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"The load balancing (LB)
feature distributes client requests sent to the system
across several servers to optimize resource utilization. In a real-world scenario, a limited number of servers
can provide service to a large number of clients."
Source : Citrix
An Introduction to Load Balancing
Load Balancing is also known as :
Load Balancing Architecture,
Load Balancing Definition,
Load Balancing Diagram,
Load Balancing Firewall,
Load Balancing Hardware,
Load Balancing Method,
Load Balancing Options,
Load Balancing Products,
Load Balancing Router,
Load Balancing Software,
Load Balancing Solution,
Load Balancing System,
Load Balancing Tools,
Load Balancing Web Servers,
Network Load Balancing,
Server Load Balancing,
Setup Load Balancing,
Simple Load Balancing,
Website Load Balancing,
Aggressive Load Balancing,
Application Load Balancing,
Best Load Balancing,
Clustering Load Balancing,
Configure Load Balancing.
As the global business environment has evolved, companies have
signiicantly expanded their reliance on remote and mobile access
to business applications over the Internet. Applications that must
be available to employees in the ield, in branch ofices and in
home ofices are increasingly delivered via a corporate intranet or
portal. Further, for many organizations, externally facing websites
are an integral component of day-in/day-out business interactions
with customers, suppliers and partners. In addition, due to the
incorporation of Web 2.0 functionality, applications have become
far more dynamic and interactive compared to their predecessors.
For all these reasons, businesses recognize they cannot function
effectively without a robust solution to ensure uninterrupted,
secure and high-performance access to network-based business
applications and corporate websites.
This reliance upon Internet-delivered applications has also
changed how businesses look at their underlying network
infrastructure. On one hand, organizations’ understanding of how
critical network infrastructure works has never been higher. On
the other, companies want to know explicitly how the network
is enhancing their ability to deliver new application services.
To a business, the network’s value is not its own availability,
performance and security, but rather its ability to improve
the availability, performance and security of the businesses
applications it serves.
As such, networks must evolve from highways designed to push
packets into more-active participants in the end-to-end delivery of
application services. For this to occur, many components of the
network will need to evolve at a fundamental level.
Traditional load balancing
What has traditionally been known as “load balancing” is one such
component. Load balancers sit at a critical junction between users
and the applications they access, which are typically hosted on
servers. They are designed to evenly distribute among available
servers the user requests that come in over the network so an
individual server does not become overwhelmed with trafic. Basic
load balancers direct trafic based on Layer 4 – the connection
layer – of the Open System Interconnection (OSI) model. Layer 4
load balancers look at the packet’s addressing information — IP
address and port number – and must support:
- Server health checks that determine whether individual servers
are “up” or “down”
- Load balancing algorithms that determine which of the “up”
application servers will receive the request
The most common algorithm is a round-robin that prompts the
load balancer to go down the list of servers from top to bottom
and then begin again. However, this assumes all requests
will have a similar load and duration, and that all servers are
available. More-advanced algorithms use factors such as server
utilization level and current-connection counts to select the most
Initially, load-balancing capabilities were built directly into the
application software or the operating system of the underlying
application server. These approaches transitioned into using
application-neutral, purpose-built, network-based appliances.
Network-based appliances enable load-balancing of all
applications, not just those with built-in functionality.
In addition to these advancements, the load balancing process
itself needs to evolve from simple packet delivery to application
delivery. The increasing demands for high availability, reliability
and security of application access are driving the need for
load balancers to provide not only traditional networking trafic
management functions, but also a comprehensive set of networklevel
and application-level services.
To ensure the business is getting the investment protection to
meet both near and longer-term requirements, today’s loadbalancing
solutions should provide the following functionality as
part of either the base offering, or as post-deployment software
- Network trafic management functionality to ensure application
availability and even distribution of load across a server farm or
- Application acceleration functionality to accelerate application
performance by 5X or more
- Application-aware delivery functionality to protect applications
and their data, control access and monitor end-user
Improving network trafic management
Layer 4 load balancing
Directing trafic based upon IP address and port number has
become standard functionality, but that doesn’t diminish its
importance. Solutions must support a wide variety of loadbalancing
algorithms that direct trafic based upon network, server
and application loads.
- Session persistence: In some cases it is important to have a
single server handle all of a user’s transactions for the length of
that session. The obvious one is online shopping. Regardless
of how the user jumps around looking at brochure content,
their shopping cart entries have to go to the same place all
the time. Session persistence ties the requests from one
client to the same server node. Common functionality used
to maintain session persistence includes cookies and header
IDs. However, maintaining persistence for the latest generation
of applications can require basing persistence on applicationspeciic
content (e.g., a transaction ID in an XML document)
carried in the payload body.
