Subject: Proposal for mapping APPN Class of Service

From: "jim (j.) cobban" <[email-masked]>
Date: Tue, 25 Jul 1995 11:49:00 -0400

As previously discussed there are some problems with mapping APPN class of
service to the quality of service characteristics of public data networks.
Gary Dudley has suggested that setting the TG characteristics in an
appropriate manner can lead us in that direction.  This memo examines this
and suggests a small modification to the default APPN COS table entries to
achieve this for a particular case.

Imagine a public data network which has two quality of service parameters:

  - priority gives preference to particular connections at each hop, but
    has a higher cost per packet.
  - end-to-end delay is used to indicate whether a particular connection
    can tolerate a satellite hop delay.

These QoS parameters can reasonably be mapped to APPN transmission group
characteristics as follows:

	Cost/byte	= 128 (low priority VC)
			  196 (high priority VC)
	Security	= 0x20 (public switched)
	delay		= 0x91 (delay-sensitive, packet-switched)
			  0x99 (not-delay-sensitive, satellite delay)

Using the published default values for the COS entries the recommended TG
characteristics will produce the following weights:

----------+-------------+-------------+-------------+-------------+
          |LoPri,ThrSens|LoPri,DelSens|HiPri,ThrSens|HiPri,DelSens|
----------+-------------+-------------+-------------+-------------+
CPSVCMG   |     210     |     180     |     210     |     210     |  
SNASVCMG  |     210     |     180     |     210     |     210     |
#CONNECT  |     210     |     180     |     210     |     210     |
#INTER    |     210     |     150     |     210     |     210     |
#INTERSC  |     210     |     150     |     210     |     210     |
#BATCH    |      90     |      90     |     180     |     180     |
#BATCHSC  |      90     |      90     |     180     |     180     |
----------+-------------+-------------+-------------+-------------+

This demonstrates a flaw in the default COS entries for interactive and
network services.  If the best available route is through a public network
these classes of service preferentially request a LOW cost circuit.  But that
means they are preferentially requesting a LOW priority circuit, when in fact
what the user generally desires for this sort of traffic is a HIGH priority
circuit.  It is therefore recommended that the COS entries for the
interactive and management services be modified to have a lower weight for
high cost (that is high priority) VCs than for low cost (that is low
priority) VCs.  In particular modify the following rows of these COS entries
as follows:

  CPSVCMG, SNASVCMG, #CONNECT, #INTER, #INTERSC
    row 5 cost/byte range 129-196 (high priority VC preference)
    row 6 cost/byte range 0-196   (no VC type preference)
    row 7 cost/byte range 129-196 (high priority VC preference)
    row 8 cost/byte range 0-255 (catch-all)

This changes the weights as follows:

----------+-------------+-------------+-------------+-------------+
          |LoPri,ThrSens|LoPri,DelSens|HiPri,ThrSens|HiPri,DelSens|
----------+-------------+-------------+-------------+-------------+
CPSVCMG   |     240     |     180     |     210     |     150     |  
SNASVCMG  |     240     |     180     |     210     |     150     |
#CONNECT  |     240     |     180     |     210     |     150     |
#INTER    |     240     |     180     |     210     |     150     |
#INTERSC  |     240     |     180     |     210     |     150     |
#BATCH    |      90     |      90     |     180     |     180     |
#BATCHSC  |      90     |      90     |     180     |     180     |
----------+-------------+-------------+-------------+-------------+

When APPN route selection performs this calculation it will always find that
the weight of a route which goes over a public data network connection with
the desired quality of service has a lower weight than an otherwise identical
VC which has a different QoS.  The combination of judicious TG
characteristics and the small modification to the COS entries gives an
appropriate order of preference for circuits with the various QoS values.
Since only rows with tolerance for at least packet delay are modified, the
behavior of the LAN and leased line portions of the APPN network is
unahcnged.