Network Working Group
Request for Comments #98
Network Information Center #5744

Edwin W. Meyer, Jr.
Thomas P. Skinner
February 11, 1971

With the ARPA Network Host-to-Host Protocol specified and at

least partially implemented at a number of sites, the question of what

steps should be taken next arises. There appears to be a widespread

feeling among Network participants that the first step should be the

specification and implementation of what has been called the "Logger

Protocol"; the Computer Network Group at project MAC agrees. The term

"logger" has been commonly used to indicate the basic mechanism to gain

access (to "login") to a system from a console. A network logger is

intended to specify how the existing logger of a network host is to

interface to the network so as to permit a login from a console attached

to another host.

        To  implement  network  login   capability   now   seems   quite

desirable.In the first place, it is natural for Network participants to

wish to learn more about the remote systems in the immediate fashion

afforded by direct use of those systems. In the second place, the

technical problems introduced by remote logins are probably less complex

than those involved with such further tasks as generalized file

transfer; thus, a Logger Protocol could be implemented relatively

quickly, furnishing additional impetus and encouragement for taking

still further steps.

In order to furnish at least a basis for discussion (and at most

an initial version of a Logger Protocol), we have prepared this

document, which attempts to present a minimal set of conditions for

basing a Logger Protocol. This proposal covers only the mechanism for

accomplishing login. What occurs following login is not discussed here,

because we feel more experimentation is necessary before any protocol

for general console communication can be established as standard. In its

absence, each site should specify its own experimental standards for

console communications following login.

Some of the points raised in this document have already reached

a certain level of consensus among network participants while at least

one point is rather new. It should be clearly understood, however, that

we feel regardless of the disposal of particular issues, Networkwide

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agreement should be reached as soon as possible on some general

protocol. This is all the more desirable in view of the fact that it is

quite likely that certain points which should be covered in this

protocol will only become apparent during the course of implementation;

therefore, the sooner a common basis for implementation can be reached,

the sooner a more rigorous protocol can be enunciated.

Before turning to 1) a discussion of the points with which to

decide the protocol should deal, and 2) specifications for the current

state of the protocolm we feel that we should acknowledge the

consideration that a case could be made for avoidingthe difficulty of

generating a Logger Protocol by simply declaring that each host may

specify its own, perhaps unique, preferences for being approached over

the Network. Although such a course is certainly possible, it does not

seem to us to be desirable. One reason for avoiding such a course is

simply that following it hamper general Network progress, in that

adressing the task of interfacing with some 20 systems is bound to more

time-consuming than to interface with "one" system, even though each

indivudual one of the former, multiple interfaces might be in some sense

simpler than the latter, single interface. Another consideration is less

pragmatic, but nonetheless important: agreement on a common protocol

would tend to foster a sense of Network "community", which would tend to

be fragmented by the local option route. After all, the Host-to-Host

Protocol could have been handled on a per-host basis as well; assumedly,

one reason why it has not had something to do with similar, admittedly

abstract considerations.

Context

Structurally, the mechanism serving to login a user over the Network

consists of two parts, one part at the using host, the other at the

serving host. The using or local host is the one to which the users

typewriter is directly connected; it contains a modulewhich channels and

transforms communications between the Network connection and the

typewriter. The serving or foreign host provides the service to be used;

it contains programming that adapts the logger and command system to use

through the Network rather than a local typewriter.

There are three different phases to a login through the network.

1 During the connection phase the users console is connected to

the serving logger through the network. This is, of course, the most important phase from the protocol viewpoint.

2 The second or dialog phase consists of a sequence of exchange

between the user and the logger that serves to identify the user and verify his right to use the system. In some hosts, this phase may be minimal or non-existent.

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3 The admission phase occurs after the user has successfully

completed the login dialog. It consists of switching his network typewriter connections from the logger to an entity providing a command processor of some sort. In some hosts this switching may be totally conceptual; in others there may be a real internal switching between entities.

