OSI/TS Configuration and Management Manual

Configuring the OSI/TS Subsystem
OSI/TS Configuration and Management Manual—424831-001
3-16
Transport Class 4 Attributes
Transport class 1 is a special case because in this class, any L4WINDOW value greater
than 1 can lead to contention problems. This can occur when the sending transport
entity waits for an acknowledgment of each DT-TPDU before sending the next one.
This is an obvious case in which both sides of the connection need to be aware of what
the other side is expecting.
If you are using a LAN, the L4WINDOW attribute controls the TPDU-resent rate and
the credit-reduction rate. These rates can be deduced from the STATS #L4 command.
The TPDU-resent rate is an indication of TPDUs lost over the LAN or a slow response
at the receiving end. The credit-reduction rate is an indication of too many TPDUs
being received by the TSP process and not yet absorbed by the application. You can
keep these two rates low by decreasing the L4WINDOW value.
If you are using X25AM, there are two window attributes you can set. L3WINDOW
specifies the number of packets that can be outstanding before an acknowledgment is
required before any more packets are sent. L2WINDOW specifies the number of Data
Link Layer frames that can be outstanding before an acknowledgment is required before
any more frames are sent. These two attributes are similar to L4WINDOW they must be
set to accommodate the size of the TPDUs, the packet size selected, and the number of
packets and frames being used to send an application message.
Transport Class 4 Attributes
Transport class 4, which implements mechanisms for error recovery, requires special
consideration. Class 4 includes a number of timeout attributes used to aid in recovery
from unsignaled errors. You should set the values for these timeout attributes high
enough that unnecessary retransmissions are not triggered and that transport connections
are not terminated unnecessarily. CCITT X.224 (ISO 8073) provides information and
recommendations on how to calculate these values. The class 4 timeout attributes
involved are as follows:
•
INACTIVETIMEOUT
•
REXMITTIMEOUT
•
WINDOWTIMEOUT
•
MAXREXMIT
These attributes are discussed below.
INACTIVETIMEOUT Attribute
The INACTIVETIMEOUT attribute defines the amount of time TSP waits before
aborting the transport connection, if no TPDU is received. You should set this value
high enough to ensure that the transport connection is not terminated unnecessarily.
REXMITTIMEOUT Attribute
The REXMITTIMEOUT attribute defines the amount of time TSP waits before
retransmitting an unacknowledged TPDU. You should set this value high enough to
ensure that TPDUs are not unnecessarily retransmitted, yet low enough to ensure that
TPDUs are retransmitted in a reasonable amount of time if any get lost.