Abstract:
A virtual computer system includes virtualization software, and one or more physical network interfaces for connecting to one or more computer networks. The visualization software supports one or more virtual machines (VMs), and exports one or more virtual network interfaces to the VM(s) to enable the VM(s) to access the computer network(s) through the physical network interface(s). The virtualization software modifies and filters network data frames from the VM(s) and from the physical network interfaces) to restrict one or more VMs to one or more virtual local area networks (VLANs) that are implemented within a VLAN topology. Restricting a VM to a VLAN Omits the broadcast domain to which the VM belongs, which may reduce security risks facing the VM. implementing the VLAN functionality within the virtualization software provides the functionality to every VM in the computer system, without requiring every VM to provide the functionality.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to virtualized computer systems, in particular, to a system and method for providing network access to a virtual computer within a physical computer. 
         [0003]    2. Description of the Related Art 
         [0004]    The advantages of virtual machine technology are widely recognized among these advantages is the ability to run multiple virtual computers (or “virtual machines”) on a single physical computer. This can make better use of the capacity of the hardware, while still ensuring that each user or application enjoys the features of a “complete,” isolated computer. A general virtual computer system is described below as background information for the invention. 
       General Virtualized Computer System 
       [0005]    As is wall known in the field of computer science, a virtual machine (VM) is a software abstraction or a “virtualization,” often of an actual physical computer system.  FIG. 1  illustrates the general configuration of a virtual computer system  700 , including one or more virtual machines (VMs) such as a first VM  200  and a second VM  200 N, each of which is installed as a “guest” on a “host” hardware platform  100 . 
         [0006]    As  FIG. 1  shows, the hardware platform  100  includes one or more processors (CPUs)  110 , system memory  130 , and a local disk  140 . The system memory is typically some form of high-speed RAM (random access memory), whereas the disk (one or mere) is typically a non-volatile, mass storage device. The hardware  100  may also include other conventional mechanisms such as a memory management unit (MMU)  150 , various registers  160  and various input/output (I/O) devices  170 . 
         [0007]    Each VM  200 ,  200 N typically includes at least one virtual CPU  210 , at least one virtual disk  240 , a virtual system memory  230 , a guest operating system  220  (which may simply be a copy of a conventional operating system), and various virtual devices  270 , in which case the guest operating system (“guest OS”) may include corresponding drivers  224 . All of the components of the VM may be implemented in software using known techniques to emulate the corresponding components of an actual computer. 
         [0008]    If the VM is property designed, then it will generally not be apparent to the user that any applications  260  running within the VM are running indirectly, that is, via the guest OS and virtual processor. Applications  260  running within the VM will typically act just as they would if run on a “real” computer, except for a decrease in running speed, which may only be noticeable in exceptionally time-critical applications. Executable files will be accessed by the guest OS from a virtual disk or virtual memory, which may simply be portions of an actual physical disk or physical memory allocated to that VM. Once an application is installed within the VM, the guest OS retrieves files from the virtual disk just as if they had been pre-stored as the result of a conventional installation of the application. The design and operation of virtual machines is well known in the field of computer science. 
         [0009]    Some interface is generally required between a VM and the underlying host platform (in particular, the CPU), which is responsible for actually executing VM-issued instructions and transferring data to and from the actual memory and storage devices. A common term for this interface is a “virtual machine monitor” (VMM), shown as a component  300 . A VMM is usually a thin piece of software that runs directly on top of a host, or directly on the hardware, and virtualizes the resources of a physical host machine. Among other components, the VMM therefore usually includes device emulators  330 , which may constitute the virtual devices  270  that the VM  200  accesses. The interlace exported to the VM may be the same as the hardware interface of the underlying physical machine, so that the guest OS cannot determine the presence of the VMM. 
         [0010]    The VMM also usually tracks and either forwards (to some form of operating system) or itself schedules and handles all requests by its VM for machine resources, as well as various faults and interrupts. A mechanism known in the art as an exception or interrupt handler  355  may therefore be included in the VMM. As is well known, such an interrupt/exception handler normally includes an interrupt descriptor table (IDT), or some similar table, which is typically a data structure that uses information in the interrupt signal to point to an entry address for a set of instructions that are to be executed when the interrupt/exception occurs. 
         [0011]    Although the VM (and thus the user of applications running in the VM) cannot usually detect the presence of the VMM, the VMM and the VM may be viewed as together forming a single virtual computer. They are shown in  FIG. 1  as separate components for the sake of clarity. 
         [0012]    Moreover, the various virtualized hardware components such as the virtual CPU(s)  210 , the virtual memory  230 , the virtual disk  240 , and the virtual device(s)  270  are shown as being part of the VM  200  for the sake of conceptual simplicity—in actual implementations these “components” are usually constructs or emulations exported to the VM by the VMM. For example, the virtual disk  240  is shown as being within the VM  200 . This virtual component, which could alternatively be included among the virtual devices  270 , may in fact be implemented as one of the device emulators  330  in the VMM. 
         [0013]    The device emulators  330  emulate the system resources for use within the VM. These device emulators will then typically also handle any necessary conversions between the resources as exported to the VM and the actual physical resources. One advantage of such an arrangement is that the VMM may be set up to expose “generic” devices, which facilitates VM migration end hardware platform-independence. For example, the VMM may be set up with a device emulator  330  that emulates a standard Small Computer System Interface (SCSI) disk, so that the virtual disk  240  appears to the VM  200  to be a standard SCSI disk connected to a standard SCSI adapter, whereas the underlying, actual, physical disk  140  may be something else. In this case, a standard SCSI driver is installed into the guest OS  220  as one of the drivers  224 . The device emulator  330  then interfaces with the driver  224  and handles disk operations for the VM  200 . The device emulator  380  then converts the disk operations from the VM  200  to corresponding disk operations for the physical disk  140 . 
       Virtual and Physical Memory 
       [0014]    As in most modern computers, the address space of the memory  130  is partitioned into pages (for example, in the x86 architecture) or other analogous units. Applications then address the memory  130  using virtual addresses (VAs), which include virtual page numbers (VPNs). The VAs are then mapped to physical addresses (PAs) that are used to address the physical memory  130 . (VAs and PAs have a common offset from a base address, so that only the VPN needs to be converted into a corresponding physical page number (PPN).) The concepts of VPNs and PPNs, as well as the way in which the different page numbering schemes are implemented and used, are described in many standard texts, such as “Computer Organization and Design: The Hardware/Software Interface,” by David A. Patterson and John L. Hennessy, Morgan Kaufmann Publishers, Inc., San Francisco, Calif., 1994, pp. 579-603 (chapter 7.4 “Virtual Memory”). Similar mappings are used in other architectures where relocatability is possible. 
         [0015]    An extra level of addressing indirection is typically implemented in virtualized systems in that a VPN issued by an application  260  in the VM  200  is remapped twice in order to determine which page of the hardware memory is intended. The first mapping is provided by a mapping module within the guest OS  230 , which translates the guest VPN (GVPN) into a corresponding guest PPN (GPPN) in the conventional manner. The guest OS therefore “believes” that it is directly addressing the actual hardware memory, but in fact it is not. 
         [0016]    Of course, a valid address to the actual hardware memory must ultimately be generated. A memory management module  360 , typically located in the VMM  300 , therefore performs the second mapping by taking the GPPN issued by the guest OS  220  and mapping it to a hardware (or “machine”) page number PPN that can be used to address the hardware memory  130 . This GPPN-to-PPN mapping may instead be done in the main system-level software layer (such as in a mapping module in a kernel  600 , which is described below), depending on the implementation. From the perspective of the guest OS, the GVPN and GPPN might be virtual and physical page numbers just as they would be if the guest OS were the only OS in the system. From the perspective of the system software, however, the is a page number that is then mapped into the physical memory space of the hardware memory as a PPN. 
       System Software Configurations in Virtualized Systems 
       [0017]    In some systems, such as the Workstation product of VMware, Inc., of Palo Alto, Calif., the VMM is co-resident at system level with a host operating system. Both the VMM and the host OS can independently modify the state of the host processor, but the VMM calls into the host OS via a driver and a dedicated user-level application to have the host OS perform certain I/O operations on behalf of the VM. The virtual compute in this configuration is thus fully hosted in that it runs on an existing host hardware platform and together with an existing host OS. 
         [0018]    In other implementations, a dedicated kernel takes the place of and performs the conventional functions of the host OS, and virtual computers run on the kernel,  FIG. 1  illustrates a kernel  800  that serves as the system software for several VM/VMM pairs  200 / 300 ,  200 N/ 300 N. Compared with a system in which VMMs run directly on the hardware platform, use of a kernel offers greater modularity and facilitates provision of services that extend across multiple VMs (for example, for resource management). Compared with the hosted deployment, a kernel may offer greater performance because it can be co-developed with the VMM and be optimized for the characteristics of a workload consisting of VMMs. The ESX Server product of VMware, inc., has such a configuration. The invention described below takes advantage of the ability to optimize a kernel as a platform for virtual computers. 
         [0019]    A kernel-based visualization system of the type illustrated in  FIG. 1  is described in U.S. patent application Ser. No. 09/877,378 (“Computer Configuration for Resource Management in Systems Including a Virtual Machine”), which is incorporated here by reference. The main components of this system and aspects of their interaction are, however, outlined below. 
