Anpois.641
net.lan
utzoo!decvax!duke!harpo!npois!srk
Wed Mar 31 14:09:58 1982
Service Specification
       I am a member of	the IEEE project 802 High Level	Interface
       subcommittee (HILI).  One of our	concerns is a specification
       of the service provided by the link layer to the	network
       layer.  Our model of interface interaction is based on
       primitives.  These are discrete,	instantaneous interface
       events which convey the information required in order to
       provide a particular service.  This is the same as the model
       that ISO	uses in	their service specifications.  However,	we
       differ from ISO in several important ways, and I	would like
       to solicit comments from	the newsgroup about them.

       The ISO (ISO/TC 97/SC 16	N697) transport	service
       specification uses the "four arrow model" of service
       primitive interaction.  For example, the	user layer passes a
       "CONNECT.request" primitive to the serving layer	to request
       that a connection be set	up.  The serving layer then passes
       an "CONNECT.indication" primitive to the	remote user layer
       to indicate the connection attempt.  The	remote user layer
       evaluates this and then passes a	"CONNECT.response"
       primitive to the	serving	layer to accept	or deny	the
       connection.  The	serving	then passes a "CONNECT.confirm"
       primitive to the	original user layer to convey the results
       of the connection attempt.

	  local	    |	serving	   |   remote
	user layer  |	 layer	   | user layer
		    |		   |
	  --------->|		   |
	   request  |		   |
		    |		   |------------>
		    |		   | indication
		    |		   |
		    |		   |<------------
		    |		   | response
	  <---------|		   |
	   confirm  |		   |
		    |		   |

       The HILI	committee uses a "three	arrow model".  For example,
       the "CONNECT.request" and "CONNECT.indication" are the same
       as above.  However, after the "CONNECT.indication" is passed
       to the remote user layer, the serving layer passes a
       "CONNECT.response" to the original user layer.  Thus the
       purpose of the response primitive is convey to the original
       user layer whether or not an indication primitive was sent
       to its peer.  (The name "response" is unfortunate since it
       conflicts with the ISO primitive, but we	couldn't think of a
       better one)

	  local	    |	serving	   |   remote
	user layer  |	 layer	   | user layer
		    |		   |
	  --------->|		   |
	   request  |		   |
		    |		   |------------>
		    |		   | indication
	  <---------|		   |
	   response |		   |
		    |		   |

       The HILI	committee feels	that the three arrow model is more
       appropriate because:

	 1.  It	makes the layers more independent, because the
	     serving layer does	not have to depend on or wait for
	     the remote	user layer to respond to an indication
	     primitive.

	 2.  The "four arrow model" interaction	is actually a user
	     layer protocol and	should not be the business of the
	     serving layer.  In	the example the	acceptance or
	     rejection of a peer connection is a user layer
	     protocol.	If the remote peer wishes to reject the
	     connection	it should do so	with a user layer PDU
	     and/or disconnect the serving layer connection.  The
	     purpose of	the serving layer should be to set up a
	     communication pipe	between	the "bottoms" of the two
	     user layers.  It should not say whether the user of
	     the pipe accepted the data	or not.

       If people want to discuss this on the net thats fine,
       otherwise sent comments to me:

	    Steven Kleiman
	    Bell Labs
	    Neptune, N.J. 07753
	    (201) 922-7276
	    ihnss!npois!srk@berkeley