Developer:LPEC

Introduction

This article describes the Linn Protocol for Eventing and Control (LPEC), which is a TCP/IP point-2-point protocol that can be used to control Linn's home network products (e.g. DS players).

The primary means of control for such products is, of course, UPnP. But  LPEC provides an alternative mechanism for accessing UPnP services over a basic "telnet-like" protocol.

The UPnP architecture subdivides product control into smaller named units called "services". A DS player, for instance, provides some standard UPnP AV services:

• AVTransport
  
• RenderingControl
  
• ConnectionManager

and additional Linn-specific services, such as:

• Playlist
• Radio
• Product

By opening a raw TCP/IP socket connection to port 23 on a Bute (or later) home network product, these same services can be controlled using LPEC.

The rest of this article describes the format of the messages that LPEC expects and delivers.  Examples are given.

Connection

LPEC can be accessed by connecting a raw socket session to port 23 on the device.
The Linn DS can only support up to 4 simultaneous sessions. If there are more than 4sessions open at the same time, the DS will ignore any commands from these new sessions, but remain in full control by the first 4 sessions.

Control

Each service contains actions, which are like methods that can be called on the device with input and output arguments.

An action can be perfomed by sending a message of the following form:

ACTION [sub-device]/[service] [version] [action] "[inarg1]" "[inarg2]" ... "[inargn]"

e.g.

ACTION MediaRenderer/RenderingControl 1 SetVolume "0" "Master" "50"

All messages sent using LPEC get a response. The response to an ACTION message is a RESPONSE message that reports the values of all the output arguments:

RESPONSE "[outarg1]" "[outarg2]" ... "[outargn]"

The SetVolume example given above has no output arguments, so the message received will simply be a <CR><LF> terminated RESPONSE message.

The following subsequent interaction better illustrates the RESPONSE message:

ACTION MediaRenderer/RenderingControl 1 GetVolume "0" "Master"

RESPONSE "50"

It should be mentioned that all input and output arguments are escaped according to XML escaping rules and enclosed in double-quotes.

Eventing

In order to subscribe to a service's events, issue a SUBSCRIBE message of the following form:

SUBSCRIBE [sub-device]/[service]

The device responds with:

SUBSCRIBE [subscription-id]

and follows this with an initial event message, which contains the current state of all the service's evented variables:

EVENT [subscription-id] 0 [var1-name] "[var1]" [var2-name] "[var2]" ... [varn-name] "[varn]"

e.g

SUBSCRIBE Ds/Product
SUBSCRIBE 49
EVENT 49 0 ProductName "Sneaky Music DS" ProductRoom "Main Room" ProductStandby "true" ProductSourceIndex "0"

Subsequent changes to the service's evented variables cause unsolicited EVENT messages to be sent from the device as follows:

EVENT [subscription-id] [sequence-no] [var1-name] "[var1]" [var2-name] "[var2]" ... [varn-name] "[varn]"

Only those variables that have changed are reported.

The sequence number is a 32-bit unsigned number which increments with each event message and wraps around to 1.

A sequence number of 0 is reserved for the initial EVENT message, which contains the current state of all the service's evented variables (see above)

By issuing multiple SUBSCRIBE messages, it is possible to subscribe to up to 16 services simultaneously.

In order to unsubscribe from a service, send an UNSUBSCRIBE message of the form:

UNSUBSCRIBE [subscription-id]

alternatively:

UNSUBSCRIBE [sub-device]/[service]

or, to unsubscribe from all previous subscriptions, just:

UNSUBSCRIBE

In all instances, the response will be:

UNSUBSCRIBE [subscription-id]

When using the form that unsubscribes from all previous subscriptions, multiple UNSUBSCRIBE responses may be received.

e.g.

UNSUBSCRIBE
UNSUBSCRIBE 62
UNSUBSCRIBE 61

Discovery

Although LPEC does not include a mechanism for discovering devices across a home network, it does contain a mechanism for discovering sub-devices within a single product.

When an LPEC connection is made, the device sends an ALIVE message for each sub-device that is currently enabled:

ALIVE [sub-device] [udn]

e.g. (from a Sneaky Music DS)

ALIVE Ds 4c494e4e-0050-c221-71e5-df000003013f
ALIVE Preamp 4c494e4e-0050-c221-71e5-df0000030133
ALIVE MediaRenderer 4c494e4e-0050-c221-71e5-df0000030171

It is possible for these sub-devices to be disabled and enabled during the normal operation of a product. If this occurs, further BYEBYE and ALIVE messages are issued accordingly.

If a device is rebooted, the appropriate BYEBYE messages are sent and the LPEC connection is closed by the device.

When a sub-device is disabled, all subscriptions to the services on that sub-device are forcibly revoked. So, a BYEBYE message is commonly preceded by a number of unsolicited UNSUBSCRIBE messages.

