Enabling planned upgrade/downgrade of network devices without impacting network sessions

Abstract

Exemplary embodiments for enabling planned network changes such as an upgrade or downgrade of a network device are disclosed. The systems and methods provide for planned upgrades and downgrades for network devices without impacting existing network sessions, by utilizing two network devices simultaneously, and creating a redirect network session for a predetermined period of time. In so doing, all network traffic may be gradually transferred to the second network device, until the sessions processed by the first network device time out. The first network device can then be taken offline for upgrade or downgrade, without any disruption to the network service or loss of network traffic.

Description

TECHNICAL FIELD

The present disclosure relates generally to data processing, and, more specifically, to the operation of a network device during planned upgrades or downgrades of the network device.

BACKGROUND

The approaches described in this section could be pursued but are not necessarily approaches that have previously been conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.

Websites, web and mobile applications, cloud computing, and various web and mobile services have been rising in popularity. Some examples of fast growing consumer services include smart phone applications, location based services, navigation services, e-book services, video applications, music applications, Internet television services, Voice over IP, and so forth. Subsequently, more and more servers hosting these applications are deployed within data networks including the Internet to accommodate the increasing computing and data storage needs. These servers are typically arranged in data centers or web farms, which may include intermediate network devices such as Application Delivery Controllers (ADC), Global Server Load Balancers (GSLB) and/or Server Load Balancers (SLB).

In a typical load balancing scenario, an application or service hosted by a group of servers is front-ended by a load balancer (LB) (also referred to herein as a LB device) which represents this service to clients as a virtual service. Clients needing the service can address their packets to the virtual service using a virtual Internet Protocol (IP) address and a virtual port. For example, www.example.com:80 is a service that is being load balanced and there is a group of servers that host this service. An LB can be configured with a virtual IP (VIP) e.g. 100.100.100.1 and virtual port (VPort) e.g. Port 80, which, in turn, are mapped to the IP addresses and port numbers of the servers handling this service. The Domain Name Service (DNS) server handling this domain can be configured to send packets to the VIP and VPort associated with this LB.

Once an ADC, or any other network device, has been deployed in a network, it may need to be upgraded or downgraded for any number of reasons. For services that need to operate continuously, any change in the intermediate network device will disrupt the flow of traffic over the network and the user's ability to access the service over the network through a client device. Data may also be lost in transit. This disruption in service may affect the quality of the service, as well as increase the response time for the client. Furthermore, once a network device has had a software update, it may need to be restarted, which will cause the existing session to be lost.

Additionally, Layer 7 network sessions may be particularly data-intensive. Thus, it may not be feasible to prepare a network device providing an application layer service for an upgrade or downgrade by copying all of the data from the first network device to a second network device. For example, a client device may be streaming a video over the network, or accessing an encrypted file. Copying the entire network session to a second network device to also provide the streaming video or encrypted file for all users may use too many resources.

Thus, a mechanism is needed whereby planned changes may be made to a network device within a network without resulting in disruption to the service being provided by the network device.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described in the Detailed Description below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

The present disclosure is related to approaches for conducting a planned upgrade or downgrade of a first network device without impacting the network sessions that are handled by that device. The method may comprise synchronizing a TCP session between a client device and the first network device with a second network device, the second device being a standby device, and creating a redirect network session for the TCP session at the second network device. During the planned network change, the second network device may become the active network device and receive the server response. The second network device may recognize that the server response is associated with the redirect network session, and redirect the server response to the first network device to be forwarded to the client device. The second network device may also handle all new service requests from the client device, since it now the active device for the service. The method may continue until all existing sessions handled by the first network device are completed or time out. The first network device may then be upgraded or downgraded without impacting any existing network sessions.

In further example embodiments of the present disclosure, the method steps are stored on a machine-readable medium comprising instructions, which when implemented by one or more processors perform the recited steps. In yet further example embodiments, hardware systems, or devices can be adapted to perform the recited steps. Other features, examples, and embodiments are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, in which like references indicate similar elements.

FIG. 1 shows an environment within which a service may be provided to a user from one or more servers over a network.

FIG. 2 shows a flow for a data packet during a planned network change for an existing service, according to an example embodiment.

