Abstract:
Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving an outgoing packet from a source virtual machine; obtaining a secret key for the source virtual machine, the secret key not being known by a destination virtual machine; obtaining a unique token derived at least partly from the secret key and a network address of the destination virtual machine; encapsulating the outgoing packet in a second packet along with the token and a token expiration time; and sending the second packet to the destination virtual machine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Under 35 U.S.C. §119, this application claims benefit of U.S. Provisional Application Ser. No. 61/432,531, filed Jan. 13, 2011, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This specification relates to providing virtual communication networks, and more specifically to virtual communication networks for virtual machines. 
     Cloud computing is network-based computing in which typically large collections of servers housed in data centers or “server farms” provide computational resources and data storage as needed to remote end users. Some cloud computing services provide access to software applications such as word processors and other commonly used applications to end users who interface with the applications through web browsers or other client-side software. Users&#39; electronic data files are usually stored in the server farm rather than on the users&#39; computing devices. Maintaining software applications and user data on a server farm simplifies management of end user computing devices. Some cloud computing services allow end users to execute software applications in virtual machines. 
     SUMMARY 
     In general, one innovative aspect of the subject matter described in this specification can be embodied in methods that include actions in a user process space of a host operating system, in which the host operating system hosts one or more distinct virtual machines each being a hardware virtualization of the data processing apparatus, performing the following: receiving an outgoing packet from a source virtual machine (VM) in the virtual machines, the outgoing packet destined for a destination VM; obtaining a secret key for the source VM, the secret key not being known by the destination VM; obtaining a unique token derived at least partly from the secret key and a network address of the destination VM; encapsulating the outgoing packet in a second packet along with the token and a token expiration time; and sending the second packet to the destination VM. Other embodiments of this aspect include corresponding systems, apparatus, and computer programs. 
     These and other aspects can optionally include one or more of the following features. The actions can further comprise: receiving the second packet; verifying the token; and de-encapsulating the second packet and providing it to the destination VM responsive to the verifying. The second packet can include a token that the destination VM can use to send a packet to the source VM. The token can be a hash-based message authentication code. A respective guest operating system can execute on each of the virtual machines. The destination VM can be on a same physical machine as the source VM. The outgoing packet can be a layer three or layer four packet. The user process space can have reduced privileges as compared to a process space in which a kernel of the host operating system executes. Obtaining the secret key for the source VM can comprise obtaining the secret key from a process which maintains a mapping between virtual machines, as identified by their respective network addresses, and the physical machine they are hosted on. 
     Particular embodiments of the subject matter described in this specification can be implemented so as to realize one or more of the following advantages. A network protocol for implementing virtual network pairs is described in which message encapsulation can be performed in a user process space of an operating system that does not require elevated privileges of the kernel process space of the operating system. This affords easy development and deployment of the protocol since an operating system kernel does not need to be rebuilt each time the protocol changes. An additional benefit of having the protocol implemented in the user process space is that is that certain classes of security vulnerabilities, such as kernel-level buffer overflows, are mitigated or rendered more difficult to exploit by an attacker because messages are encapsulated before being provided to the kernel. The encapsulation protocol can be fast since, in some implementations, there is no encryption of the message payload required. Since authentication credentials are embedded in messages, rather than in physical layer headers, the protocol has the ability to distinguish between multiple virtual network pairs per virtual machine. That is, the protocol enables a given virtual machine to communicate with one or more other virtual machines on the same or different host machine. The protocol provides protection from packet spoofing, i.e., counterfeit packets. Another advantage of the protocol is that network traffic between two virtual machines can be prevented unless explicitly authorized. 
     The details of one or more embodiments of the subject matter described in this specification are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the subject matter will become apparent from the description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an example system that provides virtual network connectivity between virtual machines. 
         FIG. 2  is a diagram illustrating example token negotiation and message sending. 
         FIG. 3  is a diagram of an example encapsulation packet. 
