Minimal FAX address format in Internet Mail
obsoletes
- rfc-2304 — Minimal FAX address format in Internet Mail
updates
- rfc-2846 — GSTN Address Element Extensions in E-mail Services
Extracted elements (16)
The term 'GSTN (Global Switched Telephone Network)' was substituted for the older 'PSTN (Public Switched Telephone Network)' to better reflect that telephony systems no longer conform to a single/few public operator paradigm, while ABNF variable names were kept unchanged for consistency.
Only digits are permitted in the sub-addr field defined by this document, which differs from T.33's use of '#' for multiple subaddresses and SPACE for padding; multiple subaddresses must be represented as separate fax-email elements.
A UA MAY accept multiple subaddress elements for the same global-phone, but it MUST generate multiple 'fax-mbox' elements when submitting the message to the MTA.
All implementations supporting FAX over e-mail address format MUST support this minimal specification.
Clients SHOULD ensure that mail routing is based only on authoritative DNS answers. Once DNS Security mechanisms (DNSSEC) become more widely deployed, clients SHOULD employ those mechanisms to verify authenticity and integrity of mail routing records.
For 'domain' strings used in SMTP transmissions, the string MUST conform to the requirements of SMTP domain specifications (RFC 821, RFC 1123). For strings used in message content headers, the string MUST conform to RFC 822 and RFC 1123.
If multiple T.33 subaddresses are present, the use of multiple 'fax-email' elements is REQUIRED, since e-mail practice uses a separate unique address for each recipient.
Implementations MUST accept the optional slash characters ('/') in fax-email addresses but SHOULD NOT generate them; these can result from translations from X.400 gateways.
The qualif-type1 element (T33S subaddress) is OPTIONAL to include, but compliant implementations MUST be able to support and correctly interpret it when present.
The qualif-type1 element for T.30/T.33 subaddresses is defined as: qualif-type1 = '/' t33-sep '=' sub-addr, where t33-sep = 'T33S' and sub-addr = 1*(DIGIT). Its use is OPTIONAL.
The service-selector for the fax service is defined as the string 'FAX', used to identify fax addresses in GSTN minimal address specifications.
IANA registration for the 'FAX' service-selector specifier, indicating the GSTN address refers to an Internet Fax device or an onramp/offramp Fax gateway. Updates the previous registration from RFC 2846.
IANA registration for the 'T33S' qualif-type1 keyword and its value (sub-addr = 1*(DIGIT)), used to specify numeric-only optional fax sub-addresses per ITU T.33. Restricted to the 'FAX' service-selector.
Since e-mail routing relies on DNS, a successful DNS attack could cause messages to be diverted via compromised MTAs or gateways. Attack vectors include DNS server compromise, counterfeit DNS responses, and incorrect additional information in unrelated query responses.
The complete fax-email address structure is: fax-email = ['"'] ['/'] fax-address ['/'] ['"'] '@' mta-I-fax, combining the fax address local-part with the I-fax device domain.
The fax-address format is: fax-address = fax-mbox [ '/T33S=' sub-addr ], where fax-mbox = 'FAX=' global-phone and global-phone = '+' 1*(DIGIT / written-sep), written-sep = ('-' / '.').