Citrix Virtual Apps and Desktops

Citrix ICA virtual channels

August 22, 2024

Contributed by:

C C

Warning:

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What are ICA virtual channels?

A large portion of the functionality and communication between the Citrix Workspace app and the Citrix Virtual Apps and Desktops servers occurs over virtual channels. Virtual channels are a necessary part of the remote computing experience with the Citrix Virtual Apps and Desktops servers. Virtual channels are used for:

Audio
COM ports
Disks
Graphics
LPT ports
Printers
Smart cards
Third-party custom virtual channels
Video

New virtual channels are sometimes released with new versions of the Citrix Virtual Apps and Desktops servers and Citrix Workspace app products to provide more functionality.

Virtual channels

A virtual channel consists of a client-side virtual driver that communicates with a server-side application. Citrix Virtual Apps and Desktops ship with various virtual channels included. They’re designed to allow customers and third-party vendors to create their own virtual channels by using one of the provided Software Development Kits (SDKs).

Virtual channels provide a secure way to accomplish various tasks. For example, an application that is running on a Citrix Virtual Apps server that is communicating with a client-side device or an application that is communicating with the client-side environment.

On the client side, virtual channels correspond to virtual drivers. Each virtual driver provides a specific function. Some are required for normal operation, and others are optional. Virtual drivers operate at the presentation layer protocol level. There can be several protocols active at any time by multiplexing channels that are provided by the Windows Station (WinStation) protocol layer.

The following functions are contained in the VirtualDriver registry value under this registry path:

HKEY_LOCAL_MACHINE\SOFTWARE\Citrix\ICA Client\Engine\Configuration\Advanced\Modules\ICA 3.0

HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Citrix\ICA Client\Engine\Configuration\Advanced\Modules\ICA 3.0 (for 64-bit)

Thinwire3.0 (Required)
ClientDrive
ClentPrinterQueue
ClentPrinterPort
Clipboard
ClientComm
ClientAudio
LicenseHandler (Required)
TWI (Required)
SmartCard
ICACTL (Required)
SSPI
TwainRdr
UserEXperience
Vd3d

Note:

You can disable specific client functionality by removing one or more of these values from the registry key. For example, if you wanted to remove the Client Clipboard, remove the word Clipboard.

This list contains the client virtual driver files and their respective functions. Citrix Virtual Apps and Citrix Workspace app for Windows use these files. They are in the form of Dynamic Link Libraries (user mode), and not Windows drivers (kernel mode) except for Generic USB as described in the Generic USB virtual channel.

vd3dn.dll – Direct3D virtual channel used for desktop composition redirection
vdcamN.dll – Bidirectional audio
vdcdm30n.dll – Client drive mapping
vdcom30N.dll - Client COM port mapping
vdcpm30N.dll – Client printer mapping
vdctln.dll – ICA controls channel
vddvc0n.dll – Dynamic virtual channel
vdeuemn.dll - End user experience monitoring
vdgusbn.dll – Generic USB virtual channel
vdkbhook.dll – Transparent key pass-through
vdlfpn.dll – Framehawk display channel over UDP like transport
vdmmn.dll – Multimedia support
vdmrvc.dll – Mobile Receiver virtual channel
vdmtchn.dll - Multi-touch support
vdscardn.dll – Smartcard support
vdsens.dll – Sensors virtual channel
vdspl30n.dll – Client UPD
vdsspin.dll – Kerberos
vdtuin.dll – Transparent UI
vdtw30n.dll – Client Thinwire
vdtwin.dll – Seamless
vdtwn.dll – Twain

Some virtual channels are compiled into other files. For example Clipboard Mapping is available in wfica32.exe

64-bit compatibility

Citrix Workspace app for Windows is 64-bit compatible. As with most of the binaries compiled for 32 bit, these client files have 64-bit compiled equivalents:

brapi64.dll
confmgr.dll
ctxlogging.dll
ctxmui.dll
icaconf.exe
icaconfs.dll
icafile.dll
pnipcn64.dll
pnsson.dll
ssoncom.exe
ssonstub.dll
vdkbhook64.dll

Generic USB virtual channel

Generic USB virtual channel implementation uses two kernel mode drivers along with the virtual channel driver vdgusbn.dll:

ctxusbm.sys
ctxusbr.sys

How ICA virtual channels work

Virtual channels are loaded in multiple ways. The Shell (WfShell for the server and PicaShell for the workstation) load some virtual channels. Some virtual channels are hosted as windows services.

Virtual channel modules loaded by the Shell, for example:

EUEM
Twain
Clipboard
Multimedia
Seamless session sharing
Time Zone

Some are loaded as kernel mode, for example:

CtxDvcs.sys – Dynamic virtual channel
Icausbb.sys – Generic USB redirection
Picadm.sys – Client drive mapping
Picaser.sys – COM port redirection
Picapar.sys – LPT port redirection

Graphics virtual channel on the server side

The ctxgfx.exe hosts the graphics virtual channel for both workstation and terminal server based sessions. Ctxgfx hosts platform specific modules that interact with the corresponding driver (Icardd.dll for RDSH and vdod.dll and vidd.dll for workstation).

For XenDesktop 3D Pro deployments an OEM graphics driver is installed for the corresponding GPU on the VDA. Ctxgfx loads specialized adaptor modules to interact with the OEM graphics driver.

Hosting specialized channels in windows services

On Citrix Virtual Apps and Desktops servers, various channels are hosted as windows services. Such hosting provides one-to-many semantics for multiple applications in a session and multiple sessions on the server. Examples of such services include:

Citrix Device Redirector Service
Citrix Dynamic Virtual Channel Service
Citrix End User Experience Monitoring Service
Citrix Location and Sensor Virtual Channel Service
Citrix MultiTouch Redirection Service
Citrix Print Manager Service
Citrix Smartcard Service
Citrix Audio Redirection Service (Citrix Virtual Desktops only)
Citrix ICA Status Channel Service

The audio virtual channel on Citrix Virtual Apps is hosted using Windows Audio service.

