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Request For Comments: 886
Proposed Standard for Message Header Munging
Thu Dec 15 03:37:52 1983
Marshall T. Rose
Department of Information and Computer Science
University of California, Irvine
Irvine, CA 92717
MRose.UCI@Rand-Relay
This memo proposes a standard for the ARPA Internet community. If
this proposal is adopted, hosts on the ARPA Internet that do message
translation would be expected to adopt and implement this standard.
Request For Comments: 886 M. Rose
Proposed Standard for Message Header Munging UCI
Introduction
This memo describes the rules that are to be used when mail is
transformed from one standard format to another. The scope of this
memo is limited to text messages (computer network mail, or
electronic mail) that traverse the ARPA Internet. This memo is not
presented as a replacement or amendment for the "Standard for the
Format of ARPA Internet Text Messages", RFC822. Rather, this memo
focuses on a particular aspect of mail, and provides a conceptual
and practical basis for implementors of transport agents and user
agents which support message munging.
Although this memo has been specifically prepared for use with the
822 standard, an understanding of the 822 standard is not required
to make use of this memo. The remainder of this section reminds
the reader of some key concepts presented in the 822 standard, and
how they relate to the perspective of this memo.
Messages are viewed as consisting of an envelope and contents. The
envelope is manipulated solely by transport agents, and contains
the information required by the transport agents to deliver the
message to its recipients. Although this memo does not address
itself directly to the envelope, we shall see that some of the
rules discussed later are applicable to the envelope.
The contents of the message consists of a rigorously structured
part, known as the headers, followed by a freely formated part,
called the body. The message body is completely uninteresting to
us. Our emphasis is strictly on the headers of the message. Each
header in the message consists of a field, its value, and a
terminating end-of-line sequence. The 822 standard discusses,
among other things, continuation lines, the syntax that is used to
distinguish between fields and values, and the syntax and semantics
of the values of various fields. For our part, we shall concern
ourselves only with the notion that the headers section consists of
one or more headers, which are divided into one or more field/value
pairs.
The term "message munging" refers to the actions taken by a
transport or user agent to transform the contents of a message from
conformance with one standard format to another. The 822 standard
refers to this as "Network-Specific Transformation". Other phrases
might be "header munging" or "mail filtering". Regardless of the
term used, the key notion is that this action transforms a message
from its current format (the source message) to the structure
required by the target standard. A "munging agent", for the
purposes of this memo, is an entity which performs message munging.
A munging agent may be part of either a transport or user agent.
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Background
As more networks connect into the ARPA Internet community, their
users will exchange computer mail messages with other Internet
hosts. Although the 822 standard must be strictly adhered to for
mail that traverses the ARPA Internet, other networks might not
internally adopt this standard. It is nevertheless desirable to
permit mail to flow between hosts which internally conform to the
standard and those which do not. The 822 standard is very clear to
indicate that:
"This standard is NOT intended to dictate the internal formats
used by sites, the specific message system features that they
are expected to support, or any of the characteristics of user
interface programs that create or read messages."
This plainly states that even hosts within the ARPA Internet, may
opt to use a different standard than 822 for their internal use,
but they are expressly required to use the 822 standard when
transferring mail to other hosts in the ARPA Internet. As such, it
is not difficult to imagine message munging becoming a common
activity among transport and user agents.
There are other reasons why message munging may become a widespread
practice. An example from CSnet will serve here. The CSnet relays
provide authorized access for mail services to the ARPA Internet
for the CSnet phonenet sites. CSnet sites are not registered with
the NIC, and hence are often absent from the host tables of ARPA
Internet sites. As a result, addresses for mailboxes on CSnet
phonenet sites are unknown to ARPA Internet sites. From an ARPA
Internet site, it would be impossible to send messages to these
addresses, since the local transport agent has no handle on the
destination hosts of the phonenet mailboxes. Obviously, even
replying to a such a message is simply not possible. To solve this
problem, the transport agents on the CSnet relays perform message
munging on mail destined for the ARPA Internet. Phonenet addresses
of the form "mbox@host" are transformed to "mbox.host@relay", where
"relay" is the ARPA Internet host name of the relay performing the
transformation. Other addresses are left alone. Agents throughout
the ARPA Internet are now able to process these addresses, since
the host-part is a known ARPA Internet host.