- Server health monitoring: Health checks to ensure a server’s
availability can prevent directing of a request to a failed server.
At a basic level, the load balancer can keep checking the
server port to determine its status. However, just because
the network and server are responding doesn’t mean the
application itself is available. Solutions should be able to
check the health of the applications themselves when marking
servers or services up or down.
Layer 7 request switching
To advance to the next generation of trafic management,
load balancers began to use Layer 7 of the OSI model – the
application layer — to read the packet payload instead of just
the addressing information to determine the best place to send
the trafic. Content switching at Layer 7 provides intelligent trafic
management, enabling application-layer information such as client
type, requested URL, cookie information and application software
usage to be used to optimize delivery, without requiring changes
to Layer 4 network addressing.
Switching at Layer 7 instead of at the connection level (Layer
4) enables better utilization of server resources. For example,
because different types of content have different requirements
for CPU usage, I/O throughput, etc., it is possible to increase
eficiency by using some servers to handle transactions, and
others to provide storage or other functions. Also, with Layer 7
request switching, certain users can be directed to higher-power
servers to provide the highest service level.
Global server load balancing
In addition to enhancing load balancing with Layer 7 request
switching, organizations can beneit from class server load
balancing (GSLB) across the entire enterprise. GSLB balances
requests from users across a geographically distributed set of
server farms based on health, load or proximity. Effective solutions
support several load-balancing algorithms (e.g., least response
time, least packets), as well as geographic proximity and network
proximity, to intelligently distribute the load across multiple
datacenters. GSLB gives network administrators the ability to
provide high availability and optimal application performance
for remote users worldwide. Other beneits include reducing
bandwidth costs and latency.
As part of a comprehensive business continuity solution, global
server load balancing transparently ensures that requests are
routed only to datacenters or failover sites that are operating
normally. GSLB technology gives IT administrators the ability
to create policies deining site health based upon site status,
connection load and packet rate. By continuously monitoring
the health of each datacenter and associated network links,
GSLB solutions maintain a global view of the entire hosting
infrastructure’s status. In the event that a hosting site cannot
meet the criteria of the health policy, further incoming requests
are automatically directed to sites still deemed healthy. When
availability is restored, new requests are transparently directed
back to the original site.
High application availability with Global Server Load Balancing
A U.S. beverage company needed a solution to loadbalance
requests from global users to access a missioncritical
application over the web. Through global server
load balancing capability, the solution directs user
requests to the optimal server, provides high availability,
maximizes server resources, and provides trafic
management for the company’s multi-site enterprise.
Accelerating application performance
Initially, users were willing to accept slow application performance
in exchange for the convenient, widely available access the Web
provides. But not any more. Users now expect applications
delivered via the Internet to offer performance similar to that of
locally deployed applications. The lexibility to add functionality that accelerates application performance has become critical
in meeting the larger business goal of successfully delivering
applications over the Internet.
All too often, problems with application performance are deemed
to be a function of server hardware after network infrastructure
has been ruled out. Although the server has a direct impact on
processing performance, it is not necessarily directly related to
application performance. There needs to be a distinction between
processing and application performance. Simply increasing
processing power in a server may have little or no effect on
application performance and scalability. The same may be said for
adding load-balanced servers to cope with increased load.
Ofloading tedious or repetitive processes from application servers
can free them to perform their main functions of serving content.
Ofloading also enables servers to scale up beyond their original
capacity while accelerating application content delivery.
Following are important technologies that can enhance basic
load balancing by reducing server workload and accelerating
- Caching: Caching static content can help relieve the burden
on servers. However, more and more applications rely upon
content that is dynamically generated each time a request is
made. In many cases, the same content (e.g., sales reports)
is repetitively generated for every user. The load balancer
can store content in a cache so that some requests can be
handled without contacting the server. Caching dynamic
content can accelerate application performance by up to 30X.
- Compression: Compressing content minimizes the amount of
data that must traverse the wire and also decreases the “back
and forth” overhead of TCP that is so sensitive to latency.
Network congestion is reduced, and applications can be
accelerated by three to ive times.
- SSL Ofload: Ofloading encryption/decryption onto a
dedicated device to reduce datacenter costs removes the
burden of SSL processing from the server. Ofloading can
signiicantly lower CPU utilization and even enable fewer
servers to handle application loads.
