RFC 1138
Mapping between X.400(1988) / ISO 10021 and RFC 822
Pages: 92
Obsoleted by: 2156 1327
Updates: 1026 0987 0822
Updated by: 1148
Obsoleted by: 2156 1327
Updates: 1026 0987 0822
Updated by: 1148
Part 2 of 3 – Pages 29 to 60
Chapter 4 -- Addressing Addressing is probably the trickiest problem of an X.400 <-> RFC 822 gateway. Therefore it is given a separate chapter. This chapter, as a side effect, also defines a textual representation of an X.400 O/R Address. Initially, we consider an address in the (human) mail user sense of "what is typed at the mailsystem to reference a mail user". A basic RFC 822 address is defined by the EBNF EBNF.822-address: 822-address = [ route ] addr-spec In an 822-MTS protocol, the originator and each recipient should be considered to be defined by such a construct. In an RFC 822 header, the EBNF.822-address is encapsulated in the 822.address syntax rule, and there may also be associated comments. None of this extra information has any semantics, other than to the end user. The basic X.400 O/R Address, used by the MTS for routing, is defined by MTS.ORAddress. In IPMS, the MTS.ORAddress is encapsulated within
IPMS.ORDescriptor. It can be seen that RFC 822 822.address must be mapped with IPMS.ORDescriptor, and that RFC 822 EBNF.822-address must be mapped with MTS.ORAddress. 4.1. A textual representation of MTS.ORAddress MTS.ORAddress is structured as a set of attribute value pairs. It is clearly necessary to be able to encode this in ASCII for gatewaying purposes. All aspects should be encoded, in order to guarantee return of error messages, and to optimise third party replies. 4.2. Basic Representation An O/R Address has a number of structured and unstructured attributes. For each unstructured attribute, a key and an encoding is specified. For structured attributes, the X.400 attribute is mapped onto one or more attribute value pairs. For domain defined attributes, each element of the sequence will be mapped onto a triple (key and two values), with each value having the same encoding. The attributes are as follows, with 1984 attributes given in the first part of the table. For each attribute, a reference is given, consisting of the relevant sections in X.402 / ISO 10021-2, and the extension identifier for 88 only attributes: Attribute (Component) Key Enc Ref Id 84/88 Attributes MTS.CountryName C P 18.3.3 MTS.AdministrationDomainName ADMD P 18.3.1 MTS.PrivateDomainName PRMD P 18.3.21 MTS.NetworkAddress X121 N 18.3.7 MTS.TerminalIdentifier T-ID N 18.3.23 MTS.OrganizationName O P/T 18.3.9 MTS.OrganizationalUnitNames.value OU P/T 18.3.10 MTS.NumericUserIdentifier UA-ID N 18.3.8 MTS.PersonalName PN P/T 18.3.12 MTS.PersonalName.surname S P/T 18.3.12 MTS.PersonalName.given-name G P/T 18.3.12 MTS.PersonalName.initials I P/T 18.3.12 MTS.PersonalName .generation-qualifier GQ P/T 18.3.12 MTS.DomainDefinedAttribute.value DD P/T 18.1
88 Attributes
MTS.CommonName CN P/T 18.3.2 1
MTS.TeletexCommonName CN P/T 18.3.2 2
MTS.TeletexOrganizationName O P/T 18.3.9 3
MTS.TeletexPersonalName PN P/T 18.3.12 4
MTS.TeletexPersonalName.surname S P/T 18.3.12 4
MTS.TeletexPersonalName.given-name G P/T 18.3.12 4
MTS.TeletexPersonalName.initials I P/T 18.3.12 4
MTS.TeletexPersonalName
.generation-qualifier GQ P/T 18.3.12 4
MTS.TeletexOrganizationalUnitNames
.value OU P/T 18.3.10 5
MTS.TeletexDomainDefinedAttribute
.value DD P/T 18.1 6
MTS.PDSName PD-SYSTEM P 18.3.11 7
MTS.PhysicalDeliveryCountryName PD-C P 18.3.13 8
MTS.PostalCode POSTCODE P 18.3.19 9
MTS.PhysicalDeliveryOfficeName PD-OFFICE P/T 18.3.14 10
MTS.PhysicalDeliveryOfficeNumber PD-OFFICE-NUM P/T 18.3.15 11
MTS.ExtensionORAddressComponents PD-EXT-D P/T 18.3.4 12
MTS.PhysicalDeliveryPersonName PD-PN P/T 18.3.17 13
MTS.PhysicalDelivery PD-O P/T 18.3.16 14
OrganizationName
MTS.ExtensionPhysicalDelivery
AddressComponents PD-EXT-LOC P/T 18.3.5 15
MTS.UnformattedPostalAddress PD-ADDRESS P/T 18.3.25 16
MTS.StreetAddress STREET P/T 18.3.22 17
MTS.PostOfficeBoxAddress PO-BOX P/T 18.3.18 18
MTS.PosteRestanteAddress POSTE-RESTANTE P/T 18.3.20 19
MTS.UniquePostalName PD-UNIQUE P/T 18.3.26 20
MTS.LocalPostalAttributes PD-LOCAL P/T 18.3.6 21
MTS.ExtendedNetworkAddress
.e163-4-address.number NET-NUM N 18.3.7 22
MTS.ExtendedNetworkAddress
.e163-4-address.sub-address NET-SUB N 18.3.7 22
MTS.ExtendedNetworkAddress
.psap-address NET-PSAP X 18.3.7 22
MTS.TerminalType NET-TTYPE I 18.3.24 23
The following keys identify different EBNF encodings, which are
associated with the ASCII representation of MTS.ORAddress.
Key Encoding
P printablestring
N numericstring
T teletex-string
P/T teletex-and-or-ps
I labelled-integer
X presentation-address
The BNF for presentation-address is taken from the specification "A
String Encoding of Presentation Address" [Kille89a].
In most cases, the EBNF encoding maps directly to the ASN.1 encoding
of the attribute. There are a few exceptions. In cases where an
attribute can be encoded as either a PrintableString or NumericString
(Country, ADMD, PRMD), either form should be mapped into the BNF.
When generating ASN.1, the NumericString encoding should be used if
the string contains only digits.
There are a number of cases where the P/T (teletex-and-or-ps)
representation is used. Where the key maps to a single attribute,
this choice is reflected in the encoding of the attribute (attributes
10-21). For most of the 1984 attributes and common name, there is a
printablestring and a teletex variant. This pair of attributes is
mapped onto the single component here. This will give a clean
mapping for the common cases where only one form of the name is used.
4.2.1. Encoding of Personal Name
Handling of Personal Name and Teletex Personal Name based purely on
the EBNF.standard-type syntax defined above is likely to be clumsy.
