Abstract:
The invention ensures that a single and consistent reply is made to an ARP request in a system of connected IP networks. When an adapter becomes active, the relative network on which it resides is determined by transmitting control packets over it and all other adapters known to the host and observing if and where responses are returned to the adapters. One adapter on a network is designated as active. If the same network contains other adapters, they are marked as standby adapters for the purpose of responding to ARP messages. Special processing is provided for offload adapters that perform there own ARP processing.

Description:
TECHNICAL FIELD 
     The invention relates generally to the field of networking. More specifically, it relates to the use of the Address Resolution Protocol (ARP) and to ensuring that only a single and consistent reply is generated in response to each ARP request. 
     BACKGROUND 
     The Address Resolution Protocol (ARP), used in TCP/IP networks such as the Internet, provides to requesting hosts a mapping between an IP address and a media access control (MAC) address. A host which needs to learn the MAC address for a given IP address broadcasts an ARP request containing the IP address to all routers and hosts in a network. The requests are received by adapters at the hosts; it is an adapter that owns an IP address and a corresponding MAC address. The requesting host learns the MAC address corresponding to an IP address by virtue of an ARP reply to an ARP request. An ARP reply is sent from the host that owns the corresponding adapter, or in some cases an adapter is arranged to perform ARP processing and it responds to ARP requests instead of the host. Such an adapter is called an offload adapter. In the remainder of this specification, host will be used to refer to both hosts that perform some kind of data processing in the traditional sense and to routers that route messages between networks or to nodes that perform both functions, 
     A host that owns multiple IP addresses that receives an ARP request will reply to the request only if the IP address in the request is the IP address of the adapter or if the adapter is explicitly configured to reply for the requested IP address. 
     A “real” IP address is one that is associated with a physical adapter. An adapter often supports thousands of network sessions with other hosts. If the adapter fails, all of the active sessions using the IP address associated with the adapter will also fail. Virtual IP addresses (VIPAs) were conceived to mitigate this problem. A VIPA is an IP address that is associated with a host, rather than with a physical adapter. Messages can be addressed to real IP addresses or to VIPAs. If a host contains multiple adapters, IP traffic addressed to a VIPA can be routed through any of the adapters. In this way, a host can provide fault tolerance after an adapter failure by routing the VIPA traffic over a different physical adapter. Virtual IP addressing is described in detail in U.S. Pat. No. 5,923,854, the contents of which are incorporated by reference herein. 
     There are two types of physical adapters, a host adapter in which the host does all of the ARP request processing for the adapter and an offload adapter that does its own ARP request processing. 
     For ease of expression, in the remainder of this document letters such as A, B, C, X, etc. other than V designate physical adapters. The letter V denotes a virtual IP address. IP-A represents the IP address of adapter A; MAC-A represents the MAC address of the adapter A associated with IP-A. IP-V denotes the virtual IP address V. VIPA and IP-V actually refer to the same thing—an IP address assigned to a host. Both of these designations are used interchangeably in this specification. 
     The traditional approach of ARP processing has a number of deficiencies. If adapters A and B are on the same physical network (i.e., all adapters on the network receive all ARP requests that any one of them receives) and both are owned by the same host, the host will not reply to ARP requests for IP-A received over adapter B. The host expects to reply to the request received over adapter A. This is a simple and effective way of preventing the generation of multiple replies to a single ARP request. However, it also means that no ARP reply will be generated if adapter A fails or is inactive. This means that adapters cannot serve as backups for one another. If a host owns IP-V and an ARP request for the MAC address assigned to IP-V arrives on adapter A, the host will not reply to the request, unless the owner has explicitly configured the system to do so. In the latter VIPA situation, if adapters A and B are on the same physical network, and A is assigned to IP-V, (explicitly configured to perform proxy ARP for IP address V), and adapter A fails, the host will no longer reply to ARP requests for V, even though it could send an ARP reply for V via adapter B. This often results in unsuccessful ARP requests. 
     The problem of providing backup adapters for offload adapters is even more difficult. For offload adapters, the host owning the offload adapter never sees an ARP request received over the offload adapter and the host likely has no knowledge of the MAC address of the offload adapter. If the offload adapter only replies to ARP requests containing its IP address, then it cannot provide any backup support for other adapters. 