FIG. 3 shows a flow for a data packet during a planned network change, utilizing dual operation of two network devices to provide the service, according to an example embodiment.

FIG. 4 shows a diagrammatic representation of a computing device for a machine in the example electronic form of a computer system, within which a set of instructions for causing the machine to perform any one or more of the methodologies discussed herein can be executed.

DETAILED DESCRIPTION

The following detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show illustrations in accordance with example embodiments. These example embodiments, which are also referred to herein as “examples,” are described in enough detail to enable those skilled in the art to practice the present subject matter. The embodiments can be combined, other embodiments can be utilized, or structural, logical, and electrical changes can be made without departing from the scope of what is claimed. The following detailed description is therefore not to be taken in a limiting sense, and the scope is defined by the appended claims and their equivalents. In this document, the terms “a” and “an” are used, as is common in patent documents, to include one or more than one. In this document, the term “or” is used to refer to a nonexclusive “or,” such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated.

Embodiments disclosed herein may be implemented using a variety of technologies. For example, the methods described herein may be implemented in software executing on a computer system or in hardware utilizing either a combination of microprocessors or other specially designed application-specific integrated circuits (ASICs), programmable logic devices like FPGA's, or various combinations thereof. In particular, the methods described herein may be implemented by a series of computer-executable instructions residing on a storage medium such as a disk drive, or computer-readable medium. It should be noted that methods disclosed herein can be implemented by a computer, e.g., a desktop computer, tablet computer, laptop computer, smartphone and so forth.

The present technology provides various methods for operation of ADCs and GSLBs in data networks such as the Internet including a plurality of switches, routers, virtual switches, web farms, host servers, and other units. The present technology provides enhanced performance of ADC and allows implementing scalable business solutions for any services, applications, clouds and organizations. Furthermore, the present technology provides a scalable, high-performance application networking platform, which can deliver superior reliability and energy efficiency at lower total cost of ownership. An ADC can also provide increased infrastructure efficiency, a faster end user experience, comprehensive Layer 4-7 feature set and flexible virtualization technologies such as Virtual Chassis System, multi-tenancy, and more for public, private and hybrid cloud environments. The ADC and GSLB may include software and/or hardware components/platforms that may vary depending on a particular application, performance, infrastructure, network capacity, data traffic parameters, and so forth. The functionality of application delivery controllers and load balancers are also described in more detail in U.S. patent application Ser. No. 13/791,760 entitled “Application Delivery Controller and Global Server Load Balancer” which is incorporated herein by reference in its entirety.

Exemplary embodiments of the presently disclosed technology are deployed on a Layer 7 TCP/IP network. A TCP network session may be established between any two devices in the network via the TCP “handshake”. This is described in more detail in U.S. patent application Ser. No. 13/413,191 entitled “System and Method for an Adaptive TCP Syn Cookie with Time Validation” which is incorporated herein by reference in its entirety.

Referring now to the drawings, FIG. 1 illustrates an environment 100 within which a service may be provided to a user from one or more servers over a network. The environment 100 may include a network 110, a client 120, a client device 130, one or more network devices 160 for distributing network traffic, and one or more servers 140. The client 120 may include a user or a host associated with the network 110.

The network 110 may include the Internet or any other network capable of communicating data between devices. Suitable networks may include or interface with any one or more of, for instance, a local intranet, a PAN (Personal Area Network), a LAN (Local Area Network), a WAN (Wide Area Network), a MAN (Metropolitan Area Network), a virtual private network (VPN), a storage area network (SAN), a frame relay connection, an Advanced Intelligent Network (AIN) connection, a synchronous optical network (SONET) connection, a digital T1, T3, E1 or E3 line, Digital Data Service (DDS) connection, DSL (Digital Subscriber Line) connection, an Ethernet connection, an ISDN (Integrated Services Digital Network) line, a dial-up port such as a V.90, V.34 or V.34bis analog modem connection, a cable modem, an ATM (Asynchronous Transfer Mode) connection, or an FDDI (Fiber Distributed Data Interface) or CDDI (Copper Distributed Data Interface) connection. Furthermore, communications may also include links to any of a variety of wireless networks, including WAP (Wireless Application Protocol), GPRS (General Packet Radio Service), GSM (Global System for Mobile Communication), CDMA (Code Division Multiple Access) or TDMA (Time Division Multiple Access), cellular phone networks, GPS (Global Positioning System), CDPD (cellular digital packet data), RIM (Research in Motion, Limited) duplex paging network, Bluetooth radio, or an IEEE 802.11-based radio frequency network.