It seems desirable to utilise the "human" conventions for encoding
these components. A syntax is defined, which is designed to provide
a clean encoding for the common cases of O/R address specification
where:
1. There is no generational qualifier
2. Initials contain only letters
3. Given Name does not contain full stop ("."), and is at least
two characters long.
4. If Surname contains full stop, then it may not be in the
first two characters, and either initials or given name is
present.
The following EBNF is defined:
encoded-pn = [ given "." ] *( initial "." ) surname
given = 2*<ps-char not including ".">
initial = ALPHA
surname = printablestring
This can be used to map from any string containing only printable
string characters to an O/R address personal name. Parse the string
according to the EBNF. The given name and surname are assigned
directly. All EBNF.initial tokens are concatenated without
intervening full stops to generate the initials.
For an O/R address which follows the above restrictions, a string can
be derived in the natural manner. In this case, the mapping will be
reversible.
For example:
GivenName = "Marshall"
Surname = "Rose"
Maps with "Marshall.Rose"
Initials = "MT"
Surname = "Rose"
Maps with "M.T.Rose"
GivenName = "Marshall"
Initials = "MT"
Surname = "Rose"
Maps with "Marshall.M.T.Rose"
Note that X.400 suggest that Initials is used to encode ALL initials.
Therefore, the proposed encoding is "natural" when either GivenName
or Initials, but not both, are present. The case where both are
present can be encoded, but this appears to be contrived!
4.2.2. Standard Encoding of MTS.ORAddress
Given this structure, we can specify a BNF representation of an O/R
Address.
std-or-address = 1*( "/" attribute "=" value ) "/"
attribute = standard-type
/ "RFC-822"
/ registered-dd-type
/ dd-key "." std-printablestring
standard-type = key-string
registered-dd-type
= key-string
dd-key = key-string
value = std-printablestring
std-printablestring
= *( std-char / std-pair )
std-char = <"{", "}", "*", and any ps-char
except "/" and "=">
std-pair = "$" ps-char
The standard-type is any key defined in the table in Section 4.2,
except PN, and DD. The value, after quote removal, should be
interpreted according to the defined encoding.
If the standard-type is PN, the value is interpreted according to
EBNF.encoded-pn, and the components of MTS.PersonalName and/or
MTS.TeletexPersonalName derived accordingly.
If dd-key is the recognised Domain Defined string (DD), then the type
and value should be interpreted according to the syntax implied from
the encoding, and aligned to either the teletex or printable string
form. Key and value should have the same encoding.
If value is "RFC-822", then the (printable string) Domain Defined
Type of "RFC-822" is assumed. This is an optimised encoding of the
domain defined type defined by this specification.
The matching of all keywords should be done in a case- independent
manner.
If the value is registered-dd-type, the value is registered with the
IANA and will be listed in the Assigned Numbers RFC, then the value
should be interpreted accordingly. This restriction maximises the
syntax checking which can be done at a gateway.
4.3. EBNF.822-address <-> MTS.ORAddress
Ideally, the mapping specified would be entirely symmetrical and
global, to enable addresses to be referred to transparently in the
remote system, with the choice of gateway being left to the Message
Transfer Service. There are two fundamental reasons why this is not
possible:
1. The syntaxes are sufficiently different to make this
awkward.
2. In the general case, there would not be the necessary
administrative co-operation between the X.400 and RFC 822
worlds, which would be needed for this to work.
Therefore, an asymmetrical mapping is defined, which can be
symmetrical where there is appropriate administrative control.
4.3.1. X.400 encoded in RFC 822
The std-or-address syntax is used to encode O/R Address information
in the 822.local-part of EBNF.822-address. Further O/R Address
information may be associated with the 822.domain component. This
cannot be used in the general case, basically due to character set
problems, and lack of order in X.400 O/R Addresses. The only way to
encode the full PrintableString character set in a domain is by use
of the 822.domain-ref syntax (i.e., 822.atom). This is likely to
cause problems on many systems. The effective character set of
domains is in practice reduced from the RFC 822 set, by restrictions
imposed by domain conventions and policy.
A generic 822.address consists of a 822.local-part and a sequence of
822.domains (e.g., <@domain1,@domain2:user@domain3>). All except the
822.domain associated with the 822.local-part (domain3 in this case)
should be considered to specify routing within the RFC 822 world, and
will not be interpreted by the gateway (although they may have
identified the gateway from within the RFC 822 world).
This form of source routing is now discouraged in the Internet
(Host Requirements, page 58 [Braden89a]).
The 822.domain associated with the 822.local-part may also identify
the gateway from within the RFC 822 world. This final 822.domain may
be used to determine some number of O/R Address attributes. The
following O/R Address attributes are considered as a hierarchy, and
may be specified by the domain. They are (in order of hierarchy):
Country, ADMD, PRMD, Organisation, Organisational Unit
There may be multiple Organisational Units.
Associations may be defined between domain specifications, and
some set of attributes. This association proceeds hierarchically.
For example, if a domain implies ADMD, it also implies country.
Subdomains under this are associated according to the O/R Address
hierarchy. For example:
=> "AC.UK" might be associated with
C="GB", ADMD="GOLD 400", PRMD="UK.AC"
then domain "R-D.Salford.AC.UK" maps with
C="GB", ADMD="GOLD 400", PRMD="UK.AC", O="Salford", OU="R-D"
There are three basic reasons why a domain/attribute mapping might
be maintained, as opposed to using simply subdomains:
1. As a shorthand to avoid redundant X.400 information. In
particular, there will often be only one ADMD per country,
and so it does not need to be given explicitly.
2. To deal with cases where attribute values do not fit the
syntax:
domain-syntax = alphanum [ *alphanumhyphen alphanum ]
alphanum = <ALPHA or DIGIT>
alphanumhyphen = <ALPHA or DIGIT or HYPHEN>
Although RFC 822 allows for a more general syntax, this
restricted syntax is chosen as it is the one chosen by the
various domain service administrations.
3. To deal with missing elements in the hierarchy. A domain
may be associated with an omitted attribute in conjunction
with several present ones. When performing the algorithmic
insertion of components lower in the hierarchy, the omitted
value should be skipped. For example, if "HNE.EGM" is
associated with "C=TC", "ADMD=ECQ", "PRMD=HNE", and omitted
organisation, then "ZI.HNE.EGM" is mapped with "C=TC",
"ADMD=ECQ", "PRMD=HNE", "OU=ZI". It should be noted that
attributes may have null values, and that this is treated
separately from omitted attributes (whilst it would be bad
practice to treat these two cases differently, they must be
allowed for).
This set of mappings need only be known by the gateways relaying
between the RFC 822 world, and the O/R Address space associated with
the mapping in question. However, it is desirable (for the optimal
mapping of third party addresses) for all gateways to know these
mappings. A format for the exchange of this information is defined
in Appendix F.
