Patent Publication Number: US-11394692-B2

Title: Distributed tunneling for VPN

Description:
CLAIM OF BENEFIT TO PRIOR APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/140,027, filed Apr. 27, 2016, now published as U.S. Patent Publication 2017/0034129. U.S. patent application Ser. No. 15/140,027 claims the benefit of Indian Patent Application No. 201641005073, filed Feb. 12, 2016. U.S. patent application Ser. No. 15/140,027 is also a Continuation In Part application of U.S. patent application Ser. No. 14/815,074, filed Jul. 31, 2015, now issued as U.S. Pat. No. 10,044,502. Indian Patent Application No. 201641005073. U.S. patent application Ser. No. 14/815,074, now issued as U.S. Pat. No. 10,044,502, and U.S. patent application Ser. No. 15/140,027, now published as U.S. Patent Publication 2017/0034129 are incorporated herein by reference. 
    
    
     BACKGROUND 
     When a user accesses application services hosted in a software defined data center (SDDC) using a mobile device over a public network such as Internet, the data traffic needs to be secured end-to-end with the help of a secure channel such as through virtual private network (VPN). The mobile device communicates with an application server running inside a VM hosted on a hypervisor within the enterprise&#39;s data center. The gateway of the data center on the data path between the remote mobile device and the application server typically act as the VPN server. A VPN server typically performs encryption and decryption for VPN channels to and from VMs within the data center. As VPN encryption and decryption are time consuming operations, VPN server can become performance bottleneck. 
     SUMMARY 
     Some embodiments provide a SDDC that uses distributed VPN tunneling to allow external access to application services hosted in the SDDC. The SDDC includes host machines for providing computing and networking resources and a VPN gateway for providing external access to those resources. Some embodiments perform VPN operations in the host machines that host the VMs running the applications that VPN clients are interested in connecting. In some embodiments, the VPN gateway does not perform any encryption and decryption operations. In some embodiments, the packet structure is such that the VPN gateway can read the IP address of the VM without decrypting the packet. 
     Some embodiments use Distributed Network Encryption (DNE) to establish a shared key for VPN encryption. DNE is a mechanism for distributed entities in a data center to share a key. The key management is done centrally from an entity called DNE Key Manager, which communicates with DNE Agents in the hypervisors using a secure control channel. The keys are synced between the Agents, which can work then onwards without requiring the DNE Key Manager to be online. 
     In some embodiments, when a packet is generated by an application at a VPN client, the VPN client encrypts the packet with VPN encryption key and processes the packet into an IPSec packet with IPSec header. The IPSec packet is then sent through the Internet to the VPN gateway of the datacenter, with the content of the packet encrypted. The VPN gateway of the data center then tunnels the packet to its destination tunnel endpoint (a host machine) by encapsulating it (under overlay such as VXLAN). The host machine that receives the tunnel packet in turn de-capsulate the packet, decrypt the packet, and forward the decrypted data to the destination VM/application. 
     In some embodiments, a VPN gateway does not perform VPN encryption or decryption. When the VPN gateway receives an encrypted VPN packet over the Internet, it identifies the destination tunnel endpoint (i.e., destination host machine) and the destination VM without decrypting the packet. In some embodiments, the VPN gateway uses information in the IP header to identify destination host machine and destination VM, and the VPN client leaves the IP header unencrypted. In some embodiments, the VPN client encrypt the IP header along with the payload of the packet, but replicates certain portion or fields (e.g., destination IP) of the IP header in an unencrypted portion of the packet so the VPN gateway would be able to forward the packet to its destination in the data center. 
     The preceding Summary is intended to serve as a brief introduction to some embodiments of the invention. It is not meant to be an introduction or overview of all inventive subject matter disclosed in this document. The Detailed Description that follows and the Drawings that are referred to in the Detailed Description will further describe the embodiments described in the Summary as well as other embodiments. Accordingly, to understand all the embodiments described by this document, a full review of the Summary, Detailed Description and the Drawings is needed. Moreover, the claimed subject matters are not to be limited by the illustrative details in the Summary, Detailed Description and the Drawings, but rather are to be defined by the appended claims, because the claimed subject matters can be embodied in other specific forms without departing from the spirit of the subject matters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several embodiments of the invention are set forth in the following figures. 
         FIG. 1  illustrates a datacenter that provides VPN services to allow external access to its internal resources. 
         FIG. 2  illustrates a VPN connection between different sites in a multi-site environment. 
         FIG. 3  illustrates the distribution of VPN traffic among multiple edge nodes in and out of a datacenter. 
         FIG. 4  illustrates the distribution of VPN traffic among multiple edge nodes between datacenters. 
         FIG. 5  illustrates an edge node of a data center serving as VPN gateway for different VPN connections. 
         FIGS. 6 a - b    conceptually illustrate the distribution of VPN encryption keys from an edge to host machines through control plane. 
         FIG. 7  conceptually illustrates a process for creating and using a VPN session. 
         FIG. 8  illustrates packet-processing operations that take place along the VPN connection data path when sending a packet from a VPN client device to a VM operating in a host machine. 
         FIG. 9  illustrates the various stages of packet encapsulation and encryption in a distributed tunneling based VPN connection. 
         FIG. 10  conceptually illustrates processes for preparing a packet for VPN transmission. 
         FIG. 11  conceptually illustrates a process for forwarding packet at a VPN gateway of a data center. 
         FIG. 12  illustrates host machines in multi-site environment performing flow-specific VPN encryption and decryption. 
         FIG. 13  conceptually illustrate the distribution of VPN encryption keys from an edge to host machines through control plane. 
         FIG. 14  conceptually illustrates a process that is performed by a host machine in a datacenter that uses VPN to communicate with external network or devices. 
         FIG. 15  illustrates packet-processing operations that take place along the data path when sending a packet from one site to another site by using VPN. 
         FIG. 16  illustrates using partial decryption of the VPN encrypted packet to identify the packet&#39;s rightful destination. 
         FIG. 17  conceptually illustrates a process for forwarding VPN encrypted packet at an edge node. 
         FIG. 18  illustrates a computing device that serves as a host machine. 
         FIG. 19  conceptually illustrates an electronic system with which some embodiments of the invention are implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention may be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. 
     Some embodiments provide a SDDC that uses distributed VPN tunneling to allow external access to application services hosted in the SDDC. The SDDC includes host machines for providing computing and networking resources and a VPN gateway for providing external access to those resources. Some embodiments perform VPN operations in the host machines that host the VMs running the applications that VPN clients are interested in connecting. In some embodiments, the VPN gateway does not perform any encryption and decryption operations. In some embodiments, the packet structure is such that the VPN gateway can read the IP address of the VM without decrypting the packet. 
     I. Distributed VPN Tunneling 
     For some embodiments,  FIG. 1  illustrates a datacenter  100  that provides VPN services to allow external access to its internal resources. The datacenter  100  is a SDDC that provides computing and/or networking resources to tenants or clients. The computing and/or network resources of the SDDC are logically organized into logical networks for different tenants, where the computing and networking resources are accessible or controllable as network nodes of these logical networks. In some embodiments, some of the computing and network resources of the SDDC are provided by computing devices that serve as host machines for virtual machines (VMs). These VMs in turn perform various operations, including running applications for tenants of the datacenter. As illustrated, the datacenter  100  includes host machines  111 - 113 . The host machine  113  in particular is hosting a VM that is running an application  123 . The datacenter  100  also has an edge node  110  for providing edge services and for interfacing the external world through the Internet  199 . In some embodiments, a host machine in the datacenter  100  is operating a VM that implements the edge node  110 . (Computing devices serving as host machines will be further described by reference to  FIG. 18  below.) 
     Devices external to the datacenter  100  can access the resources of the datacenter (e.g., by appearing as a node in a network of the datacenter  100 ) by using the VPN service provided by the datacenter  100 , where the edge  110  is serving as the VPN gateway (or VPN server) for the datacenter  100 . In the illustrated example, a device  105  external to the datacenter  100  is operating an application  120 . Such a device can be a computer, a smart phone, other types of mobile devices, or any other device capable of secure data communicating with the datacenter. The application  120  is in VPN communication with the datacenter  100  over the Internet. 
     The VPN communication is provided by a VPN connection  195  established over the Internet between a VPN client  130  and the edge node  110 . The VPN connection  195  allows the application  120  to communicate with the application  123 , even though the application  120  is running on a device external to the datacenter  100  while the application  123  is running on a host machine internal to the datacenter  100 . The VPN connection  195  is a secured, encrypted connection over the Internet  199 . The encryption protects the data traffic over the Internet  199  when it travels between the VPN client  130  and the edge  110 . 
     In some embodiments, an edge node (such as  110 ) of the data center serves as a VPN gateway/VPN server to allow external networks or devices to connect into the SDDC via a tunneling mechanism over SSL/DTLS or IKE/IPSec. In some embodiments, the VPN server has a public IP address facing the Internet and a private IP address facing the datacenter. In some embodiments, the VPN server in a SDDC is a software appliance (e.g., a VM running on a host machine) rather than a hardware network appliance. 
