Abstract:
A method for detecting and discarding stale cells following route changes in a data communication network. The data communication network comprises a transmitter, which upon detection of a failure in a route of a network, retransmits data tagged as resent data along a different route of a multi-stage switch; and a receiver, which upon detection of tagged data from the transmitter via the different route of the multi-stage switch, utilizes tagged data for data communications while discarding previously transmitted data that are not tagged to avoid data duplication.

Description:
TECHNICAL FIELD 
     The present invention relates to a data communication network, and more particularly, relates to a method for detection of stale cells following route changes in a data communication network. 
     BACKGROUND 
     A data communication network is generally consisted of a network of nodes connected by point-to-point links. Each link may be a bi-directional communication path between two connect nodes (stations) within the network. Data may be transmitted in groups called cells or packets from a source to a destination, often through intermediate nodes. In many data communication networks, cells (or data packets) between two endpoints (e.g., end stations such as computers, servers and/or I/O devices) may transverse the network along a given path to ensure that cells are delivered in the order transmitted and without duplication. A control mechanism may be used to permit the re-transmission of a cell or group of cells if the original cell is corrupted in the network due to network errors, including, for example, random noise. 
     If a particular link between two connect nodes fails within the infrastructure of the network, retransmission cells may be re-routed by the control mechanism via an alternate link that does not include the failed link (assuming such a link exists). After the failure is detected, a new route chosen by the control mechanism, duplicate old cells may still exist in the network that have not yet transversed the old route. These cells may be referred to as stale cells. These stale cells may interact with cell retransmission which can result in delivery of duplicate cells. The duplicate cells must be eliminated in order to prevent corruption of data communication between two endpoints in the network. 
     Currently, there may be two techniques commonly employed to address the duplicate or stale cell problem. The first technique seeks to include a sequence number in each cell at the transmitting endpoint. This sequence number must be sufficiently large to ensure that a unique number may be assigned to every cell that may exist in the network infrastructure at any given time. Using the sequence number, the receiving endpoint may discard any cell that contains a sequence number that has been previously-received. The major disadvantage of this technique is that, in large data communication networks, the overall performance of the network can be significantly degraded, e.g., significant computing time can be spent in bookkeeping choirs. Moreover, in many data communication networks that support multiple and/or simultaneous communication flows between two endpoints, the large sequence number can significantly affect the cost of implementing the context store required for each flow. 
     The second technique seeks to ensure a quiet period just prior to, and for a time period after, the network is configured with the new route. This quiet period must be sufficiently long to ensure that all cells that were in the network infrastructure at the time of the re-route have exited the network. During this quiet period, endpoints affected by the re-route may be prohibited from injecting new cells into the network. There may be two significant limitations to this technique, however. First, the endpoints of the network must be coordinated to prevent injection of new cells into the network. This required participation of the endpoints may increase the complexity of the switching algorithm. Second, the required quiet time may be unacceptably long, particularly in large data communication networks which operate at significant network speeds. 
     Therefore, there is a need for a more flexible, cost-effective and performance-efficient technique for recovery from faults within a network infrastructure which requires neither a quiet period, nor that the transmitting endpoint be aware that the route through the network infrastructure has changed for data communications. 
