Patent Publication Number: US-9838297-B2

Title: System and method for message routing in a network

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 10/835,991, filed Apr. 30, 2004, entitled System and Method for Message Routing in a Network, which is hereby incorporated by reference. This application is related to U.S. application Ser. No. 10/837,115, entitled System And Method For Flow Control In A Network, U.S. Pat. No. 7,627,627; U.S. application Ser. No. 10/835,821, entitled System For Addressing Network End-Points Using Route Handles, U.S. Pat. No. 7,466,699; U.S. application Ser. No. 10/835,940, entitled System For Determining Network Route Quality Using Sequence Numbers, U.S. Pat. No. 7,440,458; and U.S. application Ser. No. 10/836,881, entitled System For Selecting Routes For Retransmission In A Network, U.S. Pat. No. 7,801,029. All referenced related applications were filed Apr. 30, 2004, and are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Communication within a prior art network  100 , such as a storage area network (SAN), is illustrated in  FIG. 1 , which shows applications  101 (*), running on end-points  102 (*), communicating with their peer applications  101 (*) via respective ports  103 (*) and fabrics  105 (*). As used herein, a ‘wild card’ indicator “(*)” following a reference number indicates an arbitrary one of a plurality of similar entities. The communication infrastructure in a storage area network often includes two or more independent Fibre Channel fabrics  105 ( 1 ) and  105 ( k ) comprising multiple switches and hubs (not shown). Each end-point  102 (*) is connected to one or more Fibre Channel fabrics  105 (*) via one or more ports  103 (*), and may use any of these fabrics to communicate with other end-points  102 (*). 
     A port  103 (*) is a module such as a Fibre Channel adapter module, a portion of such a module, or a collection of circuits embedded within an end-point. Each port  103 (*) is attached to at most one Fibre Channel fabric  105 (*). A port  103 (*) may be dedicated for use by a single end-point, as is port  103 ( p ), or it may be shared by multiple end-points, as is port  103 ( 1 ). 
     Many existing networking systems select source and destination ports at connection establishment based on the lowest end-to-end hop count, and continue to use those ports for the duration of a connection. Internet Protocol (IP) performs routing within the network between the source and destination ports, but not among the ports local to the end-points. The effect of fixing a connection&#39;s source and destination ports is that no available routes that employ other source or destination ports will be used unless the connection fails. Because Fibre Channel does not provide for routing, the entire IP route is selected at connection establishment. Furthermore, in a network such as Fibre Channel, where multiple routes have equal hop count, selecting the ‘shortest’ route is a meaningless concept. In systems that select routes during connection establishment, fail-over is not rapid and usually results in application error recovery. Furthermore, in these systems, load balancing is not performed and route selection does not adapt to changing route performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing applications, running on end-points, communicating with peer applications via respective ports and Fibre Channel fabrics over a prior art network; 
         FIG. 2  is an exemplary diagram showing the relationship between TCP/IP protocol layering and protocol layering of the present system; 
         FIG. 3  is a diagram showing relationships between exemplary physical components of a route in the present system; 
         FIG. 4  is a diagram showing relationships between exemplary logical objects in the present system; 
         FIG. 5  is a diagram of a route showing half-routes; 
         FIG. 6A  is a diagram showing an exemplary relationship between routes and route sets; 
         FIG. 6B  is a diagram showing exemplary relationships between a route set management connection, a route set, and application connections; 
         FIG. 6C  is an exemplary diagram showing the relationship between a route management connection and pseudo end-points; 
         FIG. 7  is a diagram showing an overview of an exemplary set of steps performed in operation of the present system; 
         FIG. 8  is a diagram showing exemplary steps performed in establishing a path and a route management connection between two ports; 
         FIG. 9  is a diagram showing exemplary steps performed in a full route registration procedure; 
         FIGS. 10A, 10B, and 10C  are flowcharts showing exemplary steps performed in establishing a route set; 
         FIG. 11  is a flowchart illustrating exemplary steps performed in handling a request to establish a connection between end-point applications; 
         FIG. 12A  is flowchart illustrating exemplary steps performed in selecting a route for initial transmission; 
         FIG. 12B  is flowchart illustrating exemplary steps performed in selecting a route for a retry transmission; 
         FIG. 13  is a diagram showing exemplary steps and queues used in queueing route selection requests; 
         FIG. 14  is a diagram showing an exemplary routing header; 
         FIG. 15A  is a diagram illustrating exemplary routing layer processing performed in sending initial transmissions; 
         FIG. 15B  is a diagram illustrating exemplary routing layer processing performed in sending retry transmissions; 
         FIG. 16  is a diagram illustrating exemplary routing layer processing performed in receiving transmissions; and 
         FIG. 17  is a diagram showing an exemplary routing layer feedback loop used in establishing the transmission rate limit for a particular half route. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     The following definitions are applicable to the present document: 
     End-point—a locus for execution of applications on a network. 
     End-point Incarnation—the sustained, continuous operation of an end-point without loss of context. 
     Connection—a relationship between two communicating program incarnations, or processes, that is maintained where those processes are running. While the connection is established, the processes may use it to communicate. If either of the communicating processes stops (i.e., fails, exits, etc.) then the connection fails, and if restarted, the programs cannot continue to use the previously existing connection. Within this document, the following examples of connections are described:
         An application connection is a relationship between two incarnations of an application that is maintained by the end-points where those applications are running and that allows them to communicate reliably.   A route set management connection is a relationship between routing layer programs on two end-point incarnations that allows them to communicate route set management information reliably.   A route management connection is a relationship between two port layer programs on two port incarnations that allows the reliable communication of route management information.       

