Username / Password : Request For Comments

RFC Number : 915

Title : Network mail path service.

Network Working Group Marc A. Elvy
Request for Comments: 915 Harvard University
Rudy Nedved
Carnegie-Mellon University
December 1984



This RFC proposes a new service for the ARPA-Internet community and
requests discussion and suggestions for improvements. Distribution
of this memo is unlimited.


The network mail path service fills the current need of people to
determine mailbox addresses for hosts that are not part of the
ARPA-Internet but can be reached by one or more relay hosts that have
Unix To Unix Copy (UUCP) mail, CSNET mail, MAILNET mail, BITNET mail,

Anyone can use the service if they have TCP/TELNET to one of the
hosts with a mail path server.


Currently many hosts that are not connected to the ARPA-Internet
network can send mail to and receive mail from the ARPA-Internet
community. The ARPA-Internet community sends mail using mailbox
addresses of the form 'user@host' or 'local-part@domain' [1, 5]. In
an effort to provide service to hosts not connected directly to the
ARPA-Internet, mail maintainers have used the feature that the
'local-part' of the mailbox address is locally interpreted to imbed
specially encoded mail routing or relaying information. These
encoded mailbox addresses have a variety of forms and have become
common practice. For example:






It is important that people be able to communicate, but it is clear
from the rampant confusion and frustration that something must be

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provided to make it easier for people to address mail to
non-ARPA-Internet hosts. The result, for a variety of reasons, has
been the work and development of the Domain Name system and
facilities [2, 3, 7, 9], and it is expected to make mailbox addresses
be as simple as the current ARPA-Internet mailbox format (e.g.,

How do people discover the special encoded addresses for
non-ARPA-Internet host mailboxes until the domain name system is
working and covering the majority of hosts in the mail world? The
proposed solution to this problem is to provide a network service for
the ARPA-Internet and a mail service for the non-ARPA-Internet hosts
that, given a host and an optional addressing system or communication
protocol or some other piece of information, supplies the mailbox
address format for sending mail to that host. For example,
'nedved@Carnegie.MAILNET' would be translated by the server to
'nedved%Carnegie.MAILNET@MIT-MULTICS.ARPA'. This memo covers the
proposed network service.


Unless otherwise noted, all numbers are in decimal.

The term 'host', as used in this document, describes one computer
system which may have more than one name associated with it. It may
have a name for each network or mail connection it supports and may
have several nicknames or aliases for the computer and/or for each
set of network names that the computer has acquired.


The network service is a connection based application on TCP [4]. A
server listens for TCP connections on the assigned port of 117 [8].
It responds to the connection with a coded greeting message and waits
for a command line. For each command line sent to the server, the
server will respond with a coded message. The special command QUIT
causes the server to respond with a coded closing message and closes
the connection.

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One of the goals is to provide the service to as many ARPA-Internet
hosts as possible. In the current ARPA-Internet, experience has shown
that software people first implement TELNET/TCP [6] before any other
network application or protocol. Therefore, it is a sub-goal that
people be able to access the service using available programs (with
minimal modifications) that implement TELNET/TCP. Therefore,
TELNET/TCP on port 117 will work correctly. The server understands
TELNET options but refuses all option negotiations that disagree with
the NVT characteristics defined by the TELNET protocol (see [6]),
does not echo, and expects command lines to end with (ASCII
code 13 (octal 15) followed by code 10 (octal 12)). Character
echoing and line editing is expected to be handled by the user host
for the benefit of the user.

Mail systems and other programs are also expected to be able to
access and understand the service. Each command reply can have
multiple line responses with text understandable by the novice user.
Each command is encoded so as to make it easy for a program to parse
the lines and extract interesting information, such as whether the
operation was successful.


Given the developing nature of the protocol and its intent, the
command lines are composed of a command (case ignored) followed by
white space, the argument(s) and a . The white space is
required if any arguments are supplied but the arguments are
optional. White space following the command and any optional
arguments are ignored.

:= [ ] []

:= [] | |

Coded response lines have the rigid format of a 3-digit decimal code
followed by a space or a dash followed by text composed of characters
within the ASCII range 32 to 126 (octal 40 to 176) with at the
end of the line. The dash after the 3-digit code indicates at least
one more response line will be supplied while the space indicates the
current response line is the last one.


:= | '-'

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:= ASCII characters in the range 32 to 126.

Some of the successful response text to certain commands have rigid
formats so programs can extract path information. The commands that
have format restrictions are clearly noted and the response format is
documented with the command.

The response codes are in the range from 200 to 599 inclusively. The
following paragraphs provide the break down for each digit.

The first, most significant, digit is the success indicator. It
breaks down into the simple success and total failure responses but
includes the ability to communicate a temporary failure condition and
the need for further information that has worked so well for SMTP [5]
and other similiar protocols. The codes are:

2xx Positive reply.

