Truths and Myths of Presentation Server and WAN Optimization

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Oct 24, 2013 (4 years and 6 months ago)


FAQ Page 1 Date Created: April 17, 2007
Truths and Myths of Presentation Server and WAN Optimization Date Updated: May 15, 2007
©2007 Citrix Systems, Inc

Truths and Myths of Presentation Server and WAN Optimization
Product: Presentation Server
Version: 4.5
Citrix Presentation Server is relied upon by millions of end users to deliver applications securely to any location, any device
over any connection. This unique solution is achieved by an advanced proprietary protocol called the Independent
Computing Architecture (ICA), developed, owned and enhanced by Citrix Systems, Inc. Citrix has continued to enhance the
ICA protocol by including capabilities like compression, encryption, caching, and flow control.
As a significant percentage of Presentation Server customers have their applications delivered over remote WAN links,
finding ways to improve the speed and the end user experience is a goal of WAN optimization solutions. Also, by being able
to tap into such a large pool of potential users is extremely enticing for all WAN optimization organizations.
Unfortunately, many of the claims about WAN optimization improving the ICA protocol speed are based on simulations not
reflecting typical real-world scenarios. Another common issue is focused on terminology used when discussing WAN
optimization. When discussions arise about WAN optimization and ICA, the improvements are usually focused on the
following two areas: • Accelerating ICA: Actively improving the performance metrics of ICA within real world cases.
• Protecting ICA: Limiting non-ICA traffic from impacting the ICA traffic, which results in decreased performance from the
user’s perspective while working on Presentation Server. This area is often referred to as Quality of Service (QoS).
The remainder of this document explains the common ways WAN optimization solutions function and the truths and myths
associated with these techniques when integrating with Citrix’s ICA protocol.
Accelerating ICA
Common techniques for optimizing WAN communication focus on flow control and compression. Without understanding how
ICA works in-depth, it makes reasonable sense that these two items will improve ICA performance just like any other protocol
crossing the wire. However, this is not the case as the following sections will explain further.
Flow Control
Typical TCP connections protect themselves from congestion by TCP Congestion Control. As TCP has no knowledge of the
bandwidth for the WAN link, TCP Slow Start is implemented and increases the send rate for each successful roundtrip. To
help protect itself from congestion, TCP reduces the send-rate by a 50% buffer for non-acknowledged transmission, which,
combined with TCP slow start and high latencies, require significant amounts of time to get back to full link utilization. Multiple
packet losses often lead to WAN link utilizations of only a few percent of the nominal bandwidth
Part of the TCP Congestion Control is a Window Size that dictates how often acknowledgements are required that the data
sent was subsequently received by the client, thus helping to ensure data was not lost on the trip. The TCP Window Size
standard is set at 64KB of data. For every 64KB chunk of data that passes across the wire, an acknowledgement is required
before the next 64KB chunk of data can be sent.
Many WAN optimization solutions are able to improve WAN communications by removing the TCP Slow Start process and by
increasing the TCP Window Size to allow more data to pass over the WAN before an acknowledgement is required.

FAQ Page 2 Date Created: April 17, 2007
Truths and Myths of Presentation Server and WAN Optimization Date Updated: May 15, 2007
©2007 Citrix Systems, Inc