     The encryption of the VPN connection  195  is based on a key  150  that is negotiated by the edge  110  and the VPN client  130 . In some embodiments, the edge negotiates such a key based on the security policies that is applicable to the data traffic (e.g., based on the flow/L4 connection of the packets, or based on L2 segment/VNI of the packets). The VPN client  130  uses this key  150  to encrypt and decrypt data to and from the VPN connection  195  for the application  120 . Likewise, the host machine  113  uses the key  150  to encrypt and decrypt data to and from the VPN connection  195  for the application  123 . As illustrated, the application  120  produces a packet  170 . A crypto engine  160  in the VPN client  130  encrypts the packet  170  into an encrypted packet  172  by using the encryption key  150 . The encrypted packet  172  travels through the Internet to reach the edge  110  of the datacenter  100 . The edge  110  forwards the encrypted packet  172  to the host machine  113  by e.g., routing and/or encapsulating the encrypted packet. The host machine  113  has a crypto engine  165  that uses the encryption key  150  to decrypt the routed encrypted packet  172  into a decrypted packet  176  for the VM  143 , which is running the application  123 . In some embodiments, the crypto engine  165  is a module or function in the virtualization software/hypervisor of the host machine. 
     It is worth emphasizing that the encryption and the decryption of traffic across VPN connection is conducted near the true endpoint of the VPN traffic, rather than by the edge node that negotiated the encryption key of the VPN connection. In the example of  FIG. 1 , the true endpoint of the VPN traffic across the VPN connection  195  are application  120  and the application  123 . The application  123  is running on the host machine  113 , and the encryption/decryption is handled at the host machine  113  rather than at the edge node  110  (which negotiated the encryption key  150 ). In some embodiments, the machines in the datacenter are operating virtualization software (or hypervisors) in order to operate virtual machines, and the virtualization software running on a host machine handles the encryption and the decryption of the VPN traffic for the VMs of the host machine. Having encryption/decryption handled by the host machines rather than by the edge has the advantage of freeing the edge node from having to perform encryption and decryption for all VPN traffic in and out of the datacenter. Performing end-to-end VPN encryption/decryption also provides higher level of security than performing encryption/decryption at the edge because the VPN packets remain encrypted from the edge all the way to the host machine (and vice versa). 
       FIG. 1  illustrates a VPN connection that is established between a datacenter&#39;s edge node and a VPN client. In some embodiments, a computing device that is running an application that requires VPN access to a datacenter also operates the VPN client in order for the application to gain VPN access into the datacenter. In the example of  FIG. 1 , the computing device  105  external to the datacenter  100  is operating the VPN client  130  as well as the application  120  in order to establish the VPN connection  195 . In some embodiments, a physical device separate from the computing device  105  provides the VPN client functionality. In either instance, a computing device operating a VPN client is referred to as a VPN client device in some embodiments. 
     In some embodiments, a datacenter is deployed across multiple sites in separate physical locales, and these different sites are communicatively interlinked through the Internet. In some embodiments, each physical site is regarded as a datacenter and the different datacenters or sites are interlinked through the Internet to provide a multi-site environment. Some embodiments use VPN communications to conduct traffic securely between the different sites through the Internet. In some embodiments, each of the sites has an edge node interfacing the Internet, and the VPN connection between the different sites are encrypted by encryption keys negotiated between the edge nodes of different sites. The host machines in those sites in turn use the negotiated keys to encrypt and/or decrypt the data for VPN communications. 
       FIG. 2  illustrates distributed VPN tunneling between different sites in a multi-site environment  200  (or multi-site datacenter). The multi-site environment  200  includes two sites  201  and  202  (site A and site B). The site  201  has host machines  211 - 213  and an edge node  210  for interfacing the Internet  199 . The site  202  includes host machines  221 - 223  and an edge node  220  for interfacing the Internet  199 . The edge nodes  210  and  220  serve as the VPN gateways for their respective sites. 
     The host machine  212  of site A is running an application  241  and the host machine  223  is running an application  242 . The application  241  and the application  242  communicates with each other through a VPN connection  295  as the two applications  241  and  242  are running in different sites separated by the Internet  199 . The VPN connection sends traffic that are encrypted by a key  250 , which is the VPN encryption key negotiated between the edge  210  and the edge  220 . Although the edge nodes  210  and  220  negotiated the key  250  for the VPN connection  295 , the key  250  is provided to the host machines  212  and  223  so those host machines can perform the encryption/decryption for the VPN connection near the endpoints of the traffic (i.e., the applications  241  and  242 ). 
     As illustrated, a VM  231  of the host machine  212  produces a packet  270  (for the application  241 ). A crypto engine  261  in the host machine  212  encrypts the packet  270  into an encrypted packet  272  by using the encryption key  250 . The host machine  212  forwards the encrypted packet  272  to the edge  210  of the site  201  by e.g., routing and/or encapsulating the packet. The edge  210  of site A in turn sends the encrypted packet  272  to the edge  220  of site B through the Internet (by e.g., using IPSec tunnel). The edge  220  forwards the encrypted packet  272  to the host machine to the host machine  223  by e.g., routing and/or encapsulating the encrypted packet. The host machine  223  has a crypto engine  262  that uses the encryption key  250  to decrypt the encrypted packet  272  into a decrypted packet  276  for a VM  232 , which is running the application  223 . 
     By performing VPN encryption/decryption at the host machines rather than at the edge, a datacenter or site is effectively implementing a distributed VPN system in which the tasks of implementing a VPN connection is distributed to the host machines from the edge node. In some embodiments, a site or datacenter has multiple edge nodes, and the VPN traffic to and from this site is further distributed among the different edge nodes. 
       FIGS. 3 a - b    illustrates the distribution of VPN traffic among multiple edge nodes in and out of a site/datacenter. The figure illustrates a data center  301 , which can be a site in a multi-site environment. The data center  301  has edge nodes  311  and  312  as well as host machines  321 - 323 . Both edge nodes  311  and  312  are serving as VPN gateways for the data center  301 . In some embodiments, traffic of one VPN connection can be distributed across multiple VPN gateways. 
       FIG. 3 a    illustrates the two edge nodes  311  and  312  jointly serving one VPN connection between a VPN client  313  and a host machine  322 . As illustrated, the host machine  322  is operating a VM  329  and the VPN client is  313  is running an application  343 . The packet traffic between the VM  329  and the application  343  can flow through either the edge node  311  or  312 . Both the VPN client  313  and the host machine  322  use the same key  350  to encrypt and decrypt traffic, while the edge nodes  311  and  312  do not perform any encryption or decryption. 
     In some embodiments, different edge gateways can serve different VPN connections.  FIG. 3 b    illustrates the two edge nodes  311  and  312  serving two different VPN connections for two different VPN clients  314  and  315 . As illustrated, there is a first VPN connection between the host machine  322  and a VPN client  314  and a second VPN connection between the host machine  323  and a VPN client  315 . The first VPN connection uses the edge node  311  to conduct traffic between the application  344  and the VM  327 , while the second VPN connection uses the edge node  312  to conduct traffic between the application  345  and the VM  328 . These two VPN connections use different keys  351  and  352  to encrypt and decrypt traffic. The host machine  322  and the VPN client  314  use the key  351  to perform the encryption and decryption of the VPN connection between the VM  327  and the App  344 . The host machine  323  and the VPN client  315  use the key  352  to perform the encryption and decryption of the VPN connection between the VM  328  and the App  345 . 
       FIG. 4  illustrates the distribution of VPN traffic among multiple edge nodes between multiple data centers. The figure illustrates a multi-site environment  400  having sites  401  (site C) and  402  (site D). Site C has edge nodes  411  and  412  as well as host machines  421 - 423 . Site D has an edge node  413  and host machines  431 - 433 . The edge node  413  is serving as the VPN gateway for the site  402 . Both edge nodes  411  and  412  are serving as VPN gateways for the site  401 . 
     The host machine  422  of site C and the host machine  433  of site D are in VPN communication with each other for an application  429  running on the host machine  422  and an application  439  running in the host machine  433 . The encryption/decryption of the VPN traffic is performed by the host machines  422  and  433  and based on a key  450  that is negotiated between the edge nodes  411 ,  412  and  413 . The encrypted VPN traffic entering and leaving site D is only through the edge node  413 , while the same traffic entering and leaving site C is distributed among the edge nodes  411  and  412 . 
     As illustrated, a VM  442  running on the host machine  422  of site C generates packets  471  and  472  for the application  429 . A crypto engine  461  of the host machine  422  encrypts these two packets into encrypted packets  481  and  482  using the encryption key  450 . The encrypted packet  481  exits site C through the edge  411  into the Internet while the encrypted packet  482  exits site C through the edge  412  into the Internet. Both the encrypted packet  481  and  482  reaches site D through the edge  413 , which forwards the encrypted packet to the host machine  433 . The host machine  433  has a crypto engine  462  that uses the key  450  to decrypt the packets  481  and  482  for a VM  443 , which is running the application  439 . 
     In some embodiments, each edge node is responsible for both negotiating encryption keys as well as handling packet forwarding. In some embodiments, one set of edge nodes is responsible for handling encryption key negotiation, while another set of edge nodes serves as VPN tunnel switch nodes at the perimeter for handling the mapping of the outer tunnel tags to the internal network hosts and for forwarding the packets to the correct host for processing, apart from negotiating the keys for the connection. 
