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INDRA
Working
Paper
INDRA Note 1025
IEN 169
23rd January 1981
A Simple NIFTP-Based Mail System
C. J. Bennett
ABSTRACT: This INDRA Note proposes a draft mail
protocol for use in a wide variety of network
situations. This protocol draws heavily on existing
facilities and could be brought up very quickly - in
months rather than years. It could thus be used as
an interim solution until international standards
emerge. An experiment in the ARPA Catenet is
proposed, and the use of the system in the UK is
discussed.
Department of Computer Science
University College London
1. Introduction
One of the major uses of computer networks is for electronic
mail services. Now that networking technology is proliferating
rapidly, it is highly desirable that such services should be
made available with the new networks. However, despite the
recent adoption of Teletex proposals by CCITT, it may be some
time before one can expect international standards to be
finalised and widely available. Hence there is a strong need
for a simple interim scheme, which could cover the basic needs
for mail transfer across multiple networks and through
intermediate mail relays in a flexible fashion. We at UCL are
particularly concerned with this, as we see the need for a
scheme which will operate both with the facilities available
in the United States, and those rapidly becoming available in
Britain and Europe.
This note contains a proposal for such an interim protocol,
discusses the requirements on mail relays, and also the
requirements for experimental systems based on it, both in the
US and the UK. Partly because the proposed system uses ARPA
facilities, the document has a certain bias towards
familiarity with ARPA systems and terminology. In particular,
the note assumes familiarity with RFC733 mail formats
[Crocker77], the ARPA standard. However, it is believed that
the system will also be useful in other environments,
particularly the X25 and Network Independent Transport Service
[SG3/80] environments which will be available shortly in the
UK. ARPA-oriented readers should note that familiarity is
assumed with the NIFTP [HLPG81], the interim UK file transfer
standard.
Comments are solicited and welcomed.
2. Requirements
The basic requirements for an immediate mail service are as
follows:
(1) Maximum use must be made of existing standards. No
radical new mail formats, transfer protocols, etc may
be imposed. Using these facilities it should be
possible to bring up an initial service within months
rather than years.
(2) The service should be economic. In particular, only
one copy of a message should be transmitted to each
destination mail server.
Bennett [Page 1]
INDRA Note 1025 NIFTP-Based Mail
(3) The service should be independent of particular
networks.
(4) Address information must be transmitted in a way which
is usable by mail relays.
(5) Total global knowledge of mail server addresses should
not be necessary.
(6) The service should be as flexible as possible. Where
possible it should be easy to extend it to meet new
needs as they emerge.
The last requirement is perhaps the least important. It
implies that the service will be around for sufficiently long
that experimental technical advances in message processing can
be usefully incorporated. For instance, the minimum
requirement is that text messages must be transferable.
Requirement (vi) suggests that it should be possible to extend
the service simply to transfer mixed-media messages. When
assessing whether a given proposal meets this criterion, the
multi-media case will be used as a test.
No existing system in wide use meets all these constraints.
Hence it is necessary to combine elements with the right
properties from a number of sources not all of which are
currently used for mail.
It should be noted that the service defined here is not
intended to be perfect. In particular, it does not of itself
guarantee bidirectionality, endpoint reliability, or use of
address lists. These will normally be available for direct
endpoint transfer, and any problems are most likely to arise
if intermediate relays are used. It is possible to support all
these things using it, and of course they are encouraged.
However, I feel that a service adequate for ordinary use can
be achieved without defining a great deal of complicated
additional mechanism to guarantee these properties. Finally,
the problem of conversion between mail formats is not
discussed at all.
3. Basic Elements
3.1 Mail Format
Bennett [Page 2]
INDRA Note 1025 NIFTP-Based Mail
The mail format defines the standards for the appearance of
a message; it covers such things as the definition of header
fields to allow messages to be processed by a standard
program.
The mail format most readily available is the ARPANET mail
standard commonly known as RFC733, defined in [Crocker77],
which is compatible with all except the last of the objectives
listed above. RFC733 has been widely and successfully used for
many years. In practice only a subset of the standard is
actually used - in particular, the standard allows extended
addressing by specifying destination networks, but no
implementation supports this. It is proposed that the common
subset of this standard be used. The only change suggested is
that the text code should be IA5 rather than the Telnet
version of ASCII, as IA5 is an international standard. In
practice, the two codes are virtually identical. An example
of an RFC733 message, and a summary of the RFC733 syntax, is
given in Appendix I.
This standard is completely text-oriented, for both message
header (control) and message text (data) information. This
makes it readily compatible with most of the text-processing
software generally available on most interactive systems, such
as text editors, pro forma preparation etc.
Other formats may be agreed to in the near future. These may
extended facilities, such as facsimile or multi-media mail.
These can be accommodated provided any mail servers needing to
process the mail body understand the format in use. Hence it
may be necessary to provide a means for labelling the format
in use. Such a label is not defined at this time.
3.2 Mail Addresses
In any mail system it is essential to provide addresses for
the mail recipient, and usually for the mail sender as well.
While mail will frequently be transferred directly between the
sender and the recipient, there will be occasions when it will
be routed, and possibly temporarily stored, through an
intermediate mail relay.
Bennett [Page 3]
INDRA Note 1025 NIFTP-Based Mail
Mail addresses are constrained to be compatible with the
RFC733 format as generally used, viz. <username>@<hostname>.