         FIG. 4  is a flow diagram illustrating an example technique for packet de-encapsulation. 
         FIG. 5  is a schematic diagram of an example host machine. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of an example virtual machine system  100 . The system  100  includes one or more host machines such as, for example, host machine  102  and host machine  104 . Generally speaking, a host machine is one or more data processing apparatuses such as a rack mounted server or other computing devices. The data processing apparatus can be in different physical locations and can have different capabilities and computer architectures. Host machines can communicate with each other through an internal data communications network  116 . The internal network can include one or more wired (e.g., Ethernet) or wireless (e.g., WI-FI) networks, for example. In some implementations the internal network  116  is an intranet. Host machines can also communicate with devices on external networks, such as the Internet  122 , through one or more gateways  120  which are data processing apparatus responsible for routing data communication traffic between the internal network  116  and the external network  122 . Other types of external networks are possible. 
     Each host machine executes a host operating system or other software that virtualizes the underlying host machine hardware and manages concurrent execution of one or more virtual machines. For example, the host operating system  106  is managing virtual machine (VM)  110  and VM  112 , while host OS  108  is managing a single VM  114 . Each VM includes a simulated version of the underlying host machine hardware, or a different computer architecture. The simulated version of the hardware is referred to as virtual hardware (e.g., virtual hardware  110   a ,  112   a  and  114   a ). Software that is executed by the virtual hardware is referred to as guest software. If guest software executing in a VM, or the VM itself, malfunctions or aborts, other VMs executing on the host machine will not be affected. In some implementations, a single virtual machine can also have its execution distributed over multiple physical machines. A host machine&#39;s microprocessor(s) can include processor-level mechanisms to enable virtual hardware to execute software applications efficiently by allowing guest software instructions to be executed directly on the host machine&#39;s microprocessor without requiring code-rewriting, recompilation, or instruction emulation. 
     Each VM (e.g., VMs  110 ,  112  and  114 ) is allocated a set of virtual memory pages from the virtual memory of the underlying host operating system and is allocated virtual disk blocks from one or more virtual disk drives for use by the guest software executing on the VM. For example, host operating system  106  allocates memory pages and disk blocks to VM  110  and VM  112 , and host operating system  108  does the same for VM  114 . In some implementations, a given VM cannot access the virtual memory pages assigned to other VMs. For example, VM  110  cannot access memory pages that have been assigned to VM  112 . A virtual disk drive can be persisted across VM restarts. Virtual disk blocks are allocated on physical disk drives coupled to host machines or available over the internal network  116 , for example. In addition to virtual memory and disk resources, VMs can be allocated network addresses through which their respective guest software can communicate with other processes reachable through the internal network  116  or the Internet  122 . For example, guest software executing on VM  110  can communicate with guest software executing on VM  112  or VM  114 . In some implementations, each VM is allocated one or more unique Internet Protocol (IP) version 4 or version 6 addresses and one or more User Datagram Protocol (UDP) port numbers. Other address schemes are possible. 
     A VM&#39;s guest software can include a guest operating system (e.g., guest operating systems  110   b ,  112   b  and  114   b ) which is software that controls the execution of respective guest software applications (e.g., guest applications  110   c ,  112   c  and  114   c ), within the VM and provides services to those applications. For example, a guest operating system could be a variation of the UNIX operating system. Other operating systems are possible. Each VM can execute the same guest operating system or different guest operating systems. In further implementations, a VM does not require a guest operating system in order to execute guest software applications. A guest operating system&#39;s access to resources such as networks and virtual disk storage is controlled by the underlying host operating system. 
     By way of illustration, and with reference to virtual machine  110 , when the guest application  110   c  or guest operating system  110   b  attempts to perform an input/output operation on a virtual disk, initiate network communication, or perform a privileged operation, for example, the virtual hardware  110   a  is interrupted so that the host operating system  106  can perform the action on behalf of the virtual machine  110 . The host operating system  106  can perform these actions with a process that executes in kernel process space  106   b , user process space  106   a , or both. 