     SUMMARY 
     Accordingly, various embodiments of the present invention are directed to a data communication network and a method for detection of stale cells following route changes in a data communication network. The data communication network comprises a transmitter, which upon detection of a failure in a route of a network, retransmits data tagged as resent data along a different route; and a receiver, which upon detection of tagged data, utilizes tagged data for data communications while discarding previously transmitted data that are not tagged to avoid data duplication. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of exemplary embodiments of the present invention, and many of the attendant advantages of the present invention, will become readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIGS. 1A and 1B illustrate an example of a less advantageous data communication network having several nodes interconnected by corresponding links; 
     FIG. 2 illustrates an example data in groups of cells for communications according to the principles of the present invention; 
     FIG. 3 illustrates an example redundant data communication network having alternate switches for data communications between several interconnected nodes via corresponding links and alternate links according to an embodiment of the present invention; 
     FIG. 4 illustrates example cascaded switches of the redundant data communication network as shown in FIG. 3; 
     FIG. 5 illustrates an example implementation of a redundant data communication network using an example input/output (I/O) channel architecture according to an embodiment of the present invention; and 
     FIG. 6 is a block diagram of an example individual switch of a multistage switch of an example redundant data communication network according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is applicable for use with all types of data communication networks designed to link together end stations such as computers, servers, peripherals, storage devices, and communication devices for data communications. Examples of such data communication networks may include a local area network (LAN), a wide area network (WAN), a campus area network (CAN), a metropolitan area network (MAN), a global area network (GAN) and a system area network (SAN), including newly developed data networks using Next Generation I/O (NGIO) and Future I/O (FIO), now known as InfiniBand™ as set forth in the “InfiniBand™ Architecture Specification,” the InfiniBand™ Trade Association on Oct. 24, 2000, and Server Net and those networks which may become available as computer technology develops in the future. LAN system may include Ethernet, FDDI (Fiber Distributed Data Interface) Token Ring LAN, Asynchronous Transfer Mode (ATM) LAN, Fiber Channel, and Wireless LAN. However, for the sake of simplicity, discussions will concentrate mainly on exemplary use of a simple data network having several example nodes interconnected by corresponding links, although the scope of the present invention is not limited thereto. 
     Attention now is directed to the drawings and particularly to FIGS. 1A and 1B, an example data communication network having several interconnected nodes for data communications is illustrated. As shown in FIG. 1A, the data communication network  10  may include, for example, a centralized switch  100  and four different nodes A, B, C, and D. Each node may correspond to an end station including, for example, a computer, a server and an input/output (I/O) device. The centralized switch  100  may contain switch ports  0 ,  1 ,  2 , and  3  each connected to a corresponding node of the four different nodes A, B, C, and D via a corresponding physical link  110 ,  112 , 114 , and  116 . The centralized switch  100  may also include a switch table (FIG. 1B) containing re-routing information in addition to routing information using, for example, explicit routing and/or destination address routing. 
     As shown in FIG. 1B, the switch table may contain listing of destination node addresses and corresponding destination ports, for example, node D and a corresponding switch port  3  (i.e., received cells destined for node D are output from switch port  3 ) and node C and a corresponding switch port  2 . Since there is no redundancy or redundant link available for data communications, a problem such as a temporary or permanent interruption may occur if a break occurs anywhere within the data communication network of FIGS. 1A and 1B. Such a break may occur, for example, along a node port of node A or along a corresponding link  110  as illustrated by break # 1 . Similarly, a break may occur at the centralized switch  100  as illustrated by break # 2 . Likewise, a break may occur along a node port of node D or along a corresponding link  114  as illustrated by break # 3 . As shown in FIGS. 1A and 1B, when any of these example breaks # 1 , # 2  and/or # 3  occurs anywhere within the data communication network there is no way of transmitting data in groups of cells to an affected node. 
     Turning now to FIGS. 2-6, example redundant types of data communication networks having redundant links or alternate routes according to an embodiment of the present invention are illustrated. These redundant types of data communication networks may be used to ensure that data in groups of cells (or data packets as per InfiniBand™ Architecture Specification) are successfully transmitted to respective destination nodes without cell duplication or stale cells in the event of a break of some links or components in the network. Before beginning to discuss the operation of the redundant type of data communication network, discussion first turns to FIG. 2 which describes the difference between a packet and a cell of data transmitted from a source node to a destination node through switches and/or other intermediate nodes. As shown in FIG. 2, a packet may represent a large data entity of user data and may be implemented as four (4) gigabytes minus one (1) in addition to cell header information and cyclic redundancy check (CRC) information. In contrast, a cell may be a subset of a packet or via verse. In particular, each cell may correspond to 256 bytes, for example, and contain a header (HDR) including a cell tag which will be described in the following discussion. 