     To address these limitations, a host could reply to any ARP request it receives over any adapter for any IP address owned by the host. However, when multiple adapters are on the same physical network, this will result in the host sending multiple ARP replies to a single ARP request and each will contain a different MAC address. This results in a flip-flopping of MAC addresses in the network for a single IP address. This, in turn, causes serious problems for network monitoring software. This flop-flopping of MAC addresses can also lead to odd traffic behavior and performance degradation. 
     To prevent multiple ARP replies when offload adapters are not involved, a host might implement a mechanism such that when the host first receives an ARP request over adapter A, it saves a timestamp and replies to the request. If within a short time it receives the same ARP request over adapter B, the host knows that an ARP response has recently been sent; so it ignores the ARP. Communication software in the Berkley Software Distribution uses this approach. A host might also implement such a technique to prevent multiple ARP replies for VIPAs. However, this timestamping solution still produces a flip-flopping of MAC addresses in a network. This is because there is a race as to which adapter A or B first receives an ARP request. 
     Therefore, there is a need for a solution that provides exactly one ARP reply with a consistent MAC address for any ARP request in an environment in which a host uses multiple adapters to address the same physical network, without the need for any user configuration. 
     SUMMARY OF THE INVENTION 
     Two embodiments are disclosed. The first embodiment is applicable to networks that do not contain VIPAs and offload adapters. The second implementation allows both types of physical adapters (host and offload) and VIPAs to coexist. 
     The First Embodiment 
     When an adapter (A) becomes active, the owning host sends an ARP advertisement into the network over adapter A that associates the MAC address for adapter A (MAC-A) with an IP address (IP-A). This advertisement is received by all hosts in the network and they update their ARP cache table to map IP-A to MAC-A accordingly. If the advertisement is also received at the sending host over a different adapter B, then the host knows that adapter B is on in the same physical network as adapter A. Therefore, B can be designated as a backup adapter for A and A can be designated as backup adapter for B. The host maintains a backup adapter field for each adapter owned by the host where this information is maintained. When the host discovers that adapter B is in the same network as adapter A, it queries the backup adapter field. If no backup adapter has been designated for A, then the host sets B as the backup adapter for A. Likewise, the host queries the backup adapter field for adapter B and sets A as the backup adapter for B if no backup adapter has already been designated. 
     If adapter A fails or becomes inactive, the host resets the backup adapter field for any adapter it owns for which A is marked as the backup adapter. If a backup adapter B has been designated for A, the owning host also sends an ARP advertisement associating MAC-B with IP-A. This advertisement causes each host in the network to update their ARP cache table to map IP-A to MAC-B. This allows network connections originally served via adapter A to continue non-disruptively over adapter B and it also provides access to the host for subsequent new connections. Whenever the host receives an ARP request for A on adapter B, the host replies to the request with MAC-B. 
     When adapter A later becomes active, the host sends a gratuitous ARP advertisement that maps IP-A to MAC-A. This allows adapter A to re-assume responsibility for responding to ARP requests for IP-A. 
     The Second Embodiment 
     The first embodiment depends on the host receiving ARP requests to determine what networks its adapters are on. Therefore, it does not function properly in networks that include offload adapters, because it does not receive ARP requests for these adapters. The adapters handle the ARP processing. 