         [0020]    At boot-up time, an existing operating system  420  may be at system level and the kernel  800  may not yet ever be operational within the system. In such case, one of the functions of the OS  420  may be to make it possible to load the kernel  800 , after which the kernel runs on the native hardware  100  and manages system resources. In effect the kernel, once loaded, displaces the OS  420 . Thus, the kernel  600  may be viewed either as displacing the OS  420  from the system level and taking this place itself, or as residing at a “sub-system level.” When interposed between the OS  420  and the hardware  100 , the kernel  800  essentially turns the OS  420  into an “application,” which has access to system resources only when allowed by the kernel  600 . The kernel then schedules the OS  420  as if it were any other component that needs to use system resources. 
         [0021]    The OS  420  may also be included to allow applications unrelated to virtualization to run; for example, a system administrator may need such applications to monitor the hardware  100  or to perform other administrative routines. The OS  420  may thus be viewed as a “console” OS (COS). In such implementations, the kernel  600  preferably also includes a remote procedure call (RPC) mechanism to enable communication between, for example, the VMM  300  and any applications  430  installed to run on the COS  420 . 
       Actions 
       [0022]    In kernel-based systems such as the one illustrated in  FIG. 1 , there must be some way for the kernel  600  to communicate with the VMM  300 . In general the VMM  300  can call info the kernel  600  but the kernel cannot call directly info the VMM. The conventional technique for overcoming this is for the kernel to post “actions” (requests for the VMM to do something) ors an action queue stored in memory  130 . As part of the VMM code, the VMM looks at this queue periodically, and always after it returns from a kernel call and also before it resumes a VM. One typical action is the “raise interrupt” action: If the VMM sees this action it will raise an interrupt to the VM  200  in the conventional manner. 
         [0023]    As is known, for example, from U.S. Pat. No. 6,397,242 (Devine, et al. May 28, 2002), some visualization systems allow VM instructions to run directly (in “direct execution”) on the hardware CPU(s) when possible. When necessary, however, VM execution is switched to the technique known as “binary translation,” during which the VM is running in the VMM. In any systems where the VM is running in direct execution when it becomes necessary tor the VMM to check actions, the kernel must interrupt the VMM so that it will stop executing VM instructions and check its action queue. This may be done using known programming techniques. 
       Worlds 
       [0024]    The kernel  800  handles not only the various VMM/VMs, but also any other applications running on the kernel, as well as the COS  420  and even the hardware CPU(s)  110 , as entities that can be separately scheduled. In this disclosure, each schedulable entity is referred to as a “world” which contains a thread of control, an address space, machine memory, and handles to the various device objects that it is accessing. Worlds are stored in a portion of the memory space controlled by the kernel. More specifically, the worlds are controlled by a world manager, represented in  FIG. 1  within the kernel  600  as module  612 . Each world also has its own task structure, and usually also a data structure for storing the hardware state currently associated with the respective world. 
         [0025]    There will usually be different types of worlds; 1) system worlds, which are used for idle worlds, one per CPU, and a helper world that performs tasks that need to be done asynchronously; 2) a console world, which is a special world that runs in the kernel and is associated with the COS  420 ; and 3) virtual machine worlds. 
         [0026]    Worlds preferably run at the most-privileged level (for example, in a system with the x86 architecture, this will be level CPL 0 ), that is, with full rights to invoke any privileged CPU operations. A VMM, which, along with its VM, constitutes a separate world, therefore may use these privileged instructions to allow it to run its associated VM so that it performs just like a corresponding “real” computer, even with respect to privileged operations. 
       Switching Worlds 
       [0027]    When the world that is running on a particular CPU (which may be the only one) is preempted by or yields to another world, then a world switch has to occur. A world switch involves saving the context of the current world and restoring the context of the new world such that the new world can begin executing where it left off the last time that it was running. 
         [0028]    The first part of the world switch procedure that is carried out by the kernel is that the current world&#39;s state is saved in a data structure that is stored in the kernel&#39;s data area. Assuming the common case of an underlying x86 architecture, the state that is saved will typically include: 1) the exception hags register; 2) general purpose registers; 3) segment registers; 4) the instruction pointer (EIP) register: 5) the local descriptor table register; 6) the task register; 7) debug registers; 8) control registers; 9) the interrupt descriptor table register; 10) the global descriptor table register; and 11) the floating point state. Similar state information will need to be saved in systems with other hardware architectures. 
         [0029]    After the state of the current world is saved, the state of the new world can be restored. During the process of restoring the new world&#39;s stale, no exceptions are allowed to take place because, if they did the stale of the new world would be inconsistent upon restoration of the state. The same state that was saved is therefore restored. The last step in the world switch procedure is restoring the new world&#39;s code segment and instruction pointer (EIP) registers. 
         [0030]    When worlds are initially created, the saved stale area for the world is initialized to contain the proper information such that when the system switches to that world, then enough of its state is restored to enable the world to start running. The EIP is therefore set to the address of a special world start function. Thus, when a running world switches to a new world that has never run before, the act of restoring the EIP register will cause the world to begin executing in the world start function. 
         [0031]    Switching from and to the COS world requires additional steps, which are described in U.S. patent application Ser. No. 09/877,378, mentioned above. Understanding the details of this process is no necessary for understanding the present invention, however, so further discussion is omitted. 
       Memory Management in Kernel-Based System 
       [0032]    The kernel  600  includes a memory management module  616  that manages all machine memory that is not allocated exclusively to the COS  420 . When the kernel  600  is loaded, the information about the maximum amount of memory available on the machine is available to the kernel, as well as information about how much, of it is being used by the COS. Part of the machine memory is used for the kernel  600  itself and the rest is used for the virtual machine worlds. 
         [0033]    Virtual machine words use machine memory for two purposes. First, memory is used to back portions of each world&#39;s memory region, that is, to store code, data, stacks, etc. For examples the code and data for the VMM  300  is backed by machine memory allocated by the kernel  600 . Second, memory is used for the guest memory of the virtual machine. The memory management module may include any algorithms for dynamically allocating memory among the different VM&#39;s  200 . 
       Interrupt and Exception Handling in Kernel-Based Systems 
       [0034]    Interrupt and exception handling is related to the concept of “worlds” described above. As mentioned above, one aspect of switching worlds is changing various descriptor tables. One of the descriptor tables that is loaded when a new world is to be run is the new world&#39;s IDT. The kernel  600  therefore preferably also includes an interrupt/exception handier  655  that is able to intercept and handle (using a corresponding IDT in the conventional manner) interrupts and exceptions for all devices on the machine. When the VMM world is running, whichever IDT was previously loaded is replaced by the VMM&#39;s IDT, such that the VMM will handle all interrupts and exceptions. 
         [0035]    The VMM will handle some interrupts and exceptions completely on its own. For other interrupts/exceptions, it will be either necessary or at least more efficient for the VMM to call the kernel to have the kernel either handle the interrupts/exceptions itself, or to forward them to some other sub-system such as the COS. One example of an interrupt that the VMM can handle completely on its own, with no call to the kernel is a check-action IPI (inter-processor interrupt). One example of when the VMM preferably calls the kernel, which then forwards an interrupt to the COS, would be where the interrupt involves devices such as a mouse, which is typically controlled by the COS. The VMM may forward still other interrupts to the VM. 
       Device Access in Kernel-Based System 
       [0036]    In some embodiments of the invention, the kernel  600  is responsible for providing access to all devices on the physical machine. In addition to other modules that the designer may choose to load onto the system for access by the kernel, the kernel will therefore typically load conventional drivers as needed to control access to devices. Accordingly,  FIG. 1  shows a module  810  containing loadable kernel modules and drivers. The kernel  600  may interface with the loadable modules and drivers in a conventional manner, using an application program interface (API) or similar interface. 
       Example Virtual Computer System 
       [0037]      FIG. 2  shows one possible configuration for the generalized virtual computer system  700  of  FIG. 1 , which is useful for describing the invention. Thus,  FIG. 2  shows a virtual computer system  700 A that includes four VMs, namely a VM- 1   200 A, a VM- 2   200 B, a VM- 3   200 C and a VM- 4   200 D. Each of the VMs  200 A,  200 B,  200 C and  200 D may be based on a common x86 architecture (For reference, see documents related to the Intel IA-32 architecture, for example.), for example, and each of the VMs is loaded with a guest OS and a set of one or more applications. Thus, the VM- 1   200 A is loaded with a first guest OS  220 A and a first set of applications  260 A, the VM- 2   200 B is loaded with a second guest OS  220 B and a second set of applications  260 B, the VM- 3   200 C is loaded with a third guest OS  220 C and a third set of applications  280 C, and the VM- 4   200 D is loaded with a fourth guest OS  220 D and a fourth set of applications  260 D. The guest OSs  220 A,  220 B,  220 C and  220 D may be any combination of supported OSs, ranging from all four of the OSs being the same OS to each of the OSs being a different OS. For example, the guest OS  220 A may be a Windows Server 2003 OS from Microsoft Corp., the guest OS  2208  may be a Linux distribution from Red Hat, Inc., the guest OS  220 C may be a Solaris OS from Sun Microsystems, Inc., and the guest OS  220 D may also be the Windows Server 2003 OS from Microsoft Corp. The applications  260 A,  260 B,  260 C and  260 D may be any combination of supported applications, possibly with some applications being common to multiple VMs. 