The Connection Phase

The issues involved in specifying a protocol for implementing

login can be separatedintop two major parts: how to establish and

maintain the network connection between the typewriter and the logger,

and how to conduct a dialog after the connection is made. The first part

is called the Initial Connection Protocol by Harlem and Heafner in RFC

80 It in turn consists of two subparts: how to establish a connection

and how and when to destroy it.

We endorse the proposal for establishing a connection made in

RFC 80, which we summarize briefly for convenience. It is a two-step

process utilizing the NCP control messages to effect a connection

between the logger and the console of a potential user. First, the user

causes the hosts NCP to send out a "request for connection" control

message destined for the serving hosts loggers contact socket. The two

purposes of this message are to indicate to the logger that this user

wishes to initiate a login dialog and to communicate the identifiers of

the and send socket he wishes to operate for this purpose. The logger

rejects this request to free its contact socket. As the second step the

logger choses two sockets to connect to the user's sockets, and

dispatches connection requests for these. If the user accepts the

connection within a reasonable period, the connection phase is over, and

the dialog phase can begin. If the user does not respond, the requests

are aborted and the logger abandons this login attempt.

There is another part to an NCP: when and how to disconnect.

There are two basic situations when a logger should disconnect. The

first situation may arise of the serving host's volition. The logger may

decide to abandon a login attempt or a logged-in user may decide to log

out. The second situation may be due to the using host's volition or

network difficulties. This situation occurs when the serving host

receives a "close connection" control message or one of the network

error messages signifying that further transmission is impossible. This

may happen for either the "read" or the "write" connection,

Disconnecting involves both the deletion of the network connections and

the stoppage of any activity at the serving host related to that user.

If the login is in progress, it should be abandoned. If the user is

already logged in, his process should be stopped, since he no longer has

control over what it is doing. This is not intended to restrict absentee

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(i.e. consoleless) jobs.

The Dialog Phase

The second major part other than getting connected is how to

conduct the login dialog. This resolves itself into two parts: what to

say and in what form to say it. The login dialog generally consist of a

sequence of exchanges, a prompting by the logger followed by a user

reply specifying a name, a project, or password. However, exactly what

information is desired in what sequence is idiosyncratic to each host.

Rather than attempt to specify a standard sequence for this dialog, we

have taken the approach that each host may specify its own sequence, so

long as it is expressible as an exchange of messages in a basic

transmission format. A message is a set of information transmitted by

one of the parties that is sufficient for the other party to reply.By

host specification, either the logger or the user sends sends the first

message of the dialog. After that, messages are exchanged sequentially

until the dialog is completed. In this context "message" has no relation

to "IMP message".

The other issue involved in the login dialog is the format for

transmitting a message. We propose that it be transmitted as a sequence

of ASCII characters (see Specificarions) in groupings calle transaction

blocks.

1 Character Set, We feel that there should be a standard

character set for logging-in. The alternative, requiring a using host to maintain different transformation between its set and of each serving host, is a burden that can only narrow the scope of interhost usage, The character set proposed, ASCII is widely used standard. Each host must define a transformation sufficient to transform an arbitrary character sequence in the host's code into ASCII and back again, without any ambiguity, The definition of ASCII sequences to express characters not contained in ASCII is appropriate.

2 Transaction Blocks. A message is transmitted as an arbitrary

integral number of transaction blocks. A transaction block consists basically of a string of ASCII characters preceeded by a character count. (It also contains a code field. See below.) The count is included as an aid to efficiently assembling a message. Some systems do not scan each character as it is input from the console. Rather, such systems have hardware IO controllers that place input characters into a main memory buffer and interrupt the central processor only when it receives an "action" character (such as "newline"). This reduces the load on the central processor. Because such a hardware facility is not available for interpreting

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network messages this scheme is proposed as a substitute. It helps in two ways. First, a system need take no action until it receives all characters specified in the count. Second, it need not scan each character to find the end of the message. The message ends at the end of the of a transaction block.

