# Session-less Service invocation

OPC 10000-4 [§6.3](https://reference.opcfoundation.org/Core/Part4/v105/docs/6.3)
lets a Client invoke a subset of the Services without CreateSession and
ActivateSession. The request then carries either an Access Token in
`RequestHeader.authenticationToken`, or no token at all. This page shows how
to enable it on a server built with this stack.

## Contents

- [Which Services](#which-services)
- [Enabling it](#enabling-it)
- [Caller identity](#caller-identity)
- [Answers without the feature](#answers-without-the-feature)
- [Limiting concurrent requests](#limiting-concurrent-requests)
- [Taking over completely](#taking-over-completely)
- [Calling the server](#calling-the-server)

## Which Services

Session-less invocation is limited to the Service Sets §6.3.1 names:

| Service Set | Services |
| --- | --- |
| View | Browse, BrowseNext, TranslateBrowsePathsToNodeIds (not RegisterNodes / UnregisterNodes) |
| Attribute | Read, Write, HistoryRead, HistoryUpdate |
| Method | Call |
| NodeManagement | AddNodes, AddReferences, DeleteNodes, DeleteReferences |
| Query | QueryFirst, QueryNext |

Every other Service, for example CreateSubscription or Publish, needs a
Session and is answered with `Bad_SessionIdInvalid` without one.
`SessionManager.IsSessionlessService(RequestType)` returns the same decision.

A request without a Session has no Session state to keep continuation
points in: Browse returns none, and BrowseNext reports every continuation
point as invalid.

## Enabling it

With dependency injection:

```csharp
services.AddOpcUa()
    .AddServer(options => { /* … */ })
    .AddSessionlessInvocation()          // OPC 10000-4 §6.3
    .AddJwtIssuer(options => { /* … */ }); // validates the Access Tokens
```

`AddSessionlessInvocation` registers `SessionlessInvocationOptions`. The
hosted server applies them to the session manager it uses, including one
created by `AddSessionManager(...)`, unless that manager was configured
already.

Without dependency injection, set the options on the session manager:

```csharp
var sessionManager = (SessionManager)server.CurrentInstance.SessionManager;
sessionManager.SessionlessInvocation = new SessionlessInvocationOptions();
```

## Caller identity

| Request | Identity | Option |
| --- | --- | --- |
| `authenticationToken` is a String NodeId | The Access Token, validated as an issued JWT token (OPC 10000-6 §7.6.5.2.3) by the server's identity registry, the authenticators ActivateSession uses | `AcceptAccessTokens` (default `true`) |
| no `authenticationToken` | An anonymous user | `AllowAnonymous` (default `false`) |

- The roles of the identity are mapped as ActivateSession maps those of a
  Session: the roles granted by the `IRoleManager` identity-mapping rules,
  including the configured Access Token claims and the built-in
  AuthenticatedUser role, are added to the identity before the request runs.
  This also applies to the identity a `ValidateSessionLessRequest` handler sets.
  A request without a Session has no client application certificate, so it is
  not granted the TrustedApplication role and no application-based mapping.
- §6.3.1 requires encryption for the Access Token. It is only accepted over
  a SecureChannel with `SignAndEncrypt`, or over HTTPS. On any other channel
  the request is rejected.
- A token no authenticator accepts is answered with the authenticator's
  error, or `Bad_IdentityTokenRejected`.
- A request without a token is answered with `Bad_IdentityTokenInvalid`
  unless `AllowAnonymous` is set.
- With `AllowAnonymous` the request has to meet what the endpoint demands of
  CreateSession and ActivateSession; see
  [Anonymous requests and the channel](#anonymous-requests-and-the-channel).
- Session tokens are UInt32 or ByteString NodeIds. A token of that kind that
  names no Session stays `Bad_SessionIdInvalid`, which is what a Client
  whose Session expired needs to see.

Override `SessionManager.ValidateSessionlessRequestAsync` for different
rules.

