<commit_before># SRT Access Control (Stream ID) Guidelines

## Motivation

One type of information that can be interchanged when a connection is being
established in SRT is "Stream ID", which can be used in a caller-listener
connection layout. This is a string of maximum 512 characters set on the caller
side. It can be retrieved at the listener side on the newly accepted socket
through a socket option (see `SRTO_STREAMID` in [SRT API Socket Options](../API/API-socket-options.md)).

As of SRT version 1.3.3 a callback can be registered on the listener socket for
an application to make decisions on incoming caller connections. This callback,
among others, is provided with the value of Stream ID from the incoming
connection. Based on this information, the application can accept or reject the
connection, select the desired data stream, or set an appropriate passphrase for
the connection.

## Purpose

The Stream ID value can be used as free-form, but there is a recommended
convention so that all SRT users speak the same language. The intent of the
convention is to:

- promote readability and consistency among free-form names
- interpret some typical data in the key-value style

In short,

1. `SRTO_STREAMID` is designed for a caller (client) to be able to identify itself, and state what it wants.
2. `srt_listen_callback(...)` function is used by a listener (server) to check what a caller (client) has provided in `SRTO_STREAMID` **before** the connection is established.
For example, the listener (server) can check if it knows the user and set the corresponding passphrase for a connection to be accepted.
3. Even if `srt_listen_callback(...)` accepts the connection, SRT will still have one more step to check the PASSPHRASE, and reject on mismatch.
If a correct passphrase is not provided by the client (caller), the request from caller will be rejected by SRT library (not application or programmer).

**Note!** `srt_listen_callback(...)` can't check the passphrase directly for security reasons. 
The only way to make the app check the passphrase is to set the passphrase on the socket by using the `SRTO_PASSPHRASE` option. This lets SRT to reject connection on mismatch.

## Character Encoding

The Stream ID uses UTF-8 encoding.

## General Syntax

This recommended syntax starts with the characters known as an executable
specification in POSIX: `#!`.

The next two characters are:

- `:` - marks the format of the following key-value pair syntax (the only one defined currently).
- The content format, which is either:
  - `:` - the comma-separated keys with no nesting
  - `{` - like above, but nesting is allowed and must end with `}`

(Nesting means that you can have multiple level brace-enclosed parts inside.)

The form of the key-value pair is:

```js
key1=value1,key2=value2...
```

## Standard Keys

Beside the general syntax, there are several top-level keys treated as standard
keys. All single letter key definitions, including those not listed in this section,
are reserved for future use. Users can additionally use custom key definitions
with `user_*` or `companyname_*` prefixes, where `user` and `companyname` are
to be replaced with an actual user or company name.

The existing key values must not be extended, and must not differ from those described in this section.

The following keys are standard:

- `u`: **User Name**, or authorization name, that is expected to control which
password should be used for the connection. The application should interpret
it to distinguish which user should be used by the listener party to set up the
password.
- `r`: **Resource Name** identifies the name of the resource and facilitates
selection should the listener party be able to serve multiple resources.
- `h`: **Host Name** identifies the hostname of the resource. For example,
to request a stream with the URI `somehost.com/videos/querry.php?vid=366` the
`hostname` field should have `somehost.com`, and the resource name can have
`videos/querry.php?vid=366` or simply `366`. Note that this is still a key to be
specified explicitly. Support tools that apply simplifications and URI extraction
are expected to insert only the host portion of the URI here.
- `s`: **Session ID** is a temporary resource identifier negotiated with
the server, used just for verification. This is a one-shot identifier, invalidated
after the first use. The expected usage is when details for the resource and
authorization are negotiated over a separate connection first, and then the
session ID is used here alone.
- `t`: **Type** specifies the purpose of the connection. Several standard
types are defined, but users may extend the use:
  - `stream` (default, if not specified): for exchanging the user-specified
  payload for an application-defined purpose
  - `file`: for transmitting a file, where `r` is the filename
  - `auth`: for exchanging sensible data. The `r` value states its purpose.
  No specific possible values for that are known so far (FUTURE USE]
- `m`: **Mode** expected for this connection:
  - `request` (default): the caller wants to receive the stream
  - `publish`: the caller wants to send the stream data
  - `bidirectional`:  bidirectional data exchange is expected

Note that `m` is not required in the case where you don't use `streamid` to
distinguish authorization or resources, and your caller is expected to send the
data. This is only for cases where the listener can handle various purposes of the
connection and is therefore required to know what the caller is attempting to do.

