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Page 4 | AlliedWare Plus™ OS: Overview of QoS
Packet markers
There are four items that are used to mark packets as they pass through the QoS system.
z Two markers that are carried within fields of the packet itself:
z 802.
1
p: The 802.
1
p or User Priority field in the VLAN tag of an Ethernet frame. This
is a 3-bit number, so it can have a value in the range 0-7.
z DSCP: The Differentiated Services Code Point within the TOS field of an IP packet
header. This is a 6-bit number, so it can have a value in the range 0-63.
z Two items that are just used within the switch chip. These are not fields within the packets,
but are extra parameters that the packets carry with them as they pass through the QoS
system:
z Bandwidth Class: This parameter can take on the values green, yellow, or red.
Essentially it is an indicator of whether the packet is deemed to have been within the
acceptable bandwidth limit set for any particular traffic flow, or whether the packet's
traffic flow had already overflowed its acceptable limit by the time this particular
packet arrived.
A value of green indicates that the flow was within the acceptable limit when the
packet arrived, a value of yellow indicates that the flow was slightly outside its
acceptable limit when the packet arrived, and a value of red means that the flow was
well outside the limit when the packet arrived.
z Egress Queue: This indicates the egress queue that the packet is currently slated to be
placed into, if and when it finally negotiates its way through all the steps in the QoS
process and lines up in one of the queues at its eventual egress port.