Patent Publication Number: US-7711831-B2

Title: Methods, systems and computer program products for source address selection

Description:
RELATED APPLICATIONS 
     The present application is related to commonly assigned and concurrently filed U.S. patent application Ser. No. 09/862,968, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR PORT ASSIGNMENTS OF MULTIPLE APPLICATION INSTANCES USING THE SAME SOURCE IP ADDRESS”, the disclosure of which is incorporated herein by reference as if set forth fully herein. 
     FIELD OF THE INVENTION 
     The present invention relates to network communications and more particularly to network communications to a cluster of data processing systems. 
     BACKGROUND OF THE INVENTION 
     The Internet Protocol (IP) is a connectionless protocol. IP packets are routed from an originator through a network of routers to the destination. All physical adapter devices in such a network, including those for client and server hosts, are identified by an IP Address which is unique within the network. One valuable feature of IP is that a failure of an intermediate router node or adapter need not prevent a packet from moving from source to destination, as long as there is an alternate path through the network. 
     In Transmission Control Protocol/Internet Protocol (TCP/IP), TCP sets up a connection between two endpoints, identified by the respective IP addresses and a port number on each. Unlike failures of an adapter in an intermediate node, if one of the endpoint adapters (or the link leading to it) fails, all connections through that adapter generally fail and must be reestablished. If the failure is on a client workstation host, only the relatively few client connections are typically disrupted and usually only one person is inconvenienced. However, an adapter failure on a server may mean that hundreds or thousands of connections may be disrupted. On a System/390 with large capacity, the number may run to tens of thousands. 
     To alleviate this situation, International Business Machines Corporation introduced the concept of a Virtual IP Address, or VIPA, on its TCP/IP for OS/390 V2R5 (and added to V2R4 as well). Examples of VIPAs and their use may be found in U.S. Pat. Nos. 5,917,997, 5,923,854, 5,935,215 and 5,951,650. A VIPA is typically configured the same as a normal IP address for a physical adapter, except that it is not associated with any particular device. To an attached router, the TCP/IP stack on System/390 simply looks like another router. When the TCP/IP stack receives a packet destined for one of its VIPAs, the inbound IP function of the TCP/IP stack notes that the IP address of the packet is in the TCP/IP stack&#39;s Home list of IP addresses and forwards the packet up the TCP/IP stack. The “home list” of a TCP/IP stack is the list of IP addresses which are “owned” by the TCP/IP stack. Assuming the TCP/IP stack has multiple adapters or paths to it (including a Cross Coupling Facility (XCF) path from other TCP/IP stacks in a Sysplex), if a particular physical adapter fails, the attached routing network will route VIPA-targeted packets to the TCP/IP stack via an alternate route. The VIPA may, thus, be thought of as an address to the stack, and not to any particular adapter. 
     U.S. patent application Ser. No. 09/401,419 entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR AUTOMATED MOVEMENT OF IP ADDRESSES WITHIN A CLUSTER” filed Sep. 22, 1999, the disclosure of which is incorporated herein by reference as if set forth fully herein, describes dynamic virtual IP addresses (VIPA) and their use. As described in the &#39;419 application, a dynamic VIPA may be automatically moved from protocol stack to protocol stack in a predefined manner to overcome failures of a particular protocol stack (i.e. VIPA takeover). Such a predefined movement may provide a predefined backup protocol stack for a particular VIPA. VIPA takeover was made available by International Business Machines Corporation (IBM), Armonk, N.Y., in System/390 V2R8 which had a general availability date of September, 1999. 
     As described above, the Virtual IP Address (VIPA) on TCP/IP for MVS and OS/390 allows clients to establish TCP connections and send UDP datagrams to a server using an IP address that is owned by a TCP/IP stack, and reachable via any interface, but not tied to any particular adapter. This allows such connections or UDP datagram transmissions to be unaffected by a failure of one adapter owned by the TCP/IP stack, as long as at least one other device for external connectivity to the same network remains operational. 
     A TCP program on OS/390 may also initiate an outbound connection, acting as a client rather than a server for the purposes of that particular connection. Such a TCP program will typically not bind the socket to any particular local address before initiating the connection and normal TCP rules will use the address of the adapter on which the connection request is transmitted. As a result, the connection may be lost if that same adapter or link should fail while the connection is still active. 
     To provide the benefits of VIPA to outbound connections, the SOURCEVIPA function was provided to allow the customer to configure a VIPA to be associated with a group of adapters/links, and to cause TCP/IP to use the VIPA instead of the adapter address when a hosted TCP program initiates a connection without binding the socket to a particular IP address. 
     The above approach works well for a TCP program that is hosted by only one TCP/IP stack for all time and/or when the program receiving the connection request does not care what IP address is used for the source address of the connection request. There are a number of cases, however, where the traditional SOURCEVIPA approach does not meet the needs of particular applications. 
     For example, some application pairs require both members to function as both client and server, in that one partner establishes a connection to the other, which in turn establishes a connection back to the first. Such applications will often use the source and destination IP addresses to correlate the paired connections. Dynamic VIPAs (DVIPAs) were invented by IBM to address the problem of outages suffered by a TCP/IP stack or an underlying OS/390 image or S/390 processing complex. A DVIPA is a VIPA which can move from one TCP/IP stack to another, under program control, managed either by collaborating stacks or which can be made active on a TCP/IP stack in response to programming actions by an application. Since DVIPAs by their very nature may move from stack to stack, they typically cannot be used for SOURCEVIPA which must, in general, be predictable to be useful by the customer. When one of the application pair resides on OS/390 and uses a DVIPA for incoming connections, SOURCEVIPA will not result in using the same DVIPA for the return connection. The application can be programmed specifically to bind the socket for the outbound connection to the DVIPA, but this may make the resulting application less portable, and may require per-application reprogramming. 
     In a further example, some servers may need to establish connections to other servers in order to process a request from the original client. These secondary connections may cross enterprises, and may, thus, traverse multiple firewalls. When such servers are used in conjunction with Sysplex Distributor or other IP workload balancing solutions, a server instance on one TCP/IP stack could use a different SOURCEVIPA than a server instance on another stack. All intervening firewalls may, therefore, need to be configured to be aware of, and permit passage of, connection requests from any possible VIPA that might be used as a SOURCEVIPA. The benefit of Sysplex Distributor is that clients outside the Sysplex generally reach all such server instances using the same IP address. Thus, customers would, typically, prefer the server instances going to secondary servers to use the same Distributed DVIPA to reduce configuration of intervening firewalls to a single IP address, independent of where the server instances reside (or might be moved) within the Sysplex. 
     Furthermore, multiple server instances in a Sysplex Distributor environment may need to establish connections to exactly the same secondary server. A TCP connection is generally identified by source and destination IP address, and source and destination port numbers (the combination of which is known as the “connection 4-tuple”). In this case, the destination IP address and port are the same for all such connections. Programs initiating outbound connections seldom specify source IP address, and almost never specify the port. Instead, they rely on the TCP/IP stack to select a port which is at present not in use by another TCP application. Such a port assignment is known as an “ephemeral port.” When a source IP address could reside on only one TCP/IP stack, it did not matter that two different stacks might assign the same ephemeral port number, because the source IP addresses for connections to the same secondary server would be different. Now that Sysplex Distributor allows the same IP address to reside on multiple TCP/IP stacks, and for that same IP address to be used for connections to the rest of the network, it is possibile that two stacks could generate the same connection 4-tuple for connections from different Sysplex Distributor server instances to the same secondary server. 