The network 110 can further include or interface with any one or more of an RS-232 serial connection, an IEEE-1394 (Firewire) connection, a Fiber Channel connection, an IrDA (infrared) port, a SCSI (Small Computer Systems Interface) connection, a USB (Universal Serial Bus) connection or other wired or wireless, digital or analog interface or connection, mesh or Digi® networking. The network 110 may include a network of data processing nodes that are interconnected for the purpose of data communication. The network 110 may include software driven network (SDN). The SDN may include one or more of the above network types. Generally, the network 110 may include a number of similar or dissimilar devices connected together by a transport medium enabling communication between the devices by using a predefined protocol. Those skilled in the art will recognize that the present disclosure may be practiced within a variety of network configuration environments and on a variety of computing devices.

As shown in FIG. 1, the client 120 may send one or more service requests 150 to (backend) servers 140 through a client device 130. The service requests 150 may include an HTTP request, a video streaming request, a file download request, a transaction request, a conference request, or any other service provided over a network. The client device 130 may include an end user computer, mobile phone, tablet, thin client, or any other device from which a user may access the service.

The servers 140 may include a web server, a wireless application server, an interactive television server, and so forth. The network device(s) 160 may include an ADC, GSLB, LB, or any other mechanism for service load distribution. The network device 160 may balance the flow of the service requests 150 among traffic forwarding devices of the network 110. The load balancing may enhance utilization of resources and enable maximize throughput with minimum response time, hence avoiding overloading of a single server. With this technology, network traffic may be distributed among different web farms, data centers, and servers 140 located at different geographical locations. Furthermore, as will be appreciated by those skilled in the art, network device 160 may act as a master to monitor “health” and responsiveness of services hosted by the servers 140.

The network device 160 may also analyze the flow of the service requests 150 and determine which and how many traffic forwarding devices of the network 110 are needed to deliver the service requests 150 to the servers 140.

The network device 160 may also inspect incoming data packets and apply data policies or algorithms to determine the server(s) to deliver the service requests to. The policy may be a forwarding policy, security policy, service policy, or any other type of policy. In exemplary embodiments, the network device 160 may also modify the data packets as necessary before delivering the service requests 150 to the servers 140. The network device 160 may also inspect data packets and modify them as necessary in the server to client traffic direction.

FIG. 2 illustrates an exemplary flow for a data packet during a planned network change for an existing service provided to a user from one or more servers over a network. The steps of the flow may be performed in varying orders or concurrently. In various embodiments, the flow illustrated in FIG. 2 may apply to data packets of Layer 7 sessions. In flow 210, a client device 130 conducts a TCP handshake with network device A 160A and establishes a TCP network session with network device 160A. The TCP handshake may comprise the exchange of a SYN packet, SYN/ACK packet, and ACK packet, as understood by a person of ordinary skill in the art. In flow 220, the client device 130 may then send a service request to network device 160A. The service request may be for a virtual IP address, HTTP, GET, etc. In exemplary embodiments, network device 160A may then determine which server to deliver the service request to by conducting load balancing on a plurality of servers that are designated for the service. Network device 160A may then conduct a TCP handshake with the designated server 140 to establish a TCP network session between these two devices in flow 230. Alternatively, network device 160A may use an existing TCP network session with a server 140. The network device 160A may then deliver the service request to the server(s) in flow 240. The server 140 may be a server computer, load balancer, ADC, or any other network component.

During normal operation, the server 140 may then process the service request 150 and generate a server response 250, which is delivered to network device 160A, and forwarded to client device 130. One or more security, forwarding, or other policies may also be employed at any stage of this process.

