The <SessionInitiator> element is used to configure handlers that are responsible for initiating the process of authentication to the SP and establishing a session with it. This represents what single sign-on architecture refers to as an "SP-initiated" flow, where a browser starting at the application end needs to be referred to the IdP to login and then return with the appropriate information to login.

This is an advanced configuration feature. Most deployments can rely on the <SSO> shorthand element.

The specifics of the authentication request process vary by protocol, and the internal SessionInitiator API allows the SP to be extended with many request protocols. The handler is responsible for all of the details associated with making the request.

A special characteristic of this kind of handler is that it often runs implicitly as a result of the first access to a protected resource (e.g. when the requireSession content setting is in effect). For other kinds of use cases, such as the "passive/lazy session" feature that enables an application to defer the creation of a session, a simple (and extensible) protocol is implemented local to the SP software to enable applications to invoke these handlers using a standard HTTP redirect with a query string (see the NativeSPSessionCreationParameters topic).

The ability to configure multiple SessionInitiator handlers, and to combine them in chains, allows the deployer to control the selection of particular SSO protocols when more than one can be used, and to implement various approaches to IdPDiscovery (the process of figuring out which IdP to use in a federated environment).

Types of SessionInitiators

Broadly speaking, there are two kinds of SessionInitiators: protocol handlers and discovery handlers.

A protocol handler requires that the name of an IdP (that is, its entityID) be supplied to the handler so that its metadata can be obtained to determine whether and where it supports a particular protocol. The entityID can be supplied in a number of ways, including via query string, a hardcoded <SessionInitiator> attribute, or via a content setting applied to the resource.

By contrast, a discovery handler requires that the IdP not be known, and is responsible for interacting with the browser in some way to determine the IdP to use. It can do this in any way it wants to, such as examining cookies, interacting with the user, or by redirecting the browser to some other server.

Basic Configuration

An individual SessionInitiator can construct an authentication or discovery request for one of a variety of built-in protocols. One or more of these handlers can be placed into series by a "Chaining" SessionInitiator. The SP will attempt to initiate a session with each handler in the chain until one of them successfully returns something to the browser.

Typically, the deployer configures a chain in which a number of SSO protocols are listed, in order of preference, followed by a handler for some kind of "discovery" protocol that will take over if the IdP is not known or provided via a query string parameter. (In single-IdP scenarios, there may be no discovery handler.)

Although multiple handlers (or chains) can be defined, one <SessionInitiator> element is marked with isDefault="true". Automatic session initiation via the requireSession setting uses this default SessionInitiator (unless told to use a different one using the requireSessionWith setting).

The default shibboleth2.xml file contains examples for common use cases.

Advanced Configuration

Common Attributes

SAML2 SessionInitiator (Protocol Handler)

Indicated by type="SAML2", supports SAML 2.0 authentication requests. As a protocol handler, an entityID must be specified/known, which is then used to check for metadata with an <md:IDPSSODescriptor> role supporting SAML 2.0. The absence of either causes a warning to be logged and the handler otherwise ignores the request.

Attributes

Child Elements

<SessionInitiator type="SAML2">
    <samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" ID="foo" Version="2.0" IssueInstant="2012-01-01T00:00:00Z">
        <samlp:RequestedAuthnContext Comparison="exact" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
            <saml:AuthnContextClassRef>https://federation.org/ac/type1</saml:AuthnContextClassRef>
            <saml:AuthnContextClassRef>https://federation.org/ac/type2</saml:AuthnContextClassRef>
        </samlp:RequestedAuthnContext>
    </samlp:AuthnRequest>
</SessionInitiator>

Shib1 SessionInitiator (Protocol Handler)

Indicated by type="Shib1", supports Shibboleth 1.x authentication requests, an extension of the SAML 1.1 standard. As a protocol handler, an entityID must be specified/known, which is then used to check for metadata with an <md:IDPSSODescriptor> role supporting Shibboleth 1.x. The absence of either causes a warning to be logged and the handler otherwise ignores the request.

A "supporting" IdP's role element has a protocolSupportEnumeration attribute containing the value "urn:mace:shibboleth:1.0", with an accompanying <md:SingleSignOnService> with a Binding of "urn:mace:shibboleth:1.0:profiles:AuthnRequest".

