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AS2 (“Applicability Statement 2”) is an SMIME-based transfer protocol that uses HTTP/S to transmit files with end-to-end encryption and guaranteed delivery/non-repudiation (when MDNs are in use).

There are two main reasons that AS2-based transmission systems are unpopular unless specifically requested by particular partners are complexity and cost.

In terms of complexity, AS2 configurations can involve up to four different X.509 certificates on each side of a transfer, plus hostnames, usernames, passwords, URLs, MDN delivery options, timeouts and other variables.  Configuration and testing of each new partner can be a full or multi-day affair, where simpler protocols such as FTP may require hours or minutes.  To hide as much of the configuration complexity as possible from administrators, some AS2 products (such as Cleo’s Lexicom) come with dozens or hundreds of preconfigured partner profiles, but knowledge of the underlying options is still often necessary to troubleshoot and deal with periodic updates of partner credentials or workflows.

In terms of cost, AS2 products that can connect to multiple trading partners are rarely available for less than ten thousand dollars, and the ones that ship with well-developed list of partner profiles cost much more than that.   One factor that drives up this cost is that any marketable AS2 product will be “Drummond Certified“.  The cost of high-end AS2 products is driven up by the fact that compiling and keeping up an extensive library of partner profiles in an expensive endeavor in its own right.  Implementing AS2 securely across a multiple-zone network also tends to drive up costs because intermediate AS2 gateways are often required to prevent direct Internet- or partner-based access to key internal systems.

Another factor working against voluntary AS2-based implementations is transfer speed.  The use of HTTP-based encoding and the requirement that MDNs are only compared after the complete file has been delivered often tips the operational balance in favor of other technology.

AS3 was developed, in part, to cope with AS’s slow HTTP-based encoding, but other modifications (“optional profiles“) to the AS2 protocol have also been introduced to address other limitations.  For example, the optional “AS2 Restart” feature was introduced industry-wide to cope with large files whose delivery was heretofore dependent on long-lasting, unbroken HTTP streams.

Nonetheless, AS2 is considered to be the most successful and most widely adopted of any vendor-independent file transfer protocol that builds both transmission security and guaranteed delivery into the core protocol.

The default port for AS2 is port 80 or port 443.

See also “AS1” for the email-based variant, “AS3” for the FTP-based variant and “AS2 optional profiles” for additional information about available AS2 features.