     Some embodiments negotiate different encryption keys for different L4 connections (also referred to as flows or transport sessions), and each host machines running an applications using one of those L4 connections would use the corresponding flow-specific key to perform encryption. Consequently, each host machine only need to perform VPN decryption/encryption for the L4 connection/session that the host machine is running. 
     In some embodiments, one edge node can serve as the VPN gateway for multiple different VPN connections.  FIG. 5  illustrates the edge node  110  of the data center  100  serving as VPN gateway for different VPN connections. 
     II. Encryption Key Distribution 
     Some embodiments negotiate different encryption keys for different L4 connections (also referred to as flows or transport sessions), and each host machines running an applications using one of those L4 connections would use the corresponding flow-specific key to perform encryption. Consequently, each host machine only need to perform VPN decryption/encryption for the L4 connection/session that the host machine is running. 
       FIG. 5  illustrates host machines in a SDDC performing flow-specific VPN encryption and decryption. Specifically, the figure illustrates the SDDC  100  having established multiple L4 connections with multiple VPN clients, where different encryption keys encrypt VPN traffic for different flows. 
     As illustrated, the SDDC  100  has established two L4 connections (or flows)  501  and  502 . In some embodiments, each L4 connection is identifiable by a five-tuple identifier of source IP address, destination IP address, source port, destination port, and transport protocol. The L4 connection  501  (“conn  1 ”) is established for transporting data between an application  511  (“app  1   a ”) and an application  521  (“app  1   b ”). The connection  502  (“conn  2 ”) is established for transporting data between an application  512  (“app  2   a ”) and an application  522  (“app  2   b ”). The applications  511  is running in a VPN client device  591  and the application  512  is running in a VPN client device  592 , while both applications  521  and  522  are running at the host machine  114  of the data center  100 . 
     Since both L4 connections  501  and  502  are inter-site connections that require VPN encryption across the Internet, the VPN gateways of each site has negotiated keys for each of the L4 connections. Specifically, the VPN traffic of L4 connection  501  uses a key  551  for VPN encryption, while the VPN traffic of L4 connection  502  uses a key  552  for VPN encryption. 
     As the VPN client device  591  is running an application (the application  511 ) that uses the flow  501 , it uses the corresponding key  551  to encrypt/decrypt VPN traffic for the flow  501 . Likewise, as the VPN client device  592  is running an application (the application  512 ) that uses the flow  502 , it uses the corresponding key  552  to encrypt/decrypt VPN traffic for the flow  502 . The host machine  114  is running applications for both the flows  501  and  502  (i.e., applications  521  and  522 ). It therefore uses both the key  551  and  552  for encrypting and decrypting VPN traffic (for flows  501  and  502 , respectively). 
     In some embodiments, when multiple different L4 connections are established by VPN, the VPN gateway negotiates a key for each of the flows such that the VPN gateway has keys for each of the L4 connections. In some of these embodiments, these keys are then distributed to the host machines that are running applications that use the corresponding L4 connections. In some embodiments, a host machine obtain the key of a L4 connection from a controller of the datacenter when it query for resolution of destination address (e.g., performing ARP operations for destination IP address.) 
     Some embodiments distribute encryption keys to the hosts to encrypt/decrypt the complete payload originating/terminating at those hosts. In some embodiments, these encryption keys are created or obtained by the VPN gateway based on network security negotiations with the external networks/devices. In some embodiments, these negotiated keys are then distributed to the hosts via control plane of the network. In some embodiments, this creates a complete distributed mesh framework for processing crypto payloads. 
     In some embodiments, each edge node (i.e., VPN gateway) is responsible for both negotiating encryption keys as well as handling packet forwarding. In some embodiments, one set of edge nodes is responsible for handling encryption key negotiation, while another set of edge nodes serves as VPN tunnel switch nodes at the perimeter for handling the mapping of the outer tunnel tags to the internal network hosts and for forwarding the packets to the correct host for processing, apart from negotiating the keys for the connection. 
       FIGS. 6 a - b    conceptually illustrate the distribution of VPN encryption keys from an edge to host machines through control plane. The figure illustrates a datacenter  600  having several host machines  671 - 673  as well as an edge  605  (or multiple edges) that interfaces the Internet and serves as a VPN gateway for the datacenter. The datacenter  600  also has a controller (or a cluster of controllers)  610  for controlling the operations of the host machines  671 - 673  and the edge  605 . 
     The datacenter  600  is also implementing a logical network  620  that includes a logical router  621  for performing L3 routing as well as logical switches  622  and  623  for performing L2 routing. The logical switch  622  is for performing L2 switching for a L2 segment that includes VMs  631 - 633 . The logical switch  623  is for performing L2 switching for a L2 segment that includes VMs  634 - 636 . In some embodiments, these logical entities are implemented in a distributed fashion across host machines of the datacenter  600 . The operations of distributed logical routers and switches, including ARP operations in a virtual distributed router environment, are described in U.S. patent application Ser. No. 14/137,862 filed on Dec. 20, 2013, titled “Logical Router”, published as U.S. Patent Application Publication 2015/0106804. The controller  610  controls the host machines of the datacenter  600  in order for those host machines to jointly implement the logical entities  621 - 623 . 
     As illustrated, the datacenter has several on going L4 connections (flows)  641 - 643  (“Conn  1 ”, “Conn  2 ”, and “Conn  3 ”), and the edge  605  has negotiated keys  651 - 653  for these flows with remote devices or networks external to the datacenter  600 . The edge  605  negotiates the keys  651 - 653  for these flows. In some embodiments, the edge  605  provides these keys to the controller  610 , which serves as a key manager and distributes the keys  651 - 653  to the host machines in the datacenter  600 . As illustrated in  FIG. 6 a   , the host machines  671 - 672  are respectively running applications for L4 connections (flows)  641 - 643 , and the controller distributes corresponding keys  651 - 653  of those flows to the host machines  671 - 673 . 
     In addition to flow-specific VPN encryption keys, some embodiments also provide keys that are specific to individual L2 segments. In some embodiments, logical switches and logical routers can be global logical entities (global logical switch and global logical routers) that span multiple datacenters. In some embodiments, each global logical switch that spans multiple datacenter can have a VPN encryption key that is specific to its VNI (virtual network identifier, VLAN identifier, or VXLAN identifier for identifying a L2 segment). VMs operating in different sites but belonging to a same L2 segment (i.e., same global logical switch and same VNI) can communicate with each other using VPN connections that are encrypted by a VNI-specific key. As illustrated in  FIG. 6 b   , the logical switch  622  (switch A) has a corresponding VPN encryption key  654  (key A) and the logical switch  623  (switch B) has a corresponding VPN encryption key  655  (key B). These keys are also stored at the edge  605  and can be retrieved by host machines that queries for them. 
     As illustrated, the host machine  671  in the datacenter  600  is controlled by the controller  610  through control plane messages. Depending on the application that it has to run (on the VMs that it is operating), the host machine  671  receives from the controller the corresponding VPN encryption keys. As illustrated, the host machine  671  is in VPN connection with a VPN client device  681  for an application running at its VM  631 . Based on this, the host machine  671  queries the key manager  610  for the corresponding keys. The key manager  610  in turn provides the keys  651  and  654 . 
     In some embodiments, the host machine receives encryption keys when it is trying to resolve destination IP addresses during ARP operations. The controller  610  would provide the encryption key to the host machine  671  when the queried destination IP is one that requires VPN encryption (i.e., a destination IP that is in another site separated from the local site). In some embodiments, such a key can be a flow-specific key. In some embodiments, such a key can be a VNI-specific key. In some embodiments, such a key can be specific to the identity of the VPN client. 
     In some embodiments, each key is negotiated for a policy instance  690  maintained at the controller  610 . These policies in some embodiments establishes rules for each flow or for each VNI/L2 segment (e.g., the conditions for rejecting or accepting packets). The controller directs the edge to negotiate the keys based on these policies for certain flows or VNIs. 
     Some embodiments use Distributed Network Encryption (DNE) to establish a shared key for VPN encryption. DNE is a mechanism for distributed entities in a data center to share a key. The key management is done centrally from an entity called DNE Key Manager, which communicates with DNE Agents in the hypervisors using a secure control channel. The keys are synced between the Agents, which can work then onwards without requiring the DNE Key Manager to be online. 
     For some embodiments,  FIG. 7  conceptually illustrates a process for creating and using a VPN session. Specifically, the figure illustrates a sequence of communications  710 - 770  between the key manager  610 , the VM  631 , the host  671 , the VPN gateway  605 , and a VPN client device  681 . The VM  631  is operating in the host machine  671 . These communications are for creating a VPN session between the VM  631  and the VPN client device  681 , in which the VPN gateway  605  negotiated a key with the client device  681  and the key manager provides the negotiated key to the host machine  671 . 
     The communications  710  is for VPN session initiation. The VPN client device  681  initiates a VPN session with the VPN server/gateway  605  via the server&#39;s external IP address. The server gives DNS (domain name system) entries to the device. The DNS maps the URLs to the enterprise IP addresses. 
     The communications  720  and  725  are for establishing a shared key. Some embodiments uses DNE supports establishment of shared keys among the DNE Agents. The VPN server shares the keys with DNE Manager module in the NSX Manager. The DNE Manager in turns shares the keys among the DNE Agents in the Distributed Switches (DS). 