Other Issues

There are several other issues involved in the area of remote

logins which we feel should be raised, although most need not

necessarily have firm agreements reached for an intial protocal.

1 "Echoplex". Echoplex is a mode of typewriter operation in which

all typed material is directed by the computer. A key struck by a user does not print directly. Rather the code is sent to the computer, which "echoes" it back to be printed on the typewriter. To reduce complexity, there is to be no option for network echoplexing (for the login only). A using system having its typewriters operating in echoplex mode must generate a local echo to its typewriters. However, a serving system operating echoplexed should suppress the echo of the input during the login phase.

2 Correction of Mistakes. During the login dialog the user may make

a typing mistake. There is no mistake correction ecplicitly proposed here. If the message in error has not yet been
transmitted, the user can utilize the input editing conventions of either the using or the serving host. In the first case, the message is corrected before transmission; in the second, it is corrected at the serving host. If the user has made an
uncorrectlable mistake, he should abort the login and try again. To abort, he instructs the local (using) host to "close" one of the connections. The connections are disconnected as specified in the Initial Connection Protocol.

3 "Waiting". It may happen that the user may get into a login dialog

but for some reason does not complete it. The logger is left waiting for a response by the user. The logger should not wait indefinitely but after a reasonable interval (perhaps a minute) abort the login and "close" the connections according to the provisions of the Initial Connection Protocol.

4 Socket Assignments. The Initial Connection Protocol does not

specify the ownership of the sockets to be used by the logger in connecting to the user. (The use code field of the socket identifier determines ownership.) The sockets may belong to the logger or may have an arbitraryuser code not used by another process currently existing at the serving host. Under this initial

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scheme, it is not possible to implement administratively assigned user codes, because the logger must assign permanent sockets before the identity of the user is verified. A future connection protocol can avoid this problem by implementing a socket connection as a part of the admission phase. The logger would talk to the user over the logger's sockets. Following identification it would transfer the connections to the sockets belonging to the user.

5 General Console Communications. A companion paper under

preparation outlines a protocol for general console communcations between hosts. This paper will seek to adress most of the problems associated with typewriter like communications. This includes discussion of full and half duplex, character escapes, action characters and other pertinent topics. Such a protocol might not be suitable for all terminals and host systems but would include solutions to problems for many. It is not
intended as a monolithic standard, but rather as a recommendation for those sites who wish to implement a common protocol. The important point is that we feel quite a bit of actual network usage is required before all the problems are better understood. This is a prerequisite for devising a standard.

SPECIFICATIONS

Initial Connection Protocol - Connection Phase

The following sequence is as presented in RFC 80. It is restated here for completeness.

1 To intiate contact , the using process requests a connection of

his receive socket (US) to a socket (SERV) in the serving host. By convention, this socket has the 24-bit user number field set to zero. The 8-bit tag or AEN field is set to one indicating the socket gender to be that of a sending socket. There is no restriction on the choice of the socket US other than it be of of the proper gender; in this case a receive socket. As a result the using NCP sends:

User -> Server

                   8        32          32         8
                +-----+------------+------------+-----+
                | RTS |     US     |   SERV     |  P  |
                +-----+------------+------------+-----+

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over the control link one, where P is the receive link assigned by the user's NCP.

2 The serving host now has the option of accepting the request for

connection or closing the the connection.

a If he sends a close it is understood by the user that the

foreign host is unable to satisfy a request for service at this time. The serving host's NCP would send:

Server -> User

                   8        32          32
                +-----+-----------+------------+
                | CLS |    SERV   |     US     |
                +-----+-----------+------------+

with the user's NCP sending the echoing close:

User -> Server

                   8        32          32
                +-----+-----------+------------+
                | CLS |     US    |    SERV    |
                +-----+-----------+------------+

b If the serving host is willing to provide service it will

accept the connection and immediately close the connection. This results in the the serving host's NCP sending:

Server -> User

                   8        32          32
                +-----+-----------+------------+
                | STR |    SERV   |     US     |
                +-----+-----------+------------+

                   8        32          32
                +-----+-----------+------------+
                | CLS |    SERV   |     US     |
                +-----+-----------+------------+

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with the user's NCP sending the echoing close. It sends:

User -> Server

                   8        32          32
                +-----+-----------+------------+
                | CLS |     US    |    SERV    |
                +-----+-----------+------------+

It should be mentioned that the echoing closes are required by the host-to-host protocol and not by the logger initial connection protocol.

