Document authors:

  • Jonathan Moore Liles (revisions from 1 to revision 1.2 from 2013-04-06)

  • Nils Hilbricht (2020-07 initial adaptation of the NON project version)

  • Nedko Arnaudov (2022 onward, as part of LADI project)






This document describes the NON Session Manager OSC API as defined and implemented by NON project. The only difference is server-control capability which was fixed in NON project codebase before the New Session Manager schism but as of when this document is written, is not fixed at the NSM API URL, see for more info.

For definition of the new-session-manager OSC API, see

Session Management as part of the Non audio production suite

The Non Session Management API is used by the various components of the Non audio production suite to allow any number of independent programs to be managed together as part of a logical session (i.e. a song). Thus, operations such as loading and saving are synchronized.

The API comprises a simple Open Sound Control (OSC) based protocol, along with some behavioral guidelines, which can easily be implemented by various applications.

The Non project contains an program called nsmd which is an implementation of the server side of the NSM API. nsmd is controlled by the non-session-manager GUI. However, the same server-side API can also be implemented by other session managers (such as LADISH), although consistency and robustness will likely suffer if non-NSM compliant clients are allowed to participate in a session. The only dependency for client implementations liblo (the OSC library), which several Linux audio applications already link to or plan to link to in the future.

The aim of this project is to thoroughly define the behavior required of clients. This is an area where other attempts at session management (LASH and JACK-Session) have failed. Often the difficulty with these systems has been not in implementing support for them, but in attempting to interpret the confusing, ambiguous, or ill-conceived API documentation. For these reasons and more all previous attempts at Linux audio session management protocols are considered harmful.

You WILL see some unambiguous and emphatic language in this document. For the good of the user, these rules are meant to be followed and are non-negotiable. If an application does not conform to this specification it should be considered broken. Consistency across applications under session management is very important for a good user experience.

Client Behavior Under Session Management

Most graphical applications make available to the user a common set of file operations, typically presented under a File or Project menu.

These are: New, Open, Save, Save As, Close and Quit or Exit.

The following sub-sections describe how these options should behave when the application is part of an NSM session. These rules only apply when session management is active (that is, after the announce handshake described in the [NSM OSC Protocol] section. In order to provide a consistent and predictable user experience, it is critically important for applications to adhere to these guidelines.

File Menu


This option may empty/reset the current file or project (possibly after user confirmation). UNDER NO CIRCUMSTANCES should it allow the user to create a new project/file in another location.


This option MUST be disabled.

The application may, however, elect to implement an option called Import into Session, creates a copy of a file/project which is then saved at the session path provided by NSM.


This option should behave as normal, saving the current file/project as established by the NSM open message.

UNDER NO CIRCUMSTANCES should this option present the user with a choice of where to save the file.

Save As

This option MUST be disabled.

The application may, however, elect to implement an option called Export from Session, which creates a copy of the current file/project which is then saved in a user-specified location outside of the session path provided by NSM.

Close (as distinguished from Quit or Exit)

This option MUST be disabled unless its meaning is to disconnect the application from session management.

Quit or Exit

This option may behave as normal (possibly asking the user to confirm exiting).

Data Storage

Internal Files

All project specific data created by a client MUST be stored in the per-client storage area provided by NSM. This includes all recorded audio and MIDI files, snapshots, etc. Only global configuration items, exports, and renders of the project may be stored elsewhere (wherever the user specifies).

External Files

Files required by the project but external to it (typically read-only data such as audio samples) SHOULD be referenced by creating a symbolic link within the assigned session area, and then referring to the symlink. This allows sessions to be archived and transported simply (e.g. with "tar -h") by tools that have no knowledge of the project formats of the various clients in the session. The symlinks thus created should, at the very least, be named after the files they refer to (some unique component may be required to prevent collisions)

NSM OSC Protocol

All message parameters are REQUIRED. All messages MUST be sent from the same socket as the announce message, using the lo_send_from method of liblo or its equivalent, as the server uses the return addresses to distinguish between clients.

Clients MUST create thier OSC servers using the same protocol (UDP,TCP) as found in NSM_URL. liblo is lacking a robust TCP implementation at the time of writing, but in the future it may be useful.

Establishing a Connection


At launch, the client MUST check the environment for the value of NSM_URL. If present, the client MUST send the following message to the provided address as soon as it is ready to respond to the /nsm/client/open event:

/nsm/server/announce s:application_name s:capabilities s:executable_name i:api_version_major i:api_version_minor i:pid

If NSM_URL is undefined, invalid, or unreachable, then the client should proceed assuming that session management is unavailable.

api_version_major and api_version_minor must be the two parts of the version number of the NSM API as defined by this document.