     Port—a port comprises all network-specific functionality associated with a specific, single Nx_port (Fibre Channel N_port or NL_port). Also denotes a specific communications protocol layer. 
     Message—A logically contiguous array of bytes sent reliably by the sequenced message transport. A message is sent using one or more transmissions. 
     Transmission—A single-frame Fibre Channel sequence having, at least, a Fibre Channel header, network header, and a routing header. Transmissions are sent on a best-effort basis. 
     Outbound Message—an object that is used to describe a message to be sent. Acronym is OBM. 
     Outbound Sequence—an object that is used to describe a transmission to be sent. Acronym is OBS. 
     Path—a relationship between two port incarnations established by the standard Fibre Channel PLOGI extended link service. 
     Process—an incarnation of the code on a machine that is executing the code. 
     Route—a connection between the processes that represent the two end point incarnations that uses a specific path. A route relates two end-point incarnations and two port incarnations. 
     Half route—the portion of a route that delivers messages in one direction. With respect to an end-point, a half route is either outbound or inbound. A route consists of exactly two half routes. 
     Route set—a relationship between two end-point incarnations that associates the complete set of routes between those incarnations and that indicates the existence (currently, or at some previous time) of one or more routes between the end-points. A route is a member of exactly one route set. A route is established as a member of a specific route set and cannot migrate to any other route set. Termination of a route set terminates all routes belonging to the route set. 
     INTRODUCTION 
       FIG. 2  is a diagram showing, as an example, the general correspondence between TCP/IP protocol layering  20  and the protocol layering  200  of the present system. As shown in  FIG. 2 , each end-point  102  in the present system includes network layer  240 , routing layer  230 , transport layer  220 , request layer  212 , and application layer  210  plus a buffer pool management component  270 . An API (Application Program Interface)  211  makes network services available to applications  210 . Network layer  240  encapsulates one or more ports  103 (*) that are local to the network layer&#39;s end-point  102  and makes them available to that end-point. In so doing, the network layer  240  hides any physical interface (e.g., a PCI bus) between the end-point  102  and its local ports  103 (*). 
     The present system uses a routing layer  230  to effectively glue together the network  240  and transport  220  layers of a networking protocol. Routing layer  230  may be implemented in conjunction with Hewlett-Packard&#39;s SCTP (Storage Cluster Transport Protocol), for example. The routing layer  230  provides a routing or steering mechanism to direct outgoing transmissions from a connection onto the proper route, and to direct incoming transmissions (on a particular route) to the proper connection. Transport layer  220  sends and receives messages on connections and provides a sequenced-message delivery service. Request layer  212  provides the command-response service, command flow-control, and the bulk data transfer service. A buffer pool management component  270  manages buffers (not shown) used to receive unsolicited messages directly into application memory. An API (applications programming interface)  211  makes network services available to applications  101 (*). 
     The functionality that is specific to a single port  103  is called the port layer  250 . In the present exemplary embodiment, port layer  250  performs the discovery and maintenance of routes, in addition to providing mechanisms for sending and receiving transmissions as described herein. In the present embodiment, each port  103  comprises an ‘Nx_port’, which is a Fibre Channel N_port or NL_port. Each port  103  uses a driver (‘Fibre Channel services’)  260  to abstract the port hardware such that the peculiarities of the Fibre Channel port hardware  261  are hidden from the upper protocol layers. In an exemplary embodiment, an end-point  102  may use several ports  103 (*) simultaneously. 
     As shown in  FIG. 2 , the application layer  21  in the TCP/IP protocol layering scheme  20  may be considered to correspond to the application and request layers  210 / 212  of the present system. In the present protocol layering scheme  200 , the TCP/IP transport layer  22  is functionally similar to the present transport layer  220 ; the TCP/IP network layer  24  may be considered to correspond to the combination of the present network layer  240  and port layer  250 ; and the TCP/IP link layer  25  is effectively implemented within Fibre Channel Services and port hardware layers  260 / 261 . While the TCP/IP protocol does routing within its network layer  24 , it does not specifically provide for a distinct formal routing layer. 
     In the present exemplary embodiment, the protocol stack  200  provides a reliable one-way sequenced message delivery service for small messages, a reliable command-response service that uses the sequenced message delivery service to deliver commands and responses, and a reliable, high-performance bulk data transfer service that can be used in conjunction with the reliable command-response service. 
     The transport protocol used to implement transport layer  220  allows two application processes running on end-points  102 (*) to establish a connection between them and to use that connection to send one-way sequenced messages. Barring major communication failures, messages sent via the connection are delivered in order and exactly once to the connected process. A command-response service implemented by the request layer  212  (using services provided by the transport layer  220 ) allows a client process, e.g., application  101 ( 1 ), to send commands to a server process, e.g., application  101 ( 2 ), for the server process to return a response to the client process, and for the client and server to perform high-performance bulk data transfers. Commands and responses are sent via a connection established between the client and the server processes, so they are presumed by the request layer to be delivered reliably. 
     As can be seen from  FIG. 2 , the present system adds a routing layer  230  to an end-point&#39;s protocol stack between the transport  220  and network  240  layers. This routing layer  230  is aware of multiple routes and networks to other end-points  102 . As described herein, the routing layer  230  organizes the available routes to a given end-point  102 (*), measures route quality, and selects the proper route for each outgoing transmission. 
     The present system&#39;s transport layer  230  uses different routing layer functions for sending initial transmissions and retry transmissions. This distinction allows the route selection for retry transmissions to differ from route selection for initial transmissions. In addition, transport layer  230  informs the routing layer when a previous transmission may not have arrived in a timely fashion so that the routing layer can avoid the route used by the previous transmission. 
     As an adjunct to the routing process, routing layer  230  monitors the quality of each route. The resulting route quality is used to select routes for outgoing transmissions to avoid routes that are unreliable, congested, or slow. The manner in which the routing layer determines route quality is described in a related application; the routing layer&#39;s use of route quality measurements and other information to select routes is described herein. 
     Routing layer  230  automatically maintains the routes in a route set (described in detail below), adding newly discovered routes and removing those that have failed. 
     Route Components 
     Physical components governed by the present system include end-points  102 (*), ports  103 (*), and fabrics  105 . Certain relationships between these component objects are summarized in  FIG. 3 . As shown in  FIG. 3 , a route  300  comprises a pair of ports, e.g.,  103 (L, 1 ) and  103 (R,j), connected via a fabric  105  through which two end-points, e.g.,  102 ( 1 ) and  102 ( 2 ), can communicate. In the above figure, there are i*j potential routes  300  between end-point  102 ( 1 ) and end-point  102 ( 2 ), j through each port [ 103 (L, 1 ) through  103 (L, i)] directly connected to end-point  102 ( 1 ). One of these routes is shown by the bold line  300 . 
     The protocols described herein govern the creation, modification, and deletion of logical objects (structures). These logical objects describe discovered physical ports  103  and end-points  102 , and include local port, remote port, local end-point, and remote end-point objects, which are described in detail with reference to  FIG. 4 . Route, route set, discovered remote end-point, and route management connection objects are used to group and manage instances of these logical objects, as described below. The term ‘local’ is used herein to refer to logical objects that represent physical objects that can be accessed without using Fibre Channel messages. In contrast, the term ‘remote’ is used to refer to objects that are not local. Thus, a remote end-point describes an end-point that can only be accessed across a Fibre Channel, e.g., end-point  102 ( 2 ) is remote to port  103 (L, 1 ), whereas end-point  102 ( 1 ) is a local end-point relative thereto. 
       FIG. 4  is a diagram showing relationships between exemplary logical component objects (structures) and their location in port memory  420  and end-point memory  421 , located in a physical port  103 (*) and a physical end-point  102 (*), respectively. In the present system, several objects (local port, route, local end-point, and requested remote end-point) are represented by the combination of a master and a proxy. This bifurcation into masters and corresponding proxies allows these objects to be distributed between the port memory/processor and the end-point memory/processor. By maintaining both the masters and proxies, the present system will support either a distributed implementation or a single memory/processor implementation.  FIG. 4  is presented at this point to clarify subsequent references made to these system objects throughout this document. A brief description of certain objects shown in  FIG. 4  is presented below. 
     Local port objects, comprising local port master  404  and local port proxy  413 , describe the state of a directly accessible physical port. Local port objects  404 / 413  are created automatically at initialization based on physically detected port hardware. Attributes of a local port object describe the physical port hardware and its fabric login state. 
     Remote port object  401  describes the state of a local port&#39;s relationship with a physical port that is not directly accessible. Remote port objects  401  are created when they are discovered via Fibre Channel communication. Attributes of the remote port object  401  describe its port ID, port name, a local port through which it can be accessed, and port login state. 
     A local end-point object (e.g., local end-point incarnation master  414  and local end-point incarnation proxy  411 ) describes the state of a directly accessible end-point. Local end-point incarnation master object  414  is created when it is initialized on that end-point and the local end-point incarnation proxy is created when the local end-point discovers the port. Attributes of the local end-point object include the local end-point&#39;s UID (unique identifier) and IID (incarnation identifier). 
     Pseudo local end-point object  402  describes the local end-point for route management connections that is associated with a local port object  404 . It is created as a side effect of creating the local port object  404 . Likewise, a pseudo remote end-point object  403  is created as a side effect of creating a remote port object  401 . Unlike real end-points, pseudo end-points are not addressed by their UIDs; rather, they are addressed by their associated ports during connection establishment and by route handles thereafter. 
     A remote end-point object describes the state of an end-point that is not directly accessible. Discovered remote end-point objects  407  are created when they are discovered through partial route registration, and requested remote end-point master objects  418  are created when a client makes a connect request to a new end-point  102 . Requested remote end-point proxy objects  415  are created when either when a client makes a connect request or when a local port  103 (L) is discovered. 
     Attributes of the remote end-point include the remote end-point&#39;s UID. Attributes of the discovered remote end-point object  407  include references to remote ports that can be used to access the remote end-point. Attributes of the requested remote end-point object ( 415 / 418 ) include references to local end-points that have requested that routes be established to the remote end-point. 
     An end-point incarnation  410 / 411  is the sustained, continuous operation of an end-point without loss of context. Attributes of an end-point incarnation include its incarnation identifier (IID). The incarnation identifier is assumed unique over all end-point incarnations. Each time an end-point reboots, its old incarnation is destroyed and a new one is created, including a new incarnation identifier. 
     Route management connection object  405  describes a connection that is used to communicate route management information between two pseudo end-points and their corresponding ports. 
     Routing layer connection object  416  is a base class from which a transport layer connection object is derived. 
     Route set object  417  associates the routes from a local end-point incarnation  411  to a remote end-point incarnation  410 . Attributes of the route set include a list of routes, a list of clients (connections), and a remote end-point UID and incarnation identifier (IID) (the local end-point is implicit). 
     There are at least two viable route management configurations for the present system. In the first configuration, all local port and end-point objects are stored in a single memory and the processors that implement the local port and end-point methods have a method for synchronizing access to that memory. In the second configuration, the port and end-point objects may be distributed between memory and processors associated with ports  103 (*) and end-points  102 (*). 
     When the port and end-point objects are distributed, those objects are decomposed into a corresponding master object and its proxies. For example, local end-point incarnation  411  is decomposed into a local end-point incarnation master object  414  in the memory  421  of the end-point  102  and multiple local end-point incarnation proxy objects  412  in the memories  420  of the end-point&#39;s local ports  103 (L). The local end-point incarnation master  414  is created automatically during the end-point initialization, and the local end-point incarnation proxy  412  is created as a result of end-point registration with a port  103 . Similarly, the local port master object  404  is created automatically during port initialization, and the local port proxy object  413  is created during hardware discovery and modified as a result of end-point registration with the port  103 . 
     Half Routes 
     Each route  300  comprises two independent, unidirectional, half routes.  FIG. 5  is a diagram of a route  300  showing the two component half routes  500 ( 1 ) and  500 ( 2 ). From the perspective of an end-point  102 (*), one of these components is the outbound half route and the other is the inbound half route. The local end-point&#39;s outbound half route is the remote end-point&#39;s inbound half route and vice versa. For example, if end-points  102 (L) and  102 (R) are considered to be the local end-point and remote end-points, respectively, half route  500 ( 1 ) is the outbound half route for local endpoint  102 (L), and half route  500 ( 2 ) is the inbound half route for local endpoint  102 (R). 
     Route Sets 
       FIG. 6A  is a diagram showing the relationship between routes  300 (*) and a route set  600 . A route set  600  specifies the relationship between two end-points  102 (*) and catalogs the routes  300 (*) between the two end-points. A route set typically contains multiple distinct routes. As shown in  FIG. 6A , route set  600  is the set of routes  300 ( 1 ),  300 ( 2 ),  300 ( 3 ), and  300 ( 4 ) between the two end-points  102 (L) and  102 (R). A route set  600  thus groups all routes  300  between a local end-point and a remote end-point. Routing layer  230  is responsible for creating, deleting, and maintaining route sets  600 . 
     Before application processes on two end-points  102 (*) can create a connection, the end-points must first establish a route set between them. A route set  600  may persist while routes are added to or removed from the route set, provided the end-point incarnations  410 / 411  continue to exist. 
     Route Set Management Connection 
       FIG. 6B  is a diagram showing exemplary relationships between a route set management connection (RTSMC)  602 , a route set  600 , and application connections  601 ( 1 )- 601 ( 3 ), with application processes P 1 -P 5  running on end-points  102 (L) and  102 (R) and communicating via application connections  601 ( 1 )- 601 ( 3 ) between the processes. A local end-point, e.g.,  102 (L), creates routes  300 , a route set  600 , and a route set management connection to a remote end-point, e.g.,  102 (R), when it finds it necessary to create a first application connection  601 ( 1 ) with an application on the remote end-point. 
     Route Selection 
     In selecting a route  300  for an outgoing transmission, transport layer  220  implicitly constrains the selection to a route set  600  by specifying a connection on which to send a transmission. Routing layer  230  attempts to select a route  300  from that route set  600  that is reliable, not congested, and fast. When multiple routes  300  appear to be acceptable, then the routing layer  230  distributes traffic among those routes to balance their utilization. 
     Routing layer clients (i.e., transport layer entities) send and receive messages via connections established by the routing layer  230 . These connections are base-class connections from which transport layer connections are derived. Routing layer  230  provides functions that clients can use to request and abort connections. A route set management connection  602 , is used by the routing layer to manage the routing layer connections established on a route set. Route set management connections  602  are described below in detail with respect to  FIGS. 10A, 10B and 10C . Once a routing layer connection is established, the routing layer connection&#39;s send and receive functions are available to routing layer clients. 
     Operational Overview 
       FIG. 7  is a diagram showing an exemplary set of high-level operations performed in operation of the present system. Much of the process shown in  FIG. 7  is event-driven, and thus the execution of each functional block shown does not occur automatically in response to execution of a previous block in the figure. 
     Configuration Discovery 
     As show in  FIG. 7 , at step  702 , port initialization begins with the fabric login and name server registration as described in the Fibre Channel specification. In order to provide better scaling for the discovery process, the subject protocols allow two classes of Fibre Channel ports  103 (*), Class A and Class B. Class A ports register with the fabric name server as two FC4 types, while Class B ports register as only one of these two. In this way, Class A ports autonomously locate all of the Class A and Class B ports by querying the fabric name server for all ports that have registered the FC4 type used by all ports that support the subject protocols. In contrast Class B ports locate only Class A ports by querying only the FC4 type that Class B ports do not register. Class B ports can locate other Class B ports by querying a protocol-specific name server that is implemented by all Class A ports. 
     Once registration is complete, port configuration discovery is performed periodically, at step  705 , with each local port  103 (*) querying the Fabric name server (logging in if necessary) to discover newly connected ports that have registered the FC4 types used by the ports that support the subject protocols. In the steady state, configuration discovery is performed every 100 seconds, in an exemplary embodiment. However, during startup and whenever there is evidence that the system configuration may have changed, this period is changed, for example, to 10 seconds for 10 periods and then back to 100 seconds. It should be noted that the present system does not require setting the configuration discovery period precisely to the foregoing values. 
     This autonomous configuration discovery must be repeated periodically because fabric name server registrations propagate slowly, configurations change, and state change notifications are not delivered reliably. When repeated periodically, only newly discovered ports  103 (*) are processed. This is true because ports that restart or whose IDs change will spontaneously execute portions of autonomous configuration discovery and thus update configuration changes. 
     Port Login 
     Port login is used to establish a shared context between two Fibre Channel ports  103 (*). Prior to port login, the ports may only exchange port login transmissions (i.e., PLOGI/PLOGI_ACC transmissions). Successful port login establishes a path between the ports  103 (*). Port login is specified by the Fibre Channel specification. Each port maintains a set of remote port objects (structures)  401 , each of which describes the relationship between it and another port that it can access via a fabric  105 (*). 
     As shown in  FIG. 7 , at step  710 , a port  103 (*) creates a path between itself and each remote port  103 (*) that it discovered from the Fabric name server. A route management connection  603  (shown in  FIG. 6C ) is then established between the pseudo end-points  402 / 403  associated with the ports of a path, at step  715 . Route management connection  603  is a sequenced-message connection that provides a one-way sequenced message delivery service used to communicate route management information between two ports  103 (*). 
       FIG. 8  is a diagram showing exemplary steps for establishing a path and a route management connection  603  between two ports  103 (*), and also for performing a ‘partial route registration’ process. As shown in  FIG. 8  (with reference also to  FIG. 3 ), at step  805 , local port  103 (L) requests that Fibre Channel Services establish a path to remote port  103 (R) using a PLOGI transmission. Upon receipt thereof, a path to the remote port  103 (R) is established, at step  810 , and the remote port responds by sending a PLOGI_ACC transmission to the local port  103 (L), at step  815 . After receipt of the PLOGI_ACC transmission at the local port, a path from the remote port  103 (R) to the local port  103 (L) is established, at step  820 . 
     At step  825 , the local port then uses the Report Node FC-4 Types (RNFT) protocol to learn the set of FC-4 protocols supported by a particular remote port. Receipt of RNFT_ACC from the remote port  103 (R) at step  830  indicates that the remote port supports the proper Fibre Channel FC-4 protocol, and causes the local port  103 (L) to initiate process login. 
     Route Management Connection Establishment 
     The present system extends every local and remote port object  404 / 401  by attaching to it a pseudo end-point object  402 / 403  to provide a terminus for route management connections.  FIG. 6C  is an exemplary diagram showing the relationship between pseudo end-points  402 / 403  and a route management connection (RMC)  603 . The present process login protocol establishes a route management connection  603  and a route  300  between pseudo end-points  402 / 403  associated with the ports  103 (*) of a path. 
     The route management connection  603  is used to communicate: 
     partial route registration messages, 
     full route registration requests and their responses, and 
     name service queries and their responses. 
     Route management connection  603  behaves slightly differently than other connections. Differences include the following:
         the two end-points of a route management connection  603  are pseudo-end-points that correspond to ports of a path.   the route management connection  603  does not use a route set. Rather, the corresponding path constitutes the sole route used by the route management connection.   when a remote Nx_port is implicitly or explicitly logged out, all corresponding route management connections  603  are terminated immediately.       

