RFC976 - UUCP mail interchange format standard

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　　Network Working Group Mark. R. Horton
Request for Comments: 976 Bell Laboratories
February 1986

UUCP Mail Interchange Format Standard

Status of This Memo

In response to the need for maintenance of current information about
the status and progress of various projects in the ARPA-Internet
community, this RFCis issued for the benefit of community members.
The information contained in this document is accurate as of the date
of publication, but is subject to change. Subsequent RFCs will
reflect such changes.

This document defines the standard format for the transmission of
mail messages between machines in the UUCP Project. It does not
address the format for storage of messages on one machine, nor the
lower level transport mechanisms used to get the data from one
machine to the next. It represents a standard for conformance by
hosts in the UUCP zone. Distribution of this memo is unlimited.

1. Introduction

This document is intended to define the standard format for the
transmission of mail messages between machines in the UUCP Project.
It does not address the format for storage of messages on one
machine, nor the lower level transport mechanisms used to get the
data from one machine to the next. We assume remote execution of the
rmail command (or equivalent) as the UUCP network primitive
operation.

The general philosophy is that, if we were to invent a new standard,
we would make ourselves incompatible with existing systems. There
are already too many (incompatible) standards in the world, resulting
in ambiguities such as a!b@c.d which is parsed a!(b@c.d) in the old
UUCP world, and (a!b)@c.d in the Internet world. (Neither standard
allows parentheses, and in adding them we would be compatible with
neither. There would also be serious problems with the shell and
with the UUCP transport mechanism.)

Having an established, well documented, and extensible family of
standards already defined by the ARPA community, we choose to adopt
these standards for the UUCP zone as well. (The UUCP zone is that
subset of the community connected by UUCP which chooses to register
with the UUCP project. It represents an administrative entity.)
While the actual transport mechanism is up to the two hosts to
arrange, and might include UUCP, SMTP, MMDF, or some other facility,
we adopt RFC-920 (domains) and RFC-822 (mail format) as UUCP zone
standards. All mail transmitted between systems should conform to

RFC976 February 1986
UUCP Mail Interchange Format Standard

those two standards. In addition, should the ARPA community change
these standards at a later time, we intend to change our standards to
remain compatible with theirs, given a reasonable time to upgrade
software.

This document specifies an interpretation of RFC-822 and RFC-920 in
the UUCP world. It shows how the envelope should be encoded, and how
UUCP routing is accomplished in an environment of mixed
implementations.

2. Basics

Messages can be divided into two parts: the envelope and the message.
The envelope contains information needed by the mail transport
services, and the message contains information useful to the sender
and receiver. The message is divided into the header and the body.
Sometimes an intermediate host will add to the message (e.g. a
Received line) but, except in the case of a gateway which must
translate formats, it is not expected that intermediate hosts will
change the message itself. In the UUCP world, the envelope consists
of the "destination addresses" (normally represented as the argument
or arguments to the rmail command) and the "source path" (normally
represented in one or more lines at the beginning of the message
beginning either "From " or ">From ", sometimes called "From_
lines".) The RFC-822 header lines (including "From:" and "To:") are
part of the message, as is the text of the message body itself.

UUCP uses short host names, such as "ucbvax", at and below the
transport layer. We refer to these names as "6 letter names",
because all implementations of UUCP consider at least the first 6
letters significant. (Some consider the first 7 or the first 14
significant, but we must use the lowest common denominator.) UUCP
names may be longer than 6 characters, but all such names much be
unique in their first 6 letters. RFC-920 domain names, such as
"ucbvax.Berkeley.EDU", are called "domain names." The two names are
different. Upper and lower case are usually considered different in
6 letter names, but are considered equivalent in domain names. Names
such as "ucbvax.UUCP", consisting of a 6 letter name followed by
".UUCP", previously were domain style references to a host with a
given 6 letter name. Such names are being phased out in favor of
organizational domain names such as "ucbvax.Berkeley.EDU"