     To solve the problems for offload adapters, the invention uses a different technique to determine what adapters of a host are on the same networks. This technique also works for host adapters and is further arranged to accommodate VIPAs as well. In each host, the first adapter A to become active is designated as being in a first physical network (PNET 1 ). The identification assigned to the network is arbitrary. It is only necessary to differentiate each separate network for the benefit of the host. For each subsequent adapter B to become active on a host, the host sends a packet over one of the adapters of each network already known to the host with a hop count of one. In the case of the second adapter to become active, the packet would be sent over adapter A. In the preferred embodiment the packet is an ICMP (Internet Control Message Protocol) echo request, although it could be any type of packet that allows a hop count of one. The hop count of one ensures that the packet will not be forwarded off of the network by a network router. The packet will be received by adapter B only if A and B are in the same physical network on which it is sent. Therefore, if the packet is received over adapter B, it is known that adapters A and B are in the same physical network. If this occurs, adapter B is marked as being in the same network PNET 1  as adapter A. If the packet is not received over adapter B, as evidenced by an eventual timeout function, then it is known that adapter B is in a different physical network as A. In this event, adapter B is marked as being in a new network PNET 2 . In general, the algorithm to determine in which network each offload adapter resides can be stated as follows. When an adapter becomes active at a host, send a data packet with a hop count of one over one adapter that resides in each different physical network known by the host and, if the packet returns on a different adapter, add the newly active adapter to the same physical network to which the receiving adapter belongs. If the packet does not return on a different adapter, create a new physical network at the host and add the new adapter to that network. If the new adapter also happens to be an offload adapter, then the host registers the IP address in the adapter. This causes the adapter to associate the IP address with the adapter MAC address known to the adapter and also to transmit an ARP advertisement into the network. If the new adapter is a host adapter, the host sends the advertisement itself. To handle VIPAs, after the first adapter on a host becomes active, it is initially marked as owning ARP responsibility for all virtual IP addresses owned by the host for the physical network in which the adapter is located. 
     When an adapter A becomes inactive in the second embodiment, if there are other active adapters in the physical network to which A belongs, then one of the remaining adapters B in that physical network is designated to have the responsibility for replying to ARP requests for IP-A. If B is a host adapter, the host sends a gratuitous ARP advertisement request mapping IP-A with MAC-B. If B is an offload adapter, the host registers IP-A in adapter B. This causes adapter B to associate its MAC address MAC-B with IP-A; the offload adapter also sends the gratuitous ARP advertisement. In either case, other network hosts update their ARP caches so that connections to IP-A will continue non-disruptively over adapter B. The host next determines if adapter A is marked as owning responsibility for VIPAs. If it is, then that marking is removed and adapter B is marked as owning VIPA ARP responsibility for that physical network. If B is a host adapter, then for each VIPA known to the host, it sends a gratuitous ARP advertisement into the network associating IP-V with MAC-B. If B is an offload adapter, the host registers IP-V with the adapter for each known VIPA and the adapter sends the advertisements into the network. Thereafter, when the host or an offload adapter on the host receives an ARP request for A or V, the host or offload adapter replies to it with the MAC address of B. When adapter A again becomes active, adapter A will re-assume ownership of ARP replies for IP address A. 
     The claims of this application are directed to the processing of ARP requests directed to VIPAs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the Drawing, 
     FIG. 1 is used to explain the problems and solutions of the invention; the Fig. shows a block diagram of a host containing three adapters, two of which are connected to first network and the third being connected to a different network; 
     FIG. 2 pertains to the first embodiment and shows the steps executed by a host when an adapter becomes active at the host; 
     FIG. 3 pertains to the first embodiment and shows the steps executed by a host when it receives an ARP advertisement message; 
     FIG. 4 pertains to the first embodiment and shows the steps executed by a host when an adapter becomes inactive; 
     FIG. 5 pertains to the first embodiment and shows the steps executed by a host when it receives an ARP request for the MAC address of an adapter associated with an IP address; 
     FIG. 6 pertains to the first embodiment and shows the steps executed by a host when it receives a reply to an ARP request it sent; 
     FIG. 7 pertains to the second embodiment and shows the steps executed by a host when an adapter becomes active at the host; 
     FIG. 8 pertains to the second embodiment and shows the steps executed by a host when it receives an ICMP echo request; 
