Abstract:
An Infiniband (IB) router with an internal subnet architecture is disclosed. It comprises multiple port interface circuits interconnected by an internal IB subnet. The multiple port interface circuits each connect to an external IB subnet and preferably determine new local route headers (LRH) for global IB packets (i.e. packets having a global route header (GRH)). The new LRHs for externally received packets include a destination local identifier (DLID) value that identifies another port interface circuit in the router, whereas the new LRHs for internally received packets include a DLID value that identifies an end node or router in the external subnet to which the port interface circuit is attached. The internal IB subnet transports IB packets between the port interface circuits, directing them according to the contents of the LRHs. The internal subnet may take the form of an IB switch or a network of IB switches.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application relates to co-pending U.S. patent application Nos. ______ and ______ (Atty. Dkt. Nos. 2120-00500, 2120-00600), which are filed concurrently herewith. 
     
    
     
       BACKGROUND  
         [0002]    1. Field of the Invention  
           [0003]    This invention generally relates to systems and methods for implementing storage area networks. More specifically, this invention relates to a method and apparatus that provides fast and efficient routing between subnets in an Infiniband network. Further, this invention relates to a method and apparatus that couples disjoint subnets into a single logical subnet, and that may provide aliasing of IB ports to facilitate the creation of virtual subnets.  
           [0004]    2. Description of Related Art  
           [0005]    Internetworking of high-performance computers has become the focus of much attention in the data communications industry. Performance improvements in processors and peripherals, along with the move to distributed architectures such as client/server configurations, have spawned increasingly data-intensive and high-speed networking applications, such as medical imaging, multimedia, and scientific visualization. Various protocols have been developed to provide the necessary communications capacity.  
           [0006]    A protocol known as Infiniband can carry data over a given link at rates exceeding 2.5 Gbps in each direction. The Infiniband standard provides a point-to-point, switched architecture that allows many devices to concurrently communicate with high-bandwidth and low latency in a protected, remotely managed environment. An end node can communicate over multiple ports, and multiple communications paths may be used between end nodes. Properly exploited, the multiplicity of ports and paths provide both fault tolerance and increased data transfer bandwidth.  
           [0007]    An Infiniband (IB) network interconnects end nodes. Each end node may be a processor node, an I/O unit, and/or a router to another network. The IB network is subdivided into subnets that are interconnected by routers. The subnets comprise subnet managers, switches, and the end nodes linked to the switches. (Technically, a single link between two end nodes is also considered a subnet, with one of the end nodes functioning as a subnet manager for that link. However, this degenerate case is neglected herein.) Multiple links may exist between any two of the devices.  
           [0008]    Packets are directed through the IB network using either path-based (“directed route”) or destination-based addressing. Directed-route addressing is reserved for subnet management communications, and may be used before the forwarding tables have been initialized in the switches and routers. Directed-route packets include two lists of port numbers that define a path through the subnet. Each list specifies, in order, the output port of each switch along the path. One list specifies the forward route, and the other specifies the reverse route. The packets also include a direction bit to indicate which list is being followed, and a pointer to indicate the current position in the list. The reverse route list is built by the switches as the packet traverses them.  
           [0009]    In destination-based addressing, the packets include either a unicast identifier of a single destination end node, or a multicast identifier of a set of destination end nodes. A multicast set can be defined by an end node and used thereafter. The subnet manager configures the switches with routing information to specify all of the ports where a multicast packet needs to travel. Switches receiving a multicast packet will replicate the packet and send it out to each of the designated ports except the arrival port.  
           [0010]    In an Infiniband network, communication occurs at two levels: local (intra-subnet) and global (inter-subnet). Each end node has a global identifier (GID) and a shorter, local identifier (LID). For local communications within a given subnet, LIDs are sufficient to identify the source and destination nodes. For communications that pass between subnets, however, GIDs are required. End nodes in a subnet are interconnected by switches that receive and forward packets based on the LIDs. In turn, subnets are interconnected by routers that receive packets and forward the packets based on GIDs.  
           [0011]    Unlike switches, the routers must process the packets to replace the source and destination LIDs in the packet with those appropriate for the current subnet. Such processing must occur at astonishing speeds to prevent the router from becoming a bottleneck in the network. Yet, such performance commonly requires unduly expensive hardware. Consumers would benefit from an architecture that provides such performance at an affordable price. Consumers would further benefit if such a router architecture provided additional features such as connecting disjoint subnets into a single virtual subnet, thereby eliminating the need for closely-related end nodes in separate subnets to communicate at the global level. Consumers would yet further benefit from simplification and centralization of network management that the virtual subnet creation would make possible. Such benefits of virtual subnets would be facilitated if routers provided LID aliasing for end nodes in separate subnets.  
