COMNET III

COMNET III Reports and Statistics
2.15 Output Port Buffer Policy Report
NODES: OUTPUT PORT BUFFER POLICY
REPLICATION 1 FROM 0.0 TO 3600.0 SECONDS
PREEMPTION
THRESHOLD
NODE: PACKETS PACKETS PACKETS PACKETS
CONNECTED LINKS PREEMPTING PREEMPTED EXCEEDED REJECTED
________________________ __________ _________ _________ _________
Source/Destination 1:
Link1 36 36 0 0
This report summarizes the output buffer policy actions on a port-by-port basis. The
preemption and threshold values reported are specific for each output port on the node.
The output buffer on a node first determines that there is room for a packet by checking that
the specific output buffer has buffer space available to fit the incoming packet. It next checks
that the total space used for all similar output buffers on the node does not exceed the buffer
constraint in the node’s parameter set. The packet may be accepted in an output buffer if
it fits in both the port-specific buffer and in the total buffer space available on the node.
There are two other policies available at the output port to refine the initial test for admitting
the packet into an output buffer. These policies are a "Preemption Policy" and a "Threshold
Policy". These policies determine which packets may be accepted at an output buffer.
The "Preemption Policy" checks to see if the packet will be accepted into the output buffer
if the packet has passed the first check and does not initially fit in the output buffer. In this
case, if the packet has sufficient priority, it can cause lower priority packets to be ejected
from the buffer, and, thus, be blocked until there is enough space in the buffer for the incoming
packet. The lower priority packets will be ejected only if there can be enough space made
available for the incoming packet.
The "Threshold Policy" checks to see if the packet will be accepted into the output buffer
if the packet has passed the first test. This is so that if a packet fits in the buffer it can be
rejected if the buffer size is above a threshold, and the packet lacks sufficient priority to use
the congested buffer. This mechanism is commonly used for rejecting packets that are marked
as "discard eligible" when the buffers are above their threshold in frame-switching networks
such as ATM or frame-relay.
Node Name
The name of the node sending packets.
Packets Preempting
The number of packets that preempted other packets in the buffer, thus
causing those other packets to be blocked. A higher priority packet arrives
at the output buffer that is full and preempts a lower priority packet.
70