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Network Working Group                                           S. KilleRequest for Comments:  1137                    University College LondonUpdates:  RFC976                                          December 1989Mapping Between FullRFC 822 andRFC 822 withRestricted EncodingStatus of this Memo   This RFC suggests an electronic mail protocol mapping for the   Internet community and UK Academic Community, and requests discussion   and suggestions for improvements.  This memo does not specify an   Internet standard.  Distribution of this memo is unlimited.   This document describes a set of address mappings which will enable   interworking between systems operatingRFC 822 protocols in a general   manner, and those environments where transfer ofRFC 822 messages   restricts the character set which can be used in addresses.  UUCP   transfer ofRFC 822 messages is an important case of this   [Crocker82a,Horton86a].Specification   This document specifies a mapping between two protocols.  This   specification should be used when this mapping is performed on the   Internet or in the UK Academic Community. This specification may be   modified in the light of implementation experience, but no   substantial changes are expected.1.  Introduction   Some mail networks which useRFC 822 cannot support the full   character set required by all aspects ofRFC 822.  This document   describes a symmetrical mapping between fullRFC 822 addressing, and   a form for use on these networks.  Any addresses within the networks   will not use the fullRFC 822 addressing, and so any addresses   encoded according to this standard will always represent remote   addresses.  This document derives from a mapping originally specified   inRFC 987 [Kille86a], where the domain of application was more   restricted.  Two terms are now defined:   FullRFC 822      This implies full support for transfer to and from any legalRFC822 address.  In particular, the quoted-string form of local-part      must be supported (e.g., <"Joe Soap"@foo.bar>).Kille                                                           [Page 1]

RFC 1137           E-Mail Address and Quoted Strings       December 1989   RestrictedRFC 822      This implies a subset ofRFC 822 addressing.  The quoted-string      form of local-part need not be supported.  Standard UUCP mail      transfer falls into this category.  RestrictedRFC 822 is      undesirable, but in practice it exists in many places.      When a message is transferred from fullRFC 822 to restrictedRFC822, and address forms used in fullRFC 822 are involved, message      loss may occur (e.g., it may not be possible to return an error      message).  This RFC describes a quoting mechanism which may be      used to map between fullRFC 822 and restrictedRFC 822, in order      to alleviate this problem.2.  Encoding   TheRFC 822 EBNF meta notation is used.  Any EBNF definitions taken   fromRFC 822 are prefixed by the string "822.".   The following EBNF is specified.      atom-encoded    = *( a-char / a-encoded-char )      a-char          = <any CHAR except specials (other than "@"                              and "."), SPACE,                              CTL, "_", and "#">      a-encoded-char  = "_"                   ; (space)                      / "#u#"                 ; (_)                      / "#l#"                 ; <(>                      / "#r#"                 ; <)>                      / "#m#"                 ; (,)                      / "#c#"                 ; (:)                      / "#b#"                 ; (\)                      / "#h#"                 ; (#)                      / "#e#"                 ; (=)                      / "#s#"                 ; (/)                      / "#" 3DIGIT "#"   The 822.3DIGIT in EBNF.a-encoded-char must have range 0-127, and is   interpreted in decimal as the corresponding ASCII character.  The   choice of special abbreviations (as opposed to decimal encoding)   provided is based on the manner in which this mapping is most   frequently used.  There are special encodings for each of the   PrintableString characters not in EBNF.a-char, except ".".  Space is   given a single character encoding, due to its (expected) frequency of   use, and backslash as theRFC 822 single quote character.   This mapping is used to transform between the two forms of 822.word:   822.quoted-string (restrictedRFC 822) and 822.atom (restricted RFCKille                                                           [Page 2]

RFC 1137           E-Mail Address and Quoted Strings       December 1989   822).  To encode (fullRFC 822 -> restrictedRFC 822), first remove   any quoting from any 822.quoted-string.  Then, all EBNF.a-char are   used directly and all other CHAR are encoded as EBNF.a-encoded-char.   To decode (restrictedRFC 822 -> fullRFC 822): if the address can be   parsed as EBNF.encoded-atom reverse the previous mapping.  If it   cannot be so parsed, map the characters directly.3.  Application   This mapping should be used for all addresses, at the MTS or Header   level.  It is applied to the 822.local-part of the addresses.  For   example:      FullRFC 822                       RestrictedRFC 822      Steve.Kille@cs.ucl.ac.uk     <->   Steve.Kille@cs.ucl.ac.uk      "Steve Kille"@cs.ucl.ac.uk   <->   Steve_Kille@cs.ucl.ac.uk      "argle#~"@blargle            <->   argle#h##126#@blargleReferences   [Crocker82a]  Crocker, D., "Standard of the Format of ARPA Internet   Text Messages",RFC 822, August 1982.   [Horton86a]  Horton, M., "UUCP Mail Interchange Format Standard",RFC 976, February 1986.   [Kille86a]  Kille, S., "Mapping Between X.400 andRFC 822",   UK Academic Community Report (MG.19),RFC 987, June 1986.Security Considerations   Security issues are not discussed in this memo.Author's Address   Steve Kille   University College London   Gower Street   WC1E 6BT   England   Phone: +44-1-380-7294   EMail: S.Kille@Cs.Ucl.AC.UKKille                                                           [Page 3]