3. Messages#

This section specifies the structure of the transmissions that happen over a connection with SECoP (“on the wire”). This is primarily relevant for implementors of clients and/or servers, users in general will find Modules more relevant.

3.1. Data transfer#

SECoP relies on a stream transport of 8-bit bytes. Most often this will be a TCP conection. In those cases the SEC node SHOULD support as many parallel simultaneous connections as technically feasible.

“Serial port” style connections may also be used. Here, only a single connection can be used. If several connections are needed, a ‘multiplexer’ is needed. This should offer multiple TCP connections and contain the necessary logic to map requests/replies from/to those network connections onto/from the serial connection to the actual SEC node.

3.2. Transfer modes#

Essentially the connections between an ECS and a SEC node can operate in one of two modes:

Synchronous mode:: where a strict request/reply pattern is used.
Async mode:: where an update may arrive any time (even between requests and replies).

In both cases, a request from the ECS to the SEC node must be followed by an reply from the SEC node to the ECS, either indicating success of the request or flag an error.

Note

An ECS may try to send a request before it received the reply to an earlier request. This has two implications:

A SEC node may serialize requests and fulfill them strictly in order. In that case the ECS should not overflow the input buffer of the SEC node.
The second implication is that an ECS which sends multiple requests before the replies arrive, MUST be able to handle the replies arriving out-of-order. Unfortunately there is currently no indication if a SEC node is operating strictly in order or if it can work on multiple requests simultaneously.

3.3. Messages#

The received byte stream is split into messages, which are the smallest unit of communication for SECoP.

A message is essentially one line of text, coded in ASCII (may be extended to UTF-8 later if needed). A message ends with a line feed character (ASCII 10), which may be preceded by a carriage return character (ASCII 13), which must be ignored.

All messages share the same basic structure:

message_structure ::= action ( SPACE specifier ( SPACE data )? )?

i.e. a message starts with an action keyword, followed optionally by one space and a specifier (not containing spaces), followed optionally by one space and a JSON value (see RFC 8259) called data, which absorbs the remaining characters up to the final LF.

Note

Numerical values and strings appear ‘naturally’ formatted in JSON values, i.e. 5.0 or "a string".

The specifier consists of a module identifier, and for most actions, followed by a colon as separator and an accessible identifier. In special cases (e.g. describe, ping), the specifier is just a token or may be empty:

specifier ::= module | module ":" (parameter|command)

All identifiers (for properties, accessibles and modules) are composed of ASCII letters, digits and underscores, where a digit may not appear as the first character.

Identifiers starting with underscore (‘custom-names’) are reserved for custom extensions, e.g. messages or parameters not specified by the standard. The identifier length is limited (<=63 characters).

Note

Although names MUST be compared/stored case sensitive, names in each scope need to be unique when lowercased. The scopes are:

module names on a SEC node (including the group entries of those modules)
accessible names of a module (including the group entries of those parameters) - each module has its own scope
properties
names of elements in a Collection of Named Items: struct (each struct has its own scope)
names of variants in an :ref:’enum` (each enum has its own scope)
names of qualifiers

SECoP defined names are usually lowercase, though that is not a restriction (esp. not for module names).

A SEC node might implement custom messages for debugging purposes, which are not part of the standard. Custom messages start with an underscore or might just be an empty line. The latter might be used as a request for a help text, when logged in from a command line client like telnet or netcat. Messages not starting with an underscore and not defined in the following list are reserved for future extensions.

When implementing SEC nodes or ECS clients, a ‘MUST-ignore’ policy should be applied to unknown or additional parts. Unknown or malformed messages are to be replied with an appropriate ProtocolError by a SEC node. An ECS-client must ignore the extra data in such messages. See also section Future Compatibility.

3.4. Message overview#

To clarify optionality of some messages, the following table is split into two: basic messages (which MUST be implemented like specified) and extended messages which SHOULD be implemented.