     Discussion now turns to FIG. 3, which illustrates an example redundant type network having redundancy paths or alternate links (routes) used in the event of a break of some links or components in the network. As shown in FIG. 3, the example redundant type network  10 ′ may also include a second switch  200 ′ (SW 2 ) in addition to the first switch  100 ′ (SW 1 ) and the four different nodes A, B, C, and D shown in FIG.  1 . The second switch  200 ′ (SW 2 ) may be used to provide a set of redundant paths from source nodes to destination nodes, via one or more intermediate nodes, in order to ensure that data are delivered to respective destination nodes in the event of a break of some links or components in the network. Accordingly, if a break occurs along any one of the link  110  between node A and switch  100 ′ (SW  1 ) as indicated by break # 1 , or switch SI as indicated by break # 2 , or the link  114  between switch SW 1  and node D as indicated by break # 3 , the data transmitted from a source node to a destination node can be re-routed along the redundant path using the second switch  200 ′ (SW 2 ). 
     The problem sought to be solved by the present invention may not necessarily be a permanent breakage within the redundant network, but instead, a temporary blockage or breakage within such network. More particularly, for example, assuming that data are to be transmitted from source node A to destination node D, source node A may contain a predetermined hierarchy or list therein for use to send data along a preferred path (link). For example, node A may contain a predetermined listing indicating that data should first be transmitted through switch SW 1  to node D and then, if problems occur, the alternate link or path through switch  200 ′ (SW 2 ) should then be attempted. Accordingly, when node A of the redundant type network as shown in FIG. 3 first begins to transmit data to node D through switch SW 1 , and assuming that a blockage or temporary breakage occurs at any one of the locations break # 1 , break # 2  and/or break # 3 , the redundant type network of FIG. 3 may contain some types of arrangement for detecting a problem within the redundant network. 
     For example, when node A sends data to node D, node A expects an acknowledgment of receipt of such data from node D within a predetermined time period. If such an acknowledgment does not occur within a predetermined time period, node A assumes that cells (or data packets) which were not acknowledged were never received by node D, and further assumes that there is a problem occurring along the previous route selected to transmit the data. Accordingly, node A switches to an alternate predetermined route for re-routing the cells around the failed link(s) in order to ensure that data are successfully delivered to respective destination nodes in the network. In the present situation, node A attempts to retransmit the non-acknowledged cells through the switch  200 ′ (SW 2 ) to node D. However, a problem occurs if node A has re-transmitted a series of cells and then the temporary blockage or breakage becomes free and then node D begins to receive redundant or duplicate cells along or from both of the switches  100 ′ and  200 ′ (SW 1  and SW 2 ). These redundant or duplicate cells are known as stale cells and need to be discarded from the network for reliable and efficient cell relay operations. 