Examples:

```js
#!::u=admin,r=bluesbrothers1_hi
```

This specifies the username and the resource name of the stream to be served
to the caller.

```js
#!::u=johnny,t=file,m=publish,r=results.csv
```

This specifies that the file is expected to be transmitted from the caller to
the listener and its name is `results.csv`.

### Rejection Codes

The listener callback handler is also able to decide about rejecting the
incoming connection. In a normal situation, the rejection code is predefined
as `SRT_REJ_RESOURCE`. The handler can, however, set its own rejection
code. There are two number spaces intended for this purpose (as the range
below `SRT_REJC_PREDEFINED` is reserved for internal codes):

- `SRT_REJC_PREDEFINED` and above: predefined errors. Errors from this range
(that is, below `SRT_REJC_USERDEFINED`) have their definitions provided in
the `access_control.h` public header file. The intention is that applications
using these codes understand the situation described by these codes standard
way.

- `SRT_REJC_USERDEFINED` and above: to be freely defined by the application.
Codes from this range can be only understood if each application knows the
code definitions of the other. These codes should be used only after making
sure that both applications understood them.

The intention for the predefined codes is to be consistent with the HTTP
standard codes. Therefore the following sub-ranges are used:

- 0 - 99: Reserved for unique SRT-specific codes (unused by HTTP)
- 100 - 399: Info, Success and Redirection in HTTP, unused in SRT
- 400 - 599: Client and server errors in HTTP, adopted by SRT
- 600 - 999: unused in SRT

Such a code can be set by using the `srt_setrejectreason` function.

The SRT-specific codes are:

#### SRT_REJX_FALLBACK

This code should be set by the callback handler in the beginning in case
the application needs to be informed that the callback handler
actually has interpreted the incoming connection, but hasn't set a
more appropriate code describing the situation.

#### SRT_REJX_KEY_NOTSUP

Indicates there was a key specified in the StreamID string that this application
doesn't support. Note that it's not obligatory for the application to
react this way - it may chose to ignore unknown keys completely, or
to have some keys in the ignore list (which it won't interpret, but tolerate)
while rejecting any others. It is also up to the application
to decide to return this specific error, or more generally report
the syntax error with `SRT_REJX_BAD_REQUEST`.

#### SRT_REJX_FILEPATH

The resource type designates a file, and the path either has the wrong syntax
or is not found. In the case where `t=file`, the path should be specified under
the `r` key, and the file specified there must be able to be saved this way.
It's up to the application to decide how to treat this path, how to parse it,
and what this path specifically means. For the `r` key, the application should
at least handle the single filename, and have storage space available to save
it (provided a file of the same name does not already exist there).  The
application should decide whether and how to handle all other situations (like
directory path, special markers in the path to be interpreted by the
application, etc.), or to report this error.

#### SRT_REJX_HOSTNOTFOUND

The host specified in the `h` key cannot be identified. The `h` key is
generally for a situation when you have multiple DNS names for a host,
so an application may want to extract the name from the URI and set it
to `h` key so that the application can distinguish the request also by
the target host name. The application may however limit the number of
recognized services by host name to some predefined names and not
handle the others, even if this is properly resolved by DNS. In this
case it should report this error.

The other error codes are HTTP codes adopted for SRT:

#### SRT_REJX_BAD_REQUEST

General syntax error. This can be reported in any case when parsing
the StreamID contents failed, or it cannot be properly interpreted.