         SUMMARY OF THE INVENTION  
         [0012]    Accordingly, there is disclosed herein an Infiniband (IB) router with an internal subnet architecture. In one embodiment, the router comprises multiple port interface circuits that are interconnected by an internal IB subnet. The multiple port interface circuits each connect to an external IB subnet and exchange IB packets with that subnet. The port interface circuits preferably determine new local route headers (LRH) for global IB packets (i.e. packets having a global route header (GRH)). The new LRHs for externally received packets include a destination local identifier (DLID) value that identifies another port interface circuit in the router, whereas the new LRHs for internally received packets include a DLID value that identifies an end node or router in the external subnet to which the port interface circuit is attached. The internal IB subnet transports IB packets between the port interface circuits, directing them according to the contents of the LRHs. The internal subnet may take the form of an IB switch or a network of IB switches. The described architecture advantageously distributes the computational load among the router ports while providing a great deal of flexibility in the design and operation of the router. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Various aspects of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:  
         [0014]    [0014]FIG. 1 shows an exemplary Infiniband (IB) network;  
         [0015]    FIGS.  2 A- 2 G show IB packet and field formats;  
         [0016]    FIGS.  3 A- 3 C show preferred router architecture embodiments;  
         [0017]    [0017]FIG. 4 shows a functional block diagram of a port interface circuit;  
         [0018]    FIGS.  5 A- 5 F show a flowchart of a preferred routing method for the port interface circuits;  
         [0019]    [0019]FIG. 6 shows a data flow diagram for a global identifier (GID) to local identifier (LID) conversion;  
         [0020]    [0020]FIG. 7 shows a functional block diagram of router including simulated functional blocks;  
         [0021]    FIGS.  8 A- 8 B show preferred encapsulated packet formats; and  
         [0022]    [0022]FIG. 9 shows a data flow diagram for a LID to LID conversion. 
     
    
       [0023]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]    Network Architecture  
         [0025]    Turning now to the figures, FIG. 1 shows an exemplary Infiniband (IB) network. A first router  102  is coupled by a network fabric  104  to a second router  106 . Fabric  104  may comprise an IB subnet, or it may comprise some other means of transporting packets between the routers  102 ,  106  such as a local area network (LAN), a wide area network (WAN), a wireless link, or the Internet. The first router  102  is shown connected by a subnet  110  to end nodes  112 - 114 , and by a subnet  120  to end nodes  122 - 124 . Similarly, the second router  106  is shown connected by a subnet  130  to end nodes  132 - 134  and by subnet  140  to end nodes  142 - 144 .  
         [0026]    In accordance with the Infiniband Architecture Release  1 .Oa, the processor and I/O nodes are each coupled to the subnets by channel adapters. Each channel adapter may have multiple ports, and each port is assigned a global identifier (GID) and a local identifier (LID). Router ports are also assigned local identifiers, whereas switch ports are not (i.e. they are “transparent” to the original sources of the communication packets). In a preferred network embodiment, router ports may also be assigned global identifiers, i.e. they may simultaneously serve as end nodes and routers.  
         [0027]    Each channel adapter port can send and receive concurrently, and packets are channeled through virtual lanes, i.e. parallel buffers with independent flow control. The switches and routers similarly have ports with matching virtual lanes for channeling the packets. Different virtual lanes may be associated with different priorities or transportation classes.  
         [0028]    Each channel adapter further includes a subnet management agent that cooperates with the subnet manager. The subnet manager is responsible for configuring and managing switches, routers, and channel adapters, and it can be implemented as part of another device such as a channel adapter or a switch. Multiple subnet managers may be attached to a given subnet, in which case they negotiate to select one as the master subnet manager. The subnet manager discovers the subnet topology, configures each channel port with local identifiers, configures each switch with a forwarding database, assigns service levels to each virtual lane on each link, and maintains a services and end node directory for the subnet.  
         [0029]    Packet Structure  
         [0030]    In a conventional IB network, end node  112  communicates with end node  114  using a local IB packet such as that shown in FIG. 2A. The local IB packet includes fields for a local route header (LRH), a base transport header (BTH), an conditional extended transport header (ETH), an optional payload, an invariant cyclic redundancy check (ICRC), and a variant cyclic redundancy check (VCRC). Taking these in reverse order, the VCRC is a two-byte redundancy check that covers the entire IB packet. The ICRC is a four byte redundancy check that covers those portions of the packet that should not change as the packet traverses the network (i.e. BTH, ETH, payload, and GRH if there is one). The payload contains the data being transferred. The ETH is present depending on the class of service and the operation specified by the LRH and BTH, respectively. The ETH includes supplementary parameters appropriate to the circumstances, e.g. a total length of a data buffer for an RDMA (remote direct memory access) write operation. The BTH includes fields for the operation (e.g., RDMA write), packet sequence number, partition key, and destination queue.  