     The kernel process space  106   b  is virtual memory reserved for the host operating system  106 &#39;s kernel  106   d  which can include kernel extensions and device drivers, for instance. The kernel process space has elevated privileges (sometimes referred to as “supervisor mode”); that is, the kernel  106   d  can perform certain privileged operations that are off limits to processes running in the user process space  106   a . Examples of privileged operations include access to different address spaces, access to special functional processor units in the host machine such as memory management units, and so on. The user process space  106   a  is a separate portion of virtual memory reserved for user mode processes. User mode processes cannot perform privileged operations directly. 
     In various implementations, a portion of VM network communication functionality is implemented in a communication process (e.g., communication process  106   c ). In some implementations, the communication process executes in the user process space (e.g., user process space  106   a ) of a host operating system (e.g., host operating system  106 ). In other implementations, the communication process can execute in the kernel process space (e.g., kernel process space  106   d ) of the host operating system. There can be a single communication process for all VMs executing on a host machine or multiple communication processes, one for each VM executing on a host machine. In yet further implementations, some portion of the communication process executes in the user process space and another portion executes in the kernel process space. The communication process communicates with a directory service (e.g., VM registry service  118 ) in order to establish a virtual network pair (VNP) between two VMs. A virtual network pair (VNP) is a logical computer network that is implemented on top of one or more physical (wired or wireless) computer networks. A VNP routes traffic between two endpoints using one or more virtual connections or links. By way of illustration, a VNP between virtual machine  110  and virtual machine  114  would route packets sent between VNP endpoints managed respectively by communication processes  106   c  and  108   c  over internal network  116 . The VM registry service  118  is one or more data processing apparatus that execute software for keeping track of assignments of network addresses (e.g., IP addresses) to VMs, and for keeping track of network addresses (e.g., IP addresses) of host machines that the VMs are executing on. The data processing apparatus can be in different locations and can have different capabilities and computer architectures. 
       FIG. 2  is a diagram illustrating example token negotiation and message sending. Before a VM can send packets to or receive packets from other VMs, the VM (or a communication process executing on the VM&#39;s host machine) needs to obtain a secret key. A secret key is a piece of information that serves to uniquely identify a VM among all VMs connected through their respective host machines to the internal network  116 , for example. The secret key can be a number, a string of characters, other data, combinations of these, or any other string of bits of sufficient entropy. The secret key can be generated in such a way that an entity with no prior knowledge of the key has no computationally feasible way of deriving the key. The secret key can be generated using, for example, a cryptographically secure pseudorandom number generator. In some implementations, the secret key is assigned to a VM by the VM registry service  118 . Secret keys can also be assigned by another process besides the VM registry service. A given VM may not know what its secret key is and does not know the secret key of other VMs. In some implementations, the communication process (e.g., communication process  106   c ) on a given host machine (e.g., host machine  102 ) keeps track of the secret keys for VMs managed by the host operating system (e.g. host operating system  106 ). 
     By way of illustration, the communication process  204  on VM A  202 &#39;s host machine can request a secret key for VM A  202  from the VM registry service  206  by sending a message  212  to the VM registry service  206 . The request for the secret key can be sent via secure protocol that allows the VM registry service  206  to authenticate which communication process it is communicating with. Other communications between the VM registry service and communication process  204  or other communication processes can also use the same secure protocol. 
     The VM registry service  206  responds to the communication process  204  with a message  214  containing the secret key for VM A. As a further example, the communication process  208  on VM B  210 &#39;s host machine can request a secret key for VM B  210  from the VM registry service  206  by sending a message  216  to the VM registry service  206 . The VM registry service  206  responds to the communication process  208  with a message  218  containing the secret key for VM B. 