The <hostname> field defines the host used for an initial mail
transfer, and in standard RFC733 usage, no further structuring
is defined or required. If this name is understood by all
message relays along a route, then no further structuring is
ever required. If further structuring is required, it should
be through the use of element separators in the <username>.
The addressing structures encountered when mail relays must be
used are discussed further in section 5.1.
3.3 Mail Transfer
In addition to the appearance of the messages and the
identity of the parties, it is necessary to define a protocol
for transferring mail between the two. At first sight, it
would seem natural to take over ARPA-developed procedures here
too, so that one could use a complete, preexisting mail
package. Despite the great success which have attended these
procedures, and their undoubted appropriateness for the
environment for which they were designed, they do not fulfil
the needs of the wider message community, for reasons which
are discussed below.
The standard ARPANET Mail Transfer procedure [Postel76]
fulfils only the first of the requirements listed in the
introduction, and is therefore not acceptable. In particular,
it has the following failings:
(1) It is uneconomic in that it transmits one copy per
destination user.
(2) It is specific to the ARPANET.
(3) It transfers address information in a way which is
only usable for direct source/destination transfer.
(4) It requires all hosts to be aware of all host names,
and hence requires a globally understood global
address space.
(5) It can only ever handle text data. If binary data in a
mixed-media message were encoded as text symbols and a
code conversion was required between NVT-ASCII and a
local text encoding, that data would be corrupted.
Bennett [Page 4]
INDRA Note 1025 NIFTP-Based Mail
A recent ARPA proposal [Sluizer80] for a new Mail Transfer
Protocol (MTP) remedies in whole or in part some of these
deficiencies. In particular, it provides facilities for
economic uses of resources, and transfers addressing
information in a way compatible with objectives (iv) and (v).
However the MTP as it is currently proposed has some problems
of its own:
(1) The MTP allows a single copy of a message to be sent
to multiple recipients, and is thus potentially more
economic than the standard ARPA protocol. However,
this is only an option which need not be implemented.
Moreover, the mail sender can only specify one path
for a given message to pass through intermediate
relays. Thus MTP does not allow a single copy to be
sent to a relay which could then decide to create
multiple copies for different destinations at the
point of a path division.
(2) In MTP, address lists may be transferred either before
or after the message body. With the 'recipients first'
option, it is only possible to check the current
validity of the addresses - for the actual message
transfer, only total success and total failure can be
indicated. If a given recipient became inaccessible
for some reason between the two stages, the entire
transfer would fail.
(3) The MTP is defined to operate in an environment
similar to that of the ARPA Catenet. In particular, it
relies heavily on the use of Telnet command
procedures, which are both little known and little
used outside the ARPA community - especially within
Europe. It does not define the mechanisms by which it
will operate across more than one network (just as
Telnet does not), or where Telnet procedures are not
appropriate. To this extent, it is not network-
independent.
(4) The MTP provides no restart procedures to recover from
errors signalled by either the host or the
transmission medium. It is therefore only as reliable
as the weakest underlying network. For example, an
X25-based version could not recover from Resets.
Bennett [Page 5]
INDRA Note 1025 NIFTP-Based Mail
(5) It can only ever handle text data. If binary data in a
mixed-media message were encoded as text symbols and a
code conversion was required between NVT-ASCII and a
local text encoding, that data would be corrupted.
Accordingly, it is felt here that mail transfer outside the
ARPA environment should use an alternative base. It is
proposed here that mail transfer be achieved using defined
conventions with the Network Independent File Transfer
Protocol (NIFTP - [HLPG81]). This FTP is, as the name implies,
network independent, has been implemented on a number of
different existing networks, including the ARPANET and ARPA
Catenet, and has been successfully used for direct file
transfers across several intermediate networks. The revised
version will be adopted as an interim standard in Britain and
has evoked wide interest in Europe. There are numerous
implementations of the existing version, and it is expected
that the revised version will be implemented fairly rapidly
after the new specification is released, which should be
within the next two months. It thus meets the criteria of
availability, standardisation and network independence. It
remains to define a transfer procedure which meets the other
criteria.
4. Point to Point Mail Transfer
Using RFC733 mail formats and RFC733-compatible addressing,
it is necessary to define the procedure used to transfer mail
from a mail donor to a mail server with the NIFTP. This
section defines that procedure.
4.1 Mail Structure
4.1.1 Proposal
A letter shall be transferred as a single file from the mail
donor to the mail server. The file name used shall be
provided by the mail server. The structure of the file is as
follows:
<address list><one or more blank lines><mail text>
The <address list> is a list of full, explicit RFC733
Bennett [Page 6]
INDRA Note 1025 NIFTP-Based Mail
addresses to whom the mail shall be distributed by the mail
server.
The <mail text> shall conform with RFC733 formats.
4.1.2 Discussion
4.1.2.1 Address Lists
This structure is designed to fulfil the requirement that
the service be economic. The passing of the <address list>
minimises the number of copies of the message which must be
made by the donor, but allows decisions to create additional
copies to be made simply by intermediate relays.
The <address list> must contain explicit addresses as the
mail server is not necessarily able to access address lists,
and in any case requires additional mechanism to do so. The
addresses should be full (i.e. contain an explicit <hostname>
component) as the server may be a relay using address mapping
mechanisms (see below).