     The present invention seeks to simply and efficiently detect and discard these redundant or duplicate stale cells from the network for reliable cell relay operations by using a “cell tagging” technique for each communication flow between a source node and destination node individually. More particularly, the header HDR of the cell (or individual data packet) which contains data pertaining to cell tagging as shown in FIG. 2 may be used by each switch or node within the redundant type network to determine which cells are tagged and not tagged for discarding purposes. Accordingly, upon occurrence of detection of a route problem, a source node or node A as shown in FIGS. 3 and 4 may automatically enter a cell tagging mode as part of an internal re-route algorithm, and remain in such a cell tagging mode for cell tagging during a predetermined cell tagging interval. A destination node or node D may then receive tagged cells and, upon receipt of a first tagged cell, automatically enter a reject untagged cell mode as part of an internal re-route algorithm, and remain in such a reject untagged cell mode for one half (½) the cell tagging interval (for example) so as to discard all received cells that are not tagged (stale cells). The period of the cell tagging interval may be set relative to the maximum delay time for a cell to exist in all routes within a switch fabric of the redundant type network before being either delivered to a destination node or discarded from the network. This is a period known as a fabric lifetime within a switch fabric of the redundant type network that is determined by each switch within the switch fabric to insure that all stale cells are exited from the switch fabric. For purposes of this disclosure, the period of the cell tagging interval may be set as twice the fabric lifetime (for example) to ensure that there may be no overlap between the cell tagging mode of the source node and the reject untagged cell mode of the destination node. However, it should be noted that the present invention is not limited to such intervals. The cell tagging interval may be set as any discrete value greater than the fabric lifetime, as long as the destination node D, as shown in FIG. 3, for example, may be assured to be removed from the reject untagged cell mode before the source node A, as shown in FIG. 3, for example, stops tagging incoming cells in the next cell tagging mode to avoid mode overlapping. A link failure or route problem within the network may be determined, for example, by a control mechanism of a source node A utilizing a time-out of a response or by a central network manager managing general faults or link failures of the network. 
     More particularly, FIG. 3 is a very simplistic illustration of a redundant type network for the sake of clarity and brevity. Instead of just two switches  100 ′ and  200 ′ (SW 1  and SW 2 ), in fact, the network may include many switches. For example, as indicated by the dashes within the redundant type network, the path from source node A to destination node D through the switch S 2  may actually include additional switches SW 3  and SW 4 . 
     Turning now to FIG. 4, and assuming that each of the switches SW 3 , SW 2  and SW 4  which are cascaded as shown incurs a delay of, for example, 10 microseconds, a total maximum delay through the switching fabric would be 30 microseconds to transmit data from source node A to destination node D. The present invention then sets the cell tagging interval to approximately twice the maximum delay (fabric lifetime) or two times 30 microseconds equals to 60 microseconds of a cell tagging interval which is effected at node A. In contrast, the receiving node or node D only enters the cell tagging interval (upon receiving any first tagged cell) for only during the fabric lifetime, that is, one-half (½) of the maximum cell tagging interval or 60 microseconds divided by two equals the 30 microseconds. 
     When node A is in a cell tagging mode, node A may insert information data (e.g., a special code) pertaining to cell tagging into the header HDR of each cell which was not acknowledged from a destination node or node D, and then re-route each tagged cell to the destination node or node D through a redundant or alternate route (link) which is different from the previously detected problematic route based on its predetermined listing. In the example as described with reference to FIGS. 3 and 4, node A may retransmit all cells which were not acknowledged by destination node D by way of transmitting the tagged cells to node D through second switch  200 ′ (SW 2 ). Upon expiration of the cell tagging interval, the cell tagging mode of a source node or node A may be terminated automatically based on a built-in internal timer (not shown). 
     When a first tagged cell transmitted from node A arrives at node D, node D may read the cell tagging information from the header of the tagged cell and immediately enter a reject untagged cell mode for one half (½) the cell tagging interval, that is, for 30 microseconds. Thereafter, destination node or node D may assume for one half (½) cell tagging interval that only the received cells having the activated cell tagging are valid cells, and then promptly discard any redundant cells which may have received from the source node or node A through first switch  100 ′ (SW 1 ). In other words, the destination node or node D may ignore all untagged cells which may have received from the source node or node A at input ports. 
     Upon expiration of one-half (½) cell tagging interval, the reject untagged cell mode of a destination node or node D may be terminated automatically based on a built-in internal timer (not shown). Each destination node such as node D described with reference to FIGS. 3 and 4 may further include a mechanism which provides a source node or node A an indication that previously clogged or redundant cells are now being received again, and that the temporarily blocked/broken network link is now fixed. That way the source node or node A may stop cell tagging and return to using the previously broken, but now fixed network link for data communications. As a result, the redundant cells can be simply and effectively discarded from the network and the use of redundant cells can be completely avoided, thus guaranteeing an integrity of the network. 