     One advantage of OS/390 lies in serving multiple different workloads within the same operating system image and cluster (S/390 Parallel Sysplex). Sysplex Distributor allows a single IP address (the Distributed DVIPA) to be associated with all instances of a particular server application administratively with the Domain Name Server. However, the same TCP/IP stack may be a target stack for multiple Distributed DVIPAs, and many applications that use DVIPAs that are not distributed. In the absence of application programming, there is typically no way for the TCP/IP stack or the system administrator to designate which of many possible DVIPAs should be used for a particular application. Sysplex Distributor as well as dynamic VIPAs are described in U.S. patent application Ser. No. 09/640,412, entitled, “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR NON-DISRUPTIVELY TRANSFERRING A VIRTUAL INTERNET PROTOCOL ADDRESS BETWEEN COMMUNICATION PROTOCOL STACKS”, U.S. patent application Ser. No. 09/640,409, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR CLUSTER WORKLOAD DISTRIBUTION”, U.S. patent application Ser. No. 09/640,438, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR FAILURE RECOVERY FOR ROUTED VIRTUAL INTERNET PROTOCOL ADDRESSES” all filed Aug. 17, 2000, and U.S. patent application Ser. No. 09/401,419 entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR AUTOMATED MOVEMENT OF IP ADDRESSES WITHIN A CLUSTER” filed Sep. 22, 1999, the disclosures of which are each incorporated herein by reference as if set forth fully herein. VIPAs were provided in OS/390 V2R5 which had a general availability date of March, 1998, Sysplex Distributor was provided in OS/390 V2R10 which had a general availability date of September, 2000 and dynamic VIPAs were provided in OS/390 V2R8 which had a general availability date of September, 1999, both from International Business Machines Corporation, Armonk, N.Y. 
     SUMMARY OF THE INVENTION 
     Methods, systems and computer program products according to embodiments of the present invention provide for establishing a connection originated by an application executing on a data processing system in a cluster of data processing systems. A dynamic network address is associated with the application at the data processing system on which the application is executing. If a request is received for the data processing system to originate a connection that is associated with the application, the connection is established utilizing the dynamic network address associated with the application as a source address for the connection. 
     In further embodiments of the present invention, it is determined if the application has specified a network address for the requested connection. The specified network address is used to establish the connection if the application has specified a network address. Furthermore, in such embodiments, the associated dynamic network address is selectively used as the source address for the connection if the application has not specified a network address for the requested connection. 
     In additional embodiments of the present invention, determining if the application has specified a network address for the requested connection is provided by determining if a socket for the connection has been bound to a network address. 
     In still further embodiments of the present invention, the application is one of a plurality of instances of an application executing on the data processing system in the cluster of data processing systems. In such embodiments, associating a dynamic network address with the application at the data processing system on which the application is executing may be provided by associating a dynamic network address with the one of the plurality of instances of the application at the data processing system on which the one of the plurality of instances of the application is executing. Furthermore, determining if a request for the data processing system to originate a connection is associated with the application may be provided by determining if a request for the data processing system to originate a connection is associated with the one of the plurality of instances of the application. 
     In still further embodiments of the present invention, selecting a source address for a connection originated by an application executing on a data processing system in a cluster of data processing systems is provided by associating a dynamic virtual IP address (DVIPA) with the application at a communication protocol stack of the data processing system in the cluster of data processing systems so as to utilize the DVIPA as the source address for the connection originated by the application. 
     In yet further embodiments of the present invention, a DVIPA is associated with the application by receiving a connection request for a connection at the communication protocol stack, determining if the connection request received at the communication protocol stack is associated with the application and selecting the DVIPA as the source address for the connection if the connection request is from the application. Furthermore, it may be determined if the application is bound to an IP address. The IP address to which the application is bound may be selected as the source address if the application is bound to an IP address. In such embodiments, selecting the DVIPA may also be provided by selecting the DVIPA as the source address for the connection if the connection request is associated with the application and the application is not bound to an IP address. 
     In additional embodiments of the present invention, a predefined association of the DVIPA and the application is established at the communication protocol stack. In such embodiments, determining if the connection request received at the communication protocol stack is associated with the application may be provided by determining if the connection request is from the application. Furthermore, selecting the DVIPA as the source address for the connection if the connection request is from the application may be provided by selecting the DVIPA as the source address for the connection if the connection request is from the application and a predefined association of the DVIPA and the application has been established. 
     In still further embodiments of the present invention, establishing the predefined association of the DVIPA and the application may be provided by processing at the communication protocol stack a configuration statement which specifies the DVIPA and an application with which the DVIPA is associated. Furthermore, it may be determined if the DVIPA is configured for the communication protocol stack and an error message generated if the DVIPA is not configured for the communication protocol stack. It may also be determined if the DVIPA is active on the communication protocol stack and the DVIPA activated if the DVIPA is not active and if the DVIPA is in a range of DVIPAs specified for the communication protocol stack. An error message may be generated if the DVIPA is not active and is not in a range of DVIPAs specified for the communication protocol stack. 
     In yet additional embodiments of the present invention, the application is an instance of a plurality of instances of an application executing on the data processing system. Furthermore, the cluster of data processing systems may be an OS/390 Sysplex. 
     In further embodiments of the present invention, a system for establishing a connection between an application and a client includes a cluster of data processing systems, the application executing on a data processing system in the cluster of data processing systems and a communication protocol stack on the data processing system in the cluster of data processing systems, the communication protocol stack being configured to associate a dynamic virtual Internet protocol address (DVIPA) with the application so that the DVIPA is utilized as a source address for a connection request from the application. 
     In further embodiments of the present invention, communication protocol stack is further configured to determine if the application is bound to an IP address, select the IP address to which the application is bound as the source address if the application is bound to an IP address and select the DVIPA as the source address for the connection if the connection request is from the application and the application is not bound to an IP address. Additionally, the communication protocol stack may be configured to establish a predefined association of the DVIPA and the application and select the DVIPA as the source address for the connection if the connection request is from the application and a predefined association of the DVIPA and the application has been established. The communication protocol stack may also be configured to establish the predefined association of the DVIPA and the application by processing a configuration statement which specifies the DVIPA and an application with which the DVIPA is associated. 
     As will further be appreciated by those of skill in the art, the present invention may be embodied as methods, apparatus/systems and/or computer program products. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is block diagram of a cluster of data processing systems incorporating embodiments of the present invention; 
         FIG. 2  is a flowchart illustrating initialization operations according to embodiments of the present invention for source address selection; 
         FIG. 3  is a flowchart illustrating operations for source address selection according to embodiments of the present invention; 
         FIG. 4  is a flowchart illustrating operations for port selection for shared addresses according to embodiments of the present invention; 
         FIG. 5  is a flowchart illustrating operations for termination of a connection utilizing port selection according to embodiments of the present invention; 
         FIG. 6  is a block diagram of a CS/390 Sysplex incorporating Sysplex Distributor incorporating Sysplex Wide Security Associations according to embodiments of the present invention; 
         FIG. 7  is a flowchart illustrating operations for source address selection of a distributed virtual Internet Protocol address (DVIPA) according to embodiments of the present invention; 
         FIG. 8A  is a flowchart illustrating operations for initialization of cluster-wide port assignment according to embodiments of the present invention; 
         FIG. 8B  is a flowchart illustrating operations for initialization of cluster-wide port assignment with error recovery according to embodiments of the present invention; 
         FIG. 9  is a flowchart illustrating operations for cluster-wide port assignment for DVIPAs according to embodiments of the present invention; 
         FIG. 10  is a flowchart illustrating operations for termination of a connection utilizing cluster-wide port assignment according to embodiments of the present invention; 
         FIG. 11  is a flowchart illustrating operations according to embodiments of the present invention when a bind operation is requested by an application; and 
         FIG. 12  is a flowchart illustrating operations according to embodiments of the present invention for recovery from failure of a communication protocol stack utilizing cluster-wide port assignment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. Furthermore, for references to a bit being set may refer to a bit being set as a “1” or a “0” value and being reset to a “0” or a “1” value respectively. 
