In this chapter:

The Application Configuration Access Protocol (ACAP) is a companion protocol to IMAP. The two share a commonality of design, as well as complementary features. These two protocols, working together, form the basis of a vision for mobile users’ use of the Internet.

Whereas IMAP allows one to store and manipulate email on a remote server, so ACAP allows one to store and manipulate generic name-value pairs of information on a remote server. This data may be program configuration options (hence the name), address book data, or any other information that a user may wish to store.

Discussions of ACAP often include mention of the Lightweight Directory Access Protocol (LDAP). LDAP servers provide a directory service, meaning that they store name-value pairs of information and make those records available to authorized users. LDAP servers are generally used on corporate networks to provide a centralized repository for security information (such as usernames and passwords) and contact information (names, addresses, telephone numbers, etc.). Network services can then access the information in LDAP servers to avoid having to duplicate the information. Doesn’t this sound a lot like ACAP?

Corporations have historically been reluctant to place their repositories of security and contact information on Internet-accessible servers, and for good reason. Even if a protocol provides adequate security, potential bugs in implementations give cause for concern. ACAP evolved to help address this concern. ACAP allows individuals to distribute a subset of corporate information via a server that is remotely accessible. Information is compartmentalized, and in case of a security hole, only this distributed subset is compromised.

ACAP clients, unlike LDAP clients, must be able to work offline by caching information locally. An ACAP server is updated when changes are made locally, either in real time or upon the next connection. ACAP is meant to be easier for clients to implement than IMAP; ACAP servers are more complex than the clients. This will hopefully encourage many application developers to choose to implement ACAP clients.

ACAP is increasingly being implemented by manufacturers of MUAs in order to provide access to a Single point of storage for address book and configuration information. If an MUA implements both IMAP and ACAP, a user could connect to a single email account, use a single address book, and maintain consistent configuration details at home, at work, and while traveling. Of course, the price for these benefits is access to a server that provides IMAP and ACAP services.