     FIG. 9 pertains to the second embodiment and shows the steps executed by a host when a timeout occurs at a host after the host sends an ICMP echo request in an attempt to determine in what network a newly active adapter belongs; 
     FIG. 10 pertains to the second embodiment and shows the steps executed by an offload adapter when the adapter receives an ARP request for the MAC address associated with an IP address; 
     FIG. 11 pertains to the second embodiment and shows the steps executed by a host when it receives an ARP request over adapter B for the MAC address associated with IP-A; 
     FIG. 12 pertains to the second embodiment and shows the steps executed by a host it receives a reply to an ARP request; 
     FIG. 13 pertains to the second embodiment and shows the steps executed by a host when it receives an ARP advertisement; 
     FIG. 14 pertains to the second embodiment and shows the steps executed by a host when an adapter becomes inactive. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of a host containing three adapters, two of which are connected to first network and the third being connected to a different network. This Fig. illustrates the problems associated with ARP processing and helps explain the invention. The host shown in the Fig. contains three adapters D, E and F. Adapters D and E are attached to the same network, which in the Fig. is illustratively assumed to be a token ring LAN TR 1 . TR 1  has attached to it workstations WS 1 , WS 2  and WS 3 . Adapter F is attached to a different network identified as token ring LAN TR 2 . TR 2  has attached to it workstations WS 4 , WS 5  and WS 6 . In conventional ARP processing, although adapters D and E are on the same network, if D fails or becomes inactive for any reason, the host (or adapter E if it is an offload adapter) will not respond to ARP requests for D received over adapter E. This prevents E from being a backup for D, and vice versa. If the host did respond to such ARP requests for D, then without additional processing ARP replies would be generated for both adapters D and E in the normal situation, thereby resulting in multiple and inconsistent ARP replies. Assume further that the host of FIG. 1 has one or more virtual IP addresses (VIPAs) V assigned to it and that D has responsibility for responding to ARP requests for VIPAs. In this case, for the same reason as above, if D fails or becomes inactive, the host will not respond to ARP requests for V received over E. 
     Therefore, the invention is directed to solving the problem of providing backup adapters when two or more adapters on the same network, and to do it in a way that results in one and only one reply to an ARP request. Further, the invention is adapted to solve this problem for host adapters, offload adapters and VIPAs. 
     The First Embodiment 
     FIG. 2 pertains to the first embodiment in which a system contains only host adapters and specifically to the steps executed by a host when an adapter X becomes active at the host. The first embodiment relies on the receipt of ARP advertisement messages to determine the network that adapters are on. The adapter control block maintained in memory for each adapter is modified to contain a backup adapter field. This field is cleared by step  202  for the adapter X which is becoming active. Next, step  204  sends an ARP advertisement over the new adapter X. This advertisement maps IP-X to MAC-X. All hosts that are on the same network as adapter X will receive the ARP advertisement. 
     FIG. 3 shows the steps executed by every host on the same network as X when the host receives the ARP advertisement from FIG.  2 . Step  304  determines the IP address of the adapter over which the host received the advertisement. Step determines the IP address S of the new sending adapter X from the advertisement message. Step  307  determines if S is owned by this host. If the answer is no, then this host needs to update its ARP cache with the mapping contained in the advertisement. Thus, step  309  performs this by mapping IP-S from the advertisement with the MAC contained in the advertisement. If step  309  determines that S is owned by this host, then this host must determine if it received the advertisement over a adapter other than the one on which it was sent. If so, the receiving adapter is on the same network as the sending adapter X. Thus, step  308  determines if IP-R equals IP-S. If they are equal, the advertisement is ignored. If they are unequal, step  310  determines if the receiving adapter R has a backup adapter marked in the adapter control block. If it doesn&#39;t have a backup adapter, step  312  marks S as the backup adapter for R. Next, step  314  determines if S has a backup adapter. If not, then R is marked as the backup adapter for S at step  316 . This ends the processing of an ARP advertisement. Every host receiving the advertisement has updated its ARP cache and the sending host has determined if adapters S and R can act as backup adapters for each other. When an adapter X becomes inactive for any reason, then if X has a backup, all hosts must be told of backup. Also, if X is marked as backup for one or more another adapters in the host owning X, then the control blocks pertaining to the other adapters must be updated to remove X as backup. Step  402  of FIG. 4 determines if adapter X has a backup adapter Y. If so, then step  404  sends an ARP advertisement over adapter Y mapping IP-X to MAC-Y. Step  406  locates all adapter control blocks in the host owning X and clears the backup adapter field in any that has X marked as backup. 