     As will be appreciated by those of skill in the art, the present invention can take the form of an entirely hardware embodiment, an entirely software (including firmware, resident software, micro-code, etc.) embodiment, or an embodiment containing both software and hardware aspects all generally referred to herein as a “circuit” or “module.” Furthermore, the present invention can take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code means embodied in the medium for use by or in connection with an instruction execution system. In the context of this document, a computer-usable or computer-readable medium can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The computer-usable or computer-readable medium can be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a nonexhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read-only memory (CD-ROM). Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner if necessary, and then stored in a computer memory. 
     Embodiments of the present invention will now be described with reference to  FIGS. 1 through 10  which are flowchart and block diagram illustrations of operations of protocol stacks incorporating embodiments of the present invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These program instructions may be provided to a processor to produce a machine, such that the instructions which execute on the processor create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. The computer program instructions may be executed by a processor to cause a series of operational steps to be performed by the processor to produce a computer implemented process such that the instructions which execute on the processor provide steps for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     Accordingly, blocks of the flowchart illustrations and/or block diagrams support combinations of means for performing the specified functions/acts, combinations of steps for performing the specified functions/acts and program instruction means for performing the specified functions/acts. It will also be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by special purpose hardware-based systems which perform the specified functions/acts or steps, or combinations of special purpose hardware and computer instructions. 
       FIG. 1  illustrates an environment in which embodiments of the present invention may be utilized. As seen in  FIG. 1 , the client  10  communicates over the network  12  to communicate with a distributing processor  50 . The distributing processor  50  may perform workload management and may distribute network communications for connections to a common IP address shared by the servers  52  and  54  such that the client  10  may communicate with any of the servers  52  or  54  utilizing the common IP address as a destination address. The distributing processor  50  may also function as a server and, thus, be the ultimate endpoint of communications with the client  10 . 
     The servers  52  and  54 , and the distributing processor  50  may be data processing systems in a cluster of data processing systems. The distributing processor  50  and the servers  52  and  54  may also provide for the movement of IP addresses such that an IP address may be moved from data processing system to data processing system. Accordingly, other data processing systems in the cluster may become the distributing processor for the IP address, for example, in the event of failure. The common IP address may, in certain embodiments, also be a dynamic IP address. Additionally, the common IP address and/or the dynamic IP address may also be virtual IP addresses. 
     In operation, when the distributing processor  50  receives communications from the client  10  to the common IP address, the distributing processor  50  routes these communications to appropriate ones of the servers  52  or  54 . Outbound communications from the servers  52  or  54  need not be routed through the distributing processor  50 . Furthermore, outbound connections to clients utilizing the common IP address may also be initiated without going through the distributing processor  50 . For example, a connection utilizing the common IP address, such as a connection to the server  52 , may have inbound communications routed through the distributing processor  50  and to the server  52  while outbound communications are routed from the server  52  to the network  12  without passing through the distributing processor  50 . Similarly, if the server  52  initiates a connection, this connection may be initiated directly onto the network  12 . 
       FIG. 1  also illustrates a common storage  64  which may be utilized by a port selector module or circuit  61  which may select a port for use by outbound connections utilizing the common IP address as a source address for the connection based on port status information contained in the common storage  64 . The port status information may be maintained by the port selector module(s) or circuit(s)  61  so as to provide up-to-date information on the availability of a port for a given common IP address. Such port selector modules  61  may operate as described herein, possibly in cooperation with other port selector modules  61  on processing systems in the cluster, and, thereby, coordinate selection of ports for the common IP address so as to provide a port which results in a unique identification, such as a unique 4-tuple (source address, source port, destination address, destination port), for each connection utilizing the common IP address. Furthermore, cluster-wide port assignment according to embodiments of the present invention may be provided alone or in combination with source address selection as described herein. 
     As will be appreciated by those of skill in the art, while the common storage  64  may be utilized to share information which may allow cluster-wide coordinated assignment of ports, other such information sharing techniques may also be utilized. For example, information could be broadcast or otherwise transmitted to processors and the information maintained at each processor using cluster-wide port assignment. Similarly, the processors could broadcast or otherwise transmit the information to the distributing processor which could maintain port information for each common IP address for which it had distribution responsibilities. Accordingly, other mechanisms for sharing information to provide cluster-wide port assignments may be utilized while still benefitting from the teachings of the present invention. 
       FIG. 1  also illustrates the servers  52  and  54  having a source address selection module or circuit  62 . The source address selection module or circuit  62  may function as described herein to associate predefined common IP addresses with a particular instance of an application, such as the application  63  illustrated in  FIG. 1 . Such an association may allow an application which does not specify a source address for an outbound connection to utilize the common IP address for the outbound connection. Thus, even if an application is moved from one server to another, the same common IP address may be utilized. Such may be accomplished without requiring the application to specify the particular IP address. Accordingly, portability and/or flexibility of the application between servers in a cluster and/or cluster configuration may be improved. Furthermore, address selection according to embodiments of the present invention may be provided alone or in combination with cluster-wide port assignment as described herein. 
     While the present invention is described above with reference to servers, such servers may also be referred to as hosts, target hosts or target data processing systems and represent an endpoint for communications from the network. Similarly, the distributing processor may be a data processing system or other network device, or portion thereof, capable of carrying out the operations described herein. 
     Operations for initialization of a source address selection module or circuit  62  according to embodiments of the present invention will now be described with reference to  FIG. 2 . As seen in  FIG. 2 , it may be determined if a configuration specification for a data processing system, for example for a communication protocol stack of the data processing system, includes a statement which identifies a source IP address and one or more application instances which are associated with the source IP address (block  200 ). If so, the data processing system associates the application instance(s) and the specified source IP address (block  210 ). Such associations may be provided on multiple data processing systems and, in fact, the same source IP address may be associated with application instances on different data processing systems. 
       FIG. 3  illustrates operations of an address selection module or circuit  62  for source address selection for an outbound connection request according to embodiments of the present invention. As seen in  FIG. 3 , it is determined if the application instance requesting the outbound connection has specified an IP address for the connection, for example, by binding the socket for the connection to a specific IP address (block  300 ). If the application did specify an IP address, the connection request is processed in a conventional manner (block  330 ) and the connection established utilizing the IP address selected by the application (block  320 ). If, however, the application instance has not specified an IP address (block  300 ), an identification of the application instance is obtained (block  305 ) to determine if a dynamic IP address is associated with the application instance (block  310 ). If a dynamic IP address is associated with the application instance (block  310 ), the associated dynamic IP address is selected as the source address for the connection (block  315 ) and the connection is established using the selected address as the source address (block  320 ). 
     If a dynamic IP address is not associated with the application (block  310 ), a source IP address is selected utilizing conventional address selection procedures (e.g. specification of a static VIPA via a SOURCEVIPA statement)(block  325 ) and the connection is established using the conventionally selected address as the source address (block  320 ). However, other operations may be carried out, for example, an error message may be generated or other mechanisms for selecting a source address for the connection request could be carried out. Such operations are beyond the scope of the present invention and, therefore, will not be described further herein. 
