XenApp is application virtualization software that delivers centrally-hosted Windows applications to local devices without the necessity of installing them.It is the flagship product for Citrix and was formerly known under the names WinFrame, MetaFrame, and Presentation Server.
XenApp software uses FlexCast Management Architecture (FMA), a proprietary architecture for Citrix virtualization products. It delivers individual applications, as opposed to entire desktops, to devices. It is also used with XenDesktop to deliver apps as part of a complete virtual desktop environment.
With XenApp, Windows applications can be used on devices that typically could not run them, including Macintosh computers, mobile devices, and Google Chromebooks. Conversely, it enables otherwise incompatible apps to run on Windows desktops.
XenApp is accessed on all devices via Citrix Receiver. The software can be delivered from on-premises data centers[6] or public, private, or hybrid clouds

XenDesktop is a Virtual Desktop Infrastructure (VDI) product developed and sold by Citrix Systems. It allows users to remotely access and operate Microsoft Windows desktops running in a datacenter or a public or private cloud, via devices located elsewhere. Users are able to access virtual desktops and applications through Citrix Receiver.Applications are delivered and managed via XenApp.

The product’s aim is to give employees the ability to work from anywhere while cutting information technology management costs because desktops and applications are centralized. XenDesktop also aims to provide security, because data is not stored on the devices of end users, instead being saved in a centralized datacenter or cloud infrastructure. Citrix developed the software for use by medium to large enterprise customers.
XenDesktop is able to manage and deliver applications and desktops using a connection broker called Desktop Delivery Controller. It supports multiple hypervisors, including XenServer, VMware vSphere, Microsoft Hyper-V and Nutanix Acropolis to create virtual machines to run the applications and desktops. The software allows for several types of delivery methods and is compatible with multiple architectures, including desktops and servers, datacenters, and private, public or hybrid clouds. XenDesktop comes with the XenServer hypervisor and image management tool Provisioning Services. Virtualized applications can be delivered to XenDesktop virtual desktops using XenApp.

ADCs have gained traction within the last decade, largely due to increased demand for legacy load balancing appliances to handle more advanced application delivery requirements and improve application performance.

Application delivery controllers are purpose-built networking appliances whose function is to improve the performance, security and resiliency of applications delivered over the web.

Application delivery
Applications have evolved significantly over the years. The term delivery is now generally accepted as the means of bringing an application to the user in this new era of mobility and cloud. In the enterprise, business applications have moved away from desktop-bound software installed on a local server accessed by users across the LAN. Modern applications need to work across all types of networks, and at locations beyond the confines of the physical workplace.
Application delivery controllers, which are widely deployed as a key fixture in the enterprise, help applications adapt to the networks and protocols that are in place today. They also ensure that applications perform optimally, are always available and don’t present any security risks either to the user or business.

Application availability
The average consumer expects the devices and applications they interact with on a daily basis to always work, and for information to be instantly available on demand. These expectations have carried over to the types of devices and applications that they use. To satisfy today’s workers, business applications need to be as intuitive and easy to use as the ones they rely on for personal tasks and entertainment.

Many employees are no longer restricted to using locked-down, company-owned equipment, and can use personal devices to work whenever they choose. With people working at any time of the day or night, IT must make certain that workplace servers and applications are available around the clock. Enterprises invest heavily in IT infrastructure to ensure that employees always have access to applications and information when they are needed.

Of course, servers can fail for a number of reasons ranging from mechanical problems to over-utilization and security breaches. If a server goes down, applications running on it become unusable or inaccessible.
IT organizations can plan for these occurrences by building fault tolerance into their environments. Deploying additional servers in the datacenter or at a co-located site are typical failover strategies. ADCs can help ensure high availability of applications by providing seamless failover. This is done by balancing application workloads across a cluster of active servers in one or multiple sites.

How server load balancing helps
Server load balancing helps distribute traffic across a cluster of servers to optimize utilization, improve responsiveness and increase availability.

An application delivery controller employs algorithms and policies to determine how inbound application traffic is distributed. Round robin, which forwards client requests to each server in turn, is a fairly rudimentary form of load balancing. This method assumes all servers are the same: it does not take into account health or responsiveness. An administrator can implement additional policies that direct an ADC to check for a number of criteria before determining to which server an inbound request should be sent. The application delivery controller can inspect packet headers for keywords or requested file types, and direct the request to the appropriate server based on this information.

Figure 1: ADC actively monitors to ensure requests are sent to available servers

Application delivery controllers are also heavily relied upon for their monitoring capabilities. They can check a server’s health and operability beyond the standard ping. If monitoring indicates a server is experiencing an issue, or that specific health criteria needed to ensure a server’s reliability are not being met, the ADC will route traffic to an alternate server, avoiding a potential disruption (see Figure 1).

Application delivery controllers can also provide real-time and historical analysis of all user and network traffic, including metrics for round-trip times, bandwidth usage and datacenter and WAN latency. This information can assist help desk staff by minimizing the time they spend identifying the cause of an issue, and help users by providing faster resolution.