     The present process login protocol comprises an FC-4 Link Service request (SCPRLI) and an FC-4 Link Service reply (SCPRLI_ACC). The local and remote ports exchange process login protocol transmissions SCPRLI (step  835 ) and SCPRLI_ACC (step  840 ) to establish a route management connection  603  between the path&#39;s local and remote ports  103 (L)/ 103 (R), at step  845 . SCPRLI and SCPRLI_ACC correspond to the first two phases of a three-way handshake. Information carried by the SCPRLI/SCPRLI_ACC FC-4 Link Service includes the route management connection&#39;s connection identifier (connection ID) and the route&#39;s full route handle, which is used to direct messages to a remote end-point  102 (*) via a route  300 , once established. The first message to be sent on the route management connection is a SCRPR command, at step  850 , which completes the three-way handshake and completes establishment of the route management connection  603 , at step  855 , thus enabling the transmission of messages on the route management connection by the remote port  103 (R). 
     More specifically, during process login, a local port  103 (L) and a remote port  103 (R) perform the steps below to establish a route management connection:
         (1) On the local port, a route management connection object  405  and pseudo remote end-point object  403  are created that reference the remote port&#39;s remote port object  401 .   (2) On the local port, a route master object  406  is created, and a SCPRLI command is sent to the remote port  103 (R), as indicated above. The SCPRLI message contains the inbound full route handle, a connection ID, an initial sequence number, and flags describing the local port  103 (L).   (3) On the remote port, receipt of a SCPRLI message normally modifies the states of the existing route management connection and route master objects  405 / 406  to the pseudo remote end-point  403 . In the case where SCPRLI is received and those objects do not exist, they are created. Successful completion is signaled by sending a SCPRLI_ACC message containing the remote port&#39;s inbound full route handle, a connection ID, an initial sequence number, and flags describing the remote port  103 (R).   (4) On the local port, receipt of a SCPRLI_ACC message modifies the states of the route management connection and route master objects  405 / 406  to the pseudo remote end-point  403  and establishes the route management connection  603 .
 
Partial Route Registration
       

     Register Partial Routes (SCRPR) messages are sent any time there is a change in the set of end-points that are local to a port, to allow end-points  102 (*) discover one another. Once process login is complete, a SCRPR message is sent as the first sequenced message on the route management connection  603 . The message includes the remote port&#39;s inbound full route handle and the connection ID. Each port  103 (R)/ 103 (L) uses the route management connection&#39;s one-way sequenced message delivery service to supply a list of its local end-points&#39; UIDs and IIDs to the corresponding remote port  103 (L)/ 103 (R) using the partial route registration protocol (SCRPR), at steps  850 / 865 . In response, the receiving port creates or modifies a discovered remote end-point object  407  for each listed end-point and registers it in its name server database. Subsequently, an end-point  102 (*) local to the receiving port can discover the set of remote end-points  102 (*) to which it can communicate via yet-to-be established routes  300  and connections  601 . As a side effect of using the route management connection&#39;s sequenced message delivery service, each SCRPR message is acknowledged (at steps  860 / 870 ); i.e., the route management connection  603  assures that every sequenced message is ACK&#39;d. 
     As shown at step  720  in  FIG. 7 , one or more routes  300  are established between end-points  102 (*) using a full route registration protocol. At step  725 , a route set  600  is established between the end-points  102 (*). Steps  720  and  725  are described in detail in the immediately following section. 
     Full Route Registration 
     A port  103  uses full route registration protocols to establish and maintain routes  300  between its local end-point incarnations  412  and those remote end-point incarnations  409  that correspond to the logical intersection of discovered remote end-points  407  and requested remote end-points  415 . These full route registration protocols include protocols to establish (register) and destroy (deregister) routes  300 . Each active route  300  provides a mechanism for delivering transmissions between its two end-points  102 (*). 
     Full route registration and deregistration cause the creation and deletion of route objects (route masters  406 , route proxies  408 , route sets  417 , and route set management connections  419 ). As a side effect, remote end-point incarnation proxy objects  409  may be created and deleted. A remote end-point incarnation proxy object  409  is used to represent the remote end-point incarnation of a full route  300 . 
     A remote end-point incarnation may be represented as an attribute of a route master object  406  (described below in detail), but because route master objects are relatively large, it is desirable to share them. Thus, in an exemplary embodiment, a remote end-point incarnation proxy  409  is created whenever a route master&#39;s remote end-point incarnation  410  is set to a previously unknown value, and deleted whenever no route master  406  references it. 
     The full route registration protocol comprises sending a Register Full Route message (SCRFR) from a local port  103 (L) to a remote port  103 (R) and a Register Full Route Response message (SCRFR_RSP) that the remote port returns to the local port. Both the SCRFR and the SCRFR_RSP messages are sent on the route management connection  603  associated with the route&#39;s path. 
       FIG. 9  is a diagram showing exemplary steps performed in a full route registration process. As shown in  FIG. 9 , full route registration is initiated after an end-point  102 (L) has expressed a demand for routes to a remote end-point  102 (R), at step  905 , by issuing a Request Remote End-Point command to a local port  103 (*). More specifically, full route registration is triggered either by an end-point  102  issuing a Request Remote End-Point command for an end-point  102  to which a partial route has already been discovered, or by the discovery of a partial route to a remote end-point  102 (R) that was previously requested. In the process of establishing a route  300 , two ports  103 (*) exchange full route handles to use to address messages to the end-points via the route. 
     After full route registration is triggered, the local port  103 (L) first checks to see if a route master  406  already exists to the remote end-point incarnation  410  that represents either an established route or a route that is in the process of being established, and if one exists then no further action is taken. Otherwise, a route master  406  is created to track the progress of full route registration and it is linked with the remote end-point incarnation proxy  409  and the route management connection  405 . Then, the local port  103 (L) changes the route master state to NRO and notifies the local end-point  102 (L) that requested the remote end-point  102 (R) by sending a New Route Originator (NRO) event, at step  910 . The local end-point  102 (L) either refuses the new route  300  or approves it and provides the local port  103 (L) with the originator&#39;s ep_info (end-point information) structure, containing end-point information for the local end-point, to transmit to the remote end-point  102 (R). A New Route Originator (NRO) event is generated by the route master object on the port originating the SCRFR to inform the local end-point that a new route to the requested remote end-point  102 (R) specified by a remote end-point UID has been discovered through the path specified by the remote port  103 (R). 
     When a new route is approved, the local port  103 (L) looks up the route master, copies the ep_info and route proxy handle into the route master, and updates the route master state to SCRFR_SENT. The local port then allocates an OBM and uses it to construct a SCRFR message which it sends to the remote port, at step  915  via the route management connection. The SCRFR message conveys the route&#39;s local and remote end-point UIDs and IIDs, the originator&#39;s ep_info structure generated by the local end-point, and a full route handle that can be used to direct messages from the remote end-point  102 (R) to the local end-point  102 (L) via the route  300 . In the case where the new route is refused, the local port runs down the route master, and no further action is taken. 
     On receipt of the SCRFR message, the remote port  103 (R) checks to make sure that the remote end-point  102 (R) referenced in the SCRFR message has registered with the port, and if not, an OBM is allocated and prepared with a SCRFR_RSP with BADREP status. Otherwise, the remote port  103 (R) creates a route master  406  to track the progress of the full route registration and links it to a remote end-point incarnation proxy  409 . If it was not possible to create a route master, then an OBM is allocated and prepared with a SCRFR_RSP with INSRES status to indicate that the remote port has insufficient resources to process the SCRFR. 
     In the case where the route master was created, the remote port  103 (R) copies the SCRFR originator&#39;s full route handle and ep_info from the SCRFR message into the new route master. In the case where the new route master is the only route master at the remote port that describes this route, the remote port changes the route master&#39;s state to NRR and then notifies the remote end-point  102 (R) by sending a New Route Responder (NRR) event, at step  917 . In the case where the new route master duplicates an existing route master the two ports may be attempting to perform the full route registration protocol concurrently. This will be true if the existing route master is in the SCRFR_SENT state, in which case the UIDs of the route&#39;s end-points are compared and the port local to the end-point with the larger UID takes charge by running down the old route master and continuing as before by generating a NRR event at step  917 . If the end-point that originated the SCRFR that is being processed has the larger end-point UID, then an OBM is allocated and used to prepare a SCRFR_RSP with DUPLICATE status. Then, if a SCRFR_RSP message was prepared, it is sent on the route management connection and the new route master is run down. 
     The (NRR) event informs the end-point referenced in the SCRFR that a new route  300  to the remote end-point incarnation specified by the remote end-point UID and remote end-point IID (incarnation identifier) is partly established through the path specified by the remote port  103 (R). The remote end-point  102 (R) either refuses the new route  300 , or approves it and provides the port  103 (R) with a responder ep_info structure, containing end-point information for the remote end-point  102 (R), to transmit to the local end-point  102 (L). In either case, the remote port  103 (R) sends a SCRFR_RSP message, at step  925 , to the local port  102 (L) via the route management connection. Refusal causes the SCRFR_RSP status field to be set to REFUSED to indicate that the remote end-point refused the route, and the route&#39;s resources are run down. Approval updates the state of the route master  406  to SCRFR_RSP_SENT, sets the SCRFR_RSP status field to SUCCESS, and causes the message to convey the route&#39;s local and remote end-point incarnations (UID and IID), the responder&#39;s ep_info structure generated by the remote end-point, and a full route handle that can be used to direct messages from the local end-point  102 (L) to the remote end-point  102 (R) via the route  300 . 
     Also in response to receipt of the SCRFR message at step  915 , the local port  102 (L) sends an ACK  920 , either explicitly or piggybacked on the SCRFR_RSP, as part of the route management connection&#39;s protocol. 
     Receipt of the SCRFR_RSP with status field SUCCESS sent at step  925  causes the local port  103 (L) to record the responder&#39;s route handle in the route master  406 , change the state of the route master  406  to ACTIVE, and generate a Route Completed Originator (RCO) event at step  927 , thus establishing the route at both the local port  103 (L) and the local end-point  102 (L), at step  930 . The Route Completed Originator event informs the end-point that had previously approved the route  300  that the route is now complete and supplies the ep_info provided by the remote end-point. Because full route registration origination is flow controlled on each route management connection, a pending full route registration can now be started. Receipt of the SCRFR_RSP with a status field indicating anything other than SUCCESS causes the local port to run down the route master. 
     In response to receipt of the SCRFR_RSP at step  925 , local port  102 (L) sends an ACK  933  to remote port  102 (R), either explicitly or piggybacked on another available message, as part of the route management connection&#39;s protocol. Receipt of that ACK causes the remote port to change the state of the route master  406  from SCRFR_RSP_SENT to ACTIVE and generate a Route Completed Responder (RCR) event at step  935 , thus establishing the full route at the remote end-point  102 (R) at step  940 . More specifically, receipt, by the remote port  103 (R), of a SCRFR_RSP ACK (at step  933 ) or receipt of the first message on the route  300  establishes the route from the local end-point  102 (L) to the remote end-point  102 (R), at step  940 . 
     End-points  102 (*) can remove the demand for new routes to a remote end-point by issuing a Derequest Remote End-Point message. A Deregister Full Route protocol is used to terminate a route. The Deregister Full Route process is triggered either by a Delete Route request by one of the route&#39;s end-points  102 (*) or the failure of one of the route&#39;s end-point incarnations  410 / 411 . 
     Route Set and Route Set Management Connection Establishment 
     As shown in  FIG. 7 , a route set  600  and a route set management connection  602  are established between end-point incarnations  410 / 411  at steps  725  and  730 . Details of these steps are described with respect to  FIGS. 10A, 10B and 10C , which are flowcharts showing exemplary steps performed in establishing a route set  600  and a route set management connection  602 . Attributes of a route set  600  include a list of connections  601  between processes on the local end-point  102 (L) and processes on the remote end-point  102 (R) and a list of routes  300  to the remote end-point. A routing layer program running on an end-point  102 (*) provides grouping of routes  300  into route sets  600 , and uses a route set  600  to find candidate routes  300  to use for transmissions to the routing layer program running on the corresponding remote end-point  1013 (*). 
     Route set and route set management connection establishment are driven by the New Route Originator (NRO)  910 , New Route Responder (NRR)  917 , Route Completed Originator (RCO)  927 , and Route Completed Responder (RCR)  935  events that are generated by the ports  103 (*) to the end-points  102 (*) during full route registration. Thus, route set and route set management connection establishment are done by the end-points  102 (*). Ports  103 (*) are only involved in that they generate the events and communicate the ep_info data opaquely. 
     As shown in  FIG. 10A , at step  1001 , the routing layer  230  waits for a New Route Originator (NRO) or New Route Responder (NRR) event. When an NRO or NRR event is detected, a route proxy object  408  is created (at step  1005 ) that describes the new route  300 . The route proxy  408  is described in detail below. If (at step  1007 ) a new route proxy  408  was not successfully created, then the new route  300  is refused. If a new route proxy  408  was created, then the UID and IID of the route proxy&#39;s remote end-point are used to find a corresponding route set among existing route sets  600 (*) at step  1010 . If a corresponding route set is found, then the new route proxy  408  is grouped with the other route proxies to the same end-point incarnation by attempting to add the route proxy to the found route set  600  at step  1015 . If no corresponding route set  600  presently exists, then the local end-point routing layer  230  attempts to create a route set  600  to the remote end-point  102 (R) described in the new route event, at step  1012 . If (at step  1017 ) a new route set  600  was successfully created, then the new route proxy is added to that route set at step  1015  as its first member, otherwise the new route  300  is refused, at step  1055 . If the attempt to add the new route to the route set  600  (step  1015 ) failed (test at step  1020 )(e.g. the route set was full), the new route proxy  408  is deleted and the new route is refused, at step  1055 . 
     If the new route  300  was added in response to an NRR event (at step  1025 ), then the supplied route index is copied from the supplied ep_info, and the route set management connection (RTSMC)  602  is updated accordingly, at steps  1030  and  1035 , respectively. An ep_info structure is then created for the remote end-point  102 (*) at step  1040 , and processing continues at step  1042 , described below. If the new route  300  was added in response to an NRO event, then the route set management connection is updated at step  1027 , and an ep_info structure is created for the remote end-point  102 (*) at step  1029 . 
     When two end-point incarnations  410 / 411  attempt to establish a route set management connection  602  between them simultaneously, only one RTSMC  602  should be created. When attempted on a single route  300 , the SCRFR protocol resolves this conflict and generates a NRR event at only one end-point  102 (*). However, when route set management connection establishment is attempted on two separate routes, NRR events will occur at both end-points  102 (L)/ 102 (R). To avoid creating two route set management connections  602 , end-point UIDs are compared, and only the end-point  102 (*) with the higher UID approves the route  300 . Note that this UID comparison must be done with the same sense as that done in the SCRFR protocol in order to avoid refusing both attempts to create a route. Thus, at step  1042 , if a duplicate route set management connection  602  exists in the requested state and the remote end-point UID has a value which is not greater than the local end-point UID, then the new route proxy  408  is deleted and the new route is refused, at step  1055 . 
     Certain event conditions constitute a conflicting route set management connection  602  and thus cause an existing RTSMC  602  to be run down. The corresponding event is then processed as if the existing RTSMC were unknown. Those conditions include the following:
         A NRR event whose originator and responder end-point UIDs and IIDs match those of an existing route set  600 , but whose conn_id_requester does not match the RTSMC&#39;s outbound connection_id (field  1408  in the routing header, described below). This implies that a stale RTSMC  602  exists at the responder.   A RCO event whose conn_id_acceptor does not match the RTSMCs outbound connection_id. This implies that a stale RTSMC  602  exists at the originator.       

