What is server offload and why do I need it?

One of the tasks of an enterprise architect is to design a framework atop which developers can implement and deploy applications consistently and easily. The consistency is important for internal business continuity and reuse; common objects, operations, and processes can be reused across applications to make development and integration with other applications and systems easier. Architects also often decide where functionality resides and design the base application infrastructure framework. Application server, identity management, messaging, and integration are all often a part of such architecture designs.

Rarely does the architect concern him/herself with the network infrastructure, as that is the purview of “that group”; the “you know who I’m talking about” group. And for the most part there’s no need for architects to concern themselves with network-oriented architecture. Applications should not need to know on which VLAN they will be deployed or what their default gateway might be. But what architects might need to know – and probably should know – is whether the network infrastructure supports “server offload” of some application functions or not, and how that can benefit their enterprise architecture and the applications which will be deployed atop it.

WHAT IT IS

Server offload is a generic term used by the networking industry to indicate some functionality designed to improve the performance or security of applications. We use the term “offload” because the functionality is “offloaded” from the server and moved to an application network infrastructure device instead. Server offload works because the application network infrastructure is almost always these days deployed in front of the web/application servers and is in fact acting as a broker (proxy) between the client and the server. Server offload is generally offered by load balancers and application delivery controllers.

You can think of server offload like a relay race. The application network infrastructure device runs the first leg and then hands off the baton (the request) to the server. When the server is finished, the application network infrastructure device gets to run another leg, and then the race is done as the response is sent back to the client.

There are basically two kinds of server offload functionality:

Protocol processing offload
Protocol processing offload includes functions like SSL termination and TCP optimizations. Rather than enable SSL communication on the web/application server, it can be “offloaded” to an application network infrastructure device and shared across all applications requiring secured communications. Offloading SSL to an application network infrastructure device improves application performance because the device is generally optimized to handle the complex calculations involved in encryption and decryption of secured data and web/application servers are not.

TCP optimization is a little different. We say TCP session management is “offloaded” to the server but that’s really not what happens as obviously TCP connections are still opened, closed, and managed on the server as well. Offloading TCP session management means that the application network infrastructure is managing the connections between itself and the server in such a way as to reduce the total number of connections needed without impacting the capacity of the application. This is more commonly referred to as TCP multiplexing and it “offloads” the overhead of TCP connection management from the web/application server to the application network infrastructure device by effectively giving up control over those connections. By allowing an application network infrastructure device to decide how many connections to maintain and which ones to use to communicate with the server, it can manage thousands of client-side connections using merely hundreds of server-side connections. Reducing the overhead associated with opening and closing TCP sockets on the web/application server improves application performance and actually increases the user capacity of servers. TCP offload is beneficial to all TCP-based applications, but is particularly beneficial for Web 2.0 applications making use of AJAX and other near real-time technologies that maintain one or more connections to the server for its functionality.

Protocol processing offload does not require any modifications to the applications.
Application-oriented offload
Application-oriented offload includes the ability to implement shared services on an application network infrastructure device. This is often accomplished via a network-side scripting capability, but some functionality has become so commonplace that it is now built into the core features available on application network infrastructure solutions.

Application-oriented offload can include functions like cookie encryption/decryption, compression, caching, URI rewriting, HTTP redirection, DLP (Data Leak Prevention), selective data encryption, application security functionality, and data transformation. When network-side scripting is available, virtually any kind of pre or post-processing can be offloaded to the application network infrastructure and thereafter shared with all applications.

Application-oriented offload works because the application network infrastructure solution is mediating between the client and the server and it has the ability to inspect and manipulate the application data.

The benefits of application-oriented offload are that the services implemented can be shared across multiple applications and in many cases the functionality removes the need for the web/application server to handle a specific request. For example, HTTP redirection can be fully accomplished on the application network infrastructure device. HTTP redirection is often used as a means to handle application upgrades, commonly mistyped URIs, or as part of the application logic when certain conditions are met.

Application security offload usually falls into this category because it is application – or at least application data – specific. Application security offload can include scanning URIs and data for malicious content, validating the existence of specific cookies/data required for the application, etc… This kind of offload improves server efficiency and performance but a bigger benefit is consistent, shared security across all applications for which the service is enabled.

Some application-oriented offload can require modification to the application, so it is important to design such features into the application architecture before development and deployment. While it is certainly possible to add such functionality into the architecture after deployment, it is always easier to do so at the beginning.

WHY YOU NEED IT

Server offload is a way to increase the efficiency of servers and improve application performance and security. Server offload increases efficiency of servers by alleviating the need for the web/application server to consume resources performing tasks that can be performed more efficiently on an application network infrastructure solution. The two best examples of this are SSL encryption/decryption and compression. Both are CPU intense operations that can consume 20-40% of a web/application server’s resources. By offloading these functions to an application network infrastructure solution, servers “reclaim” those resources and can use them instead to execute application logic, serve more users, handle more requests, and do so faster.

Server offload improves application performance by allowing the web/application server to concentrate on what it is designed to do: serve applications and putting the onus for performing ancillary functions on a platform that is more optimized to handle those functions. Server offload provides these benefits whether you have a traditional client-server architecture or have moved (or are moving) toward a virtualized infrastructure. Applications deployed on virtual servers still use TCP connections and SSL and run applications and therefore will benefit the same as those deployed on traditional servers.