3xx Intermedate reply. Positive acknowlegement but more
information is neccessary.

4xx Temporary error. Try again later.

5xx Permanent error. Do not retry.

The second digit is used to classify the response to provide a flavor
for certain types of success. The flavor is apparent in providing the
response on whether a host name is known by a domain name server or
not. The codes are:

x0x Command related response.

x1x Connection related response.

x2x Database related response.

x3x Domain transition related response.

x4x Data added response.

x5x Data deleted response.

x6x Data modified response.

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The minimum implementation is the support of three commands: HELP,
PATH and QUIT. The HELP command provides some level of documentation
and possibly lists the known addressing or communication protocols.
The PATH command takes as a required argument a user name or id
followed by a '@', followed by a domain style host name whose domain
components may be an addressing protocol, a communication
environment, or an unofficial or colloquial domain.

S: (server listens on port 117)
U: (user connects to port 117)
S: 210-Welcome to the CMU network mail path service.
S: 210 Type 'HELP' for help.
U: help
S: 200-The server currently knows about the following mail worlds:
S: 200-Use the PATH command with '' to get the
S: 200 ARPA-Internet mail address.
U: path root@inria.uucp
S: 220 philabs!mcvax!inria!root@SEISMO.ARPA
U: quit
S: 211 Bye bye.
S: (server closes connection)


The protocol is designed to provide a flexible but conservative
mechanism for providing responses and adding experimental or extended

The server responds with a message indicating the status of the
server and optional information.

210 Greeting message indicating the server is ready.

410 The server is down for some unknown reason for a short

510 The server is unavailable.


The server can respond with general help information about the
server, about the specific topic described by 'arg', or it can

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indicate that something is temporarily wrong with the HELP
facility. It is strongly recomended that the general HELP
command documentation be implemented and expanded.

200 General or specific documentation given.

220 Documentation given from a database.

421 Service temporarily unavailable.

501 Command not implemented or topic not known.


The server normally responds with either the mail path that
will work for the given mailbox address or indicates the domain
style host name is unknown. If the database is in transition or
inconsistent, a temporary or permanent error can be supplied.

220 Rigid format route given.

230 Rigid format route given. Domain servers should be

420 Database problems. Try again later.

501 Invalid argument form or null argument given.

520 No such host found in database.

521 Host name is ambiguous.

When a route is supplied with the 2xx success responses. It has a
fixed format with a one-line response. The format is as follows:


The 'local-part' and 'domain' components are defined under the
SMTP protocol [5] and are intended to be used over SMTP


Respond and close the server down.

211 Close the connection down.

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One special code is reserved and is used for a special case. The code
is 412 and is sent when the server has been waiting for a response
for more then 2 minutes and has decided to timeout the connection.
After the '412 ' is sent, the server may close or
possibly abort the connection.

Because of the somewhat experimental nature of the server, additional
commands are expected to be added as they become needed. No
restrictions are placed on the names of these experimental commands
other then the must not conflict with the basic commands and are not
allowed to be abbreviated (i.e., 'SEAR' can not be used for


It is important to understand that the server is an aid to users that
may have minimal amount of information about the host. Therefore the
PATH command takes domain style host names that may be complete or
incomplete specifications for the host and may be common or
colloquial domain names. The servers look through the entire database
for anything that matches and try to find the best answer
disregarding any local domain information. If several hosts have the
same nickname or alias and lack distinguishing domain components, the
server returns an error response containing all of the hosts found.
Some implementation may even break down the host name and indicate in
error messages that even though it did not find the host, it found
something else that might be what the user wanted.


As mentioned previously, the mail path service is not intended to be
a replacement or a parallel service to the domain name system. It is
a stop gap measure and, when most of the domain name system is in
place, will probably be disabled on some or most of the hosts with
the service.

Mail systems should check the domain name servers for the specified
host before trying a mail path server. The mail path servers should
be modified when one or more domain servers are in place to check if
a host is part of the domain system and to generate an error or an
indication (but still include the path information) if a host is
found to be a part of the domain system.

The names used by the mail path servers have no official standing in
the ARPA-Internet community and have colloquial origins. The domain
name components are based on the adminstrative entities involved
whereas many of the current unofficial common domain style names for

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non-ARPA-Internet hosts are based on the protocol used, the relay
host used, or some acronym that someone dreamed up. Only a few of
the current domain style names that are privately in use are expected
to be used by the ARPA-Internet community when the domain name
service is in use by the majority of the ARPA-Internet community.


The greatest problem with the new service, as implemented, is that it
reports paths from the service host rather than from the user's host.
This is due to the nature of software. It would be more convenient
if it reported a correct path from the caller's host, but this would
require a different method of database management (a method which
could quickly compute the path from the caller's machine or a machine
which would be willing to keep updated databases for each host (which
is impractical)).