#### SRT_REJX_UNAUTHORIZED

Authentication failed, which makes the client unauthorized to access the
resource. This error, however, confirms that the syntax is correct and
the resource has been properly identified. Note that this cannot be
reported when you use a simple user-password authentication
method because in this case the password is verified only after the
listener callback handler accepts the connection. This error is rather
intended to be reported in case of `t=auth` when the authentication
process has generated some valid session ID, but then the session
connection has specified a resource that is not within the frames
of that authentication.

#### SRT_REJX_OVERLOAD

The server is too heavily loaded to process your request, or you
have exceeded credits for accessing the service and the resource.
In HTTP the description mentions payment for a service, but
it is also used by some services for general "credit" management
for a client. In SRT it should be used when your service is doing
any kind of credit management to limit access to selected clients
that "have" enough credit, even if the credit is something the client
can recharge itself, or that can be granted depending on available
service resources.

#### SRT_REJX_FORBIDDEN

Access denied to the resource for any reason. This error is
independent of an authorization or authentication error (as reported
by `SRT_REJX_UNAUTHORIZED`). The application can decide which
is more appropriate. This error is usually intended for
a resource that should only be accessed after a successful
authorization over a separate auth-only connection, where the query
in StreamID has correctly specified the resource identity and mode,
but the session ID (in the `s` key) is either (a) not specified, or (b) does
specify a valid session, but the authorization region for this
session does not embrace the specified resource.

#### SRT_REJX_NOTFOUND

The resource specified in the `r` key (in combination with the `h` key)
is not found at this time. This error should be only reported if the
information about resource accessibility is allowed to be publicly
visible. Otherwise the application might report authorization
errors.

#### SRT_REJX_BAD_MODE

The mode specified in the `m` key in StreamID is not supported for this request.
This may apply to read-only or write-only resources, as well for when interactive
(bidirectional) access is not valid for a resource.

#### SRT_REJX_UNACCEPTABLE

Applies when the parameters specified in SocketID cannot be satisfied for the
requested resource, or when `m=publish` but the data format is not acceptable.
This is a general error reporting an unsupported format for data that appears to 
be wrong when sending, or a restriction on the data format (as specified in the 
details of the resource specification) such that it cannot be provided 
when receiving.

#### SRT_REJX_CONFLICT

The resource being accessed (as specified by `r` and `h` keys) is locked for
modification. This error should only be reported for `m=publish` when the
resource being accessed is read-only because another client (not necessarily
connected through SRT):

- is currently publishing into this resource
- has reserved this resource ID for publishing

Note that this error should be reported when there is no other reason for
having a problem accessing the resource.

#### SRT_REJX_NOTSUP_MEDIA

The media type is not supported by the application. The media type is
specified in the `t` key. The currently standard types are
`stream`, `file` and `auth`. An application may extend this list, and
is not obliged to support all of the standard types.

#### SRT_REJX_LOCKED

The resource being accessed is locked against any access. This is similar to
`SRT_REJX_CONFLICT`, but in this case the resource is locked for reading 
and writing. This is for when the resource should be shown as existing and 
available to the client, but access is temporarily blocked.

#### SRT_REJX_FAILED_DEPEND

The dependent entity for the request is not present. In this case the
dependent entity is the session, which should be specified in the `s`
key. This means that the specified session ID is nonexistent or it
has already expired.

#### SRT_REJX_ISE

Internal server error. This is for a general case when a request has
been correctly verified, with no related problems found, but an
unexpected error occurs after the processing of the request has started.

#### SRT_REJX_UNIMPLEMENTED

The request was correctly recognized, but the current software version
of the service (be it SRT or any other software component) doesn't
support it. This should be reported for a case, when some features to
be specified in the StreamID request are supposed to be supported in a
predictable future, but the current version of the server does not
support it, or the support for this feature in this version has been
temporarily blocked. This shouldn't be reported for existing features that are
being deprecated, or older features that are no longer supported
(for this case the general `SRT_REJX_BAD_REQUEST` is more appropriate).