The source-routing solution to this problem will hopefully be
replaced by domain handling when domains are implemented in the ARPA
Internet. When this is the case, phonenet addresses of the form
"mbox@host" will become "mbox@host.CSNET". Despite this change,
(which cannot help but be for the better, as the use of
source-routing leads to a plethora of problems), message munging
will still occur as it will most likely be necessary to add domain
names during message transmission (see section 6.2.2 of the 822
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standard).
For an alternate reason, consider that it is not unlikely for users
to wish to transform mail from their archives which conforms to an
older standard to the current standard. There could be many
reasons for this, although a common one would be that a user wishes
to re-introduce the message into the transport system. Although
the aged message was perfectly valid when it was composed (e.g., in
the days of the 733 standard), it might no longer conform to the
current standard (i.e., 822). In this case, a user agent would
have to perform message munging, in order to make the message
acceptable to the local transport agent.
To summarize, even under the most "homogeneous" of environments,
message munging will still be required on the part of transport and
user agents, under certain conditions.
Section 3.4.10 of the 822 standard briefly discusses the topic of
"Network-Specific Transformations". In short, the 822 standard
envisions a message traversing net-A to reach net-B as going
through three phases:
o Transformation
The message is made to conform to net-A's standards
o Transformation Reversal
Net-A's idiosyncrasies are removed and the message now
conforms to the 822 standard
o Transformation
The message is made to conform to net-B's standards
This memo concerns itself solely with this section of the 822
standard. The 822 standard presents end-of-line sequences as an
example of an area where transformation might occur. Although this
is a valid concern, our emphasis deals with constructs of higher
semantics: fields and structured field values.
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Scope
This memo does not specify the particular transformation rules that
should be used when munging a message from one standard to another.
Rather, this memo attempts to make clear the policies that are to
be followed when implementing any munging agent for the ARPA
Internet. The derivation of the formulas specific to message
munging between two given standards is left to the implementors of
such munging systems or to the writers of future RFCs. As such,
this memo can be considered to present the philosophy and
conceptual basis of message munging in the ARPA Internet.
NOTE: It is critical that this position be understood. The
actual policies used by domain-specific munging agents is
completely beyond the scope of this memo.
For ease of explanation, some of the examples in this memo use
message munging between the ARPA Internet and the USENET
distribution network as an example. This memo should NOT be
considered to specify how this particular munging activity should
take place. Instead, this context has been chosen for its
familiarity and simplicity.
Message Decomposition
A munging agent concerns itself with transforming a message in
conformance with a source standard to a message in conformance with a
target standard. This transformation occurs at various levels. Four
of these are presented here.
o Field Transformation
For two standards, some fields may convey identical semantics
but have different names. As standards progress, for
example, the names of fields may change, but the presence of
those fields and their contents continue to have the same
meaning. For example, prior to 822 standard, some mailers
considered the Remailed- prefix to have semantics equivalent
to the 822 standard's Resent- prefix. In this circumstance,
one aspect of message munging would be to simply substitute
the field names.
o Value Transformation
The value of certain fields may be viewed as containing
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structured components. The syntax and semantics of these
components may differ significantly between two formats. In
this circumstance, one aspect of message munging would be to
transform components from one representation to another.
o Field/Value Combination
The semantics of a given header in a particular standard may
not be directly expressed using a single header from another
standard. In this circumstance, one aspect of message munging
would be to map the field/value of a header in the source
message to any number of headers in the target message (or
vice-versa). As expected, further complication could result
by performing value transformation in addition to one-to-many
or many-to-one field transformation.
o Header Ordering
Some standards may require that fields appear in a particular
order in the headers part of the message. Others make no
requirements as to the order in which the fields appear. In
this circumstance, one aspect of message munging from the
latter to the former standard would be to capture the essential
information from the source message in order to construct the
first field of the target message. As expected, further
complication could result by requiring several field/values be
consulted in the source message before sufficient context is
present to construct the first field of the target message.
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Canonical Forms
Fundamental to the activity of transformation is the notion of a
canonical form. For a given message standard, each field and
structured field value may be thought of as an object with a
particular semantics that is representable by one or more strings.