### Anonymous requests and the channel

§6.3.1 lets a Server skip the Access Token and assume an anonymous user "if
application authentication through the SecureChannel is sufficient". A request
that `AllowAnonymous` admits therefore runs only if its channel authenticates
the application the way the endpoint demands for CreateSession:

| Endpoint | The request needs | Otherwise |
| --- | --- | --- |
| opc.tcp or opc.wss with a security mode or policy other than None | the client certificate of the SecureChannel, which opc.tcp has validated while it opened the channel | `Bad_SecurityChecksFailed` |
| HTTPS (REST, binary or JSON) with a security mode or policy other than None, or with `HttpsMutualTls` set | the TLS client certificate, which the HTTPS listener has validated during the TLS handshake | `Bad_SecurityChecksFailed` |
| HTTPS without `HttpsMutualTls`, SecurityMode None | nothing | |
| any, with user token policies that include no Anonymous policy | an anonymous user token policy on the endpoint, as ActivateSession requires it | `Bad_IdentityTokenRejected` |
| unknown (no endpoint on the channel) | an endpoint | `Bad_SecurityModeInsufficient` |

The endpoints of an HTTPS listener without `HttpsMutualTls` offer no Anonymous
user token policy, so on them an anonymous Session-less request is rejected
as an anonymous Session is.

HTTPS clients are not required to present a certificate (OPC 10000-6 §7.4.1),
and HTTPS authenticates the application when a Session is created. With
`HttpsMutualTls` set, a Client that wants an anonymous Session-less call
presents its application certificate in the TLS handshake. A request that
carries an Access Token does not need the client certificate: the token
authenticates the caller, and HTTPS keeps it confidential.

## Answers without the feature

Without `AddSessionlessInvocation` and without a
`ValidateSessionLessRequest` handler, a request that carries no
`authenticationToken` is answered with `Bad_ServiceUnsupported`, as §6.3.1
prescribes for a Server that does not support session-less invocation.
Requests with a token that names no Session stay `Bad_SessionIdInvalid`.

## Limiting concurrent requests

A request of a Session is accounted for by its Session. A session-less request
has none, but it still occupies a worker and keeps its channel active. The
[resource isolation](ResourceIsolation.md) of the server limits channels,
queued requests and incomplete messages. Session-less requests share these
queue and execution stages with the requests of Sessions. `AddSessionlessInvocation`
adds a ceiling on the number of Session-less requests that run at the same
time. The ceiling does not reserve any capacity: it neither keeps a share of
the queue or the workers free for Sessions nor gives Session-less requests a
share of their own, so Session-less requests below the ceiling still compete
with the requests of Sessions for the shared stages:

| Option | Default | Limits |
| --- | --- | --- |
| `MaxConcurrentRequests` | 64 | Session-less requests that run at the same time, over all channels |
| `MaxConcurrentRequestsPerChannel` | 16 | The same on one channel |

```csharp
services.AddOpcUa()
    .AddServer(options => { /* … */ })
    .AddSessionlessInvocation(options =>
    {
        options.MaxConcurrentRequests = 128;
        options.MaxConcurrentRequestsPerChannel = 32;
    });
```

- Zero does not limit. A negative value is rejected.
- A request counts from the check of its identity until its Service has
  completed, failed or was cancelled. A request that is rejected while its
  identity is checked, for example because its Access Token is not accepted,
  returns its place at once.
- A request over a limit is answered with `Bad_ServerTooBusy`: "The Server does
  not have the resources to process the request at this time." The client can
  retry the same request later. Nothing was executed.
- The channel of an opc.tcp request is its SecureChannel. The HTTPS bindings
  have no SecureChannel per connection, so their requests are grouped by the
  network address of the peer. Callers behind one address, such as a proxy,
  share the limit.
- Requests that carry a Session token, and requests the feature does not
  accept (see [Which Services](#which-services)), are not counted.
- The limits are read for every request, so changing the options applies to the
  next request. Requests that run keep their place.

## Taking over completely

A handler on `ISessionManager.ValidateSessionLessRequest` decides before the
built-in handling. It receives the token and the request type and sets the
identity or an error. The Service Set limit above applies before the
handler is called. When `AddSessionlessInvocation` is used as well, the
request limits above apply before the handler too. A handler without
`AddSessionlessInvocation` runs without a request limit.

## Calling the server

Over opc.tcp, any client channel without a Session works, for example a
`SessionClient` on a channel from `ClientChannelManager`:

```csharp
var client = new SessionClient(channel, telemetry);
ReadResponse response = await client.ReadAsync(
    new RequestHeader { AuthenticationToken = new NodeId(accessToken, 0) },
    0,
    TimestampsToReturn.Neither,
    [new ReadValueId { NodeId = VariableIds.Server_ServerStatus_State, AttributeId = Attributes.Value }],
    ct);
```

Over the [REST binding](WebApi.md) the token is a String NodeId in the JSON
request header:

```json
{
  "RequestHeader": { "AuthenticationToken": "s=<access-token>" },
  "NodesToRead": [ { "NodeId": "i=2259", "AttributeId": 13 } ]
}
```