     Sometimes a host sends an ARP request into a network to request the host owning an adapterX with IP address IP-X to reply with its MAC address MAC-X. FIG. 5 shows the steps executed by a host when it receives an ARP request associated with IP-A over adapter B. Step  502  determines if IP-A equals IP-B. That is,  502  determines if the request is received over the same adapter as the IP address contained in the request. If the answer is yes, step  506  returns a conventional ARP reply over the adapter mapping IP-A to MAC-A. If the answer is no, conventional hosts will not generate a reply. However, step  504  of the invention determines if a backup adapter B is marked in the A control block. If not, nothing more can be done. However, if A has a backup, step  505  determines if adapter A is active. If it is, then it is assumed that a reply will be made to the request that is received over adapter A. Thus, no further processing is done on this request. However, If adapter A is not active, then step  506  sends an ARP reply to the requester mapping IP-A to MAC-B. 
     FIG. 6 shows the steps executed by a host when it receives a reply to an ARP request. At step  602 , the ARP cache maintained by the host receiving the reply conventionally updates its ARP cache to associate the IP address in the reply to the MAC address in the reply. 
     The Second Embodiment 
     The second embodiment relies principally on sending and receiving ICMP (Internet Control Message Protocol) messages with a hop count of one to determine which of separate networks contain specific adapters. This embodiment is also arranged to handle offload adapters and VIPAs. 
     FIG. 7 is the initial figure of the second embodiment and shows the steps executed by a host when an adapter X becomes active at the host. X should regain ownership of its IP address if another backup adapter has been previously given responsibility (IP-X associated with MAC Y). Step  702  determines if a backup adapter is specified in the adapter X control block. If the answer is yes, step  708  determines if the backup adapter is an offload adapter. If that answer is yes, at step  710  the host sends a command to the adapter X to un-register IP-X with MAC-Y. This causes the adapter X to remove any association of IP-X with MAC-Y. Step  704  clears the backup adapter field in the X control block so that no other adapter is marked as backup for X. Any possible backup adapter for X will be determined dynamically shortly. Step  706  determines if X is an offload adapter. If it is, step  712  sends a command to the adapter X to register IP-X with MAC-X. This causes the adapter to send an ARP advertisement into the network containing this association. If the adapter is not an offload adapter, step  714  sends the advertisement into the network itself. Next begins the operation of determining what adapters are on what network. Step  716  determines if this adapter X is the first adapter to become active on this host. If there are no other active adapters on this host, then this host knows of no other network other than the network on which X is located, so there is no need to determine the network to which X belongs relative to other active adapters. In this case, step  724  creates a new and first network control block for a network PNET-X (where X in this case is 1) and links the network control block to the adapter X control block. All that is known now is that adapter X is active and that it resides in some network designated as PNET- 1 . Since X is the only active adapter on this host, it is marked at step  726  as owning all virtual IP addresses adapters on this host for this physical network. An alternative when an adapter becomes active is to assign VIPA responsibility to any one of the adapters known to the host at that time. Step  728  determines if X is an offload adapter. If so, step  730  sends a message to adapter for each VIPA owned by the host, Each message maps IP-V with the adapter X. As a result, adapter X sends an ARP advertisement message into the network mapping each IP-V to its MAC address MAC-X. If the adapter is not an offload adapter, step  732  sends the ARP advertisements into the network itself. 
     Returning to step  716 , if there are active adapters on this host other than X, then it is desired to determine on which, if any, of these networks X resides. For each physical network known to the host (as evidenced by network control blocks created by the host) step  718  selects one adapter and sends an ICMP message to that adapter via the new adapter X. The ICMP message is marked with a hop count of one to prevent routers and other hosts from transmitting the packet off of the physical network. In the preferred embodiment, the ICMP message used is an echo request, although any message with a hop count of one can be used. Also, step  718  saves the IP address of each selected destination adapter in a list. When all of the echo requests have been sent at step  718 , step  720  starts a timer. The IP address of the new adapter X is included in the timer setup as a parameter to be delivered if a timeout occurs. That is the end of this processing. The network occupied by adapter X will be determined by a reply to the echo request or a timeout of the timer activated by step  720 . 