4.1.2.2 Possible Extensions
There are two simple extensions which may be desirable:
(1) To distinguish between message formats. This requires
simply the addition of an extra field in the mail
file, and the definition of text encodings. Such a
field should be inserted between the <address list>
and the <message text>. The use of other formats will
particularly affect the design of mail relays.
(2) Mailbagging. The file may contain several messages
separated by defined message delimiters. (A simple
one, which is widely used in message files on UNIX
systems in the ARPANET, is ^A^A<cr>. Another
alternative, preferred here, is to insert a
delimitered character count, encoded in IA5, as in
TENEX.) Mailbagging also has other implications. For
instance, if the mail donor wishes to initiate a
mailbagged transfer and it knows the name of an
existing mailbag at the server from a previous
transfer, it may append the file to the existing
Bennett [Page 7]
INDRA Note 1025 NIFTP-Based Mail
mailbag. The advantages of a mailbagging system are
for further study.
4.1.2.3 Alternative Structures
Two alternative structures were considered, both involving
the explicit separation of the address list from the mail
text.
The first was to require the donor to specify the file name,
which would be the address list. This has a number of
disadvantages:
(1) The NIFTP allows a maximum string length of 255
characters for string-valued attributes. This would
allow at most about 12-20 addresses.
(2) It would be difficult to trace the progress of a
message through a series of relays. With an explicit
and known filename, it would be possible to initiate
manual or automatic tracing procedures.
(3) It is unlikely that most mail servers or relays would
be able to use such a filename directly. The internal
filename would be created using local mappings. The
potential costs of these mappings could be very high.
The second alternative considered was to transfer the
address list and mail text as two separate files. This also
has disadvantages:
(1) The two files must be linked, e.g. by requiring that
an Action Message be passed on STOP or STOPACK
containing the filename to be used for the text
portion of the transfer.
(2) An additional level of error handling procedure must
be defined, to cover cases such as the loss of a
portion of the message text, or the arrival of two
address lists with no intermediate message text.
Bennett [Page 8]
INDRA Note 1025 NIFTP-Based Mail
These problems are avoided by the mechanism proposed. By
sending the address list and the body as a single file,
address lists of arbitrary length can be sent; their link to
the text is assured; maximum use can be made of the NIFTP
reliability mechanisms; and the donor can be assured that the
mail has at least reached the server host.
The new MTP proposal from the ARPA community has a fairly
similar proposal, but allows as an additional option the
possibility of transmitting the text before the addressing
data, arguing that in some cases this is more efficient for
the destination host. Although it is conceivable that this may
be true for MTP given the details of the MTP mechanisms, I
think it is most unlikely that any advantage would be gained
in the current context; moreover, in order to achieve it
additional mechanism is required to specify which option is
being used. Hence it has not been proposed.
4.2 Mail Server Identification
4.2.1 Proposal
The mail server will be identified by its transport service
subaddress. This subaddress will be network-specific and
possibly host-specific; for instance, on the ARPANET it will
be an NCP server socket, on the ARPA Catenet a TCP port, on
public data networks an X25 or Transport Service subaddress as
appropriate.
If the mail donor and server are not on the same transport
service, it is the responsibility of the intermediate virtual
call gateways to perform any address transformations required,
e.g. mapping the NCP mail socket to the TCP mail port.
4.2.2 Discussion
This proposal is in line with the recommendation in the
NIFTP specification for distinguishing different services
utilising NIFTP. An alternative, which is not favoured here,
is to reserve a value for the Username attribute, such as
MAIL.
Bennett [Page 9]
INDRA Note 1025 NIFTP-Based Mail
4.3 Mail Server Communication
4.3.1 Proposal
Synchronisation shall be achieved via the establishment of
an virtual connection between the mail donor and the mail
server. This connection may be used for one or more mail
transfers. The connection may have one or more segments, where
each segment may use a different transport protocol.
4.3.2 Discussion
This is normal NIFTP usage, and is essentially a restatement
of the network-independence criterion.
Normally, the donor and server will use the same transport
protocol, and no additional procedures need be used.
Exceptionally, the mail donor and server may not be on the
same transport service. In this case direct NIFTP file
transfer is still possible, but an additional degree of
support is needed, through the provision of one or more
virtual call gateways. At the least, the following services
are necessary:
(1) One-to-one connection mapping.
(2) Addressing procedures. This could be any acceptable
procedure, e.g. source routing, creation of a
hierarchical address space, or mapping of transport
service subaddress to destination host.
(3) Call request facility. This carries all the addressing
information necessary for establishing an end-to-end
path.
(4) Call accept facility. This is necessary to confirm
that an end-to-end path has been established.
(5) Data transfer. This should commence only after a call
accept has been received.
(6) Push preservation. Data should be forwarded if any
push indication (e.g. TCP EOL, X25 No More Data) is
received. The gateway may decide to forward data in
other circumstances.
Bennett [Page 10]
INDRA Note 1025 NIFTP-Based Mail
(7) Closure propagation. If a connection closure is
received from one side, it shall be mapped into the
appropriate closure procedure on the other.
(8) Reset propagation. If any network in the path is
capable of generating resets, these must be forwarded
in some fashion by the gateway. For instance, an X25
RESET may be mapped into TCP URGENT. If resets cannot
be generated this may be ignored by the gateway.
It should be noted that the address transformations mentioned
above need not be visible to the mail donor and server, and do
not in any circumstances require address processing of the
mail text at the virtual call gateway. For bidirectionality,
however, it is necessary that the donor and server can
recognise their respective hostnames as embedded in the mail
file.