     When one end-point incarnation  410 / 411  attempts to establish a route set management connection  602  via two routes simultaneously, no special action is needed because both NRR events will reference the same conn_id_requester. 
     An end-point  102  can come up, go down, and then come back up with a new incarnation  410 / 411  fast enough so that the NRO and NRR events can arrive from the second (new) incarnation before those of the first incarnation. Therefore, both route sets  600  are initially allowed to be established, after which the stale one will fail naturally because it will be unable to deliver messages. When an end-point  102  has a choice of more than one route set  600  to a given end-point destination, preference is given to the last route set to become established. In addition, to accelerate the demise of the stale route set, when an end-point successfully sends a message on a RTSMC  602  and receives a response to that message, then the RTSMC is known to be functioning and any other route sets that exist to other incarnations of that end-point are stale and can be run down. 
     At step  1044 , if there is no conflicting established route set management connection  602 , then the state of the route proxy  408  is changed to indicate that the corresponding route  300  is in the process of being established, at step  1045 . At step  1050 , the ep_info structure that was constructed at step  1040  or  1029  (and mentioned in the description of  FIG. 9 ) is passed to the port for transmission to the remote end-point and the route  300  is approved. Included in the ep_info structure are the inbound half route&#39;s identifier and the route&#39;s local route index (local_route_index), which is the index of the corresponding route proxy  408  in the local route set&#39;s route_proxies array (an attribute of the route set object  417 , described below). The local route index value is sent to the remote end-point  102 (R) during full route registration, where it is used by the remote end-point&#39;s routing layer  230  to identify the described route in the routing headers of subsequent messages sent on a particular route set  600 . 
     If (at step  1044 ) there is a conflicting established route set management connection  602 , then the new route proxy  408  is deleted at step  1046 , the existing route set  600  is run down at step  1048 , and an attempt is made to create a route proxy object  408  to describe the new route  300 , at step  1005 , and the above-described process is repeated. 
     As shown in  FIG. 10B , at step  1060 , the routing layer  230  waits for a Route Completed Originator (RCO) event or Route Completed Responder (RCR) event. When either an RCO or RCR event is detected, then the corresponding route proxy and route set objects  408 / 417  are located, at step  1062  as the states of these objects will be changed in response to the RCO/RCR event. If (at step  1063 ) a Route Completed Originator event was detected, then the supplied route index is recorded from the supplied ep_info structure at step  1066 , and the state of the route proxy  408  is changed to active, at step  1067 . The route set management connection  602  is then updated accordingly, at step  1068 . At this point, the state of the route set management connection  602  may change in response to receiving the RCO event. In the situation wherein the RTSMC  602  is in the ‘requested’ state, and an RCO event is received in conjunction with an unknown connection ID, the state of the RTSMC is changed to established. 
     At step  1069 , if there is a conflicting established route set management connection, then the route set is run down, at step  1080 , and processing continues at step  1076 ; otherwise, processing continues at step  1070 . 
     If a Route Completed Responder event was detected at step  1063 , then the state of the route proxy is changed to active, at step  1064 , and the route set management connection is then updated accordingly, at step  1065 . Here, the state of the route set management connection  602  may change in response to receiving various events. In the situation wherein the RTSMC  602  is in the ‘accepted’ state, and an RCR event is received, the state of the RTSMC is changed to established. 
     At step  1070 , if the route set management connection  602  was successfully established, then a check is made for connections  601  waiting for route sets and their connection establishment process is restarted, at step  1072 . If the route set management connection  602  was not successfully established, then step  1072  is skipped. 
     At step  1076 , a check is made to see if there are any outbound sequences (OBSs) waiting for an acceptable route and, if so, their route selection process is restarted (see  1325 ). At step  1078 , the ep_info structure is passed to the port for inclusion in the SCRFR_RSP message, as indicated above with respect to  FIG. 9 . 
     As shown in  FIG. 10C , at step  1080 , routing layer  230  waits for a Route Deleted event, which is generated to remove a failed route  300  from a route set  600 . When the network layer  240  delivers a route deleted event to the routing layer  230 , it removes a failed route  300  from a route set  600 . In addition, a route set  600  is automatically deleted as a result of events including the following:
         no connection has existed on the route set  600  for a substantial period, e.g., on the order of an hour. This indicates that there is no demand for the route set;   no route has existed within the route set  600  for a substantial period, e.g., on the order of an hour. This indicates the route set cannot be supplied; or   no connection exists on the route set  600  and no route exists in the route set.       