#### SRT_REJX_GW

The server acts as a gateway and the target endpoint rejected the
connection. The reason the connection was rejected is unspecified.
The gateway cannot forward the original rejection code from the
target endpoint because this would suggest the error was on the
gateway itself. Use this error with some other mechanism to report
the original target error, if possible.

#### SRT_REJX_DOWN

The service is down for maintenance. This can only be reported
when the service has been temporarily replaced by a stub that is only
reporting this error, while the real service is down for maintenance.

#### SRT_REJX_VERSION

Application version not supported. This can refer to an application feature
that is unsupported (possibly from an older SRT version), or to a feature
that is no longer supported because of backward compatibility requirements.

#### SRT_REJX_NOROOM

The data stream cannot be archived due to a lack of storage space. This is
reported when a request to send a file or a live stream to be archived is
unsuccessful. Note that the length of a file transmission is usually
pre-declared, so this error can be reported early. It can also be reported when
the stream is of undefined length, and there is no more storage space
available.

## Example

An example of Stream ID functionality and the listener callback can be
found under `tests/test_listen_callback.cpp`.

A listener can register a callback to be called in the middle of accepting a
new socket connection:

```c++
srt_listen(server_sock, 5);
srt_listen_callback(server_sock, &SrtTestListenCallback, NULL);
```

A callback function has to be implemented by the upstream application. In the
example below, the function tries to interpret the Stream ID value first according
to the Access Control guidelines and to extract the username from the `u` key.
Otherwise it falls back to a free-form specified username. Depending on the user,
it sets the appropriate password for the expected connection so that it can be
rejected if the password isn't correct. If the user isn't found in the
database (`passwd` map) the function itself rejects the connection. Note that
this can be done by both returning -1 and by throwing an exception.

```c++
int SrtTestListenCallback(void* opaq, SRTSOCKET ns, int hsversion,
    const struct sockaddr* peeraddr, const char* streamid)
{
    using namespace std;

    // opaq is used to pass some further chained callbacks

    // To reject a connection attempt, return -1.

    static const map<string, string> passwd {
        {"admin", "<PASSWORD>"},
        {"user", "<PASSWORD>"}
    };

    // Try the "standard interpretation" with username at key u
    string username;

    static const char stdhdr [] = "#!::";
    uint32_t* pattern = (uint32_t*)stdhdr;
    bool found = -1;

    // Extract a username from the StreamID:
    if (strlen(streamid) > 4 && *(uint32_t*)streamid == *pattern)
    {
        vector<string> items;
        Split(streamid+4, ',', back_inserter(items));
        for (auto& i: items)
        {
            vector<string> kv;
            Split(i, '=', back_inserter(kv));
            if (kv.size() == 2 && kv[0] == "u")
            {
                username = kv[1];
                found = true;
            }
        }

        if (!found)
        {
            cerr << "TEST: USER NOT FOUND, returning false.\n";
            return -1;
        }
    }
    else
    {
        // By default the whole streamid is username
        username = streamid;
    }

    // When the username of the client is known, the passphrase can be set
    // on the socket being accepted (SRTSOCKET ns).
    // The remaining part of the SRT handshaking process will check the
    // passphrase of the client and accept or reject the connection.

    // When not found, it will throw an exception
    cerr << "TEST: Accessing user '" << username << "', might throw if not found\n";
    string exp_pw = passwd.at(username);

    cerr << "TEST: Setting password '" << exp_pw << "' as per user '" << username << "'\n";
    srt_setsockflag(ns, SRTO_PASSPHRASE, exp_pw.c_str(), exp_pw.size());
    return 0;
}
```
<commit_msg>Improved Access Control general syntax description<commit_after># SRT Access Control (Stream ID) Guidelines

## Motivation

One type of information that can be interchanged when a connection is being
established in SRT is "Stream ID", which can be used in a caller-listener
connection layout. This is a string of maximum 512 characters set on the caller
side. It can be retrieved at the listener side on the newly accepted socket
through a socket option (see `SRTO_STREAMID` in [SRT API Socket Options](../API/API-socket-options.md)).