The remaining attributes are encoded on the LHS, using the EBNF.std-
or-address syntax. For example:
/I=J/S=Linnimouth/GQ=5/@Marketing.Widget.COM
encodes the MTS.ORAddress consisting of:
MTS.CountryName = "TC"
MTS.AdministrationDomainName = "BTT"
MTS.OrganizationName = "Widget"
MTS.OrganizationalUnitNames.value = "Marketing"
MTS.PersonalName.surname = "Linnimouth"
MTS.PersonalName.initials = "J"
MTS.PersonalName.generation-qualifier = "5"
The first three attributes are determined by the domain Widget.COM.
Then, the first element of OrganizationalUnitNames is determined
systematically, and the remaining attributes are encoded on the LHS.
In an extreme case, all of the attributes will be on the LHS. As the
domain cannot be null, the RHS will simply be a domain indicating the
gateway.
The RHS (domain) encoding is designed to deal cleanly with common
addresses, and so the amount of information on the RHS should be
maximised. In particular, it covers the Mnemonic O/R Address using a
1984 compatible encoding. This is seen as the dominant form of O/R
Address. Use of other forms of O/R Address, and teletex encoded
attributes will require an LHS encoding.
There is a further mechanism to simplify the encoding of common
cases, where the only attributes to be encoded on the LHS is a (non-
Teletex) Personal Name attributes which comply with the restrictions
of 4.2.1. To achieve this, the 822.local-part shall be encoded as
EBNF.encoded-pn. In the previous example, if the GenerationQualifier
was not present, the encoding J.Linnimouth@Marketing.Widget.COM would
result.
From the standpoint of the RFC 822 Message Transfer System, the
domain specification is simply used to route the message in the
standard manner. The standard domain mechanisms are are used to
select appropriate gateways for the corresponding O/R Address space.
In most cases, this will be done by registering the higher levels,
and assuming that the gateway can handle the lower levels.
4.3.2. RFC 822 encoded in X.400
In some cases, the encoding defined above may be reversed, to give a
"natural" encoding of genuine RFC 822 addresses. This depends
largely on the allocation of appropriate management domains.
The general case is mapped by use of domain defined attributes. A
Domain defined type "RFC-822" is defined. The associated attribute
value is an ASCII string encoded according to Section 3.3.3 of this
specification. The interpretation of the ASCII string depends on the
context of the gateway.
1. In the context of RFC 822, and RFC 1034
[Crocker82a, Mockapetris87a], the string can be used
directly.
2. In the context of the JNT Mail protocol, and the NRS
[Kille84a, Larmouth83a], the string should be interpreted
according to Mailgroup Note 15 [Kille84b].
3. In the context of UUCP based systems, the string should be
interpreted as defined in [Horton86a].
Other O/R Address attributes will be used to identify a context in
which the O/R Address will be interpreted. This might be a
Management Domain, or some part of a Management Domain which
identifies a gateway MTA. For example:
C = "GB"
ADMD = "GOLD 400"
PRMD = "UK.AC"
O = "UCL"
OU = "CS"
"RFC-822" = "Jimmy(a)WIDGET-LABS.CO.UK"
OR
C = "TC"
ADMD = "Wizz.mail"
PRMD = "42"
"rfc-822" = "Postel(a)venera.isi.edu"
Note in each case the PrintableString encoding of "@" as "(a)". In
the second example, the "RFC-822" domain defined attribute is
interpreted everywhere within the (Private) Management Domain. In
the first example, further attributes are needed within the
Management Domain to identify a gateway. Thus, this scheme can be
used with varying levels of Management Domain co-operation.
4.3.3. Component Ordering
In most cases, ordering of O/R Address components is not significant
for the mappings specified. However, Organisational Units (printable
string and teletex forms) and Domain Defined Attributes are specified
as SEQUENCE in MTS.ORAddress, and so their order may be significant.
This specification needs to take account of this:
1. To allow consistent mapping into the domain hierarchy
2. To ensure preservation of order over multiple mappings.
There are three places where an order must be specified:
1. The text encoding (std-or-address) of MTS.ORAddress as used
in the local-part of an RFC 822 address. An order is needed
for those components which may have multiple values
(Organisational Unit, and Domain Defined Attributes). When
generating an 822.std-or-address, components of a given type
shall be in hierarchical order with the most significant
component on the RHS. If there is an Organisation
Attribute, it shall be to the right of any Organisational
Unit attributes. These requirements are for the following
reasons:
- Alignment to the hierarchy of other components in RFC
822 addresses (thus, Organisational Units will appear
in the same order, whether encoded on the RHS or LHS).
Note the differences of JNT Mail as described in
Appendix B.
- Backwards compatibility with RFC 987/1026.
- To ensure that gateways generate consistent addresses.
This is both to help end users, and to generate
identical message ids.
Further, it is recommended that all other attributes are
generated according to this ordering, so that all attributes
so encoded follow a consistent hierarchy.
There will be some cases where an X.400 O/R address of this
encoding will be generated by an end user from external
information. The ordering of attributes may be inverted or
mixed. For this reason, the following heuristics may be
applied:
- If there is an Organisation attribute to the left of
any Org Unit attribute, assume that the hierarchy is
inverted.
- If an inversion of the Org Unit hierarchy generates a
valid address, when the preferred order does not,
assume that the hierarchy is inverted.
2. For the Organisational Units (OU) in MTS.ORAddress, the
first OU in the SEQUENCE is the most significant, as
specified in X.400.
3. For the Domain Defined Attributes in MTS.ORAddress, the
First Domain Defined Attribute in the SEQUENCE is the most
significant.
Note that although this ordering is mandatory for this
mapping, there are NO implications on ordering significance
within X.400, where this is a Management Domain issue.
4.3.4. RFC 822 -> X.400
There are two basic cases:
1. X.400 addresses encoded in RFC 822. This will also include
RFC 822 addresses which are given reversible encodings.
2. "Genuine" RFC 822 addresses.
The mapping should proceed as follows, by first assuming case 1).
STAGE I.
1. If the 822-address is not of the form:
local-part "@" domain
Go to stage II.
NOTE:It may be appropriate to reduce a source route address
to this form by removal of all bar the last domain. In
terms of the design intentions of RFC 822, this would
be an incorrect action. However, in most real cases,
it will do the "right" thing and provide a better
service to the end user. This is a reflection on the
excessive and inappropriate use of source routing in
RFC 822 based systems. Either approach, or the
intermediate approach of stripping only domain
references which reference the local gateway are
conformant to this specification.
