An MDN (“Message Disposition Notification”) is the method used by the AS1, AS2 and AS3 protocols (the “AS protocols”) to return a strongly authenticated and signed success or failure message back to the senders of the original file. Technically, MDNs are an optional piece of any AS protocol, but MDNs’ critical role as the provider of the “guaranteed delivery” capability in all of the AS protocols means that MDNs are usually used.
Depending on the protocol used and options selected, the MDN will be returned to the sender in one of the following ways:
Via the same HTTP/S stream used to post the original file: AS2 senders may request that MDNs are sent this way. This type of transfer is popularly called “AS2 with synchronous MDNs” (or “AS2 sync” for short). When small files are involved, this type of transfer is the fastest AS protocol transfer currently available.
Via a separate HTTP/S stream back to the sender’s server: AS2 senders may request that MDNs are sent this way. This type of transfer is popularly called “AS2 with ansynchronous MDNs” (or “AS2 async” for short). This type of transmission is slightly more resiliant to network hiccups and long processing turnaround times of large files than “AS2 sync” transmissions.
Via email: All AS1 MDNs are returned this way. AS2 async senders may also request that MDNs are sent this way.
Via FTP: All AS3 MDNs are returned this way.
Full MDNs (the signed responses) are sometimes retained by the sender and/or recipient as irrefutable proof of guaranteed delivery. The use of X.509 certificates to authenticate and sign both the original file transmission and the MDN receipt often allows MDNs to rise to the level of legally binding nonrepudiation in many jurisdictions.