Note

For clarification, the symbol “␣” is used here instead of a space character. <elem> refers to the element elem which is defined in another section.

Basic messages (implementation is mandatory).#
Message intent	Message kind	Message elements
identification	request	`*IDN?`
	reply	`ISSE&SINE2020,SECoP,<version>,<add-info>`
description	request	`describe`
	reply	`describing␣.␣<structure-report>`
activate updates	request	`activate`
	reply	`active`
deactivate updates	request	`deactivate`
	reply	`inactive`
heartbeat	request	`ping␣<identifier>`
	reply	`pong␣<identifier>␣<data-report>`
read request	request	`read␣<module>:<parameter>`
	reply	`reply␣<module>:<parameter>␣<data-report>`
change value	request	`change␣<module>:<parameter>␣<value>`
	reply	`changed␣<module>:<parameter>␣<data-report>`
execute command	request	`do␣<module>:<command>` (argumentless commands)
		`do␣<module>:<command>␣<value>`
	reply	`done␣<module>:<command>␣<data-report>`
value update event	event	`update␣<module>:<parameter>␣<data-report>`
error reply	reply	`error_<action>␣<specifier>␣<error-report>`

All these messages must be implemented. For example, change is mandatory, even if only readonly accessibles are present. In this case, a change message will naturally be replied with an error_change message with an Error class of “ReadOnly” and not with a “ProtocolError”.

Extended messages (implementation is optional).#
Message intent	Message kind	Message elements
logging	request	`logging␣<module>␣<loglevel>`
	reply	`logging␣<module>␣<loglevel>`
	event	`log␣<module>:<loglevel>␣<message-string>`
activate updates,	request	`activate␣<module>`
module-wise	reply	`active␣<module>`
deactivate updates,	request	`deactivate␣<module>`
module-wise	reply	`inactive␣<module>`
heartbeat,	request	`ping`
with empty identifier	reply	`pong␣␣<data-report>`

Theory of operation:

After a connection between an ECS and a SEC node is established, the client must verify that the SEC node is speaking a supported protocol by sending an identification request and checking the answer from the SEC node to comply. If this check fails, the connection is to be closed and an error reported.

The second step is to query the structure of the SEC node by an exchange of description messages. After this step, the ECS knows all it needs to know about this SEC node and can continue to either stick to a request/reply pattern or activate updates. In any case, an ECS should correctly handle updates, even if it didn’t activate them, as that may have been performed by another client on a shared connection.

3.5. Message intents#

3.5.1. Identification#

The syntax of the identification message differs a little from other messages, as it is meant to be compatible with IEEE 488.2. The identification request *IDN? is meant to be sent as the first message after establishing a connection. The reply consists of 4 comma separated fields, where the second and third field determine the used protocol.

In this and in the following examples, messages sent to the SEC node are marked with “> “, and messages sent to the ECS are marked with “< “.

Example:

> *IDN?
< ISSE&SINE2020,SECoP,V2019-09-16,v1.0

So far the SECoP version is given like V2019-09-16, i.e. a capital “V” followed by a date in year-month-day format with 4 and 2 digits respectively. The add-info field was used to differentiate between draft, release candidates and final. It is now used to indicate a release name.

3.5.2. Description#

The next messages normally exchanged are the description request and reply. The reply contains the “structure report”, i.e. a JSON object describing the name of modules exported and their parameters, together with the corresponding properties. This is explained in detail in Descriptive Data.

Example:

> describe
< describing . {"modules":{"t1":{"interface_classes":["TemperatureSensor","Readable"],"accessibles":{"value": ...

The dot (second item in the reply message) is a placeholder for extensibility reasons and may be changed in a later revision. A client implementing the current specification MUST ignore it.

Note

This reply might be a very long line, no raw line breaks are allowed in the JSON part. Clients MUST implement a reasonable buffer size for these replies or use a streaming JSON decoder.