As of SRT version 1.3.3 a callback can be registered on the listener socket for
an application to make decisions on incoming caller connections. This callback,
among others, is provided with the value of Stream ID from the incoming
connection. Based on this information, the application can accept or reject the
connection, select the desired data stream, or set an appropriate passphrase for
the connection.

## Purpose

The Stream ID value can be used as free-form, but there is a recommended
convention so that all SRT users speak the same language. The intent of the
convention is to:

- promote readability and consistency among free-form names
- interpret some typical data in the key-value style

In short,

1. `SRTO_STREAMID` is designed for a caller (client) to be able to identify itself, and state what it wants.
2. `srt_listen_callback(...)` function is used by a listener (server) to check what a caller (client) has provided in `SRTO_STREAMID` **before** the connection is established.
For example, the listener (server) can check if it knows the user and set the corresponding passphrase for a connection to be accepted.
3. Even if `srt_listen_callback(...)` accepts the connection, SRT will still have one more step to check the PASSPHRASE, and reject on mismatch.
If a correct passphrase is not provided by the client (caller), the request from caller will be rejected by SRT library (not application or programmer).

**Note!** `srt_listen_callback(...)` can't check the passphrase directly for security reasons. 
The only way to make the app check the passphrase is to set the passphrase on the socket by using the `SRTO_PASSPHRASE` option. This lets SRT to reject connection on mismatch.

## Character Encoding

The Stream ID uses UTF-8 encoding.

## General Syntax

The recommended syntax starts with the characters known as an executable specification in POSIX: `#!`.

The next character defines the format used for the following key-value pair syntax.
At the moment, there is only one supported syntax identified by `:` and described below.

Everything that comes after a syntax identifier is further referenced as the content of the Stream ID.

The content starts with a `:` or `{` character identifying its format:

- `:` : comma-separated key-value pairs with no nesting,
- `{` : a nested block with one or several key-value pairs that must end with a `}` character. Nesting means that multiple level brace-enclosed parts are allowed.

The form of the key-value pair is

~~~
key1=value1,key2=value2,...
~~~

## Standard Keys

Beside the general syntax, there are several top-level keys treated as standard
keys. All single letter key definitions, including those not listed in this section,
are reserved for future use. Users can additionally use custom key definitions
with `user_*` or `companyname_*` prefixes, where `user` and `companyname` are
to be replaced with an actual user or company name.

The existing key values must not be extended, and must not differ from those described in this section.

The following keys are standard:

- `u`: **User Name**, or authorization name, that is expected to control which
password should be used for the connection. The application should interpret
it to distinguish which user should be used by the listener party to set up the
password.
- `r`: **Resource Name** identifies the name of the resource and facilitates
selection should the listener party be able to serve multiple resources.
- `h`: **Host Name** identifies the hostname of the resource. For example,
to request a stream with the URI `somehost.com/videos/querry.php?vid=366` the
`hostname` field should have `somehost.com`, and the resource name can have
`videos/querry.php?vid=366` or simply `366`. Note that this is still a key to be
specified explicitly. Support tools that apply simplifications and URI extraction
are expected to insert only the host portion of the URI here.
- `s`: **Session ID** is a temporary resource identifier negotiated with
the server, used just for verification. This is a one-shot identifier, invalidated
after the first use. The expected usage is when details for the resource and
authorization are negotiated over a separate connection first, and then the
session ID is used here alone.
- `t`: **Type** specifies the purpose of the connection. Several standard
types are defined, but users may extend the use:
  - `stream` (default, if not specified): for exchanging the user-specified
  payload for an application-defined purpose
  - `file`: for transmitting a file, where `r` is the filename
  - `auth`: for exchanging sensible data. The `r` value states its purpose.
  No specific possible values for that are known so far (FUTURE USE]
- `m`: **Mode** expected for this connection:
  - `request` (default): the caller wants to receive the stream
  - `publish`: the caller wants to send the stream data
  - `bidirectional`:  bidirectional data exchange is expected

Note that `m` is not required in the case where you don't use `streamid` to
distinguish authorization or resources, and your caller is expected to send the
data. This is only for cases where the listener can handle various purposes of the
connection and is therefore required to know what the caller is attempting to do.