2. Attempt to parse EBNF.domain as:
*( domain-syntax "." ) known-domain
Where EBNF.known-domain is the longest possible match in a
list of supported mappings (see Appendix F). If this fails,
and the EBNF.domain does not explicitly identify the local
gateway, go to stage II. If it succeeds, allocate the
attributes associated with EBNF.known-domain, and
systematically allocate the attributes implied by each
EBNF.domain-syntax component. If the domain explicitly
identifies the gateway, allocate no attributes.
3. If the local-part contains any characters not in
PrintableString, go to stage II.
4. If the 822.local-part uses the 822.quoted-string encoding,
remove this quoting. Parse the (unquoted) 822.local-part
according to the EBNF EBNF.std-or-address. If this parse
fails, parse the local-part according to the EBNF
EBNF.encoded-pn. The result is a set of type/value pairs.
If the values generated conflict with those derived in step
2 (e.g., a duplicated country attribute), the domain should
be assumed to be an RFC 987 gateway. In this case, take
only the LHS derived attributes. Otherwise add LHS and RHS
derived attributes together.
5. Associate the EBNF.attribute-value syntax (determined from
the identified type) with each value, and check that it
conforms. If not, go to stage II.
6. Ensure that the set of attributes conforms both to the
MTS.ORAddress specification and to the restrictions on this
set given in X.400. If not go to stage II.
7. Build the O/R Address from this information.
STAGE II.
This will only be reached if the RFC 822 EBNF.822-address is not
a valid X.400 encoding. If the address is an 822-MTS recipient
address, it must be rejected, as there is a need to interpret
such an address in X.400. For the 822-MTS return address, and
any addresses in the RFC 822 header, they should now be encoded
as RFC 822 addresses in an X.400 O/R Name:
1. Convert the EBNF.822-address to PrintableString, as
specified in Chapter 3.
2. The "RFC-822" domain defined attribute should be generated
from this string.
3. Build the rest of the O/R Address in the local Management
Domain agreed manner, so that the O/R Address will receive a
correct global interpretation.
Note that the domain defined attribute value has a maximum length
of MTS.ub-domain-defined-attribute-value-length (128). If this is exceeded by a mapping at the MTS level, then the gateway should reject the message in question. If this occurs at the IPMS level, then the action should depend on the policy being taken, which is discussed in Section 5.1.3. 4.3.5. X.400 -> RFC 822 There are two basic cases: 1. RFC 822 addresses encoded in X.400. 2. "Genuine" X.400 addresses. This may include symmetrically encoded RFC 822 addresses. When a MTS Recipient O/R Address is interpreted, gatewaying will be selected if there a single "RFC-822" domain defined attribute present. In this case, use mapping A. For other O/R Addresses which: 1. Contain the special attribute. AND 2. Identifies the local gateway or any other known gateway with the other attributes. Use mapping A. In other cases, use mapping B. NOTE: A pragmatic approach would be to assume that any O/R Address with the special domain defined attribute identifies an RFC 822 address. This will usually work correctly, but is in principle not correct. Mapping A 1. Map the domain defined attribute value to ASCII, as defined in Chapter 3. Mapping B This will be used for X.400 addresses which do not use the explicit RFC 822 encoding. 1. For all string encoded attributes, remove any leading or trailing spaces, and replace adjacent spaces with a single space.
2. Noting the hierarchy specified in 4.3.1, determine the
maximum set of attributes which have an associated domain
specification. If no match is found, allocate the domain as
the domain specification of the local gateway, and go to
step 4.
3. Following the 4.3.1 hierarchy and noting any omitted
components implied by the mapping tables (see Appendix F),
if each successive component exists, and conforms to the
syntax EBNF.domain-syntax (as defined in 4.3.1), allocate
the next subdomain. At least one attribute of the X.400
address should not be mapped onto subdomain, as
822.local-part cannot be null.
4. If the remaining components are personal-name components,
conforming to the restrictions of 4.2.1, then EBNF.encoded-
pn should be derived to form 822.local-part. In other cases
the remaining components should simply be encoded as a
822.local-part using the EBNF.std-or-address syntax. If
necessary, the 822.quoted-string encoding should be used.
If the derived 822.local-part can only be encoded by use of
822.quoted-string, then use of the mapping defined
in [Kille89b] may be appropriate. Use of this mapping is
discouraged.
4.4. Repeated Mappings
The mappings defined are symmetrical and reversible across a single
gateway. The symmetry is particularly useful in cases of (mail
exploder type) distribution list expansion. For example, an X.400
user sends to a list on an RFC 822 system which he belongs to. The
received message will have the originator and any 3rd party X.400 O/R
Addresses in correct format (rather than doubly encoded). In cases
(X.400 or RFC 822) where there is common agreement on gateway
identification, then this will apply to multiple gateways.
When a message traverses multiple gateways, the mapping will always
be reversible, in that a reply can be generated which will correctly
reverse the path. In many cases, the mapping will also be
symmetrical, which will appear clean to the end user. For example,
if countries "AB" and "XY" have RFC 822 networks, but are
interconnected by X.400, the following may happen: The originator
specifies:
Joe.Soap@Widget.PTT.XY
This is routed to a gateway, which generates:
C = "XY"
ADMD = "PTT"
PRMD = "Griddle MHS Providers"
Organisation = "Widget Corporation"
Surname = "Soap"
Given Name = "Joe"
This is then routed to another gateway where the mapping is reversed
to give:
Joe.Soap@Widget.PTT.XY
Here, use of the gateway is transparent.
Mappings will only be symmetrical where mapping tables are defined.
In other cases, the reversibility is more important, due to the (far
too frequent) cases where RFC 822 and X.400 services are partitioned.