         [0031]    The LRH is shown in FIG. 2E. It provides the necessary information to the switches for routing the packet, and it is included at the beginning of every packet. The LRH begins with a four-bit field indicating the virtual lane that the packet is traveling on. This field can change from link to link. The next field is a four-bit field that indicates the link version, i.e. the general packet format. The next field is the service level, and the switch uses it to determine which virtual lane to use for this packet. The next field is a 2-bit reserved field, which is ignored. This is followed by a 2-bit “next link header” field that indicates the header following the LRH, i.e. GRH, BTH, RWH, IPv6. The next field is a 16-bit DLID field that specifies the LID of the port to which the subnet delivers the packet. If the packet is to be routed to another subnet, then this is the LID of the router. The DLID field is followed by a 5-bit reserved field, which in turn is followed by an 11-bit packet length field. The packet length field indicates the number of 4-byte words in the packet, excluding only the VCRC field. Finally, the LRH concludes with a 16-bit SLID field containing the LID of the port that injected the packet into the subnet.  
         [0032]    Accordingly, the LRH provides the necessary information for routing within the subnet, and the local IB packet of FIG. 2A is sufficient for local communications. However, inter-subnet communications require more information, and end node  112  conventionally must use a different packet type to communicate with end node  124  or end node  134 . FIG. 2B shows a packet of this type, i.e. a global IB packet. It includes the same fields as the local IB packet, but additionally includes a forty-byte global route header (GRH) that immediately follows the LRH. The GRH provides the necessary information for routers to route the packet between subnets, and is shown in FIG. 2F.  
         [0033]    The GHR includes IP Version, TClass, Flow Label, Payload Length, Next Header, Hop Limit, Source GID and Destination GID. The IP Version field indicates the version of the GRH (currently set to six). The TClass field is used to communicate service level end-to-end, i.e. across subnets. The Flow Label field may be used to identify a sequence of packets that must be delivered in order. The Payload Length field indicates the number of bytes, beginning after the GRH and counting up to the VCRC or any zero-padding bytes that precede the VCRC. The Next Header field indicates what header (if any) follows the GRH. The Hop Limit field indicates the number of routers that a packet is allowed to transit before being discarded. The Source GID field identifies the port that injected the packet into the global fabric, and the Destination GID field identifies the final destination port of the packet.  
         [0034]    In addition to local IB and global IB packets, the end node  112  may also transmit raw datagrams of two types: Ethertype and IPv6. FIG. 2C shows the Ethertype datagram packet, which includes the raw header (RWH) shown in FIG. 2G. The raw header includes a 16-bit Ethertype field that identifies the transport protocol service data unit contained in the payload. The IPv6 datagram packet is shown in FIG. 2D. Raw datagram packets allow IB networks to carry non-IB transport protocols. The Ethertype datagram packet bridges non-IB communications within the subnet, whereas IPv6 datagram packets will pass through routers.  
         [0035]    Router Architecture  
         [0036]    In the preferred embodiments, IB routers  102 ,  106  have an internal subnet architecture as shown in FIG. 3A. As shown, a router  302  is coupled to multiple subnets  304 - 308 . Each of the ports of router  302  presents a port interface circuit  314 - 318 , which may be implemented as a two-port router  314 - 318 . These port interface circuits  314 - 318  are coupled via an internal IB subnet  310 . This internal subnet architecture offers high performance and a great deal of versatility.  
         [0037]    [0037]FIG. 3B shows a functional block diagram of preferred embodiment of an  8 -port router.  
         [0038]    The internal subnet is implemented by an  8  port IB switch  310 . The port interface circuits  314 ,  316 ,  318  are preferably implemented by application-specific integrated circuits (ASICs) described further below. Connection modules  313 ,  315 ,  317  are provided for coupling the respective port interface circuits  314 ,  316 ,  318  to physical IB links. The connection modules provide the conversions between digital format and the signal format suitable for the physical IB links.  
         [0039]    The preferred 8-port router embodiment includes an embedded processor  320  and memory  322  that operate to configure and support the operation of the switch and port interface circuits. A boot bus  324  (such as an industry-standard architecture (ISA) bus) couples the embedded processor  320  to peripherals such as a boot flash memory  326 , a user flash memory  328 , and a complex programmable logic device (CPLD)  330 . These peripherals provide firmware support for embedded processor  320  and initialize the system when power is initially supplied to the router.  
         [0040]    A serial bus  332  (such as an I 2 C bus) couples the embedded processor  320  (preferably via a multiplexer  333 ) to low-level peripherals such as programmable input/output  334 , a real time clock  336 , serial electrically erasable programmable read only memories (SEEPROMs)  338 ,  340 , connection modules  313 ,  315 ,  317 , and a configuration portion of switch  310 . The programmable I/O  334  are processor controlled latches generally used to detect switch positions or other user input signals, and used to drive light-emitting diodes or other output means. The real time clock  336  tracks a current date and time, and may be further configured to provide timer and watchdog functions. SEEPROM  338  may be used to store configuration parameters, and SEEPROM  340  may be used to store configuration information for switch  310 . The connection modules include status registers and may further include programmable operating parameters that can be accessed via bus  332 .  