4.4 Mail Transfer
4.4.1 Proposal
The transfer may be initiated by either the mail donor or
the mail server.
The file will be transferred by the NIFTP using IA5 text
codes. If the file only contains a text message, then the Data
Type attribute will indicate text only.
If the transfer is initiated by the mail donor, then the
Mode of Access used will be 'Replace or Make' and the Filename
attribute on the SFT will indicate 'no value available'; the
server should supply a value on the RPOS for reporting
purposes.
If the transfer is initiated by the mail server, then the
Mode of Access used will be 'Read Only'. The donor will
nevertheless often wish to delete the file after it has been
successfully transferred. The Filename attribute on the SFT
will supply a value.
No other constraints are imposed on the use of NIFTP
attributes.
Bennett [Page 11]
INDRA Note 1025 NIFTP-Based Mail
4.4.2 Discussion
This section defines the actual mail transfer procedure, and
places minimal restrictions on NIFTP use.
Alternative structures lead to greater restrictions or
special interpretation of NIFTP attributes, or both.
If a message format other than RFC733 is used, then mixed-
media transfers may be possible. The NIFTP procedure would
then have to be modified. If the file contains a mixed-media
message, then the Data Type attribute will indicate either
mixed file or mixed records as appropriate, and the binary
formats for the non-text portions will be negotiated.
Mailbagging may also require extension. In this case, the
mode of access used by a mail donor initiating the transfer
could be 'Append or Make', though this is not recommended.
Other facilities which may be useful are: data compression,
text formatting, record structuring, restart and resumption
recovery facilities, account name, various passwords etc.
4.5 Reliability
The NIFTP has several grades of recovery action, which can
be exploited to ensure that a message will be delivered to a
server despite the occurence of system or communication
errors. However, the successful delivery of a message to a
server does not guarantee it will be successfully delivered to
the recipient. If it cannot be delivered, notification should
be sent to the sender by the server forced to discard it,
where this is possible. This notice will take the form of a
message, and the sender's address will be determined from
inspection of the appropriate fields (i.e. "Reply-to:" and
"From:") in the message. A non-delivery notice should make
maximum use of the NIFTP reliability procedures to ensure that
it itself is delivered.
5. Mail Relays
For a number of reasons it may not be possible to transfer a
message direct between the sender and the receiver. In
particular, they may not use the same mail transfer procedure.
Bennett [Page 12]
INDRA Note 1025 NIFTP-Based Mail
In such cases, mail must be staged through an intermediate
mail server, which acts as a mail relay.
The function of the relay is to redistribute received mail
to the destinations or to other mail relays. I do not intend
to specify a mail manipulation protocol here, but it is
necessary to discuss the functions which must be provided, and
the options which are available, in order to determine to what
extent it is possible to allow variation and still provide an
acceptable service. Since the actions of other mailing systems
cannot be specified here, these functions will be discussed,
where necessary, in relation to the NIFTP-based system
described above.
It is important to distinguish these relays from the virtual
call gateways discussed above. Either may be used at the
interface between two transport services, but the virtual call
gateway gives the minimum facilities which must be provided at
this point, and is invisible to the endpoint mail transfer.
Mail relays must be used when different mail transfer
protocols are used by mail donor and recipient, and will be
visible to both the sender and recipient; in this case a
virtual call gateway will be totally inadequate.
5.1 Address Processing
It is not possible to prescribe an addressing format for use
by relays, except that it be RFC733-compatible. The actions
to be taken by the relay on processing addresses are dependent
on local conventions and private agreements. It is expected
that there are three major types of address processing which
may occur:
(1) The address of the next stage may be defined by a
received <hostname> component, which is understood by
the relay to map to the name of some other host. For
example, the name 'Linington@Cambridge', if received
at UCL from the US, might cause the mail to be
transferred to Cambridge, or to some intermediate
relay.
(2) If no <hostname> is received (which can only happen
from a non-NIFTP mailing system), the address of the
next stage may be defined by a received <username>
component, which is understood by the relay to map to
Bennett [Page 13]
INDRA Note 1025 NIFTP-Based Mail
the name of some other relay host, or to the
<username> and <hostname> of the final user. For
example, the name 'DCPU', if received at UCL from the
US, might be understood to map to
'Linington@Cambridge', and be forwarded. This form is
not encouraged as user names must then become widely
known.
(3) If the <hostname> is the relay itself, and the
destination is not at the relay, then either case (ii)
applies, or the username is structured in some fashion
understood by the relay. A recommended form is
through the use of % as a separator, which leads to a
source-routed form, e.g:
Linington%Cambridge%PSS@UCL.
On reception at UCL from the US, the <hostname>
component will be discarded, the last % converted to
an @, and the structured name becomes:
Linington%Cambridge@PSS. The relay then injects the
message into the appropriate mailing system over PSS,
subject to the constraints of that system.
Any or all of these approaches may coexist. As a general
approach, I prefer the third. All suffer from the
disadvantages that they are not global, and that address
transformation may be required. The user who uses relays must
use an address format he is sure will be understood along the
route. In practice, however, it is unlikely that more than one
or two relays will be involved in the transfer of most
messages.
The address processing at mail relays, which may affect the
text of the message received, must be carefully distinguished
from the address processing which may be required at virtual
call gateways between the donor and server, which does not.