     FIG. 8 shows the steps executed by a host if and when it receives an ICMP echo request. If a request is received over an adapter on the list from  718 , then it is known that the adapter X over which the message was sent in the same network as the adapter receiving the message. Step  802  determines if this receiving host sent the ICMP echo request. If it did not, then the echo request offers no information as to what networks the adapters on this host belong. Therefore, the echo request is processed in the conventional way at step  808 . If the echo request was sent by this receiving host, step  804  determines if this request contains an IP address that is on the list generated at step  718 . If the answer is no, then again the echo request is just processed conventionally at step  808 . If the IP address is on the list, it is now known that the adapter X, whose IP address is in the echo request, is in the same network as the receiving adapter. Step  806  sets the network field of the adapter X control block to point to the same network to which the receiving adapter points. Since an adapter can only be in one network, step  807  cancels the timer initiated at  718 . Processing of the echo request is completed by step  808 . It is now known which of the active adapters on this host has the capability of acting as backup for the new adapter. An actual backup adapter is not selected at this time. That decision is reserved in the preferred embodiment until it is necessary. This is discussed below with respect to FIG.  14 . 
     FIG. 9 shows the steps executed by a host as a result of a timeout initiated at step  720 . A timeout means that the newly active adapter X from FIG. 7 is not in any network presently known to the host. Therefore, the host needs to create a new network control block and link this adapter into it. The IP address of the new adapter X is delivered to step  900  when the timeout occurs. Step  902  determines if the adapter X control block is already linked to a network control block. If it is, then the timeout is ignored. If it is not, step  902  branches control at  904  to step  723  of FIG. 7 to create a new network identification PNET-X for this adapter and to link the adapter control block to the new PNET network control block. 
     FIG. 10 shows the steps executed by an offload adapter B when it receives an ARP request for the MAC address associated with an IP address IP-A. Step  1002  determines if the host has registered the address IP-A with the offload adapter. If the host has not so registered, the adapter ignores the ARP request. Otherwise, the adapter responds at step  1004  with an ARP reply mapping IP-A to MAC-B. 
     FIG. 11 shows the steps executed by a host when it receives an ARP request over adapter B for the MAC address associated with IP-A. Step  1102  compares IP-A with IP-B to determine if the request is received over the same adapter to which the request pertains. If so, then step  1106  replies to the ARP request in the conventional way mapping IP-A to MAC-A. If IP-A does not match IP-B, step  1104  determines if adapter B is marked as a backup for adapter A. If it is, then again step  1106  replies to the request, but in this case it maps IP-A to MAC-B. At  1104 , if adapter B is not marked as backup for adapter A, step  1108  determines if IP-A is a VIPA address. If IP-A is a VIPA address, then step  1110  determines if adapter B is designated as owning responsibility for VIPAs for that physical network. If it is, then step  1106  replies to the ARP request, mapping MAC-B to the virtual IP address IP-A. 
     FIG. 12 shows the steps executed by a host it receives a reply to an ARP request for the MAC address associated with IP-A. Step  1202  updates the ARP cache of the host in a conventional way to map the MAC address in the reply to the IP address in the reply. 
     FIG. 13 shows the steps executed by a host when it receives an ARP advertisement. Step  1302  also updates the host ARP cache table in a conventional way. 
     FIG. 14 shows the steps executed by a host when an adapter X becomes inactive. Step  1402  determines if there is another adapter on this host that also is in the same network as adapter X. This is determined by examining the network control blocks that are linked to the adapter control blocks for adapters that share the same network. If there is no other sharing adapter, no further processing is necessary. If there is, ARP caches and backup indications maintained by this host and other hosts need to be updated. Step  1406  picks the sharing adapter B or one of the sharing adapters B if there are more than one to backup adapter X and updates the backup field in the adapter X control accordingly. Step  1408  determines if adapter B is an offload adapter. If so, step  1410  registers IP-X with adapter B to cause the adapter to advertise to the network a mapping of IP-x to MAC-Y. Otherwise, the host performs the advertising at step  1416 . Step  1412  determines if adapter X has been designated as owning responsibility for VIPAs. If not, then processing is complete. If yes, step  1418  marks the backup adapter B as now owning VIPA responsibility. Step  1420  now determines if adapter B is an offload adapter. If so, step  1422  registers IP-V with the backup adapter B for each VIPA known to the host. This causes adapter B to broadcast an ARP advertisement to the network for each of these VIPAs mapping it to MAC-B. If adapter B is not an offload adapter, at step  1424  the host sends these advertisement messages into the network to complete the processing required for this inactive adapter X.