     The communications  730  shows a packet from the VPN client device  681  to the VPN server  605 . The VPN stack on the device encrypts and encapsulates the data, which is destined to the VM  631  in the data center, and sends the encapsulated payload to the VPN server&#39;s external IP address. The encapsulation is such that the VPN server  605  can authenticate the payload and find out the VM&#39;s IP address. 
     The communications  740  shows a packet from the VPN server  605  to the host  671  of VM  631 . After the VPN server  605  has authenticated the payload, it removes the encapsulation. The VPN server  605  reads the destination IP address and forwards the packet to the VM  631 . 
     The communications  750  shows a packet from the host  671  to the application VM  631 . The hypervisor in the host  671  gets the packet and uses DNE to decrypt the packet and send the decrypted packet to the VM  631 . 
     The communications  760  shows a packet from the VM  631  to the host  671 . The L2 packet originating from the VM  631  destined to the VPN client device  681  is forwarded to the hypervisor in the host  671 . The DNE in the hypervisor encrypts the IP datagram and inserts an authentication header. 
     The communications  765  shows a packet from the host  671  to the VPN server  605 . The L2 packet is forwarded to the VPN server&#39;s internal IP address. This packet may be encapsulated in an overlay protocol such as VXLAN on its way to the VPN server. The VPN server de-capsulate the overlay if such encapsulation is applied. 
     The communications  770  shows a packet from the VPN server  605  to the VPN client device  681 . The VPN server  605  encapsulates the L2 payload in another IP packet and sends it to the device over the public IP network (e.g., Internet). The VPN stack in the VPN client device  681  authenticates the packet, removes the encapsulation, decrypts the data, and hands it over to its IP stack. 
     III. VPN Data Path 
     As mentioned above, in order to send data packets from its originating application/VM to its destination application/VM through VPN connection and tunnels, the packet has to go through a series of processing operations such as encryption, encapsulation, decryption, and de-capsulation. In some embodiments, when a packet is generated by an application at a VPN client, the VPN client encrypts the packet with VPN encryption key and processes the packet into an IPSec packet with IPSec header. The IPSec packet is then sent through the Internet to the VPN gateway of the datacenter, with the content of the packet encrypted. The VPN gateway of the data center then tunnels the packet to its destination tunnel endpoint (a host machine) by encapsulating it (under overlay such as VXLAN). The host machine that receives the tunnel packet in turn de-capsulate the packet, decrypt the packet, and forward the decrypted data to the destination VM/application. 
     In some embodiments, a VPN gateway does not perform VPN encryption or decryption. When the VPN gateway receives an encrypted VPN packet over the Internet, it identifies the destination tunnel endpoint (i.e., destination host machine) and the destination VM without decrypting the packet. In some embodiments, the VPN gateway uses information in the IP header to identify destination host machine and destination VM, and the VPN client leaves the IP header unencrypted. In some embodiments, the VPN client encrypt the IP header along with the payload of the packet, but replicates certain portion or fields (e.g., destination IP) of the IP header in an unencrypted portion of the packet so the VPN gateway would be able to forward the packet to its destination in the data center. 
     For some embodiment,  FIG. 8  illustrates packet-processing operations that take place along the VPN connection data path when sending the packet  170  from the VPN client device  130  to the VM  143  operating in the host machine  113 . The packet  170  originates at the application  120  of the VPN client device  130 , travels through the edge node  110  of the data center  100  to reach the host machine  113  and the VM  143 . 
     The figure illustrates the packet  170  at five sequential stages labeled from ‘1’ through ‘5’. At the first stage labeled ‘1’, the App  120  produces the packet  170 , which includes the application data  872  and IP header  871 . In some embodiments, such header can includes destination IP address, source IP addresses, source port, destination port, source MAC address, and destination MAC address. 
     At the second stage labeled ‘2’, the VPN client  130  has identified the applicable VPN encryption key for the packet  170 . In some embodiments, this encryption key is the shared key negotiated by the VPN gateway  110  with the VPN client  130 . The VPN client then encrypts the application data  872  along with the IP header  871 . However, since the VPN gateway  110  does not perform VPN encryption/decryption at all, the VPN client  130  leaves certain fields of the IP header unencrypted. As illustrated, the VPN client  130  stores destination IP  879  in an unencrypted portion of the packet so the VPN gateway  110  would be able to use the unencrypted destination IP field to forward the packet to its destination without performing VPN decryption. 
     At the third stage labeled ‘3’, the VPN client  130  creates a VPN encapsulated packet  172  having a VPN encapsulation header  874  for transmission across the Internet. In some embodiments, the VPN encapsulation packet  172  is encapsulated according to a tunneling mechanism over SSL/DTLS or IKE/IPSec. In some embodiments, the VPN encapsulated packet  172  is an IPSec packet and the VPN encapsulation header is an IPSec Tunnel Mode header. In embodiments, the VPN encapsulated packet comprises a SSL header. In some embodiments, the VPN encapsulation header includes an outer TCP/IP header that identifies the external address (or public address) of the VPN gateway  110 . The VPN client  130  then sends the VPN encapsulated packet  172  (with the encrypted IP header  871 , the encrypted application data  872 , unencrypted destination IP  879 , and the VPN encapsulation header  874 ) to the VPN gateway  110  of the data center  100 . 
     At the fourth stage labeled ‘4’, the VPN gateway  110  of the data center  100  receives the VPN encapsulated packet  172 . The VPN gateway  110  in turn uses the unencrypted (or exposed) destination IP  879  to identify destination host machine and the destination VM of the packet. No decryption of the packet is performed at the VPN gateway  110 . The VPN gateway  110  then creates an overlay header  875  based on the destination IP  879 . This overlay header is for encapsulating the packet  170  (with encrypted IP header  871  and encrypted application data  872 ) for an overlay logical network. In some embodiments, the host machines and the edge gateways of the data center communicates with each other through overlay logical networks such as VXLAN, and each host machine and gateway machine is a tunnel endpoint in the overlay logical network (a tunnel endpoint in a VXLAN is referred to as VTEP). The VPN encapsulation is removed. The edge then tunnels the encapsulated packet to the destination host machine  113 . 
     At the fifth stage labeled ‘5’, the host machine  113  strips off the overlay header  875  and decrypt the packet  170  (i.e., the IP header  871  and the application data  872 ) for delivery to the destination VM  143 . 
     For some embodiments,  FIG. 9  illustrates the various stages of packet encapsulation and encryption in a distributed tunneling based VPN connection. The figure illustrates seven different stages  901 - 907  of packet traffic between the App  120  and the VM  143 . Each stage shows the structure the packets traversing along the data path. 
     The stage  901  shows the structure of a packet  971  produced by the app  120  before any encryption and encapsulation. As illustrated, the packet includes payload  905  and IP header  910 , both of which are unencrypted. 
     The stage  902  shows the structure of the packet  971  after the crypto engine  160  has encrypted the packet for VPN. As illustrated, the payload  905  is encrypted and the crypto engine  160  has added an SSL header  920  to the packet. At least a portion of the IP header  910  (e.g., destination IP address) remains unencrypted. 
     The stage  903  shows the structure of the packet  971  as its is transmitted by the VPN client  130  for the VPN gateway  110 . The packet at the stage  903  has an outer TCP/IP header  930  that identifies the external IP address of the VPN gateway. This external IP address is used to forward the packet toward the data center across the Internet. In some embodiments, the outer TCP/IP header is part of a VPN encapsulation header as described by reference to  FIG. 8  above. 
     The stage  904  shows the structure of the packet  971  that has arrived at the VPN gateway  110 . The VPN gateway has removed the external TCP/IP header  930  from the packet. The VPN gateway has also created an L2 header  940  based on unencrypted IP address  910 . The SSL header  920  and the encrypted payload  905  remain in the packet. 
     The stage  905  shows the structure of the packet  971  as it is encapsulated by the VPN gateway  110  for transmission over an overlay logical network (e.g., VXLAN). As illustrated, the packet has overlay encapsulation header  950 . The overlay encapsulation header identifies the destination host machine  113 , which is a tunnel endpoint in the overlay logical network. 
     The stage  906  shows the structure of the packet  971  after it has arrived at the host machine  113 . The host machine  113  as tunnel endpoint (VTEP) removes the encapsulation header  950 . The SSL header  920  and the encrypted payload  905  remain in the packet along with L2 header  940  and IP address  910 . 
     The stage  907  shows the structure of the packet after the crypto engine  165  of the host machine  113  has decrypted it. The crypto engine has removed the SSL header  920  as well as decrypted the payload  905 . The L2 header  940  and the IP header  940  remains in the packet and are used by the host machine to forward the packet to the VM  143  (through L2 switch and/or L3 router in the hypervisor). 
       FIG. 10  conceptually illustrates processes  1001  and  1002  for preparing a packet for VPN transmission. Both processes are for sending a packet to a VPN gateway or edge of the data center so the VPN gateway can forward the packet to its destination. 
     In some embodiments, a host machine performs the process  1001  when sending a packet from a VM in a data center to a VPN client. The process  1001  starts when it receives (at  1010 ) a packet from a VM. 