       FIG. 4  illustrates operations of a port selector module or circuit  61  for cluster-wide port assignment according to embodiments of the present invention. As seen in  FIG. 4 , the source address of an outbound connection request is evaluated to determine if a common IP address is specified as the source address (block  400 ). If the source address is not a common IP address (block  400 ), conventional port assignment may be utilized and the connection opened using conventional techniques (block  440 ). If, however, the source address is a common IP address (block  400 ), it may be determined if cluster-wide port assignment is provided for the common IP address (block  410 ). If cluster-wide port assignment is not provided for the common IP address (block  410 ), a local port assignment process may be used to select a port which is identified locally as available (block  430 ) and the selected port may be identified locally as unavailable (block  435 ) and the connection opened utilizing the selected port (block  425 ). 
     If, however, cluster-wide port assignment is provided for the common IP address (block  410 ), the common storage  64  is accessed to select an unused port for the connection (block  415 ). The selected port is identified in the common storage  64  as used or unavailable (block  420 ) so that other data processing systems will not select the same port. The connection is then opened using the common IP address and the selected port (block  425 ). 
       FIG. 5  illustrates operations of the port selector module or circuit  61  when a connection is terminated. As seen in  FIG. 5 , the source address of the connection is evaluated to determine if a common IP address is specified as the source address (block  500 ). If the source address is not a common IP address (block  500 ), conventional connection termination may be utilized (block  525 ). If, however, the source address is a common address (block  500 ), the connection may be terminated (block  505 ) and it may be determined if cluster-wide port assignment is provided for the common IP address (block  510 ). If cluster-wide port assignment is not provided for the common IP address (block  510 ), the port is identified locally as available (block  520 ). If, however, cluster-wide port assignment is provided for the common IP address (block  510 ), the common storage  64  is accessed and updated to reflect that the port is now an unused port (block  515 ) so as to make the port available for use in a subsequent connection. 
     In particular embodiments of the present invention, source address selection and/or cluster-wide port assignments may be provided in a Sysplex cluster utilizing Sysplex Distributor. The Sysplex Distributor was provided in OS/390 V2R10 (General Availability of September, 2000) and is described in detail in commonly assigned U.S. patent application Ser. No. 09/640,409, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR CLUSTER WORKLOAD DISTRIBUTION”, U.S. patent application Ser. No. 09/640,412, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR NON-DISRUPTIVELY TRANSFERRING A VIRTUAL INTERNET PROTOCOL ADDRESS BETWEEN COMMUNICATION PROTOCOL STACKS” and U.S. patent application Ser. No. 09/640,438, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR FAILURE RECOVERY FOR ROUTED VIRTUAL INTERNET PROTOCOL ADDRESSES”, the disclosures of which are incorporated herein by reference as if set forth fully herein. 
     In Sysplex Distributor, a single IP address is associated with a plurality of communication protocol stacks in a cluster of data processing systems by providing a routing protocol stack which associates a Virtual IP Address (VIPA) and port with other communication protocol stacks in the cluster and routes communications to the VIPA and port to the appropriate communication protocol stack. VIPAs capable of being shared by a number of communication protocol stacks are referred to herein as “dynamic routable VIPAs”. While the present invention is described with reference to a specific embodiment in a System/390 Sysplex, as will be appreciated by those of skill in the art, the present invention may be utilized in other systems where clusters of computers utilize virtual addresses by associating an application or application group, rather than a particular communications adapter, with the addresses. Thus, the present invention should not be construed as limited to the particular exemplary embodiments described herein. 
     A cluster of data processing systems is illustrated in  FIG. 6  as a cluster of nodes in Sysplex  10 . As seen in  FIG. 6 , several data processing systems  20 ,  24 ,  28 ,  32  and  36  are interconnected in a Sysplex  10 . The data processing systems  20 ,  24 ,  28 ,  32  and  36  illustrated in  FIG. 6  may be operating system images, such as MVS images, executing on one or more computer systems. While the present invention will be described primarily with respect to the MVS operating system executing in a System/390 environment, the data processing systems  20 ,  24 ,  28 ,  32  and  36  may be mainframe computers, mid-range computers, servers or other systems capable of supporting dynamic routable Virtual IP Addresses as described herein. 
     As is further illustrated in  FIG. 6 , the data processing systems  20 ,  24 ,  28 ,  32  and  36  have associated with them communication protocol stacks  22 ,  26 ,  30 ,  34  and  38 , which may be TCP/IP stacks. The communication protocol stacks  22 ,  26 ,  30 ,  34  and  38  have been modified to incorporate a VIPA distribution function  23  as described herein for providing dynamic routable VIPAs so as to provide a single IP address for multiple communication protocol stacks. 
     While each of the communication protocol stacks  22 ,  26 ,  30 ,  34  and  38  illustrated in  FIG. 6  incorporate the VIPA distribution function  23 , not all communication protocol stacks in a Sysplex need incorporate the VIPA distribution function  23 . Thus, the present invention may be carried out on any system where two or more communication protocol stacks in a cluster of data processing systems support dynamic routable VIPAs. 
     As is further seen in  FIG. 6 , the communication protocol stacks  22 ,  26 ,  30 ,  34  and  38  may communicate with each other through a coupling facility  40  of the Sysplex  10 , for example, utilizing XCF messaging. Furthermore, the communication protocol stacks  22  and  38  may communicate with an external network  44  such as the Internet, an intranet, a Local Area Network (LAN) or Wide Area Network (WAN) utilizing the Enterprise System Connectivity (ESCON)  42 . Thus, a client  46  may utilize the network  44  to communicate with an application executing on an MVS image in Sysplex  10  through the communication protocol stacks  22  and  38  which may function as routing protocol stacks as described herein. 
     As is further illustrated in  FIG. 6 , as an example of utilization of the present invention and for illustration purposes, data processing system  20  has associated with it communication protocol stack  22  which is associated with MVS image MVS 1 which has application APP A executing on MVS image MVS 1 and utilizing communication protocol stack  22  to allow access to, for example, client  46  through network  44 . Similarly, data processing system  24  has associated with it communication protocol stack  26  which is associated with MVS image MVS 2 which has a second instance of application APP A and an instance of application APP B executing on MVS image MVS 2 which may utilize communication protocol stack  26  for communications. Data processing system  28  has associated with it communication protocol stack  30  which is associated with MVS image MVS 3 which has a second instance of application APP B executing on MVS image MVS 3 which may utilize communication protocol stack  30  for communications. Data processing system  32  has associated with it communication protocol stack  34  which is associated with MVS image MVS 4 which has a third instance of application APP A executing on MVS image MVS 4 which may utilize communication protocol stack  34  for communications. Finally, data processing system  36  has associated with it a communication protocol stack  38  which is associated with MVS image MVS 5 which has a third instance of application APP B executing on MVS image MVS 5 which may utilize communication protocol stack  38  for communications. Furthermore, each of the protocol stacks  22 ,  26 ,  30 ,  34  and  38  are communication illustrated as including an IP address selection module or circuit (SIP)  25  and a cluster-wide port assignment module or circuit (CLP)  27 . 