That is, each of these strings has an identical semantics, as they
all refer to the same object. For example, in terms of the 822
standard, the two strings
The Protocol Police <NetSer@UCI>
NetSer@UCI
are semantically equivalent. For the purposes of this memo, a
fully-qualified canonical form of an object is thought of as the
simplest string that represents the full and complete meaning of an
object. The meaning of "simple" is, of course, open to
interpretation. In some cases, "simplest" may mean "shortest". In
other cases, a longer, but unabbreviated string may be "simpler"
than a shorter string. Regardless of this, a canonical form is a
representation of an object. This representation contains the
smallest amount of information required to fully describe the
meaning of the object.
It is not difficult to determine what a canonical form should
describe for different objects. In terms of the 822 standard, the
following should be considered as minimal definitions of canonical
forms:
object specifies contains
------ --------- --------
field field-name name
address mailbox local-part
domain-part
date date-time date-part
time-part
In terms of USENET, the following might be considered as minimal
definitions of canonical forms:
object specifies contains
------ --------- --------
field field-name name
address mailbox user
route
date date-time date-part
time-part
NOTE: This memo clearly has no authority to specify the
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minimal canonical forms for USENET. The above table is listed
solely for the benefit of the examples which follow.
Conceptually, transformation of fields and structured field values
occurs between canonical forms. That is, to transform an address,
one reduces the string representing the object to its canonical
form, to capture the essence of its meaning, and this form is then
transformed, somehow, to the equivalent canonical form for the
target standard. This target canonical form can later be output
using a string representation.
NOTE: This memo does not require that canonical forms be
represented or otherwise implemented as strings. Nor does
this standard require that strings be used during the
transformation process. Thinking of a canonical form as a
string is a convenient formalism only, not an implementational
requirement.
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Transformation Rules
All of the forms of message decomposition discussed above may now
be viewed as transformation between canonical forms. Hence, it now
becomes necessary to consider how canonical forms should be
manipulated during transformation. That is, what rules are to be
followed when constructing an equivalent canonical form? There are
several guidelines that must be followed, that we will characterize
in the following fashion:
o Preservation of information
All attempts must be made to preserve all information
contained in the original canonical form. This information
can be highly useful to the recipients of munged messages.
Obviously, for two widely-differing formats, this may not be
possible. For example, some standards may not have a group
addressing notation such as the one present in the 822
standard, e.g., the notation
List: Support@UCI, ZOTnet@UCI;
might not be permitted. If one were to consider membership in
a group as part of an address' canonical form, then this
portion of the canonical form could not be transformed to the
other standard.
The 822 standard supports a liberal commenting convention
which might prove quite useful in preserving information.
Implementors may wish to consider capturing the original
information in commentary text. For example, if the USENET
address
mark@cbosgd.UUCP (Mark Horton)
had the USENET canonical of
user: mark
route: ucbvax!ihnp4!cbosgd
and if the corresponding 822 canonical was
local-part: ucbvax!ihnp4!cbosgd!mark
domain-part: USENET.UCI
then it would not be unreasonable for an implementation to
output this canonical form as
"mark@cbosgd.UUCP" <ucbvax!ihnp4!cbosgd!mark@USENET.UCI>
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NOTE: Implementors should exercise extreme caution in
using a policy such as this. Information placed between
commentary delimiters must still conform to the target
standard at the syntactic level.
Note however that in the above example, the commentary
information "(Mark Horton)" was discarded. This practice is
strongly discouraged. Although the canonical form for an
object does not rely on commentary information as a necessary
part, implementors are encouraged to preserve this information
whenever possible.
Finally, preservation of information requires preservation of
case at all costs. Only under the most restrictive of
circumstances should an implementation change the case of the
strings output for a canonical form.
o Re-Formatting
Ideally, the target message should have the exact horizontal
and vertical padding as the source message. Because a string
representing the source canonical form of an object may not be
of the same length as the string representing the target
canonical form, the number of characters on each physical and
logical line in the headers may be different.