     The process identifies (at  1015 ) the destination IP address of the packet. The process then identifies (at  1020 ) an encryption key based on the identified destination IP address. In some embodiments, this encryption key is negotiated by the VPN gateway and distributed by a key manager/controller as described in Section II. The process then encrypts (at  1025 ) the payload of the packet but leaves the destination IP address unencrypted or exposed. In some embodiments, the process encrypts the entire IP header of the packet but replicates the destination IP address in an unencrypted region of the packet. 
     The process encapsulates ( 1030 ) the packet for transmission to the VPN gateway. In some embodiments, the host machine is a tunnel endpoint in an overlay logical network (e.g., VXLAN), and the process encapsulates the packet according to the overlay logical network in order to forward the packet to the VPN gateway, which is also a tunnel endpoint in the overlay logical network. In some embodiments, the encapsulation identifies the internal address (or private address) of the VPN gateway. The process then forwards (at  1035 ) the encapsulated packet with encrypted payload to the VPN gateway. The process  1001  then ends. 
     In some embodiments, a VPN client performs the process  1002  when sending a packet from an app running on the VPN client device to a VM in a data center. The process  1002  starts when it receives (at  1050 ) payload to be transmitted. In some embodiments, the VPN client receives the payload from an application running on the device that needs to communicate with a corresponding application running in the VM in the data center. 
     The process identifies (at  1055 ) the destination IP address of the packet. The process then identifies (at  1060 ) an encryption key based on the identified destination IP address. In some embodiments, this encryption key is negotiated by the VPN gateway and distributed by a key manager/controller as described in Section II. The process then encrypts (at  1065 ) the payload of the packet but leaves the destination IP address unencrypted or exposed. In some embodiments, the process encrypts the entire IP header of the packet but replicates the destination IP address in an unencrypted region of the packet. 
     The process then attaches (at  1070 ) an outer TCP/IP header to the packet. This header identifies the outer IP address of the VPN gateway as its destination. The process then forwards (at  1075 ) the encrypted packet toward the VPN gateway (e.g., via the Internet). The process  1002  then ends. 
       FIG. 11  conceptually illustrates a process  1100  for forwarding packet at a VPN gateway of a data center. The process starts when it receives (at  1105 ) a VPN encrypted packet at the VPN server/gateway, which is an edge node of the data center. In some embodiments, such encryption is according to SSL (secure socket layer) or TLS (transport layer security) protocol. 
     The process then identifies (at  1110 ) the destination address from an unencrypted portion of the packet. In some embodiments, the VPN gateway does not perform any VPN encryption or decryption (because encryption and decryption operations are distributed to the host machines hosting the end machines/VMs). The unencrypted destination address allows the VPN gateway to identify the destination of the packet without having to perform any decryption. In some embodiments, the unencrypted destination address is an IP address, and the entire IP header of the packet is unencrypted. In some embodiments, the IP header of the packet is encrypted, but the addresses that are needed for identification of destination (e.g., destination IP) is replicated to an unencrypted portion of the packet. 
     Next, the process determines (at  1115 ) whether the VPN encrypted packet is an outgoing packet to a VPN client external to the data center, or an incoming packet to the data center and destined for an application running in a VM hosted by a host machine. Some embodiments make this determination based on the destination address identified from the unencrypted portion of the packet. If the packet is an incoming packet destined for a VM operating in the data center, the process proceeds to  1120 . If the packet is an outgoing packet destined for a VPN client external to the data center, the process proceeds to  1160 . 
     At  1120 , the process has determined that the VPN encrypted packet is an incoming packet from an external VPN client. The incoming packet has a VPN encapsulation header (including an outer TCP/IP header) identifying an external address (or public address) of the VPN gateway. The process removes the VPN encapsulation header from the packet. The process also identifies (at  1130 ) the destination endpoint (e.g., VTEP) and the VNI (virtual network identifier) based on the identified destination address. In some embodiments, the VPN gateway has configuration data that associates address of VMs (L2 MAC address or L3 IP address) with VTEP address of corresponding host machines. 
     The process then encapsulates (at  1140 ) the packet according to the identified VNI and destination endpoint. The process then tunnels (at  1150 ) the encapsulated packet to the identified VTEP, which is also the host machine that hosts the destination VM. The process  1100  then ends. Once the packet reaches its destination tunnel endpoint, the host machine strips the encapsulation, decrypt the VPN encryption, and forward the payload to the VM. 
     At  1160 , the process has determined that the VPN encrypted packet is an outgoing packet from a host machine of the data center. The outgoing packet is encapsulated according to an overlay logical network that allows the packet to be tunneled to the VPN gateway. The process then removes the encapsulation. The process also attaches (at  1170 ) a VPN encapsulation header (including an outer TCP/IP header) based on the identified destination address from the unencrypted portion of the packet. The VPN encapsulation header identifies the VPN client for the destination application. The process then forwards the packet to the VPN client based on the VPN encapsulation header. The process  1100  then ends. Once the packet reaches the destination VPN client, the VPN client device remove the VPN encapsulation header, decrypts the payload and delivers the application data. 
     IV. Partial Decryption at Edge Node 
     In some embodiments, the edge of a data center stores VPN encryption keys that it has negotiated. In order to forward packets to their rightful destination within a datacenter, the edge in some embodiments use the negotiated keys to decrypt at least a portion of each incoming VPN encrypted packet to expose the destination of the encrypted packet. This is necessary for some embodiments in which the identity of the destination (e.g., its VNI, MAC address, IP address, etc.) is in the encrypted payload of a VPN encrypted packet. In some of these embodiments, the edge uses information in the header of the VPN encrypted packet to identify the corresponding decryption key and then use the identified key to decrypt and reveal the destination information of the packet. 
       FIG. 12  illustrates host machines in multi-site environment performing flow-specific VPN encryption and decryption. Specifically, the figure illustrates a multi-site environment having established multiple L4 connections across different sites using VPN, where different encryption keys encrypt VPN traffic for different flows. 
     As illustrated, the multi-site environment  200  has established two L4 connections (or flows)  1201  and  1202 . In some embodiments, each L4 connection is identifiable by a five-tuple identifier of source IP address, destination IP address, source port, destination port, and transport protocol. The L4 connection  1201  (“conn  1 ”) is established for transporting data between an application  1211  (“app  1   a ”) and an application  1221  (“app  1   b ”). The connection  1202  (“conn  2 ”) is established for transporting data between an application  1212  (“app  2   a ”) and an application  1222  (“app  2   b ”). The applications  1211  is running in the host machine  212  and the application  1212  is running in the host machine  213 , while both applications  1221  and  1222  are running in site B at the host machine  223 . 
     Since both L4 connections  1201  and  1202  are inter-site connections that require VPN encryption across the Internet, the VPN gateways of each site has negotiated keys for each of the L4 connections. Specifically, the VPN traffic of L4 connection  1201  uses a key  1251  for VPN encryption, while the VPN traffic of L4 connection  1202  uses a key  1252  for VPN encryption. 
     As the host machine  212  is running an application (the application  1211 ) that uses the flow  1201 , it uses the corresponding key  1251  to encrypt/decrypt VPN traffic for the flow  1201 . Likewise, as the host machine  213  is running an application (the application  1212 ) that uses the flow  1202 , it uses the corresponding key  1252  to encrypt/decrypt VPN traffic for the flow  1202 . The host machine  223  is running applications for both the flows  1201  and  1202  (i.e., applications  1221  and  1222 ). It therefore uses both the key  1251  and  1252  for encrypting and decrypting VPN traffic (for flows  1201  and  1202 , respectively). 
     As mentioned, VPN encryption keys are generated based on the negotiation between the VPN gateways (i.e., edge nodes of datacenters/sites). In some embodiments, when multiple different L4 connections are established by VPN, the VPN gateway negotiates a key for each of the flows such that the VPN gateway has keys for each of the L4 connections. In some of these embodiments, these keys are then distributed to the host machines that are running applications that use the corresponding L4 connections. In some embodiments, a host machine obtain the key of a L4 connection from a controller of the datacenter when it query for resolution of destination address (e.g., performing ARP operations for destination IP address.) In some embodiments, a VPN gateway that negotiated a key also keeps a copy of the key for subsequent partial decryption of packets for identifying the destination of the packet within the data center. 
       FIG. 13  conceptually illustrate the distribution of VPN encryption keys from an edge to host machines through control plane. The figure illustrates a datacenter  1300  having several host machines  1371 - 1373  as well as an edge  1305  (or multiple edges) that interfaces the Internet and serves as a VPN gateway for the datacenter. The datacenter  1300  also has a controller (or a cluster of controllers)  1310  for controlling the operations of the host machines  1371 - 1373  and the edge  1305 . 
     The datacenter  1300  is also implementing a logical network  1320  that includes a logical router  1321  for performing L3 routing as well as logical switches  1322  and  1323  for performing L2 routing. The logical switch  1322  is for performing L2 switching for a L2 segment that includes VMs  1331 - 1333 . The logical switch  1323  is for performing L2 switching for a L2 segment that includes VMs  1334 - 1336 . In some embodiments, these logical entities are implemented in a distributed fashion across host machines of the datacenter  1300 . The controller  1310  controls the host machines of the datacenter  1300  in order for those host machines to jointly implement the logical entities  1321 - 1323 . 
