Patent Application: US-76334091-A

Abstract:
a method and apparatus for using switched telecommunications services to emulate a local area network medium . the method and apparatus convert a public switched network or an equivalent private network into a lan cabling method for connecting distant devices using the same communications software as used in traditionally wired lans .

Description:
three classes of datagrams are typically submitted to a lan medium : directed datagrams , multicast datagrams and all stations broadcast datagrams . the lan emulator manages lan datagram traffic by a set of logical channels between every pair of nodes that exchanges datagrams . the actual transmission of datagrams between nodes is provided by a physical channel . the lan emulator only requires a physical channel between nodes when datagrams are actively being exchanged over a logical channel ( between two nodes ). since the hardware interface to the public network ( e . g . isdn ) provides a limited number of physical channels , the lan emulator provides a monolithic interface to the higher layer protocol process . that process interacts with a single entity , the lan emulator , while the total transmission service may be provided by more than one hardware interface to the public network . all types of datagrams ( directed , multicast , and broadcast ) intended for transmission are potentially subject to one or more filtering mechanisms . a filter can either leave the datagram unchanged or remove the datagram from any further transmit processing . datagrams that remain unchanged are termed ordinary or unfiltered datagrams , while datagrams that are removed by a filter are termed filtered datagrams . each filter typically acts on a specific class or type of datagram . one generic filtering mechanism used in the invention is termed rate suppression . rate suppression acts on certain types of datagrams which contain repetitive information , and functions by passing only a certain percentage or ratio of those datagrams it recognizes . the purpose of rate suppression filtering is to minimize transmission charges for those datagrams whose content does not change or changes very slowly over time . another generic filtering mechanism used in the invention is termed response spoofing . response spoofing acts on those packets which contain repetitive information , but which require a response from the destination or destinations . the response spoofing filter not only removes these datagrams from further transmission processing , but also simulates the response that would be expected from the destination ( s ), and delivers the spoofed response to the higher layer protocol processes . with reference to fig2 an embodiment of the invention for conveying a directed datagram with a specific destination address through the lan emulator interface to emulate the transmission of a directed datagram on a lan is described . as mentioned above , a protocol stack is a set of protocols for the various layers . with the disclosed embodiment , and with reference to fig2 a high layer protocol process submits a directed datagram to the lan emulator interface 10 accompanied by the appropriate lan mac layer source and destination addresses . for every device there is at least one lan mac address and a corresponding switched network address . this disclosed embodiment describes a direct interface from higher layers to the mac interface , as is commonly found in systems implemented for personal computers . an llc interface may be required in some systems . the difference between an llc interface and a mac interface is not significant with respect to the disclosed invention . a directed datagram filter 15 makes a determination of the datagram type by comparing the datagram type with the contents of a directed datagram filter list 25 . this is a list of protocol specific datagrams specific to the application system . when there is a matching datagram type in the directed datagram filter list 25 , the datagram is marked as one which may be discarded from the transmission queue or enqueued for a spoofed response at a later time . the marking is based upon the actions specified within the directed datagram filter list 25 . the datagram is then passed to a channel classifier 20 for further qualification . however , datagrams marked for removal from the transmission queue are not given to the channel classifier 20 and so are not sent to any destination node . instead , they are either discarded or sent to the datagram response handler 30 , where a spoofed response is formatted and eventually delivered to the higher layer protocol process . the channel classifier 20 receives unfiltered directed datagrams from the directed datagram filter 15 . using the lan mac address as a search key , the classifier 20 retrieves a switched network address and an associated node channel status from a network definition table 35 . the network definition table 35 has an entry for each node on the emulated lan . a node may have multiple entries with different lan mac addresses . each entry includes , but is not limited to : the lan mac address , which is the node lan address the device would have if it was connected to a conventional lan ; the switched network address , which is the address of the device on the switched network ( e . g ., for devices using isdn , the public network number on the public switched telephone network ); the node type , which is a descriptor which describes the node type or function ; suspension timer values , which are the parameters that control the suspension of the network connection ; preferred service parameters , which specify the preferred types of transmission service when a connection is created ; node channel status parameter , which reflects the operational status of a remote node ; and broadcast service selectors , which are parameters that specify the methods by which broadcast messages are distributed to remote nodes . one class of preference is the type of service . the effective bandwidth delivered is determined by the preferred service parameter of the network definition table . the channel classifier can decide whether to transmit low priority information on low bandwidth channels , such as the d - channel packet switched service of isdn . devices connected to an isdn may select , among others , a b - channel circuit switched service , b - channel packet switched service , d - channel packet switched service , or ho circuit switched service . another class of preference is the minimum and maximum throughput desired . with the disclosed invention , multiple instances of a physical interface may be used under a single service layer . the upper layer protocols can therefore be presented with a single logical service layer while the actual transmission service may be delivered by more than one physical interface . it is possible to synthesize higher speed transmission service by combining multiple physical interfaces under a single service interface . thus a device that requires higher speed transmission may specify the minimum