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Page 3 | AlliedWare Plus™ OS: Overview of QoS
Which products and software version does this
Note apply to?
z Products: SwitchBlade x908, x900-12XT/S, and x900-24 series switches
z Software versions: AlliedWare Plus version 5.2.1-0.1 and above
The process flow and methodology of the QoS
system
Before discussing the details of the various processes that comprise the QoS system, it is
desirable to first get a picture of what the processes are, and the order in which they are
applied to the packets passing through the system.
Therefore, this section discusses what the QoS system is really trying to do to packets, and
how it keeps track of what it has decided about any given packet.
The QoS system does the following things:
z decides which egress queue to send a packet to
z decides whether to drop the packet or attempt to forward it
z updates markers in the packet for downstream devices to use
z controls the relative priorities of the egress queues
In general, the main aim of all the processes in the QoS system is to work out which egress
queue a particular packet should be put into.
There are several factors that can affect this choice of egress queue, so packets need to be
put through several processes, so that each of the competing factors has its opportunity to
exert its influence on the final choice of egress queue.
In some cases, the system can decide to simply discard certain packets at some steps in the
process.
Additionally, the QoS system often has an obligation to update certain fields within a packet,
to indicate to downstream devices how they should deal with the packet when it gets to
them.
So, we have this multi-stage process, and the eventual fate of a packet will depend on the sum
total of the various decisions that were made about it at various stages in the process. In
order to keep track of the outcomes of those decisions, a packet needs to be marked so that
at any point in the process it is possible to know the net effect of the decisions that have been
made on it so far.