     Utilizing the above described system configuration as an example, the VIPA distribution function  23  with cluster-wide port assignment and/or source IP address selection will now be described. The VIPA distribution function  23  allows for protocol stacks which are defined as supporting DVIPAs to share the DVIPA and communicate with the network  44  through a routing protocol stack such that all protocol stacks having a server application which is associated with the DVIPA will appear to the network  44  as a single IP address. Such dynamically routable VIPAs may be provided by designating a protocol stack, such as protocol stack  22 , as a routing protocol stack, notifying other protocol stacks of the routing protocol stack and having other protocol stacks notify the routing protocol stack when an application which binds to the DVIPA is started. 
     The communication protocol stacks  22 ,  26 ,  30 ,  34  and  38  may be configured as to which stacks are routing stacks, backup routing stacks and server stacks. Different DVIPAs may have different sets of backup stacks, possibly overlapping. The definition of backup stacks may be the same as that for the VIPA takeover function described in U.S. patent application Ser. No. 09/401,419, entitled “METHODS, SYSTEMS AND COMPUTER PROGRAM PRODUCTS FOR AUTOMATED MOVEMENT OF IP ADDRESSES WITHIN A CLUSTER” which is incorporated herein by reference as if set forth fully herein. 
     Utilizing the system illustrated in  FIG. 6  for the present example, the application APP A is associated with a DVIPA VA1 which may be associated with the respective first second and third instances of APP A; and application APP B likewise has a DVIPA VB1 associated with the respective first, second and third instances of APP B. 
     Configuration of a dynamic routable VIPA may be provided by a definition block established by a system administrator for each routing communication protocol stack  22  and  38 . Such a definition block is described in the above referenced United States Patent Applications and defines dynamic routable VIPAs for which a communication protocol stack operates as the primary communication protocol stack. Backup protocol stacks may be defined as described for the VIPA takeover procedure. Thus, the definition block “VIPADynamic” may be used to define dynamic routable VIPAs. Within the VIPADynamic block, a definition may also be provided for a protocol stack supporting moveable VIPAs. All of the VIPAs in a single VIPADEFine statement should belong to the same subnet, network, or supernet, as determined by the network class and address mask. VIPAs may also be defined as moveable VIPAs which may be transferred from one communication protocol stack to another. 
     Similarly, within the definitions, a protocol stack may be defined as a backup protocol stack and a rank (e.g. a number between 1 and 254) provided to determine relative order within the backup chain(s) for the associated dynamic routable VIPA(s). A communication protocol stack with a higher rank will take over the dynamic VIPAs before a communication protocol stack with a lower rank. 
     Within the VIPADYNamic block, a VIPA may be defined as a dynamic routable VIPA based on a VIPA address and a portlist which is a list of ports for which the DVIPA will apply. Alternatively, all ports may be considered to be usable for workload distribution when used with a dynamic routable VIPA. Also provided in the definition is a list of protocol stacks which will be included as server stacks in routing communications directed to the DVIPA. The IP addresses which define the potential server stacks may be XCF addresses of the protocol stacks or may be designated “ALL.” If “ALL” is designated, then all stacks in the Sysplex are candidates for distribution. This may include future stacks which are not active when the routing stack is initialized. Thus, if ALL is specified, a protocol stack may be added to the DVIPA without disruption of operations and without user intervention to redefine the stack in the VIPADynamic block. In addition to the above definitions, a range of IP addresses may be defined as DVIPAs utilizing the VIPARange definition. At initialization or profile changes, the communication protocol stacks  22 ,  26 ,  30 ,  34  and  38  communicate to partner communication protocol stacks the complete list of dynamic routable VIPAs, their associated potential servers and list of ports and the primary and backup definitions for the communication protocol stack. 
     When a communication protocol stack  22 ,  26 ,  30 ,  34 ,  38  receives the DVIPA information it notes if it is identified as a candidate target protocol stack or as a backup stack. If the receiving communication protocol stack is a candidate target stack, it monitors its applications and sends a message to the defined routing stack when an application instance is bound to the DVIPA and listens on a defined port. If the receiving communication protocol stack is a backup stack, it stores the DVIPA information for use in the event of failure of the primary routing stack. 
     In addition to the conventional configuration statements provided for dynamically routeable VIPAs, an additional configuration statement is provided to provide source address selection for DVIPAs according to embodiments of the present invention. Thus, a configuration statement of the form: 
     SOURCEIPA ipaddr job [job . . . ], 
     may be provided. 
     The first parameter, ipaddr, is a Dynamic VIPA which is either (a) configured on the same TCP/IP stack with VIPADEFINE or VIPABACKUP, (b) falls within a range of DVIPAs specified in a VIPARANGE statement on the same stack, (c) is a Distributed DVIPA specified in a VIPADISTRIBUTE statement where the TCP/IP stack of the SOURCEIPA statement is a target stack for the Distributed DVIPA, or (d) is already defined on the stack at the time the SOURCEIPA configuration statement is processed. 
     The second parameter (and subsequent parameters) are used to identify an application instance or a group of application instances to be associated with ipaddr, using the same syntax and semantics as the jobname parameter of the current PORT statement. The PORT statement on TCP/IP generally allows a port and/or IP address of a particular application to be characterized and restricted. The application may be identified by job name, the user ID under which the job is executed, or a group of user IDs. 
     As described in more detail below, when an application instance identified in a SOURCEIPA configuration statement initiates an outbound connection, and does not bind the socket to a particular source address before initiating the connection request, the communications protocol stack may use the designated ipaddr as the source address instead of normal SOURCEVIPA processing or using the IP address of the selected outbound link. If the IP address is not active at the time of the connection request, but lies within a range of IP addresses specified by VIPARANGE, the address will be activated as a Dynamic VIPA. 
     Thus, a system administrator may configure a particular application to use a particular IP address on all TCP/IP stacks where an instance of the application might execute, now or in the future (allowing for application movement, whether planned or failure to restart on another stack), when the IP address is a Dynamic VIPA. An application on a TCP/IP stack may be configured to use the appropriate DVIPA as the source IP address for outbound connections, independent of any other applications on the same stack. If an application consists of more than one instance, they may all be configured to use the same source IP address for outbound connections. The system administrator may configure different application to use a different IP address. 
     In addition to the inclusion of the new SOURCEIPA statement, to provide cluster-wide port assignment. the VIPADEFINE and VIPARANGE statements may be modified to include a new keyword: CLUSTERPORTS. As described in more detail below, when a DVIPA is activated via VIPADEFINE with CLUSTERPORTS, a corresponding structure is created in the coupling facility  40  if it does not already exist, and an entry is created for the DVIPA if such an entry does not already exist. If CLUSTERPORTS is added via VARY OBEY, when the DVIPA is already active, the stack will scan the connection table, and indicate as in use all port numbers that are already used as local ports in a connection using the DVIPA. If this DVIPA is also a Distributed DVIPA, then the routing stack will also examine the Connection Routing Hash Table (CRHT) and update the coupling facility entry on behalf of all target stacks. 
     When a VIPARANGE configuration statement with the CLUSTERPORTS keyword is processed, the communication protocol stack searches its list of IP addresses to find active ones within the designated range. For each such IP address, the connection table is searched for connections to the DVIPA, and the coupling facility structure and corresponding entry are created as described above with reference to the VIPADEFINE. 
     Returning to the example of  FIG. 6 , for MVS1 to MVS5, the VIPADEFine statements may be: 
     MVS1: VIPADEFine MOVEable IMMEDiate CLUSTERPORTS DVA1
         VIPADISTribute DVA1 PORT 60 DESTIP XCF1, XCF2, XCF4   SOURCEIPA DVA1 APPA       

     MVS5: VIPADEFine MOVEable IMMEDiate CLUSTERPORTS DVB1
         VIPADISTribute DVB1 PORT 60 DESTIP ALL   VIPADISTribute DVA1 PORT 60 DESTIP XCF2, XCF3, XCF4   SOURCEIPA DVA1 APPA   SOURCEIPA DVB1 APPB
 
For purposes of illustration, the respective address masks have been omitted because they are, typically, only significant to the routing daemons.