On the server side, all client virtual channels are routed through the WinStation driver, Wdica.sys. On the client side, the corresponding WinStation driver, built into wfica32.exe, polls the client virtual channels. This image illustrates the virtual channel client-server connection.

Virtual channel connections

This overview contains a client-server data exchange using a virtual channel.

The client connects to the Citrix Virtual Apps and Desktops server. The client passes information about the virtual channels it supports to the server.
The server-side application starts, obtains a handle to the virtual channel, and optionally queries for additional information about the channel.
The client virtual driver and server-side application pass data using the following two methods:
- If the server application has data to send to the client, the data is sent to the client immediately. When the client receives the data, the WinStation driver de-multiplexes the virtual channel data from the ICA stream and immediately passes it to the client virtual driver.
- If the client virtual driver has data to send to the server, the data is sent the next time the WinStation driver polls it. When the server receives the data, it is queued until the virtual channel application reads it. There is no way to alert the server virtual channel application that data was received.
When the server virtual channel application is completed, it closes the virtual channel and frees any allocated resources.

Creating your own virtual channel using the Virtual Channel SDK

Note:

Citrix SDKs are available in the Citrix Developer portal at https://developer.cloud.com.

Creating a virtual channel using the Virtual Channel SDK requires intermediate programming knowledge. Use this method to provide a major communication path between the client and the server. For example, if you are implementing usage of a device on the client side, such as a scanner, to be used with a process in the session.

Note:

The Virtual Channel SDK requires the WFAPI SDK to write the server side of the virtual channel.

Because of enhanced security for Citrix Virtual Apps and Desktops, you must specify which virtual channels are allowed to be opened in an ICA session. For more information, see Virtual channel allow list policy settings.

Creating your own virtual channel using the ICA Client Object SDK

Creating a virtual channel using the ICA Client Object (ICO) is easier than using the Virtual Channel SDK. Use the ICO by creating a named object in your program using the CreateChannels method.

Important:

Because of enhanced security starting with the 10.00 version of the Citrix Receiver for Windows and later (and Citrix Workspace apps for Windows), you must take an extra step when creating an ICO virtual channel.

Pass-through functionality of virtual channels

Most virtual channels that Citrix provides operate unmodified when you use the Citrix Workspace app for Windows within an ICA session (also known as a pass-through session). There are considerations when using the client in extra hops.

The following functions operate the same way in single or multiple hops:

Client COM port mapping
Client drive mapping
Client printer mapping
Client UPD
End user experience monitoring
Generic USB
Kerberos
Multimedia support
Smartcard support
Transparent key pass-through
Twain

As the inherent nature of latency and factors such as compression and decompression and rendering being performed at each hop, performance might be affected with each additional hop that the client undergoes. The affected areas are:

Bidirectional audio
File transfers
Generic USB redirection
Seamless
Thinwire

Important:

By default, the client drives mapped by an instance of the client running in a pass-through session are restricted to the client drives of the connecting client.

Pass-through functionality of virtual channels between a Citrix Virtual Desktop session and a Citrix Virtual App session

Most virtual channels provided by Citrix operate unmodified when you use Citrix Workspace app for Windows within an ICA session on a Citrix Virtual Desktops server (also known as a pass-through session).

Specifically, on the Citrix Virtual Desktops server, there is a VDA hook that runs picaPassthruHook. This hook makes the client think it’s running on a CPS server, and placing the client into its traditional pass-through mode.

We support the following traditional virtual channels and their functionality:

Client
Client COM port mapping
Client drive mapping
Client printer mapping
Generic USB (limited due to performance)
Multimedia support
Smartcard support
SSON
Transparent key pass-through

Security and ICA virtual channels

Securing usage is an important part of planning, developing, and implementing virtual channels. There are several references to specific areas of security located throughout this document.

Best practices

Open virtual channels when you Connect and Reconnect. Close virtual channels when you log off and Disconnect.

Keep the following guidelines in mind when you create scripts that use virtual channel functions.

Naming the Virtual Channels:

You can create a maximum of 32 virtual channels. Seventeen of the 32 channels are reserved for special purposes.

Virtual channel names must not be more than seven characters in length.
The first three characters are reserved for the vendor name, and the next four for the channel type. For example, CTXAUD represents the Citrix audio virtual channel.

Virtual channels are referred to by a seven-character (or shorter) ASCII name. In some previous versions of the ICA protocol, virtual channels were numbered. The numbers are now assigned dynamically based on the ASCII name, making implementation easier. Users who are developing virtual channel code for internal use only can use any seven-character name that does not conflict with existing virtual channels. Use only numbers and upper and lowercase ASCII. Follow the existing naming convention when adding your own virtual channels. There are several predefined channels. The predefined channels begin with the OEM identifier CTX and are for use only by Citrix.

Double-Hop Support:

Virtual Channel	Is double hop supported?
Audio	No
Browser Content Redirection	No
CDM	Yes
CEIP	No
Clipboard	Yes
Continuum (MRVC)	No
Control VC	Yes
HTML5 Video Redirection (v1)	Yes
Keyboard, Mouse	Yes
MultiTouch	No
NSAPVC	No
Printing	Yes
SensVC	No
Smartcard	Yes
Twain	Yes
USB VC	Yes
WAYCOM devices -K2M using USB VC	Yes
Webcam Video Compression	Yes
Windows Media Redirection	Yes