During a planned network change (such as an upgrade or downgrade of network device 160A), the network administrator may choose to remove network device 160A as the active device for any reason, and configure an additional network device 160B as the active device. Network device 160B may be a surplus network device previously connected to the network 110, or a new device added to the network for the planned network change. Network device 160B may have previously been configured as a backup for network device 160A, or may have been an active device for a different virtual service.

In an exemplary embodiment, once network device 160B has been configured as the active device for the virtual service, the server response 250 is delivered to network device 160B via flow 260. However, since network device 160B does not have the open TCP session with client device 130 for the service request 150, it will not recognize the network session for the server response 250. As such, network device 160B may not know where to deliver server response 250 and may simply drop the data packet(s) from the server, resulting in a loss of data. When client device 130 does not receive server response 250 after a designated amount of time, it may have to re-send service request 150 to start the process of requesting the service again.

FIG. 3 illustrates an exemplary embodiment of a flow for a data packet during a planned network change, utilizing dual operation of two network devices to provide the service over the network. The steps of the flow may be performed in varying orders or concurrently. In various embodiments, the flow illustrated in FIG. 3 may apply to data packets of Layer 7 sessions.

In preparation for a planned network change, such as an upgrade or downgrade, of network device 160A, a second network device 160B may be deployed as a standby or backup device to the active network device 160A. In exemplary embodiments, the standby network device 160B may be upgraded or downgrade first, before being deployed in the network. Alternatively, standby network device 160B may be upgraded or downgraded after network device 160A.

In an example embodiment, a client device 130 may conduct a TCP handshake with network device 160A and establish a TCP network session with network device 160A in flow 310. The TCP handshake may comprise the exchange of a SYN packet, SYN/ACK packet, and ACK packet, as understood by a person of ordinary skill in the art. In flow 320, network device 160A may then send information to the standby network device 160B to create a similar, local session on network device 160B. Network device 160B may then create a redirect TCP session, such that it can recognize incoming traffic originally destined for network device 160A as needing to be redirected to network device 160A. The redirect session created at network device 160B may comprise all of the data contained in the network session, or may contain only certain identifying information needed to recognize the network session. For example, the redirect session may contain only the source IP address, destination IP address, source port, destination port, and network protocol. Fewer or additional components may be a part of the redirect session, as will be understood by a person of ordinary skill in the art.

In flow 330, client device 130 may then send a service request 150 to network device 160A, since it is still the active network device for the service. The service request may be for a virtual IP address, HTTP, GET, etc. In exemplary embodiments, network device 160A may determine which server to deliver the service request to by conducting load balancing on a plurality of servers that are designated for the service. Network device 160A may then conduct a TCP handshake with the designated server 140 to establish a TCP network session between these two devices in flow 340. Alternatively, network device 160A may use an existing TCP network session with a server 140. The network device 160A may then deliver the service request to the server(s) in flow 350. The server 140 may be a server computer, load balancer, ADC, or any other network component.

In an example embodiment, the network administrator may decide to switch the active network device for the virtual service from network device 160A to network device 160B, in order to prepare network device 160A for upgrade. When the server 140 processes the service request 150, it may generate a server response 250, which may then be delivered to network device 160B in flow 360, since this is now the active device for the service. Network device 160B may then recognize information in the data packet(s) of server response 250 as being a part of the redirect session that originated from network device 160A. Network device 160B may then redirect server response 250 to network device 160A in flow 370, since that is the device for which the client device 130 has established the TCP session. In exemplary embodiments, network device 160B may also conduct a session lookup to match the received data in flow 360 with the redirect session entry created in flow 320. Network device 160B may match one or more of source IP address, destination IP address, source port, destination port, protocol, or any other information from the network session. The server response 250 may then be delivered to client device 130 in flow 380. One or more security, forwarding, or other policies may also be employed at any stage of this process as well.

In various embodiments, after network device 160B has been configured as the active device for the service, a new service request 150 from client device 130 may be directed to network device 160B, since it is now the active device. In these embodiments, network device 160B may receive the service request 150 from client device 130 and deliver the request to the server 140. Network device 160B may also receive the server response 250 and deliver it to the client device 130. However, a server response 250 from a service request 150 that was previously received by network device 160A, may continue to be redirected to network device 160A. As such, the two network devices 160A and 160B may both be handling service requests from client device 130 for a period of time.