Two different levels of addressing are involved here - the
former is visible only in mail, the latter only within a
particular file transfer.
5.2 Return Paths
The system provides no guarantee that a message can be
replied to, although this will normally be possible. Relays
Bennett [Page 14]
INDRA Note 1025 NIFTP-Based Mail
can provide assistance in the following ways, both of which
involve the processing of the header content of the message:
(1) Each relay can insert a "Via: " field in the message.
(2) Each relay can add its name to a "Reply-to: " field,
thus building up a return source route. The name added
must be the name known to the message system into
which it is forwarding the mail.
The general problem is to allow replies to be sent to
recipients other than the sender. One possibility is to allow
replies to be redistributed by the original sending host, if
that host is willing to do so. Alternatively, intermediate
relays could process the names of "To: " and "Cc: " fields in
the message to provide a shorter path. This problem is exactly
analogous to the "third-party addressing" problem of the UK
Transport Service [SG3/80], and the techniques discussed in
that document could be used here.
Although this procedure requires relays to process the
message text, programs to do such processing already exist
which could be used for this purpose. If such processing is
not done, it is necessary to insist that replies can only be
sent if the recipient knows the location of the sender, which
for full answerability amounts to a requirement for a global
address space. This contravenes one of the stated limitations
on the system.
5.3 Economy
Where the mail system protocols allow, the relay should
minimise the number of copies of a message injected into the
system. Thus a relay may receive a single copy of a message
destined for several different hosts, some of which may be
only accessible through another relay. For each host which can
be reached directly the relay will send a single copy of the
message; for the remaining hosts, a single copy will be sent
to the next relay which will redistribute it in turn.
This minimisation may require considerable intelligence to
do properly, and may not always be practicable. If the relay
is receiving mail from a system which creates one copy per
user, and injecting it into a system similar to the NIFTP-
Bennett [Page 15]
INDRA Note 1025 NIFTP-Based Mail
based system described here, there is no easy option. One
possibility is to parse the header to identify, so far as
possible, how many copies of the message it may expect to
receive, detect the duplicates, and discard them. Another is
for a relay to create a mail list and instruct a recipient to
"Reply-to: <mail list name>@<relay>". It must then have
criteria for deciding how long to keep such lists around. No
doubt other equally inspiring schemes can be devised.
5.4 Reliability
Relays should make use of the mail system to give delivery
failure notifications, as described above. There is, however,
no guarantee that the return path can be constructed.
6. The ARPA Mail Transition Plan: A Case Study
In this section a proposal is made for using the NIFTP-based
protocol to allow mail transfer between ARPANET users who have
only NCP-based mail transfer available to them, and those who
have only TCP-based network access.
6.1 Current Proposals
The current proposals for ARPANET mail transition centre on
the implementation of the MTP discussed above, and the
definition of relay procedures between NCP-based mailing
systems and TCP-based mailing systems [Postel80]. In general,
these relays fit into the context discussed in the previous
section, and most of the comments of the current proposal on
the complexity of maintaining relays, mapping tables etc are
fully endorsed here.
Aside from the features of the MTP, there are a two specific
points that need discussion:
(1) As noted in the October 1980 Internet meeting, the
transition plan requires that user names be unique
throughout the ARPA Catenet (and hence a growing
portion of the ARPANET), in order to allow ARPA mail
from a standard ARPANET NCP-based mail server to be
sent to a relay for forwarding to the ARPA Catenet.
This is clearly unacceptable, and can most simply be
Bennett [Page 16]
INDRA Note 1025 NIFTP-Based Mail
avoided by allowing the relays to parse, and if
necessary modify the header fields in the message
text. Such solutions are preferable.
(2) The solution is inextensible. The transition plans
assumes that transfer of mail between two MTP servers
on hosts using only NCP and TCP must be achieved
through an MTP-based mail relay at a site with access
to both. This is rather wasteful, since essentially
the same protocol is involved in both cases. A similar
relay is required if other transport services, or
network services such as X25, require mail service.
6.2 NIFTP and the Transition Plan
The major fault of the current transition arrangements
relevant here is that a complex message relay is required even
for hosts which both talk MTP. This is not the case for the
NIFTP-based scheme outlined here. All that is necessary is a
relatively simple virtual call gateway at an NCP/TCP
transition point, along the lines discussed above. Such a
gateway has been built and its feasability demonstrated
[Bennett80]. Moreover, it has been demonstrated that the NIFTP
can be readily extended to non-Catenet networks, whereas it is
far from clear that this is true for the MTP-based scheme,
because of its reliance on Telnet.
The advantage of an explicitly network-independent approach
is that mail transition can now be entirely divorced from
NCP/TCP transition. The development of future mail protocols
can carry on without requiring an immediate, new, solution for
the Catenet. Any host with NIFTP can do direct mail transfers
using existing formats.
Of course, very few ARPANET hosts have access to NIFTP,
although this is easy to change. However, it is not necessary
that they should. There is indeed a staging problem to be
solved - the staging between NIFTP-based mail and current ARPA
mail. This must take place along the lines discussed above,
but once solved, it is solved, in theory, not just for NCP and
TCP but for any transport protocol.
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INDRA Note 1025 NIFTP-Based Mail
6.3 A Proposal
It is believed that an NIFTP scheme of this sort can be
built and proliferated very quickly. The following components
already exist:
(1) NIFTP implementations written to a transport service
interface for TOPS20 DEC20s, UNIX PDP-11s (Version 6),
and MOS LSI-11s. These need to be upgraded to conform
to the new specification of the protocol. The first is
available above TCP and NCP; the second will shortly
be accessible above TCP and X25; and the third is
available above TCP.