Load balancing servers across multiple sites
Load balancing is a critical service in any high-traffic datacenter, but an application delivery controller can also redirect traffic to a cluster of servers located in an entirely different datacenter. This is called global server load balancing. The servers in the other datacenter can be front-ended by another ADC, which works in tandem with the first appliance. These sites can be configured in either active-passive or active-active mode. In the latter, both sites are actively supporting inbound traffic. Each application delivery controller detects which datacenter is closest to a given user, and routes the client request to a server in that datacenter. This process minimizes latency and round trip times for the user’s request and ensures a better experience.

This configuration also supports business continuity if a datacenter suffers a shutdown. When traffic is routed to that datacenter, the ADC will divert it to an available ADC in a co-located site that can direct traffic to a viable server resource.

Application performance
If applications do not perform to users’ expectations, their productivity can be severely compromised. An application delivery controller can employ an array of mechanisms to improve application performance, especially over mobile and high-latency networks.

SQL database load balancing is one mechanism that can deliver performance gains. SQL load balancing uses many of the same techniques employed for load balancing TCP traffic, but applies this intelligence at the database level. It uses policy-driven logic for each SQL transaction, improving the number of requests and connections that can be handled within the database cluster.
Other common app performance optimization services offered by application delivery controllers are offloading of server-intensive tasks, connection multiplexing, compression and caching.

SSL and TLS are mainstays for doing business on the web. Managing traffic encrypted with new ciphers is very CPU intensive. Application delivery controllers can handle exceedingly high volumes of encrypted and unencrypted traffic. The ADC manages certificates and decrypts traffic before it reaches the server.
TCP multiplexing is an effective method for handling high volumes of inbound server requests. TCP multiplexing maintains active connections between the ADC and the servers. As traffic hits the ADC, it routes requests using these open channels, which eliminates the inefficient “open-close” overhead for each transaction that can negatively impact server performance.

Performance optimization on mobile networks
Application delivery controllers can also provide performance benefits across mobile networks. Web pages designed for high-speed Internet links often fail to deliver the same user experience on a mobile device connecting over a bandwidth-constrained network.

Several creative mechanisms enable an application delivery controller to optimize web content delivery over mobile networks. Domain sharding is one example. Connection-layer optimization is applied to a single domain. Content on each page is broken down into a sequence of subdomains that allow a larger number of channels to be opened simultaneously, which decreases page load time and improves performance.

Application delivery controllers have visibility into the content that is being delivered, and can further optimize delivery of web pages containing large images by converting GIF files into more-efficient PNG formats.
The other large components of a web page include extensive scripts and cascading style sheet (CSS) files, which ADCs can compress by removing unnecessary characters and white space.

When compressed, files traverse the network at a much faster rate, so download times are significantly reduced.

Application and user security
Delivery over the web has introduced new threats and vulnerabilities that traditional LAN-bound applications never had to contend with. As workers become more mobile and require remote access to applications and data, IT must devise more-stringent safeguards against external attacks and data leakage.

Application delivery controllers serve as the natural entry point or gateway to the network. They authenticate each user attempting to access an application. If the application is SaaS based, the ADC can validate a user’s identity using an on-premises Active Directory data store that eliminates the need to store credentials in the cloud. Not only is this process more secure, it also enhances the user experience by providing single sign-on capabilities across multiple applications.
SAML, the XML-based protocol, is now widely used to simplify the application login process. The ADC can act as a SAML agent, authorizing users via any data stores where their identity can be confirmed. Some applications allow the use of credentials from sites such as Facebook or Google+ to validate identity before granting access. ADCs can act as a SAML identity or service provider in this respect.

Distributed Denial-of-Service (DDoS) attacks have become rampant.1 Enterprise web properties, specifically, are being targeted with the intent of overwhelming their servers and disrupting their ability to conduct business. The ADC can implement rate-limiting measures to protect internal server resources from being targeted by these specially designed attacks. When an unusually massive surge of inbound requests occurs, the ADC can throttle these requests and minimize the amount of available bandwidth they consume, or reject the request entirely.
Application delivery controllers have converged load balancing and advanced Layer 7 protection, which traditionally were only available as standalone solutions. Application firewalls can inspect data packet headers for suspicious content or malicious scripts that may not be detected by network firewall (See Figure 2).

Figure 2. Layer 7 protection beyond a network firewall’s capabilities

1 Jonathan Keane. DDoS Attacks Hit Record Numbers in Q2 2015. Digital Trends. August 19, 2015. http://www.digitaltrends.com/computing/ddos-attacks-hit-record-numbers-in-q2-2015/

An application delivery controller can support both positive and negative security models. When an ADC is placed in “learning” mode, it can analyze traffic to determine usage patterns that signify normal behavior. If a malicious inbound request is sent, for example, using SQL injection or cross-site scripting, it will automatically flag that request and block it. It can also employ signature-based protection via integration with third- party security providers such as Qualys. Combining these protection methods allows the ADC to use a comprehensive hybrid security model for applications and users.

What’s next for ADCs?
ADCs already provide tremendous value to IT organizations ensuring the secure delivery of applications and data to the user. However, they are expected to continue advancing as applications evolve. Software-defined networking has placed increased demands on application delivery controllers to function “as a service.” As network protocols become more application centric, ADCs must also adapt and become more “self-automated” to provide seamless optimization and protection for every type of application.