         [0038]    The VM- 1   200 A is supported by a first VMM  300 A, the VM- 2   200 B is supported by a second VMM  300 B, the VM- 3   200 C is supported by a third VMM  300 C, and the VM- 4   200 D is supported by a fourth VMM  300 D. Each of the VMMs  300 A,  300 B,  300 C and  300 D may be substantially the same as the VMM  300  described above in connection with FIG  1 . Thus, in particular, each of the VMMs  300 A,  300 B,  300 C and  300 D may include the interrupt handler  355 , the device emulators  330  and the memory management unit  350  that are illustrated in  FIG. 1 . All of the VMMs  300 A,  300 B,  300 C and  300 D are supported by the same kernel  600  that was illustrated in  FIG. 1  and that was partially described above, including the memory management unit  618 , the world manager  612  and the interrupt/exception handler  655 . The virtual computer system  700 A of  FIG. 2  also includes the set of loadable modules and drivers  610  that were illustrated in  FIG. 1 , along with the system hardware  100 . The virtual computer system  700 A of  FIG. 2  may also include the console OS  420  and the applications  430  shown in  FIG. 1 , although these units are not shown in  FIG. 2  for simplicity. 
         [0039]      FIG. 2  also shows the virtual computer system  700 A being connected to one or more computer networks  20  by a first network interface card (NIC)  180 A and a second NIC  180 B. The network(s)  20  may be a simple network, such as a local area network (LAN) based on any of a variety of networking technologies, or it may be an interconnection of multiple networks using one or more networking technologies, including zero or more LANs and zero or more wide area networks (WANs). The NICs  180 A and  180 B are appropriate for the system hardware  100  and for the network(s)  20  to which the virtual computer system  700 A is connected. 
         [0040]    The description in this patent generally assumes the use of the popular Ethernet networking technology for simplicity, although it may also be applied to other networking technologies, including other layer 2 technologies of the Open System Interconnection (OSI) model. There are numerous books available on Ethernet technology and a large variety of other networking and internetworking technologies. In this patent, the word Ethernet is generally used to refer to any of the variations of Ethernet technology, including, in particular, the standard IEEE (Institute of Electrical and Electronics Engineers, Inc.) 802.3 interfaces operating at 1 megabit per second (Mbps), 10 Mbps, 100 Mbps, 1 gigabit per second (Gbps) and 10 Gbps. Thus, it the network  20  is an Ethernet network, then the NICs  180 A and  180 B are Ethernet cards that are compatible with the system hardware  100 . The system hardware  100  may constitute a conventional server computer cased on the x86 architecture, for example. In this case, the NICs  180 A and  180 B may be Intel PRO/100 Ethernet NiCs, Intel PRO/1000 Gigabit Ethernet NICs, or various other NICs, including possibly a combination of different types of NICs from the same or from different manufacturers. 
         [0041]    In general terms, the virtual computer system  700 A comprises virtualization software that supports the VMs  200 A,  200 B,  200 C and  200 D and enables the VMs to operate within the system hardware  100  and to utilize the resources of the system hardware. In the particular virtual computer system illustrated in  FIG. 2 , the visualization software comprises the kernel  600 , the loadable modules and drivers  610  and the VMMs  300 A,  300 B,  300 C and  300 D. In other virtual computer systems, the virtualization software may comprise other software modules or other combinations of software modules. Of particular relevance to this invention, the virtualization software in the virtual computer system  700 A of  FIG. 2  enables the VMs  200 A,  200 B,  200 C and  200 D to access the computer network(s)  20  through the NICs  180 A and  180 B. A similar virtual computer system that enabled VMs to access computer networks through physical NIC(s) of a physical computer system was described in U.S. patent application Ser. No. 10/865,779, emitted “Managing Network Data Transfers in a Virtual Computer System” (the &#39;779 application), which is incorporated here by reference. The virtualization software of the virtual computer system  700 A may be substantially the same as corresponding software modules of the virtual computer system described in the &#39;779 application, except as described below. 
         [0042]    Similar to the virtual computer system of the &#39;779 application,  FIG. 2  shows two NIC drivers  680 A and  680 B in the modules and drivers  610 . The NIC driver  680 A operates as a driver for the NIC  180 A and the NIC driver  680 B operates as a driver for the NIC  180 B. Each of the NIC drivers  680 A and  680 B may be substantially the same as a conventional, basic NIC driver for the corresponding NIC  180 A or  180 B. The NIC drivers  680 A and  680 B are specific to the particular types of NICs used as the NICs  180 A and  180 B, respectively. If the two NICs are of the same type, then the corresponding NIC drivers may be separate instances of the same NIC driver. For example, for a Linux platform, if the NICs are both Intel PRO/100 Ethernet NICs, then the NIC drivers may be separate instances of the e 100  driver from Intel. As is well known, the NIC drivers control the NICs, and provide an interface with the NICs. In other implementations of this Invention, there may be a larger number of NICs  180  and corresponding NIC drivers  680 , or there could be a smaller number of each. 
         [0043]    One of the device emulators  330  (see  FIG. 1 ) within each of the VMMs  300 A,  300 B,  300 C and  300 D emulates one or more NICs to create virtual NICs for the VMs  200 A,  200 B,  200 C and  200 D. Thus, in the virtual computer system of  FIG. 2 , a device emulator  330  within the VMM  300 A supports a virtual MIC  280 A for the VM  200 A, a device emulator  330  within the VMM  300 B supports a virtual NIC  280 B for the VM  200 B, a device emulator  330  within the VMM  300 C supports a virtual NIC  280 C for the VM  200 C, and a device emulator  330  within the VMM  300 D supports a first virtual NIC  280 D for the VM  200 D and a second virtual NIC  280 E for the VM  200 D. The device emulators  330  preferably emulate the NICs in such a way that software within the VMs  200 A,  200 B,  200 C and  200 D, as well as a user of the VMs, cannot tell that the virtual NICs  280 A,  280 B,  280 C,  230 D and  280 E are not actual, physical NICs. Techniques for emulating a NIC in this manner are well known in the art. The virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E may all be generic NICs, they may all be specific NICs, such as Intel PRO/100 Ethernet NICs, for example, or they may be a combination of generic NICs and/or specific NICs of one or more different types. 
         [0044]    The virtual NICs are preferably widely supported NICs, having drivers available for a large number and variety of OSs, such as the PCnet Lance Ethernet driver, from Advanced Micro Devices, Inc., which is built into all OSs that are common at this time. A NIC driver that is appropriate for each of the virtual NICs and the corresponding guest OSs is loaded as one of the drivers  224  (see  FIG. 1 ), if it is not already resident in the corresponding guest OS. Thus, a NIC driver  281 A that is appropriate for the virtual NIC  280 A and the guest OS  220 A is loaded as a driver  224  in the VM  200 A, a NIC driver  281 B that is appropriate for the virtual NIC  280 B and the guest OS  220 B is loaded as a driver  224  in the VM  200 B, a NIC driver  281 C that is appropriate for the virtual NIC  280 C and the guest OS  220 C is loaded as a driver  224  in the VM  200 C, and a NIC driver  281 D that is appropriate for the virtual NICs  280 D and  280 E and the guest OS  220 D is loaded as a driver  224  in the VM  200 D. Here, the virtual NICs  280 D and  280 E are assumed to be of the same type for simplicity, so that the corresponding NIC drivers may be separate instances of the same NIC driver  281 D, although the virtual NICs  280 D and  280 E may alternatively be of different types, requiring different NIC drivers. The NIC drivers  281 A,  281 B,  281 C and  281 D may be standard NIC drivers for use with the corresponding emulated virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E, or they may be custom NIC drivers that are optimized for the virtual computer system  700 A. 
         [0045]    From the perspective of the guest OSs  220 A,  220 B,  220 C and  220 D, the guest applications  260 A,  260 B,  260 C and  260 D, and the users of any of this guest software, the respective VMs  200 A,  200 B,  200 C and  200 D preferably appear to be conventional physical computers, end the virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E preferably appear to be conventional physical NICs connected to the network(s)  20 . Thus, guest software and/or users of the guest software may attempt to communicate with other computers over the network(s)  20  in a conventional manner. For example, the VM  200 A may implement an email server that is accessible through the network(s)  20 . A client computer attached to the network(s)  20  may communicate with the VM  200 A to retrieve email messages, and the VM  200 A may respond, as appropriate. These communications would involve one or more incoming network data frames that would arrive from the client computer at one or both of the NICs  180 A and  180 B and that must be forwarded to the VM  200 A, along with one or more outgoing network data frames that would be sent out by the VM  200 A and that must be transmitted to the client computer through one or both of the NICs  180 A and  180 B. One of the functions of the virtualization software of the virtual computer system  700 A is to facilitate these communications by the VMs  200 A,  200 B,  200 C and  200 D over the network(s)  20  by conveying incoming and outgoing network data frames between the VMs and the physical NICs  180 A and  180 B. 
         [0046]    As described in the &#39;779 application, a NIC manager  842  plays an important role in enabling software entities within the virtual computer system  700 A to communicate over the network(s)  20 . In  FIG. 2 , the NIC manager  642  is shown as being implemented within the kernel  600 , although the NIC manager  642  may alternatively be implemented as a driver within the modules and drivers  610 . Thus, the NIC manager  842  receives outgoing network data frames, from the VMs  200 A,  200 B,  200 C and  200 D and forwards them to the NIC drivers:  680 A and  680 B for transmission onto the network by the respective NICs  180 A and  180 B. The NIC manager  642  also receives incoming network data frames from the NICs through the NIC drivers, and routes them to the appropriate destinations, such as the VMs  200 A,  200 B,  200 C and  200 D, based on the layer  2  and/or layer  3  destination address(es) contained in the data frames. For example, for internet protocol (IP) data over an Ethernet network, the NIC manager  642  routes the data frames based on the medium access control (MAC) address and/or the IP address. 