and maximum number of transmission channels to be used when communication is established with a remote node . for purposes of description of the invention , the logical channel is the connection service delivered to the upper layers , and the physical channel is the means by which datagrams are delivered . the datagram is discarded by channel classifier 20 when there is no entry in the table 35 that has the destination lan mac address . for lan mac addresses that are in the network definition table 35 , the four possible values for node channel status are : registered with a logical channel and assigned physical channel ( s ), registered with a logical channel and no physical channels , registered , or not registered . where the node channel status for a case is registered node with datagram traffic on both a logical channel and its physical channels , it is considered a connection that is completely active . an unfiltered datagram is immediately submitted to the datagram dispatcher 40 . for an unfiltered datagram where the node channel status is registered node with datagram traffic on the logical channel but with no associated physical channel , there is an attempt to establish a connection to the destination node on a new physical channel . this is accomplished by giving the datagram to channel manager 45 which attempts to establish a connection on a physical channel and then to send the datagram over that connection . the channel manager 45 manages the process of establishing a connection through both a logical and new physical channel . node registration is a mac management function that occurs at the time the lan emulator is initialized . when a node is registered with another node it means that it will respond to a request for connection . it also means that another node may attempt a connection with it . there is no implication that the connection attempt will be successful ; a connection attempt may fail because the node &# 39 ; s circuit resources may be occupied at the time of the attempt . node de - registration occurs when the lan emulator is shut down . this involves the lan emulator sending de - registration messages to its connection partners . de - registration is not mandatory . a node may also be de - registered when an attempt to connect to it fails because that node is no longer active . datagrams addressed to a node which is not registered are discarded by the channel classifier 20 . as mentioned above , the channel manager 45 is responsible for initiating a connection on a logical and / or a physical channel . if there is a request for connection on a pre - existing logical channel , the channel manager 45 establishes connections on the requisite physical channel ( s ). for a connection request when no logical channel exists , the channel manager 45 will set up a new logical channel as well as new physical channels . a logical channel may be supported by more than one connection to the same destination through multiple physical channels . the channel manager 45 uses a user preference contained within the network definition table 35 to determine the number of physical channel resources to allocate for a given connection attempt to the destination node . there are three possible occurrences when there is an attempt to allocate a physical channel on behalf of the logical channel . in the first , there are either no physical channels available for conveying the datagram or there is a network problem that prevents a call from being offered to the destination node . in the second , there are physical channels available , but when a call request is placed to the remote node at the destination switched network address , a rejected call response gets returned . in these two cases , the logical channel is optionally torn down and network definition table maintenance is performed . the third possibility is that the call request is accepted by the node at the destination switched network address , at which point the connection is considered physically active when the node channel status field shows that no logical or physical channel exists , there is a choice for an appropriate course of action which is dependent upon whether a connection on a physical channel is possible , and which requires a destination node to have registered its lan mac address and to be able to accept a connection request . in this case , a logical channel and logical channel reference number are assigned after the physical channel is set up and a connection established . the channel manager 45 will also monitor traffic on the physical channel , and during periods when there is no traffic , it may disconnect the physical channel while maintaining the logical channel . when new datagram traffic begins , the channel manager 45 will reassign a physical channel . during this process a physical channel is described as being suspended and later resumed . the channel manager 45 will monitor external events such as incoming calls and determine whether a logical channel will release its physical channel in order to reassign it to the new call . this may be termed release of bandwidth on demand . one consequence of the invention &# 39 ; s embodiment of suspend and resume is that the channel manager 45 may maintain more logical channels than the maximum number of physical channels possible . this is known as channel over - subscription . the channel manager 45 may associate an idle physical channel to a new logical channel while still maintaining the logical channel that had originally used the physical channel . as a consequence of over - subscription , there may be periods when there is more demand for active channels than there is supply of preferred physical channels . in such cases the channel manager 45 will assign alternate ( i . e ., slower and / or more expensive ) physical channels to carry the data . over - subscription is also tied to the concept of a monolithic interface since the effectiveness of over - subscription is enhanced by the use of a larger number of physical channels . the directed datagram dispatcher 40 receives a list of physical channels to which a datagram must be transmitted . it then manages datagram delivery by sending it through those individual channels . usually there is only one element in the channel list . if multiple channels are listed and the packet size is sufficiently large , the datagram dispatcher 40 may fragment the datagram and send the marked fragments on different physical channels to the same logical destination . with reference to fig3 the mechanisms employed to convey an all stations broadcast datagram through the lan emulator interface to emulate a local area network are described . an all stations broadcast datagram has a lan mac destination address with a format that is a special case in that all bits are set to one in each of the address &# 39 ; s octets . this transmission is emulated by sending the datagram to all eligible members of a finite list of recipients . as with the emulation mechanism for the directed datagram , for an all stations broadcast , the higher layer