Network Working Group                                          J. Wong
Request for Comments: 117                                         UCLA
NIC #5826                                                 7 April 1971

                 Some Comments on the Official Protocol

               [Categories B.1, C.1, C.2, C.3, C.4, C.5]

 Document No. 1 and NWG/RFC No. 107 gave a very detailed description of
connection establishment, connection termination and flow control over
the Network.  Throughout the implementation of the NCP it was
discovered that the handling of ERR control commands, messages of
types other than 0 (regular), 4 (nop), and 5 (rfnm), and messages with
the From-imp bit on are not well discussed so that problems arise when
they occur.

The Protocol is not complete if the above situations are not handled
clearly, and the Host-Host Protocol Glitch Cleaning Committee should
take this into consideration.  In this document, experience with these
unfavorable situations and suggestions for handling are given:

1.  ERR Control Commands

In Document No. 1, the following error conditions are described:

     a.  Illegal Op. code.
     b.  End of message encountered before all expected parameters.
     c.  Bad socket polarity within commands.
     d.  Link number not in the range of 0 <= L < 32.
     e.  A request (other than RTS/STR) on a non-existent socket.
     f.  A request (ALL, GVB, RET, INR, INS) on a non-existent link
     g.  Transmit over non-existent link number.

Other error conditions are:

     h.  A request (GVB, RET, INR, INS) on an existent link, but
         connection is not established.

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     i.  Transmit over an existent link, but connection is not
     j.  ALL or GVB on a send connection.
     k.  RET on a receive connection.
     l.  An attempt to send more than the allocated number of bits or
     m.  ECO, ERP, ERR commands do not have the defined number of bits
        of data.

In Document No. 1, each site is supposed to document the information
on their ERR command.  No one has done that so far, and the main
reason is we are not sure of what information is important.  In
NWG/RFC No. 107, the text portion of the ERR Commands is decided to
have a fixed length of 80 bits because 80 bits is long enough to hold
the longest non-ERR command.  In some of the above error conditions,
more information than the command itself is desirable.  It was noted
that these error conditions arise very often in the experimental stage
of the NCP.  If every NCP is operating properly, none of them should
ever occur.  The ERR commands are therefore, an excellent debugging
tool for the protocol.  So it is desirable to define a set of possible
error conditions, and for each condition, define a set of arguments in
the corresponding ERR command so that enough information is given to
tell what's wrong.  The suggested arguments for each situation (a - m)
are listed below:

     a.  1.  Op. code in error.
         2.  Part of message following op. code (A maximum of 72

     b, c, d, e, f.
         1.  The command in error.

     g.  1.  Link number,
         2.  Beginning of message (A maximum of 72 bits),

     h.  1.  Command in error.
         2.  Socket numbers for the connection.
         3.  Status of the connection.

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     i.  1.  Link number,
         2.  Beginning of message (A maximum of 72 bits),
         3.  Socket numbers for the connection.
         4.  Status of the connection.

     j, k.
         1.  Command in error.
         2.  Socket numbers for the connection.

     l.  1.  Link number.
         2.  Beginning of message (A maximum of 72 bits).
         3.  Number of bits sent.
         4.  Number of bits allocated.
         5.  Number of messages allocated.

     m.  1.  The Command in error.

Each of the ERR commands should have a special error code (8 bits) to
tell the error type, an 80-bits field to store the command in error,
and additional fields for socket numbers and other information.

2.  Imp-to-host messages of types other than 0, 4, and 5.

From the BBN report 1822, the following message types will cause
difficulty in the implementation of the Protocol.

     a.  Type 2 - Imp going down.
     b.  Type 7 - Destination host or imp dead.
     c.  Type 9 - Incomplete transmission.

It was discovered that on sending a message to a site whose imp or
host is not running, a Type 7 or Type 9 message is returned.  This
can happen in two situations:

     a.  The foreign host or imp is not up at all.
     b.  Some connections have been established, and the foreign host
         or imp goes down.

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The first situation does not cause much problem because the NCP has no
entry in its table corresponding to this site.

The second situation is more complicated, because if the table entries
for the connections to the dead host are not cleared, by the time this
host comes up again, the table entries still exist and the information
will be very misleading.  One suggestion to solve this problem is:

     a.  Whenever a NCP comes up, it send a RESET Control Command to
         every other site.
     b.  Associated with each site there is a bit called the up-bit.
         If a RESET-reply command is received from some site, the
         corresponding up-bit is set to 1.  Race condition can be
         avoided by ignoring all messages from sites which have not
         returned the RESET-reply command.
     c.  Messages can only be sent to sites with the up-bit on.
     d.  If a RESET control command is received, the Table entries
         corresponding to the site are cleared, a RESET-reply is
         immediately returned, and the up-bit for the site is set.
     e.  The up-bit is reset to 0 when a Type 7 or Type 9 message is
         received from a particular site.

The above solution will handle the Type 2 messages also.  When a host
receives a Type 2 message, there is no way for it to tell the other
NCP's that its imp is going down.  Subsequent messages to this host
will return a Type 7 or 9 message.  The solution above will then come
into effect.

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With the introduction of the RESET and RESET-reply Control command,
the ECO and ERP control command are no longer important and should be

3.  Messages with the From-imp bit on.

These kinds of messages are not discussed at all.  Some statistical
measurements have been made on messages with the From-imp bit on.  We
should classify what these messages represent.

       [ This RFC was put into machine readable form for entry ]
         [ into the online RFC archives by Randy Dunlap 4/97]

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