Abstract:
A data transmission system and method for transmitting packetized data from an Internet Protocol (IP) host, having at least an IP layer and a network layer, to a plurality of workstations by the intermediary of an IP network, wherein the IP host is connected to the IP network via a layer 2 network interfacing the IP network with a set of routers. The IP host further includes a Multiple Address Resolution Protocol (MARP) layer between the IP layer and the network layer for selecting one of the set of routers in response to the next hop IP addresses provided by the IP layer to the MARP layer when a packet of data is be transmitted from the IP host to one of the workstations.

Description:
TECHNICAL FIELD 
     The present invention relates in general to data processing systems, and in particular, to a new way for loading balancing outgoing Internet Protocol (IP) packets from an IP host such as a large Web server, and relates in particular to a multiple Address Resolution Protocol (MARP) functionality for an IP data transmission system. 
     BACKGROUND INFORMATION 
     Modern digital networks are made to operate over different transmission media and interconnect upon request a very large number of users (e.g., hosts) in applications through fairly complex digital communication networks. 
     Do to the large variety of users&#39; profiles and distributed applications, the traffic is becoming more and more bandwidth consuming, non deterministic and requiring more connectivity. This has been the driver for the emergence of fast packet switching techniques in which data from different origins are chopped into fixed or variable length packets or datagrams, and then transferred, over high-speed digital networks, between a data source and target terminal equipment. 
     Several types of networks have been installed throughout the world, which need to be interconnected for example (e.g., via so-called routers) to optimize the possibilities of organizing traffic between source hosts and target hosts located anywhere in the world. This is made possible by using so-called Internetworking. 
     Internetwork (also referred to as Internet) facilities use a set of networking protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP) developed to allow cooperating host computers to share resources across the Internetwork. TCP/IP is a set of data communication protocols that are referred to as the Internet protocol (IP) suite. Because TCP and IP are the best known, it has become common to use the term TCP/IP to refer to the whole protocol family. TCP and IP are two of the protocols in this suite. Other protocols of the suite are User Datagram Protocol (UDP), Address Resolution Protocol (ARP), Real Time Protocol (RTP), etc. 
     An Internet may thus be a collection of heterogeneous and independent networks using TCP/IP, and connected together by routers. The administrative responsibilities for the Internet (e.g., to assign IP addresses and domain names) can be within a single network, for example a Local Area Network (LAN), or distributed among multiple networks. 
     When the communication of data has to be established from a source host to a particular computer IP destination over an IP network, there are a number of methods to determine the first hop router of the network leading towards this destination. These include running (or snooping) dynamic routing protocol such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF) version, running an Internet Control Message Protocol (ICMP) router discovery client or using a statically configured default route. 
     Running a dynamic routing protocol on every end-host may be infeasible for a number of reasons, including administrative overhead, processing overhead, security issues, or lack of a protocol implementation for some platforms. Neighbor or router discovery protocols may require active participation by all hosts on a network, leading to large timer values to reduce protocol overhead in face of a large number of hosts. This can result in significant delay in the detection of a lost (i.e., dead) neighbor, which may introduce unacceptably long “black hole” periods. 
     The use of a statically configured default route is quite popular, it minimizes configuration and processing overhead on the end-host, and is supported by virtually every IP implementation. This mode of operation is likely to persist as Dynamic Host Configuration Protocols (DHCP) are deployed, which typically provide configuration for an end-host IP address and default gateway. However, this creates a single point of failure. Loss of the default router results in a catastrophic event, isolating all end hosts that are unable to detect any alternate path that may be available. 
     One solution to solve this problem is to allow hosts to appear to use a single router and to maintain connectivity even if the actual first hop router they are using fails. Multiple routers participate in this protocol and in concert create the illusion of a single virtual router. The protocol insures that one and only one of the routers is forwarding packets on behalf of the virtual router. End hosts forward their packets to the virtual router. The router forwarding packets is known as the active router. A standby router is selected to replace the active router should it fail. The protocol provides a mechanism for determining active and standby routers using the IP addresses on the participating routers. If an active router fails, a standby router can take over without a major interruption in the host&#39;s connectivity. 
