30  are part of the same AS. 
   In order to receive BGP information from the divert-from router, it is sufficient that guard device  22  be an iBGP neighbor of the router. The divert-from router will then propagate all the BGP announcements that it receives to the guard device, as provided by BGP convention. 
   On the other hand, IGP information is not automatically propagated in this manner. Therefore, in order to share in the IGP announcements that the divert-from router receives, guard device  22  may be connected by tunnels to the same next-hop routers as is the divert-from router. Tunnels  71  and  73 , as shown in  FIG. 4 , may be used for this purpose. Via these tunnel connections, the guard device will receive the same IGP routing information with respect to next-hop routers  32  and  34  as will router  28 , which is connected to routers  32  and  34  by physical connections. 
   As noted above, when server  30  does not belong to the same AS as the divert-from router, guard device  22  may need both IGP and BGP information in order to determine the next-hop router for forwarding traffic to the server. In this case, the guard device gathers BGP information in order to learn the route (or routes) from its own AS to server  30 , and uses IGP information to learn the routes within the AS that connect with these external routes. The guard device combines these pieces of information in order to identify the next-hop routers. 
   While gathering the desired IGP and BGP information during the next-hop learning process, guard device  22  generally avoids announcing any routing information of its own or receiving any traffic through tunnels  71  and  73  other than routing announcements. For these purposes, guard device  22  is typically configured not to redistribute any information that it learns from iBGP, and tunnels  71  and  73  are assigned high routing costs, so that routers do not attempt to route traffic through them. 
   The methods described above are useful when the divert-from router uses automatic routing protocols in making its routing decisions. Alternatively or additionally, guard device  22  may query the divert-from router directly in order to determine the next hop on the route to server  30  (and to other protected network elements). This sort of querying may be used regardless of whether automatic routing protocols or static routing is used to configure the divert-from router. 
   A number of different management protocols may be used to query the divert-from router. For example, guard device  22  may send a query to the router using a Telnet or Secured Shell (SSH) connection. In this case, the guard device sends the command: “show ip route &lt;target IP address&gt;,” causing the divert-from router to respond with the IP address of the next-hop router to the target IP address. Alternatively, the guard device may use a Simple Network Management Protocol (SNMP) query, or may use a Remote Shell (RSH) or Remote Command (RCMD) protocol to request the next-hop information from the router, without establishing a remote session with the router as required by Telnet or SSH. 
   Although methods and devices for traffic diversion are described hereinabove in the context of protecting a target address from receiving malicious traffic, the principles of the present invention may similarly be used in other applications of traffic diversion and filtering. For example, diversion may be useful in analysis of traffic flows for purposes such as detecting anomalies in data flows, billing, forensic analysis and law enforcement, and traffic engineering. Diversion has the advantage in these applications that the computer that is to analyze the traffic receives only the part of the traffic that is relevant for analysis, and is not burdened with filtering all the traffic on a given network link. In other words, diversion permits the analyzing computer (such as guard device  22 ) to operate at relatively low communication bandwidth, and to focus its resources on the particular analysis task at hand. 
   Diversion of this sort may also be used to permit the analyzing computer to process a number of different flows cyclically, one after the other. For this purpose, the diverting router (such as router  28 ) may cycle through the different flows, beginning by diverting the first flow to the analyzing computer for a certain period of time, then stopping diversion of the first flow and starting diversion of the second flow for its allotted time period, and so on through the entire cycle. 
   It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.