- TCP Multiplexing: TCP overhead can slow any application’s
performance. TCP optimization reduces the number of client
connections each application server has to deal with while
optimizing server response. The result is a server that can
support an increased number of users. This can extend the life
of existing hardware while delivering application content with
much better performance.
- TCP optimization: Latency, network congestion and TCP
overhead can slow any application’s performance. In order to
minimize the unnecessary TCP transmissions and round trips
that increase network congestion, solutions should support
network optimizations such as limited transmit and fast
retransmit, windows scaling, selective acknowledgement and
Using caching to optimize server performance
A government organization overseeing public transport
was experiencing huge growth in trafic needed to ensure
its public website could cope with more users, as well
as unpredictable spikes in Web trafic. Because much
of the information requested on the site is repetitive, the
solution caches all commonly required content — such
this burden from the Web server infrastructure optimized
existing servers and considerably improved response
times. Up to 40 percent of all server requests are
delivered entirely from the cache.
Reducing server load with TCP multiplexing
An online media provider sought a way to support
dramatic increases in online trafic during major
sports events without over-investing in servers. A TCP
multiplexing solution that consolidated multiple user TCP
sessions into fewer sessions on the Web servers allowed
the servers to focus on processing user requests. As
a result, the servers’ load dropped dramatically, while
throughput experienced a strong increase — all without
the cost of adding more servers. The company was able
to reduce the number of servers required to support its
Web site by 66 percent.
For networks to provide true business value, they must advance
from merely transporting network packets from point A to point
B to actively improving the applications themselves. To do this,
solutions must be able to inspect all aspects of application trafic,
take action based upon this inspection, and potentially change
or act on behalf of the applications themselves. With these
capabilities, the network becomes an enabler of overall business
agility and lexibility.
One of the most important capabilities is improved application
security. Network irewalls and authentication solutions have
largely secured the network itself. However, applications
themselves remain surprisingly vulnerable to attack. Cross-site
scripting, buffer overlows, SQL injection and other common
hacking techniques are continually used to steal valuable customer
and corporate data from applications.
As application services are rolled out to employees, agents,
customers and contractors on a global basis, the need to inely
control which users have access to which functions in which
applications has increased. SSL VPNs have emerged as the de
facto method for providing trusted application access. Integrating
SSL VPNs and application security with load balancing and trafic
management strengthens end-to-end application security and
simpliies the IT infrastructure. Another important functionality that
improves application delivery is end-user performance monitoring
- SSL VPN with granular access control: With the prevalence
of Internet threats, organizations need to control who is
accessing corporate applications and what actions they are
taking with each application. By integrating special SSL VPN
technology with granular access control into a load-balancing
solution, administrators can control both access and actions
(such as downloading, printing or saving) of remote and mobile
users who want to connect to applications over the Internet,
mitigating the risk of opening the corporate network to threats.
- Application irewall: With over 70% of successful Internet
attacks now exploiting application vulnerabilities, network
irewalls are not enough. Standard irewalls are designed to
restrict access to certain ports or services that an administrator
doesn’t want unauthorized people to access. In contrast,
application irewalls are often called “deep packet inspection firewalls” because they look at all content within every request
and response. Some application irewalls look for certain
attack signatures to try to identify a speciic attack that an
intruder may be sending, but this only protects against known
attacks. True application-layer defense protects against known
and unknown attacks.
Improved security of access by home-based workers
A newspaper publishing company wanted to enable
employees to connect to network resources from
home computers to meet tight deadlines – without
compromising security. An advanced SSL VPN solution
allows IT administrators to deine granular access
policies for different users. End-point analysis allows
them to thoroughly check each device that connects to
the network and ensure it meets security requirements.
With these security capabilities the company agreed
to let workers monitor content and make changes
necessitated by late-breaking news from home. Work is
performed more quickly, deadlines are met and people
do not have to drive into the ofice.
In many respects, the inal frontier of successful application
delivery is obtaining direct feedback on how the applications
themselves are performing. Ultimately, applications and networks
are only as good as the perception of the people who use
them. Directly monitoring end users’ actual experience with an
application is critical to understanding how effectively users are
served, and how well the network is working. The point in the
network where load balancers are traditionally deployed is an ideal
“junction” for performing this monitoring. As such, solutions should
provide the ability to directly and transparently measure and track
Fundamental changes are affecting networks, particularly the
sweeping transformations of Web 2.0. Because Web 2.0 is
driving greater user participation, openness and network effects,
tomorrow’s networks will require infrastructure that is agile, lexible
and dynamic. Is your organization prepared for these changes with
a load-balancing solution that can optimize application performance,
ensure high application availability and provide tools to safeguard
data and improve the application experience? Citrix® NetScaler®
offers a powerful and comprehensive solution to these challenges.