     Another similar approach is the use of Virtual Router Redundancy Protocol (VRRP) designed to eliminate the single point of failure inherent in the static default routed environment. VRRP specifies an election protocol that dynamically assigns responsibility for a virtual router to one of the VRRP routers on a LAN. The VRRP router controlling the IP address (es) associated with a virtual router is called the Master and forwards packets sent to these IP addresses. The election process provides dynamic fail-over in the forwarding responsibility should the Master become unavailable. Any of the virtual router&#39;s IP addresses on a LAN can then be used as the default first hop router by the end-hosts. The advantage gained by using VRRP is a higher availability default path without requiring configuration of dynamic routing or router discovery protocols on every end-host. 
     Unfortunately, the two solutions above cannot provide load balancing for a given host&#39;s traffic because only the router that answered the ARP is used. Also, customers are reluctant to change their main router configuration to enable such a function. Clearly there is a need for a method of providing load balancing for traffic on the Internet. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is to provide a data transmission system including an Internet Protocol (IP) network wherein it is the IP host that selects directly the default router thereby improving load balancing and high availability. 
     Another object of the invention is to enable an IP source host to be aware of the availability of a set of candidate default routers and to select one of them dynamically, insuring both load balancing and high availability. Another object of the invention is a method of selecting a router amongst a set of routers for an IP host in a data transmission system including an IP network. 
     Therefore, the invention relates to a data transmission system for transmitting packetized data from of an IP host having at least an IP layer and a network layer to a plurality of workstations by the intermediary of an IP network and wherein the IP host is connected to the IP network via a layer  2  network interfacing the IP network by a set of routers, the IP host further including a multiple Address Resolution Protocol (MARP) layer between the IP layer in the network layer for selecting one of the set of routers in response to the next hop IP address provided by the IP layer to the MARP layer when a packet of data is to be transmitted from the IP host to one of the workstations. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the invention will be better understood by reading the following more particular description of the invention in conjunction with the accompanying drawings wherein: 
         FIG. 1  is prior art which illustrates schematically a data transmission system wherein an Internet Protocol (IP) host can select one router from a set of routers; 
         FIGS. 2A and 2B  illustrate respectively the Multiple Address Resolution Protocol (MARP) table and the Address Resolution Protocol (ARP) table used in combination to achieve a method according to the invention; 
         FIG. 3  is a flow chart of a method of selecting a router according to the invention; and 
         FIG. 4  is illustrates schematically a data transmission system wherein an Internet Protocol (IP) host can select one router amongst a set of routers according to the invention showing a MARP layer. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted in as much as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
       FIG. 1  illustrates a data transmission system wherein an Internet Protocol (IP) host  10  has to transmit data to one or several work stations  12 ,  14  via and IP network  16  such as the Internet. It can be assumed that IP host  10  is connected to IP network  16  by means of a layer  2  network such as a Local Area Network(LAN)  18  which is interfacing to IP network  16  by a set of input routers  20 ,  22  and  24 . The IP packets are routed over the IP network  16  via a plurality of routers (not shown) and then by an output router  26  connected directly (or by means of a layer  2  network) to workstations  12  or  14 . 
     As illustrated in  FIG. 1 , to communicate over the IP network  16 , IP host  10  must implement a layered set of protocols  28  referred as the Internet protocol suite. Without the invention, the protocol suite would be used as follows;
         the application layer  30  (level  5 ) generates a data stream to be sent and passes this data stream to a transport layer;   the transport layer (level  4 ) such as Transmission Control Protocol (TCP) layer  32 , segments the data stream into packets and passes the packet to the IP layer  34  for routing to the destination IP address with an added TCP header;   the IP layer  34  finds the next hop IP address based upon the destination IP address. Normally, with the IP Host which does not run a routing protocol, this address is a default entry that leads to a default router;   IP layer  34  passes the packet to the network layer (not shown) with an added IP header information. As a side parameter, the IP layer informs the network layer of the next hop IP address; the network layer resolves the next hop IP address into a network address of the default router using the ARP protocol and transmits packets over the IP network  16 .       

       FIG. 4  illustrates an embodiment of the present invention where a new layer, multiple ARP (MARP)  36  is introduced between IP layer  34  and the network layer. IP layer  34  passes the packet and the next hop IP addresses to MARP layer  36  instead of the network layer. As explained below, this MARP layer  36  runs an algorithm to determine the best physical router (e.g., from exemplary routers  20 ,  22  or  24 ) based on parameters defined in packets such as source and destination addresses and ports. 