RFC976 February 1986
UUCP Mail Interchange Format Standard

2.1 Hybrid Addresses

There are (among others) two major kinds of mailing address syntax
used in the UUCP world. The a!b!c!user ("bang paths") is used by
older UUCP software to explicitly route mail to the destination. The
user@domain ("domain") syntax is used in conformance to RFC-822.
Under most circumstances, it is possible to look at a given address
and determine which sort of address it is. However, a hybrid address
with a ! to the left of an @, such as a!b@c, is ambiguous: it could
be interpreted as (a!b)@c.d or a!(b@c.d). Both interpretations can
be useful. The first interpretation is required by RFC-822, the
second is a de-facto standard in the UUCP software.

Because of the confusion surrounding hybrid addresses, we recommend
that all transport layer software avoid the use of hybrid addresses
at all times. A pure bang syntax can be used to disambiguate, being
written c.d!a!b in the first case above, and a!c.d!b in the second.
We recommend that all implementations use this "bang domain" syntax
unless they are sure of what is running on the next machine.

In conformance with RFC-822 and the AT&T Message Transfer
Architecture, we recommand that any host that accepts hybrid
addresses apply the (a!b)@c.d interpretation.

2.2 Transport

Since SMTP is not available to much of the UUCP domain, we define the
method to be used for "remote execution" based transport mechanisms.
The command to be "remotely executed" should read

rmail user@domain ...

with the message on the standard input of the command. The
"user@domain" argument must conform to RFC-920 and RFC-822. More
than one address argument is allowed, in order to save transmission
costs for multiple recipients of the same message.

An alternative form that may be used is

rmail domain!user

where "domain" contains at least one period and no !'s. This is to
be interpreted exactly the same as user@domain, and can be used to
transport a message across old UUCP hosts without fear that they
might change the address. The "user" string can contain any
characters except "@". This character is forbidden because it is
unknown what an intermediate host might do to it. (It is also

RFC976 February 1986
UUCP Mail Interchange Format Standard

recommended that the "%" character be avoided, since some hosts treat
"%" as a synonym for "@".) However, to route across hosts that don't
understand domains, the following is possible

rmail a!b!c!domain!user

A "domain" can be distinguished from a 6 letter UUCP site name
because a domain will contain at least one period. (In the case of
single level domains with no periods, a period should be added to the
end, e.g. Mark.Horton@att becomes "att.!Mark.Horton". A translator
from ! to @ format should remove a trailing dot at the end of the
domain, if one is present.) We don't expect this to happen, except
for local networks using addresses like "user@host".

A simple implementation can always generate domain!user syntax
(rather than user@domain) since it is safe to assume that gateways
are class 3 (Classes are explained in section 3.5).

2.3 Batch SMTP

Standard conforming implementations may optionally support a protocol
called "Batch SMTP". SMTP (Simple Mail Transfer Protocol) is the
ARPA community standard mail transfer protocol (RFC-821). It is also
used on BITNET and Mailnet. While SMTP was designed to be
interactive, it is possible to batch up a series of commands and send
them off to a remote machine for batch execution. This is used on
BITNET, and is appropriate for UUCP. One advantage to BSMTP is that
the UNIX shell does not get involved in the interpretation of
messages, so it becomes possible to include special characters such
as space and parentheses in electronic messages. (Such characters
are expected to be popular in X.400 addresses.)

To support BSMTP on UNIX, a conforming host should arrange that mail
to the user "b-smtp" is interpreted as Batch SMTP commands. (We use
b-smtp instead of bsmtp because bsmtp might conflict with a login
name.) Since many mail systems treat lines consisting of a single
period as an "end of file" flag, and since SMTP uses the period as a
required end of file flag, and to strip off headers, we put an extra
"#" at the beginning of each BSMTP line. On a sendmail system, an
easy way to implement this is to include the alias

b-smtp: "|egrep '^#' | sed 's/^#//' | /usr/lib/sendmail -bs"

which will feed the commands to an SMTP interpreter. A better
solution would appropriately check for errors and send back an error
message to the sender.