The 822 standard supports a header folding convention which
permits long field/value pairs to be represented on more than
one physical line. When a new canonical form is output to the
target message, it is possible that the resulting field/value
pair may be longer than the number of characters that
antiquated display devices can present on a single line. The
822 standard suggests 65 or 72 characters-per-line as a metric
for this limitation. Although not required, message munging
agents may re-fold headers if (and only if) this limitation is
exceeded. Note however that under no circumstances should a
header be re-folded if it was not munged. Refolding without
munging may occur on behalf of some transport or user agent,
but it may not occur on behalf of a munging agent. Put more
simply, this memo does not authorize or forbid such activity,
although it does discourage it.
o Error Recovery
The preceding discussion has made been under the assumption
that the objects composing the field/value pairs of the source
message have conformed to the source standard. It is an
unfortunate reality that this may not be the case. In fact,
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for those standards which are poorly specified (if at all),
determining that an object is improperly constructed might be
quite difficult. In addition, it is possible, though
hopefully extremely improbable that a target canonical form
does not exist for a particular source canonical form. In
these cases, munging agents must be able to recover.
At this point, we introduce two extension fields for the 822
standard. As such, these fields are hereby designated as
"reserved" and may not be used for other purposes. These
fields are:
Illegal-Field
Illegal-Object
The syntax of these fields is as follows:
munge-field =
"Illegal-Field" ":" *text
/ "Illegal-Object" ":" *text
munge-object =
<a "field-name", the exact field-names which are
valid will be presented later>
The semantics of these fields are as follows:
An Illegal-Field header should be introduced when a
header-line which does not conform to the source standard is
found in the source message. Illegal-Field should be used
only when a header-line is so poorly formed as to prevent
recognition of the field in the header-line. For example, if
the line lacks a colon, or has a poorly formed field-name,
then it should not be output to the target message and a new
header-line should be introduced in its place. This
header-line has Illegal-Field as its field and contains the
offending line as its value. Illegal-Field should not be used
if the field can be identified, but the value is poorly
formed.
An Illegal-Object header should be introduced when an object
in the source message can not be parsed into a canonical form,
or if the canonical form it represents has no corresponding
target canonical form. The offending object should not be
output to the target message in the header-line in which it
occurs. If the header-line now contains no objects, then the
header-line should not be output to the target message as
well. Then, an Illegal-Object field should be introduced into
the target message. The value of this Illegal-Object field
should at the very minimum contain the name of the field that
contained the object, the object in question, and an
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explanation as to why the object was illegal. Alternately,
the value of this Illegal-Object field should consist of the
entire header-line (field and value) that contained the object
in question along with an explanation as to why the object was
illegal.
NOTE: In the circumstance where multiple objects exist
in a single header-line in the source message, and one of
those objects is determined to be illegal, the actual
policy used in determining how much information can be
considered to be "uncorrupted" is left to the
implementors. Munging agents which use sophisticated
parsers may attempt to recover in mid-stream (so to
speak) and continue parsing objects on the header-line.
Other agents may wish to continue recover with the next
header-line in the source message. Regardless of the
policy used, the agent must present the contents of the
entire header-line in the associated Illegal-Object
header.
Implementations should not take extraordinary measures to
perform syntax/semantics checking of the source message --
only those fields which must be examined should be rigorously
checked. This memo strongly discourages any additional
examination. It is not the intention of this memo to suggest
that composing agents should produce messages which do not
conform to the source standard. A composing agent should not
expect a munging agent to enforce adherence to the source
standard.
o Introduction of Information
Munging agents are authorized to introduce a "Received" header
into the target message when a message is transformed.
NOTE: Adding a "Received" header is entirely optional.
This memo strongly recommends that this header be
introduced whenever some munging (translation of addresses
and/or dates) occurs.
NOTE: Although this memo does not specify the position
that the introduced header should have in relation to the
other fields in the target message, it is strongly
recommended that the introduced header be grouped with
the other "Received" headers, at the very beginning of
the message.