         [0047]    When a software entity within one of the VMs  200 A,  200 B,  200 C or  200 D wants to send an outgoing data frame to the network(s)  20 , the data frame is sent to the respective NIC driver  281 A,  281 B,  281 C or  281 D, so that the respective NIC driver can send the data frame onto the network(s)  20  using the respective virtual NIC  280 A,  280 B,  280 C,  280 D or  280 E, with the virtual NICs appearing to be connected directly to the network(s)  20 . Instead of going directly out onto the network(s)  20 , however, the data frame is first forwarded to the NIC manager  642 . As an example, a software entity within the VM  200 A may send an outgoing data frame to the NIC driver  281 A for transmission on the network(s)  20 . This data frame is forwarded from the NIC driver  281 A to the NIC manager  642 . This forwarding of data frames can be accomplished in a variety of ways. For example, if the virtual NIC  280 A emulated by the device emulator  330  (see  FIG. 1 ) is a standard NIC that provides direct memory access (DMA) capabilities, and the NIC driver  281 A (see  FIG. 2 ) is a standard NIC driver for that particular type of NIC, then the NIC driver  281 A attempts to set up the NIC  280 A to perform a DMA transfer of the data frame. The device emulator  330  responds by communicating with the NIC driver  281 A and performing the transfer of data, making it appear to the NIC driver  281 A that the virtual NIC  280 A performed the DMA transfer, as expected. The emulator  330  then provides the data frame to the NIC manager  642  for routing through one of the NIC drivers  680 A and  680 B and the corresponding NIC  180 A or  180 B onto the network. For example, the emulator  330  may copy the data frame from a memory page that is controlled by the VM  200 A to a memory page that is controlled by the kernel  600 , and which is accessible to the NIC manager  642 , and, more particularly, to the NIC drivers  680 A and  680 B. 
         [0048]    Similarly, for an incoming data frame to the VM  200 A the NIC manager  642  receives the data frame from one of the NIC drivers  680 A or  680 B and forwards the data frame to the device emulator  330  within the VMM  300 A. The device emulator  330  places the data frame in an appropriate location in memory and generates an appropriate interrupt to the guest OS  220 A to cause the NIC driver  281 A to retrieve the data frame from memory. A person of skill in the art will also understand now to emulate the virtual NICs  280 A,  280 B,  280 C,  230 D and  230 E in this manner to facilitate the transfer of data frames between the NIC drivers  281 A,  281 B,  281 C and  281 D and the NIC manager  642 . A person of skill in the an will also understand how to minimize the number of times that data frames are copied in transferring data between the NIC drivers  281 A,  281 B,  281 C and  281 D and the network(s)  20 , depending on the particular implementation. For example, for an outgoing data frame, it may be possible to set up the physical NICs  180 A and  180 B to perform DMA transfers directly from the NIC drivers  281 A,  281 B,  281 C and  281 D, to avoid any unnecessary copying of the data. 
         [0049]    For this description, suppose that the virtual computer system  700 A is connected to an Ethernet network and that each of the physical NICs  180 A and  180 B and each of the virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E are Ethernet cards, in this case, each of the virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E preferably has a MAC address that is unique, at feast within the virtual computer system  700 A, and preferably also within the local network to which the virtual computer system  700 A is connected. Then, for example, any outgoing data frames from the VM  200 A will contain the MAC address of the virtual NIC  280 A in the source address field of the Ethernet frame, and any Incoming data frames for the VM  200 A will contain the same MAC address, or a broadcast or multicast address, in the destination address field of the Ethernet frame. Each of the NICs  180 A and  180 B may be placed in a promiscuous mode, which causes the NICs to receive all incoming data frames and forward them to the respective NIC drivers  680 A and  680 B, even if they don&#39;t contain the MAC address of the respective NIC. This ensures that the NIC manager  642  receives data frames containing the MAC address of each of the virtual NICs  230 A,  280 B,  280 C,  280 D and  280 E. The NIC manager  642  then routes incoming data frames to the appropriate VMs  200 A,  200 B,  200 C and  200 D, based on the MAC address that is contained in the destination field of the Ethernet frame. The NIC manager  642  is generally able to transmit data frames from the VMs  200 A,  200 B,  200 C and  200 D through the NIOs  180 A and  180 B, using the MAC address of the respective virtual NIC  280 A,  280 B,  280 C,  280 D or  280 E within the source field of the Ethernet frame. In other words, the physical NICs  180 A and  180 B generally transmit outgoing data frames onto the network, even if the data frames do not contain the MAC address of the physical NICs in the source address field. 
         [0050]    Incoming data frames may also be routed to other destinations within the virtual computer system  700 A, such as to an application  430  (see  FIG. 1 ), as appropriate. Similarly, other entities within the virtual computer system  700 A may generate outgoing data frames for transmission on the attached network. For example, on behalf of an application  430 , a NIC driver within the COS  420  (see  FIG. 1  again), possibly in coordination with the NIC manager  642 , may insert the MAC address of one of the NICs  180 A or  180 B into the source field of the Ethernet header of an outgoing data frame. Then, responsive incoming data frames destined for the application  430  will contain the same MAC address, or a broadcast or multicast address, in the destination field of the Ethernet frame. Using these techniques, the NIC drivers  281 A,  281 B,  281 C and  281 D within the guest OSs  220 A,  2208 ,  220 C and  220 D, the NIC driver within the COS  420 , the virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E, the device emulators  330 , the NIC manager  642 , the NIC drivers  680 A and  680 B, and the physical NICs  180 A and  180 B are able to transfer both incoming data frames and outgoing data frames between numerous different software entities within the virtual computer system  700 A and numerous different software entities on the network. 
         [0051]    One of the primary functions of the NIC manager  642  is to decide which outgoing data frames will be routed over each of the physical NICs  180 A and  180 B. As described in the &#39;779 application, the NIC manager  642  operates in coordination with a VM manager  660  and a resource manager  662 , which are additional units of the kernel  600 , as illustrated in  FIG. 2 . The VM manager  660  and the resource manager  662  may be combined into a single software unit or that may be implemented as separate units as illustrated in  FIG. 2 . The VM manager  660  and the resource manager  662  are illustrated and described as separate units herein simply because they have distinct functions. The VM manager  680  performs high-level functions related to the control and operation of the VMs  200 A,  200 B,  200 C and  200 D. For example, the VM manager  660  may initialize a new VM, suspend an active VM, terminate a VM or cause a VM to migrate to another physical computer system. The VM manager  660  may perform these actions in response to a variety of stimuli or conditions, such as in response to commands from a system administrator at a control console, in response to conditions within a VM or in response to other conditions within the virtual computer system  700 A. 
         [0052]    The resource manager  682  generally allocates system resources between the multiple VMs  200 A,  200 B,  200 C and  200 D, as well as between the other worlds within the virtual computer system. For example, the resource manager  662  schedules and manages access to the CPU(s), the memory, the network resources and any accessible data storage resources. The resource manager  662  may allow a system administrator to specify various levels of service that are to be provided to each of the VMs for each of the system resources. For example, an application  430  running on the COS  420  (see  FIG. 1 ) may provide a user interface to a system administrator, enabling the system administrator to control numerous system parameters, including the levels of service of system resources for the multiple VMs  200 A,  200 B,  200 C and  200 D. The resource manager  662  then works with other units within the computer system  700 A to provide the requested levels of service. 
         [0053]    The NIC manager  642  preferably obtains and evaluates a variety of NIC management information and VM-specific information received from the VM manager  660 , the resource manager  682  and other sources, in deciding whether a data frame is to be transferred onto the network(s)  20 , queued for transferring at a later time, or discarded; and, if a decision is made to transfer the data frame, the NIC manager  642  also decides over which NIC  180 A or  180 B the data frame is to be transferred. 
         [0054]    As also described in the &#39;779 application, the NIC manager  642  preferably provides NIC teaming capabilities such as failover and failback functions, along with a load distribution function, when making these decisions. A wide variety of algorithms may be implemented in making data frame routing decisions. One such algorithm involves sending outgoing data frames from each VM in the virtual computer system over a different physical NIC. For example, if there are two VMs in the system and two physical NICs, one VM&#39;s data frames would be routed over the first physical NIC and the other VM&#39;s data frames would be routed over the other physical NIC. As described in the &#39;779 application, such an algorithm provides greater isolation between the operation of the different VMs in the system. This algorithm is not possible, however, in many virtual computer systems, because the number of VMs in such systems exceeds the number of physical NICs in the systems. For example, in the virtual computer system  700 A of  FIG. 2 , there are tour VMs  200 A,  200 B,  200 C and  200 D, but only two physical NICs  180 A and  180 B. In this case, there is no way to give each VM in the system its own physical NIC. 