3.5.3. Activate Updates#

The parameterless “activate” request triggers the SEC node to send the values of all its modules and parameters as update messages (initial updates). When this is finished, the SEC node must send an “active” reply (global activation).

This initial update is to help the ECS establish a copy of the ‘assumed-to-be-current’ values. The values transferred are not necessarily read fresh from the hardware, check the timestamps!

Note

An ECS MUST be able to handle the case of an extra update occurring during the initial phase, i.e. it must handle the case of receiving more than one update for any valid specifier.

A SEC node might accept a module name as second item of the message (module-wise activation), activating only updates on the parameters of the selected module. In this case, the “active” reply also contains the module name.

A SEC node not implementing module-wise activation MUST NOT send the module name in its reply to an module-wise activation request, and MUST activate all modules (fallback mode).

3.5.4. Update#

When activated, update messages are delivered without explicit request from the client. The value is a data-report, i.e. a JSON array with the value as its first element, and an JSON object containing the Qualifiers as its second element.

If an error occurs while determining a parameter, an error_update message has to be sent, which includes an Error report stating the problem.

Example:

> activate
< update t1:value [295.13,{"t":150539648.188388,"e":0.01}]
< update t1:status [[400,"heater broken or disconnected"],{"t":1505396348.288388}]
< active
< error_update t1:_heaterpower ["HardwareError","heater broken or disconnected",{"t":1505396349.20}]
< update t1:value [295.14,{"t":1505396349.259845,"e":0.01}]
< update t1:value [295.13,{"t":1505396350.324752,"e":0.01}]

The example shows an activate request triggering an initial update of two values: t1:value and t1:status, followed by the active reply. Also, an error_update for a parameter _heaterpower is shown. After this, two more updates on the t1:value show up with roughly 1 second between each.

Note

It is vital that all initial updates are sent, before the ‘active’ reply is sent! An ECS may rely on having gotten all values.

To speed up the activation process, polling and caching of all parameters on the SEC node is advised, i.e. the parameters should not just be read from hardware for activation, as this may take a long time.

Another example with a broken sensor:

> activate
< error_update t1:value ["HardwareError","Sensor disconnected", {"t":1505396348.188388}]}]
< update t1:status [[400,"Sensor broken or disconnected"],{"t":1505396348.288388}]
< active

Here the current temperature can not be obtained. An error_update message is used instead of update.

3.5.5. Deactivate Updates#

A parameterless message. After the “inactive” reply no more updates are delivered if not triggered by a read message.

Example:

> deactivate
< update t1:value [295.13,{"t":1505396348.188388}]
< inactive

Remark

The update message in the second line was sent before the deactivate message was treated. After the “inactive” message, the client can expect that no more untriggered update message are sent, though it MUST still be able to handle (or ignore) them, if they still occur.

The deactivate message might optionally accept a module name as second item of the message for module-wise deactivation. If module-wise deactivation is not supported, the SEC node should ignore a deactivate message which contains a module name and send an error_deactivate reply. This requires the ECS being able to handle update events at any time!

It is not clear if module-wise deactivation is really useful. A SEC node supporting module-wise activation does not necessarily need to support module-wise deactivation.

3.5.6. Read Request#

With the read request message, the ECS may ask the SEC node about a reasonable recent ‘current’ value. In most cases this means that the hardware is read to give a fresh value. However, there are use cases where either an internal control loop is running anyway in which case it is perfectly fine to return the internally cached value. When it can take a long time to actually obtain a fresh value, it is also acceptable to return the most recently obtained value. In any way, the timestamp qualifier should indicate the time the value was obtained.

Example:

> read t1:value
< reply t1:value [295.13,{"t":1505396348.188}]
> read t1:status
> reply t1:status [[100,"OK"],{"t":1505396348.548}]

3.5.7. Change Value#

The change value message contains the name of the module or parameter and the value to be set. The value is JSON formatted. As soon as the set-value is read back from the hardware, all clients, having activated the parameter/module in question, get an “update” message. After all side-effects are communicated, a “changed” reply is then send, containing a data-report of the read-back value.