The syntax may be used to source route. THIS IS STRONGLY
DISCOURAGED. For example:
X.400 -> RFC 822 -> X.400
C = "UK"
ADMD = "Gold 400"
PRMD = "UK.AC"
"RFC-822" = "/PN=Duval/DD.Title=Manager/(a)Inria.ATLAS.FR"
This will be sent to an arbitrary UK Academic Community gateway by
X.400. Then it will be sent by JNT Mail to another gateway
determined by the domain Inria.ATLAS.FR (FR.ATLAS.Inria). This will
then derive the X.400 O/R Address:
C = "FR"
ADMD = "ATLAS"
PRMD = "Inria"
PN.S = "Duval"
"Title" = "Manager"
Similarly:
RFC 822 -> X.400 -> RFC 822
"/C=UK/ADMD=BT/PRMD=AC/RFC-822=jj(a)seismo.css.gov/"
@monet.berkeley.edu
This will be sent to monet.berkeley.edu by RFC 822, then to the AC
PRMD by X.400, and then to jj@seismo.css.gov by RFC 822. 4.5. Directory Names Directory Names are an optional part of O/R Name, along with O/R Address. The RFC 822 addresses are mapped onto the O/R Address component. As there is no functional mapping for the Directory Name on the RFC 822 side, a textual mapping should be used. There is no requirement for reversibility in terms of the goals of this specification. There may be some loss of functionality in terms of third party recipients where only a directory name is given, but this seems preferable to the significant extra complexity of adding a full mapping for Directory Names. 4.6. MTS Mappings The basic mappings at the MTS level are: 1) 822-MTS originator -> MTS.PerMessageSubmissionFields.originator-name MTS.OtherMessageDeliveryFields.originator-name -> 822-MTS originator 2) 822-MTS recipient -> MTS.PerRecipientMessageSubmissionFields MTS.OtherMessageDeliveryFields.this-recipient-name -> 822-MTS recipient 822-MTS recipients and return addresses are encoded as EBNF.822- address. The MTS Originator is always encoded as MTS.OriginatorName, which maps onto MTS.ORAddressAndOptionalDirectoryName, which in turn maps onto MTS.ORName. 4.6.1. RFC 822 -> X.400 From the 822-MTS Originator, use the basic ORAddress mapping, to generate MTS.PerMessageSubmissionFields.originator-name (MTS.ORName), without a DirectoryName. For recipients, the following settings should be made for each component of MTS.PerRecipientMessageSubmissionFields. recipient-name This should be derived from the 822-MTS recipient by the basic ORAddress mapping.
originator-report-request
This should be set according to content return policy, as
discussed in Section 5.2.
explicit-conversion
This optional component should be omitted, as this service
is not needed.
extensions
The default value (no extensions) should be used.
4.6.2. X.400 -> RFC 822
The basic functionality is to generate the 822-MTS originator and
recipients. There is information present on the X.400 side, which
cannot be mapped into analogous 822-MTS services. For this reason,
new RFC 822 fields are added for the MTS Originator and Recipients.
The information discarded at the 822-MTS level should be present in
these fields. There may also be the need to generate a delivery
report.
4.6.2.1. 822-MTS Mappings
Use the basic ORAddress mapping, to generate the 822-MTS originator
(return address) from MTS.OtherMessageDeliveryFields.originator-name
(MTS.ORName). If MTS.ORName.directory-name is present, it should be
discarded.
The 822-MTS recipient is conceptually generated from
MTS.OtherMessageDeliveryFields.this-recipient-name. This is done by
taking MTS.OtherMessageDeliveryFields.this-recipient-name, and
generating an 822-MTS recipient according to the basic ORAddress
mapping, discarding MTS.ORName.directory-name if present. However,
if this model was followed exactly, there would be no possibility to
have multiple 822-MTS recipients on a single message. This is
unacceptable, and so layering is violated. The mapping needs to use
the MTA level information, and map each value of
MTA.PerRecipientMessageTransferFields.recipient-name, where the
responsibility bit is set, onto an 822-MTS recipient.
4.6.2.2. Generation of RFC 822 Headers
Not all per-recipient information can be passed at the 822-MTS level.
For this reason, two new RFC 822 headers are created, in order to
carry this information to the RFC 822 recipient. These fields are
"X400-Originator:" and "X400-Recipients:".
The "X400-Originator:" field should be set to the same value as the
822-MTS originator. In addition, if MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName) contains MTS.ORName.directory-name then this Directory Name should be represented in an 822.comment. Recipient names, taken from each value of MTS.OtherMessageDeliveryFields.this-recipient-name and MTS.OtherMessageDeliveryFields.other-recipient-names should be made available to the RFC 822 user by use of the "X400-Recipients:" field. By taking the recipients at the MTS level, disclosure of recipients will be dealt with correctly. If any MTS.ORName.directory-name is present, it should be represented in an 822.comment. If MTS.OtherMessageDeliveryFields.orignally-intended-recipient-name is present, then it should be represented in an associated 822.comment, starting with the string "Originally Intended Recipient". In addition, the following per-recipient services from MTS.OtherMessageDeliveryFields.extensions should be represented in comments if they are used. None of these services can be provided on RFC 822 networks, and so in general these will be informative strings associated with other MTS recipients. In some cases, string values are defined. For the remainder, the string value may be chosen by the implementor. If the parameter has a default value, then no comment should be inserted. requested-delivery-method physical-forwarding-prohibited "(Physical Forwarding Prohibited)". physical-forwarding-address-request "(Physical Forwarding Address Requested)". physical-delivery-modes registered-mail-type recipient-number-for-advice physical-rendition-attributes physical-delivery-report-request "(Physical Delivery Report Requested)". proof-of-delivery-request "(Proof of Delivery Requested)".
4.6.2.3. Delivery Report Generation If MTA.PerRecipientMessageTransferFields.per-recipient-indicators requires a positive delivery notification, this should be generated by the gateway. Supplementary Information should be set to indicate that the report is gateway generated. 4.6.3. Message IDs (MTS) A mapping from 822.msg-id to MTS.MTSIdentifier is defined. The reverse mapping is not needed, as MTS.MTSIdentifier is always mapped onto new RFC 822 fields. The value of MTS.MTSIdentifier.local-part will facilitate correlation of gateway errors. To map from 822.msg-id, apply the standard mapping to 822.msg-id, in order to generate an MTS.ORAddress. The Country, ADMD, and PRMD components of this should be used to generate MTS.MTSIdentifier.global-domain-identifier. MTS.MTSIdentifier.local-identifier should be set to the 822.msg-id, including the braces "<" and ">". If this string is longer than MTS.ub-local-id-length (32), then it should be truncated to this length. The reverse mapping is not used in this specification. It would be applicable where MTS.MTSIdentifier.local-identifier is of syntax 822.msg-id, and it algorithmically identifies MTS.MTSIdentifier. 4.7. IPMS Mappings All RFC 822 addresses are assumed to use the 822.mailbox syntax. This should include all 822.comments associated with the lexical tokens of the 822.mailbox. In the IPMS O/R Names are encoded as MTS.ORName. This is used within the IPMS.ORDescriptor, IPMS.RecipientSpecifier, and IPMS.IPMIdentifier. An asymmetrical mapping is defined between these components. 4.7.1. RFC 822 -> X.400 To derive IPMS.ORDescriptor from an RFC 822 address. 1. Take the address, and extract an EBNF.822-address. This can be derived trivially from either the 822.addr-spec or 822.route-addr syntax. This is mapped to MTS.ORName as described above, and used as IMPS.ORDescriptor.formal-name. 2. A string should be built consisting of (if present):
- The 822.phrase component if the 822.address is an
822.phrase 822.route-addr construct.
- Any 822.comments, in order, retaining the parentheses.
This string should then be encoded into T.61 us a human
oriented mapping (as described in Chapter 3). If the string
is not null, it should be assigned to
IPMS.ORDescriptor.free-form-name.