       

     In the above illustration, XCF1 is an XCF address of the TCP/IP stack on MVS1, XCF2 is an XCF address of the TCP/IP stack on MVS2 and XCF3 is an XCF address of the TCP/IP stack on MVS4. Note that, for purposes of the present example, definitions for MVS2, MVS3, and MVS4 are not specified. Such may be the case because the protocol stacks for these MVS images are candidate target protocol stacks and are not identified as routing protocol stacks and, therefore, receive their dynamic routable VIPA definitions from the routing protocol stacks. As such, MVS2, MVS3 and MVS4 may include SOURCEIPA statements for the DVIPAs for which they are target communication protocol stacks. Thus, MVS2, MVS3 and MVS4 may include the following statements: 
     MVS2: SOURCEIPA DVA1 APPA
         SOURCEIPA DVB1 APPB       

     MVS3: SOURECEIPA DVB1 APPB 
     MVS4: SOURCEIPA DVA1 APPA 
     Additional VIPA definitions may also be provided, however, in the interests of clarity, such definitions have been omitted. 
     Operations for source IP address selection and cluster-wide port assignment will now be described with reference to the above described example and to the flowcharts of  FIGS. 7 through 10 . Operations for initialization of the source selection aspects of the present invention, e.g. processing of the above definition statements, are substantially the same as described with reference to  FIG. 2 . In such initialization, the source IP address statements may be the SOURCEIPA statements described above. 
     Turning to  FIG. 7 , operations for selection of a source IP address according to embodiments of the present invention are illustrated. As seen in  FIG. 7 , when a request for an outbound connection is received, it is determined if the application has bound the socket for the connection to specified IP address (block  700 ). If so, the IP address to which the socket is bound is used for the IP address of the connection (block  705 ). If the socket for the connection has not been bound to a specific IP address (block  700 ), it is determined if a SOURCIPA statement has been included for the application requesting the connection (block  710 ). If no SOURCEIPA statement is included for the application (block  710 ), conventional address selection may be utilized (block  715 ). 
     If a SOURCEIPA statement is included for the application (block  710 ), it is determined if the specified address is a DVIPA (block  720 ). If the specified address in the SOURCEIPA statement is not a DVIPA (block  720 ), it is determined if the specified address has previously been defined for the communication protocol stack receiving the request (block  725 ). If not, an error code may be returned in response to the connection request (block  740 ) and operations terminate. If the specified address has previously been defined for the communication protocol stack receiving the request (block  725 ), the specified address is used for the connection (block  730 ). 
     Returning to block  720 , if the specified address in the SOURCEIPA statement is a DVIPA, it is determined if the DVIPA has been configured on the communication protocol stack receiving the request (block  735 ). If not, an error code may be returned in response to the connection request (block  740 ) and operations may be terminated. If the DVIPA is configured (block  735 ), it is determined if the DVIPA is active on the communication protocol stack receiving the request (block  745 ). If the DVIPA is active, the DVIPA is used as the source address for the connection (block  755 ). If not, it is determined if the DVIPA is within a range of a VIPARANGE statement for the communication protocol stack (block  750 ). If not, an error code may be returned (block  740 ) and operations may be terminated. If the DVIPA is within a range of a VIPARANGE statement for the communication protocol stack (block  750 ), the DVIPA is activated (block  760 ) and the DVIPA is used as the source address for the connection (block  755 ). 
     Returning to the example of  FIG. 6 , if MVS2 receives a connection request from APP A for a socket which is not bound to a specific IP address the SIP module  25  of communication protocol stack  26  will select DVA1 as the source address for the connection request. This is because MVS2 has a SOURCEIPA statement which identifies DVA1 as a DVIPA for use with APP A and the communication protocol stack  26  is identified as a target communication protocol stack in a VIPADISTribute statement. Furthermore, as described in more detail below, because DVA1 is defined with the CLUSTERPORTS parameter, the CLP module  27  of the communications protocol stack  26  will carry out the operations described below to select an ephemeral port for the connection. 
     As described above, collaboration among cluster communication protocol stacks may be needed to ensure unique connection 4-tuples when multiple instances of the same application, running on multiple stacks, connect to the same external server (same external IP address and port). This coordination may be accomplished using the coupling facility  40  or other shared-memory facility. A structure may be defined for the CF  40  for this purpose, with an entry for each unique DVIPA. The entry will contain a structure (which could, for example, be a bit map) that indicates which ephemeral ports are currently in use for outbound TCP connections using the DVIPA as a source IP address. 
     While operations for selecting a source address are illustrated in  FIG. 7  in a particular order, other sequences of operations could also be utilized. For example, block  720  could check for an active DVIPA and, if active, operations could continue at block  755  with selection of the active DVIPA as the source address. If the address was not an active DVIPA the operations at block  725  and after could be modified to be determined if the address was defined as a static VIPA or if it was defined in a VIPARANGE statement. If the address was a static VIPA it would be selected as the source address. If the address was defined by a VIPARANGE statement the DVIPA would be activated. Thus, embodiments of the present invention should not be construed as limited to the particular sequence of operations illustrated in  FIG. 7  but is intended to cover any sequence of operations which allows for the selection of a DVIPA as a source address for a connection. 
       FIG. 8A  illustrates operations for initialization of the cluster-wide port assignment according to embodiments of the present invention. As seen in  FIG. 8A , it is determined if CLUSTERPORTS is specified for the DVIPA being initialized (block  800 ). As described above, this may be accomplished by including the CLUSTERPORTS parameter in a VIPADEFine statement. If CLUSTERPORTS is not specified for the DVIPA (block  800 ), then operations according to embodiments of the present invention may terminate. If CLUSTERPORTS is specified (block  800 ), an entry is created in a structure in the coupling facility  40  for the DVIPA or DVIPAs (block  805 ). As described above, the structure will keep track of the availability of ports for the DVIPA. In particular, the structure may take the form of a bitmap for each DVIPA with each bit corresponding to a port such that, for example, a “1” in the bit location indicates a port is available and a “0” indicates that a port is unavailable. 
     It is also determined if the DVIPA is a distributed DVIPA (block  810 ). Such may be the case, for example, if a VIPADISTribute statement is associated with the DVIPA. If the DVIPA is a distributed DVIPA (block  810 ), the connection routing table for the DVIPA is searched on behalf of the target stacks to obtain port information for connections to the target stacks (block  815 ). If the CLUSTERPORTS parameter is added via a VARY OBEY (block  825 ), the connection table of the communication protocol stack is scanned for ports of active DVIPAs (block  835 ) and the coupling facility is updated with the port information obtained at block  815  and/or block  835  (block  840 ). 
     Returning to block  810 , if the DVIPA is not a distributed DVIPA, it is determined if the CLUSTERPORTS parameter is part of a VIPARange statement (block  820 ). If not, operations continue with block  825  as described above. If the CLUSTERPORTS is part of a VIPARange statement (block  820 ), all active DVIPAs are determined and marked for termination processing (block  830 ). Again, the connection table is scanned for ports of active DVIPAs (block  835 ) and the coupling facility  40  is updated with the DVIPA port information (block  840 ). 