         [0041]    The embedded processor  320  is preferably coupled to the switch  310  and port interface circuits  314 ,  316 ,  318  by a PCI (peripheral component interconnect) bus  342 . The processor preferably operates as the subnet manager for the switch and port interface circuits, and may further operate as a subnet manager for virtual switches “embedded” in the port interface circuits as described further below. The processor further operates to configure the forwarding tables of the switch and port interface circuits, and provides other standard services described in the IB specification (e.g. general service agents). The processor preferably still further provides error condition handling and performance monitoring.  
         [0042]    The PCI bus  342  may further couple the processor to a PCI-to-CardBus bridge  344 . The bridge  344  allows the processor to access removable PC Cards  346 . Users can easily upgrade the router using such cards, e.g. to add memory, to update software, or to unlock enhanced features.  
         [0043]    The router in FIG. 3B uses a single switch to implement the internal subnet, but no such limitation is necessary or implied by this. On the contrary, the internal subnet may be implemented as any IB compliant subnet. FIG. 3C shows an exemplary implementation of a sixteen-port router. Each of the sixteen ports has a corresponding port interface circuit (i.e. a two-port router)  342 , and the port interface circuits are interconnected by an arrangement of six 8-port switches  344 .  
         [0044]    [0044]FIG. 4 shows a functional block diagram of the preferred embodiment of the port interface circuits  314 . The port interface circuits are preferably built around a crossbar switch  402  that routes IB packets between Send Queue Adapters (SQA)  406  and Receive Queue Adapters (RQA)  404 . The SQA  406  has an input interface containing eight virtual lanes that can be used to assign buffer credits for packets that are being injected into the switch  402 . The input interface of the SQA maps the service level (SL) of the packet to a virtual lane based on an IB-compliant SL-to-VL mapping table. In addition, the SQA calculates ICRC and VCRC fields while sending. The RQA  404  implement an arbitration mechanism in accordance with the IB specification (see vol. 1, chapter 7), and validates the ICRC of incoming packets.  
         [0045]    The port interface circuits  314  further include two router logic circuits  408  coupled to respective RQA/SQA pairs. The router logic  408  comprises memory buffers, hardwired buffer controllers and packet header extractors, and embedded RISC processors. The router logic  408  processes the packet headers, determines new packet headers, and routes the outgoing packets to the appropriate IB link control logic  410 . The packet header processing performed by the router logic includes key verification, packet filtering, GID to LID conversion, and statistics gathering.  
         [0046]    The link control logic  410  receives packets from the IB transceiver (in connection module  313 ). For these packets, the link control logic  410  performs a DLID lookup to determine which of the two router logic units to send the packet to. By default, the first link control logic sends to the first router logic, and the second control logic sends to the second router logic. The control logic also performs a service level to virtual lane (SL to VL) mapping based on the packet SL and the destination router logic. The packet is then provided to the SQA for delivery to the selected router logic.  
         [0047]    A PCI port  412  is provided for interfacing with the PCI bus. The PCI port allows access to the embedded registers, buffers, look-up tables, and memory for data and instruction code for processors embedded in the router logic  408 . The PCI port can access these locations directly or by using IB packet communications via the crossbar switch  402 .  
         [0048]    Operation  
         [0049]    FIGS.  5 A- 5 F show a flowchart of a preferred routing method to be performed by router logic  408 . Beginning in block  501 , the router logic receives a valid IB packet (invalid IB packets are processed separately and discarded). In block  502 , the routing logic determines if the DLID is the permissive address (i.e. 0×FFFF), and if so, it further determines whether the SLID is set to an appropriate value in block  503  before forwarding the packet in block  504  to switch port  0  (i.e. the Subnet Manager Agent (SMA) for router  302 ). If the SLID is not appropriate for a permissively routed packet, then in block  505  the router logic  408  saves the local route header (LRH), discards the rest of the packet, and alerts the SMA.  
         [0050]    Recall that the SMA function is performed by embedded processor  320 . To forward the packet, the router logic  408  may post an interrupt to the processor, which can then retrieve the packet via the PCI bus  342 . Alternatively, the router logic  408  may set a register bit that is periodically polled by the processor, or the router logic  408  may send the packet to a memory-based buffer for the processor. The local route headers (and, if available, the global route headers) of discarded packets may be provided to processor in a similar manner.  