     As illustrated, the datacenter has several on going L4 connections (flows)  1341 - 1343  (“Conn  1 ”, “Conn  2 ”, and “Conn  3 ”), and the edge  1305  has negotiated keys  1351 - 1353  for these flows with remote devices or networks external to the datacenter  1300 . The edge  1305  negotiates the keys  1351 - 1353  for these flows and stores the negotiated keys  1351 - 1353  at the edge  1305 . In some embodiments, these keys are distributed to those host machines by the controller  1310 . As illustrated in  FIG. 13 , the host machines  1371 - 1372  are respectively running applications for L4 connections (flows)  1341 - 1343 , and the controller distributes corresponding keys  1351 - 1353  of those flows to the host machines  1371 - 1373 . 
     For some embodiments,  FIG. 14  conceptually illustrates a process  1400  that is performed by a host machine in a datacenter that uses VPN to communicate with external network or devices. The process  1400  starts when it receives (at  1410 ) an outgoing packet to be forwarded from an application running on a VM. 
     The process then identifies (at  1420 ) the destination IP address of the outgoing packet and determines (at  1430 ) whether the destination IP address need to be resolved, i.e., whether the next hop based on the destination IP address is known. In some embodiments, the next hop is identified by its VNI and MAC address. In some embodiments, the next hop is behind a virtual tunnel and the packet is to be forwarded according to a tunnel endpoint address (VTEP), which can corresponds to another host machine or physical router in the network. If the next hop address is already resolved, the process proceeds to  1440 . If the next hop address is not resolved, the process proceeds to  1435 . 
     At  1435 , the process performs ARP in order to receive the necessary address resolution information from the controller. Such information in some embodiments includes the VNI, the MAC address, and/or the VTEP of next hop. In some embodiments, such information also includes VPN encryption key if the data is to be transmitted via a VPN connection. In some embodiments, such information includes a remote network&#39;s topology using host tags so that the secure overlay traffic travels directly to host machines in the remote networks where the workload is located. The process then proceeds to  1440 . 
     At  1440 , the process determines if VPN encryption is necessary for the next hop. Some embodiments make this determination based on the earlier ARP response from  1435 , which informs the process whether packet has to be encrypted for VPN and provides a corresponding key if encryption is necessary. Some embodiments make this determination based on security policy or rules applicable to the packet. If the VPN encryption is necessary, the process proceeds to  1445 . Otherwise the process proceeds to  1450 . 
     At  1445 , the process identifies the applicable VPN encryption key and encrypts the packet. In some embodiments, the host machine may operate multiple VMs having applications requiring different encryption keys (e.g., for packets belonging to different flows or different L2 segments.) The process would thus use information in packet (e.g., L4 flow identifier or L2 segment identifier) to identify the correct corresponding key. The process then proceeds to  1450 . 
     At  1450 , the process encapsulates the (encrypted) packet according to the resolved next hop information (i.e., the destination VTEP, MAC address, and VNI) so the packet can be tunneled to its destination. The process then forwards (at  1460 ) the encapsulated packet to its destination, i.e., to the edge so the edge can forward the packet to the external device through the Internet. After forwarding the encapsulated packet, the process  1400  ends. 
     As mentioned above by reference to  FIGS. 1 and 2 , in order to send data packets from its originating application/VM to its destination application/VM through VPN connection and tunnels, the packet has to go through a series of processing operations such as encryption, encapsulation, decryption, and de-capsulation. In some embodiments, when a packet is generated by an application at a particular datacenter or site, the host machine running the application encrypts the packet with VPN encryption key and then encapsulates the packet (using overlay such as VXLAN) in order to tunnel the to the edge. The edge in turn processes the packet into an IPSec packet with IPSec header. The IPSec packet is then sent through the Internet to another datacenter or site, with the content of the packet encrypted. The edge of the other site then tunnels the packet to its destination tunnel endpoint (a host machine) by encapsulating it (under overlay such as VXLAN). The host machine that receives the tunnel packet in turn de-capsulate the packet, decrypt the packet, and forward the decrypted data to the destination VM/application. In some embodiments, the edge of the other site uses its stored negotiated keys to decrypt a portion of the packet in order to identify the destination tunnel endpoint in that other site. 
     For some embodiment,  FIG. 15  illustrates packet-processing operations that take place along the data path when sending a packet  1570  from one site (the site  201 ) to another site (the site  202 ) by using VPN. The packet  1570  originates at the VM  231  of the host machine  212 , travels through the edge node  210  of site  201  and the edge node  220  of the site  202  to reach the host machine  223  and the VM  232 . 
     The figure illustrates the packet  1570  at five sequential stages labeled from ‘1’ through ‘5’. At the first stage labeled ‘1’, the VM  231  produces the packet  1570 , which includes the application data  1571  and IP header data  1572 . In some embodiments, such header can includes destination IP address, source IP addresses, source port, destination port, source MAC address, and destination MAC address. The packet  1570  is not encrypted at operation ‘1’. In some embodiments, the information in the IP header refers to topologies of the source datacenter (i.e., the site  201 ) that the security policy of the datacenter may not want to reveal, and hence the subsequent VPN encryption operations will encrypt the IP header as well as the application data. 
     At the second stage labeled ‘2’, the host machine  212  has identified the applicable VPN encryption key for the packet  1500  based on the content of the IP header  1571  (e.g., by identifying the flow/L4 connection or by identifying the VNI/L2 segment). The host machine then encrypted the IP header  1571  and well as the application data  1572  (shown in hash). Furthermore, based on the information of the IP header  1571 , the host machine has encapsulated the packet  1570  for an overlay logical network (e.g., VXLAN) with an overlay header  1573  in order to tunnel the packet to the edge  210  of site  201 . 
     At the third stage labeled ‘3’, the edge  210  receives the tunneled packet and strips off the overlay header  1573 . The edge then creates an IPSec packet for transmission across the Internet. The IPSec packet includes an IPSec Tunnel Mode header  1574  that is based on the information in the stripped off overlay header  1573 . This IPSec header  1574  includes information that can be used to identify the VPN encryption key (e.g., in the SPI field of the IPSec header). The edge  210  then sends packet  1570  (with the encrypted IP header  1571 , the encrypted application data  1572 , and their corresponding IPSec Tunnel Mode header  1573 ) toward the edge  220  of the site  202 . 
     At the fourth stage labeled ‘4’, the edge  220  of the site  202  uses the information in the IPSec Tunnel Mode header to  1573  to identify the key used for the encryption and decrypt enough of the IP header  1571  in order to create an overlay header  1575 . This overlay header is for encapsulating the packet  1570  (with encrypted IP header  1571  and encrypted application data  1572 ) for an overlay logical network (e.g., VXLAN). The edge then tunnels the encapsulated packet to the host machine  223 . 
     At the fifth stage labeled ‘5’, the host machine  223  strips off the overlay header  1575  and decrypt the packet  1570  (i.e., the IP header  1571  and the application data  1572 ) for delivery to the destination VM  232 . 
     As mentioned, the encryption keys used by the host machines to encrypt and decrypt VPN traffic are edge-negotiated keys. The edge as VPN gateway negotiates these keys according to security policies of the tenant or the logical network that is using the VPN connection, specific to a L4 connection or a L2 segment (logical switch). The controller then distributes negotiated keys to the host machines so the host machine performs the actual encryption and decryption. The edge is in turn tasked with forwarding the incoming encrypted VPN traffic to their rightful destinations. 
     However, in order to forward packets to their rightful destination within a datacenter, the edge in some embodiments nevertheless has to use the negotiated keys to decrypt at least a portion of each incoming VPN encrypted packet in order to reveal the destination of the encrypted packet. This is necessary for some embodiments in which the identity of the destination (e.g., its VNI, MAC address, IP address, etc.) is in encrypted payload of a VPN encrypted packet. In some of these embodiments, the edge uses information in the header of the VPN encrypted packet to identify the corresponding decryption key and then use the identified key to decrypt and reveal the destination information of the packet. 
       FIG. 16  illustrates using partial decryption of the VPN encrypted packet to identify the packet&#39;s rightful destination. The figure illustrates the forwarding of a VPN encrypted packet  1670  by the edge  220  of the datacenter  202 . The received VPN encrypted packet  1670  is an IPSec packet arriving at the edge  220  from the Internet from another datacenter. As the packet  1670  arrives at the edge  220 , it has an encrypted payload  1671  and an unencrypted IPSec header  1672 . The payload  1671  includes both IP header  1673  and application data  1683 . 
     Since the header  1672  of the IPSec is an IPSec tunnel mode header that is not encrypted, it can be read directly by the edge  220 . The IPSec tunnel mode header  1672  includes field that identifies the flow or L4 connection that the packet  1670  belongs to. In some embodiments in which the VPN encrypted packet is an IPSec packet, the SPI field of the IPSec header provides the identity of the flow. The edge  220  in turn uses the identity of the flow provided by the IPSec header to select/identify a corresponding encryption key  252 . 
     The edge  220  in turn uses the identified key  252  to decrypt a portion of the encrypted payload  1671  of the packet  1670 , revealing the first few bytes (e.g., the header portion)  1673  of the payload. In some embodiment, the edge  220  would halt the decryption operation once these first few bytes are revealed in some embodiments. Based on the revealed bytes, the edge determines the identity of the destination and encapsulates the encrypted payload  1671  into an encapsulated packet  1674  by adding an overlay header  1676 . In some embodiments, this encapsulation is for tunneling in overlay logical network such as VXLAN. The encapsulated packet  1674  is tunneled to the destination host machine  222 . 