         [0055]    Suppose in the virtual computer system  700 A of  FIG. 2  that the NIC manager  842  is configured to route data frames from the VM- 1   200 A and the VM- 2   200 B through the first physical NIC  180 A and to route data frames from the VM- 3   200 C and the VM- 4   200 D through the second physical NIC  180 B. In this case, the VM- 1   200 A and the VM- 2   200 B are sharing the first physical NIC  180 A, while the VM- 3   200 C and the VM- 4   200 D are sharing the second physical NIC  180 B. The NIC manager  642  may also restrict incoming data frames in a similar way, so that, if an incoming broadcast data frame is received at the first NIC  180 A but not at the second NIC  180 B (for example, if the NICs are connected to different networks), then the data frame is delivered to the VM- 1   200 A and the VM- 2   200 B, but not to the VM- 3   200 C or the VM- 4   200 D. Although using this algorithm improves the isolation between some of the VMs, each of the physical NICs  180 A and  180 B is still shared between multiple VMs. As another alternative, suppose that a critical application requiring consistent, reliable network access is executing in the VM- 1   200 A. In this case, the physical NIC  180 A may be dedicated for use by the VM- 1   200 A, while the other VMs  200 B,  200 C and  200 D all use the other physical NIC  180 B for network access. This algorithm improves the isolation of the VM- 1   200 A, but the remaining VMs must still share a physical NIC. 
         [0056]    This sharing of physical NICs causes a possible security risk, with the degree of risk varying, depending on the implementation and use of the virtual computer system. For example, suppose that a first VM in a virtual computer system is afflicted with a virus or some other malicious software while a second VM is running an important application. Or, even worse, suppose that the first VM is being actively used by a sophisticated hacker while an important application is running in the second VM. Any ability within the first VM to tap into the network traffic of the second VM increases the risk of compromising the second VM. 
         [0057]    The same risk of sharing NICs also exists if the NIC manager  642  implements other algorithms for routing data frames between the VMs  200 A,  200 B,  200 C and  200 D and the physical NICs  180 A and  180 B, likely to an even greater extent. For example, if the NIC manager  842  implements a “round robin” algorithm, so that outgoing data frames from all of the VMs are sent alternately over the first NIC  180 A and the second NIC  180 B, each of the VMs  200 A,  200 B,  200 C and  200 D uses both of the NICs  180 A and  180 B. The NIC manager  842  must generally forward incoming broadcast data frames received at either of the NICs  180 A or  180 B to all of the VMs  200 A,  200 B,  200 C and  200 D. In this case, all of the VMs in the system will have some access to the network traffic of each of the other VMs. 
         [0058]    What is needed, therefore, is a technique for improving the isolation between the network traffic of multiple VMs in a virtual computer system, where the number of VMs in the system exceeds the number of physical NICs in the system. 
       SUMMARY OF THE INVENTION 
       [0059]    The invention comprises a method that is performed in a virtual computer system. The virtual computer system may comprise virtualization software that supports a first virtual machine (VM) and a second VM. The virtualization software may provide the first VM with a first virtual network interface for accessing a first computer network through a first physical network interface and it may provide the second VM with a second virtual network interface for accessing a second computer network through a second physical network interface. The method comprises: receiving outgoing network data frames from the first VM at the first virtual network interface and writing a first VLAN identifier into the outgoing network data frames before conveying the outgoing network data frames to the first computer network through the first physical network interface; receiving outgoing network data frames from the second VM at the second virtual network interface and writing a second VLAN identifier into the outgoing network data frames before conveying the outgoing network data frames to the second computer network through the second physical network interface; receiving incoming network data frames from the first computer network through the first physical network interface and, for those incoming network data frames that include the first VLAN identifier, removing the first VLAN identifier from the incoming network data frames and conveying the incoming network data frames to the first VM through the first virtual network interface; and receiving incoming network data frames from the second computer network through the second physical network interface and, for those incoming network data frames that include the second VLAN identifier, removing the second VLAN identifier from the incoming network data frames and conveying the incoming network data frames to the second VM through the second virtual network interface. 
         [0060]    The invention also comprises a software module containing computer executable code for operating m a virtual computer system. The virtual computer system may comprise a physical computer on which the software module executes, a physical network interface for providing access to one or more computer networks, and a visualization software also executing on the physical computer. The virtualization software may support a first virtual machine (VM) and a second VM that also operate in the virtual computer system. The visualization software may provide a first virtual network interface to the first VM to provide the first VM with access to one or more of the computer networks through the physical network interface. The visualization software may also provide a second virtual network interface to the second VM to provide the second VM with access to one or more of the computer networks through the physical network interface. The software module comprises computer executable code operating to restrict the network access of the first VM to a first virtual local area network (VLAN) and to restrict the network access of the second VM to a second VLAN. The executable code further comprises: a first set of code that writes a first VLAN identifier for the first VLAN into outgoing network data frames from the first VM after the outgoing data frames are received by the visualization software from the first virtual network interface and that writes a second VLAN identifier for the second VLAN into outgoing network data frames from the second VM after the outgoing data frames are received by the visualization software from the second virtual network interface; and a second set of code that prevents incoming network data frames from reaching the first VM unless they include the first VLAN identifier and that prevents incoming network data frames from reaching the second VM unless they include the second VLAN identifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0061]      FIG. 1  illustrates the main components of a generalized, kernel-based, virtual computer system. 
           [0062]      FIG. 2  illustrates selected components of a more specific implementation of the virtual computer system of  FIG. 1 , where the virtual computer system is connected to one or more data networks by a pair of network interface cards. 
           [0063]      FIG. 3  illustrates the virtual computer system of  FIG. 2  connected to a bridged network implementing a virtual local area network (VLAN) topology. 
           [0064]      FIG. 4  illustrates the logical configuration of the VLAN topology of  FIG. 3 . 
           [0065]      FIG. 5  illustrates one particular approach for implementing this invention in a virtual computer system. 
       
    
    
     DETAILED DESCRIPTION  
       [0066]    The invention relates to managing data transfers between a virtual computer system and a computer network. The virtual computer system may be any of a wide variety of virtual computer systems implemented in any of a wide variety of physical computer systems. The computer network may be any of a wide variety of computer networks, including a combination of various types of networks. The physical computer system is connected to the computer network by one or more NICs, or other devices for connecting a physical computer system to a computer network. The invention is described below in one particular embodiment, but it can also be implemented in a wide variety of other embodiments. 
         [0067]    The invention comprises adding a VLAN coordinator  640 , as illustrated in  FIG. 2 , or some other functionally comparable software module, to a virtual computer system. Alternatively, the functionality of the VLAN coordinator  640  may be implemented at least partially in hardware. Also, the VLAN coordinator  640  is represented as a discrete unit within the kernel  600  to distinguish its functionality from other functions performed by software modules within the kernel  600 . The functions of the VLAN coordinator  640  may be performed by a discrete software module or by a software module that also performs some other function(s) within the kernel  600  or within some other visualization software. The VLAN coordinator  640  may be integrated into the kernel  600  or other visualization software, or it may be a separate module that is added to the visualization software, possibly as one of the loadable modules and drivers  610 . 
         [0068]    The VLAN coordinator  640  places one or more of the VMs in a virtual computer system onto one or more VLANs. For example, for current Ethernet networks, the VLAN coordinator  640  may use VLANs according to the IEEE 802.1Q standard for “Virtual Bridged Local Area Networks.” The invention is described below in relation to the IEEE 802.1Q implementation of VLANs, although it also applies to other possible implementations. For the following description, a person of skill in the an is assumed to have a working knowledge of the IEEE 802.1Q standard and related technologies. A reader should refer to that standard, as needed, as well as to numerous other books and other documents related to the IEEE 802.1Q standard and related technologies. 
         [0069]    The VLAN coordinator  640  may place VMs on particular VLANs in coordination with the respective VMs, or without any coordination. The VLAN coordinator  640  may isolate VMs to particular VLANs without any software on the VMs or any user of the VMs even “knowing” of the operation of the VLAN coordinator  640 . The software on the VMs or users of the VMs may not realize that the local networks to which the VMs are connected are virtual networks instead of real, physical networks. The possibilities for implementing the VLAN coordinator  840  vary widely and the possibilities for how the VLAN coordinator  840  implements VLANs in a virtual computer system also vary widely. 
         [0070]    As described above, the virtual computer system  700 A of  FIG. 2  comprises four VMs  200 A,  200 B,  200 C and  200 D connected to one or more networks  20  by a pair of physical NICs  180 A and  180 B. Each of the VMs includes one or more virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E. Software entities within each of the VMs may use their respective virtual NIC(s) to communicate over the network(s)  20 , as if the virtual NICs were connected directly to the networks. The virtualization software within the virtual computer system  700 A conveys outgoing network data frames from the respective NIC drivers  281 A,  281 B,  281 C and  281 D to the physical NICs  180 A and  180 B and it conveys incoming network data frames from the physical NICs to the respective NIC drivers. During this process, the VLAN coordinator  640  modifies the data frames as required to implement its VLAN restrictions. The IEEE 802.1Q standard, for example, specifies an optional VLAN identifier field that may be added to a date frame to indicate a VLAN to which the data frame&#39;s transmission is to be limited. The VLAN coordinator  640  controls the VLANs to which the VMs belong by adding VLAN identifiers to data frames, by deleting VLAN identifiers from data frames and/or by modifying VLAN identifiers in data frames, as required, and depending on the circumstances. 