     FIG. 5 illustrates an example implementation of a redundant data communication network having an example input/output (I/O) channel architecture for data communications according an embodiment of the present invention. As shown in FIG. 5, the redundant data communication network includes a multi-stage switch  10 ′ comprised of a plurality of switches, including, for example, first and second switches  100 ′ and  200 ′ (SW 1  and SW 2 ) of FIG. 3 for allowing host systems and target systems to communicate to a large number of other host systems and target systems. In addition, any number of end stations, switches and links may be used for relaying data in groups of cells between the end stations and switches via corresponding links. For example, node A may represent a host system  320  as shown in FIG.  5 . Similarly, node B may represent another network, including, but not limited to, local area network (LAN), Ethernet, ATM and fibre channel network. Node C may represent an input/output (I/O) device  360 . Likewise, node D may represent a remote system  380  such as a computer or a server. Alternatively, nodes A, B, C, and D of FIG. 3 may also represent individual switches of the multi-stage switch  10 ′ which serve as intermediate nodes between the host system  320  and the target systems  340 ,  360  and  380 . 
     The multi-state switch  10 ′ may include a central network manager  150  connected to all the switches for managing all network management functions. However, the central network manager  150  may alternatively be incorporated as part of either the host system  322 , the second network  340 , the I/O device  360 , or the remote system  380  for managing all network management functions. In either situation, the central network manager  150  may be configured for learning network topology, determining the switch table or forwarding database, detecting and managing faults or link failures in the network and performing other network management functions. When there may be faults or link failures in the network, the central network manager  150  may alert the source node or node A to automatically enter a cell tagging mode for cell tagging purposes, that is, for re-routing or re-transmitting the same cells to a destination node or node D around the failed links where alternate links exist in the network while discarding redundant or stale cells from the switch. 
     Separately, the central network manager  150  may also be used to determine the maximum delay time (fabric lifetime) that a cell may exist in the switch fabric before being either delivered to a destination node or discarded from the switch fabric. As described with reference to FIGS. 3 and 4, the fabric lifetime may be utilized to insure that all stale cells are exited from the switch fabric. The fabric lifetime may be determined by first calculating a switch cell lifetime, that is, the time a cell may exist in a given switch, then calculating the maximum time a cell may exist in all routes within the switch fabric. 
     A host channel adapter (HCA)  322  may be used to provide an interface between a memory controller (not shown) of the host system  320  and a multi-stage switch  10 ′ via high speed serial links. Similarly, target channel adapters (TCA)  342 ,  362  may be used to provide an interface between the multi-stage switch  10 ′ to an I/O controller of either an I/O device  360  or a remote system  380  via high speed serial links. Separately, another host channel adapter (HCA)  382  may also used to provide an interface between a memory controller (not shown) of the remote system  380  and the multi-stage switch  10 ′ via high speed serial links. 
     FIG. 6 is a block diagram of an example individual switch of a multi-stage switch  10 ′ of an example redundant data communication network according to an embodiment of the present invention. Each switch  100 ′ or  200 ′ of the multi-stage switch  10 ′ may include a switch table or forwarding database  202 , a switch manager  204 , a memory  206 , a relay function unit  208 , a plurality of switch ports, including ports  1 -n (shown as ports  220 ,  222  and  224 ), and a plurality of receive (Rx) and transmit (Tx) queues  230 - 240 . A switch manager  204  may be a logic or processor used for managing all functions of a switch. For example, the switch manager  204  may relay cells between the host system  320  and the target systems  340 ,  360 ,  380  and, if serving as an intermediate node within the network, may re-route or re-transmit the same cells to target systems  340 ,  360 ,  380  around the failed links in the event of a break in the links or components of the network while discarding redundant or stale cells from the switch fabric. A memory  206  may optionally contain a re-route or cell tagging information for re-routing cells around the failed links where alternate links exist in the network when serving as an intermediate node between a source node and a destination node. A relay function unit  208  may be provided for relaying received cells to specific destination ports. The receive (Rx) and transmit (Tx) queues  230 - 240  may be connected between the relay function unit  268  and each switch port  220 ,  222  and  224 . 