The dual operation of network devices 160A and 160B may continue until all of the existing sessions from network device 160A have timed out or are completed. Once network device 160A has finished processing all of its existing sessions, it may be ready for upgrade or downgrade. Existing sessions from network device 160A may also be marked to time out faster or slower so that the planned network change can occur at a specific time. By waiting until network device 160A no longer has any open network sessions, the device may be upgraded or downgraded without any resulting data loss in the network. Additionally, client device 130 may receive the service seamlessly and without disruption over the network.

While embodiments have been described herein in the context of a single client 120 and client device 130, it will be understood by persons of ordinary skill in the art that any number of clients and client devices may be able to access a service of a network in a similar manner. Additionally, while the example embodiments have been depicted in FIGS. 2 and 3 with a single active network device and a single standby network device, any number of network devices may be utilized in a similar manner.

FIG. 4 shows a diagrammatic representation of a machine in the example electronic form of a computer system 400, within which a set of instructions for causing the machine to perform any one or more of the methodologies discussed herein may be executed. In various example embodiments, the machine operates as a standalone device or may be connected (e.g., networked) to other machines. In a networked deployment, the machine may operate in the capacity of a server or a client machine in a server-client network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a PC, a tablet PC, a set-top box (STB), a cellular telephone, a portable music player (e.g., a portable hard drive audio device such as a Moving Picture Experts Group Audio Layer 3 (MP3) player), a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein.

The example computer system 400 includes a processor or multiple processors 402 (e.g., a central processing unit (CPU), a graphics processing unit (GPU), or both), a main memory 404 and a static memory 406, which communicate with each other via a bus 408. The computer system 400 may further include a video display unit 410 (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)). The computer system 400 may also include an alpha-numeric input device 412 (e.g., a keyboard), a cursor control device 414 (e.g., a mouse), a disk drive unit 416, a signal generation device 418 (e.g., a speaker), and a network interface device 420.

The disk drive unit 416 includes a non-transitory computer-readable medium 422, on which is stored one or more sets of instructions and data structures (e.g., instructions 424) embodying or utilized by any one or more of the methodologies or functions described herein. The instructions 424 may also reside, completely or at least partially, within the main memory 404 and/or within the processors 402 during execution thereof by the computer system 400. The main memory 404 and the processors 402 may also constitute machine-readable media.

The instructions 424 may further be transmitted or received over a network 426 via the network interface device 420 utilizing any one of a number of well-known transfer protocols (e.g., Hyper Text Transfer Protocol (HTTP)).

While the computer-readable medium 422 is shown in an example embodiment to be a single medium, the term “computer-readable medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database and/or associated caches and servers) that store the one or more sets of instructions. The term “computer-readable medium” shall also be taken to include any medium that is capable of storing, encoding, or carrying a set of instructions for execution by the machine and that causes the machine to perform any one or more of the methodologies of the present application, or that is capable of storing, encoding, or carrying data structures utilized by or associated with such a set of instructions. The term “computer-readable medium” shall accordingly be taken to include, but not be limited to, solid-state memories, optical and magnetic media, and carrier wave signals. Such media may also include, without limitation, hard disks, floppy disks, flash memory cards, digital video disks, random access memory (RAMs), read only memory (ROMs), and the like.

The example embodiments described herein can be implemented in an operating environment comprising computer-executable instructions (e.g., software) installed on a computer, in hardware, or in a combination of software and hardware. The computer-executable instructions can be written in a computer programming language or can be embodied in firmware logic. If written in a programming language conforming to a recognized standard, such instructions can be executed on a variety of hardware platforms and for interfaces to a variety of operating systems. Although not limited thereto, computer software programs for implementing the present method can be written in any number of suitable programming languages such as, for example, Hypertext Markup Language (HTML), Dynamic HTML, Extensible Markup Language (XML), Extensible Stylesheet Language (XSL), Document Style Semantics and Specification Language (DSSSL), Cascading Style Sheets (CSS), Synchronized Multimedia Integration Language (SMIL), Wireless Markup Language (WML), Java™, Jini™, C, C++, Perl, UNIX Shell, Visual Basic or Visual Basic Script, Virtual Reality Markup Language (VRML), ColdFusion™ or other compilers, assemblers, interpreters or other computer languages or platforms.