Note that if the application intends to register JACK clients, application_name MUST be the same as the name that would normally be passed to jack_client_open. For example, Non-Mixer sends "Non-Mixer" as its application_name. Applications MUST NOT register their JACK clients until receiving an open message; the open message will provide a unique client name prefix suitable for passing to JACK. This is probably the most complex requirement of the NSM API, but it isn’t difficult to implement, especially if the application simply wishes to delay its initialization process breifly while awaiting the announce reply and subsequent open message.

capabilities MUST be a string containing a colon separated list of the special capabilities the client possesses. e.g. :dirty:switch:progress:

executable_name MUST be the executable name that the program was launched with. For C programs, this is simply the value of argv[0]. Note that hardcoding the name of the program here is not the same as using, as the user may have launched the program from a script with a different name using exec, or have created a symlink to the program. Getting the correct value in scripting languages like Python can be more challenging.

Table 1. Available Client Capabilities




client is capable of responding to multiple open messages without restarting


client knows when it has unsaved changes


client can send progress updates during time-consuming operations


client can send textual status updates


client has an optional GUI


The server will respond to the client’s announce message with the following message:

/reply "/nsm/server/announce" s:message s:name_of_session_manager s:capabilities

message is a welcome message.

The value of name_of_session_manager will depend on the implementation of the NSM server. It might say "Non Session Manager", or it might say "LADISH". This is for display to the user.

capabilities will be a string containing a colon separated list of special server capabilities.

Presently, the server capabilities are:

Table 2. Available Server Capabilities




client-to-server control


server responds to /nsm/server/broadcast message


server responds to optional-gui messages—if this capability is not present then clients with optional-guis MUST always keep them visible

A client should not consider itself to be under session management until it receives this response. For example, the Non applications activate their "SM" blinkers at this time.

If there is an error, a reply of the following form will be sent to the client:

/error "/nsm/server/announce" i:error_code s:error_message

The following table defines possible values of error_code:

Table 3. Response codes




General Error


Incompatible API version


Client has been blacklisted.

Server to Client Control Messages

Compliant clients MUST accept the client control messages described in this section. All client control messages REQUIRE a response. Responses MUST be delivered back to the sender (NSM) from the same socket used by the client in its announce message (by using lo_send_from) AFTER the action has been completed or if an error is encountered. The required response is described in the subsection for each message.

If there is an error and the action cannot be completed, then error_code MUST be set to a valid error code (see [Error Code Definitions]) and message to a string describing the problem (suitable for display to the user).

The reply can take one of the following two forms, where path MUST be the path of the message being replied to (e.g. "nsm/client/save":

/reply s:path s:message
/error s:path i:error_code s:message


There is no message for this. Clients will receive the Unix SIGTERM signal and MUST close cleanly IMMEDIATELY, without displaying any kind of dialog to the user and regardless of whether or not unsaved changes would be lost. When a session is closed the application will receive this signal soon after having responded to a save message.


/nsm/client/open s:path_to_instance_specific_project s:display_name s:client_id

path_to_instance_specific_project is a path name assigned to the client for storing its project data.

The client may append to the path, creating a sub-directory, e.g. / or simply append the client’s native file extension (e.g. .non or .XML). The same transformation MUST be applied to the name when opening an existing project, as NSM will only provide the instance specific part of the path.

If a project exists at the path, the client MUST immediately open it.

If a project does not exist at the path, then the client MUST immediately create and open a new one at the specified path or, for clients which hold all their state in memory, store the path for later use when responding to the save message.

No file or directory will be created at the specified path by the server. It is up to the client to create what it needs.

For clients which HAVE NOT specified the :switch: capability, the open message will only be delivered once, immediately following the announce response.

For clients which HAVE specified the :switch: capability, the client MUST immediately switch to the specified project or create a new one if it doesn’t exist.

Clients which are incapable of switching projects or are prone to crashing upon switching MUST NOT include :switch: in their capability string.

If the user the is allowed to run two or more instances of the application simultaneously (that is to say, there is no technical limitation preventing them from doing so, even if it doesn’t make sense to the author), then such an application MUST PRE-PEND the provided client_id string to any names it registers with common subsystems (e.g. JACK client names). This ensures that multiple instances of the same application can be restored in any order without scrambling the JACK connections or causing other conflicts. The provided client_id will be a concatenation of the value of application_name sent by the client in its announce message and a unique identifier. Therefore, applications which create single JACK clients can use the value of client_id directly as their JACK client name. Applications which register multiple JACK clients (e.g. Non-Mixer) MUST PRE-PEND client_id value to the client names they register with JACK and the application determined part MUST be unique for that (JACK) client.

For example, a suitable JACK client name would be: $CLIENT_ID/track-1

Note that this means that the application MUST NOT register with JACK (or any other subsystem requiring unique names) until it receives an open message from NSM. Likewise, applications with the :switch: capability should close their JACK clients and re-create them with using the new client_id. Re-registering is necessary because the JACK API does currently support renaming existing clients, although this is a sorely needed addition.

A response is REQUIRED as soon as the open operation has been completed. Ongoing progress may be indicated by sending messages to /nsm/client/progress.