3. IPMS.ORDescriptor.telephone-number should be omitted.
If IPMS.ORDescriptor is being used in IPMS.RecipientSpecifier,
IPMS.RecipientSpecifier.reply-request and
IPMS.RecipientSpecifier.notification-requests should be set to
default values (none and false).
If the 822.group construct is present, any included 822.mailbox
should be encoded as above to generate a separate IPMS.ORDescriptor.
The 822.group should be mapped to T.61, and a IPMS.ORDescriptor with
only an free-form-name component built from it.
4.7.2. X.400 -> RFC 822
Mapping from IPMS.ORDescriptor to RFC 822 address. In the basic
case, where IPMS.ORDescriptor.formal-name is present, proceed as
follows.
1. Encode IPMS.ORDescriptor.formal-name (MTS.ORName) as
EBNF.822-address.
2a. If IPMS.ORDescriptor.free-form-name is present, convert it
to ASCII (Chapter 3), and use this as the 822.phrase
component of 822.mailbox using the 822.phrase 822.route-addr
construct.
2b. If IPMS.ORDescriptor.free-form-name is absent. If
EBNF.822-address is parsed as 822.addr-spec use this as the
encoding of 822.mailbox. If EBNF.822-address is parsed as
822.route 822.addr-spec, then a 822.phrase taken from
822.local-part should be added.
3. If IPMS.ORDescriptor.telephone-number is present, this
should be placed in an 822.comment, with the string "Tel ".
The normal international form of number should be used. For
example:
(Tel +44-1-387-7050)
4. If IPMS.ORDescriptor.formal-name.directory-name is present,
then a text representation should be placed in a trailing
822.comment.
5. If IPMS.RecipientSpecifier.report-request has any non-
default values, then an 822.comment "(Receipt Notification
Requested)", and/or "(Non Receipt Notification Requested)",
and/or "(IPM Return Requested)" should be appended to the
address. The effort of correlating P1 and P2 information is
too great to justify the gateway sending Receipt
Notifications.
6. If IPMS.RecipientSpecifier.reply-request is True, an
822.comment "(Reply requested)" should be appended to the
address.
If IPMS.ORDescriptor.formal-name is absent, IPMS.ORDescriptor.free-
form-name should be converted to ASCII, and used as 822.phrase within
the RFC 822 822.group syntax. For example:
Free Form Name ":" ";"
Steps 3-6 should then be followed.
4.7.3. IP Message IDs
There is a need to map both ways between 822.msg-id and
IPMS.IPMIdentifier. This allows for X.400 Receipt Notifications,
Replies, and Cross References to reference an RFC 822 Message ID,
which is preferable to a gateway generated ID. A reversible and
symmetrical mapping is defined. This allows for good things to
happen when messages pass multiple times across the X.400/RFC 822
boundary.
An important issue with messages identifiers is mapping to the exact
form, as many systems use these ids as uninterpreted keys. The use
of table driven mappings is not always symmetrical, particularly in
the light of alternative domain names, and alternative management
domains. For this reason, a purely algorithmic mapping is used. A
mapping which is simpler than that for addresses can be used for two
reasons:
- There is no major requirement to make message IDs "natural"
- There is no issue about being able to reply to message IDs.
(For addresses, creating a return path which works is more
important than being symmetrical).
The mapping works by defining a way in which message IDs generated on one side of the gateway can be represented on the other side in a systematic manner. The mapping is defined so that the possibility of clashes is is low enough to be treated as impossible. 4.7.3.1. 822.msg-id represented in X.400 IPMS.IPMIdentifier.user is omitted. The IPMS.IPMIdentifier.user- relative-identifier is set to a printable string encoding of the 822.msg-id with the angle braces ("<" and ">") removed. 4.7.3.2. IPMS.IPMIdentifier represented in RFC 822 The 822.domain of 822.msg-id is set to the value "MHS". The 822.local-part of 822.msg-id is built as: [ printablestring ] "*" [ std-or-address ] with EBNF.printablestring being the IPMS.IPMIdentifier.user- relative-identifier, and std-or-address being an encoding of the IPMS.IPMIdentifier.user. If necessary, the 822.quoted-string encoding is used. For example: <"147*/S=Dietrich/O=Siemens/ADMD=DBP/C=DE/"@MHS> 4.7.3.3. 822.msg-id -> IPMS.IPMIdentifier If the 822.local-part can be parsed as: [ printablestring ] "*" [ std-or-address ] and the 822.domain is "MHS", then this ID was X.400 generated. If EBNF.printablestring is present, the value is assigned to IPMS.IPMIdentifier.user-relative-identifier. If EBNF.std-or-address is present, the O/R Address components derived from it are used to set IPMS.IPMIdentifier.user. Otherwise, this is an RFC 822 generated ID. In this case, set IPMS.IPMIdentifier.user-relative-identifier to a printable string encoding of the 822.msg-id without the angle braces. 4.7.3.4. IPMS.IPMIdentifier -> 822.msg-id If IPMS.IPMIdentifier.user is absent, and IPMS.IPMIdentifier.user- relative-identifier mapped to ASCII and angle braces added parses as 822.msg-id, then this is an RFC 822 generated ID. Otherwise, the ID is X.400 generated. Use the
IPMS.IPMIdentifier.user to generate an EBNF.std-or-address form
string. Build the 822.local-part of the 822.msg-id with the syntax:
[ printablestring ] "*" [ std-or-address ]
The printablestring is taken from IPMS.IPMIdentifier.user-relative-
identifier. Use 822.quoted-string if necessary. The 822.msg-id is
generated with this 822.local-part, and "MHS" as the 822.domain.
4.7.3.5. Phrase form
In "Reply-To:" and "References:", the encoding 822.phrase may be used
as an alternative to 822.msg-id. To map from 822.phrase to
IPMS.IPMIdentifier, assign IPMS.IPMIdentifier.user-relative-
identifier to the phrase. When mapping from IPMS.IPMIdentifier for
"Reply-To:" and "References:", if IPMS.IPMIdentifier.user is absent
and IPMS.IPMIdentifier.user-relative-identifier does not parse as
822.msg-id, generate an 822.phrase rather than adding the domain MHS.
4.7.3.6. RFC 987 backwards compatibility
The mapping proposed here is different to that used in RFC 987, as
the RFC 987 mapping lead to changed message IDs in many cases.
Fixing the problems is preferable to retaining backwards
compatibility. An implementation of this standard is encouraged to
recognise message IDs generated by RFC 987.
Chapter 5 -- Detailed Mappings
This chapter gives detailed mappings for the functions outlined in
Chapters 1 and 2. It makes extensive use of the notations and
mappings defined in Chapters 3 and 4.