     When a Route Deleted event is detected, at step  1082 , the corresponding route proxy and route set objects  408 / 417  are located. At step  1084 , the route proxy  408  is removed from the route set  600  and deleted. 
     At step  1086 , if the route set  600  is now empty, then (at step  1088 ) if the route set has no client connections, the route set is deleted, otherwise a timer is started which will delete the route set if no routes  300  are added before the timer expires. If the route set  600  is not empty at step  1086 , then Route Deleted event processing terminates, at step  1090 . 
     Application Connection Establishment 
     As shown in  FIG. 7 , a connection  601  between local and remote end-point applications  101 (*) is established at step  735 . These application connections are derived from a routing layer connection base class. The aspects of the application connection described in this section are actually those provided by the routing layer connection, and thus only the routing layer connection is described subsequently. 
     As explained above, route set  600  also provides a route set management connection  602  that is used to manage the routing layer connections  601  established on the route set. This includes the communication of Connect Request and Connect Accept messages used to establish the connections as well as the Connect Abort message used to destroy the connections. 
     During routing layer connection establishment, routing layer  230  assigns connection IDs. Once the connection is established, the request layer copies the connection ID into the routing header of each outbound message, and dispatches incoming messages to their proper client based on the connection ID in the received routing header. The routing header ( 1400 ) is described in detail with respect to  FIG. 14 , below. Each routing layer connection  601  is established on a route set  600 . The route set  600  defines the set of available routes  300  that can be used by the connection  601 . 
     Routing layer  230  relies on the transport layer  220  to send and receive messages on a route set management connection  602 . In this role, the routing layer  230  is merely a client making use of transport layer functionality. Thus, the transport layer  220  must allow each connection  601  to have a different client. 
       FIG. 11  is a flowchart illustrating exemplary steps performed in handling a request to establish a connection  601  between end-point applications  101 (*). As described in detail in the present section, the routing layer  230  establishes an application connection  601  with a three-way handshake beginning with a Connect Request message on the route set management connection  602 . Receipt of a Connect Accept message completes the handshake at the requester and receipt of either the connect accept ACK or the first message on the routing layer connection  601  completes the handshake at the acceptor. 
     As shown in  FIG. 11 , at step  1105 , when routing layer  230  receives a request to establish a connection  601  to a remote end-point  102 (*), the routing layer must first find an established route set  600  to that remote end-point. This is accomplished by a standard software lookup procedure. 
     At step  1110 , if no established route set  600  exists, the routing layer will attempt to create a route set  600  by issuing a request remote end-point message to each of the network layer&#39;s local ports at step  1120 . This triggers an autonomous full route registration process in the network layer&#39;s local port, as described above with respect to  FIG. 9 . From this point on, route establishment and route set establishment are driven by events generated by the network layer  240 . 
     At step  1125 , a routing layer routine waits for a route set  600  to the requested remote end-point  418  to be established. After the route set  600  and a route set management connection  602  are established to the requested remote end-point  418 , then an OBM is allocated by the client wanting to send the message, at step  1130 . 
     At step  1110 , if an established route set  600  exists to the remote end-point  102 (R), then an OBM (outbound message object) is allocated by the client wanting to send the message, at step  1130 , and processing continues as described below with respect to step  1135 . 
     In the present case, routing layer  230  is the client. An OBM is an object that is shared between the routing layer and its client, and used by the routing layer&#39;s client to describe a message that is to be sent by the routing layer. The OBM is passed to the routing layer in various functions that select routes and send messages. In the present case, the routing layer allocates an OBM so that it can send a Connect Request message on a route set management connection. Once the OBM is allocated, the routing layer calls a Make Connect Request virtual function, at step  1135 , to allow the routing layer&#39;s client to add its information to the Connect Request message being constructed within the OBM, at step  1140 . The Connect Request message is then sent on the route set management connection  602  to the remote end-point  102 (R), at step  1145 . 
     When the Connect Request message is received by routing layer  230  at the remote end-point  102 (R), the routing layer delivers it to the transport layer entity to which the message is addressed, at step  1150 . The transport (or a higher layer) then determines whether or not a matching Connect Accept message is pending, at step  1155 . If there is a pending Connect Accept, then an OBM is allocated by the routing layer, at step  1165 , and the routing layer calls a MakeConnectRequest virtual function to allow the routing layer&#39;s client to add its information to the Connect Accept message, at step  1170 . The Connect Accept message is then sent on the route set management connection  602 , at step  1175 . 
     If no matching Connect Accept is pending when the Connect Request message arrives at the receiver at the remote end-point  102 (R), then the routing layer allocates a OBM and responds with a Connect Reject message, at step  1160 . 
     When the Connect Accept message is received, at step  1180 , the routing layer application connection  601  becomes established at the requestor, and the routing layer&#39;s client is notified with a ConnectRequestDone virtual function associated with the connection. 
     The routing layer application connection  601  becomes established at the acceptor when the ACK to the Connect Accept message is received (step  1185 ) or the first message is received on the new connection by the remote end-point  102 (R) (step  1190 ). A ConnectAcceptDone virtual function then notifies the routing layer&#39;s client that the connection has been successfully established. 
     Route Selection 
     Once a connection, including an application connection  601  or a route set management connection  602 , is established between end-points  102 (*), the clients of that connection can use it to exchange messages reliably. Each message (e.g., a Connect Request message) is preferably sent via its initial transmission, and retry transmissions are used by the transport layer to assure that messages are delivered reliably in the same manner employed by TCP. In the present system, the transport layer  220  requests that the routing layer  230  send these transmissions and the routing layer selects a route  300  for each transmission and then sends the transmissions via the selected route. 
     The present route selection method is based on the notion of acceptable routes. An acceptable route is one that meets all of the following constraints:
         the outbound half route  500  is enabled and in the active state;   the outbound half route&#39;s transmission rate is below its transmission rate limit; and   the outbound half route&#39;s local port  103 (L) has the resources required to send a transmission, including an available outbound sequence (OBS) object, which is the primary object describing a transmission to be sent.       

     As shown in  FIG. 7 , at step  740 , a route  300  between two end-points  102 (*) is selected in response to a route selection request. Route selection is done each time the routing layer  230  sends a transmission, which may be the initial transmission of a message, a retry transmission of a message, or an ACK. The route selection process is described in detail with respect to  FIGS. 12A and 12B . Before making a route selection request, the routing layer client must first allocate and construct an OBM that describes the outbound message to send, and this OBM must be available for queueing in the case where not acceptable route is available. 
       FIG. 12A  is flowchart illustrating an algorithm comprising exemplary high-level steps performed in selecting a route  300  for an initial transmission. As shown in  FIG. 12A , when selecting a route for an initial transmission, the last route  300  used within the route set  600  is selected if it is still acceptable (step  1110 ) and has been used for fewer than some small number of consecutive transmissions CTmax (step  1215 ). The value of CTmax is selected by the implementer, but is typically approximately 5, and tends to amortize the selection computation over that number of transmissions. 
     At step  1225 , a determination is made as to whether there are any acceptable routes  300  in the route set  600 . If no acceptable routes are found, then at step  1230 , the OBM supplied in the route selection request is queued in the routing layer. If there are a plurality of acceptable routes  300  in the route set  600  that are equally underutilized (step  1235 ), then a route is selected from these equally underutilized acceptable routes at step  1240 , that is next in route set order starting with the last route used; otherwise, the least utilized acceptable route from the route set  600  is selected at step  1245 . 
       FIG. 12B  is flowchart illustrating exemplary high-level steps performed in selecting a route  300  for a retry transmission. As shown in  FIG. 12B , at step  1250 , a determination is made as to whether there are any acceptable routes  300  in the route set. If acceptable routes exist, then, at step  1265 , the route  300  is selected which is the next acceptable route in route set route order relative to the route used for the previous transmission of the message being retried. This procedure assures that for any given message, all acceptable routes are tried before any are retried. If no acceptable routes  300  exist in the route set  600 , then at step  1260 , the OBM supplied in the route selection request is queued within the routing layer  230 . 
     When a route&#39;s state changes such that it may now be acceptable, the route selection algorithm of  FIG. 12A /B is executed, and queued route selection requests may complete asynchronously. The passing of time will automatically lower a route&#39;s transmission rate below its transmission rate limit and when the other constraints are met, then transmissions will be sent at the route&#39;s transmission rate limit. A method of controlling the transmission rate for a given half route  500  is described below with respect to  FIG. 17 . 
     Queueing 
       FIG. 13  is a diagram showing exemplary steps and queues used in queueing route selection requests. As shown in  FIG. 13 , when a route selection request is received by the routing layer  230  ( FIG. 12A , step  1230 , or  FIG. 12B , step  1260 ), a determination is made as to whether any acceptable routes  300  are presently available within the specified route set  600 , at step  1305 . If an acceptable route  300  exists, then route selection proceeds in accordance with the steps shown in either  FIG. 12A  or  FIG. 12B . When no acceptable routes  300  are available within the specified route set  600 , the OBMs supplied in the requests to select routes are queued within the routing layer  230 , at step  1315 . When a route&#39;s state changes such that it may now be acceptable, the route selection algorithm described with respect to  FIG. 12N / 11 B is executed and queued route selection requests may complete asynchronously. Because an acceptable route must have a transmission rate that is below its transmission rate limit, the routing layer is able to control transmission rate using its queueing mechanism. This feature is used to back off transmission rate when a route becomes congested. 
     Each route selection request issued to the routing layer  230  is described by an outbound message (OBM) object that the routing layer may queue. When no acceptable route is available, the request&#39;s OBM is placed on one of the following prioritized FIFO queues:
         1. A route_proxy.send_specific_route queue  1320  is used to queue requests for which the route has already been determined. Although no route selection is done in this case, route status must be acceptable and an outbound sequence (OBS) must be allocated; these are effectively route selection functions. These requests, for which the route has already been determined, are given highest priority because the delivery of an End_MSE (end multi-sequence exchange) frees resources and because the local resource allocation is constrained.   2. A route_set.send_retry_sfe queue  1321  is used to queue route selection requests for retry SFEs (single frame exchanges). These requests are given second highest priority because they indicate that connection message delivery is probably stalled at the remote end-point.   3. A route_set.send_initial_sfe queue  1322  is used to queue route selection requests for sends of initial SFEs. These requests are given third highest priority because they will stall the connection&#39;s message transmission unless satisfied.   4. A route_set.send_start_mse queue  1323  is used to queue route selection requests for Start_MSEs (start multi-sequence exchanges).       

     When a route&#39;s state changes such that it may now be acceptable (step  1325 ), the route selection algorithm of  FIG. 11A / 11 B is executed with the OBMs at the heads of these queues for the corresponding route proxy  408  and route set  600  in the priority order listed above, at step  1330 . If the route selection algorithm indicates that a route  300  is acceptable for an OBM, the OBM is dequeued. Then an OBS is allocated from the route&#39;s local port  103 (L), and after the OBS is prepared it is transmitted via the local port  103 (L). 
     When the route selection algorithm determines that a route  300  is unacceptable because no OBS is available on the route&#39;s local port  102 (L), the corresponding route set  600  is queued to the local port. When an OBS becomes available on that local port, route sets  600 (*) are dequeued and their route selection algorithm is run until one consumes the available OBS. Because a route set  600  may need to be queued to several local ports  103 (*) simultaneously, the unacceptable route&#39;s route proxy  408 , instead of the route set, is queued to the local port  103 (L). Instead of dequeueing a route set  600 , a route proxy  408  is dequeued along with all other route proxies in that route&#39;s route set  600  that use the same local port  103 (L). 
     Route Order 
     The two end-points  102 (*) of a route set  600  each determine an order for the routes  300  in the route set. There are several reasons for route ordering. First, when a route  300  becomes unacceptable, it is desirable to quickly select a next route from the same route set that is largely independent of the old route. Since route independence is relatively static, a next route can be predetermined for each route. In addition, when a series of two or more routes becomes unacceptable, it is desirable to quickly select a next route from the same route set that is largely independent of each route in the series of unacceptable routes. All routes of a route set may be placed in a circular queue to facilitate this route selection. 
     Each end-point  102 (*) determines route order independently, using locally available information, and then communicates that order to the corresponding remote end-point. The remotely determined route order is used to select the next route for transmission because it may include information about route independence that is not available locally. When a new route  300  is added to a route set  600  or removed from a route set, the route set&#39;s route order changes and the new route order is communicated via a route set management connection message. While changes in route set order may cause transient anomalies in route selection, these anomalies do not cause incorrect network behavior and should disappear quickly. 
     Route Order Report (ROR) messages are sent on the route set management connection  602  to communicate a route set&#39;s locally determined route order to the remote end-point  102 (R). The ROR is a one-way sequenced message that includes a count field indicating the number of routes  300  in a route set  600  and an array containing an ordered list of the sender&#39;s remote_route_index values of the routes in the sender&#39;s local route set route order. The remote_route_index comprises indicia of the remote route proxy  408  for a given route  300  in the remote route set object&#39;s route_proxies array. 
     Route order determination is not architecturally constrained, because it does not affect correctness of operation. However, route order can have a significant effect on performance. In general, successive routes  300  in a route set  600  should share as few common components or resources as possible. This is desirable in error recovery, to maximize chances of a retry succeeding. It is also desirable for load balancing, where the goal is to spread the load as uniformly as possible across the available components. The manner in which the route ordering is determined is described in a related application. 
     Routing Header 
     All routing layer transmissions include a routing header  1400 . The routing layer  230  uses information in the routing header  1400  to monitor transmissions on each half route  500  to determine the half route&#39;s quality, by determining whether a half route is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. Inbound half route quality is computed at the receiving routing layer and converted to a transmission rate limit, which is communicated back to the transmitting routing layer in the routing header where it is used by a route selection algorithm (e.g., the algorithm of  FIGS. 12A /B) to choose the best route  300  of a route set  600  for sending future transmissions. Finally, the routing header contains the routing layer connection&#39;s connection_id that was discussed in reference to  FIG. 11 . 
     In an alternative embodiment, the routing layer places information into the routing header of each transmission sent on a route set that describes multiple (or even all) routes of that route set. However, the result is expensive both in transmission header efficiency and processing time to send and receive the transmission. In an exemplary embodiment, the routing layer  230  places monitoring information that describes a single route of the route set in the routing header  1400  of each transmission sent on that route set  600 , and in the steady state, the single route described is rotated among the routes of that route set in a round-robin fashion. Thus, the described route may differ from the route used to send the transmission. 
     For a given transmission, the route  300  described in the routing header  1400  is called the described route (DR). In contrast, the route  300  on which the transmission is sent and received is called the receive route (RR). Routing header  1400  includes the following information about the described route:
         the DR&#39;s index, which uniquely identifies the DR to the receiver;   the RSN of the last transmission sent on the outbound half DR;   the time since the last transmission was sent on the outbound half DR;   information that signals that the transmission rate limit (TRL) should be reset on the receiver&#39;s inbound half DR; and   the computed TRL for the transmitter&#39;s inbound half DR.       

