The purpose of an on-ramp gateway in store-and-forward fax is to receive faxes from the PSTN or standard fax devices. The on-ramp gateway performs the following actions:
- Converts a fax message into a TIFF file.
- Creates a standard Multipurpose Internet Mail Extension (MIME) e-mail message.
- Attaches the TIFF file to the e-mail message.
- Forwards the e-mail message and attachment to the messaging infrastructure of a designated SMTP server, where the message is stored.
The on-ramp gateway uses the sending Message Transfer Agent (MTA) and dial peers to receive fax calls from the PSTN and to define delivery parameters for the resulting e-mail message to which the fax TIFF file is attached. MTAs define the following elements of e-mail messages to which fax TIFF files are attached:
- Subject of the message
- Destination mail server
- Return path
- Postmaster (default mail station for undeliverable messages)
- E-mail header information
- Address to which any disposition notices are sent
A DSN message notifies the sender of an e-mail message that contains a fax TIFF image about the status of that message. DSNs are automatically generated by the SMTP server and are described in RFC 1891, RFC 1892, RFC 1893, and RFC 1894. The following states can be reported to the sender:
- Delay--Message delivery was delayed.
- Success--Message was successfully delivered to the recipient mailbox.
- Failure--SMTP server was unable to deliver the message to the recipient.
The on-ramp gateway security controls who can send fax messages over the packet network. On-ramp accounting keeps track of who uses the packet network resources and how long they use them. On-ramp security and accounting are facilitated by authentication, authorization, and accounting (AAA) security services using RADIUS or TACACS+ as the local security protocol. On-ramp gateway faxing is a client of either the RADIUS or the TACACS+ authentication server. User information is forwarded to the AAA interface, and authentication requests are forwarded to the security server.
Authentication must be completed before the first page of the faxed material is accepted by the Fax Application Process (FAP). If a response is not received from the AAA server before the first page is received, the fax modem or voice card disconnects the call.
RADIUS attributes define specific AAA elements in a user profile. The user profile is stored on the RADIUS server. The Cisco implementation of RADIUS supports Internet Engineering Task Force (IETF) and vendor-proprietary attributes. IETF RADIUS attribute 26 enables vendors to support extended attributes not suitable for general use. The Cisco fax applications use the RADIUS implementation of vendor-specific options in the recommended format.
The "RADIUS Vendor-Specific Attributes" appendix lists the supported vendor-specific options (subtype numbers from 3 through 21) using IETF RADIUS attribute 26 and the Cisco vendor-ID company code of 9.
There are two kinds of off-ramp fax messages:
- Faxes that originate in the PSTN. On entering a packet network, these faxes are converted to TIFF files that are attached to e-mail messages for their transit through the network.
- Faxes that originate from e-mail messages on a PC in the packet network.
Either type can be delivered to a PC on the network before reaching an off-ramp gateway. Upon reaching the off-ramp gateway, however, both types are converted to standard Group 3 fax format for transmission through the PSTN to terminating fax machines.
A basic e-mail operation that store-and-forward fax supports is MDN (return receipt). An MDN is sent to an e-mail originator when the e-mail recipient opens a fax e-mail. MDNs are described in RFC 2298, which also states that e-mail recipients must be able to disable the automatic generation of MDNs.
MDNs are initiated by the sending e-mail client. Return receipts are generated by the receiving e-mail client. Most PC-based e-mail software applications, such as Eudora, Netscape Messenger, and Microsoft Outlook, support MDNs.
Off-ramp security controls who can send outgoing fax messages and is facilitated by AAA security services using either RADIUS or TACACS+. Authentication begins as soon as a fax e-mail message header is received from the e-mail server on the off-ramp gateway. The off-ramp gateway does not dial the destination fax device until authentication for each fax mail is successfully completed.