rfc705.txt

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	    of the two foreign sockets.

Error CODES in REPLY

		Category	Type	   Meaning

		   3		 11	INDEX unknown
		   3		 12	PATH unknown
		   3		 13	CODE invalid

4:  MESSAGE								4e

Format

	MESSAGE INDEX PATH COUNT PAD TEXT

Use

This command is sent by either the Host or the FE to transmit data on the
specified path and index.

Parameters

	a)  MESSAGE is an 8-bit field with value 4.

	b)  INDEX is a 16-bit field specifying the index.


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	c)  PATH is an 8-bit field specifying the path.  Note that the value
	    0 is used in the broadcast option (see Section 3j).

	d)  COUNT is a 16-bit field specifying the number of bits of data
	    in the TEXT field.

	e)  PAD is an n-bit field, where n is an installation parameter.
	    It contains only padding (in the present protocol specification)
	    and can be used to enable the host to have the TEXT field start
	    on a convenient boundary.

	f)  TEXT is a field containing COUNT bits of data being transmitted
	    on this path.

Error CODES in REPLY

	Category	Type		Meaning

	   2		  1	This option not implemented	   
           3		 12	PATH unknown
	   3		 14	No connection opened in this direction
	   3		 15	PATH blocked at this time, resent later
	   3		 16	PATH suspended at this time, resent later
	   3		 17	PATH closed
	   3		 17	COUNT too large
	   4		  3	Error in transmitting data, resend command
	   4		  4	Data lost, resent command.

5:  INTERRUPT								4f
Format

	INTERRUPT INDEX PATH CODE
Use

This command is sent by either the Host or the FE.

Its most common use is to pass the information that a terminal user has
pressed his INT (or ATTN or Control-C) key, thereby requesting his 
applications program to quit what it is doing for him.[5]


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Parameters

	a)  INTERRUPT is a 8-bit field with value 5.

	b)  INDEX is a 16-bit field specifying the index.

	c)  PATH is an 8-bit field specifying the path on which the
	    INTERRUPT is transmitted.  Note that the value 0 is used in
	    the broadcast option (see Section 3j).

	d)  CODE is a 16-bit field.  It has no defined meanings as yet
	    and should contain 0.

Error CODES in REPLY

	Category	Type		Meaning

	   2		  1	This option not implemented
	   3		 11	INDEX unknown
	   3		 12	PATH unknown
	   3		 14	No connection opened in this direction
	   3		 15     PATH blocked at this time, resend later
	   3		 17	PATH closed.

6:  END									  4g

Format

	END INDEX PATH CODE

Use

This command is sent by either the Host or the FE, to terminate a connection.
If PATH is 0, then the index and all its paths are terminated, otherwise just
the specified path of the index is terminated.

Parameters

	a)  END is an 8-bit field with value 6.

	b)  INDEX is a 16-bit field specifying the index.


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	c)  PATH is an 8-bit field containing the path to be closed, or 0 if
	    the whole index is to be closed.

	d)  CODE is a 16-bit field indicating the reason for the closure:

		Value	     Meaning

		  0	Normal close
		  1	Retries exhausted
		  2	Foreign Host failure
		  3	Foreign IMP failure
		  4	Network failure
		  5	Local IMP failure.

	    The "Retries exhausted" code indicates that the FE has been 
	    retrying a transmission to the foreign host without success.

Error CODES in REPLY

	Category	Type		Meaning

	   3		 11	INDEX unknown
	   3		 12	PATH unknown
	   3		 13	CODE unknown
	   3		 15	PATH blocked at this time, resend later
	   3		 17	PATH closed.

7:  REPLY								   4h

Format

	REPLY INDEX PATH CODE

Use

This command is sent by both the Host and the FE to acknowledge receipt of 
every other type of command (including those on index 0, see Section 4) and/or
to unblock that particular direction of an opened path for the transmission
of another command.

Note that the INDEX and PATH fields contain exactly the same as those of the
command being replied to.


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Parameters

	a)  REPLY is an 8-bit field with value 7.

	b)  INDEX is a 16-bit field specifying the index.

	c)  PATH is a 8-bit field specifying the path.

	d)  CODE is a 16-bit field indicating the success/failure of the
	    command being REPLYed to, and the sender's readiness for more
	    commands on the same path.  It is divided into four subfields -
	    STATUS, COMMAND, CATEGORY, and TYPE.	

		1)  STATUS is 4 bits wide
	        
		    bit 0 (right-most) - READY
		    bit 1	       - NOT-READY
		    bit 2              - ACK
		    bit 3	       - NAK

	        ACK=1 indicates that the sender (of the REPLY) has accepted
		the command (being REPLYed to).  NAK=1 indicates that the
		sender has discarded the command (with the reason given by
		the settings of the other bits of the CODE field).