protocol process submits a datagram to the lan emulator interface 110 with an all stations broadcast destination lan mac address . because there are several uses for an all station broadcast datagram that are entirely application dependent , several selection mechanisms may be used for separating broadcast datagrams of different origin and directing them into appropriate courses of action . for example , some datagrams may be entirely blocked from transmission by filter 115 , some may have a transmission frequency attenuated by the filter , and the filter may have no effect on the transmission frequency of others . the metric for these filters may be a function of either cost , throughput performance , propagation delay or some other factor . the broadcast filter 115 compares the broadcast type against a broadcast filter list 125 to determine whether the broadcast datagram is a candidate for filtering or is an ordinary broadcast datagram . the filtering mechanism 115 affects the transmission frequency of broadcasts either by not transmitting any of the datagrams or by transmitting over time 1 of every n datagrams for each type submitted . the broadcast filter list 125 either specifies a spoofing response or contains the parameters and metrics that specify the appropriate attenuation rate for each type of broadcast datagram . the value of this rate is zero for blocked broadcasts or is expressed as a ratio for reduced broadcast frequencies . the filtering mechanism 115 does not affect the transmission frequency of unfiltered broadcasts , which are automatically given to the channel classifier 120 . the channel classifier 120 receives all unfiltered broadcasts and some reduced frequency broadcasts and qualifies the broadcast eligibility for transmission . when the channel classifier 120 receives a broadcast datagram , it determines a list of destinations to which to send the broadcast and then passes that list to the channel manager 145 to handle delivery . the channel classifier 120 will enqueue unfiltered broadcasts for delivery through all channels that are eligible for this service . this is regulated by the broadcast service parameter of the network definition table 135 . the functions of the channel classifier 120 may be accomplished by software , hardware , or through packet replication services provided by the network . there may be several reference criteria that are useful for deciding which destinations receive a particular broadcast datagram , and the network definition table 135 can hold values for these criteria including node type and node channel status . the channel manager 145 receives information from the network definition table 135 to discriminate between those logical channels with active physical channels and those for which a physical channel must be first reestablished . for logical channels with active physical channels , the channel manager 145 submits the broadcast frame to the datagram dispatcher 140 with a request for transmission . the datagram dispatcher 140 receives a list of physical channels to which a broadcast must be transmitted and then manages broadcast delivery by sending the broadcast through the individual physical channels . for logical channels without active physical channels , the channel manager 145 enqueues the broadcast datagram for delivery to those logical channels than can establish a new physical channel . with reference to fig4 the mechanisms employed to convey a multicast datagram through a lan emulator interface to emulate a local area network are described . a multicast destination mac address is represented by a ` 1 ` in the individual / group bit ( the lsb ) and a ` 0 ` in the universal / local administration bit ( the lsb + 1 ) of the first octet , thus denoting a group address within the universally administered address space . as with broadcasts , this class of transmission is emulated by sending the datagram to a list of recipients . the list may be same as the list maintained for the all stations broadcast datagram . again , as with the directed datagrams and broadcast datagrams , the higher layer protocol process submits a datagram with a multicast lan mac address to the lan emulator interface 210 , which in turn submits it , after filtering in filter 215 based on information in the multicast filter list 225 , to the channel classifier 220 . when the channel classifier 220 receives a multicast that is enqueued for transmission , it uses the network table 235 to determine a list of destinations to which to send the multicast and passes that list to the channel manager 245 to handle delivery . there are several reference criteria that may be useful for deciding which destinations receive a particular multicast datagram . the network definition table 235 holds values for these criteria which include node type , node channel status , and multicast filter lists . channel manager 245 examines the network definition table 235 to discriminate between those logical channels with active physical channels and those for which a physical channel must be first reestablished . for logical channels with active physical channels , the channel manager 245 submits the multicast frame to the datagram dispatcher 240 with a request for transmission . the datagram dispatcher 240 receives a list of channels to which a multicast must be transmitted . it the manages multicast delivery by sending it through the individual channels . it should be apparent to one skilled in the art that the invention contains a message receive function which will perform complementary processing on received messages . all received directed datagrams , as well as broadcast and multicast datagrams , will be passed up to the higher layer protocol process . it should also be readily apparent to one skilled in the art that the disclosed invention is not limited to any specific computer architecture or hardware device . for example , the disclosed embodiment works not only on ibm compatible personal computers with the novell network operating system , but with other computer architectures as well , including ibm microchannel personnel computers , sun sparc workstations , apple macintosh computers , minicomputers and mainframes . in addition , although the embodiment disclosed herein simulates lans over isdn , the invention is applicable to other switched networks such as pre isdn switched digital networks , x . 25 networks , and frame relay networks . although it may be preferred to implement the described procedures using software , they can also be implemented using well - known hardware elements . similarly , the disclosed invention can be applied to other communication devices which use lan interfaces , including lan bridges and routers . although an embodiment of the invention has been illustrated and described , it is anticipated that various changes and modifications will be apparent to those skilled in the art , and that such changes may be made without departing from the scope of the invention as defined by the following claims 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