ADFS SessionInitiator (Protocol Handler)

The ADFS handler is only available if the adsfs.so extension library is loaded by the SP.

Indicated by type="ADFS", supports Microsoft ADFS authentication requests, a subset of the WS-Federation passive requester profile. As a protocol handler, an entityID must be specified/known, which is then used to check for metadata with an <md:IDPSSODescriptor> role supporting ADFS. The absence of either causes a warning to be logged and the handler otherwise ignores the request.

A "supporting" IdP's role element has a protocolSupportEnumeration attribute containing the value "http://schemas.xmlsoap.org/ws/2003/07/secext", with an accompanying <md:SingleSignOnService> with a Binding of "http://schemas.xmlsoap.org/ws/2003/07/secext".

SAMLDS SessionInitiator (Discovery Handler)

Indicated by type="SAMLDS", refers the browser to a discovery service (DS) supporting the proposed SAML 2.0 IdP Discovery Protocol. As a discovery handler, no entityID can be known (or the handler will silently ignore the request, since discovery would serve no purpose).

The design of this protocol results in a redirect back to the SP with a single entityID selected, if one is chosen by the user. The handler implementation causes this redirect to be returned to itself in a manner that allows the handler to be configured in a "chain" with one or more protocol handlers. Used alone, the SAMLDS handler will simply display an error when the result is returned because it cannot itself cause an authentication request to be generated.

Attributes

WAYF SessionInitiator (Discovery Handler)

Indicated by type="WAYF", refers the browser to a Shibboleth WAYF service. As a discovery handler, no entityID can be known (or the handler will silently ignore the request, since discovery would serve no purpose).

The design of this protocol results in the browser leaving the SP until a successful authentication response is returned. This is a legacy technique that limits the ability of an SP to effectively support newer protocols and more advanced features.

Attributes

Cookie SessionInitiator (Discovery Handler)

Indicated by type="Cookie", checks for a cookie maintained as part of the SP's IdP history feature and uses it to obtain the entityID to use for later SessionInitiator handlers in a chain. This handler doesn't actually cause a response to the browser, but it generally runs first in a chain, and allows the entityID to be set before other handlers run. As a discovery handler, no entityID can be known (or the handler will silently ignore the request, since discovery would serve no purpose).

The SP's IdP history can be enabled via the idpHistory attribute on the <Sessions> element.

Attributes

Form SessionInitiator (Discovery Handler)

Indicated by type="Form", displays an HTML template containing a form to prompt the user for the entityID to use. As a discovery handler, no entityID can be known (or the handler will silently ignore the request, since discovery would serve no purpose).

This is a simple substitute for referring the user to another site, which is generally incapable of addressing scenarios involving multiple sets of unrelated IdPs. This handler can be combined with the Transform SessionInitiator to enable the user's input to be turned from something simpler into an entityID.

Attributes

Chaining SessionInitiator

Identified by type="Chaining", wraps a sequence of SessionInitiator handlers so that they run in series. The series ends when a handler indicates that a response to the browser was returned. If no response is sent, an error results.

Options specified via attributes on the surrounding element will apply to all the embedded handlers (if not overridden inside them).

Child Elements

Transform SessionInitiator

Identified by type="Transform", transforms an entityID according to a set of permutations until IdP metadata can be found. No specific protocol support is assumed; the first entityID for which a valid <md:IDPSSODescriptor> can be found terminates the handler's activity.

This handler doesn't actually cause a response to the browser, but it generally runs first in a chain, and allows the entityID to be manipulated before other handlers run. It serves a variety of purposes, from transforming user input into an entity to acting as a kind of "redirect" mechanism that turns one entityID into another.

Attributes

Child Elements

Example

The example tries a sequence of transforms that allows any of the following to be turned into an InCommon IdP name (currently a URN containing a domain name):

<SessionInitiator type="Transform">
    <Subst>urn:mace:incommon:$entityID</Subst>
    <Regex match=".+@(.+)">urn:mace:incommon:$1</Regex>
    <Regex match="^[^.]+\.(.+)">urn:mace:incommon:$1</Regex>
</SessionInitiator>