ActiveSync Library Technical Information

This page is designed to give anyone working on the ActiveSync library in Horde 5.x a general overview of logic flow and what happens where and when.

General Library Structure

The logic for handling ActiveSync requests is split between 3 different libraries currently - Horde_Rpc, Horde_Core, and obviously Horde_ActiveSync. Horde_Rpc only handles the initial request and basically just passes on control to Horde_ActiveSync so this page will focus only on the other two libraries.

Horde_ActiveSync

This library contains the main logic for decoding WBXML from EAS requests, passing the request to the appropriate controller and then sending properly encoded responses back to the client.

TODO general description of the classes

Horde_Core

Core contains any code specific to handling Horde Groupware collections. This is where requests for information and changes to information are actually handled.

Factories

Horde_Core_Factory_ActiveSyncServer

Creates the main Horde_ActiveSync object. Injects the Horde_Core_ActiveSync_Driver, the WBXML encoder/decoder objects, the Horde_ActiveSync_State_[Sql|Mongo] storage object and the Horde_Controller_Request object.

Horde_Core_Factory_ActiveSyncState

Creates the state storage handler. The name is misleading (and will change in Horde 6) as this class has evolved to be more of a general storage handler and now handles more than just device state. We will assume a Sql storage backend for this document.

Horde_Core_Factory_ActiveSyncBackend

Creates the Horde_Core_ActiveSync_Driver backend driver. Injects Horde_Core_ActiveSync_Connector, Horde_ActiveSync_Imap_Adapter (if needed), the state storage driver, and the Horde_Core_ActiveSync_Auth object.

TODO ....

Protocol Overview

It's beyond the scope to explain the ActiveSync protocol in detail. For that, there is the official documentation. The most useful of these are

MS-ASCMD

Details every command/request and it's schema.

MS-ASHTTP

Documents the requirements and flow of the HTTP protocol as used in the EAS protocol.

There are also the individual documents for each collection type, such as MS-ASEMAIL, MS-ASTASKS etc... These documents are the best place to start when trying to track down issues such as "Protocol Error" issues with certain clients.

The basic bit to know about the EAS protocol is that it is encoded using WBXML. That is, binary encoded XML data. Again, the structure/schema of WBXML data is beyond the scope of this page. Instead of decoding the entire request first and then handling it, Horde_ActiveSync decodes the data and handles it on the fly. That is, we decode each individual message/object as it comes in and handle it in-line, so to speak. This is to avoid keeping more in memory than is necessary. The same is true for encoding - we don't wait to enocde the entire response - we encode and send the response to the output stream as soon as we can. The actual encoding/decoding is done in the Horde_ActiveSync_Wbxml_* classes. The codepages/schema is defined in //Horde_ActiveSync_Wbxml::.

Logic Flow

I have a long-standing item on my todo list to generate an activity diagram for the program flow of an EAS request, but in the meantime here is a description of what happens.

EAS requests hit rpc.php first. There are a number of ways that these are differentiated from other RPC requests, but the main give-away is the REQUEST_URI containing Microsoft-Server-ActiveSync. A few things happen here before continuing. First, EAS requests are session-less, meaning that the entire Horde environment needs to be setup for each request. This is handled by explicitly setting $session_control = 'None' to force the use of the Null session driver in Horde. From there, we perform the same logic and checks like every other Horde RPC request. This includes having to initialize the Horde environment with NO authentication.

From here, we instantiate a Horde_Rpc_ActiveSync object and inject a Horde_ActiveSync object (which is created using Horde_Core_Factory_ActiveSyncServer).

Here, we perform some sanity checking on the request and sniff out what type of request we are handling. OPTIONS and Autodiscover requests are handled a little differently, but for now we will concentrate on the "normal" request handling.

The client must send certain data, apart from the WBXML structure, with each request. This data is either present as "normal" GET variables or is sent as BASE64 encoded binary data sent in QUERY_STRING (see Horde_ActiveSync::getGetVars()). The format of this binary data is beyond the scope of this page, but the data is decoded in Horde_ActiveSync_Utils::decodeBase64().

Cmd

This is the command or request type. E.g., SYNC, PING, FOLDESYNC

DeviceId

This is a unique identifier for the client. This value is only unique to the client, not to the account. I.e., the same physical device/application will have the same DeviceId. Multiple users can be associated with the same DeviceId.

Flow is turned over to Horde_ActiveSync::handleRequest(). This is where the interesting stuff starts to happen.

Authentication

First, we perform some checks, normalization, and call the versionCallback hook if it's present. If all is well, we finally attempt authentication. Authentication to Horde is a massive topic and it's only complicated more by layering ActiveSync on top of it. I will try to give a brief overview here. For a more detailed description of the general Horde authentication layer, see Jan's excellent series of posts on his blog.

Thanks to broken clients, different supported authentication mechanisms and other idiosyncrasies, we need to perform some magic to make sure we have the user's credentials. For this, there is Horde_ActiveSync_Credentials. This class, when constructed and injected with the Horde_ActiveSync object, will have two properties set: username and password. For the purposes of this page, we will assume a typical setup where the client properly sends credentials using HTTP BASIC and we are NOT using X509 certificates.

Once we have found the credentials, we call Horde_ActiveSync::authenticate(). This performs a few checks then ultimately passes control to Horde_Core_ActiveSync_Driver::authenticate(). Since some broken clients always send the email address as the username, plus the fact the Autodiscover requests ALWAYS use the email address, we need to normalize the username to the correct form. This is handled in Horde_Core_ActiveSync_Driver::getUsernameFromEmail() and is partially affected by the $GLOBALS['conf']['activesync']['autodiscovery'] setting (yes, this name is misleading since it is now used for more than just autodiscovery).

Once in Horde_Core_ActiveSync_Driver::authenticate() we use the Horde_Core_ActiveSync_Auth object that was injected when the driver was created to perform the actual authentication. The reason for this authentication wrapper is to allow for combinations of a "normal" Horde auth driver along with a transparent driver like X509 to support clients that allow for certificates along WITH credentials. For this page, we will assume a "normal" authentication.

Life Cycle of a Client.

What we will describe is the life cycle of a client-server pairing from the initial connection to be able to synchronize changes. Let's start with a fresh, never before connected client. The first thing that happens is the OPTIONS request. This essentially tells the server what protocol versions the client supports and the server responds with (among other things) the version that it will be using.