• Myth: Improving flow control through the aforementioned processes will improve ICA performance as ICA will be
able to use more of the WAN connection and send larger packets across the wire.
• Truth: The ICA protocol is a self-tuning and highly interactive protocol consisting of many small packets. Part of the
self-tuning aspect of the ICA protocol is it already manages the TCP window size to the optimal setting based on
current conditions. Providing this feature in another product will have little, if any, positive impact on ICA
However, most WAN links contain additional protocols in addition to ICA. Citrix WANScaler can have a positive
impact on the responsiveness of ICA by using its adaptive TCP flow control functionality to improve overall WAN link
utilization. WANScaler’s implementation of TCP Congestion Control is setup in such a way that if a collision occurs
on the wire, the send rate is only slightly impacted or not at all, thus allowing the protocols to continue to use the full
bandwidth of the link.
WANScaler also modifies the TCP Window Size for the WAN link, based on the Window Scaling process. This
modification means more data passes before pauses, thus allowing for faster data traversal. Both of these
enhancements to WAN link utilization can improve ICA responsiveness as the purchased bandwidth will be more
fully utilized.
Compression is the process of encoding information using fewer bytes than the uncompressed version of the data using
different schemes to get the best compression ratios. There are multiple forms of compression. The goal of each is to replace
a large chunk of data with a significantly smaller chunk of data using table lookups, tokens, or some other method. By
compressing the data traversing the WAN connection, a larger amount of data can be sent over the same connection. The
ways in which compression is implemented are different between solution providers.
• Myth A: The ICA protocol can be compressed more efficiently using a dedicated WAN optimization solution,
providing a noticeable decrease in bandwidth usage.
• Fact A: The ICA protocol is already highly compressed by Citrix Presentation Server. Compressing a compressed
data stream will not decrease the size. In fact, compressing data that is already compressed has been shown to
actually increase the size of the data! Citrix WANScaler does not fall victim to this fate as WANScaler verifies the
compression output from its multi-level compression engine never falls below a 1:1 compression ratio, thus ensuring
the data does not increase in size.
Additionally, Presentation Server compresses the ICA protocol by performing multiple levels of compression on the
data before it hits the wire. First, ICA captures all bitmaps and compresses them with an engine optimized for bitmap
images. Secondly, Presentation Server utilizes another compression engine specifically for the ThinWire virtual
channel, which is responsible for screen refreshes. Finally, the entire data stream is then transmitted through a
general purpose compression engine optimizing the data as it is placed onto the wire. Adding another layer of
compression based on WAN optimization solutions will only add latency and could potentially increase the size.
• Myth B: By utilizing a WAN optimization solution instead of using the built-in compression algorithms within the ICA
protocol, the Presentation Server’s scalability will be increased allowing the server to host more users.
• Fact B: The ICA protocol is scrambled with an integrated encryption algorithm. The traffic changes based on the
public/private key combination for the particular session. With encryption enabled for the ICA protocol, any WAN
optimization product will not be able to compress the ICA traffic effectively based on repeating bit streams
(compression history). Building a compression history on encrypted traffic with changing crypto-keys is just an
exercise in futility. However, the ICA protocol is still capable of being compressed with the built–in compression
engine inherent within the ICA protocol.
• Myth C: By disabling encryption and compression within the ICA protocol, the WAN optimization solution will be able
to compress the data. By not requiring the Presentation Server to perform these functions, single server scalability
should be improved allowing for more simultaneous user sessions on the server.

FAQ Page 3 Date Created: April 17, 2007
Truths and Myths of Presentation Server and WAN Optimization Date Updated: May 15, 2007
©2007 Citrix Systems, Inc

• Fact C: Encryption or data scrambling cannot be disabled in Presentation Server through the supplied administration
tools. The administration tool only allows encryption to be set to one of the following: Basic, 40bit, 56bit, and 128bit.
Even Basic encryption still scrambles the data, thus it is still encrypted.

If one was able to disable ICA encryption and compression, there would not be an increase in single server
scalability. The CPU utilization savings one would see by disabling these ICA features would be offset by the
increased workload on the CPU for putting more data onto the wire. Also, it is not a best practice to disable
encryption for production deployments due to security and compliance issues which may arise as a result.
• Myth D: Users who print frequently and utilize the ICA protocol for print traffic (the print traffic is encapsulated within
the ICA protocol) will see improvements in their session speed with a WAN optimization solution as the print traffic is
• Fact D: The printing infrastructure and algorithms were completely revised for Presentation Server 4.0 and higher.
The print traffic is now highly compressed as it passes through the ICA protocol. The history buffer has been
increased significantly to 8MB as resource constraints were removed due to a re-architecture. A larger Presentation
Server printing session history buffer allows the current and subsequent print jobs to re-use parts of the already
transmitted data. As an example, this document has graphics in the header of each page. These graphics will incur
a lot of data in the transmission during printing. With the new history buffers, the data representing the header
graphics are only sent once to the client and then re-used over and over again for each page. A huge reduction in
network communication.
The new printing subsystem incorporates its own reducer engine (compression engine) separate from the other
channels within the ICA protocol. This reducer was specifically designed to compress print traffic. As an example,
print jobs destined for the same printer often have many repeated elements from previous print jobs even though the
documents are completely different. The dedicated reducer is capable of taking advantage of this and reducing the
size of the job.
These enhancements in Presentation Server 4.0+ will significantly improve printing within ICA virtual channels. As
the ICA protocol is proprietary, WAN optimization solutions will not be able to look inside of the ICA packet to
optimize the print information.
• Myth E: Users who copy large files through the ICA protocol will see improvements in their session speed as the file
transfers are compressed with a WAN optimization solution.
• Fact E: Since the ICA protocol is already compressed by default, adding a new level of compression will not help in
reducing the amount of data crossing the wire. As the ICA protocol is proprietary, third party WAN optimization
products are not able to look inside of the ICA protocol to optimize directory browsing information and improve file
transfers. Finally, the ICA protocol was enhanced in Presentation Server 4.0 and higher and is now capable of
caching information so directory browsing will be faster from the user perspective.
Protecting ICA (QoS)
Does Citrix WANScaler fit within a Presentation Server environment? YES!!
Numerous organizations have expressed concern that a user printing or performing a file transfer, whether it be within or
outside of the ICA stream, impacts all Presentation Server sessions at the site. As portions of the ICA protocol are for highly
interactive traffic, it is imperative to protect the high-priority virtual channels of ICA from other non-ICA traffic. This is the core
goal of protecting ICA traffic with Quality of Service (QoS).
There are two areas of focus for QoS as it relates to the ICA Protocol:
• Protecting ICA from Other Traffic: The WAN connection is a free-for-all. Every protocol is equal and is trying to get
across the link as fast as possible. Although this sounds fair, it is not a best practice. Certain protocols are highly
interactive and require fast traversal/response times, like portions of ICA. If the WAN link is fully utilized, then ICA
will have to wait its turn to cross the wire, which will slow down a user’s computing experience. Citrix WANScaler is