     Before an initial packet from one VM to another is transmitted, a VNP between the two VMs is established. In various implementations, the communication process on a given host machine is responsible for establishing VNPs. For example, communication process  106   c  is responsible for establishing VNPs for VM  110  and VM  112 . Likewise, communication process  108   c  can do the same for VM  114 . Each VM can communicate with one or more other VMs using a separate VNP for each. Referring again to  FIG. 2  and by way of example, VM A  202  attempts to transmit a packet  220  to VM B  210 . In some implementations, the packet is an IP version 4 or version 6 packet. In other implementations, the packet is an Open Systems Interconnection Reference Model layer 3 or higher protocol packet, such as, for example, UDP, Transmission Control Protocol (TCP), and so on. Other types of packets are possible. The packet is intercepted by the communication process  204  of the host operating system on which VM A  202  is executing. The communication process  204  determines that a VNP between VM A  202  and VM B  210  has not yet been established. This being the case, the communication process  204  requests a token to be used to communicate with VM B  210  from the VM registry service  206 . A token is required in order to establish a unidirectional VNP from a source VM to a destination VM. The token request  222  can contain the secret key for VM A and a network address (e.g., IP address) of the destination VM B  210 , for example. 
     In response to the request  222 , the VM registry service  206  uses the secret key S_a of VM A  202  to look up or determine the following attributes of VM A  202 : 
     IP_VM_a, the IP address assigned to VM A; 
     Phys_Port_a, the UDP port assigned to VM A on VM A&#39;s host machine; 
     Phys_IP_a, the IP address of VM A&#39;s host machine; and 
     expiry_a_b, the validity period of the token which, in some implementations, is the current time plus a time-to-live (TTL). The TTL can be on the order of minutes (e.g., 10 minutes) or other granularities. In some implementations, expiry_a_b is set to a value (e.g., −1) to indicate that the token never expires. 
     In some implementations, the VM registry service  206  verifies that the request  222  was actually transmitted from Phys_IP_a and otherwise denies the request. In further implementations, the VM registry service  206  can consult a traffic policy to determine if VM A  202  should be allowed to communicated with VM B  210  and, if not, denies the request. 
     In various implementations, the VM registry service  206  computes the token T_a_b for traffic from VM A  202  to VM B  210  as follows ( FIG. 2  at  224 ):
 
 T   —   a   —   b =TruncMAC( S   —   b ,Phys_IP —   a |Phys_IP —   b |Phys_Port —   a |Phys_Port —   b |IP_VM —   a |IP_VM —   b |expiry —   a   —   b )
 
     Where ‘|’ denotes concatenation, S_b is VM B  210 &#39;s secret key, and TruncMAC is a Message Authentication Code (MAC) function (e.g., HMAC-SHA1, or TBD) that has been truncated, for example, to 64 bits. Other MAC functions are possible. In some implementations, all of the fields being concatenated are fixed size, i.e., the concatenation is uniquely determined without the need for separators. Although the fields are concatenated in a certain order, other field orderings are possible. 
     The VM registry service  206  returns the tuple (Phys_IP_b, Phys_Port_b, T_a_b, expiry_a_b) to communication process  204  in a response  226  to the request  222 , where Phys_IP_b is the IP address of the host machine on which VM B  210  is executing and Phys_Port_b is the UDP port on VM B&#39;s host machine that has been reserved for receiving traffic directed to VM B  210 . Once the communication process  204  receives the tuple, the packet  220  can be encapsulated and transmitted to VM B&#39;s host machine using the Phys_IP_b and Phys_Port_b as the destination address of the packet. Future packets destined for VM B  210  can reuse the tuple information until expiry_a_b has passed to. Once the expiry_a_b has passed (if it is not set to a value indicating that the token never expires), a new token can be obtained as described above, for example. In order for VM B  210  to send packet to VM A  202 , a token is needed to establish a unidirectional VNP from VM B  210  to VM A  202 . The same process can be followed as outlined above or, alternatively, a packet sent from VM A to VM B can include the token. This alternative is described further below. 