         [0071]    As a first example, suppose that the VM- 1   200 A is to be placed on a first VLAN, having a VLAN identifier value of 1. Suppose further that the software within the VM- 1   200 A does not include a VLAN identifier in outgoing data frames. Before outgoing data frames are transmitted to the network(s)  20  over the physical NICs  180 A and  180 B, the VLAN coordinator  640  adds a VLAN identifier value of 1 to the outgoing data frames. Now, any outgoing data frames from the VM- 1   200 A will be restricted to the first VLAN. Mors specifically, any broadcast data frames sent out by the VM- 1   200 A will only be delivered to other network, entitles that belong to the first VLAN. 
         [0072]    For any incoming data frames received at the NICs  180 A and  180 B that are to be delivered to the VM- 1   200 A, any VLAN identifiers in the data frames will typically be removed, because the software within the VM will not be expecting incoming data frames to contain VLAN identifiers. Also, incoming broadcast data frames will only be delivered to the software of the VM- 1   200 A if the data frames contain the VLAN identifier value of 1. By modifying outgoing and incoming data frames in this manner, the VLAN coordinator has effectively limited the local network of the VM- 1   200 A to other network entities on the first VLAN. The software in the VM- 1   200 A, as well as any user of the VM, need not even know, however, that the VM&#39;m local network has been restricted by the imposition of a VLAN. The only things that are seen by the guest OS  220 A, for example, may be that outgoing data frames are sent to the NIC driver  281 A without any VLAN identifiers and that incoming data frames are received from the NIC driver  281 A without any VLAN identifiers. It may simply appear to the software within the VM- 1   200 A that the local physical network to which the VM is connected comprises only the network entities in the first VLAN. 
         [0073]    Now suppose, however, that a user of the VM- 1   200 A, who may not even be aware that the apparent network to which the VM is connected is virtual instead of physical, attempts to place the VM on a second VLAN using the software within the VM. Perhaps the same user or a different user is also attempting to place one or more other VMs or other network entities on the second VLAN too. Thus, in effect, the users may be attempting to implement the second VLAN within the first VLAN, to include a subset of the network entities that belong to the first VLAN. Suppose therefore that the software within the VM- 1   200 A includes a VLAN identifier value of 2 within outgoing data frames that are sent to the NIC driver  281 A. The VLAN coordinator  640  may attempt to facilitate this second VLAN within the first VLAN, depending on the implementation and the situation. Suppose, for example, that another VM on the first VLAN (either on the same physical computer or on a separate one) is also placed on the second VLAN using the software within the VM. 
         [0074]    Now, the same virtualization software may be supporting both of these VMs, the two VMs may be supported by separate copies or instances of the same virtualization software, or the two VMs may be supported by different visualization software. In any case, the virtualization software for the VM- 1   200 A and the visualization software for the other VM may permit the VMs to implement the second VLAN if the visualization software is the same, or if there is sufficient coordination between the visualization software of the two VMs. For example, the VLAN coordinators  640  for each of the VMs may allow both outgoing data frames and incoming data frames that include the VLAN identifier value of 2 to pass through without modification, so that the VMs can implement their own second VLAN within the first VLAN implemented by the virtualization software. Alternatively, if the virtualization software for either of the VMs is already using the VLAN identifier value of 2 for another VLAN, the visualization software for the two VMs may select a different VLAN identifier value to use for the second VLAN. For example, for outgoing data frames that are to be restricted to the second VLAN, each VLAN coordinator  640  may modify the VLAN identifier from a value of 2 to a value of 3 and, for incoming data frames that are restricted to the second VLAN, the VLAN coordinators may modify the VLAN identifier from a value of 3 to a value of 2. Using this approach, the two VMs believe that they have created a second VLAN using the VLAN identifier value of 2, but, on the network(s)  20 , the second VLAN is still created, but with a VLAN identifier value of 3 instead of 2. This implementation may be useful, for example, in distinguishing between the use of private VLANs, using private VLAN identifiers, and public VLANs, using public VLAN identifiers. For example, private VLANs may be set up using VLAN identifier values of 4 and 5, while corresponding public VLANs are set up using VLAN identifier values of  1004  and  1005 , respectively. In this case, the virtualization software would replace the VLAN identifier values of 4 and 5 of the private VLANs with the VLAN Identifier values of  1004  and  1005 , respectively, of the public VLANs when conveying data frames from the private VLANs to the public VLANs, and the virtualization software would replace the VLAN identifier values of  1004  and  1005  of the public VLANs with the VLAN identifier values of 4 and 5, respectively, of the private VLANs when conveying data frames from the public VLANs to the private VLANs. 
         [0075]    The possible VLAN configurations for different virtual computer systems may vary widely too, depending or the circumstances. For example, to achieve the greatest isolation between multiple VMs in a system, it may be advantageous to place each of the VMs on a different VLAN, to minimize the amount of network traffic that each VM sees that relates to the other VMs in the system. Thus, for example, in the virtual computer system  700 A of  FIG. 2 , the VM- 1   200 A may be placed on a first VLAN having a first VLAN identifier, the VM- 2   200 B may be placed on a second VLAN having a second VLAN identifier, the VM- 3   200 C may be placed on a third VLAN having a third VLAN identifier, and the VM- 4   2000  may be placed on a fourth VLAN having a fourth VLAN identifier. 
         [0076]    Placing each VM in a system on a separate VLAN may mitigate the risks involved with sharing a physical NIC between multiple VMs. The added security gained by using different VLANs may justify changing the routing algorithm used by the NIC manager  842  to one that improves the throughput of the network connection. For example, suppose the NIC manager  642  of  FIG. 2  is configured to use the physical NIC  180 A for network traffic related to the VM- 1   200 A and the VM- 2   200 B and to use the physical NIC  180 B for network traffic related to the VM- 3   200 C and the VM- 4   200 D. While this algorithm limits the sharing of physical NICs by multiple VMs in comparison to other possible algorithms, it may also be less efficient in using the potential network bandwidth provided by the physical NICs  180 A and  180 B. For example, at some times, the physical NIC  180 A may be flooded with network traffic for the VMs  200 A and  200 B while the physical NIC  180 B is idle and, at other times, the physical NIC  180 B may be flooded with network traffic for the VMs  200 C and  200 D while the physical NIC  180 A is idle. The physical NICs  180 A and  180 B would be able to provide a greater combined throughput if the network traffic were distributed more evenly over the NICs. 
         [0077]    Now suppose instead that the NIC manager  842  routes network traffic through the NICs  180 A and  180 B according to a different algorithm that is more effective in balancing the load between the physical NICs, such as by sending each outgoing data frame to the NIC that has the lightest estimated load, regardless of the source of the data frame. Such an algorithm may be preferable even if it leads to more sharing of physical NICs because of the gain in throughput for the physical NICs. 
         [0078]    There are certainly other reasons, however, why one might want to put multiple VMs on the same VLAN, whether the VMs are running on the same physical computer system or on different ones. For example, suppose that the VM- 2   200 B and the VM- 3   200 C are running applications that need to communicate with one another over the network connection, in this case, the VMs  200 B and  200 C may be placed on the same VLAN, while other VMs in the computer system are placed on one or more other VLANs. If desired, however, the network traffic for the VMs  200 B and  200 C may be split up, so that one physical NIC handles the traffic of one of the VMs and the other physical NIC handles the traffic of the other VM. Using separate NICs for the network traffic of these two VMs may tend to alleviate the added risk of putting the two NICs on the same VLAN. 
         [0079]    There are a variety of factors, including possible network communications between VMs and the trustworthiness of the different VMs, that may lead to a wide variety of NIC usage configurations, including different algorithms for routing network traffic to the physical NICs in the computer system and selecting VLAN configurations, along with various other considerations. The NIC usage configuration of a computer system may be established by a variety of means, including giving a system administrator an ability to select or specify the various configuration parameters, such as the algorithm used by the NIC manager  842  and the VLAN configuration. Depending on the particular circumstances, a system administrator may choose an appropriate tradeoff between security risks and an efficient use of the network connection. 
         [0080]      FIG. 3  shows the virtual computer system  700 A having an example VLAN configuration and being connected through the physical NICs  180 A and  180 B to an example configuration of network entities implementing an example VLAN topology.  FIG. 3  shows the virtual computer system  700 A including a visualization software  360 . As described above, for this particular implementation, the visualization software  380  comprises the kernel  600 , the loadable modules and drivers  610  and the VMMs  300 A,  300 B,  300 C and  300 D which are all shown in  FIG. 2 . The visualization software  360  supports the VMs  200 A,  200 B,  200 C and  200 D, as described above. The VM- 1   200 A includes the virtual NIC  280 A, the VM- 2   200 B includes the virtual NIC  280 B, the VM- 3   200 C includes the virtual NIC  280 C, and the VM- 4   200 D includes the virtual NICs  280 D and  280 E. The VLAN configuration for the virtual computer system  700 A is as follows; the VM- 1   200 A is restricted to a first VLAN (VLAN- 1 ) having a VLAN identifier of 1, the VM- 2   200 B and the VM- 3   200 C are both restricted to a second VLAN (VLAN- 2 ) having a VLAN identifier of 2. and the VM- 4   200 D has access to both the first VLAN and the second VLAN, with access to the first VLAN being provided through the virtual NIC  280 D and access to the second VLAN being provided through the visual NIC  280 E. 