       FIG. 8B  illustrates operations for initialization of the cluster-wide port assignment which may also provide for error recovery for distribute VIPAs (e.g. DRVIPAs) according to embodiments of the present invention. The embodiments illustrated in  FIG. 8B  provide a cluster-wide port availability structure in the coupling facility  40  for each DVIPA for which CLUSTERPORTS is specified and a stack specific port usage structure which indicates which ports are used by which stacks for which the DVIPA is defined. As described in more detail below, the stack specific port information may be used in the event of the failure of a stack to update the cluster-wide port availability structure to make the ports of the failed stack available for use. The operations of  FIG. 8B  may be carried out by both the routing communication protocol stack and the target communication protocol stacks of the DRVIPA. 
     As seen in  FIG. 8B , it is determined if CLUSTERPORTS is specified for the DVIPA being initialized (block  800 ). As described above, this may be accomplished by including the CLUSTERPORTS parameter in a VIPADISTRIBUTE statement. If CLUSTERPORTS is not specified for the DVIPA (block  800 ), then operations according to embodiments of the present invention may terminate. If CLUSTERPORTS is specified (block  800 ), it is determined if a structure which tracks cluster-wide port assignments for the DVIPA has been created in the coupling facility  40  for the DVIPA or DVIPAs (block  850 ). If not, the cluster-wide port assignments structure is created in the coupling facility  40  (block  855 ) and a stack specific port assignment structure is also created in the coupling facility  40  (block  865 ). As described above, the structure will keep track of the availability of ports for the DVIPA. In particular, the cluster-wide structure may take the form of a bitmap for each DVIPA with each bit corresponding to a port such that, for example, a “1” in the bit location indicates a port is available and a “0” indicates that a port is unavailable. The stack specific structure may take the form of a bitmap for each stack with each bit corresponding to a port such that, for example, a “1” in the bit location indicates a port is in use and a “0” indicates that a port is available. Both structures may be initialized to indicate that all ports are available. Alternatively, the structured may take the form of an enumerated list of available ports within the cluster and an enumerate list of used ports for a stack. 
     Returning to block  850 , if the cluster-wide structure has previously been created, it is determined if the stack specific structure has been created (block  860 ). If not, the stack specific structure is created in the coupling facility  40  and initialized to indicate that all ports are available (block  865 ). 
     It is also determined if the CLUSTERPORTS parameter is part of a VIPARange statement (block  870 ). If not, operations continue with block  875  as described below. If the CLUSTERPORTS is part of a VIPARange statement (block  870 ), all active DVIPAs are determined and marked for termination processing (block  880 ). The connection table is scanned for ports of active DVIPAs (block  880 ) and the coupling facility  40  is updated with the DVIPA port information for both the cluster-wide structure and the stack specific structure (block  890 ). 
     If the CLUSTERPORTS parameter is added via a VARY OBEY (block  875 ), the connection table of the communication protocol stack is scanned for ports of active DVIPAs (block  885 ) and the coupling facility is updated with the port information for connections identified in block  885  (block  890 ) as described above. In particular embodiments of the present invention, the CLUSTERPORTS parameter may only be added by a VARY OBEY command if there are no active connections for a distributed VIPA. Thus, there should be no conflicts between ports of active connections. However, in other embodiments of the present invention, conflicts between connections with the same port may be resolved by showing the port as unavailable in the cluster-wide structure and in use in each of the stack specific structures until all connections utilizing the port have terminated. At that time the port could be marked as available. For example, a count of connections which use a port could be maintained and the structures indicate use of the port until the count was decremented to zero by the termination of connections using the port. 
     Operations for port assignment when an application initiates an outbound connection are illustrated in  FIG. 9 . At block  900 , it is determined if the socket of the connection request is bound to a DVIPA. If not, conventional connection operations may be utilized (block  950 ). If the socket is bound to a DVIPA (block  900 ), it is determined if CLUSTERPORTS is specified for the DVIPA (block  905 ). If not, conventional port selection techniques may be utilized (block  910 ). If CLUSTERPORTS is specified for the DVIPA (block  905 ), it is determined if the socket is bound to a specific port or if an ephemeral port, such as ports greater than 1024, is to be selected (block  915 ). For example, binding the socket to port 0 may indicate that an ephemeral port is to be selected when a connection request is made. If the socket is bound to a specific port other than port 0, the specified port is selected for use (block  920 ) and may be identified locally as unavailable for use in another connection (block  930 ). The selected port is used to open the connection (block  945 ). 
     If the socket is not bound to a specific port and an ephemeral port is to be used (block  915 ), the structure or structures for the DVIPA are retrieved from the coupling facility  40  and a lock placed on the structure(s) to prevent other communications protocol stacks from accessing the structure (block  925 ). This may prevent two stacks from simultaneously selecting the same port. An available port is selected for the connection and is identified as used or unavailable in the structure of the embodiments of  FIG. 8A  and, in the embodiments of  FIG. 8B , the port is identified as unavailable in the cluster-wide structure and in use in the stack specific structure (block  935 ). The updated structure or structures are restored to the coupling facility  40  and the structure(s) unlocked to allow access to the structure by other communication protocol stacks (block  940 ). The selected port is then used to open a connection using the DVIPA (block  945 ). 
       FIG. 10  illustrates operations according to embodiments of the present invention when a connection is terminated. As seen in  FIG. 10 , it is determined if a DVIPA is specified as the source address for the connection (block  1000 ). If not, conventional termination operations may be utilized (block  1035 ). If the connection has a DVIPA as its source address (block  1005 ), the connection is terminated and appropriate tables are updated as would be the case with a conventional DVIPA (block  1005 ). It is also, however, determined if CLUSTERPORTS is specified for the DVIPA (block  1010 ). If not, no additional operation need be performed. If CLUSTERPORTS is specified (block  1010 ), the structure or structures are retrieved from the coupling facility  40  and access to the structure(s) is locked (block  1015 ). The structure in the embodiments of  FIG. 8A  is updated to identify the port of the connection which is terminating as available and, in the embodiments of  FIG. 8B , the port is identified as available in the cluster-wide structure and not in use in the stack specific structure (block  1020 ). The structure or structures are restored to the coupling facility  40  and unlocked to allow other communications protocol stacks access to the structure(s) (block  1025 ). 
       FIG. 11  illustrates operations according to embodiments of the present invention when a communication protocol stack receives a bind request from an application. As seen in  FIG. 11 , the communication protocol stack may determine if the bind request specifies a port (block  1100 ). If so, it may be determined if CLUSTERPORTS is defined for the address to which the bind request is directed (block  1102 ). If CLUSTERPORTS is specified (block  1102 ), the structure in the coupling facility  40  is checked to determine if the specified port is available (block  1105 ). If the port is not available (block  1105 ), the bind request is rejected and an error code is returned to the requesting application (block  1110 ). If the port is available (block  1105 ), bind request may be marked to update the coupling facility  40  to reflect that the port is in use if the bind operation is successful and, in embodiments having stack specific structures in the coupling facility  40 , the stack specific structure would also updated (block  1115 ). 
     If the bind request is not rejected due to specification of an unavailable port (blocks  1100 ,  1102  and  1105 ), then it is determined if the address is a DVIPA (block  1120 ). If the specified address in the bind request is not a DVIPA (block  1120 ), it is determined if the specified address has previously been defined for the communication protocol stack receiving the request (block  1125 ). If not, an error code may be returned in response to the bind request (block  1140 ) and operations terminate. If the specified address has previously been defined for the communication protocol stack receiving the request (block  1125 ), the bind operation is completed using the specified address and, if the request is marked for update of the coupling facility  40  (see block  1115 ), the coupling facility is updated to reflect that the port specified in the bind request is not available (block  1130 ). 