Two minor problems exist with the database used by the software. Many
relay hosts exist in several different protocol or addressing name
spaces but under different names. The current software cross
referencing for the multiple protocol relay hosts is done by hand,
but, given the seeming reliability of these relay hosts, the problem
does not appear to be significant. The second problem is that the
data should be collected from the actual relay hosts to ensure
correctness, but in many cases this is impossible.


Find a route to CMU-CC-TE in the CARNEGIE part of MAILNET for user id

S: (server listens on port 117)
U: (user connects to port 117)
S: 210-Welcome to the CMU network mail path service
S: 210 Type 'HELP' for help.
U: quit
S: 211 Bye bye.
S: (server closes connection)

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Find a route to a host which has an unknown addressing system or
communication protocol and for which the name may be an alias:

S: (server listens on port 117)
U: (user connects to port 117)
S: 210-Welcome to the CMU network mail path service
S: 210 Type 'HELP' for help.
U: path mss@dartvax
S: 220 mss%dartmouth@CSNET-RELAY.ARPA
U: quit
S: 211 Bye bye.
S: (server closes connection)

Find a route to a host that is known by a very long domain style name
but is not in the current ARPA-Internet host tables:

S: (server listens on port 117)
U: (user connects to port 117)
S: 210-Welcome to the CMU network mail path service
S: 210 Type 'HELP' for help.
U: path
S: 220 rob%vax1.cent.lanc@UCL-CS.ARPA
U: quit
S: 211 Bye bye.
S: (server closes connection)

Find a route to a host without any additional information and the
name is discovered to be ambiguous:

S: (server listens on port 117)
U: (user connects to port 117)
S: 210-Welcome to the CMU network mail path service
S: 210 Type 'HELP' for help.
U: path brad@pitt
S: 521-Several hosts found under the name of 'pitt', try one of:
S: 521-brad@pitt.UUCP
S: 521-brad@pitt.CSNET
U: path brad@pitt.CSNET
S: 220 brad%pitt@CSNET-RELAY.ARPA
U: quit
S: 211 Bye bye.
S: (server closes connection)

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The original protocol was documented by Marc Elvy for a server that
he and Alan Langerman built. The server used the pathalias software
created by Steve Bellovin, as modified by Peter Honeyman and Robert
T. Morris, to maintain the host to host connection database. The
software provided a way for people to make sense out of the jungle of
UUCP hosts. The Info-Nets@MIT-MC mailing list, created and maintained
by Robert Krawitz, made the CMU and Harvard mail path projects aware
of each other and the people on the list provided many of the mail
relay databases that are in use by the mail path servers. The
original server may be accessed through TCP port 117 on
-- the 'pathto' program that runs under 4.2BSD UNIX may be obtained
as a front end to the server from RFC915@HARVARD.ARPA.

The current protocol scope was changed by Rudy Nedved to cover
BITNET, CSNET, MAILNET and other 'mail networks' and further refined
by Marc Elvy, Alan Langerman and others.

Comments should be sent to RFC-915@HARVARD.ARPA or mailed (via the
U.S. Postal Service) to:

Marc A. Elvy
108 Aiken Computation Laboratory
33 Oxford Street
Harvard University
Cambridge, MA 02138

(617) 495-5849

Rudy Nedved
Department of Computer Science
Carnegie-Mellon University
Schenley Park
Pittsburgh, PA 15213

(412) 578-7685

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

[2] Mockapetris, P. 'Domain Names - Concepts and Facilities'.
RFC 882, USC/Information Sciences Institute, Novemeber, 1983.

[3] Mockapetris, P. 'Domain Names - Implementation Specification'.
RFC 883, USC/Information Sciences Institute, Novemeber, 1983.

[4] Postel, J. 'Transmission Control Protocol- DARPA Internet
Program Protocol Specification'. RFC 793, USC/Information
Sciences Institute, September, 1981.

[5] Postel, J. 'Simple Mail Transfer Prootcol'. RFC 821,
USC/Information Sciences Institute, August, 1982.

[6] Postel, J., and J. Reynolds. 'Telnet Protocol Specification'.
RFC 854, USC/Information Sciences Institute, May, 1983.

[7] Postel, J. 'Domain Name System Implementation Schedule'.
RFC 897, USC/Information Sciences Institute, Feburary, 1984.

[8] Reynolds, J., and J. Postel. 'Assigned Numbers'. RFC 923,
USC/Information Sciences Institute, October, 1984.

[9] Su, Z., and Postel, J. 'The Domain Naming Convention for
Internet User Applications'. RFC 819, SRI International,
August, 1982.

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