RFC976 February 1986
UUCP Mail Interchange Format Standard

An example BSMTP message from seismo.CSS.GOV to cbosgd.ATT.COM is
shown here. This sample is the file shipped over the UUCP link for
in put to the command "rmail b-smtp". Note that the RFC- 822 message
is between the DATA line and the period line. The envelope
information is passed in the MAIL FROM and RCPT TO lines. The name
of the sending system is in the HELO line. The actual envelope
information (above the # lines) is ignored and need not be present.

From foo!bar Sun Jan 12 23:59:00 1986 remote from seismo Date:
Tue, 18 Feb 86 13:07:36 EST
From: mark@ucbvax.Berkeley.EDU
Message-Id: <8602181807.AA10228@mark@ucbvax.Berkeley.EDU> To:
b-smtp@cbosgd.ATT.COM

#HELO seismo.CSS.GOV
#MAIL FROM:<mark@ucbvax.Berkeley.EDU>
#RCPT TO:<mark@cbosgd.ATT.COM>
#DATA
#Date: Tue, 18 Feb 86 13:07:36 EST
#From: mark@ucbvax.Berkeley.EDU
#Message-Id: <8602181807.AA10228@mark@ucbvax.Berkeley.EDU> #To:
mark@cbosgd.ATT.COM
#
#This is a sample message.
#.
#QUIT

2.4 Envelope

The standard input of the command should begin with a single line

From domain!user date remote from system

followed immediately by the RFC-822 format headers and body of the
message. It is possible that there will be additional From_ lines
preceding this line - these lines may be added, one line for each
system the message passes through. It is also possible that the
"system" fields will be stacked into a single line, with many !'s in
the "user" string. The ">" character may precede the "From". In
general, this is the "envelope" information, and should follow the
same conventions that previous UUCP mail has followed. The primary
difference is that, when the system names are stacked up, if
previously the result would have been a!b!c!mysys!me, the new result
will be a!b!c!mysys!domain!me, where domain will contain at least one
period, and "mysys" is often the 6 letter UUCP name for the same

RFC976 February 1986
UUCP Mail Interchange Format Standard

system named by "domain". If the "domain!" is redundant, it may be
omitted from the envelope, either in the source path or in the
destination address.

The receiving system may discard extra "From_" lines if it folds the
information into a a single From_ line. It passes the
path!domain!user along as the "envelope" information containing the
address of the sender of the message, and possibly preserves the
forwarding date and system in a newly generated header line, such as
Received or Sent-By. (Adding Received using this information is
discouraged, since the line appears to have been added on a different
system than the one actually adding it. That other system may have
actually included a Received line too! The Sent-By line is similar to
Received, but the date need not be converted into RFC-822 format, and
the line is not claimed to have been added by the system whose name
is mentioned.)

If the receiving system passes the message along to another system,
it will add a "From_" line to the front, giving the same user@domain
address for the sender, and its own name for the system. If the
receiving system stores the message in a local mailbox, it is
recommended that a single "From_" line be generated at the front of
the message, keeping the date (in the same format, since certain mail
reading programs are sensitive to this format), and not using the
"remote from system" syntax.

Note - if an intermediate system adds text such as "system!" to the
front of a "user@domain" syntax address, either in the envelope or
the body, this is a violation of this standard and of RFC-822.

2.5 Routing

In order to properly route mail, it is sometimes necessary to know
what software a destination or intermediate machine is running, or
what conventions it follows. We have tried to minimize the amount of
this information that is necessary, but the support of subdomains may
require that different methods are used in different situations. For
purposes of predicting the behavior of other hosts, we divide hosts
into three classes. These classes are:

Class 1 old-style UUCP ! routing only. We assume that the host
understands local user names:

rmail user

RFC976 February 1986
UUCP Mail Interchange Format Standard

and bang paths

rmail host1!host2!user

but we assume nothing more about the host. If we have
no information about a host, we can treat it as class 1
with no problems, since we make no assumptions about
how it will handle hybrid addresses.