Examples:

```js
#!::u=admin,r=bluesbrothers1_hi
```

This specifies the username and the resource name of the stream to be served
to the caller.

```js
#!::u=johnny,t=file,m=publish,r=results.csv
```

This specifies that the file is expected to be transmitted from the caller to
the listener and its name is `results.csv`.

### Rejection Codes

The listener callback handler is also able to decide about rejecting the
incoming connection. In a normal situation, the rejection code is predefined
as `SRT_REJ_RESOURCE`. The handler can, however, set its own rejection
code. There are two number spaces intended for this purpose (as the range
below `SRT_REJC_PREDEFINED` is reserved for internal codes):

- `SRT_REJC_PREDEFINED` and above: predefined errors. Errors from this range
(that is, below `SRT_REJC_USERDEFINED`) have their definitions provided in
the `access_control.h` public header file. The intention is that applications
using these codes understand the situation described by these codes standard
way.

- `SRT_REJC_USERDEFINED` and above: to be freely defined by the application.
Codes from this range can be only understood if each application knows the
code definitions of the other. These codes should be used only after making
sure that both applications understood them.

The intention for the predefined codes is to be consistent with the HTTP
standard codes. Therefore the following sub-ranges are used:

- 0 - 99: Reserved for unique SRT-specific codes (unused by HTTP)
- 100 - 399: Info, Success and Redirection in HTTP, unused in SRT
- 400 - 599: Client and server errors in HTTP, adopted by SRT
- 600 - 999: unused in SRT

Such a code can be set by using the `srt_setrejectreason` function.

The SRT-specific codes are:

#### SRT_REJX_FALLBACK

This code should be set by the callback handler in the beginning in case
the application needs to be informed that the callback handler
actually has interpreted the incoming connection, but hasn't set a
more appropriate code describing the situation.

#### SRT_REJX_KEY_NOTSUP

Indicates there was a key specified in the StreamID string that this application
doesn't support. Note that it's not obligatory for the application to
react this way - it may chose to ignore unknown keys completely, or
to have some keys in the ignore list (which it won't interpret, but tolerate)
while rejecting any others. It is also up to the application
to decide to return this specific error, or more generally report
the syntax error with `SRT_REJX_BAD_REQUEST`.

#### SRT_REJX_FILEPATH

The resource type designates a file, and the path either has the wrong syntax
or is not found. In the case where `t=file`, the path should be specified under
the `r` key, and the file specified there must be able to be saved this way.
It's up to the application to decide how to treat this path, how to parse it,
and what this path specifically means. For the `r` key, the application should
at least handle the single filename, and have storage space available to save
it (provided a file of the same name does not already exist there).  The
application should decide whether and how to handle all other situations (like
directory path, special markers in the path to be interpreted by the
application, etc.), or to report this error.

#### SRT_REJX_HOSTNOTFOUND

The host specified in the `h` key cannot be identified. The `h` key is
generally for a situation when you have multiple DNS names for a host,
so an application may want to extract the name from the URI and set it
to `h` key so that the application can distinguish the request also by
the target host name. The application may however limit the number of
recognized services by host name to some predefined names and not
handle the others, even if this is properly resolved by DNS. In this
case it should report this error.

The other error codes are HTTP codes adopted for SRT:

#### SRT_REJX_BAD_REQUEST

General syntax error. This can be reported in any case when parsing
the StreamID contents failed, or it cannot be properly interpreted.

