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RFC 98

Updated by RFC 123

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

                        Logger Protocol Proposal

                          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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RFC 98                  Logger Protcol Proposal                 Feb 1971

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.


   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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RFC 98                  Logger Protcol Proposal                 Feb 1971

      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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RFC 98                  Logger Protcol Proposal                 Feb 1971

(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

   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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RFC 98                  Logger Protcol Proposal                 Feb 1971

      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

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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RFC 98                  Logger Protcol Proposal                 Feb 1971

    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

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.


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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RFC 98                  Logger Protcol Proposal                 Feb 1971

    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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RFC 98                  Logger Protcol Proposal                 Feb 1971

        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

       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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RFC 98                  Logger Protcol Proposal                 Feb 1971

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

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RFC 98                  Logger Protcol Proposal                 Feb 1971

        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.


        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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