     In an exemplary embodiment, the routing header  1400  contains the following fields, which are transmitted in the order listed. All fields except the connection ID describe a route in the route set known as the described route or DR. These fields are organized within an exemplary routing header as shown in  FIG. 14 , and described below.
     dr_index_key This 2-bit field (‘key’  1401  in  FIG. 14 ) is a key that is used to validate the dr_index field at the receiver. Its value is determined by the receiver and is communicated to the transmitter during full route registration. When a transmission is received in which this value does not match the expected value then the described route information in the transmission is ignored.   dr_tre This 2-bit field (‘tre’  1402  in  FIG. 14 ) specifies the transmitter&#39;s route epoch for the outbound half DR. The receiver compares this value to the last dr_tre value received for the inbound half DR. If it differs, the receiver resets the inbound half DR&#39;s transmit rate limit to one transmission per E_D_TOV (defined in FC-FLA V2.7).   dr_rre This 2-bit field (‘rre’  1403  in  FIG. 14 ) is a copy of the last dr_tre received by the transmitter for its inbound half DR. When this field equals the receiver&#39;s outbound half DR route epoch, then the local end-point knows that the remote end-point has received its last route epoch update for the DR.   dr_trlc This 8-bit field  1404  is a compressed form of the transmit rate Limit (TRL) computed by the transmitter for its inbound half DR. This value is used by the receiver to limit the transmit rate of its outbound half DR.   dr_itt This field  1405  contains an 11-bit unsigned integer representing the number of milliseconds that have elapsed since the last transmission on the transmitter&#39;s outbound half DR. If the elapsed time exceeds 2047 milliseconds, then this field contains a value of 2047.   dr_index This field  1406  contains a 5-bit unsigned integer representing the index of the inbound half DR at the receiver. This index is supplied to the transmitter during full route registration. The receiver uses this value to determine which inbound half route is being described.   dr_rsn This field  1407  contains a 32-bit unsigned integer representing the routing sequence number of the transmitters outbound half DR. This value is the number of transmissions sent on the route since it was established, modulo 2 32 .   connection_id This field  1408  contains the 32-bit outbound connection ID of the connection on which the message is being sent. The receiver uses this value as an inbound connection ID to find the destination connection.   

     End-points  102 (*) monitor the quality of each inbound half route  500  using data supplied by the remote end-point  102 (R) via the routing header  1400 . The receiving end-point combines all said data into a single metric called the transmission rate limit (TRL). The TRL is indicative of whether a half route is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. Each TRL computed by the receiving end-point is fed back to the transmitting end-point via a field in the routing header  1400 , which, in one embodiment, is sent in compressed form as the dr_trlc field  1404 . At the transmitter, the TRL is used to place an upper bound on the rate of transmissions issued on the outbound half route  500  and by a route selection algorithm (e.g., the algorithm of  FIGS. 12A /B) to choose the best route  300  of a route set  600  for sending future transmissions. In this capacity, the ratio TR/TRL is used to determine whether a route is acceptable and as a measure of route utilization. These processes are described below in detail with respect to  FIGS. 12 and 17 . 
     Send Processing 
       FIGS. 15A and 15B  are diagrams illustrating exemplary routing layer processing performed in sending transmissions. The routing layer client sends a transmission via a routing layer connection in two phases. First, the routing layer client selects a route  300 , and then the routing layer client sends the transmission on that route. The routing layer provides separate procedures for selecting and sending the initial transmission of a message, shown in  FIG. 15A , and for selecting and sending retry transmissions of a message, shown in  FIG. 15B . 
     As shown in  FIG. 15A , the route selection phase for an initial transmission begins at step  1502 , in which the routing layer client supplies a reference to the routing layer connection and a outbound message (OBM) structure that describes the message to be transmitted. At step  1505 , the routing layer connection state is validated as suitable for sending transmissions, i.e., the connection state must be established. At step  1508 , a route is selected for an initial transmission as described in  FIG. 12A , queueing for an acceptable route if necessary as described in  FIG. 13 . Once route selection completes, the routing layer must check to see that a route was indeed selected at step  1511 , because it is possible, for example, that the route set failed while the OBM was queued for an acceptable route. In the case where a route was selected, an outbound sequence (OBS) structure must be allocated from the route&#39;s local port for describing the transmission at step  1514 , and linked to the OBM. The selected route is recorded in that OBS at step  1517 , and the selected route is recorded in the OBM as the last route tried for transmissions of the message described therein at step  1520 . Finally, at step  1523 , the routing layers estimate of the selected route&#39;s outbound half route transmission rate is updated and control is returned to the routing layer client at step  1526 . 
     Before the routing layer client requests that the routing layer send the initial transmission at step  1529 , the routing layer client may update its connection state. For example, the routing layer client might start a timer that triggers a retry transmission if the initial transmission is not acknowledged before the timer expires. When the routing layer client does request that the initial transmission be sent, the routing layer starts by initializing the OBS that was previously linked to the OBM at step  1514 . At step  1532 , the route&#39;s route master handle is inserted in the OBS so that when the OBS is passed into the port layer then the port layer can rapidly find the outbound route. At step  1535 , a scatter-gather list (SGL) is copied to the OBS that describes the OBM buffer that holds the transmission&#39;s routing, transport, and request layer headers plus the application payload buffer. Thus, the port can transmit all of this data directly from the end-point&#39;s buffers without having to first copy it into a contiguous buffer. At step  1538 , the described route (DR) is selected, and the routing header described route fields are filled in using data from the selected described route. Next, the routing header&#39;s connection ID  1408  is filled in using data from the routing layer connection, at step  1541 . The receive route (RR) is then updated by incrementing its RSN at step  1544 . Next, the network layer  240  is used to send the transmission via the selected route&#39;s local port  103 (L), at step  1547 . Once the network layer completes its attempt to send the transmission, the status is checked. In the case where the network layer was unable to successfully send the transmission, then the receive route (RR) is updated at step  1550  to indicate that it is seriously congested. Regardless of the send completion status, the OBS is unlinked from the OBM and de-allocated at step  1553  and control is returned to the routing layer client. 
     As shown in  FIG. 15B , the route selection phase for a retry transmission begins at step  1564  in which the routing layer client supplies a reference to the routing layer connection and a outbound message (OBM) structure that describes the message to be retransmitted. At step  1567 , the routing layer connection state is validated as suitable for sending transmissions, i.e., the connection state must be established. Because this is a retry transmission, it is very likely that the previous transmission of this message failed, thus at step  1570  the TRL of the half route indicated by the OBM&#39;s last route tried (set in step  1520 ) is decreased and the half route&#39;s significant event flag is set. At step  1573 , a route is selected for a retry transmission as described in  FIG. 12B , queueing for an acceptable route if necessary as described in  FIG. 13 . Once route selection completes the route selection phase for a retry, transmission continues, and the steps shown in bracket  1580  are performed exactly as corresponding steps  1511 - 1523  (described with respect to  FIG. 15A ) for an initial transmission. 
     The send transmission phase for a retry transmission begins at step  1587 . The routing layer starts by initializing the last OBS that was linked to the OBM. At step  1590 , the route&#39;s route master handle is inserted in the OBS so that when the OBS is passed into the port layer then the port layer can rapidly find the outbound route. At step  1593 , a scatter-gather list (SGL) is copied to the OBS that describes the buffer that holds the transmission&#39;s routing, transport, and request layer headers plus the application payload buffer. In contrast to the initial transmission, the SGL for the retry transmission describes a buffer for the headers that is in the OBS instead of the OBM, and the request and transport headers are copied from the OBM to the OBS in step  1596 . This allows the routing layer to modify the routing header without risking modifying said header of the initial transmission of the message which may not be complete at the time of the retransmission. Once the header copy completes the send transmission phase for a retry, transmission continues, and the steps shown in bracket  1598  are performed exactly as corresponding steps  1538 - 1556  (described with respect to  FIG. 15A ) for an initial transmission. 
     Receive Processing 
       FIG. 16  is a diagram illustrating exemplary routing layer processing performed in receiving transmissions. As shown in  FIG. 16 , the routing layer performs the following exemplary steps with respect to each received transmission. Initially, at step  1605 , context is established from a pool buffer descriptor, which includes indicia of the route over which the transmission was received, and information supplied by the receiving port about how the transmission was received. In an exemplary embodiment of the present system, communicating applications  101 (*) create buffer pools, containing pool buffers, to directly receive messages bound for them. When an application  101  requests or accepts a connection  601 , it specifies the buffer pool into which messages received on that connection should be stored. In an exemplary embodiment, end-point memory  421  is used for the buffer pools, as applications  101  are executed in that memory space. However, when a pool buffer (i.e., a buffer within the buffer pool) is posted for receive, a description of that buffer is passed to the associated port  103  and that memory becomes shared between the end-point  102  and the port until the pool buffer is consumed. This allows the port to receive a transmission directly into the pool buffer without the need to copy the data from the port memory  420  to end-point memory  421 . 
     Given the route over which the transmission was actually received, the routing layer is able to determine the corresponding RR and the route set  600  at step  1605 . The DR is determined at step  1608  from the route set itself and dr_index  1406  and dr_index_key  1401  (in the routing header). At step  1610 , the route set, DR, RR, and the routing header data are used to compute a new transmission rate limit (TRL) for the inbound half DR. Next, at step  1615 , the transmission rate limit (TRL) for the outbound half DR is updated using the dr_trlc  1404  that was fed back from the other end-point via the routing header. Since updating this TRL may have caused the DR to become acceptable, a check is made for OBMs queued for route selection. Then, at step  1618 , the reception rate estimate, round-trip-time estimate, inbound TRL, and expected RSN for RR are updated. The destination routing layer connection whose inbound connection ID matches connection_id  1408  (in the routing header) is then located, at step  1620 . At step  1623 , if the destination routing layer connection is in the accepted state then transition the routing layer connection to the established state and run down any existing route sets to other incarnations of the same remote end-point. At step  1625 , if the destination routing layer connection state is suitable for receiving transmissions (e.g., established), then the transmission is delivered to the routing layer connection&#39;s client at step  1630  by passing the pool buffer descriptor to it; otherwise, the transmission is discarded, at step  1635  by returning the pool buffer descriptor to its buffer pool. 
     Route Monitoring 
     The routing layer  230  monitors transmissions on each half route  500 (*) to determine the half route&#39;s quality. Routing layer  230  determines whether a half route  500  is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. This route quality information is used by a route selection algorithm (e.g., the algorithm of  FIGS. 12A /B) to choose the best route  300  of a route set  600  for sending future transmissions. 
     More specifically, routing layer  230  monitors the quality of each inbound half route  500  using data supplied by the remote end-point&#39;s routing layer via the routing header that is included in each transmission. All half route quality information is combined into a single metric called the transmission rate limit (TRL) (See step  1610 ,  FIG. 16 ). The TRL is computed by the receiving end-point&#39;s routing layer and is fed back to the transmitting end-point where it is used by the routing layer to place an upper bound on the rate of transmissions issued on the outbound half route. When not limiting, the rate of transmissions divided by TRL indicates the degree of outbound half route underutilization, a metric the routing layer uses to select a route for an initial transmission. 
       FIG. 17  is a diagram showing an exemplary routing layer feedback loop  1710 / 1715 / 1720 / 1725  that is executed to control the transmission rate for each half route  500  and to supply route quality information to be used by the transmitting routing layer for route selection. 
     In operation, transmissions  1710  are sent from transmitting end-point  102 ( 1 ) via outbound half route  500  to receiving end-point  102 ( 2 ). The routing layer at end-point  102 ( 2 ) monitors the quality of each inbound half route  500  using data supplied by a transmitting end-point  102 ( 1 ) via the routing header  1400  of each received transmission  1710 . In an exemplary embodiment, all half route quality information is combined by the receiving routing layer at block  1715  (see step  1610 ,  FIG. 16 ) to establish a transmission rate limit (TRL)  1404 . Transmission rate limit determination is described in detail in a related application. 
     After TRL  1404  is computed by the receiving end-point  102 ( 2 ), it is fed back to the transmitting end-point in the dr_trlc field of the routing header  1400 , via the next message  1720  that describes that half route  500 , where it is used by the routing layer to place an upper bound on the rate of transmissions issued on the outbound half route  500 . 
     In an exemplary embodiment, a routing layer program  1703  continually determines a current value for the transmission rate (TR)  1701  on each outbound half route by passing a sequence of inter-transmission times through a low-pass digital filter and inverting the result (see step  1523 ,  FIG. 15A ). Block  1725  functions as a limiter in which the transmitting end-point  102 ( 1 ) compares the current value for TR  1701  with the value of TRL  1404  fed back from the remote end-point  102 ( 2 ) to determine whether to send the next transmission on that outbound half route or another route within the route set, or to delay it. When the current TR  1701  is no greater than TRL  1404 , the transmission is sent without imposition of a delay. In an exemplary embodiment, when the current TR  1701  exceeds TRL  1404 , no transmissions will be sent on the outbound half route  500 ( 1 ) because the route will not be acceptable and thus not selected (see  FIG. 12 ). However, the next transmission may be sent on another route of the route set without delay if another route is acceptable. In the case where no route of the route set is acceptable, the OBM is queued as described in  FIG. 13 , at step  1315 . As time passes, the intertransmission time will increase and thus the transmission rate  1701  of an unacceptable route will decrease until it reaches a value that is equal to or less than the present TRL  1404 , at which time the route will become acceptable again. This mechanism effectively limits the transmission rate to be no greater than TRL. 
     To avoid continually computing a half route&#39;s transmission rate when its transmission rate exceeds its transmission rate limit, a timer is started whose expiration time is computed to be the time when TR will equal TRL. When the timer expires, TR is updated. Then, if the route is acceptable and OBMs are waiting, route selection is triggered, as described above with respect to  FIGS. 12A /B and  13 . 
     When not limiting, the transmission rate  1701  divided by the transmission rate limit  1404  indicates the degree of outbound half route underutilization. This metric is used by initial route selection at step  1245 , in  FIG. 12 . 
     In the case where an end-point  102 (*) detects a serious problem with an outbound half route, a mechanism is provided for communicating that event to a corresponding remote end-point  102 (R). For example, if a transmission sent to the network layer  240  times out because the network layer detects so much congestion on the network that it cannot send the transmission within E_D_TOV (see  FIG. 15A , step  1550 ), this mechanism allows the transmitting end-point to request that the remote end-point  102 (R) set the inbound half route&#39;s TRL  1702  to one transmission per E_D_TOV. 
     Routing Sequence Numbers 
     The routing layer  230  counts transmissions sent on each outbound half route  500  from the time the full route  300  was established. These counts are called routing sequence numbers (RSNs) and they facilitate monitoring half routes  500 . Each transmission on a route set  600  is uniquely identified by the route  300  on which it is sent, its direction or sending end-point  102 , and its routing sequence number. The sending end-point  102  knows this information for each transmission. 
     In an alternative embodiment, each transmission includes its own RSN (i.e. the RSN of the outbound half route on which the transmission was sent), which allows both sending and receiving end-points  102  to share knowledge of each transmission&#39;s unique identity. In particular, upon receiving a transmission, the end-point  102  can readily determine whether any previous transmissions sent on the same route  300  are missing by comparing the RSN just received to the highest RSN previously received. 
     The above concept may be extended across routes  300  in a route set  600 . In an alternative embodiment, each transmission describes not just its own RSN, but the RSN for every outbound half route  500  in the route set  600 . These RSNs identify the most recent transmission sent on each outbound half route  500 . Together the RSNs identify all transmissions that have been sent in a particular direction on the route set  600  prior to the subject transmission. For each transmission that it receives, a receiving end-point  102  determines which prior transmissions it has received (on all routes of the route set) and which are missing. 
     The receiving end-point  102  cannot always distinguish a missing transmission as lost or delayed; both are transmissions that the receiving end-point  102  expects to receive but has not received. However, because Fibre Channel fabrics (and communication fabrics in general) are unlikely to deliver transmissions out of order when sent via a single route  300 , the receiving end-point  102  can use the manner in which it learned of a missing transmission to distinguish those that were probably lost from those that were probably delayed. 
     When evidence of a missing transmission is obtained on the same route  300  as the missing transmission, then the transmission is probably lost. For example, suppose an end-point  102  had received all transmissions through RSN N on route A, and then received RSN N+5 on the same route. The end-point may then conclude that the four transmissions RSN N+1 through RSN N+4 were probably lost. 
     When evidence of a missing transmission is obtained on a different route  300  than the missing transmission, then the transmission is first considered probably delayed, and then after a period of time equal to R_A_TOV, it is considered lost. For example, suppose an end-point  102  has received all transmissions through RSN N on route A, and then received a transmission on route B specifying that the most recent RSN sent on route A was N+10. In this case, the receiving end-point may initially conclude that the ten transmissions N+1 through N+10 are probably delayed. If the end-point next received transmission N+5 on route A, it would then conclude that the four transmissions N+1 through N+4 were probably lost, the transmission N+5 was certainly delayed, and the five transmissions N+6 through N+10 are probably still be delayed. If no further transmissions are received on route A for R_A_TOV, the end-point may then conclude that transmissions N+6 through N+10 are certainly lost. 
     As each transmission is received, the receiving end-point  102  compares an RSN included in the transmission with that expected for the corresponding inbound half route  500 . Thus, the end-point must store an expected RSN for each inbound half route. If it were not for lost transmissions, the expected RSN would simply be the count of transmissions received on the inbound half route  500 . However, because transmissions may be lost, the expected RSN is estimated using the following procedure. 
     When a transmission is received that includes the RSN of the half route  500  on which the transmission was sent, the inbound half route&#39;s expected RSN is updated: If the received RSN is greater than or equal to the inbound half route&#39;s expected RSN, the inbound half route&#39;s expected RSN is set to one higher than the received RSN. Otherwise, the transmission must have been received out of order and the expected RSN is left unchanged. 
     Because out-of-order delivery on a single route  300  is unlikely, it is not necessary to include, in every transmission, the RSN of the half route  500  on which the transmission is sent. When a transmission is received that does not include the RSN of the half route  500  on which it was sent, the receiver simply increments the inbound half route&#39;s expected RSN. Furthermore, because changes in route quality tend to occur infrequently, it is not necessary to include, in every transmission, the RSN of every other outbound half route of the receive route&#39;s (RR) route set. In an exemplary embodiment, each transmission sent on outbound route RR includes the RSN of one of the outbound routes, the described route (DR), of the receive route&#39;s route set, and the described route is rotated among the routes of the route set. 
     Significant Events 
     Most events that cause changes in half route quality metrics are known as significant events. When a significant event occurs, it becomes desirable to describe the affected route  300  to the remote end-point promptly. Normally, the routes  300  in a route set  600  are described in round-robin order, but following a significant event, the corresponding route is given a higher priority for being described. This is done with a significant_event flag (an attribute of the route proxy object  408 ) that is set when a significant event occurs on that route  300  and which is cleared when the route is described. 
     The following events are considered to be significant events:
         The routing layer client indicates that an ACK response timeout occurred on a route  300 . In response, the outbound half route&#39;s transmission rate limit (TRL) is reduced by half and its significant_event flag is set. Because the ACK may have been sent on any route, the routing layer  230  cannot conclude with certainty that the indicated half route  500  has lost or delayed any transmissions. By changing the outbound half route&#39;s TRL, the local end-point  102 (L) is immediately discouraged from using that route further, but when the remote end-point  102 (R) provides an updated TRL, this change will be dismissed, since the remote end-point has more definitive quality data about the outbound half route  500 .   Any time the outbound half route  500  selected for an initial transmission differs from the route set&#39;s last_rtp. The value for last_rtp indicates the route proxy  408  of the last route  300  that was used to send a transmission on this route set  600 ; this value is included as a field in the route set object  417 , described in detail below. In response, the significant_event flags are set for the selected route  300  and last_rtp, and last_rtp is updated to be the selected route. This is done to describe changes that motivated selecting a new route  300  and to assure that routes describe themselves frequently.   Any time the outbound half route  500  selected for a retry transmission differs from the route set&#39;s last_rtp. In response, the significant_event flag is set for last_rtp and last_rtp is updated to be the selected route  300 . The significant event flag is not set for the selected route so that the route used for the previous try will be described as soon as possible.   Any time a half-route&#39;s local port changes state, the significant_event flags for all associated outbound half routes are set. In addition, when a local port changes state from link-up to link-down, the TRLs for all associated half-routes are set to zero, and when a local port changes state from link-down to link-up, the TRLs for all associated half routes are set to their default values.
 