#### SRT_REJX_UNAUTHORIZED

Authentication failed, which makes the client unauthorized to access the
resource. This error, however, confirms that the syntax is correct and
the resource has been properly identified. Note that this cannot be
reported when you use a simple user-password authentication
method because in this case the password is verified only after the
listener callback handler accepts the connection. This error is rather
intended to be reported in case of `t=auth` when the authentication
process has generated some valid session ID, but then the session
connection has specified a resource that is not within the frames
of that authentication.

#### SRT_REJX_OVERLOAD

The server is too heavily loaded to process your request, or you
have exceeded credits for accessing the service and the resource.
In HTTP the description mentions payment for a service, but
it is also used by some services for general "credit" management
for a client. In SRT it should be used when your service is doing
any kind of credit management to limit access to selected clients
that "have" enough credit, even if the credit is something the client
can recharge itself, or that can be granted depending on available
service resources.

#### SRT_REJX_FORBIDDEN

Access denied to the resource for any reason. This error is
independent of an authorization or authentication error (as reported
by `SRT_REJX_UNAUTHORIZED`). The application can decide which
is more appropriate. This error is usually intended for
a resource that should only be accessed after a successful
authorization over a separate auth-only connection, where the query
in StreamID has correctly specified the resource identity and mode,
but the session ID (in the `s` key) is either (a) not specified, or (b) does
specify a valid session, but the authorization region for this
session does not embrace the specified resource.

#### SRT_REJX_NOTFOUND

The resource specified in the `r` key (in combination with the `h` key)
is not found at this time. This error should be only reported if the
information about resource accessibility is allowed to be publicly
visible. Otherwise the application might report authorization
errors.

#### SRT_REJX_BAD_MODE

The mode specified in the `m` key in StreamID is not supported for this request.
This may apply to read-only or write-only resources, as well for when interactive
(bidirectional) access is not valid for a resource.

#### SRT_REJX_UNACCEPTABLE

Applies when the parameters specified in SocketID cannot be satisfied for the
requested resource, or when `m=publish` but the data format is not acceptable.
This is a general error reporting an unsupported format for data that appears to 
be wrong when sending, or a restriction on the data format (as specified in the 
details of the resource specification) such that it cannot be provided 
when receiving.

#### SRT_REJX_CONFLICT

The resource being accessed (as specified by `r` and `h` keys) is locked for
modification. This error should only be reported for `m=publish` when the
resource being accessed is read-only because another client (not necessarily
connected through SRT):

- is currently publishing into this resource
- has reserved this resource ID for publishing

Note that this error should be reported when there is no other reason for
having a problem accessing the resource.

#### SRT_REJX_NOTSUP_MEDIA

The media type is not supported by the application. The media type is
specified in the `t` key. The currently standard types are
`stream`, `file` and `auth`. An application may extend this list, and
is not obliged to support all of the standard types.

#### SRT_REJX_LOCKED

The resource being accessed is locked against any access. This is similar to
`SRT_REJX_CONFLICT`, but in this case the resource is locked for reading 
and writing. This is for when the resource should be shown as existing and 
available to the client, but access is temporarily blocked.

#### SRT_REJX_FAILED_DEPEND

The dependent entity for the request is not present. In this case the
dependent entity is the session, which should be specified in the `s`
key. This means that the specified session ID is nonexistent or it
has already expired.

#### SRT_REJX_ISE

Internal server error. This is for a general case when a request has
been correctly verified, with no related problems found, but an
unexpected error occurs after the processing of the request has started.

#### SRT_REJX_UNIMPLEMENTED

The request was correctly recognized, but the current software version
of the service (be it SRT or any other software component) doesn't
support it. This should be reported for a case, when some features to
be specified in the StreamID request are supposed to be supported in a
predictable future, but the current version of the server does not
support it, or the support for this feature in this version has been
temporarily blocked. This shouldn't be reported for existing features that are
being deprecated, or older features that are no longer supported
(for this case the general `SRT_REJX_BAD_REQUEST` is more appropriate).