Remarks

If the value is not stored in hardware, the “update” message can be sent immediately.
The read-back value should always reflect the value actually used.
A client in async mode may get an update message before the changed message, both containing the same data report.

Example on a connection with activated updates. Qualifiers are replaced by {...} for brevity here.

> read mf:status
< reply mf:status [[100,"OK"],{...}]
> change mf:target 12
< update mf:status [[300,"ramping field"],{...}]
< update mf:target [12,{...}]
< changed mf:target [12,{...}]
< update mf:value [0.01293,{...}]

The status changes from “idle” (100) to “busy” (300). The ECS will be informed with a further update message on mf:status, when the module has finished ramping. Until then, it will get regular updates on the current main value (see last update above).

Note

It is vital that all ‘side-effects’ are realized (i.e. stored in internal variables) and be communicated before the ‘changed’ reply is sent!

Correct handling of side-effects:

To avoid difficult to debug race conditions, the following sequence of events should be followed whenever the ECS wants to initiate an action:

ECS sends the initiating message request (either change or do) and awaits the response.
SEC node checks the request and if it can be performed. If not, SEC node sends an error-reply (sequence done). If nothing is actually to be done, continue to point 4.
If the action is fast-finishing, it should be performed and the sequence should continue to point 4. Otherwise the SEC node ‘sets’ the status code to BUSY and instructs the hardware to execute the requested action. Also an update status event (with the new BUSY status-code) MUST be sent to ALL activated clients (if any). From now on, all read requests will also reveal a BUSY status-code. If additional parameters are influenced, their updated values should be communicated as well.
SEC node sends the reply to the request of point 2 indicating the success or failure of the request.

Note

An error may be replied after the status was set to BUSY if triggering the intended action failed (communication problems?).
When the action is finally finished and the module no longer to be considered BUSY, an update status event to IDLE MUST be sent, also subsequent status queries should reflect the now no longer BUSY state. Of course, all other parameters influenced by this should also communicate their new values.

An ECS establishing more than one connection to the same SEC node and which may process the update event message from point 3 after the reply of point 4 MUST query the status parameter synchronously to avoid the race-condition of missing the (possible) BUSY status-code.

Temporal order should be kept wherever possible!

3.5.8. Execute Command#

Actions can be triggered with a command. If an action needs significant time to complete (i.e. longer than a fraction of a second), the information about the duration and success of such an action has to be transferred via the status parameter.

If a command is specified with an argument, the actual argument is given in the data part as a JSON value. This may be also a JSON object if the datatype of the argument specifies that (i.e. the type of the single argument can also be a struct, tuple or an array, see Data Info). The types of arguments must conform to the declared datatypes from the datatype of the command argument.

A command may have a return value, which may also be structured. The “done” reply always contains a data-report with the return value. If no value is returned, the data part is set to “null”. The “done” message should be returned quickly, the time scale should be in the order of the time needed for communications. Still, all side-effects need to be realized and communicated before sending the done message.

Note

If a command does not require an argument, an argument MAY still be transferred as JSON null. A SEC node MUST accept and treat the following two messages the same:

do <module>:<command>
do <module>:<command> null

An ECS SHOULD only generate the shorter version.

Example:

> do t1:stop
< done t1:stop [null,{"t":1505396348.876}]

> do t1:stop null
< done t1:stop [null,{"t":1505396349.743}]

3.5.9. Error Reply#

Contains an error class from the list below as its second item (the specifier). The third item of the message is an Error report, containing the request message (minus line endings) as a string in its first element, a (short) human readable text as its second element. The third element is a JSON object, containing possibly implementation specific information about the error (stack dump etc.).