Thus, methods and systems for enabling planned upgrades and downgrades of a network device with minimum to no impact on network sessions are disclosed. Although embodiments have been described with reference to specific example embodiments, it will be evident that various modifications and changes can be made to these example embodiments without departing from the broader spirit and scope of the present application. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

Claims

1. A method for enabling a planned upgrade or a planned downgrade of a first network device, the method comprising: establishing, at the first network device, a TCP session with a client device;receiving, at the first network device, a request for a virtual service over a network by the client device;creating a redirect network session for the TCP session at a second network device, the second network device being a standby device for the first network device;delivering, by the first network device, the request for the virtual service over a network to a server;receiving, from a network administrator, a change designating the second network device as an active device for the virtual service;receiving a server response associated with the virtual service at the second network device;recognizing, at the second network device, that the server response is associated with the redirect network session;redirecting, by the second network device, the server response to the first network device in accordance with the redirect network session; andsending, by the first network device, the server response to the client device.

2. The method of claim 1, wherein the redirect network session comprises at least one of a source IP address, a destination IP address, a source port, destination port, and protocol.

3. The method of claim 1, wherein the service includes a virtual IP address.

4. The method of claim 1, wherein the request for the virtual service includes a GET request.

5. The method of claim 1, wherein the delivery of the request for the virtual service over the network to the server by the first network device further comprises load balancing of a plurality of servers.

6. The method of claim 1, wherein the recognizing, at the second network device, that the server response is associated with the redirect network session further comprises conducting a session lookup at the second network device to determine the TCP session that corresponds to the server response received.

7. The method of claim 1, wherein the first network device includes an application delivery controller.

8. The method of claim 1, wherein the first network device includes a global server load balancer.

9. The method of claim 1, wherein the second network device includes an application delivery controller.

10. The method of claim 1, wherein the second network device includes a global server load balancer.

11. The method of claim 1, further comprising upgrading the first network device after the redirect network session times out.

12. A system for enabling a planned upgrade or a planned downgrade of a first network device during a TCP session, the first network device being configured to: establish a TCP session with a client device;receive, by the client device, a request for a virtual service over a network;deliver the request for the virtual service over the network to a server;receive a change request from a network administrator designating a second network device as an active device for the virtual service;receive a redirected server response associated with the virtual service from the second network device; andsend the redirected server response to the client device.

13. The system of claim 12, wherein the virtual service includes a virtual IP address.

14. The system of claim 12, wherein the request for the virtual service includes a GET request.

15. The system of claim 12, wherein the first network device is further configured to load balance a plurality of servers before delivering the request for the virtual service over the network to the server.

16. The system of claim 12, wherein the first network device incudes an application delivery controller.

17. The system of claim 12, wherein the first network device includes a global server load balancer.

18. The system of claim 12, further comprising a second network device configured to: create a redirect network session for the TCP session of the first network device;receive a server response;recognize that the server response is associated with the redirect network session; andredirect the server response to the first network device.

19. The system of claim 18, wherein the redirect network session comprises at least one of a source IP address, a destination IP address, a source port, destination port, and protocol.

20. A non-transitory processor-readable medium having instructions stored thereon, which when executed by one or more processors, cause the one or more processors to perform the following operations: establishing at a first network device, a Transmission Control Protocol (TCP) session with a client device;receiving, at the first network device, a request for a virtual service over a network by the client device;creating a redirect network session for the TCP session at a second network device, the second network device being a standby device for the first network device;delivering, by the first network device, the request for the virtual service over a network to a server;receiving a change from a network administrator designating the second network device as an active device for the virtual service;receiving a server response associated with the virtual service at the second network device;recognizing, at the second network device, that the server response is associated with the redirect network session;redirecting, by the second network device, the server response to the first network device in accordance with the redirect network session; andsending, by the first network device, the server response to the client device.