Errors

After sending an ACTION, SUBSCRIBE, or UNSUBSCRIBE message, it is possible to receive an ERROR message of the form:

ERROR [code] "[description]"

The following errors are defined:

ERROR 101 "Command not recognised"
ERROR 102 "Service not specified"
ERROR 103 "Service not found"
ERROR 104 "Version invalid"
ERROR 105 "Version not specified"
ERROR 106 "Version not supported"
ERROR 107 "Method not specified"
ERROR 108 "Method execution exception"
ERROR 201 "Boolean argument invalid"
ERROR 202 "String argument invalid"
ERROR 203 "Unsigned numeric argument invalid"
ERROR 204 "Signed numeric invalid"
ERROR 205 "Binary argument invalid"
ERROR 206 "Invalid argument escaping"
ERROR 301 "Argument list incomplete"
ERROR 302 "Argument not quoted"
ERROR 303 "Argument incomplete"
ERROR 401 "Already subscribed"
ERROR 402 "Client has too many subscriptions"
ERROR 403 "Service has too many subscriptions"
ERROR 404 "Subscription not found"
ERROR 405 "Service not subscribed"
ERROR 406 "Invalid XML escaping"

Service-specific ERROR messages may also be sent.

Examples

Play, Pause, Skip a DS player using the UPnP AVTransport service:

ACTION MediaRenderer/AVTransport 1 Play "0" "1"

ACTION MediaRenderer/AVTransport 1 Pause "0"

ACTION MediaRenderer/AVTransport 1 Next "0"

ACTION MediaRenderer/AVTransport 1 Previous "0"

Get the volume of a Linn pre-amplifier using the Linn Preamp service

ACTION Preamp/Preamp 1 Volume

RESPONSE "40"

Set mute on a Linn pre-amplifier using the Linn Preamp service

ACTION Preamp/Preamp 1 SetMute "true"

RESPONSE

Select an Input on the DS/DSI/DSM

ACTION Ds/Product 1 SetSourceIndex "0"

RESPONSE

SetSourceIndex	Linn DS	Sekrit DSI	Kiko	Akurate DSM
0	Playlist	Playlist	Playlist	Playlist
1	Radio	Radio	Radio	Radio
2	UpnpAv	UpnpAv	UpnpAv	UpnpAv
3	Songcast	Songcast	Songcast	Songcast
4	NetAux	NetAux	NetAux	NetAux
5	---	Analog	Analog	Analog1
.... etc
9	---	---	HDMI 1	SPDIF1
.... etc
15	---	---	---	HDMI 1
.... etc

See attachment below for more detail: LPEC examples

Other Considerations

If a control point closes an LPEC session, all outstanding subscriptions are automatically cleaned up by the device.

Each product supports only one concurrent LPEC session.

There is a bug which means that the first command sent after initial ALIVE messages are received will generate an error. All subsequent commands will work as expected. To work around this it is suggested that blank command is sent immediately after ALIVE messages ar received to clear this spurious error response.

Services and Actions Discovery

LPEC doesn't implement a service discovery protocol.

The specific set of services and actions implemented on each device is dependent on the device type.  The simplest method of determining which ACTIONs are available over LPEC is to discoverthe services via UPnP which can then be trivially mapped onto LPEC ACTIONs.  This can be done as follows:

• Use UPnP service discovery tools. (such as Intel's Device Spy or Device Monitor Intel Intel tools or Open-Source Dev Tools)
• Inspecting the device and service description directly from the device, (eg, a DS device on IP address 192.168.0.123:55178 is described by

  http://192.168.0.123:55178/Ds/device.xml

  From this description we can (for example) see that the device has a "Media" service, which is described here,

  http://192.168.0.123:55178/Ds/Media/service.xml

• ...or if you know the device type, you can inspect the published service description online.

UPnP documentation for the Linn DS products can be found in Openhone.org

Command Summary

LPEC examples

Sent

Invoke Action	ACTION [sub-device]/[service] [version] [action] "[inarg1]" "[inarg2]" ... "[inargn]"
Subscribe Service	SUBSCRIBE [sub-device]/[service]
Unsubscribe Id	UNSUBSCRIBE [subscription-id]
Unsubscribe Service	UNSUBSCRIBE [sub-device]/[service]
Unsubscribe All	UNSUBSCRIBE

Received

Action Response	RESPONSE "[outarg1]" "[outarg2]" ... "[outargn]"
Subscribe Response	SUBSCRIBE [subscription-id]
Initial Event	EVENT [subscription-id] 0 [var1-name] "[var1]" [var2-name] "[var2]" ... [varn-name] "[varn]"
Event	EVENT [subscription-id] [sequence-no] [var1-name] "[var1]" [var2-name] "[var2]" ... [varn-name] "[varn]"
Unsubscribe Response	UNSUBSCRIBE [subscription-id]
Sub-Device Enabled	ALIVE [sub-device] [udn]
Sub-Device Disabled	BYEBYE [sub-device] [udn]
Error	ERROR [code] "[description]"