         [0051]    Returning to block  502 , if the packet is not a permissively routed packet, then in block  506 , the router logic determines if the DLID is a multicast address. If so, then the method branches to the multicast process (see FIG. 5F). Otherwise, in block  507 , the router logic performs a DLID lookup in the local forwarding tables associated with the input port that received the packet. The local forwarding table maps the LID to a port number and port type. As explained further below with reference to FIG. 7, the router  302  may implement the functionality of multiple IB network units including a router and multiple virtual switches. (Virtual switches are a preferred mechanism for the router to provide a “switched” path between separate portions of a virtual subnet.) Accordingly, the port type may be “router” or “switch”.  
         [0052]    Next, the results of the input port forwarding table lookup are tested. In block  508 , the router logic  408  tests in block  509  to determine if the egress port is port  0  (i.e. a directed-route packet), and if so, the router logic verifies that the original local route header SID and DID are valid for a packet directed to port  0 . If so, then in block  510  the router logic forwards the packet to the processor  320 , which provides the control functionality of internal switches, virtual switches, and the overall router. If the SID/DID values are not valid, the router logic drops the packet and alerts the processor in block  505 .  
         [0053]    Returning to block  508 , if the egress port is not port  0 , then in block  511 , the router logic  408  tests the output port type. If the output port type is “router”, the router logic  408  treats the packet in a conventional fashion, i.e. in block  512  it verifies the validity of the source LID and virtual lane. If either is invalid, the packet is dropped in block  505 ; otherwise, the router logic determines in block  513  whether the packet has an IB packet format (i.e. FIGS. 2A or  2 B). If not, then the router logic treats the packet as a raw packet as shown in FIG. 5F. If the packet is an IB packet, then in block  514 , the router logic verifies that the packet includes a global route header (i.e. FIG. 2B). If not, the router logic drops the packet and reports an error in block  505 . If so, the router logic operates on the packet as shown in FIG. 5B.  
         [0054]    Returning to block  511 , if the output port type is not “router”, the router logic branches in block  515  to FIGS. 5C or  5 D based on the necessary encapsulation type. As described below with reference to the implementation of virtual subnets, the router logic may encapsulate the packet using either of the raw packet formats shown in FIGS. 2C and 2D. One format is suitable for local routing within the router  302 , whereas the other format is appropriate for inter-router communication.  
         [0055]    Turning now to FIG. 5B, the router logic performs a field lookup using the SGID in block  517 . This preferably allows the router logic to implement a fine-grained protection scheme based on the packet&#39;s combination of source and destination, and may further allow counting of packets between specific pairs of end nodes to enable accounting for accounting and analysis of bandwidth utilization. In block  518 , the router logic tests to determine whether a match was found in the GID-LID table. If not, the packet is dropped in block  505 . Otherwise, the router logic performs a GID-LID table lookup using the destination GID in block  519 .  
         [0056]    Referring momentarily to FIG. 6, the lookup proceeds as follows. The destination GID is provided to a “lookup table”  602 . This table is preferably implemented as a B-tree search in parallel with a hash table index search to minimize the number of lookups needed. The use of two parallel lookups also allows one to be optimized for exact matches (which usually occur at the final subnet), and the other to be optimized for longest-prefix matches (which usually occur in intermediate routers where a range of addresses is mapped to the exit port of the next subnet). In any event, the output from the lookup table is preferably a set of output ports of router  302  that may be used by the packet to reach the destination GID. In a preferred embodiment, the lookup table holds up to four output ports for each GID. The set of output ports is provided to multiplexer  604 , which uses an output signal from multipath table  606  to select one of the output ports. The selected output port is sent to LID table  608  to determine the internal subnet LID of the output port.  
         [0057]    The output of multipath table  606  is determined by applying both the Flow Label and the TClass values from the packet&#39;s global route header to the table input. This mechanism allows the router to support multiple paths to the desired destination, and the path selection can be based on a software-defined combination of these header values.  
         [0058]    The TClass value is further applied to a TClass table  610 , which maps the TClass value to a service level for the internal subnet. The service level in turn is applied to a VL table  612  to determine a virtual lane for the packet. These new values (LID, SL, VL) along with the LID of the outgoing link controller  410  are used to build a new local route header that is applied to the IB packet before it is sent over the internal subnet  310 . Values for other fields in the new LRH may be obtained in a similar fashion.  
         [0059]    Returning to FIG. 5B, the router logic tests the results of the table lookup in block  520  to determine if a match was found. If a match is found, the router logic verifies in block  521  the access properties of the SGID and DGID (e.g. whether they are in the same partition and are allowed to communicate with each other). If the access properties are not valid, the packet is dropped in block  505 . Otherwise, in block  522 , the TClass is used to determine the LID of the output port as described above with reference to FIG. 6.  