     At an exemplary destination workstation  12 , a reciprocal protocol suite  38  is implemented. Namely, the network layer passes the IP packets to IP layer  40  which transfers the packets to TCP layer  42  reassembling them into a data stream communicated to the application layer  44 . Note that workstation  12  does not include a MARP layer since such a layer is not required for receiving data. However, exemplary workstation  12  could also be an IP Host  10  provided with such a MARP layer used by embodiments of the present invention to transmit IP packets over the network in the same way as illustrated by IP host  10 . 
     The MARP layer  36  operates with a table called the MARP table illustrated in  FIG. 2A . The MARP table maps the next hop IP address into a set of candidate IP addresses corresponding to candidate routers amongst the set of routers (illustrated in  FIG. 4  as  20 ,  22  and  24 ) interfacing to the IP network  16 . In the simplest form, there is only one entry in the MARP table for the default router, the default router points to the set of candidate routers; which may act as default routers. The candidate routers associated with the IP addresses in a MARP table can either be configured to the MARP layer  36  via a configuration tool, or be dynamically acquired using a learning protocol such as an extension to the Dynamic Host Configuration Protocol (DHCP). 
     As some ones of the candidate routers may not be active at a given time, the MARP layer  36  uses the ARP table  206  provided by the network layer. The ARP table  206  is illustrated in  FIG. 2B . The ARP table  206  maps the IP addresses  201  provided by the MARP table  205  into network addresses  202 . 
     Referring now to  FIG. 3 , the method steps used in the selection of an active router are illustrated in a flow diagram. When an IP packet  50  is to transmit over the network, the MARP layer  36  is called by the IP layer  34  and the next hop IP address (usually that of the default router) is provided as a parameter for looking up the MARP table (step  51 ). If the next hop IP address matches an entry in the MARP table (step of  52 ), an associated list of candidate routers is built (step  54 ). The candidate routers are then checked one by one in the ARP table (step  56 ). A determination is made (step  58 ) of the candidate routers which have a recent entry in the ARP table, and these routers are selected as active candidate routers in step  62 . Note that, if no active candid routers can be determined (step  58 ), the package is destroyed in step  60  and a return is executed in step  68 . 
     Out of the list of the active candidate routers, the MARP layer  36  selects (step  62 ) one IP address corresponding to a candidate router that is passed to the network layer as a substitute of the original next hop IP address as selected by the IP layer  34 . In an embodiment of the present invention, selection is performed on a per packet basis, without a history of previous selection, but this is not the only possible selection algorithm. Other techniques like round robin or byte wise weighting mechanisms may be used alternatively. A hash coding technique as described in European Patent Application 98480062.3 may be use in order to stick a TCP connection to a same candidate router as long as the candidate topology is left unchanged. The hash coding technique uses the destination IP address and a pair of ports in packets. These are mingled with a candidate router&#39;s IP addresses, one by one. The highest resulting hash value is selected. Weight coefficients may be used to modify the statistical expectancy of each individual candidate, in order to match their capacity. After a candidate router is selected in step  62 , IP packets are sent to the network layer (step  64 ) so they can be transmitted to the candidate router which has been selected. The IP packet is sent to the network layer in step  64  and a return to step  50  is executed in step  66 . It should be noted that the IP packet will be sent directly to the network layer when no matches have been found (step  52 ) when looking up the next hop address in the MARP table. The IP packet will be sent directly to the network layer, because in this case, the next hop IP address corresponds to a router or a host which is not required to be substituted. 
     The MARP layer  36  only uses candidates that are already present in the ARP table. As a consequence, the MARP layer  36  uses an out-of-band technique to insure that the ARP table is correctly filled with all the up-to-date information. In embodiments of the present invention, periodic void packets, like ICMP echo, are transmitted to the non-active routers, which are candidate routers that were not present in the ARP table. Upon such packets the ARP function in a network layer will automatically refresh the entry by using the ARP protocol. Also, at an initialization time, one such packet is sent to all the configuration routers to preset the ARP table before a single data is issued by an application layer. 
     The ARP function insures the freshness of the ARP table by aging the entries and flushing the older ones. To maintain the status of active candidate routers, methods in embodiments of the present invention consist in resetting the age of an entry each time a packet is received from a matching network address. Also, if an entry gets old and before it is flushed by ARP, MARP may flush the ARP table entry before it passes a packet to the network layer with the next hop IP address pointing to that router. Again this forces the network layer to use ARP procedures to check for the router availability. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.