Character Set

The character set used in conducting the login dialog is

standard ASCII as documented in "American National Standard Code for

Information Interchange", ANS X3, 4-1968, American National Standard

Institute, October, 1968. A logger at a serving host may demand any kind

of input that can be expressed as a string of one or more ASCII

characters. It similarly, it may output any such string.

All ASCII characters are legal, including the graphics and

control characters. However, it is proposed that the only standard way

of indicating the end of a console line be the line feed character

(012). This is in accordance with an anticipated change to the ASCII

standard.

Currently the ASCII standard permits two methods of ending a

line. One method defines a single character, line feed (012), as

incorporating the combined functions of line space and carriage return

to the lefthand margin. The second method, implicitly permitted by

ASCII, uses the two character sequence line feed (012) and carriage

return (015) to perform the same function.

There is a proposal that the ASCII standard be changed to

include a return to the left-hand margin in all vertical motion

characters of at least one full space (line feed, vertical tab and new

page). This will disallow the dual character sequence to end a line.

It is suggested that a character in a hostst character set not

having any ASCII equivalnet be represented by the ASCII two character

sequence ESC (033) and one of the ASCII characters. Each host should

publish a list of the escape sequence it has defined.

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Transaction Block Format

All textual messages exchanged between user and logger are to

consist of one or more "transaction blocks". Each transaction block is a

sequence of 8-bit elements in the following format:

                <code> <count> <char1> ... <charn>

<code> is an 8-bit element that is not interpreted in this

protocol. In the proposed general console communications protocol, this field specifies communication modes or special characteristics of this transaction block. Here

           <code> is to be zero.

<count> is an 8-bit element that specifies the number of character

elements that follow in this transaction block. It is interpreted as a binary integer which has a permissible range between 0 and 127. The most significant bit is zero.

<chari> is an 8-bit element containing a standard 7-bit ASCII

character right-adjusted. The most significant bit is zero. The number of <chari> in the transaction block is governed by the <count> field. A maximum of 127 and minimum of zero characters are permitted in a single transaction block.

The most significant bit of each of these elements is zero,

effectively limiting each of these elements to seven bits of

significance. The reason for doing this is twofold: the eighth bit of

the <chari> elements is specifically reserved for future expansion, and

it was desired to limit all the elements so as to permit certain

implementations to convert the incoming stream from 8-bit elements to

7-bit elements prior to decoding.

With one exception, there is to be no semantic connotation

attached with the division of a logger-user message into one or more

transaction blocks. The character string comprising the message to be

transmitted may be divided and apportioned among multiple transaction

blocks according to the whim of the sending host. If less than 128

characters in length, the message may be sent as a single transaction

block. The exception is that separate messages may not appear in the

same transaction block. That is, a message must start at the beginning

of a transaction block and finish at the end of one. Note also that

there is no syntactic device for specifying the last transaction block

of a message. It is presumed that the logger end user both have

sufficient knowledge of the format to know when all of a message has

arrived

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Note that the first 8-bits of data transmitted through a newly

established connection must be a type code as specified in Protocol

Document 1. This type code must be sent prior to the first transaction

block and should be discarded by the receiving host.

Acknowledgments

Robert Bressler, Allen Brown, Robert Metcalfe, and Michael

Padlipsky contributed directly to the establishment of the ideas

presented here. Thanks are due Michael Padlipsky and others for

editorial comments.

[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Carl Moberg 1/98 ]

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