What is AS2? It stands for Applicability Standard #2, an RFC, via the W3C, that came into being in the mid 2000′s. AS2 is a way to transfer EDI (or any) files securely from point to Point, with acknowledgement. AS2 is different than FTP or the secure FTP variants, because it is not interactive, like FTP, and does not allow access to host system directories – which FTP does allow unless you lock it down in any number of ways.
AS2 is attractive because it defines a strict state based transfer – certain things have to happen in a order, before an MDN acknowledgement is generated synchronously (right away) or asynchronously (not right away). As2 rides on top of http, or secure http, and the EDI MIME file is PKI encrypted with certificates. All of this is standardized; what is not standardized are the sender and receiver port numbers to use (a glaring omission) and other conventions that should have been spelled out as best practices, which Todd has started delving into in his As2 Best Practices Articles (see link below).
See Todd’s excellent post on AS2 for an explanation. We are catching a ton of AS2 keyword traffic, so check out http://www.ld.com/as2-part-1-what-is-it/ as it never hurts to review.