Example:

> read tx:target
< error_read tx:target ["NoSuchModule","tx is not configured on this SEC node", {}]
> change ts:target 12
< error_change ts:target ["NoSuchParameter","ts has no parameter target", {}]
> change t:target -9
< error_change t:target ["BadValue","requested value (-9) is outside limits (0..300)", {}]
> meas:volt?
< error_meas:volt?  ["ProtocolError","unknown action", {}]

Error Classes:

Error classes are divided into two groups: persisting errors and retryable errors. Persisting errors will yield the exact same error message if the exact same request is sent at any later time. A retryable error may give different results if the exact same message is sent at a later time, i.e. depends on state information internal to either the SEC node, the module or the connected hardware.

Persisting errors#
`ProtocolError`	A malformed Request or on unspecified message was sent. This includes non-understood actions and malformed specifiers. Also if the message exceeds an implementation defined maximum size. Note: this may be retryable if induced by a noisy connection.
`NoSuchModule`	The action can not be performed as the specified module is non-existent.
`NoSuchParameter`	The action can not be performed as the specified parameter is non-existent.
`NoSuchCommand`	The specified command does not exist.
`ReadOnly`	The requested write can not be performed on a readonly value.
`WrongType`	The requested parameter change or command can not be performed as the argument has the wrong type, e.g. a string where a number is expected, or a struct doesn’t have all required members.
`RangeError`	The requested parameter change or command can not be performed as the argument value is not in the allowed range specified by the `datainfo` property. This also happens if an unspecified enum variant is tried to be used, the size of a blob or string does not match the limits given in the descriptive data, or if the number of elements in an array does not match the limits given in the descriptive data.
`BadJSON`	The data part of the message can not be parsed, i.e. the JSON data is not valid JSON.
`NotImplemented`	A (not yet) implemented action or combination of action and specifier was requested. This should not be used in productive setups, but is very helpful during development.
`HardwareError`	The connected hardware operates incorrectly or may not operate at all due to errors inside or in connected components.

Retryable errors#
`CommandRunning`	The command is already executing. The request may be retried after the module is no longer BUSY.
`CommunicationFailed`	Some communication (with hardware controlled by this SEC node) failed.
`TimeoutError`	Some initiated action took longer than the maximum allowed time.
`IsBusy`	The requested action can not be performed while the module is BUSY or the command still running.
`IsError`	The requested action can not be performed while the module is in error state.
`Disabled`	The requested action can not be performed while the module is disabled.
`Impossible`	The requested action can not be performed at the moment.
`ReadFailed`	The requested parameter can not be read just now.
`OutOfRange`	The value read from the hardware is out of sensor or calibration range.
`InternalError`	Something that should never happen just happened.

Note

This list may be extended, if needed. Clients should treat unknown error classes as generic errors.

3.5.10. Logging#

Logging is an optional message, i.e. a SEC node is not forced to implement it.

logging

followed by a specifier of <modulename> and a string in the JSON part which is either “debug”, “info”, “error” or “off”. This is supposed to set the ‘logging level’ of the given module (or the whole SEC node if the specifier is empty) to the given level:

This scheme may also be extended to configure logging only for selected parameters of selected modules.

“off”: Remote logging is completely turned off.
“error”: Only errors are logged remotely.
“info”: Only ‘info’ and ‘error’ messages are logged remotely.
“debug”: All log messages are logged remotely.

A SEC node should reply with an Error report (ProtocolError) if it doesn’t implement this message. Otherwise it should mirror the request, which may be updated with the logging-level actually in use. I.e. if an SEC node does not implement the “debug” level, but “error” and “info” and an ECS requests “debug” logging, the reply should contain “info” (as this is ‘closer’ to the original request than “error”) or “off”. Similarly, if logging of a too specific item is requested, the SEC node should activate the logging on the least specific item where logging is supported. E.g. if logging for <module>:<param> is requested, but the SEC node only supports logging of the module, this should be reflected in the reply and the logging of the module is to be influenced.

Note

It is not foreseen to query the currently active logging level. It is supposed to default to "off".

log

followed by a specifier of <modulename>:<loglevel> and the message to be logged as JSON string in the datapart. This is an asynchronous event only to be sent by the SEC node to the ECS which activated logging.