About Citrix NetScaler
Citrix® NetScaler® from Citrix Systems, Inc., is an ideal solution
for any enterprise organization seeking basic and advanced load
balancing capability combined with application performance
enhancement, improved application security and increased
application availability for users. Citrix NetScaler integrates all the
critical functionality of Layer 4-7 network trafic management,
application acceleration and application-aware delivery in a single
- Load balancing: NetScaler delivers ine-grained direction of
client requests to ensure optimal distribution of trafic to servers.
In addition to Layer 4 addressing information (protocol and
port number), trafic management policies can be based on
application content. For example, administrators can segment
application trafic based upon information contained within
an HTTP request body or TCP payload, as well as Layer 4-7
header information such as URL, application data type or
cookie. Numerous load-balancing algorithms and extensive
server health checks provide greater application availability by
ensuring client requests are directed only to correctly behaving
- Web application acceleration: Citrix NetScaler accelerates
Web application performance by up to ive times by leveraging
multiple acceleration technologies including data compression
and caching of static and dynamic content. NetScaler TCP
optimizations overcome the issues caused by high latency
and congested network links and are transparent to clients
and servers, accelerating the delivery of any application while
requiring little or no coniguration.
- Application-aware delivery: NetScaler protects applications
from application-layer attacks, helping to prevent the theft
and leakage of valuable corporate and customer data. The
latest version, 8.0, includes application irewall technology
that proactively protects against application-layer attacks
and helps prevent theft and leakage of valuable corporate
and customer data. It also includes real-time and historical
page-level monitoring of the end-user experience with
application performance. NetScaler 8.0 makes secure access
to applications easier by tightly integrating SSL technology
that automatically responds to each user scenario with the
appropriate level of application access, including control of
actions such as print, save and edit.
Glossary of terms
Application firewall — An enhanced irewall that limits access to the operating system (OS) of a computer. Conventional irewalls merely
control the low of data to and from thecentral processing unit (CPU) , examining each packet and determining whether or not to forward it
toward a particular destination. An application irewall offers additional protection by controlling the execution of iles or the handling of data
by speciic applications.
Application-layer attack — Targets application servers by deliberately causing a fault in a server’s operating system or applications, which
results in the attacker gaining the ability to bypass normal access controls.
Caching — Local storage of remote data on a ile server, which is designed to reduce network transfers and therefore increase speed of
Compression — Encoding data to take up less storage space and less bandwidth for transmission.
Content switching – Allows trafic management to be based on application-layer content such as the information contained in the body of
a TCP or HTTP request.
Global server load balancing, GSLB, (also known as global traffic management) — The load balancer distributes load to a
geographically distributed set of server farms based on health, server load or proximity.
Layer 4 (Transport layer of the Open System Interconnection model) — Provides transparent transfer of data between end systems,
or hosts, and is responsible for end-to-end error recovery and low control.
Layer 7 – (Application layer of the Open System Interconnection model) – Deines the services that directly support applications, such
as software for network management, electronic mail or ile transfers. It interfaces directly to and performs common application services for
the application processes.
Load balancing — A technique performed by load balancers to spread work between many computers, processes, hard disks or other
resources in order to get optimal resource utilization and decrease computing time.
SSL offloading — Relieves a Web server of the processing burden of encrypting and/or decrypting trafic sent via SSL, the security
protocol that is implemented in every Web browser. The processing is ofloaded to a separate device designed speciically to perform SSL
SSL VPN — Provides a comprehensive, secure remote access technology for remote users without the use of additional remote client
software, but instead uses common client technology and industry-standard Secure Sockets Layer technology for content privacy.
TCP optimization — Reduces the number of client connections each application server has to deal with while optimizing server responses.
Web 2.0 applications — Deliver software as a continually updated service that gets better the more that people use it, consuming and
remixing data from multiple sources – including individual users.
Web server farm — A redundant cluster of several Web servers serving a single IP address.
Table of Contents
- Traditional load balancing
- improving network trafic management
- Accelerating application performance
- Application-aware delivery
- About Citrix NetScaler
- Glossary of terms
- Appendix A