     Outgoing packets (e.g., outgoing packet  220 ) such as IPv4 and IPv6 packets are encapsulated by the communication process, or another process, using the obtained token. Encapsulation of an outgoing packet takes place before packet transmittal. In some implementations, the encapsulation packet is a UDP datagram. Other types of encapsulation packets are possible. By way of illustration, an outgoing packet is encapsulated in the data portion  302   e  of a UDP packet  302  as shown in  FIG. 3 . The UDP packet  302  has a header consisting of a source port number  302   a  (16 bits), a destination port number  302   b  (16 bits), a length field  302   c  (16 bits) which is the length of the data portion  302   e , and a checksum  302   d  (16 bits). In various implementations, the source port  302   a  is set to Phys_Port_a and the destination port  302   b  is set to Phys_Port_b. The data portion  302   e  is variable length and contains the outgoing packet. In some implementations, the data portion  302   e  can be encrypted. Symmetric or asymmetric encryption key algorithms can be used to encrypt some or all of the data portion  302   e , for example. The encryption keys can be distributed by a VM registry service, e.g., VM registry service  118  of  FIG. 1 . In some implementations, a conventional key negotiation protocol, e.g., the Diffie-Hellman protocol, is used to encrypt the data portion  502   e . The data portion  302   e  contains VNP packet  304  that includes the token T_a_b  304   a  (64 bits), the expiry time expiry_a_b  304   b  (32 bits), the outgoing packet  304   c  (variable length), and an optional options field  304   d  (variable length). Other lengths for the VNP packet fields are possible. In addition, the fields of the VNP packet  304  can occur in a different order than that which is illustrated in  FIG. 3 . As described above, the outgoing packet can be, for instance, an OSI Reference Model layer 2 or higher protocol packet. In some implementations, the outgoing packet is an IP packet. Other outgoing packet types are possible including, for example, Ethernet packets. Once encapsulation is complete, the communication process or another process on the respective host machine&#39;s operating system can transmit the UDP packet  302  in an IP packet with a source IP address of Phys_IP_a and a destination IP address of Phys_IP_b. Alternatively, the encapsulation packet can be transmitted using a different protocol. 
     In some implementations, the VNP packet includes an options field  304   d . The options field  304   d  allows a sending VM to convey additional information to a receiving VM. If the encapsulation packet is a UDP packet, as illustrated in  FIG. 3 , the length of the options field can be calculated based on the difference of the value specified in the length  302   c  field and the combined size of the token  304   a , expiry  304   b , and encapsulated IP packet  304   c  fields. The size of the encapsulated IP packet  304   c  is available in the header of the packet  304   c . The options field  304   d  can be omitted from the VNP packet  304  if its inclusion would push the size of the encapsulation packet (e.g., UDP packet  302 ) above the maximum transmission unit (MTU) which would result in packet fragmentation. In further implementations a field such as a bit flag, for example, can be added to the VNP packet  304  to indicate the presence of an options field  304   d.    
     The options field  304   d  can be used to send a return VNP token T_b_a to the receiving VM in order to avoid the receiving VM having to obtain a token in order to reply to the sending VM. The communication process  204  for VM A  202  has all information on hand that is needed to compute T_b_a; in particular, the token is signed with VM A  202 &#39;s secret key S_a. The return token T_b_a can likely be included in the first packet sent between VM A  202  and VM B  210  since, in the case of TCP, the first packet is a small SYN packet which leaves sufficient room for the options field  304   d  containing the return token without the resulting encapsulation packet exceeding MTU. The options field  504   d  can also be used to send various protocol extensions, to exchange encryption keys, to send control flow information, and so on. 
     Returning to  FIG. 2 , the encapsulation packet is then sent  230  to the destination host machine for VM B  210  where it is intercepted by the communication process  208  (or by another process). The communication process  208  de-encapsulates the received message ( FIG. 2  at  232 ) and then delivers the original packet to the VM B  210 . This can be done by injecting the IP packet into the network stack as an Ethernet frame, for example. 