(2) A simple NCP/TCP virtual call gateway, for NIFTP
support, on a TOPS20. This was built for demonstration
purposes, and needs some modification for general use.
(3) Message relays for heterogenous mail systems, in the
form of the MMDF system developed by the University of
Delaware [Crocker79], for UNIX. Such relays must be
built in any case for the MTP scheme.
The following components are needed:
(1) Specification and agreement to the virtual call
extensions needed for direct NCP/TCP file transfer. If
these are done using a subset of the protocol proposed
for implementing the UK Transport Service above TCP
[Bennett80a] (and a similar protocol for NCP), then
direct mail transfers from NCP sites to sites on the
UK public network PSS could also be done.
(2) Allocation of NIFTP-mail server sockets and ports.
Again, for extension to the UK public network,
Transport Service and/or X25 subaddresses must also be
defined.
(3) Agreement on an addressing scheme to allow transfer
from ARPA mail to NIFTP-based sites. It is proposed
that a structured user name of the form outlined above
be used.
(4) Implementation of an NIFTP mail channel in a form
suitable for incorporation under MMDF by UCL.
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INDRA Note 1025 NIFTP-Based Mail
(5) At least one system in the US supporting MMDF which
can act as a relay between ARPA mail and NIFTP mail,
using the NIFTP channel supplied by UCL.
(6) Code interfacing the transport service interface of
the UNIX NIFTP directly to UNIX TCP (and possibly NCP)
implementations, supplied by the US MMDF site.
This allows us to define the minimum configuration necessary
to provide NIFTP-based mail transfer between UCL systems and
systems in the CONUS ARPANET using either the current ARPANET
mail transfer protocol or the new MTP. The path would be as
follows:
(1) UCL donor passes mail to UCL UNIX NIFTP in core.
(2) NIFTP initiates a connection to the remote NIFTP,
which is which is driven as a mail channel by an MMDF
system (e.g. SRI-UNIX or UDEL-EE). This path will be:
through the UCL UIPP protocol to the Front End; thence
via TCP through UCLNET, SATNET and the CONUS ARPANET,
either directly to the remote system, or to a TCP/NCP
virtual call gateway resident at ISIE (which in turn
forwards the NIFTP traffic to the remote MMDF server
through NCP).
(3) The remote MMDF injects the mail into either the
standard ARPANET mail channel or an MTP channel; at
the remote end of which it is delivered to the user's
mailbox.
In addition, it would be highly desirable to construct an
MMDF-like system which could act as a multi-channel mail relay
on TOPS20 systems in the ARPANET. All relevant mail channels
could be driven through it in a precisely similar manner.
However, rather more work is required to make this service
available.
The above discussion ignores MTP-based mail altogether. This
has been done for illustrative purposes only - I have no doubt
that the current transition plans will be implemented, though
possibly not quite in their current form. In practice, the two
systems could coexist quite happily. The main impact of
allowing the two systems to proceed in parallel would be in
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INDRA Note 1025 NIFTP-Based Mail
the design of mail relays. The more mail transfer systems
exist, the more important it is that relays be designed in a
'mail-independent' fashion. The advantages of this have
already been demonstrated for the UNIX MMDF. The same
principles should be followed in the design of relays for
other computer systems.
7. NIFTP-Based Mail in the UK.
Within Britain, the problems of building a mail system along
these lines are rather different, but on the whole less
complex. The basic communications facilities are only now
coming into widespread use, and the investment in higher level
protocols is much lower. However, the higher level protocols
which are coming into use are much friendlier to the system,
for the following reasons:
(1) NIFTP is already available on most widely used machine
types in Britain. Implementing the mail transfer
procedure is a trivial additional exercise.
(2) The UK transport service proposals assume the need for
network interconnection to provide a semantically
equivalent service rather than a superimposed common
protocol. Hence the need for extensibility through
virtual call gateways is widely accepted.
(3) Because there is no heavy investment in first
generation protocols, there is no absolute requirement
for mail relays at this stage.
The major need is for message processing and preparation
programs, as such programs are not widely available in the UK,
except for UNIX systems. In particular, these should be based
on RFC733 procedures. For many system types these may be
available from similar systems in the US; others would have to
be developed from scratch.
8. References
[Bennett80] - Bennett, C. J.: The Design and Implementation of
Bennett [Page 20]
INDRA Note 1025 NIFTP-Based Mail
Transnetwork Systems. UCL TR 65. Available from
Univeristy College London.
[Bennett80a] - Bennett, C. J.: Realisation of the Yellow Book
Above TCP. INDRA Note 965. Available from University
College London.
[Crocker77] - Crocker, D. H., Vittal, J. J., Pogran, K. T.,
Henderson Jr, D. A.: Standard for the Format of ARPA
Network Messages. RFC733. Available from SRI
International, Menlo Park, California. Obsoletes earlier
versions.
[Crocker79] - Crocker, D. H., Szurkowski, E. S., Farber, D.
J.: An Internetwork Memo Distribution Capability - MMDF.
Proc. 6th Data Comm. Symp.
[HLPG81] - HLPG/FTPIG: A Network Independent File Transfer
Protocol. Available from DCPU, National Physical
Laboratory, Teddington, UK. Obsoletes earlier versions.