     Once the encapsulated packet  1674  reaches the host machine  222 , the host machine uses the VPN encryption key  252  to decrypt the encrypted payload  1671 . If the host machine  222  does not have the key, it would perform an ARP like operation and queries the controller for the key based on either the VNI or the destination IP. The decryption results in a decrypted payload  1675 , which is provided to the destination VM  262 . 
     For some embodiments,  FIG. 17  conceptually illustrates a process  1700  for forwarding VPN encrypted packet at an edge node. In some embodiments, the process  1700  is performed by an edge of the datacenter such as the edge node  220 . 
     The process  1700  starts when it receives (at  1710 ) a packet from outside of the network/datacenter. In some embodiments, the payload of this packet is encrypted based on a VPN encryption key. In some embodiments, the packet is an IPSec packet. 
     Next, the process identifies ( 1720 ) a VPN encryption key based on the header data of the packet. In some embodiments in which the packet is an IPSec packet, the header of the IPSec packet is not encrypted. Such a packet header in some embodiments includes information that can be used to identify VPN encryption key. In some embodiments, these indication includes the flow/L4 connection of the IPSec packet. Consequently, the process is able to identify the encryption key based on the indication provided by the header by e.g., using the flow identifier of the IPSec packet to identify the corresponding VPN encryption key. 
     The process then uses ( 1730 ) the identified key to decrypt the starting bytes of the encrypted payload in order to reveal these bytes to the edge node. In some embodiments, the starting bytes of the encrypted payload include information that can be used to determine the next hop after the edge node, information such as destination IP address, destination VNI, destination VTEP, destination MAC address, etc. The process then uses the decrypted bytes to identify (at  1740 ) the next hop information. In some embodiments, the process performs L3 routing operations based on the information in the revealed bytes (e.g., destination IP address) in order to identify the destination VNI, destination VTEP, or next hop MAC. 
     Next, the process encapsulates ( 1750 ) packets based on the identified VNI. In some embodiments, the encrypted payload of the IPSec is encapsulated under VXLAN format based on the earlier identified information (e.g., destination VNI and VTEP). 
     The process then forwards ( 1760 ) the encapsulated packet to the identified destination (e.g., a host machine as the VTEP). The process  1700  then ends. 
     V. Computing Device and Virtualization Software 
       FIG. 18  illustrates a computing device  1800  that serves as a host machine or edge gateway (i.e., VPN gateway or VPN server) for some embodiments of the invention. The computing device  1800  is running virtualization software that implements a physical switching element and a set of physical routing elements. (i.e., MPSE and MPREs). 
     As illustrated, the computing device  1800  has access to a physical network  1890  through a physical NIC (PNIC)  1895 . The host machine  1800  also runs the virtualization software  1805  and hosts VMs  1811 - 1814 . The virtualization software  1805  serves as the interface between the hosted VMs and the physical NIC  1895  (as well as other physical resources, such as processors and memory). Each of the VMs includes a virtual NIC (VNIC) for accessing the network through the virtualization software  1805 . Each VNIC in a VM is responsible for exchanging packets between the VM and the virtualization software  1805 . In some embodiments, the VNICs are software abstractions of physical NICs implemented by virtual NIC emulators. 
     The virtualization software  1805  manages the operations of the VMs  1811 - 1814 , and includes several components for managing the access of the VMs to the physical network (by implementing the logical networks to which the VMs connect, in some embodiments). As illustrated, the virtualization software includes several components, including a MPSE  1820 , a set of MPREs  1830 , a controller agent  1840 , a VTEP  1850 , a crypto engine  1875 , and a set of uplink pipelines  1870 . 
     The VTEP (VXLAN tunnel endpoint)  1850  allows the host machine  1800  to serve as a tunnel endpoint for logical network traffic (e.g., VXLAN traffic). VXLAN is an overlay network encapsulation protocol. An overlay network created by VXLAN encapsulation is sometimes referred to as a VXLAN network, or simply VXLAN. When a VM on the host  1800  sends a data packet (e.g., an ethernet frame) to another VM in the same VXLAN network but on a different host, the VTEP will encapsulate the data packet using the VXLAN network&#39;s VNI and network addresses of the VTEP, before sending the packet to the physical network. The packet is tunneled through the physical network (i.e., the encapsulation renders the underlying packet transparent to the intervening network elements) to the destination host. The VTEP at the destination host decapsulates the packet and forwards only the original inner data packet to the destination VM. In some embodiments, the VTEP module serves only as a controller interface for VXLAN encapsulation, while the encapsulation and decapsulation of VXLAN packets is accomplished at the uplink module  1870 . 
     The controller agent  1840  receives control plane messages from a controller or a cluster of controllers. In some embodiments, these control plane message includes configuration data for configuring the various components of the virtualization software (such as the MPSE  1820  and the MPREs  1830 ) and/or the virtual machines. In the example illustrated in  FIG. 18 , the controller agent  1840  receives control plane messages from the controller cluster  1860  from the physical network  1890  and in turn provides the received configuration data to the MPREs  1830  through a control channel without going through the MPSE  1820 . However, in some embodiments, the controller agent  1840  receives control plane messages from a direct data conduit (not illustrated) independent of the physical network  1890 . In some other embodiments, the controller agent receives control plane messages from the MPSE  1820  and forwards configuration data to the router  1830  through the MPSE  1820 . In some embodiments, the controller agent  1840  also serve as the DNE agent of the host machine, responsible for receiving VPN encryption keys from a key manager (which can be the controller). Distribution of encryption keys under DNE is described by reference to  FIG. 14  above. 
     The MPSE  1820  delivers network data to and from the physical NIC  1895 , which interfaces the physical network  1890 . The MPSE also includes a number of virtual ports (vPorts) that communicatively interconnects the physical NIC with the VMs  1811 - 1814 , the MPREs  1830  and the controller agent  1840 . Each virtual port is associated with a unique L2 MAC address, in some embodiments. The MPSE performs L2 link layer packet forwarding between any two network elements that are connected to its virtual ports. The MPSE also performs L2 link layer packet forwarding between any network element connected to any one of its virtual ports and a reachable L2 network element on the physical network  1890  (e.g., another VM running on another host). In some embodiments, a MPSE is a local instantiation of a logical switching element (LSE) that operates across the different host machines and can perform L2 packet switching between VMs on a same host machine or on different host machines. In some embodiments, the MPSE performs the switching function of several LSEs according to the configuration of those logical switches. 
     The MPREs  1830  perform L3 routing on data packets received from a virtual port on the MPSE  1820 . In some embodiments, this routing operation entails resolving L3 IP address to a next-hop L2 MAC address and a next-hop VNI (i.e., the VNI of the next-hop&#39;s L2 segment). Each routed data packet is then sent back to the MPSE  1820  to be forwarded to its destination according to the resolved L2 MAC address. This destination can be another VM connected to a virtual port on the MPSE  1820 , or a reachable L2 network element on the physical network  1890  (e.g., another VM running on another host, a physical non-virtualized machine, etc.). 
     As mentioned, in some embodiments, a MPRE is a local instantiation of a logical routing element (LRE) that operates across the different host machines and can perform L3 packet forwarding between VMs on a same host machine or on different host machines. In some embodiments, a host machine may have multiple MPREs connected to a single MPSE, where each MPRE in the host machine implements a different LRE. MPREs and MPSEs are referred to as “physical” routing/switching element in order to distinguish from “logical” routing/switching elements, even though MPREs and MPSE are implemented in software in some embodiments. In some embodiments, a MPRE is referred to as a “software router” and a MPSE is referred to a “software switch”. In some embodiments, LREs and LSEs are collectively referred to as logical forwarding elements (LFEs), while MPREs and MPSEs are collectively referred to as managed physical forwarding elements (MPFEs). 
     In some embodiments, the MPRE  1830  includes one or more logical interfaces (LIFs) that each serves as an interface to a particular segment (L2 segment or VXLAN) of the network. In some embodiments, each LIF is addressable by its own IP address and serve as a default gateway or ARP proxy for network nodes (e.g., VMs) of its particular segment of the network. In some embodiments, all of the MPREs in the different host machines are addressable by a same “virtual” MAC address (or vMAC), while each MPRE is also assigned a “physical” MAC address (or pMAC) in order indicate in which host machine does the MPRE operate. 
     The crypto engine  1875  applies encryption key to decrypt incoming data from the physical network and to encrypt outgoing data to the physical network  1890 . In some embodiments, a controller sends the encryption key to the virtualization software  1805  through control plane messages, and the crypto engine  1875  identifies a corresponding key from among the received keys for decrypting incoming packets and for encrypting outgoing packets. In some embodiments, the controller agent  1840  receives the control plane messages, and the keys delivered by the control plane messages is stored in a key store  1878  that can be accessed by the crypto engine  1875 . 
     The uplink module  1870  relays data between the MPSE  1820  and the physical NIC  1895 . The uplink module  1870  includes an egress chain and an ingress chain that each performs a number of operations. Some of these operations are pre-processing and/or post-processing operations for the MPRE  1830 . The operations of LIFs, uplink module, MPSE, and MPRE are described in U.S. patent application Ser. No. 14/137,862 filed on Dec. 20, 2013, titled “Logical Router”, published as U.S. Patent Application Publication 2015/0106804. 