         [0060]    If no match is found in block  520 , then in block  523 , the routing logic uses the destination GID to perform a lookup in a subnet-forwarding table. The forwarding table will provide the internal subnet LID of the appropriate router port to move the packet one hop closer to the subnet containing the packet&#39;s ultimate destination. Once the LID has been found, then in block  524  the router logic updates counters (for measuring traffic flow characteristics), and preferably runs one or more filters. Filters are programmable tests that are based on selected packet header fields and that have programmable outcomes (e.g., whether a counter should be incremented, whether a packet should be dropped, whether a packet should be passed to the subnet manager). In block  525 , the router logic is given an opportunity to discard the packet if output port limits are being exceeded. (The filters may be used to enforce traffic limits.)  
         [0061]    In block  525 , the router logic begins building the new local route header by replacing the original destination LID with the LID determined from blocks  522  or  523 . In block  527 , the original source LID is replaced with the internal subnet LID of the output port from the port interface circuit. In block  528 , a new service level value is determined from the TClass value in the original header, and in block  529 , this service level used to determine a virtual lane value for the header. The new local route header is now complete.  
         [0062]    In block  530 , the router logic determines whether the packet is entering the internal subnet from this port interface circuit. If not, i.e. if the packet is exiting router  302  from this port interface circuit, then the router logic recalculates the VCRC value for the packet and the packet is dispatched. If the packet is entering the internal subnet, then the router logic checks the hop count in block  531 . If no further hops are allowed the router logic discards the packet; otherwise, the router logic decrements the hop count by one in block  532 . The router logic then recalculates the VCRC value for the packet and dispatches the packet.  
         [0063]    Thus, FIGS.  5 A- 5 B show a preferred method for routing a global IB packet in a more-or-less normal manner. Before discussing FIGS.  5 C- 5 E, it would be helpful to describe some additional preferred functionality of router  302 . It was mentioned in the discussion of FIG. 1 that a conventional IB network requires end node  112  to use global routing to communicate with end node  124  or with end node  134 . In a preferred embodiment, however, router  102  also offers the functionality of a switch, thereby allowing physically separate IB subnets  110 ,  120  to be combined into a single logical subnet (hereafter referred to as a local virtual private subnet). Further, in the preferred embodiment, router  102  cooperates with router  106  to provide the functionality of a switch that allows physically separate IB subnets  110 ,  130  to be combined into a single logical subnet (hereafter referred to as a remote virtual private subnet).  
         [0064]    [0064]FIG. 7 shows a functional block diagram of preferred embodiments of routers  102  and  106  as perceived by IB devices external to the routers. As the subnet manager probes router  102  to discover what subnet  110  is linked to, it encounters a switch  704  emulated by router  102 . Further probing reveals to the subnet manager that the switch  704  is coupled only to subnet  110  and to a router  702 . The subnet manager does not investigate network topology beyond router  702 . Similarly, the subnet manager for subnet  120  finds an emulated switch  706  coupled to a router  702 , the subnet manager for subnet  130  finds an emulated switch  708  coupled to a router  703 , and the subnet manager for subnet  140  finds an emulated switch  710  coupled to a router  703 .  
         [0065]    When it is desired to couple subnet  110  to subnet  120 , router  102  creates a virtual switch  712  that couples switch  704  to switch  706 . (This may be done through appropriate programming of the tables described previously.) Switch  704  notifies the subnet manager for subnet  110  that a connection event has occurred, thereby prompting the subnet manager to explore the topology of the “newly connected” portion of the subnet. Similarly, switch  706  notifies the subnet manager of subnet  120  that a connection event has occurred, thereby prompting the subnet manager to discover the “newly connected” subnet units. If desired, the router  102  can operate as a filter, thereby allowing the subnet  110  access to only selected portions of subnet  120 , and vice versa for subnet  120 .  
         [0066]    Likewise, when it is desired to couple subnet  110  to subnet  130 , routers  102  and  103  each create a virtual switch or, more preferably, they cooperate to create a single virtual switch  714 . The created virtual switches couple switch  704  to switch  708 . As before, switches  704  and  704  notify their respective subnets of a connection event, and the subnet managers of the respective subnets are allowed to “see” past the router into the other subnet.  
         [0067]    The above-described technique is not limited to the connection of just two subnets. Rather, a virtual switch can couple together multiple subnets, although locally connected subnets are preferably coupled together by a virtual switch separate from a virtual switch that couples a local subnet to a remote subnet.  
         [0068]    Because the virtual switches are not physical, the packets travel through one or more routers to move between the switches that are supposedly connected by the virtual switches. However, the packets that are supposed to be carried by the virtual switches may have only local route headers (LRH) to indicate their source and destination. To preserve the LRH information, the router logic  408  is preferably configured to encapsulate the original packets in a larger packet that travels through the internal subnet. The router logic  408  at the exit port from the subnet can then de-encapsulate the original packet and dispatch it to the destination subnet as if it had moved unchanged across a virtual switch.  