     Returning to block  1120 , if the specified address in the bind request is a DVIPA, it is determined if the DVIPA has been configured on the communication protocol stack receiving the request (block  1135 ). If not, an error code may be returned in response to the connection request (block  1140 ) and operations may be terminated. If the DVIPA is configured (block  1135 ), it is determined if the DVIPA is active on the communication protocol stack receiving the request (block  1145 ). If the DVIPA is active, the DVIPA is used as the source address for the connection (block  1155 ). If not, it is determined if the DVIPA is within a range of a VIPARANGE statement for the communication protocol stack (block  1150 ). If not, an error code may be returned (block  1140 ) and operations may be terminated. If the DVIPA is within a range of a VIPARANGE statement for the communication protocol stack (block  1150 ), the DVIPA is activated (block  1160 ) and the bind operation is completed using the DVIPA as the source address and, if the request is marked for update of the coupling facility  40  (see block  1115 ), the coupling facility is updated to reflect that the port specified in the bind request is not available (block  755 ). 
     As described above with respect to  FIG. 7 , while operations for performing a bind operation are illustrated in  FIG. 11  in a particular order, other sequences of operations could also be utilized. For example, block  1120  could check for an active DVIPA and, if active, operations could continue at block  1155  with completion of the bind operation using the active DVIPA. If the address was not an active DVIPA the operations at block  1125  and after could be modified to be determined if the address was defined as a static VIPA or if it was defined in a VIPARANGE statement. If the address was a static VIPA it would be used to complete the bind operation. If the address was defined by a VIPARANGE statement the DVIPA would be activated. Thus, embodiments of the present invention should not be construed as limited to the particular sequence of operations illustrated in  FIG. 11 . 
       FIG. 12  illustrated operations for recovery from the failure of a communication protocol stack. Preferably, the operations of  FIG. 12  are carried out by a routing communication protocol stack or a backup routing communication protocol stack if the failing communication protocol stack is a routing communication protocol stack. As seen in  FIG. 12 , the communication protocol stacks are notified of the failure of other communication protocol stacks in the cluster (block  1200 ), for example, by XCF messages. Upon notification of the failure of the communication protocol stack, the cluster-wide structure and the stack specific structure for the failing communication protocol stack are obtained from the coupling facility and locked to deny other communication protocol stacks access to the structures (block  1205 ). The ports identified in the stack specific structure as in use are then identified as available in the cluster-wide structure and the stack specific structure is deleted (block  1210 ). 
     In embodiments of the present invention where the cluster-wide structure and stack specific structure are bitmaps with a “1” in the cluster-wide structure indicating a port is available and a “1” in the stack specific structure indicating that a port is in use, the two bitmaps may be logically ORed together to update the cluster-wide structure. In embodiments with opposite polarity logic, the bitmaps may be logically ANDed together. In embodiments where the cluster-wide structure is an enumerated list of the available ports and the stack specific structure which is an enumerated list of the ports in use, the stack specific structure could be appended to the cluster-wide structure. 
     In any event, the cluster-wide structure is restored to the coupling facility  40  and unlocked so as to allow other communication protocol stacks access to the structure (block  1215 ). The stack specific structure need not be restored to the coupling facility  40  and, in fact, should be removed from the coupling facility  40 , as the connections to the failing stack are no longer valid. If the connections are restarted on another communication protocol stack, the structures will be updated at that time. 
     In the Sysplex embodiments described above, the number of simultaneously active outbound connections across the cluster for a particular DVIPA may be 65,536, minus the number of ports that are restricted or otherwise in use on the stack. Thus, up to approximately 63,000 active connections may be initiated by outbound connection requests from the total set of application instances using a particular DVIPA. However, in alternative embodiments of the present invention, the entire connection routing hash table for the DVIPA may be stored in the coupling facility  40  or other shared memory. A communication protocol stack would then allocate a possible ephemeral port, construct the resulting connection routing entry (consisting of source and destination IP addresses and ports for the connection), and search the connection routing hash table to see if such an entry already exists. If the entry does not exist, the entry is added to the connection routing hash table and the connection request is issued with that ephemeral port. The ephemeral port is not marked as in use as normal, however, so that the same ephemeral port may be used when connecting using the same source DVIPA, but to a different destination IP address and/or port. In such embodiments, the limitation on simultaneous active outbound connections from the set of applications using the same DVIPA as source IP address is approximately 63,000 for each destination IP address/port pair. 
     As described herein, embodiments of the present invention may automatically allow designation of source IP address at an application instance granularity, irrespective of (current or future) location of the application instance within a cluster, and may do so in a manner which may reduce the need for specific programming in the application and specific configuration of the server instance. Thus, embodiments of the present invention may allow an administrator to designate the source IP address to be used for any application instance for outbound connection requests, designate that different application instances should use different source IP addresses for their outbound connection requests, or specify that several application instances should use the same source IP address for outbound connection requests. When several application instances on different TCP/IP stacks within the cluster use the same source IP address for outbound connection requests, coordinated assignment of ports may also be provided. 
     Thus, embodiments of the present invention may allow a cluster multiple instances of a TCP/IP application making outbound connection requests to present appearance of a single application instance to partner applications and networks outside the cluster. This may be accomplished in a manner which may reduce the need for special application-level programming or customization while possibly reducing the risk of connection failure due to inappropriate assignment of the same source port number by two different stacks to two different connections to the same server instance outside the cluster. 
     Both source IP address selection and cluster-wide port assignment may be utilized in combination to deploy a server application with inbound and outbound connection request handling in multiple instances using the same IP address for the local address for all connections. However, as described above, source IP address selection and cluster-wide port assignment may be utilized separately. For example, if there is only one instance of an application using an IP address as the designated source IP address for outbound connection requests, cluster-wide port assignment may not be needed. Similarly, if a server application initiates connections back to a client that has connected to the server using the same source IP address on the server connection as the client specified as a destination address on the connection the client initiated, then cluster-wide port assignment may be utilized to support parallel connections to the same client from two different instances of the cluster server application. However, because the application specifies the appropriate source IP address on its own, source IP address selection by the stack may not be needed. 
     While the present invention has been described with respect to the VIPA distribution, the cluster-wide port assignment and the source address selection functions being a part of the communication protocol stack, as will be appreciated by those of skill in the art, such functions may be provided as function, objects or applications which are separate from the communication protocol stack and which may cooperate with the communication protocol stacks. Furthermore, the present invention has been described with reference to particular sequences of operations. However, as will be appreciated by those of skill in the art, other sequences may be utilized while still benefitting from the teachings of the present invention. Thus, while the present invention is described with respect to a particular division of functions or sequences of events, such divisions or sequences are merely illustrative of particular embodiments of the present invention and the present invention should not be construed as limited to such embodiments. 
     Furthermore, while the present invention has been described with reference to particular embodiments of the present invention in a System/390 environment, as will be appreciated by those of skill in the art, the present invention may be embodied in other environments and should not be construed as limited to System/390 but may be incorporated into other systems, such as a Unix or other environments, by associating applications or groups of applications with an address rather than a communications adapter. Thus, the present invention may be suitable for use in any collection of data processing systems which allow sufficient communication to all of the systems for the use of dynamic virtual addressing. Accordingly, specific references to System/390 systems or facilities, such as the “coupling facility,” “ESCON,” “Sysplex” or the like should not be construed as limiting the present invention. 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.