NWG/RFC# 695                       MCK 5-JUL-75 15:38  32908
Official Change in Host-Host Protocol

                                      Mark Krilanovich     1

         Official Change in Host-Host Protocol             2

This is an official change to the Host to Host
protocol, this document should be filed with the
protocol specification (nic -- 8246,) in the Current
Network Protocols Notebook (nic -- 7104,).                 3

                                         -- Jon Postel    3a

This document corrects an ambiguity in the current
host-host protocol, concerning the ERR command.
Paragraph "f", page 35, of NIC 8246 defines the
meaning of an ERR command with error code of 5 to be
"socket (link) not connected".  The error code is
stated to apply to two cases, one in which a control
command other than STR or RTS refers to a socket that
is neither fully open nor fully closed, and the other
in which a (non-control) message arrives over a link
not being used for a connection.                           4

The difficulty arises from the fact that the contents
of the "data" field of the ERR command has distinctly
different formats in the two cases.  In the first, it
is a host-host command, and in the second it is a
message header.  There is no reliable way for the code
in the NCP receiving the ERR command (or a human
reviewing an error log) to distinguish between the two
cases, and therefore fullest use cannot be made of the
ERR command.                                               5

The two cases are now defined to have different error
codes.  In addition, a new error code is defined,
meaning "invalid host leader received".  Therefore,
paragraph "f" under "ERR - Error detected" is now
replaced by the following:                                 6

f.  Request on a non-open socket (Error code = 5)          7

NWG/RFC# 695                       MCK 5-JUL-75 15:38  32908
Official Change in Host-Host Protocol

   A request other than an STR or RTS was made for a
   socket (perhaps referenced by link number) that is
   not party to an fully established connection.  The
   socket's inappropriate state could either be that
   only one RFC has been sent (not yet open) or that
   only one CLS has been sent (not yet closed).  The
   "data" field contains the command in error; the
   value of any fill necessary is zeros.                  7a

g.  Message on an unknown link (Error code = 6)            8

   A message was received over a user link which is
   not currently being used for any connection.  The
   contents of the "data" field are the message header
   followed by the first eight bits of text, if any,
   or zeros.                                              8a

h.  Invalid host header (Error code = 7)                   9

   A message was received either over the control link
   or a valid user link that had a host header with
   invalid format. Examples of when this subtype would
   be appropriate are the following:  the M1 or M2
   fields were non-zero, the byte size was invalid
   (not 8 for a control link, zero for any link), or
   the declared length (byte size times byte count)
   exceeded the actual length.  The contents of the
   "data" field is the message header padded with
   eight bits of zeros.                                   9a

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