[Postel76] - Postel, J. B.: Mail Protocol. NIC 29588.
Available from SRI International, Menlo Park, California.
[Postel80] - Postel, J. B., Cerf, V. G.: Mail Transition Plan.
RFC773. Available from SRI International, Menlo Park,
California.
[SG3/80] - PSS/SG3: A Network Independent Transport Service.
Available from the DCPU, National Physical Laboratory,
Teddington, UK.
[Sluizer80] - Sluizer, S., Postel, J. B.: A Mail Transfer
Protocol. RFC772. Available from SRI International, Menlo
Park, California.
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INDRA Note 1025 NIFTP-Based Mail
APPENDIX I
RFC733 Formats
Below is an example of an RFC733 message taken from the
specification.
Date: 26 August 1976 1430-EDT
From:George Jones<Group at Host>
Sender:Secy at SHOST
To:Al Neuman at Mad-Host,
Sam Irving at Other-Host
Message-ID: <some string at SHOST>
This is an example of an RFC733 message. Both
simpler and more complex headers are possible.
The entire RFC733 syntax specification is summarised in
the following listing, extracted from the original
specification:
A. ALPHABETICAL LISTING OF SYNTAX RULES
address = host-phrase / quoted-string
/ (*phrase "<" address ">" )
/ (*phrase ":" address ";" )
/ (":" ("Include" / "Postal" / atom) ":" address)
ALPHA = <any TELNET ASCII alphabetic character>
atom = 1*<any CHAR except specials and CTLs>
CHAR = <any TELNET ASCII character>
comment = "(" *(ctext / comment / quoted-pair) ")"
CR = <TELNET ASCII carriage return>
CRLF = CR LF
ctext = <any CHAR excluding "(", ")", CR, LF and
including linear-white-space>
CTL = <any TELNET ASCII control character and DEL>
date = 1*2DIGIT ["-"] month ["-"] (2DIGIT /4DIGIT)
date-field = "Date" ":" date-time
date-time = [ day-of-week "," ] date time
day-of-week = "Monday" / "Mon" / "Tuesday" / "Tue"
/ "Wednesday" / "Wed" / "Thursday" / "Thu"
/ "Friday" / "Fri" / "Saturday" / "Sat"
/ "Sunday" / "Sun"
delimiters = specials / comment / linear-white-space
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INDRA Note 1025 NIFTP-Based Mail
DIGIT = <any TELNET ASCII digit>
extension-field = <Any field which is defined in a document
published as a formal extension to this
specification>
field = field-name ":" [ field-body ] CRLF
fields = date-field originator-fields *optional-field
field-body = field-body-contents
[CRLF LWSP-char field-body]
field-body-contents = <the TELNET ASCII characters making up the
field-body, as defined in the following sections,
and consisting of combinations of atom, quoted-
string, and specials tokens, or else consisting of
texts>
field-name = fnatom *(LWSP-char [fnatom])
fnatom = 1*<any CHAR, excluding CTLs, SPACE, and ":">
host-indicator = 1*( ("at" / "@") node )
host-phrase = phrase host-indicator
hour = 2DIGIT [":"] 2DIGIT [ [":"] 2DIGIT ]
HTAB = <TELNET ASCII horizontal-tab>
LF = <TELNET ASCII linefeed>
linear-white-space = 1*([CRLF] LWSP-char)
LWSP-char = SPACE / HTAB
mach-id = "<" host-phrase ">"
mailbox = host-phrase / (phrase mach-id)
message = fields *(CRLF *text)
month = "January" / "Jan" / "February" / "Feb"
/ "March" / "Mar" / "April" / "Apr"
/ "May" / "June" / "Jun"
/ "July" / "Jul" / "August" / "Aug"
/ "September" / "Sep" / "October" / "Oct"
/ "November" / "Nov" / "December" / "Dec"
node = word / 1*DIGIT
optional-field =
"To" ":" address
/ "cc" ":" address
/ "bcc" ":" address
/ "Subject" ":" *text
/ "Comments" ":" *text
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INDRA Note 1025 NIFTP-Based Mail
/ "Message-ID:" mach-id
/ "In-Reply-To"":" (phrase / mach-id)
/ "References:" (phrase / mach-id)
/ "Keywords" ":" phrase
/ extension-field
/ user-defined-field
originator-fields =
( "From" ":" mailbox
["Reply-To:" address] )
/ ( "From" ":" 1 address
"Sender" ":" mailbox
["Reply-To:" address] )
phrase = 1*word
quoted-pair = "
quoted-string = <"> *(qtext / quoted-pair) <">
qtext = <any CHAR except <">, CR, or LF and including
linear-white-space>
SPACE = <TELNET ASCII space>
specials = "(" / ")" / "<" / ">" / "@"/ "," / ";" / ":"
/ "
text = <any CHAR, including bare CR and/or bare LF, but
NOT including CRLF>
time = hour zone
user-defined-field = <Any field which has not been defined in
this specification or published as an extension to
this specification; names for such fields must be
unique and may be preempted by published
extensions>
word = atom / quoted-string
zone = ( ("+" / "-") 4DIGIT )
/ ( ["-"] (1ALPHA
/ "GMT" / "NST" / "AST" / "ADT" / "EST" / "EDT"
/ "CST" / "CDT" / "MST" / "MDT" / "PST" / "PDT"
/ "YST" / "YDT" / "HST" / "HDT" / "BST" / "BDT" ))
<"> = <TELNET ASCII quote mark>
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INDRA Note 1025 NIFTP-Based Mail
APPENDIX II
UCL Mail System Plans for 1981
The following is an extract from the UCL annual report
for the Ministry of Defence, summarising our planned
activities in 1981.