5.1. RFC 822 -> X.400
5.1.1. Basic Approach
A single IP Message is generated. The RFC 822 headers are used to
generate the IPMS.Heading. The IP Message will have one IA5
IPMS.BodyPart containing the RFC 822 message body.
Some RFC 822 fields cannot be mapped onto a standard IPM Heading
field, and so an extended field is defined in Section 5.1.2. This is
then used for fields which cannot be mapped onto existing services.
The message is submitted to the MTS, and the services required can be
defined by specifying MTS.MessageSubmissionEnvelope. A few
parameters of the MTA Abstract service are also specified, which are
not in principle available to the MTS User. Use of these services allows RFC 822 MTA level parameters to be carried in the analogous X.400 service elements. The advantages of this mapping far outweigh the layering violation. 5.1.2. X.400 Extension Field An IPMS Extension is defined: rfc-822-field HEADING-EXTENSION VALUE RFC822Field ::= id-rfc-822-field RFC822Field ::= IA5String The Object Identifier id-rfc-822-field is defined in Appendix D. To encode any RFC 822 Header using this extension, the RFC822Field should be set to the 822.field omitting the trailing CRLF (e.g., "Fruit-Of-The-Day: Kiwi Fruit"). Structured fields should be unfolded. There should be no space before the ":". The reverse mapping builds the RFC 822 field in a straightforward manner. 5.1.3. Generating the IPM The IPM (IPMS Service Request) is generated according to the rules of this section. The IPMS.IPM.body usually consists of one IPMS.BodyPart of type IPMS.IA5TextbodyPart with IPMS.IA5TextBodyPart.parameters.repertoire set to the default (ia5) which contains the body of the RFC 822 message. The exception is where there is a "Comments:" field in the RFC 822 header. If no specific 1988 features are used, the IPM generated should be encoded as content type 2. Otherwise, it should be encoded as content type 22. The latter will always be the case if extension heading fields are generated. When generating the IPM, the issue of upper bounds must be considered. At the MTS and MTA level, this specification is strict about enforcing upper bounds. Three options are available at the IPM level. Use of any of these options conforms to this standard. 1. Ignore upper bounds, and generate messages in the natural manner. This assumes that if any truncation is done, it will happen at the recipient UA. This will maximise transfer of information, but may break some recipient UAs. 2. Reject any inbound message which would cause a message
violating constraints to be generated. This will be robust,
but may prevent useful communication.
3. Truncate fields to the upper bounds specified in X.400.
This will prevent problems with UAs which enforce upper
bounds, but will sometimes discard useful information.
These choices have different advantages and disadvantages, and the
choice will depend on the exact application of the gateway.
The rest of this section concerns IPMS.IPM.heading (IPMS.Heading).
The only mandatory component of IPMS.Heading is the
IPMS.Heading.this-IPM (IPMS.IPMIdentifier). A default should be
generated by the gateway. With the exception of "Received:", the
values of multiple fields should be merged (e.g., If there are two
"To:" fields, then the mailboxes of both should be used).
Information should be generated from the standard RFC 822 Headers as
follows:
Date:
Ignore (Handled at MTS level)
Received:
Ignore (Handled at MTA level)
Message-Id:
Mapped to IPMS.Heading.this-IPM. For these, and all other
fields containing 822.msg-id the mappings of Chapter 4 are
used for each 822.msg-id.
From:
If Sender: is present, this is mapped to
IPMS.Heading.authorizing-users. If not, it is mapped to
IPMS.Heading.originator. For this, and other components
containing addresses, the mappings of Chapter 4 are used
for each address.
Sender:
Mapped to IPMS.Heading.originator.
Reply-To:
Mapped to IPMS.Heading.reply-recipients.
To: Mapped to IPMS.Heading.primary-recipients
Cc: Mapped to IPMS.Heading.copy-recipients.
Bcc: Mapped to IPMS.Heading.blind-copy-recipients.
In-Reply-To:
If there is one value, it is mapped to
IPMS.Heading.replied-to-IPM, using the 822.phrase or
822.msg-id mapping as appropriate. If there are several
values, they are mapped to IPMS.Heading.related-IPMs, along
with any values from a "References:" field.
References:
Mapped to IPMS.Heading.related-IPMs.
Keywords:
Mapped onto a heading extension.
Subject:
Mapped to IPMS.Heading.subject. The field-body uses the
human oriented mapping referenced in Chapter 3 from ASCII to
T.61.
Comments:
Generate an IPMS.BodyPart of type IPMS.IA5TextbodyPart with
IPMS.IA5TextBodyPart.parameters.repertoire set to the
default (ia5), containing the value of the fields, preceded
by the string "Comments: ". This body part should precede
the other one.
Encrypted:
Mapped onto a heading extension.
Resent-*
Mapped onto a heading extension.
Note that it would be possible to use a ForwardedIPMessage
for these fields, but the semantics are (arguably) slightly
different, and it is probably not worth the effort.
Other Fields
In particular X-* fields, and "illegal" fields in common
usage (e.g., "Fruit-of-the-day:") are mapped onto a heading
extension, unless covered by another section or appendix of
this specification. The same treatment should be applied to
RFC 822 fields where the content of the field does not
conform to RFC 822 (e.g., a Date: field with unparsable
syntax).
5.1.4. Mappings to the MTS Abstract Service
The MTS.MessageSubmissionEnvelope comprises
MTS.PerMessageSubmissionFields, and
MTS.PerRecipientMessageSubmissionFields. The mandatory parameters
should be defaulted as follows.
MTS.PerMessageSubmissionFields.originator-name
This is always generated from 822-MTS, as defined in
Chapter 4.
MTS.PerMessageSubmissionFields.content-type
Set to the value implied by the encoding of the IPM (2 or
22).
MTS.PerRecipientMessageSubmissionFields.recipient-name
These will always be supplied from 822-MTS, as defined in
Chapter 4.
Optional components should be left out, and default components
defaulted, with two exceptions. For
MTS.PerMessageSubmissionFields.per-message-indicators, the following
settings should be made:
- Alternate recipient should be allowed, as it seems desirable
to maximise the opportunity for (reliable) delivery.
- Content return request should be set according to the issues
discussed in Section 5.2.
MTS.PerMessageSubmissionFields.original-encoded-information-types
should be made a set of one element
BuiltInEncodedInformationTypes.ia5-text.
The MTS.PerMessageSubmissionFields.content-correlator should be
encoded as IA5String, and contain the Subject:, Message-ID:, Date:,
and To: fields (if present). This should include the strings
"Subject:", "Date:", "To:", "Message-ID:", and appropriate folding.
This should be truncated to MTS.ub-content-correlator-length (512)
characters. In addition, if there is a "Subject:" field, the
MTS.PerMessageSubmissionFields.content-identifier, should be set to a
printable string representation of the contents of it, truncated to
MTS.ub-content-id-length (16). Both are used, due to the much larger
upper bound of the content correlator, and that the content id is
available in X.400(1984).