       FIG. 4  is a flow diagram illustrating an example technique  400  for packet de-encapsulation as can be performed by the communication process  208  or another process on the destination host machine. The de-encapsulation process can be performed without the need to communicate with the VM registry service  118 . With reference to  FIG. 2 , when an encapsulation packet is received fields from the packet are extracted in order to validate the packet ( 402 ). The source and destination IP addresses and ports of the UDP packet are extracted: Phys_IP_a, Phys_IP_b, Phys_Port_a, and Phys_Port_b. The source and destination address of the encapsulated IP datagram are also extracted: IP_VM_a, IP_VM_b. Finally, the token and expiry are extracted from the encapsulation header: T_a_b and expiry_a_b. 
     If expiry_a_b is less than the current time minus a skew ( 404 ), the encapsulation packet is discarded ( 416 ). In some implementations, the skew is an estimate of likely clock skew between host machine clocks (e.g., a few seconds). The skew can be set to zero if host machine clocks are synchronized. Otherwise, if expiry_a_b is greater than the current time plus skew and TTL ( 406 ), the encapsulation packet is also discarded ( 416 ). A check is performed to verify whether the host operating system is hosting a VM having the address VM_IP_b ( 408 ). If not, the encapsulation packet is discarded ( 416 ). The extracted fields and VM B  210 &#39;s secret key S_b are used to calculate T_a_b_actual as follows ( 410 ):
 
 T   —   a   —   b _actual=TruncMAC( S   —   b ,Phys_IP —   a |Phys_IP —   b |Phys_Port —   a |Phys_Port —   b |IP_VM —   a |IP_VM —   b |expiry —   a   —   b ).
 
     T_a_b actual is then compared to the token T_a_b from the encapsulation packet ( 412 ). If T_a_b actual is the same as T_a_b then the encapsulated packet (e.g., encapsulated IP packet  304   c ) is extracted from the encapsulation packet (e.g., encapsulation packet  302 ) and is delivered to VM B  210  ( 414 ). Otherwise, the encapsulation packet is discarded ( 416 ). 
     In further implementations, and with reference to  FIG. 1 , the gateway  120  can serve as an encapsulation gateway to allow virtual machines on the internal network  116  to communicate with hosts on the external network  122 . When the gateway  120  receives an encapsulated packet from a VM (e.g., VM  114 ) destined for the external network  122 , the gateway  120  will validate the token of the VNP packet as described above with reference to  FIG. 4 , for example. If the token validation fails, the packet is discarded. If validation succeeds, the encapsulation packet is de-encapsulated and the encapsulated packet is extracted and injected into the gateway  120 &#39;s network stack where it is subject to normal routing. Upon receiving a non-encapsulated packet from the external network  122  destined for a VM, the gateway  120  obtains a token (e.g., from the VM registry service  118  or from a local cache of tokens) for the destination of the packet and then encapsulates the packet as described above with reference to  FIG. 2 , for example. The gateway  120  then transmits the encapsulated packet to the destination VM&#39;s host machine. If a VM queries the VM registry service  118  for a token that is for a destination computer that is not on the internal network  116 , the VM registry service  118  responds with a tuple (Phys_IP_b, Phys_Port_b, T_a_b, expiry_a_b) where Phys_IP_b, Phys_Port_b and T_a_b are for the gateway  120 , rather than the ultimate destination, so that the VM&#39;s packets are routed through, and de-encapsulated by, the gateway  120 . In some implementations, the VM can use this tuple to send packets to any destination on the external network  122  thus avoiding having to obtain a tuple from the VM registry service  118  for each unique destination. 