When introducing a "Received" field, three phrases, which are
normally optional in such a field, should be specified by the
munging agent. These are:
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"from" domain # the source domain
"by" domain # the target domain
"with" protocol # the munging agent's host
For example, suppose we have a munging agent on the UCI host,
and that this agent services a USENET/ARPA boundary. When the
UCI host gets a message from the USENET domain for the ARPA
domain, the following happens: First, the UCI mailsystem would
prepend the header:
Received: from nma by UCI with UUCP; 15 Dec 83 03:53:00 PST
Second, the munging agent, when transforming the message,
would prepend the header:
Received: from USENET by ARPA with UCI; 15 Dec 03:54:00 PST
Finally, the UCI mailsystem would then deliver the message to
the appropriate ARPA mailsystem, which in turn would prepend
the header:
Received: from UCI by ISIF with SMTP; 15 Dec 83 03:55:00 PST
This example might be a bit clearer if the domains were
qualified a bit more. The first three lines of the message
could look like this:
Received: from UCI.ARPA by ISIF.ARPA; 15 Dec 83 03:55:00 PST
Received: from USENET by ARPA with UCI; 15 Dec 03:54:00 PST
Received: from nma.USENET by UCI.USENET; 15 Dec 83 03:53:00 PST
The key point to notice is that the munging agent used the
"from" and "by" clauses to denote the domain boundary that was
crossed, and used the "with" clause to denote itself. Since
the agent is munging the message according to some set of
transformation rules, it is actually using a "mail protocol",
and as such is justified in identifying itself in the "with"
clause.
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Objects of Interest
At present, only three types of objects are of interest: fields,
addresses, and dates. In the context of the 822 standard,
header-lines containing the following fields are to be viewed as
appropriate for transformation:
Address Fields: From, Sender, Reply-To, To, cc, Bcc,
and any of these fields with the Resent- prefix
Date Fields: Date, Resent-Date
Hence the definition of munge-object, in 822 terms, is:
munge-object =
"From"
/ "Sender"
/ "Reply-To"
/ "To"
/ "cc"
/ "Bcc"
/ "Resent-From"
/ "Resent-Sender"
/ "Resent-Reply-To"
/ "Resent-To"
/ "Resent-cc"
/ "Resent-Bcc"
/ "Date"
/ "Resent-Date"
NOTE: The list of munge-objects is extensible. For the
purposes of this memo, the above fields are defined as the
MINIMUM list of munge-objects for the 822 standard.
Implementors are encouraged to introduce other fields to the
list of munge-objects as their munging agents require. These
additions should also be registered with the revisions of this
memo as they gain popularity.
For the purposes of the remainder of this memo, an address
header-line is defined as any header-line in the source message
whose field component is one of the fields listed above as an
address field. Further, a date header-line is defined as any
header-line in the source message whose field component is one of
the fields listed above as an date field.
If address munging is performed, then all addresses contained in
all address header-lines must be munged. It is expressly forbidden
to perform address munging on the source message and without
performing address munging on every address header-line. Further,
it is expressly forbidden to munge some, but not all, of the
addresses in any address header-line. All addresses in all of the
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message's address header-lines must be address munged. If address
munging is not performed, then these header-lines need not be
considered for munging.
A similar requirement is made for date munging. If date munging is
performed, then all instances of header-lines whose field is Date
or Resent-Date must be fully date munged.
NOTE: Certain fields are to be excluding from munging of any
sort, all munging agents must preserve their contents exactly.
At present, there is one such field: "Received". This contents
of this field should ALWAYS be preserved for trace and
debugging purposes.
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Bibliography
D.H. Crocker, "Standard for the Format of ARPA Internet Text
Messages", RFC 822, (August, 1982).
M.R. Horton, "Standard for the Interchange of USENET Messages", RFC
850, (June, 1983).
M.T. Rose, "Achieving Interoperability between two Domains --
Connecting the ZOTnet and UUCP Computer Mail Networks", Technical
Report Number 201, Department of Information and Computer Science,
University of California, Irvine, (January, 1983).
P.V. Mockapetris, "Domain Names -- Concepts and Facilities", RFC
882, (November, 1983).
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Appendices
Minimal Canonical Forms
This memo defines the minimal canonical forms for the 822 standard.
Implementations may wish to augment these forms with additional
information that may be present in the source message. An earlier
example suggested that group membership might be part of an
address' canonical form. Further, since the 822 standard permits
routes to be specified in addresses, e.g.,
Fred Rated <@ISI-Troll.ARPA,@UCI-750a.UCI: FRated@UCI-750b>
Perhaps they too should be considered part of the 822 address'
canonical form?
This memo makes no such requirement, if implementations wish to
make use of this additional information, then they are free to do
so. This practice is neither encouraged nor discouraged. In short
the spirit of this memo is to require those minimal components
required by the 822 standard, nothing more.
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