RTC Client API General Registry Settings

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Windows Mobile Not SupportedWindows Embedded CE Supported


The RTC Client API uses the registry to control encryption and the type of service (TOS, or DiffServ) for audio packets generated by the RTC Client API.

The HKEY_LOCAL_MACHINE\Comm\RTC\PortRange registry key configures Port Manager settings.

Value : type Description

Enabled : REG_DWORD

Enables Port range as defined by subsequent port key values


MaxSipDynamicPort: REG_DWORD

Used for setting the port range for SIP signaling traffic.


MaxMediaPort : REG_DWORD

Used for setting the port range for audio/video RTP and RTCP traffic.

The HKEY_LOCAL_MACHINE\Comm\RTC\QoS registry key configures SIP and Media Stack QoS settings.

Value : type Description

Disabled : REG_DWORD

When = 1, disables setting of DSCP field in IP header and layer 2 tagging. By default DSCP QoS is On.


Defines the DSCP index value to be used for SIP packets. By default, DSCPAudio (index 5) is used. DSCPAudio corresponds to 0x38 in the IP DSCP/TOS field.


Defines the DSCP index value to be used for Audio RTP packets. By default, DSCPAudio (index 5) is used. DSCPAudio corresponds to 0x38 in IP DSCP/TOS field.


Defines the DSCP index value to be used for Audio RTCP packets. By default, DSCPAudio (index 5) is used. DSCPAudio corresponds to 0x38 in the IP DSCP/TOS field.

The HKEY_LOCAL_MACHINE\Comm\RTC registry key sets the minimum Expiry time that RTC allows from the server. RTC sends this value to the server in the SIP REGISTER message. The Server can request a longer Expiry time and RTC will accept the change. If the Server requests a shorter Expiry time, RTC will reject it and registration will fail.

Value : type Description

MinExpiresTimeoutinSec : REG_DWORD

Sets the minimum Expiry time. The registry key value can be set from 5 to 9999999 seconds. If the registry key is not set, a default value of 19 seconds is used. RTC will reject any value from the server that is less than the value set or less than the default value.

The HKEY_LOCAL_MACHINE\Comm\RTC\DeviceSettings registry key configures the Media Stack devices settings.

Value : type Description

DeviceSSMode : REG_DWORD

If set to 0, will disable silence suppression and all audio packets, including silence will be transmitted during an audio call


Default is 0.

If set to 1, disables Auto Gain Control (AGC) in RTC.

The HKEY_LOCAL_MACHINE\Comm\RTC\Threads registry key configures Media Stack performance tuning.

Value : type Description

EngineThreadsPriority : REG_DWORD

Changes the priority at which media stack thread is running (default is 221). Acceptable value is between 1 and 255. Caution should be used in changing the default priority as this may impact the way audio data is captured and rendered and may also affect other components running on the system.

TimerFrequency : REG_DWORD

Sets the polling time period for the RTC media engine stack. The registry key value can be set between 10000 (1 millisecond (ms)) and 1000000 (100 ms). The default value is 5 ms (value 50000).

The HKEY_LOCAL_MACHINE\Comm\RTC\NAT registry key configures Media Stack NAT settings.

Value : type Description

EnableEarlyUDPPackets : REG_DWORD

When set to 1, enables sending Early UDP packets on RTP and RTCP ports, whenever RTC stack detects a change in remote address/port. This enables opening of ports on a NAT, incase RTC client is behind one. This way, traffic from the remote client can be received through the NAT.

The HKEY_LOCAL_MACHINE\Comm\RTC\DTMF registry key configures Media Stack in-band DTMF settings.

Value : type Description


Sets the time gap that RTC should introduce between sending 2 in-band DTMF tones. The default value is 250ms and the registry key is settable in ms between 100 and 1000.

DisableOOBDTMFPlayback : REG_DWORD

Disables playback of Out of Band DTMF events. By default, RTC media stack plays back these events with the corresponding DTMF tones

OOBDTMFPlaybackDuration : REG_DWORD

Controls the duration of the Out of Band DTMF tone playback. The registry key can be set between 10 and 500 ms. By default, tones played back are of 200 ms.

Applies to Windows Embedded CE 6.0 R2

The HKEY_LOCAL_MACHINE\Comm\RTC\Session Timers registry key enables and disables the session timers. It also modifies the default and minimum values.

Value : type Description

SessionTimerOn : REG_DWORD

Enables and disables the session timers. The value 0x0 disables the session timers. Any other value will enable the session timers. The default value is 0x0.

SessionExpire : REG_DWORD

Sets the length of time, in seconds, that a session should last without being refreshed. The value should be equal to or greater than MinimumSessionExpire. The default value for SessionExpire is 1800 seconds.

MinimumSessionExpire : REG_DWORD

Sets the minimum value in seconds. The default value is 90 seconds.

> [!NOTE] > The value for MinimumSessionExpire must not be less than 90 seconds.

The HKEY_LOCAL_MACHINE\COMM\RTC\Register specifies alternate behavior for the REGISTER request.

Value : type Description

Expires : REG_DWORD

This value is only valid for the first REGISTER request.

If set to 0, the Expires header will not be added to the REGISTER message. If the value is greater than 0, the value specifies the number of seconds before the registration times out. If the value is greater than 0 and less than 5, the value will be set to 5 implicitly. The recommended value is 3600 seconds. The default value is 0.

MaxRetryBackOffLimitInSec : REG_DWORD

Sets the back off limit for Register retries when the registration fails. Registration failures can occur for many reason, for example socket errors and IP address changes. If the registration fails, RTC starts a registration cycle to re-register. For every registration attempt failure, RTC will back off exponentially, until the back off limit is reached.

The default value is 600 seconds.

The HKEY_LOCAL_MACHINE\Comm\RTC\SymmetricResponseRouting registry key enables and disables symmetric response routing. If symmetric response routing is enabled, the client can add an rport extension to SIP messages. This allows SIP messages to traverse through a symmetric National Address Translator (NAT) when the messages are sent over unreliable unicast transport.

The rport extension is only added to messages sent over UDP/unicast when this feature is enabled. The parameter will not be added if the feature is disabled or if messages are sent over UDP/multicast, TCP or TLS.

Notice that when the rport extension is enabled, symmetric UDP signaling is also implicitly enabled. This setting is associated with the RTCIF_ENABLE_SYMMETRIC_UDP_SIGNALING bit of the init flag.

Value : type Description

ResponsePort : REG_DWORD

If set to 0, the rport extension is disabled. If the value is non-zero the rport extension is enabled. The default value is 0, or disabled.

The HKEY_LOCAL_MACHINE\Comm\RTC\UnsolicitedNotification registry key configures the treatment for Unsolicited Notification message by the RTC stack.

Value : type Description

Response : REG_DWORD

Configures the RTC stack's response to unsolicited Notification messages.

Value Response
0 If the application is listening for these types of notification, return 200-OK, otherwise return 481.
1 Always return 481.
All other values Reserved.

The default value is 0.

The HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\EventLog\VOIP registry key enables RTC event logging. Set the registry keys to the following values.

Value : type Description

Format : String

Specifies the format. Set to PlainText.

Output : String

Output type. Set the type to file.

File : String

Specifies the name of the file. Set the file type to "txt." For example, set the file name to "VoipLog.txt".


Specifies the max size in hexadecimal. Set MaxSize to 12C00.

EventMessageFile : REG_DWORD

Specifies the dll for the event message file. Set the value to "evtlgres.dll".

See Also


RTC Client API Registry Settings
RTC Client API Audio Codec Registry Settings