Route Master/Proxy Objects
       

     The combination of the route master object  406  and the route proxy object  408  describes the state of a route  300  from the local end-point incarnation  411  to a remote end-point incarnation  410 . The route master object  406  describes the portion of the route that is important to the local port  103 (L) including the associated route management connection  603  [which implies the remote port  103 (R)], the full route handle, references to the local and remote end-point incarnations  411 / 410 , state, and a reference to the corresponding route proxy  408 . 
     The route proxy object  408  describes the portion of the route that is important to the end-point  102  including a reference to the route&#39;s route set  600 , the route&#39;s index within the route set, the route&#39;s order within the route set, the routing sequence number for the outbound half route, the transmission rate limit for the outbound half route, information used to measure the quality of the inbound half route, state, and a reference to the corresponding route master object  406 . 
     Route Master Object 
     Each port  103  maintains route master objects  406 , each of which describes a relationship between a local end-point incarnation  411 , a remote end-point incarnation  410 , a remote port  103 (R), and (by implication) the local port  103 (L). A route master object  406  is created whenever a port  103 (*) discovers a unique route  300  between a requested remote end-point  415  and one of the requested remote end-point&#39;s local end-point incarnation proxies  412 . This may occur because either a new partial route is discovered to an existing requested remote end-point  415 , or a local end-point incarnation  411  issues a request remote end-point message to an end-point  102  for which a partial route already exists. 
     A route master object  406  is also created whenever a port  103  receives a Register Full Routes (SCRFR) message. A route master object  406  is deleted whenever any of its references become invalid, which may occur in the following cases:
         The associated path becomes invalid because either the remote port  103 (R) is implicitly logged out (i.e., becomes not usable) or the associated route management connection  603  fails, or a SCRPR arrived on that path that omitted the route master&#39;s remote end-point incarnation  410 .   The associated local end-point incarnation proxy  412  is deleted because the local end-point  102 (L) either deregistered or reregistered.   The associated route proxy  408  is explicitly deleted.       