Plans for the message systems work in 1981 have not
been finalised, as some areas depend on decisions and
choices which have not yet been made. However, a rough
scheme is as follows:
(1) Complete installation of MMDF in local systems
(January). Supportive tasks will be required
throughout the year, eg bugfixing as found,
transferring and reconfiguring MMDF for new systems
(eg the 11/44) as they become available.
(2) Issue specification of proposed NIFTP-based mail
channel (January)
(3) Either
implement NIFTP-based mail channel over TCP under
MMDF; if possible, after upgrading UNIX NIFTP to the
1980 specification. Optionally (but preferably) the
UNIX NIFTP should also use Yellow Book TCP commands.
or
implement or obtain MTP mail channel over TCP under
MMDF. If one is obtained from an outside source it
must be modified to access TCP through the UIPP, and
very probably modified so that it can be driven as a
channel through MMDF. (1st - 3rd quarter)
(4) Depending on the choice made above, appropriate
supportive action must be taken. (2nd to 4th quarter
and beyond)
(1) NIFTP-based channel
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INDRA Note 1025 NIFTP-Based Mail
(1) This should be released to an MMDF
site in the US (either SRI or the
University of Delaware) which could
act as a relay.
(2) Help and assistance as required to
interface NIFTP directly to TCP or NCP
in the remote system.
(3) TCP/X25 virtual call gateway to allow
access through IPSS and PSS. This
should be in existance in any case.
(2) MTP channel
(1) TCP/X25 virtual call gateway, as
above.
(2) Possibly TCP/NCP virtual call
gateways. This depends on the progress
of MTP/ARPA mail relays at TCP/NCP
boundaries as required by the ARPA
Mail Transition Plan.
(5) Replace MSG message processing system by MH, to allow
remote access to UCL mail handling. (3rd/4th quarter
and beyond).
While the above lists the primary path to providing
mail services, there are a number of subsidiary or
optional pathways which will also be undertaken, if
necessary or desirable. These include the following:
(6) Continue efforts to provide ARPANET mail access
through a terminal channel. (January/February).
(7) Undertake necessary activity to incorporate
TOPS20/TENEX systems into either the NIFTP or MTP
based scheme above. The latter case should presumably
involve very little UCL activity. The former case
requires the following from us (1st to 4th quarter and
beyond):
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INDRA Note 1025 NIFTP-Based Mail
(1) Optionally (but preferably) the upgrading of
the TOPS20 NIFTP to the 1980 specification.
(2) Optionally (but preferably) the interfacing of
the NIFTP to a Yellow Book TCP Service.
(3) The design and development of a mail relay
system analogous to MMDF for TOPS20. Even if
it is decided that UCL will go the NIFTP path,
such a relay may well be developed by a US
ARPA contractor for MTP support. In this case,
our task is to interface the NIFTP channel on
TOPS20 to this relay.
(8) There may arise interest in providing mail servers
within the UK community, eg on SRCNet or PSS. Such
services are more likely to be NIFTP-based than MTP-
based (though in the longer term Teletex is a more
favoured candidate than either). In this case the
following UCL activities would be required (2nd/3rd
quarter to 4th quarter and beyond):
(1) NIFTP-based mail channel over Yellow Book over
X25.
(2) The use of the UCL MMDF server as an actual
mail relay, if Catenet sites are using an MTP
channel.
(3) A TCP/X25 virtual call convertor if they are
not.
(9) Additionally, UCL will continue to take an active
interest in message standardisation activity, in areas
such as Teletex, IFIP WG6.5, etc.
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INDRA Note 1025 NIFTP-Based Mail
Table of Contents
1. Introduction..................................................1
2. Requirements..................................................1
3. Basic Elements................................................2
3.1 Mail Format...............................................2
3.2 Mail Addresses............................................3
3.3 Mail Transfer.............................................4
4. Point to Point Mail Transfer..................................6
4.1 Mail Structure............................................6
4.1.1 Proposal.............................................6
4.1.2 Discussion...........................................7
4.1.2.1 Address Lists...................................7
4.1.2.2 Possible Extensions.............................7
4.1.2.3 Alternative Structures..........................8
4.2 Mail Server Identification................................9
4.2.1 Proposal.............................................9
4.2.2 Discussion...........................................9
4.3 Mail Server Communication.................................9
4.3.1 Proposal.............................................9
4.3.2 Discussion...........................................10
4.4 Mail Transfer.............................................11
4.4.1 Proposal.............................................11
4.4.2 Discussion...........................................11
4.5 Reliability...............................................12
5. Mail Relays...................................................12
5.1 Address Processing........................................13
5.2 Return Paths..............................................14
5.3 Economy...................................................15
5.4 Reliability...............................................16
6. The ARPA Mail Transition Plan: A Case Study...................16
6.1 Current Proposals.........................................16
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INDRA Note 1025 NIFTP-Based Mail
6.2 NIFTP and the Transition Plan.............................17
6.3 A Proposal................................................17
7. NIFTP-Based Mail in the UK....................................20
8. References....................................................20
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