         [0069]    The preferred packet encapsulation formats are shown in FIGS. 8A, 8B. For packets traveling within a single router (e.g. between end nodes  112 ,  124 ), the packet is preferably encapsulated in a raw datagram Ethertype packet format as shown in FIG. 8A. The original packet has a new local route header prepended, followed by a raw header (RWH) and an extended raw header (ERWH). The original VCRC is replaced by a new VCRC, which is calculated with the new headers included. The local route header directs the packet through the internal subnet to the exit port of the router, and the raw header indicates that the packet encapsulates an original packet. The extended raw header preferably includes a field identifying the originating subnet, and may include a security field to prevent unauthorized use of this feature.  
         [0070]    Packets that need to travel through more than one router (e.g. between end nodes  112 ,  134 ) are preferably encapsulated in a raw datagram IPv6 packet format as shown in FIG. 8B. The original packet is prepended with a new local route header, a global route header, and a global raw header (GRWH). The local route header directs the packet through the internal subnet to the exit port of the router. The global route header directs the packet from there to the exit port of the target router, and the global raw header indicates that the packet encapsulates an original packet, and may also include a security field to maintain the privacy of the virtual subnet.  
         [0071]    Turning now to FIG. 5C, the router logic  408  reaches block  536  if it has determined that a packet is to be encapsulated for the internal subnet only. In block  536 , the router logic verifies that the source is allowed access to the targeted destination, and if not, the router logic drops the packet in block  505 . Otherwise, in block  537 , the router logic begins encapsulation of the original packet in a raw datagram Ethertype packet. To do this, the router logic prepends a raw datagram header and an extended raw header. In block  538 , the router logic performs a lookup to determine the internal subnet LID of the appropriate exit port, which will be the new DLID value in the new local route header. In block  539 , the new local route header is prepended to the packet, and the SLID value is set to the LID of the input port. The service level value in the original LRH is then used to determine a new service level value for internal subnet travel in block  540 . In block  541 , the local next header (LNH) value is set to zero to indicate a raw datagram. In block  542 , the source subnet value is set for the extended raw header. In block  543 , the new service level is used to determine a virtual lane value. The VCRC is then recalculated in block  544 . In block  545  the counters are updated and the filters run. In block  546 , the router logic decides whether to drop the packet due to excess loading, and in block  547  the encapsulated packet is sent off through the internal subnet.  
         [0072]    [0072]FIG. 5D is reached if the router logic has determined that a packet is to be encapsulated for inter-router travel. In block  550 , the router logic verifies the access properties for the source and destination LID ports. If the access is not allowed, the packet is dropped in block  505 . The packet is also dropped if the router logic determines in block  551  that traffic is excessive. In block  552 , the router logic begins the encapsulation process by prepending a global raw header. In block  553 , the router logic performs a lookup with the DLID to determine the GID of the router that is attached to the destination port.  
         [0073]    Turning momentarily to FIG. 9, the lookup procedure is shown. The router logic  408  provides the DLID value to a local forwarding table  902  to obtain two values. One value indicates the router port that the packet should exit from, and this is applied to a target port LID table  904  to determine the internal subnet LID for the corresponding port interface circuit. The internal subnet LID will be the destination LID value in the new local route header. The second value is an index into the LID-GID table  906 . The router logic can determine the GID of the destination router by applying the index to the LID-GID table  906 .  
         [0074]    Returning to FIG. 5D, the router logic in block  554  sets the destination GID value in the new global route header with the GID from the lookup in block  553 . In block  555 , the source GID value is set to the GID of the current port interface circuit  314 . In block  556 , the service level value for the new local route header and the TClass value for the global route header are determined as functions of the original service level value. In block  557 , the Flow Label is set equal to the DGID value, and in block  558 , the next header value is set to indicate a custom header format. In block  559 , the internal subnet LID of the destination port is determined in accordance with FIG. 9. In block  560 , the LRH is prepended to the packet with the LID of the current port interface circuit used as the source LID, and the DLID value from the lookup. In block  561 , the local next header value is set to indicate that the packet is an IPv6 raw datagram. In block  562 , the virtual lane is determined, and in block  563 , the VCRC is recomputed.  
         [0075]    The router logic reaches FIG. 5E for all non-IB packets (i.e. for raw datagrams). In block  566 , the router logic tests to determine if the packet is an IPv6 datagram. If not, the router logic tests to determine if the packet is an encapsulated packet by first checking in block  567  whether the packet has the custom header format, and by then checking whether the extended raw header identifies the subnet attached to the port interface circuit. If not, the packet is dropped in block  505 . Otherwise, the original packet is de-encapsulated in block  569 . In block  570 , the counters are updated and the filters run. In block  571 , the router logic may determine that the packet should be dropped if there is too much traffic, and if so, the packet is dropped in block  505 .  
         [0076]    In block  572 , the router logic performs a lookup in the outgoing forwarding table to verify that the destination LID is in the external subnet connected to the port interface circuit. In block  573 , the router logic verifies that the target is so connected, and if not, the router logic drops the packet in block  505 . Otherwise, the router logic uses the service level value to determine the virtual lane in block  574 , and in block  575 , the router logic recalculates the VCRC value.  
