intro.me

<B>Digital的Unix操作系统VAX 4.2源码</B>

.pp
It is somewhat harder to integrate a new channel\**
.(f
\**The MMDF equivalent of a
.i sendmail
.q mailer.
.)f
into MMDF.
In particular,
MMDF must know the location and format
of host tables for all channels,
and the channel must speak a special protocol.
This allows MMDF to do additional verification
(such as verifying host names)
at submission time.
.pp
MMDF strictly separates the submission and delivery phases.
Although
.i sendmail
has the concept of each of these stages,
they are integrated into one program,
whereas in MMDF they are split into two programs.
.sh 2 "Message Processing Module"
.pp
The Message Processing Module (MPM)
discussed by Postel [Postel79b]
matches
.i sendmail
closely in terms of its basic architecture.
However,
like MMDF,
the MPM includes the network interface software
as part of its domain.
.pp
MPM also postulates a duplex channel to the receiver,
as does MMDF,
thus allowing simpler handling of errors
by the mailer
than is possible in
.i sendmail .
When a message queued by
.i sendmail
is sent,
any errors must be returned to the sender
by the mailer itself.
Both MPM and MMDF mailers
can return an immediate error response,
and a single error processor can create an appropriate response.
.pp
MPM prefers passing the message as a structured object,
with type-length-value tuples\**.
.(f
\**This is similar to the NBS standard.
.)f
Such a convention requires a much higher degree of cooperation
between mailers than is required by
.i sendmail .
MPM also assumes a universally agreed upon internet name space
(with each address in the form of a net-host-user tuple),
which
.i sendmail
does not.
.sh 1 "EVALUATIONS AND FUTURE PLANS"
.pp
.i Sendmail
is designed to work in a nonhomogeneous environment.
Every attempt is made to avoid imposing unnecessary constraints
on the underlying mailers.
This goal has driven much of the design.
One of the major problems
has been the lack of a uniform address space,
as postulated in [Postel79a]
and [Postel79b].
.pp
A nonuniform address space implies that a path will be specified
in all addresses,
either explicitly (as part of the address)
or implicitly
(as with implied forwarding to gateways).
This restriction has the unpleasant effect of making replying to messages
exceedingly difficult,
since there is no one
.q address
for any person,
but only a way to get there from wherever you are.
.pp
Interfacing to mail programs
that were not initially intended to be applied
in an internet environment
has been amazingly successful,
and has reduced the job to a manageable task.
.pp
.i Sendmail
has knowledge of a few difficult environments
built in.
It generates ARPANET FTP/SMTP compatible error messages
(prepended with three-digit numbers
[Neigus73, Postel74, Postel82])
as necessary,
optionally generates UNIX-style
.q From
lines on the front of messages for some mailers,
and knows how to parse the same lines on input.
Also,
error handling has an option customized for BerkNet.
.pp
The decision to avoid doing any type of delivery where possible
(even, or perhaps especially, local delivery)
has turned out to be a good idea.
Even with local delivery,
there are issues of the location of the mailbox,
the format of the mailbox,
the locking protocol used,
etc.,
that are best decided by other programs.
One surprisingly major annoyance in many internet mailers
is that the location and format of local mail is built in.
The feeling seems to be that local mail is so common
that it should be efficient.
This feeling is not born out by
our experience;
on the contrary,
the location and format of mailboxes seems to vary widely
from system to system.
.pp
The ability to automatically generate a response to incoming mail
(by forwarding mail to a program)
seems useful
(\c
.q "I am on vacation until late August...." )
but can create problems
such as forwarding loops
(two people on vacation whose programs send notes back and forth,
for instance)
if these programs are not well written.
A program could be written to do standard tasks correctly,
but this would solve the general case.
.pp
It might be desirable to implement some form of load limiting.
I am unaware of any mail system that addresses this problem,
nor am I aware of any reasonable solution at this time.
.pp
The configuration file is currently practically inscrutable;
considerable convenience could be realized
with a higher-level format.
.pp
It seems clear that common protocols will be changing soon
to accommodate changing requirements and environments.
These changes will include modifications to the message header
(e.g., [NBS80])
or to the body of the message itself
(such as for multimedia messages
[Postel80]).
Experience indicates that
these changes should be relatively trivial to integrate
into the existing system.
.pp
In tightly coupled environments,
it would be nice to have a name server
such as Grapvine
[Birrell82]
integrated into the mail system.
This would allow a site such as
.q Berkeley
to appear as a single host,
rather than as a collection of hosts,
and would allow people to move transparently among machines
without having to change their addresses.
Such a facility
would require an automatically updated database
and some method of resolving conflicts.
Ideally this would be effective even without
all hosts being under
a single management.
However,
it is not clear whether this feature
should be integrated into the
aliasing facility
or should be considered a
.q "value added"
feature outside
.i sendmail
itself.
.pp
As a more interesting case,
the CSNET name server
[Solomon81]
provides an facility that goes beyond a single
tightly-coupled environment.
Such a facility would normally exist outside of
.i sendmail
however.
.sh 0 "ACKNOWLEDGEMENTS"
.pp
Thanks are due to Kurt Shoens for his continual cheerful
assistance and good advice,
Bill Joy for pointing me in the correct direction
(over and over),
and Mark Horton for more advice,
prodding,
and many of the good ideas.
Kurt and Eric Schmidt are to be credited
for using
.i delivermail
as a server for their programs
(\c
.i Mail
and BerkNet respectively)
before any sane person should have,
and making the necessary modifications
promptly and happily.
Eric gave me considerable advice about the perils
of network software which saved me an unknown
amount of work and grief.
Mark did the original implementation of the DBM version
of aliasing, installed the VFORK code,
wrote the current version of
.i rmail ,
and was the person who really convinced me
to put the work into
.i delivermail
to turn it into
.i sendmail .
Kurt deserves accolades for using
.i sendmail
when I was myself afraid to take the risk;
how a person can continue to be so enthusiastic
in the face of so much bitter reality is beyond me.
.pp
Kurt,
Mark,
Kirk McKusick,
Marvin Solomon,
and many others have reviewed this paper,
giving considerable useful advice.
.pp
Special thanks are reserved for Mike Stonebraker at Berkeley
and Bob Epstein at Britton-Lee,
who both knowingly allowed me to put so much work into this
project
when there were so many other things I really should
have been working on.
.+c
.ce
REFERENCES
.nr ii 1.5i
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.ul
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RFC 733,
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.ul
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.ul
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.ul
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.ul
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.ul
A Structured Format for Transmission of Multi-Media Documents.
RFC 767.
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