     A route master object  406  is also deleted whenever a route  300  is deregistered via SCDFR. Finally, incomplete route master objects  406  are deleted when SCRFR is rejected via a SCRFR_RSP and when the new route event is refused. 
     Exemplary attributes associated with a route master object  406  include the following:
     full_route_handle The full route handle received in the SCRFR or SCRFR_RSP message used to establish this route  300 . It is used to direct messages to the remote end-point  102 (R) via the route.   local_end_point_incarnation_proxy Handle of the associated local end-point incarnation proxy  412  or pseudo local end-point  402  (the null handle is used to reference the pseudo local end-point).   remote_end_point_incarnation_proxy Handle of the associated remote end-point incarnation proxy  409  or pseudo remote end-point  403 .   remote_port Handle of the associated remote port object  401 .   route_management_connection Handle of the associated route management connection object  603 .   route_proxy Handle of the associated route proxy  408 .   state state of the route master object  406 . Values for the route master state include:
       Unknown: Implies that the route master does not exist.   NewRouteOrig: Implies that the route master has generated a new_route_originator event and is awaiting a response.   NewRouteResp: Implies that the route master has generated a new_route_responder event and is awaiting a response.   SCRFR_Sent: Implies that the route master has sent a SCRFR message and is awaiting a response.   SCRFR_RSP_Sent: Implies that the route master has sent a SCRFR_RSP message and is awaiting its ACK. Active implies the normal operational state of the route master.   RunDown: Implies that the route master is in the process of being deleted, but still exists because references to it still exist. On explicit creation, the value of this attribute is set to NewRouteOrig. On creation due to receiving a SCRFR message, the value of this attribute is set to NewRouteResp.
 
Route Proxy Object
   
       

     The route proxy object  408  is a structure that describes a single route  300 . It is a proxy of the route master object  406 . The route proxy object  408  describes the state of a route  300  from the local end-point  102  to the remote end-point  102  through a specific path. Attributes of the route proxy  408  include its route_set, rtp_handle, rtm_handle, local_port_proxy, and state. These attributes are involved in route selection and monitoring. Route proxy objects  408  are created and deleted in response to events (new route originator, new route responder, route deleted) generated by route masters  406  on local ports  103 (L). Route proxy objects  408  are also deleted when the local port  103 (L) fails. 
     Exemplary attributes associated with a route proxy object  408  include the following:
     rtp_handle Handle that the route master  406  uses to identify the route proxy object  408 .   expected_rsn The expected value of the routing sequence number that will be received in the next transmission received on this route that describes itself.   highest_described_rsn The highest RSN received that described this route. It is used to calculate transmission delay likelihood. On creation, this value is set to zero. subsequently, it is updated each time a transmission is received that describes this route in which the described RSN exceeds this value.   inbound_trlc Transmission rate limit of the inbound half  500 ( 1 ) of this route  300  in compressed form. This value is computed locally and communicated to the remote end-point  102 (R) where it is used to limit the transmission rate of the corresponding outbound half route  500 (O).   last_receive_time The time when the last transmission was received on this route. Used to calculate reception_rate_est.   last_send_time The time when the last transmission was sent on this route. Used for calculating transmission_rate_est.   local_port_proxy The local port proxy  413  for the port  103  that contains this route proxy&#39;s route master  406 .   local_route_epoch This route proxy&#39;s route epoch. The route epoch is incremented to cause the remote end-point  102 (R) to reset its TRL computation for the inbound half route  500 ( 1 ).   local_route_index Index of this route proxy  408  in the local route set&#39;s route_proxies array. This value is sent to the remote end-point  102 (R) during full route registration. The remote end-point uses it to identify this route  300  in route headers  1400  that it transmits.   local_route_index_key Opaque key used to validate the local_route_index. This value is sent to the remote end-point  103 (R) during full route registration and a copy is received in the routing header  1400  when the transmission describes the route  300 . When a received transmission&#39;s local_route_index_key does not match this attribute, then the described route information is ignored.   next_described_route The next route  300  in the route set  600  to describe in the routing header  1400  of a transmission sent on this route.   next_route The next route in the route set&#39;s route order. It is a forward link in the circular list of all routes  300  of the route set  600 .   one_shot_flag Used when transmssion_rate_est&gt;decompressed (outbound_trlc) to make the route acceptable for one transmission.   outbound_trlc Transmission rate limit of the outbound half  500 (O) of this route  300  in compressed form. it is used to determine whether a route is acceptable during route selection. On creation, this value is set to SCR_TRLC_DEFAULT. Subsequently, it is updated when a received transmission describes this route. It may also be updated when the local end-point  103 (L) suspects that the outbound half route&#39;s quality is low.   reception_iat_est Estimate of the mean transmission receive inter-arrival time of the outbound half  500 (O) of this route  300 . On creation, this value is set to scr_irr_scale. Subsequently, it is updated each time a transmission is received on this route.   remote_route_epoch The last value of transmitter_route_epoch described for the inbound half  500 ( 1 ) of this route  300 . This value is sent in the routing header  1400  of each transmission that describes this route. On creation, this value is set to zero. Subsequently, it is updated when a received transmission describes this route.   remote_route_index Index of the remote route proxy  408  for this route  300  in the remote route set&#39;s route_proxies array. It is copied to the routing header  1400  in each transmission that describes this route.   remote_route_index_key Opaque key used to validate the remote_route_index. This value is received from the remote end-point  102 (R) during full route registration and is transmitted in the routing header  1400  with the remote_route_index.   round_trip_time_est Estimate of the round-trip-time for this route  300 . Its value is a number of milliseconds*256 expressed as an unsigned 32-bit integer. On creation, this value is set to 256 (1 millisecond).   route_set The route set  600  to which the route proxy  408  belongs. On creation, its value is set to null. However, every route proxy  408  is added to a route set  600  immediately after creation and only removed from a route set immediately prior to rundown.   rsn The route proxy&#39;s routing sequence number. Its value is the count of transmissions originated at this end of the route expressed as an unsigned 32-bit integer.   rtm_handle Handle that identifies the route master object  406  that corresponds to this route proxy  408  within the scope of a local port  103 (L). Its value is supplied by the route master  406  when the route proxy  408  is created and cannot be modified subsequently.   send_specific_route_queue Queue of obms waiting for this route  300  to send a transmission. Used when an allocateobs operation requires a specific route but that route is not acceptable.   significant_event A binary flag indicating that the route  300  has experienced a significant event since the route was described.   state State of the route proxy object  408 .   timer A general-purpose timer for the route used for a variety of purposes.   transmission_iat_est Estimate of the mean transmission send inter-arrival time of the outbound half  500  of this route. On creation, this value is set to scr_irr_scale. Subsequently, it is updated each time a route selection algorithm selects this route for a transmission.   transmission_rate_est Estimate of the transmission rate of the outbound half  500 (O) of this route. On creation, this value is set to zero. subsequently, it is updated each time the route selection algorithm selects this route for a transmission.   transmissions_received The count of transmissions received at this end of the route expressed as an unsigned 32-bit integer. This attribute is used as a performance counter, i.e., expected_rsn-1-transmissions_received gives the total transmissions lost on this route.
 
Route Set Object
   

     A route set object  417  associates the routes  300  from a local end-point incarnation  411  to a remote end-point incarnation  410 . Attributes of the route set object  417  include a list of routes  300 , a list of clients (connections)  601 , and a remote end-point UID and IID (the local end-point  102 (L) is implicit). 
     A route set  600  groups the complete set of route proxies  408  that correspond to approved routes  300  between this local end-point incarnation  411  and a remote end-point incarnation  410 . The route set object  417  is used to manage the creation and deletion of routes  300  between a pair of end-points  102 (*) and groups the routes that exist between those end-points. 
     As indicated above, a route set  600  is created when a route proxy  408  is created to a remote end-point incarnation  410  for which no route set yet exists. A route set  600  is deleted immediately when it has neither route proxies nor client connections. Stale route sets  600  are also deleted using policies based on an empty timeout (SCR_E_TOV) and a demand timeout (SCR_D_TOV). 
     When a route set  600  has clients but no route proxies  408  for a continuous period (SCR_E_TOV) of time, it will enter the rundown state and issue Derequest Remote End-Point commands to each local port  103 (L) to stop the creation of new routes  300 . In an exemplary embodiment, the value for SCR_E_TOV is 600,000 (i.e., ten minutes). The route set object  417  then generates a route_set_failed event to each of its client connections  601 . As each client connection aborts, it will remove its route set reference and thus allow the route set  600  to be deleted. SCR_E_TOV is used to control local policy that recovers resources from empty route sets. Specifically, when a route set  600  has no routes for SCR_E_TOV continuous milliseconds, it may be forced into rundown. 
     Exemplary attributes associated with a route set object  417  include the following:
     client_list Unordered list of client connections  601  that use this route set  600 .   demand timer Timer to time SCR_D_TOV. On creation, this timer is started. It is cancelled when client_list transitions from empty to nonempty. It is restarted when client_list transitions from nonempty to empty.   establishment_time Time when the route set&#39;s RTSMC  602  entered the established state. This is used to determine the most recently established route set  600  when selecting between multiple route sets to the same remote end-point  102 (R).   empty_timer Timer to time SCR_E_TOV. On route set creation, this timer is started. It is cancelled when route_proxies transitions from empty to nonempty. It is restarted when route_proxies transitions from nonempty to empty.   last_rtp Indicates the route proxy  408  of the last route  300  that was used to send a transmission on this route set  600 . On creation, this value is set to NULL. When the first route is added, it becomes a reference to that route. When the last route is removed then it becomes NULL.   last_rtp_count Count of the consecutive route set transmissions sent on route last_rtp. On creation, this value is set to zero. The route selection process zeros it each time it changes last_rtp and increments it each time a transmission is sent on the route set.   lepim Specifies the local end-point incarnation master  414  to which this object belongs.   max_r_a_tov The maximum value of the member routes&#39; local_port_proxy.r_a_tov.   remote_end_point_iid Incarnation identifier (IID) of the route set&#39;s remote end-point incarnation  410 .   remote_end_point_UID UID of the route set&#39;s remote end-point incarnation  410 .   route_index_keys Array of opaque keys used to verify dr_index values in received Transmissions. The route_proxy.local_route_index is used to index this array. On creation, the entries in this array are set to zero. When a route proxy  408  is added to the route set  600 , the corresponding route_index_keys value is incremented and is copied into route_proxy.local_route_index_key.   route_order_is_arbitrary True implies that the routes  300  within the route set  600  have not been ordered since the route membership last changed. On creation, this value is set to false. Subsequently, it is set to true each time a route is added to or remove from the route set. It is cleared when the routes within the route set are assigned an order.   route_proxies Array of handles of the route proxies  408  that belong to this route set. The route_proxy.local_route_index is used to index this array.   rtsmc The route set&#39;s management connection object. It may be stored either as a reference to a transport layer connection or as an object embedded in the route set object  417 .   send_initial_sfe_queue Queue of OBMs waiting for a route  300  to send an initial SFE on this route set  600 . Used for route selection when no acceptable route exists.   send_retry_sfe_queue Queue of OBMs waiting for a route  300  to send a retry SFE on this route set  600 . Used to select a route when no acceptable route exists.   send_start_mse_queue Queue of OBMs waiting for a route  300  to send a Start_MSE on this route set  600 . Used to select a route when no acceptable route exists.   significant_events Array of bits indicating which routes  300  of the route set  600  have experienced a significant event since the route was described.   state State of the route set object  417 .
 
Route Management Connection Object
   

     The route management connection  603  is a sequenced-message connection that provides a one-way sequenced message delivery service. Each message payload sent on an established route management connection  603  is prefixed with network, routing, and transport headers, as in the case of a ‘plain’ connection. All route management connection message payloads are received in the header buffer portion of a pool buffer. 
     The route management connection object  405  tracks the number of attempts to complete process login with retries_remaining, which, in an exemplary embodiment, is initially set to a value of 8. When retries_remaining reaches zero, the route management connection Connect Request operation fails. In response, route management deletes the corresponding pseudo remote end-point object  403  and remote port object  410 . 
     Exemplary attributes associated with a route management connection (RMC) object  405  include a system buffer pool handle, a full route handle, retries_remaining, the RMC connection ID, and the state of the connection. 
     Certain changes may be made in the above methods and systems without departing from the scope of the present system. It is to be noted that all matter contained in the above description or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. For example, the network shown in  FIG. 1  may be constructed to include components other than those shown therein, and the components may be arranged in other configurations. The elements and steps shown in  FIGS. 3-17  may also be modified in accordance with the methods described herein, without departing from the spirit of the system thus described. In addition, fabric  105 (*) may be any type of switched network, including the Internet.