#### SRT_REJX_GW

The server acts as a gateway and the target endpoint rejected the
connection. The reason the connection was rejected is unspecified.
The gateway cannot forward the original rejection code from the
target endpoint because this would suggest the error was on the
gateway itself. Use this error with some other mechanism to report
the original target error, if possible.

#### SRT_REJX_DOWN

The service is down for maintenance. This can only be reported
when the service has been temporarily replaced by a stub that is only
reporting this error, while the real service is down for maintenance.

#### SRT_REJX_VERSION

Application version not supported. This can refer to an application feature
that is unsupported (possibly from an older SRT version), or to a feature
that is no longer supported because of backward compatibility requirements.

#### SRT_REJX_NOROOM

The data stream cannot be archived due to a lack of storage space. This is
reported when a request to send a file or a live stream to be archived is
unsuccessful. Note that the length of a file transmission is usually
pre-declared, so this error can be reported early. It can also be reported when
the stream is of undefined length, and there is no more storage space
available.

## Example

An example of Stream ID functionality and the listener callback can be
found under `tests/test_listen_callback.cpp`.

A listener can register a callback to be called in the middle of accepting a
new socket connection:

```c++
srt_listen(server_sock, 5);
srt_listen_callback(server_sock, &SrtTestListenCallback, NULL);
```

A callback function has to be implemented by the upstream application. In the
example below, the function tries to interpret the Stream ID value first according
to the Access Control guidelines and to extract the username from the `u` key.
Otherwise it falls back to a free-form specified username. Depending on the user,
it sets the appropriate password for the expected connection so that it can be
rejected if the password isn't correct. If the user isn't found in the
database (`passwd` map) the function itself rejects the connection. Note that
this can be done by both returning -1 and by throwing an exception.

```c++
int SrtTestListenCallback(void* opaq, SRTSOCKET ns, int hsversion,
    const struct sockaddr* peeraddr, const char* streamid)
{
    using namespace std;

    // opaq is used to pass some further chained callbacks

    // To reject a connection attempt, return -1.

    static const map<string, string> passwd {
        {"admin", "<PASSWORD>"},
        {"user", "<PASSWORD>"}
    };

    // Try the "standard interpretation" with username at key u
    string username;

    static const char stdhdr [] = "#!::";
    uint32_t* pattern = (uint32_t*)stdhdr;
    bool found = -1;

    // Extract a username from the StreamID:
    if (strlen(streamid) > 4 && *(uint32_t*)streamid == *pattern)
    {
        vector<string> items;
        Split(streamid+4, ',', back_inserter(items));
        for (auto& i: items)
        {
            vector<string> kv;
            Split(i, '=', back_inserter(kv));
            if (kv.size() == 2 && kv[0] == "u")
            {
                username = kv[1];
                found = true;
            }
        }

        if (!found)
        {
            cerr << "TEST: USER NOT FOUND, returning false.\n";
            return -1;
        }
    }
    else
    {
        // By default the whole streamid is username
        username = streamid;
    }

    // When the username of the client is known, the passphrase can be set
    // on the socket being accepted (SRTSOCKET ns).
    // The remaining part of the SRT handshaking process will check the
    // passphrase of the client and accept or reject the connection.

    // When not found, it will throw an exception
    cerr << "TEST: Accessing user '" << username << "', might throw if not found\n";
    string exp_pw = passwd.at(username);

    cerr << "TEST: Setting password '" << exp_pw << "' as per user '" << username << "'\n";
    srt_setsockflag(ns, SRTO_PASSPHRASE, exp_pw.c_str(), exp_pw.size());
    return 0;
}
```

{'fraction_non_alphanumeric': 0.05426867876741241, 'fraction_numerical': 0.003271422541156606, 'mean_word_length': 4.198162369720241, 'pattern_counts': {'":': 0, '<': 49, '<?xml version=': 0, '>': 15, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
{'dir': 'git-commits-cleaned', 'id': '2862806', 'n_tokens_mistral': 9810, 'n_tokens_neox': 9449, 'n_words': 5656}