Filter
Top Products
Page 7 | AlliedWare Plus™ OS: Overview of QoS
Policing
Policing involves measuring the bandwidth used by a policer and comparing the measurement
to the bandwidth limits that have been set for the policer.
The policing process allocates a temporary bandwidth class value to packets. It is important
to note that the policing process does not overwrite the bandwidth class value that the
packet is already carrying around with it. Instead, an extra, temporary, bandwidth class
marker is attached to the packets.
When traffic first enters the switch, it is all marked with bandwidth class green, simply
because it has not been policed yet. Packets can be assigned a new bandwidth class at the
Premarking stage, but this is not done on the basis of actual measurement of bandwidth use.
At the policing stage, a policer's bandwidth usage is constantly monitored to see how well it
conforms to the limits set for it, and the individual packets within the flow are assigned to a
temporary bandwidth class depending on the policer's conformance to its limits at that time.
So, while a policer is still within its bandwidth limit, all the packets that have been classified to
that policer are marked with a temporary bandwidth class of green. If a policer starts to
exceed its limit, then the packets in that policer are given a temporary bandwidth class of
yellow. If it starts seriously exceeding its limits, then the packets’ temporary marking is
bandwidth class red.
The actual algorithms used to determine whether a policer is slightly exceeding its bandwidth
limit or seriously exceeding the limit are described later in this document.
Limiting or remarking (dropping non-conformant
packets)
Based on the temporary bandwidth class assigned to a packet at the policing stage, one of
two actions can be taken:
z the packet can be dropped if it is was assigned to bandwidth class red by the policing
process, or
z the packet can be remarked with new QoS property values.
The first of these two actions is straightforward; the user can choose to simply drop packets
if the policer exceeds the bandwidth limits set for it to the extent that packets are assigned
to bandwidth class red.
Remarking is a little more complex as it is not done solely on the basis of the bandwidth class
that the packet has been assigned to; the packet's current DSCP value, and its temporary
bandwidth class are used to determine the new values for all four QoS properties for the
packet (that is, new values for the DSCP, VLAN tag user priority, bandwidth class, and egress
queue can be specified). The new values are taken from the user-configurable policed-dscp
map.