     As illustrated by  FIG. 18 , the virtualization software  1805  has multiple MPREs for multiple different LREs. In a multi-tenancy environment, a host machine can operate virtual machines from multiple different users or tenants (i.e., connected to different logical networks). In some embodiments, each user or tenant has a corresponding MPRE instantiation of its LRE in the host for handling its L3 routing. In some embodiments, though the different MPREs belong to different tenants, they all share a same vPort on the MPSE  1820 , and hence a same L2 MAC address (vMAC or pMAC). In some other embodiments, each different MPRE belonging to a different tenant has its own port to the MPSE. 
     The MPSE  1820  and the MPRE  1830  make it possible for data packets to be forwarded amongst VMs  1811 - 1814  without being sent through the external physical network  1890  (so long as the VMs connect to the same logical network, as different tenants&#39; VMs will be isolated from each other). Specifically, the MPSE performs the functions of the local logical switches by using the VNIs of the various L2 segments (i.e., their corresponding L2 logical switches) of the various logical networks. Likewise, the MPREs perform the function of the logical routers by using the VNIs of those various L2 segments. Since each L2 segment/L2 switch has its own a unique VNI, the host machine  1800  (and its virtualization software  1805 ) is able to direct packets of different logical networks to their correct destinations and effectively segregates traffic of different logical networks from each other. 
     VI. Electronic Device 
     Many of the above-described features and applications are implemented as software processes that are specified as a set of instructions recorded on a computer readable storage medium (also referred to as computer readable medium). When these instructions are executed by one or more processing unit(s) (e.g., one or more processors, cores of processors, or other processing units), they cause the processing unit(s) to perform the actions indicated in the instructions. Examples of computer readable media include, but are not limited to, CD-ROMs, flash drives, RAM chips, hard drives, EPROMs, etc. The computer readable media does not include carrier waves and electronic signals passing wirelessly or over wired connections. 
     In this specification, the term “software” is meant to include firmware residing in read-only memory or applications stored in magnetic storage, which can be read into memory for processing by a processor. Also, in some embodiments, multiple software inventions can be implemented as sub-parts of a larger program while remaining distinct software inventions. In some embodiments, multiple software inventions can also be implemented as separate programs. Finally, any combination of separate programs that together implement a software invention described here is within the scope of the invention. In some embodiments, the software programs, when installed to operate on one or more electronic systems, define one or more specific machine implementations that execute and perform the operations of the software programs. 
       FIG. 19  conceptually illustrates an electronic system  1900  with which some embodiments of the invention are implemented. The electronic system  1900  can be used to execute any of the control, virtualization, or operating system applications described above. The electronic system  1900  may be a computer (e.g., a desktop computer, personal computer, tablet computer, server computer, mainframe, a blade computer etc.), phone, PDA, or any other sort of electronic device. Such an electronic system includes various types of computer readable media and interfaces for various other types of computer readable media. Electronic system  1900  includes a bus  1905 , processing unit(s)  1910 , a system memory  1925 , a read-only memory  1930 , a permanent storage device  1935 , input devices  1940 , and output devices  1945 . 
     The bus  1905  collectively represents all system, peripheral, and chipset buses that communicatively connect the numerous internal devices of the electronic system  1900 . For instance, the bus  1905  communicatively connects the processing unit(s)  1910  with the read-only memory  1930 , the system memory  1925 , and the permanent storage device  1935 . 
     From these various memory units, the processing unit(s)  1910  retrieves instructions to execute and data to process in order to execute the processes of the invention. The processing unit(s) may be a single processor or a multi-core processor in different embodiments. 
     The read-only-memory (ROM)  1930  stores static data and instructions that are needed by the processing unit(s)  1910  and other modules of the electronic system. The permanent storage device  1935 , on the other hand, is a read-and-write memory device. This device is a non-volatile memory unit that stores instructions and data even when the electronic system  1900  is off. Some embodiments of the invention use a mass-storage device (such as a magnetic or optical disk and its corresponding disk drive) as the permanent storage device  1935 . 
     Other embodiments use a removable storage device (such as a floppy disk, flash drive, etc.) as the permanent storage device. Like the permanent storage device  1935 , the system memory  1925  is a read-and-write memory device. However, unlike storage device  1935 , the system memory is a volatile read-and-write memory, such a random access memory. The system memory stores some of the instructions and data that the processor needs at runtime. In some embodiments, the invention&#39;s processes are stored in the system memory  1925 , the permanent storage device  1935 , and/or the read-only memory  1930 . From these various memory units, the processing unit(s)  1910  retrieves instructions to execute and data to process in order to execute the processes of some embodiments. 
     The bus  1905  also connects to the input and output devices  1940  and  1945 . The input devices enable the user to communicate information and select commands to the electronic system. The input devices  1940  include alphanumeric keyboards and pointing devices (also called “cursor control devices”). The output devices  1945  display images generated by the electronic system. The output devices include printers and display devices, such as cathode ray tubes (CRT) or liquid crystal displays (LCD). Some embodiments include devices such as a touchscreen that function as both input and output devices. 
     Finally, as shown in  FIG. 19 , bus  1905  also couples electronic system  1900  to a network  1965  through a network adapter (not shown). In this manner, the computer can be a part of a network of computers (such as a local area network (“LAN”), a wide area network (“WAN”), or an Intranet, or a network of networks, such as the Internet. Any or all components of electronic system  1900  may be used in conjunction with the invention. 
     Some embodiments include electronic components, such as microprocessors, storage and memory that store computer program instructions in a machine-readable or computer-readable medium (alternatively referred to as computer-readable storage media, machine-readable media, or machine-readable storage media). Some examples of such computer-readable media include RAM, ROM, read-only compact discs (CD-ROM), recordable compact discs (CD-R), rewritable compact discs (CD-RW), read-only digital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a variety of recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD-RW, etc.), flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.), magnetic and/or solid state hard drives, read-only and recordable Blu-Ray® discs, ultra density optical discs, any other optical or magnetic media, and floppy disks. The computer-readable media may store a computer program that is executable by at least one processing unit and includes sets of instructions for performing various operations. Examples of computer programs or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. 
     While the above discussion primarily refers to microprocessor or multi-core processors that execute software, some embodiments are performed by one or more integrated circuits, such as application specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs). In some embodiments, such integrated circuits execute instructions that are stored on the circuit itself. 
     As used in this specification, the terms “computer”, “server”, “processor”, and “memory” all refer to electronic or other technological devices. These terms exclude people or groups of people. For the purposes of the specification, the terms display or displaying means displaying on an electronic device. As used in this specification, the terms “computer readable medium,” “computer readable media,” and “machine readable medium” are entirely restricted to tangible, physical objects that store information in a form that is readable by a computer. These terms exclude any wireless signals, wired download signals, and any other ephemeral signals. 
     In this document, the term “packet” refers to a collection of bits in a particular format sent across a network. One of ordinary skill in the art will recognize that the term packet may be used herein to refer to various formatted collections of bits that may be sent across a network, such as Ethernet frames, TCP segments, UDP datagrams, IP packets, etc. 
     This specification refers throughout to computational and network environments that include virtual machines (VMs). However, virtual machines are merely one example of data compute nodes (DCNs) or data compute end nodes, also referred to as addressable nodes. DCNs may include non-virtualized physical hosts, virtual machines, containers that run on top of a host operating system without the need for a hypervisor or separate operating system, and hypervisor kernel network interface modules. 
     VMs, in some embodiments, operate with their own guest operating systems on a host using resources of the host virtualized by virtualization software (e.g., a hypervisor, virtual machine monitor, etc.). The tenant (i.e., the owner of the VM) can choose which applications to operate on top of the guest operating system. Some containers, on the other hand, are constructs that run on top of a host operating system without the need for a hypervisor or separate guest operating system. In some embodiments, the host operating system uses name spaces to isolate the containers from each other and therefore provides operating-system level segregation of the different groups of applications that operate within different containers. This segregation is akin to the VM segregation that is offered in hypervisor-virtualized environments that virtualize system hardware, and thus can be viewed as a form of virtualization that isolates different groups of applications that operate in different containers. Such containers are more lightweight than VMs. 
     Hypervisor kernel network interface modules, in some embodiments, is a non-VM DCN that includes a network stack with a hypervisor kernel network interface and receive/transmit threads. One example of a hypervisor kernel network interface module is the vmknic module that is part of the ESXi™ hypervisor of VMware, Inc. 
     One of ordinary skill in the art will recognize that while the specification refers to VMs, the examples given could be any type of DCNs, including physical hosts, VMs, non-VM containers, and hypervisor kernel network interface modules. In fact, the example networks could include combinations of different types of DCNs in some embodiments. 
     While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. In addition, a number of the figures (including  FIGS. 10, 11, and 14 ) conceptually illustrate processes. The specific operations of these processes may not be performed in the exact order shown and described. The specific operations may not be performed in one continuous series of operations, and different specific operations may be performed in different embodiments. Furthermore, the process could be implemented using several sub-processes, or as part of a larger macro process. Thus, one of ordinary skill in the art would understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.