         [0081]    The physical NICs  180 A and  180 B are connected to a VLAN switch  22 A, which is further connected to a VLAN switch  22 B by a VLAN bridge  24 . VLAN switches and VLAN bridges are described in the IEEE 802.10 standard. Thus, according to the IEEE 802.1Q standard, all end stations connected to the VLAN switches  22 A and  22 B form a VLAN topology  40 . Besides the virtual computer system  700 A, other end stations that are connected to the VLAN switches  22 A and  22 B include a second physical computer (PC- 2 )  702 B that includes a NIC  704 B, a third physical computer (PC- 3 )  702 C that includes a NIC  704 C, a fourth physical computer (PC- 4 )  702 D that includes a NIC  704 D, and a fifth physical computer (POS)  702 E that includes a NIC  704 E. 
         [0082]    Each of the physical computers  702 B,  702 C,  702 D and  702 E is restricted to either the VLAN- 1  or the VLAN- 2 . The physical computers may be restricted to their respective VLANs in any of a variety of ways, for example, each of the physical computers  702 B,  702 C,  702 D and  702 E may include an OS that provides VLAN functionality, such as a recent Linux distribution that includes an 802.1Q driver. More specifically, the physical computer  702 C and the physical computer  702 D are placed on the first VLAN, while the physical computer  702 B and the physical computer  702 E are placed on the second VLAN. Thus, as illustrated in  FIG. 3 , the broadcast domain of the first VLAN includes the VM- 1   200 A, the VM- 4   200 D, the physical computer  702 C and the physical computer  702 D, while the broadcast domain of the second VLAN includes the VM- 2   200 B, the VM- 3   200 C, the VM- 4   200 D, the physical computer  702 B and the physical computer  702 E. The VLAN configuration illustrated in  FIG. 3  provides improved security to the end stations on the first VLAN from the end stations on the second VLAN, and to the end stations on the second VLAN from the end stations on the first VLAN. In particular, the VLAN configuration provides improved security to the VM- 1   200 A from the VM- 2   200 B and the VM- 3   200 C, and to the VM- 2   200 B and the VM- 3   200 C from the VM- 1   200 A.  FIG. 3  also shows that the VLAN switch  228  is also connected to a router  26 , which is attached to one or more other network(s)  28 . 
         [0083]    Other computer systems can have a wide variety of other physical network configurations and a wide variety of VLAN configurations. For example, other embodiments can include multiple virtual computer systems, with each virtual computer system including one or more VMs. The virtualization software supporting the VMs in each of these virtual computer systems may be different from one another, or some or all of the virtual computer systems may have substantially the same virtualization software. In the case of a computer system including multiple VMs in each of multiple virtual computer systems, each of the virtual computer systems may implement its own VLAN configuration, just as the virtual computer system  700 A has its own VLAN configuration. In effect, each VM in each of the virtual computer systems is an end station in the VLAN topology for the entire bridged network. A system administrator may choose which end stations are on each VLAN, regardless of whether each end station is a physical computer, a VM or some other network entity. 
         [0084]      FIG. 4  illustrates the logical configuration of the VLAN topology  40  of  FIG. 3 . Thus, the VLAN topology  40  comprises a VLAN- 1   42 A and a VLAN- 2   426 . The VLAN- 1   42 A includes a logical switch  30 A that represents the VLAN switching implemented by the virtualization software  360  within the virtual computer system  700 A, the VLAN switches  22 A and  22 B and the VLAN bridge  24  with respect to data frames on the VLAN- 1   42 A. Similarly, the VLAN- 2   42 B includes a logical switch  308  that represents the VLAN switching implemented by the virtualization software  360  within the virtual computer system  700 A, the VLAN switches  22 A and  228  and the VLAN bridge  24  with respect to data frames on the VLAN- 2   42 B.  FIG. 4  also shows the logical switches  30 A and  308  feeing connected to the router  28 , which is connected to the other network(s)  28 . 
         [0085]    As shown in  FIG. 4 , the VLAN- 1   42 A includes the VM- 1   200 A, the physical computer  702 D, the physical computer  702 C and the VM- 4   200 D. The VM- 1   200 A is shown as being connected to the logical switch  30 A through the virtual NIC  280 A, the physical computer  702 D is shown as being connected to the logical switch  30 A through the physical NIC  704 D, the physical computer  702 C is shown as being connected to the logical switch  30 A through the physical NIC  704 C, and the VM- 4   2000  is shown as being connected to the logical switch  30 A through the virtual NIC  280 D. 
         [0086]    Similarly,  FIG. 4  also shows that the VLAN- 2   428  includes the VM- 2   200 B, the VM- 3   200 C, the physical computer  702 B, the physical computer  702 E and the VM- 4   200 D. The VM- 2   200 B is shown as being connected to the logical switch  30 B through the virtual NIC  280 B, the VM- 3   200 C is shown as being connected to the logical switch  30 B through the virtual NIC  280 C, the physical computer  702 B is shown as being connected to the logical switch  30 B through the physical NIC  704 B, the physical computer  702 E is shown as being connected to the logical switch  30 B through the physical NIC  704 E, and the VM- 4   200 D is shown as being connected to the logical switch  308  through the virtual NIC  280 E. Thus, again, the VM- 4   200 D is connected to the VLAN- 1   42 A through the virtual NIC  280 D and to the VLAN- 2   42 B through the virtual NIC  280 E. As another alternative, the VM- 4   200 D could be connected to both the VLAN- 1   42 A and to the VLAN- 2   42 B through a single virtual NIC. 
         [0087]      FIG. 4  shows more clearly the broadcast domains of the VLAN- 1   42 A and the VLAN- 2   42 B that are implemented within the VLAN topology  40 . Any data frame that is broadcast over the VLAN- 1   42 A is received by the VM- 4   200 D, the VM- 1   200 A, the physical computer  702 D and the physical computer  702 C. Any data frame that is broadcast over the VLAN- 2   42 B is received by the VM- 4   200 D, the physical computer  702 B, the VM- 2   200 B, the VM- 3   200 C, and the physical computer  702 E.  FIG. 4  also straws more clearly which end stations are excluded from the broadcast domains of the respective VLANs  42 A and  42 B, which makes it more difficult for them to cause problems in those VLANs. For example, malicious software in the VM- 2   200 B would have a harder time breaking into the VM- 1   200 A because the VM- 2   200 B does not receive broadcast data frames related to the VM- 1   200 A, even though both VMs execute within the same physical computer. 
         [0088]      FIG. 5  illustrates a particular method that is used in a commercial embodiment of the invention for allowing a system administrator to configure VLANs in a virtual computer system.  FIG. 5  shows the virtual computer system  700 A. Including the VM- 1   200 A with the virtual NIC  280 A, the VM- 2   200 B with the virtual NIC  280 B, the VM- 3   200 C with the virtual NIC  280 C and the VM- 4   200 D with the virtual NICs  280 D and  280 E.  FIG. 5  also shows the VLAN switch  22 A to which the virtual computer system  700 A is connected. The virtual computer system  700 A and the VLAN switch  22 A may be the same as described above. 
         [0089]      FIG. 5  also shows a virtual switch  282 , which is an emulated device supported by the visualization software  380  (see  FIG. 3 ). The virtual switch  282  is presented to a system administrator or other user of the virtual computer system  700 A through a user interface to allow the administrator to assign VMs to VLANs, as desired. The administrator may create one or more “port groups” within the virtual switch  282 , where each port group corresponds to a VLAN to which VMs may be assigned. The administrator may also specify a VLAN identifier to be used for the VLAN associated with a port group. Using the same example VLAN configuration as described above, the administrator of the virtual computer system  700 A may create a first VLAN port  284 A that uses the VLAN identifier for the VLAN- 1   42 A and a second VLAN port  284 B that uses the VLAN identifier for the VLAN- 2   42 B, as illustrated in  FIG. 5 . Now the system administrator may specify connections between the virtual NICs  280 A,  280 B,  280 C,  280 D and  280 E and the VLAN pods  284 A and  284 B. As shown in  FIG. 5 , the virtual NIC  280 A and the virtual NIC  280 D are connected to the VLAN- 1  port  284 A, which indicates to the virtualization software  360  that outgoing data frames from these virtual NICs are to be sent out on the VLAN- 1   42 A and that incoming data frames will only be delivered to these virtual NICs if they were received from the VLAN- 1   42 A. Similarly, the virtual NIC  280 B, the virtual NIC  280 C and the virtual NIC  280 E are connected to the VLAN- 2  port  284 B, which indicates to the virtualization software  360  that outgoing data frames from these virtual NICs are to be sent out on the VLAN- 2   428  and that incoming data frames will only be delivered to these virtual NICs if they were received from the VLAN- 2   42 B. Thus, just like in  FIGS. 3 and 4 , the VM- 1  is connected to the VLAN- 1   42 A through the virtual NIC  280 A, the VM- 2  is connected to the VLAN- 2   42 B through the virtual NIC  280 B. the VM- 3  is connected to the VLAN- 2   428  through the virtual NIC  280 C, the VM- 4  is connected to the VLAN- 1   42 A through the virtual NIC  280 D. and the VM- 4  is connected to the VLAN- 2   42 B through the virtual NIC  280 E. 
         [0090]    As described above, creating multiple VLANs within a VLAN topology, with each VLAN defining a different broadcast domain that generally includes a subset of the end stations within the VLAN topology, and restricting VMs to one or more of these VLANs generally improves the security situation for a virtual computer system. Under this invention, the actions required to restrict a VM to a VLAN are performed by virtualization software within a virtual computer system, so that software entities within the VMs don&#39;t have to do anything to gain the benefits of using VLANs, and may not even be aware of the use of VLANs. Guest software can send network data frames to a virtual NIC and receive data frames from a virtual NIC, without using any VLAN identifiers. The visualization software typically handles the VLAN functionality while data frames are being transferred between the virtual NIC and a physical NIC that is actually connected to a physical computer network. 