The client MUST respond to the open message with:

/reply "/nsm/client/open" s:message


/error "/nsm/client/open" i:error_code s:message
Table 4. Response codes




General Error


An existing project file was found to be corrupt


A new project could not be created


Unsaved changes would be lost


Operation cannot be completed at this time



This message will only be delivered after a previous open message, and may be sent any number of times within the course of a session (including zero, if the user aborts the session).

/reply "/nsm/client/save" s:message


/error "/nsm/client/save" i:error_code s:message
Table 5. Response codes




General Error


Project could not be saved


Operation cannot be completed at this time

Server to Client Informational Messages

Session is Loaded

Accepting this message is optional. The intent is to signal to clients which may have some interdependence (say, peer to peer OSC connections) that the session is fully loaded and all their peers are available. Most clients will not need to act on this message. This message has no meaning when a session is being built or run—only when it is initially loaded. Clients who intend to act on this message MUST not do so by delaying initialization waiting for it.


This message does not require a response.

Show Optional Gui

If the client has specified the optional-gui capability, then it may receive this message from the server when the user wishes to change the visibility state of the GUI. It doesn’t matter if the optional GUI is integrated with the program or if it is a separate program \(as is the case with SooperLooper\). When the GUI is hidden, there should be no window mapped and if the GUI is a separate program, it should be killed.


No response is message is required.

Client to Server Informational Messages

Optional GUI

If the client has specified the optional-gui capability, then it MUST send this message whenever the state of visibility of the optional GUI has changed. It also MUST send this message after it’s announce message to indicate the initial visibility state of the optional GUI.

It is the responsibility of the client to remember the visibility state of its GUI across session loads.


No response will be delivered.


/nsm/client/progress f:progress

For potentially time-consuming operations, such as save and open, progress updates may be indicated throughout the duration by sending a floating point value between 0.0 and 1.0, 1.0 indicating completion, to the NSM server.

The server will not send a response to these messages, but will relay the information to the user.

Note that even when using the progress feature, the final response to the save or open message is still REQUIRED.

Clients which intend to send progress messages should include :progress: in their announce capability string.



Some clients may be able to inform the server when they have unsaved changes pending. Such clients may optionally send is_dirty and is_clean messages.

Clients which have this capability should include :dirty: in their announce capability string.

Status Messsages

/nsm/client/message i:priority s:message

Clients may send miscellaneous status updates to the server for possible display to the user. This may simply be chatter that is normally written to the console. priority should be a number from 0 to 3, 3 being the most important.

Clients which have this capability should include :message: in their announce capability string.

Error Code Definitions

Table 6. Error Code Definitions

Symbolic Name

Integer Value





















Client to Server Control

If the server publishes the :server-control: capability, then clients can also initiate action by the server. For example, a client might implement a Save All option which sends a /nsm/server/save message to the server, rather than requiring the user to switch to the session management interface to effect the save.

Server Control API

The session manager not only manages clients via OSC, but it is itself controlled via OSC messages. The server responds to the following messages.

All of the following messages will be responded to, at the sender’s address, with one of the two following messages:

/reply s:path s:message
/error s:path i:error_code s:message

The first parameter of the reply is the path to the message being replied to. The /error reply includes an integer error code (non-zero indicates error). message will be a description of the error.

The possible errors are:

Table 7. Responses




General Error


Launch failed


No such file


No session is open


Unsaved changes would be lost


  • /nsm/server/add s:executable_name

    • Adds a client to the current session.


  • /nsm/server/save

    • Saves the current session.


  • /nsm/server/open s:project_name

    • Saves the current session and loads a new session.


  • /nsm/server/new s:project_name

    • Saves the current session and creates a new session.


  • /nsm/server/duplicate s:new_project

    • Saves and closes the current session, makes a copy, and opens it.


  • /nsm/server/close

    • Saves and closes the current session.


  • /nsm/server/abort

    • Closes the current session WITHOUT SAVING


  • /nsm/server/quit

    • Saves and closes the current session and terminates the server.

List projects

  • /nsm/server/list

    • Lists available projects. One /reply message will be sent for each existing project.

Client to Client Communication

If the server includes :broadcast: in its capability string, then clients may send broadcast messages to each other through the NSM server. Clients may send messages to the server at the path /nsm/server/broadcast.

The format of this message is as follows:

/nsm/server/broadcast s:path [arguments...]

The message will then be relayed to all clients in the session at the path path (with the arguments shifted by one).

For example the message:

/nsm/server/broadcast /tempomap/update "0,120,4/4:12351234,240,4/4"

Would broadcast the following message to all clients in the session (except for the sender), some of which might respond to the message by updating their own tempo maps.

/tempomap/update "0,120,4/4:12351234,240,4/4"

The Non programs use this feature to establish peer to peer OSC communication by symbolic names (client IDs) without having to remember the OSC URLs of peers across sessions.