		NOT-READY=1 indicates that the sender (of the REPLY) is
		willing to receive an END or INTERRUPT on this path.  
		READY=1 indicates that MESSAGE commands will also be received.

		Normally only one REPLY command will be sent for each
		other command.  However MESSAGE, INTERRUPT, RESPONSE and
		invalid END commands can be replied to by a REPLY with
		ACK (or NAK)=1 and NOT-READY=1 and another REPLY, some
		time later, with READY=1. [6]

		The ACK and NAK bits are mutually exclusive and should
		never both be on simultaneously, and similarly the READY
		and NOT-READY bits.

		Note that the READY/NOT-READY bit settings are only
 		relevant when a path is open.


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		2)  COMMAND is 4 bits wide.  It indicates the command for
                    which this is a REPLY :

			Value	     Meaning

			  0	any of the following
			  1	BEGIN
			  2	LISTEN
			  3	RESPONSE
			  4	MESSAGE
			  5	INTERRUPT
			  6	END

		    The value 0 is defined for cases where a Host does not
		    wish to incur any overhead required to fill in the
		    non-zero value.

    		3)  CATEGORY is 3 bits wide.  It specifies the category of 
		    the error indicated by the ACK bit being off :

			Value	     Meaning

			  1	Command not recognized
			  2	Option not implemented
			  3	Invalid
			  4	Action failed.

		    Its value is relevant only when NAK=1.

		4)  TYPE is 5 bits wide.  It specifies which error occurred.
		    Its value is relevant only when NAK=1.  The possible values
		    and meanings for the various errors and their corresponding
		    CATEGORY subfield values are given under the description
		    of each command.

Sequencing								  4i

Once communication between the Host and the FE has been established and each 
side is "Ready for Business" (see Section 4b) the Host may at any time send
BEGIN or LISTEN commands for new indexes.  The FE will acknowledge a BEGIN or


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LISTEN with a REPLY and the index is then set-up providing that the REPLY
indicates no errors.  Other BEGIN or LISTEN commands for the new paths on the
same index may be sent at any time after the index is set-up.

The FE will send a RESPONSE command for each path on completion of its attempts
to fulfill the Host's instructions.  If an attempt failed (indicated by the
CODE field) then the path remains closed and another BEGIN or LISTEN for that 
path can be sent.  If the attempt was successful, then MESSAGE or INTERRUPT 
commands can be sent after the Host has REPLYED to the RESPONSE.

An INTERRUPT or END command may be sent on any opened path after receiving
a REPLY for the previous command on the same path in the same direction.  A
MESSAGE command may be sent if in addition the READY bit was on in the last
REPLY command.

New paths on the same index may be opened at any time after the index has
been set-up, or particular paths may be ENDed and then have new BEGINs or
LISTENs for them issued.  An index remains set-up, even if all its paths are
closed, until an END command with PATH=0 is issued for the index.

Communication between the Host and the FE is terminated by an END with INDEX=0
and this will abort any remaining open paths and indexes.

Broadcasting								  4j

Broadcasting is an optional feature of the protocol.  If it has been enabled
by the installation parameter, then the Host may send a MESSAGE or INTERRUPT 
command on a particular index with PATH=0.  This instructs the FE to send the 
data contained in the TEXT field of the MESSAGE command (or an interrupt) on
every network connection corresponding to an open path of the specified index.

This feature will only occur on MESSAGEs from the Host to the FE (since no
utilization of it in the other direction is envisaged).

A broadcast MESSAGE is replied to with one or two REPLYs in the same way
as any other MESSAGE command.  Flow control within the index is maintained
as if broadcast MESSAGEs were sent on a separate path, i.e., flow control
on other paths is not directly affected.


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Note that for a broadcast MESSAGE command the FE will perform translation
on the data for each path in accordance with the BEGIN or LISTEN which
initiated that path.  Thus, care must be taken when all paths of the
particular index do not have the same format on the Host side specified in 
their TRANS-TYPE (see Section 6b).

Index 0									  5

Introduction								  5a

Index 0 provides a control link between the Host and the FE, and thus has no
network connections directly associated with it.  The commands on this index
are used to establish and terminate the connection between Host and FE and to
control other indexes.

Path 0									  5b

Path 0 of Index 0 is used to pass global commands - i.e., those which do not
refer to any particular index or path.  The currently defined commands are :

	MESSAGE INDEX=0 COUNT PAD TEXT

	    where TEXT = COMMAND [PARM1] [PARM2]