       FIG. 5  is a schematic diagram of an example host machine. The host machine  500  generally consists of a data processing apparatus  502 . The data processing apparatus  502  can optionally communicate with one or more other computers  590  through a network  580 . While only one data processing apparatus  502  is shown in  FIG. 5 , multiple data processing apparatus can be used in one or more locations. The data processing apparatus  502  includes various modules, e.g. executable software programs. One of the modules is the kernel  506  of a host operating system (e.g., host operating system  106 ). A communication process module  504  (e.g., communication process  106   c ) is configured to establish VNPs, encapsulate packets and to de-encapsulate packets. A virtual machine module  508  (e.g., virtual machine  110 ) includes virtual hardware (e.g., virtual hardware  110   a ), a guest operating system (e.g., guest operating system  110   b ), and guest applications (guest applications  110   c ). Although several software modules are illustrated, there may be fewer or more software modules. Moreover, the software modules can be distributed on one or more data processing apparatus connected by one or more networks or other suitable communication mediums. 
     The data processing apparatus  502  also includes hardware or firmware devices including one or more processors  512 , one or more additional devices  514 , a computer readable medium  516 , a communication interface  518 , and optionally one or more user interface devices  520 . Each processor  512  is capable of processing instructions for execution within the data processing apparatus  502 . In some implementations, the processor  512  is a single or multi-threaded processor. Each processor  512  is capable of processing instructions stored on the computer readable medium  516  or on a storage device such as one of the additional devices  514 . The data processing apparatus  502  uses its communication interface  518  to communicate with one or more computers  590 , for example, over a network  580 . Examples of user interface devices  520  include a display, a camera, a speaker, a microphone, a tactile feedback device, a keyboard, and a mouse. The data processing apparatus  502  can store instructions that implement operations associated with the modules described above, for example, on the computer readable medium  516  or one or more additional devices  514 , for example, one or more of a floppy disk device, a hard disk device, an optical disk device, or a tape device. 
     Embodiments of the subject matter and the operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on computer storage medium for execution by, or to control the operation of, data processing apparatus. Alternatively or in addition, the program instructions can be encoded on an artificially-generated propagated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus for execution by a data processing apparatus. A computer storage medium can be, or be included in, a computer-readable storage device, a computer-readable storage substrate, a random or serial access memory array or device, or a combination of one or more of them. Moreover, while a computer storage medium is not a propagated signal, a computer storage medium can be a source or destination of computer program instructions encoded in an artificially-generated propagated signal. The computer storage medium can also be, or be included in, one or more separate physical components or media (e.g., multiple CDs, disks, or other storage devices). 
     The operations described in this specification can be implemented as operations performed by a data processing apparatus on data stored on one or more computer-readable storage devices or received from other sources. 
     The term “data processing apparatus” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, or combinations, of the foregoing The apparatus can include special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as web services, distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub-programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
     The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform actions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit). 
     Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for performing actions in accordance with instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device, e.g., a mobile telephone, a personal digital assistant (PDA), a mobile audio or video player, a game console, a Global Positioning System (GPS) receiver, or a portable storage device (e.g., a universal serial bus (USB) flash drive), to name just a few. Devices suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
     To provide for interaction with a user, embodiments of the subject matter described in this specification can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube) or LCD (liquid crystal display) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user&#39;s client device in response to requests received from the web browser. 
     Embodiments of the subject matter described in this specification can be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation of the subject matter described in this specification, or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (“LAN”) and a wide area network (“WAN”), an inter-network (e.g., the Internet), and peer-to-peer networks (e.g., ad hoc peer-to-peer networks). 
     The computing system can include clients and servers. A client and server are generally remote from each other and typically interact through a communication network. The relationship of client and server arises by virtue of computer programs running on the respective computers and having a client-server relationship to each other. In some embodiments, a server transmits data (e.g., an HTML page) to a client device (e.g., for purposes of displaying data to and receiving user input from a user interacting with the client device). Data generated at the client device (e.g., a result of the user interaction) can be received from the client device at the server. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular embodiments of particular inventions. Certain features that are described in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. 
     Thus, particular embodiments of the subject matter have been described. Other embodiments are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results. In addition, the processes depicted in the accompanying figures do not necessarily require the particular order shown, or sequential order, to achieve desirable results. In certain implementations, multitasking and parallel processing may be advantageous.