Class 2 Old style UUCP ! routing, and 4.2BSD style domain
parsing. We assume the capabilities of class 1, plus
the ability to understand

rmail user@domain

if the "domain" is one outside the UUCP zone which
the host knows about. Class 2 hosts do not necessarily
understand domain!user or have routers. Hosts in non-

UUCP RFC-920 domains are considered class 2, even though
they may not understand host!user.

Class 3 All class 1 and 2 features are present. In addition,
class 3 hosts must be able to route UUCP mail for hosts
that are not immediately adjacent and also understand
the syntax

rmail domain!user

as described above. All gateways into UUCP must be
class 3.

This document describes what class 3 hosts must be able to process.
Classes 1 and 2 already exist, and will continue to exist for a long
time, but are viewed as "older systems" that may eventually be
upgraded to class 3 status.

3. Algorithm

The algorithm for delivering a message to an address "user@domain"
over UUCP links can be summarized as follows:

a. If the address is actually of the form @domain1:user@domain2,
the "domain" used for the remainder should be "domain1"
instead of "domain2", and the bang form reads
domain1!domain2!user.

RFC976 February 1986
UUCP Mail Interchange Format Standard

b. Determine d: the most specific part of "domain" that is
recognized locally. This part will be a suffix of "domain".
This can be done by scanning through a table with entries that
go from specific to general, comparing entries with "domain"
to see if the entries are at the tail of "domain". For
example, with the address "mark@osgd.cb.att.com", if the local
host recognizes "uucp" and "att.com", d would be "att.com".
The final entry in the table will be the null string, matching
any completely unrecognized domain.

c. Look in the found table entry for g: the name of the
"gateway", and for r: a UUCP !-style route to reach g. G is
not necessarily directly connected to the local host, but
should be viewed as a gateway into the d domain. (The values
of g and r for a given d may be different on different hosts,
although g will often be the same.)

d. Look at the beginning of r to find the "next hop" host n. N
will always be directly connected to the local host.

e. Determine, if possible, the class of g and n.

f. Create an appropriate destination string s to be interpreted
by n. (See below.)

g. Pass the message off to n with destination information s.

In an environment with other types of networks that do not use
UUCP ! parsing, the table will probably contain additional
information, such as which type of link to use. The path
information may be replaced in other environments by information
specific to the network.

The first entries in the table mentioned in part (b) are normally
very specific, and allow well known routes to be constructed
directly instead of routing through the domain tree. The domain
tree should be reserved for cases where no better information is
available, or where traffic is very light, or where the default
route is the best available. If a better route is available, that
information can be put in the table. If a host has any
significant amount of traffic sent to a second host, it is
normally expected that the two hosts will set up a direct UUCP
link and make an entry in their tables to send mail directly, even
if they are in separate domains. Routing tables should be
constructed to try to keep paths short and inexpensive for as much
traffic as possible.

RFC976 February 1986
UUCP Mail Interchange Format Standard

Here are some hints for the construction of the destination string
n (step f above.) The "envelope recipient" information (the
argument(s) to rmail) may be in either domain ! form
(host.com!user) or domain @ form (user@host.com) as long as the
sending site is sure the next hop is class 3. If the next hop is
not class 3, or the sending site is not sure, the ! form should be
used, if possible, since it is hard to predict what the next hop
would do with a hybrid address.

If the gateway is known to be class 3, domain ! form may be used,
but if the sending site is not sure, and the entire destination
string was matched in the lookup (rather than some parent domain),
the 6 letter ! form should be used: r!user, for example:
dumbhost!host!user. If the gateway appears to actually be a
gateway for a subdomain, e.g. because a parent domain was matched,
(such as the address user@host.gateway.com, where host.gateway.com
was not found but gateway.com was) it can be assumed to be at
class 3. This allows routes such as
dumbhost!domain!host.domain.com!user to be used with a reasonable
degree of safety. If a direct link exists to the destination
host, the user@domain syntax or the domain!user syntax may be
used.