Example:

# note: empty specifier -> select all modules
> logging  "error"
# SEC node confirms
< logging  "error"
< log mod1:debug "polling value"
< log mod1:debug "sending request..."
...

Another example:

# enable full logging of mod1
> logging mod1 "debug"
# SEC node can only log errors, logging of errors of mod1 is now active
< logging mod1 "error"
< log mod1:error "value par1 can not be determined, please refill read-out liquid"
...
> logging mod1 false
< logging mod1 false

3.5.11. Heartbeat#

In order to detect that the other end of the communication is not dead, a heartbeat may be sent. The second part of the message (the id) must not contain a space and should be short and not be re-used. It may be omitted. The reply will contain exactly the same id.

A SEC node replies with a pong message with a data-report of a null value. The Qualifiers part SHOULD only contain the timestamp (as member “t”) if the SEC node supports timestamping. This can be used to synchronize the time between ECS and SEC node.

Note

The qualifiers could also be an empty JSON-object, indicating lack of timestamping support.

For debugging purposes, when id in the ping request is omitted, in the pong reply there are two spaces after pong. A client SHOULD always send an id. However, the client parser MUST treat two consecutive spaces as two separators with an empty string in between.

Example:

> ping 123
< pong 123 [null, {"t": 1505396348.543}]

Related Issues

SECoP Issue 003 Timestamp Format

SECoP Issue 007 Time Synchronization

3.6. Compatibility#

This specification defines a set of requests and replies above. Only those messages are ALLOWED to be generated by any software complying to this specification:

Any ECS is allowed to generate the following messages:

Any SEC node is allowed to generate the following messages:

The specification is intended to grow and adopt to new needs. (Related issue: SECoP Issue 038 Extension mechanisms.) To futureproof the the communication, the following messages MUST be parsed and treated correctly (i.e. the ignored_value part is to be ignored).

Any SEC node MUST accept the following messages and handle them properly:

Any ECS MUST accept the following messages and handle them accordingly:

As a special case, an argumentless command may also by called without specifying the data part. In this case an argument of null is to be assumed. Also, an argumentless ping is to be handled as a ping request with an empty token string. The corresponding reply then contains a double space. This MUST also be parsed correctly.

Similarly, the reports need to be handled like this:

Data report:

Error report:

Essentially this boils down to:

ignore additional entries in the list-part of reports
ignore extra keys in the qualifiers, structure report and error report mappings
ignore message fields which are not used in the definition of the messages (i.e. for describe)
treat needed, but missing data as null (or an empty string, depending on context)
if a specifier contains more “:” than you can handle, use the part you understand, ignore the rest (i.e. treat activate module:parameter as activate module, ignoring the :parameter part)
same for error class (i.e. treat WrongType:MustBeInt as WrongType, ignoring the :MustBeInt part)
upon parsing a value, when you know it should be one element from an Enumerated Type: enum (which SHOULD be transported as integer), if you find a string instead and that string is one of the names from the Enum, use that entry.
check newer versions of the specification and check the issues as well, as the above may change

Complying to these rules maximizes the possibility of future + backwards compatibility.

Note

Also check SECoP Issue 036 Dynamic units as it may have implications for a certain implementation.

3.6.1. Handling Timeout Issues#

If a timeout happens, it is not easy for the ECS to decide on the best strategy. Also there are several types of timeout: idle-timeout, reply-timeout, etc… Generally speaking: both ECS and SEC side need to be aware that the other side may close the connection at any time! On reconnect, it is recommended that the ECS sends a *IDN? and a describe message. If the responses match the responses from the previous connection, the ECS should continue without any internal reconfiguring, as if no interruption happened. If the response of the description does not match, it is up to the ECS to handle this.

Naturally, if the previous connection was activated, an activate message has to be sent before it can continue as before.

SECoP Issue 006 Keep Alive