     The switch manager  204  may communicate with the central network manager  150  as shown in FIG. 5 to receive information for switch initialization (e.g., to receive the switch address) and to receive and download the forwarding database  202 . The switch manager  204  may also manage the forwarding or relaying of cells between links using either destination address routing, explicit routing or a combination of the two. In addition, when serving as an intermediate node between a source node and a destination node, the switch manager  204  may also control the re-route or cell tagging operation in the event of a break of some links or components in the network in order to insure that all stale cells are discarded from the switch fabric in the manner described with reference to FIGS. 3-4. 
     For example, if a source node and a destination node of the example redundant data communication network represent a host system  320  and a remote system  380  respectively as shown in FIG. 5, then the host system  320  may be set to automatically enter a cell tagging mode in response to detection of a link failure or route problem within the network, and the remote system  380  may be set to automatically enter a reject untagged cell mode in response to receipt of a first tagged cell from the host system  320 . A link failure or route problem within the network may be determined, for example, by either a time-out mechanism of the host system  320  or by the central network manager  150  of the multi-stage switch  10 ′. If the central network manager  150  is configured to inform the source node or host system  320  of the link failure or route problem of the network, then the time-out mechanism may not be needed for the source node or host system  320  to detect such a link failure. Other link failure detection mechanisms may also be used instead of the time-out mechanism of the host system  320  or the central network manager  150  of the multi-stage switch  10 ′ as long as link failures of the network are detected for cell tagging purposes. 
     The host system  320  may contain a built-in timer (not shown) set for cell tagging during a cell tagging interval. Upon expiration of the cell tagging interval, the host system  320  may be automatically reverted from the cell tagging mode back to normal operation. Likewise, the remote system  380  may contain a built-in timer (not shown) set for rejecting untagged cells (i.e., stale cells) during a reject untagged cell mode having a period one half (½) the cell tagging interval. Again, the cell tagging interval may be set as twice the fabric lifetime to ensure that there may be no overlap between the cell tagging mode of the source node and the reject untagged cell mode of the destination node. However, any discrete value greater than the fabric lifetime may be suitable as long as the destination node may be assured to be removed from the reject untagged cell mode before the source node stops tagging incoming cells in the next cell tagging mode to avoid mode overlapping for each communication flow between the source node and the destination node of the network infrastructure. 
     The remote system  380  may also contain a mechanism which informs the host system  320  that the temporary link failure in the network is now fixed. That way the host system  380  may stop cell tagging and return to using the previously broken, but now fixed network link for data communications. As a result, the redundant cells can be simply and effectively discarded from the network and the use of redundant cells can be completely avoided, thus guaranteeing an integrity of the network. 
     As described from the foregoing, the present invention advantageously provides a unique cost-effective and performance-efficient solution for recovery from faults or link failures within a network infrastructure which requires neither a quiet period, nor that the transmitting endpoint be aware that the route through the network infrastructure has changed for data communications. Switch elements of a multi-stage switch permit routing and alternate routing of data in groups of cells between endpoints efficiently on an individual communication flow basis. 
     While there have been illustrated and described what are considered to be exemplary embodiments of the present invention, it will be understood by those skilled in the art and as technology develops that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. For example, the present invention is applicable to all types of redundant type networks, including, but not limited to, Next Generation Input/Output (NGIO), InfiniBand™ ATM, SAN (system area network, or storage area network), server net, Future Input/Output (FIO), fiber channel, Ethernet). Many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from the scope thereof. Therefore, it is intended that the present invention not be limited to the various exemplary embodiments disclosed, but that the present invention includes all embodiments falling within the scope of the appended claims.