5.1.5. Mappings to the MTA Abstract Service
There is a need to map directly onto some aspects of the MTA Abstract
service, for the following reasons:
- So the the MTS Message Identifier can be generated from the
RFC 822 Message-ID:.
- So that the submission date can be generated from the
822.Date.
- To prevent loss of trace information.
- To prevent RFC 822/X.400 looping caused by distribution
lists or redirects.
The following mappings are defined.
Message-Id:
If this is present, the
MTA.PerMessageTransferFields.message-identifier should be
generated from it, using the mappings described in
Chapter 4.
Date:
This is used to set the first component of
MTA.PerMessageTransferFields.trace-information
(MTA.TraceInformationElement). The 822-MTS originator
should be mapped into an MTS.ORAddress, and used to derive
MTA.TraceInformationElement.global-domain-identifier. The
optional components of
MTA.TraceInformationElement.domain-supplied-information are
omitted, and the mandatory components are set as follows:
MTA.DomainSuppliedInformation.arrival-time
This is set to the date derived from Date:
MTA.DomainSuppliedInformation.routing-action
Set to relayed.
The first element of
MTA.PerMessageTransferFields.internal-trace-information
should be generated in an analogous manner, although this
may later be dropped (see the procedures for "Received:").
Received:
All RFC 822 trace is used to derive
MTA.PerMessageTransferFields.trace-information and
MTA.PerMessageTransferFields.internal-trace-information.
Processing of Received: lines should follow processing of
Date:, and should be done from the the bottom to the top of
the RFC 822 header (i.e., in chronological order). If other
trace elements are processed (Via:, X400-Received:), care
should be taken to keep the relative ordering correct. The
initial element of
MTA.PerMessageTransferFields.trace-information will be
generated already (from Date:).
Consider the Received: field in question. If the "by" part
of the received is present, use it to derive an
MTS.GlobalDomainIdentifier. If this is different from the
one in the last element of
MTA.PerMessageTransferFields.trace-information
(MTA.TraceInformationElement.global-domain-identifier)
create a new MTA.TraceInformationElement, and optionally
remove
MTA.PerMessageTransferFields.internal-trace-information.
This removal should be done in cases where the message is
being transferred to another MD where there is no bilateral
agreement to preserve internal trace beyond the local MD.
The trace creation is as for internal trace described below,
except that no MTA field is needed.
Then add a new element (MTA.InternalTraceInformationElement)
to MTA.PerMessageTransferFields.internal-trace-information,
creating this if needed. This shall be done, even if
inter-MD trace is created. The
MTA.InternalTraceInformationElement.global-domain-identifier
should be set to the value derived. The
MTA.InternalTraceInformationElement.mta-supplied-information
(MTA.MTASuppliedInformation) should be set as follows:
MTA.MTASuppliedInformation.arrival-time
Derived from the date of the Received: line
MTA.MTASuppliedInformation.routing-action
Set to relayed
The MTA.InternalTraceInformationElement.mta-name should be
taken from the "by" component of the "Received:" field,
truncated to MTS.ub-mta-name-length (32). For example:
Received: from computer-science.nottingham.ac.uk by
vs6.Cs.Ucl.AC.UK via Janet with NIFTP id aa03794;
28 Mar 89 16:38 GMT
Generates the string:
vs6.Cs.Ucl.AC.UK
Note that before transferring the message to some ADMDs, additional
trace stripping may be required, as the implied path through multiple
MDs would violate ADMD policy. Two extended fields must be mapped, in order to prevent looping. "DL-Expansion-History:" is mapped to MTA.PerMessageTransferFields.extensions.dl-expansion-history. "Redirection-History:" is mapped to MTA.PerRecipientMessageTransferFields.extensions.redirection-history. 5.1.6. Mapping New Fields This specification defines a number of new fields for Reports, Notifications and IP Messages in Section 5.3. As this specification only aims to preserve existing services, a gateway conforming to this specification does not need to map these fields to X.400, with the exception of "DL-Expansion-History" and "Redirection-History" described in the previous section. However, it is usually desirable and beneficial to do so, particularly to facilitate support of a message traversing multiple gateways. These mappings may be onto MTA, MTS, or IPMS services. 5.2. Return of Contents It is not clear how widely supported the X.400 return of contents service will be. Experience with X.400(1984) suggests that support of this service may not be universal. As this service is expected in the RFC 822 world, two approaches are specified. The choice will depend on the use of X.400 return of contents withing the X.400 community being serviced by the gateway. In environments where return of contents is widely supported, content return can be requested as a service. The content return service can then be passed back to the end (RFC 822) user in a straightforward manner. In environments where return of contents is not widely supported, a gateway must make special provision to handle return of contents. For every message passing from RFC 822 -> X.400, content return request will not be requested, and report request always will be. When the delivery report comes back, the gateway can note that the message has been delivered to the recipient(s) in question. If a non-delivery report is received, a meaningful report (containing some or all of the original message) can be sent to the 822-MTS originator. If no report is received for a recipient, a (timeout) failure notice should be sent to the 822-MTS originator. The gateway may retransmit the X.400 message if it wishes. When this approach is taken, routing must be set up so that error reports are returned through the same MTA. This approach may be difficult to use in conjunction with some routing strategies.
5.3. X.400 -> RFC 822 5.3.1. Basic Approach A single RFC 822 message is generated from the incoming IP Message, Report, or IP Notification. All IPMS.BodyParts are mapped onto a single RFC 822 body. Other services are mapped onto RFC 822 header fields. Where there is no appropriate existing field, new fields are defined for IPMS, MTS and MTA services. The gateway mechanisms will correspond to MTS Delivery. As with submission, there are aspects where the MTA (transfer) services are also used. In particular, there is an optimisation to allow for multiple 822-MTS recipients. 5.3.2. RFC 822 Settings An RFC 822 Service requires to have a number of mandatory fields in the RFC 822 Header. Some 822-MTS services mandate specification of an 822-MTS Originator. Even in cases where this is optional, it is usually desirable to specify a value. The following defaults are defined, which should be used if the mappings specified do not derive a value: 822-MTS Originator If this is not generated by the mapping (e.g., for a Delivery Report), a value pointing at a gateway administrator should be assigned. Date: A value will always be generated From:If this is not generated by the mapping, it should be assigned equal to the 822-MTS Originator. If this is gateway generated, an appropriate 822.phrase should be added. At least one recipient field If no recipient fields are generated, a field "To: list:;", should be added. This will ensure minimal RFC 822 compliance. When generating RFC 822 headers, folding should be used in an appropriate manner.
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