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Network Working Group S. Kille
Request for Comments: 1137 University College London
Updates: RFC 976 December 1989
Mapping Between Full RFC 822 and RFC 822 with
Restricted Encoding
Status 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 operating RFC 822 protocols in a general
manner, and those environments where transfer of RFC 822 messages
restricts the character set which can be used in addresses. UUCP
transfer of RFC 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 use RFC 822 cannot support the full
character set required by all aspects of RFC 822. This document
describes a symmetrical mapping between full RFC 822 addressing, and
a form for use on these networks. Any addresses within the networks
will not use the full RFC 822 addressing, and so any addresses
encoded according to this standard will always represent remote
addresses. This document derives from a mapping originally specified
in RFC 987 [Kille86a], where the domain of application was more
restricted. Two terms are now defined:
Full RFC 822
This implies full support for transfer to and from any legal RFC
822 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
Restricted RFC 822
This implies a subset of RFC 822 addressing. The quoted-string
form of local-part need not be supported. Standard UUCP mail
transfer falls into this category. Restricted RFC 822 is
undesirable, but in practice it exists in many places.
When a message is transferred from full RFC 822 to restricted RFC
822, and address forms used in full RFC 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 full RFC 822 and restricted RFC 822, in order
to alleviate this problem.
2. Encoding
The RFC 822 EBNF meta notation is used. Any EBNF definitions taken
from RFC 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 the RFC 822 single quote character.
This mapping is used to transform between the two forms of 822.word:
822.quoted-string (restricted RFC 822) and 822.atom (restricted RFC
Kille [Page 2]
RFC 1137 E-Mail Address and Quoted Strings December 1989
822). To encode (full RFC 822 -> restricted RFC 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 (restricted RFC 822 -> full RFC 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:
Full RFC 822 Restricted RFC 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#@blargle
References
[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 and RFC 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.UK
Kille [Page 3]