         [0091]    There are other ways to configure and use VLANs in a computer system comprising one or more virtual computer systems, however, which could also provide some of these same benefits. For example, each of the VMs could be loaded with a guest OS that can provide VLAN functionality, such as a recent distribution of Linux with an 802.1Q driver. Then, the guest OS could perform the actions required to use a VLAN for network communications. For example, the guest OS  220 A within the VM- 1   200 A could add the VLAN identifier for the VLAN- 1   42 A to all outgoing data frames sent to the virtual NIC  280 A and only deliver incoming data frames received at the virtual NIC  280 A to other software entities within the VM  200 A if the data frames include the VLAN identifier for the VLAN- 1   42 A. 
         [0092]    Providing the VLAN functionality within the visualization software, however, may be more advantageous than providing the functionality from within the VMs. The VLAN coordinator  640  provides the functionality to all of the VMs  200 A,  2008 ,  200 C and  200 D. so that the software within each of the VMs need not have any VLAN capabilities to realize the benefits of using VLANs. This way, even VMs running older OSs that don&#39;t support VLANs can still derive these benefits. Also, if the visualization software  360  can take advantage of other VLAN features, such as hardware VLAN acceleration/offloading, then all of the VMs can benefit from these other features, even if the software within some of the VMs would not otherwise have the capability of using the other VLAN features. 
         [0093]    Another advantage that is realized by implementing the VLAN functionality in the visualization software layer, instead of within the guest OS, is that a VM can be dynamically switched from one VLAN to another without interrupting any services, if the guest OS were implementing the VLAN functionality; the network interface within the VM would need to be brought down before the switch could be made to a different VLAN, and then the network interface would need to be reconfigured. Bringing down a network interface in this manner can cause substantial adverse effects. For example, if a web server application is executing within a VM and the network interface within the VM must be brought down, any client sessions that were being hosted by the web server may be abruptly terminated, or they may be rerouted to other sockets, possibly on another VM or another physical computer. Abruptly terminating client sessions may certainly cause a variety of different adverse effects, such as interrupting a client&#39;s download of a large file, for example. Then, each of the client sessions that has been terminated must generally be restarted by the different clients. Rerouting the client sessions may also cause adverse effects, however, such as delays, inefficiencies and an increase in complexity, which increases the likelihood of a connection failure. In contrast, the visualization software  360  can simply switch the VLAN identifies that it writes into outgoing data frames and that it looks for in incoming frames, without stopping any services, in the web server example, ongoing client sessions may be able to continue without any noticeable interference. 
         [0094]    Implementing the VLAN functionality in the virtualization software layer, instead of within the guest OS, may be particularly advantageous in situations where a VM may be migrated from one physical computer to another. The migration of VMs is disclosed in U.S. patent application Ser. No. 10/318,217 (“Virtual Machine Migration”), which is incorporated here by reference. A VM may be migrated to a different physical computer within the same network or even on a different network. Depending on the circumstances, it may be necessary or advantageous (a) to switch the VM to a different VLAN when it is migrated to the new physical computer, (b) to restrict the VM to a specific VLAN when it previously was not restricted to a VLAN, or (c) to remove a VLAN restriction. In any of these cases, implementing the VLAN functionality in the visualization software layer may enable the switch in VLAN status to be completed without any interference in the operation of the VM or in the VM&#39;s network connection. In the web server example, depending on the particular implementation, a VM may be migrated to a different physical computer and the VLAN status of the VM may be changed, all without disrupting any ongoing client sessions. 
         [0095]    Finally, implementing VLAN functionality within the virtualization software  380 , instead of within the guest OSs, enables different priorities and different security policies to be implemented for different VLANs. For example, if the operation of the end stations on the VLAN- 1   42 A is considered a higher priority than the operation of the end stations on the VLAN- 2   42 B, additional security measures may be taken with respect to data frames on the VLAN- 1   42 A. For example, the visualization software  360  can monitor all incoming data frames containing the VLAN identifier for the VLAN- 1   42 A for a Denial of Service (DoS) attack, without expending additional processor cycles monitoring incoming data frames on the VLAN- 2   428 . The &#39;779 application contains a more detailed description of DoS attacks. As described in the &#39;779 application, performing such a DoS defection in the visualization software  380  instead of within a VM may be advantageous, because it may prevent the VMs from being adversely affected by an attack. 
         [0096]    As described above and in the &#39;779 application, the NIC manager  842  preferably provides failover and failback functions, along with a load distribution function, in deciding how outgoing data frames are routed through the physical NICs  180 A and  180 B. To provide these functions and make these decisions, the NIC manager  642  is preferably able to determine whether each of the physical NICs  130 A and  180 B is functioning properly, and whether the connections from the physical NICs to the network(s) are functioning property. This determination may be made in a conventional manner. 
         [0097]    It would also be beneficial, however, if the NIC manager  642  could determine additional information about the network(s) to which the virtual computer system is connected, such as the operational status of one or more data links or network components within these networks. This additional information can be used by the NIC manager  642  to make better decisions regarding the routing of outgoing data frames, as described in the &#39;779 application, and if can be used by the NIC manager  642  and other software units for other purposes. There are also known ways of obtaining some of this additional information. 
         [0098]    In addition, however, depending on the implementation, the NIC manage  642  may be able to determine additional information about the status of network components and data links by sending data frames out onto the network using the physical NICs  180 A and  180 B and one or more of the VLANs that are established for use by the VMs in the virtual computer system. For example, assuming that the physical NICs  180 A and  180 B are connected to different switches, the NIC manager  842  can send out data frames for each of the VLAN IDs used by the VMs using the physical NIC  180 A, for example, and determine which of the data frames are received at the other physical NIC  180 B. Based on the physical configuration of the networks, the VLAN topology for each VLAN ID used by the VMs and the data frames received at the physical NIC  180 B, a network administrator may obtain clues regarding the status of different network components and data links within the network. In particular, for example, network failures often result from mistakes made by network administrators in configuring the various components of a network, and, when VLANs are used in the network, these configuration errors often affect some, but not all, of the VLANs. In this case, testing the network operation for each of the VLANs used in the virtual computer system often provides very useful information regarding the operation of the network. Based on the test results and the network configuration, a network administrator may be able to narrow down a configuration error to one or just a few network components, for example. 
         [0099]    As a more specific example implementation, suppose that a virtual computer system has three physical NICs, namely a first physical NIC, a second physical NIC and a third physical NIC, each connected to a different switch. Suppose further that the virtual computer system uses four different VLANs having a first VLAN  10 , a second VLAN ID, a third VLAN ID and a fourth VLAN ID, respectively. In this case, in one example implementation, a NIC manage within the virtual computer system may send data frames out over each of the four VLANs for all possible pairings of the physical NICs. Specifically, for each of the four VLANs, the NIC manager may send data frames (1) out through the first physical NIC that are addressed to the second physical NIC, (2) out through the first physical NIC that are addressed to the third physical NIC, (3) out through the second physical NIC that are addressed to the first physical NIC, (4) out through the second physical NIC that are addressed to the third physical NIC, (5) out through the third physical NIC that are addressed to the first physical NIC, and (6) out through the third physical NIC that are addressed to the second physical NIC. 
         [0100]    Based on which data frames are received at the respective destinations, the NIC manager and/or a network administrator may be able to draw various conclusions regarding failures within the computer network, and take appropriate actions in response to the detected failures. For example, suppose that data frames that are sent out through the first physical NIC using the first VLAN ID are not received at either the second physical NIC or the third physical NIC, but data frames that are sent out through the first physical NIC using the other VLANs are generally received at both the second physical NIC and the third physical NIC. These results suggest that there is some sort of problem related to the first VLAN ID, somewhere along the path from the first physical NIC. For example, a switch along that path may be configured in such a way that the switch doesn&#39;t allow data frames having the first VLAN ID to pass through. For example, if the first physical NIC and the second physical NIC were connected to the same switch and the third physical NIC were connected to a different switch, then these test results would suggest that the first VLAN ID may be disabled at the switch port to which the first physical NIC is connected. In response to such a situation, the NIC manager may ensure that any data frames from any of the VMs in the virtual computer system that are on the first VLAN are sent out through either the second physical NIC or the third physical NIC, and not the fast physical NIC. An alert may also be generated to a network administrator, allowing the network administrator to track down and resolve the problem. 
         [0101]    As another example, suppose that no data frames are received at any of the physical NICs for the first VLAN, but data frames are generally received at all three physical NiCs for the other VLANs, in this case, the NIC manager may communicate this information to a VLAN coordinator within the virtual computer system, which may switch the first VLAN to use a fifth VLAN ID, instead of the first VLAN ID. A wide variety of other test results are also possible, and, depending on the results, the network configuration and the sophistication of the visualization software, a variety of other remedial actions are also possible. Thus, the NIC manager, the VLAN coordinator and/or other software modules within the virtualization software may be configured to apply a specified policy, for example, for automatically detecting and responding to problems encountered in the network, especially for problems related to VLAN configurations. In any case, the test results may also be used to assist a network administrator in pinpointing and correcting a wide variety of network problems.