OSI/TS Configuration and Management Manual

Managing a Transport Connection
OSI/TS Configuration and Management Manual—424831-001
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Flow Control
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Upon receipt of a T-EXPEDITED DATA request, OSI/TS sends the TSDU in an
ED-TPDU. No segmentation and reassembly is performed on expedited data. ED-
TPDUs are placed ahead of other TPDUs in the output queue of a transport
connection endpoint.
•
Upon receipt of an incoming ED-TPDU from a remote peer, OSI/TS passes a
T-EXPEDITED DATA indication to the transport user and acknowledges receipt by
sending an EA-TPDU to the remote peer.
Flow Control
OSI/TS provides host-to-host flow control on class-2, class-3, and class-4 connections.
For class-0 and class-1 connections, it is assumed that flow control is provided by the
subnetwork.
The mechanism for flow control depends on consecutive numbering of DT-TPDUs.
Each correspondent transport entity maintains a transmit window that represents a
sequence of DT-TPDUs. OSI/TS transmits a DT-TPDU only if its number falls within
the window.
The boundaries of a transmit window are its upper and lower edges. Upon entering the
data-transfer phase, OSI/TS initializes its transmit window as follows:
•
The lower edge is set to 0.
•
The upper edge is the value of the initial credit parameter negotiated in connection-
establishment TPDUs. Initial credit is a configured parameter.
During data transfer, OSI/TS adjusts the boundaries of the transmit window whenever
data is acknowledged by the remote peer. Each received AK-TPDU redefines the lower
edge of the transmit window and specifies an amount of transmit credit. Upon receipt of
an AK-TPDU, OSI/TS adjusts the lower edge of the transmit window and calculates the
upper edge by adding the credit.
A received AK-TPDU can cause OSI/TS to raise or leave unchanged, but never lower,
either edge of the transmit window. The window can be closed when a series of
AK-TPDUs reduces the amount of credit to 0. AK-TPDUs can reduce credit only by the
same amount that the lower edge is raised. When credit is reduced to 0, the window is
closed. A later AK-TPDU can re-open the window by increasing the amount of credit.
OSI/TS invokes flow control by not transmitting an AK-TPDU when it cannot obtain
buffer space for the incoming DT-TPDU.
RJ-TPDUs, which are used only in protocol classes 1 and 3, provide the remote peer
entity with additional control over the transmit window. Like AK-TPDUs, RJ-TPDUs
define the lower edge of the transmit window and specify an amount of credit. A
received RJ-TPDU can raise or leave unchanged, but never decrease, the lower edge of
the window. Unlike AK-TPDUs, however, RJ-TPDUs are not restricted in the rate by
which credit can be reduced. An RJ-TPDU can close the transmit window from the top
by reducing credit to 0, while holding the lower edge constant. After transmitting an
RJ-TPDU, OSI/TS does not consider a received DT-TPDU outside the window to be an
error.