FAQ Page 4 Date Created: April 17, 2007
Truths and Myths of Presentation Server and WAN Optimization Date Updated: May 15, 2007
©2007 Citrix Systems, Inc

able to provide QoS by assigning different traffic to five separate traffic classes. During configuration, each class is
given and guaranteed a percentage of the link. If a particular class is not able to fully utilize their percentage, then
other traffic will use the available bandwidth until the class has enough traffic to send that will maximize its link
utilization. By creating a traffic class with a high percentage of the link and assigning ICA to the queue, ICA will be
guaranteed a large portion of the WAN link at all times, thus protecting it from other traffic bursts.
• Prioritizing ICA Virtual Channels: The ICA protocol is divided into different virtual channels, each responsible for
different types of communication like screen refreshes, printing, clipboard mapping, etc. Additionally, each virtual
channel is assigned to a different priority class (real-time, interactive, bulk data transfer, background). From the
user’s perspective, the real-time class is the most critical as this class contains items like screen refreshes and
keyboard/mouse feedback. If the real-time class’s traversal across the WAN is delayed, the user will notice a poor
computing experience. The highly-interactive classes must be guaranteed a portion of the WAN connection so the
traffic can make it across the wire as quickly as possible. Citrix WANScaler is able to integrate the different ICA
priority classes into the five different WANScaler priority queues, thus helping to ensure portions of the ICA protocol
receives the most logical connection size.
The Citrix ICA protocol is highly interactive, requiring constant, albeit small, communication between client and server. Too
many times have Citrix Presentation Server users endured slow or dropped sessions while other users at the same remote
site consume the valuable and limited network bandwidth by printing large documents, downloading files or watching
streaming media. As millions of users daily rely on the ICA protocol to help them do their jobs, it is extremely beneficial to
accommodate the ICA protocol’s needs in the network architecture, which includes:
• Priority over other protocols
• A clean WAN link with little congestion
Citrix WANScaler is able to give the ICA protocol priority over other less-critical protocols with its Quality of Service (QoS)
capabilities. WANScaler can guarantee a percentage of the WAN link to ICA, a specific virtual channel within ICA, or to a set
of protocols that includes ICA. With this capability, the ICA protocol will always be able to provide constant updates between
client and server. Being able to provide prioritization down to the ICA virtual channel allows an organization to guarantee the
greatest percentage of WAN bandwidth to ICA screen updates and a much smaller percentage to ICA printing. This is
prioritization at the protocol and virtual channel level.
Citrix WANScaler can easily clean up WAN links with its adaptive TCP flow control and its Multi-Level Compression engine.
Using these technologies on non-ICA protocols will help to reduce overall congestion by allowing the link to overcome TCP
WAN challenges like TCP Slow Start and TCP Window Size that reduce application performance
Less data will traverse the WAN link due to the highly efficient Multi-Level Compression engine that is independent of the
protocol, file, object or application. WANScaler implements different types of compression based on the history and types of
data being sent. In one example, as data passes through, large chunks of data are replaced with tokens pointing to specific
locations within memory or on disk. The token is sent to the receiving WANScaler that uses the token to look up the stored
data and forwards it onto the client. The client receives a large chunk of data, but the WAN connection only had a small
chunk of data passing across. By combining these two technologies into a single solution, all traffic crossing the WAN will
benefit due to some traffic being compressed and additional traffic being optimized.
Citrix WANScaler is a single solution that provides compression, TCP congestion control and prioritization benefits for every
protocol, including the Citrix ICA protocol.

FAQ Page 5 Date Created: April 17, 2007
Truths and Myths of Presentation Server and WAN Optimization Date Updated: May 15, 2007
©2007 Citrix Systems, Inc

The information in this publication is subject to change without notice.
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The exclusive warranty for Citrix products, if any, is stated in the product documentation accompanying such products.
Citrix does not warrant products other than its own.
Product names mentioned herein may be trademarks and/or registered trademarks of their respective companies.
Copyright © 2007 Citrix Systems, Inc., 851 West Cypress Creek Road, Ft. Lauderdale, Florida 33309-2009 U.S.A.
All rights reserved.

Version History
Daniel Feller (Sr. Architect) .1 Document created April 24, 2007
Daniel Feller (Sr. Architect) .2 Updated based on feedback April 30, 2007
Daniel Feller (Sr. Architect) .3 Final Updates May 9, 2007
Daniel Feller (Sr. Architect) 1.0 Final Release May 15, 2007

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