All hosts conforming to this standard are class 3, and all
subdomain gateways must be class 3 hosts.

4. Example

Suppose host A.D.COM sends mail to host C.D.COM. Let's suppose that
the 6 letter names for these hosts are aname and dname, and that the
intermediate host to be routed through has name bname.

The user on A types

mail user@c.d.com

The user interface creates a file such as

Date: 9 Jan 1985 8:39 EST
From: myname@A.D.COM (My Name)
Subject: sample message
To: user@c.d.com

This is a sample message

and passes it to the transport mechanism with a command such as

RFC976 February 1986
UUCP Mail Interchange Format Standard

sendmail user@c.d.com < file

The transport mechanism looks up a route to c.d.com. It does not
find c.d.com in its database, so it looks up d.com, and finds that
the path is bname!dname!%s, and that c.d.com is a class 3 host.
Plugging in c.d.com!user, it gets the path bname!dname!c.d.com!user.
(If it had found c.d.com with path bname!cname!%s, it would have
omitted the domain from the resulting path: bname!cname!user, since
it is not sure whether the destination host is class 1, 2, or 3.)

It prepends a From_ line and passes it to uux:

uux - bname!rmail dname!c.d.com!user < file2

where file2 contains

From A.D.COM!user Wed Jan 9 12:43:35 1985 remote from aname Date:
9 Jan 1985 8:39 EST
From: myname@A.D.COM (My Name)
Subject: sample message
To: user@c.d.com

This is a sample message

(Note the blank line at the end of the message - at least one blank
line is required.) This results in the command

rmail dname!c.d.com!user

running on B. B prepends its own from line and passes the mail
along:

uux - dname!rmail c.d.com!user < file3

where file3 contains

From nuucp Wed Jan 9 12:43:35 1985 remote from bname >From
A.D.COM!user Wed Jan 9 11:21:48 1985 remote from aname Date: 9
Jan 1985 8:39 EST
From: myname@A.D.COM (My Name)
Subject: sample message
To: user@c.d.com

This is a sample message

RFC976 February 1986
UUCP Mail Interchange Format Standard

The command

rmail c.d.com!user

is run on C, which stacks the From_ lines

From bname!aname!A.D.COM!user Wed Jan 9 12:43:35 1985 Date: 9
Jan 1985 8:39 EST
From: myname@A.D.COM (My Name)
Subject: sample message
To: user@c.d.com

This is a sample message

and stores the message locally, probably in this same format.

5. Summary

Hosts conforming to this standard should accept all of the following
forms:

rmail localuser (no !%@ in user)
rmail hosta!hostb!user (no !%@ in user)
rmail user@domain (only . in domain)
rmail domain!user (at least 1 . in domain)
rmail domain.!user (in case domain has no dots)

The "envelope" portion of the message ("From_" lines) should conform
to existing conventions, using ! routing. The "heading" portion of
the message (the Word: lines such as Date:, From:, To:, and Subject:)
must conform to RFC-822. All header addresses must be in the @ form.
The originating site should ensure that the addresses conform to
RFC-822, since no requirement is placed on forwarding sites or
gateways to transform addresses into legal RFC-822 format. (Such
forwarding sites and gateways should NOT, however, change a legal
RFC-822 address such as user@domain into an illegal RFC-822 address
such as gateway!user@domain, even if forwarding to a class 1 UUCP
host.)

6. References

[1] Postel, J., "Simple Mail Transfer Protocol", RFC-821,
USC/Information Sciences Institute, August, 1982.

[2] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC-822, Department of Electrical Engineering,
University of Delaware, August, 1982.

RFC976 February 1986
UUCP Mail Interchange Format Standard

[3] Postel, J., and J. K. Reynolds, "Domain Requirements", RFC-920,
USC/Information Sciences Institute, October, 1984.

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