         [0077]    Returning to block  566 , if the packet is an IPv6 datagram, then in block  576  the router logic performs a lookup in the GID-LID table using the destination GID. In block  577 , the router logic determines if a match was found, and if not, the procedure moves to block  523  (FIG. 5B) to do a lookup for the next hop. Otherwise, the router logic tests the DGID value to determine if it equals the, GID value of the port interface circuit in block  578 . If not, the procedure moves to block  517  (FIG. 5B) to test the SGID value. Otherwise, the router logic performs a series of tests before de-encapsulating the packet.  
         [0078]    In block  579 , the router logic performs a lookup in the GID-LID table using the SGID value. In block  580 , the router logic verifies that a match was found. In block  581 , the router logic verifies that the access properties are valid, and in block  582 , the router logic verifies that the target of the original packet is in the subnet attached to the port interface circuit. The router logic drops the packet if any of these tests fail; otherwise, it de-encapsulates the packet in block  583 , and proceeds to block  572 .  
         [0079]    The router logic reaches FIG. 5F for multicast packets. In blocks  585 ,  586 , the router logic performs a lookup using the SGID in the GID-LID table and verifies that a match is found. In block  587 ,  588 , the router logic performs a lookup using the DGID in the global multicast table and verifies that a match is found. In block  589 , the router logic verifies that the access properties are valid, and in block  590 , the router logic updates the counters and runs the filters. In block  591 , the router logic chooses whether to retain the packet in view of the traffic load, and in block  592 , the router logic sets the destination LID to a multicast value. In block  593 , the source LID value in the local route header is set to the LID of output port of the port interface circuit. In block  594 , a service level is determined from the TClass value, and in block  595 , that service level is used to determine a virtual lane. In block  596 , the router logic determines whether the port interface circuit is the one through which the multicast packet is entering the router, and if so, then in blocks  597 ,  598  the router logic verifies and decrements the hop count value. In block  599 , the VCRC is recalculated.  
         [0080]    To permit the routing of local packets within the router (i.e. encapsulation), the routers preferably advertise a maximum transfer unit (MTU) size that is smaller than what is internally supported. This to enable the encapsulation of local packets, within RAW local packets. These packets are routed to the final router port in a RAW format, and de-encapsulated by the target router port, before injecting the packet into the subnet.  
         [0081]    Aliasing  
         [0082]    Thus, the above-described routing method provides for the connection of physically separate subnets into a single virtual subnet. In a traditional subnet, one subnet manager is selected as a master subnet manager, and it coordinates the configuring of the subnet. While the router preferably supports this model, the master subnet manager has to operate on the remote subnet via the router, which may cause an undesired amount of management traffic flow through the router. Further, there may be circumstances in which it is desired to make only a portion of the remote subnet part of the virtual subnet.  
         [0083]    Accordingly, a preferred model is also supported in which each physical subnet is managed by a subnet manager that is attached to that subnet. When the router connects a remote subnet (e.g. subnet  120 ) or a portion thereof to a given subnet (e.g. subnet  110 ), the subnet manager for subnet  110  “configures” the devices made visible to subnet  110 . Part of the configuration process is the assignment of local identifiers (LIDs), which are likely to be different from the LIDs assigned to the devices by the subnet manager for subnet  120 . The reverse is also true, in that the subnet manager for subnet  120  assigns LIDs to the accessible devices in subnet  110 , and those LIDs are typically different from the LIDs assigned by the subnet manager for subnet  110 .  
         [0084]    The router  102  preferably supports this behavior through the use of LID re-mapping. The router logic in a port interface circuit receives a packet from subnet  110  that is addressed to the subnet  110  LID for a device in a remote subnet. The router logic determines that the packet needs to be encapsulated, and determines that the destination LID needs to be changed to the remote subnet LID for the targeted device. The router logic performs this change to the original local route header. For local raw datagram encapsulation, the packet is then encapsulated, and a field is included in the extended raw header to provide the GID of the end node that originated the packet (LRH:SLID). The port interface circuit that receives the encapsulated packet de-encapsulates the packet and determines the appropriate source LID value for the remote subnet.  
         [0085]    For IPv6 datagram encapsulation, the port interface circuit that receives a packet addressed to a LID of a remote subnet replaces both the destination LID and source LID fields with appropriate values for the remote subnet. The packet is then encapsulated and transmitted as before (see FIG. 5D).  
         [0086]    LID remapping allows a single end node (or a set of end nodes if desired) to be virtually included in a given subnet. This may advantageously simplify communications between that node and the given subnet, and may further provide a means of limiting access by end nodes in the given subnet to other end nodes in the remote subnet.  
         [0087]    Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.