QoS and QoS Policies
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Quality of Service Guide
3HE 11014 AAAC TQZZA Edition: 01
3.6.6 Packet Byte Offset (PBO)
Packet byte offset (PBO), or internal headerless rate, allows 7705 SAR schedulers
to operate on a modified packet size by adding or subtracting a certain number of
bytes. The actual packet size remains the same but schedulers take into account the
modified size as opposed to the actual size of the packet. One of the main uses of
the packet byte offset feature is to allow scheduling, at access ingress, to be carried
out on the received packet size without taking into account service (for example, PW,
MPLS) or internal overhead. Transport providers who sell bandwidth to customers
typically need the 7705 SAR shapers/schedulers to only take into account the
received packet size without the added overhead in order to accurately calculate the
bandwidth they need to provide to their customers. Packet byte offset addresses this
requirement. Another common use is at egress where port shapers can take into
account four additional bytes, associated with Ethernet FCS.
Packet byte offset is configured under QoS profiles. Packet size modification might
be desired to accommodate inclusion or exclusion of certain headers or even fields
of headers during the scheduling operation. The packet size that the schedulers take
into account is altered to accommodate or omit the desired number of bytes. Both
addition and subtraction options are supported by the packet-byte-offset command.
The actual packet size is not modified by the command; only the size used by ingress
or egress schedulers is changed. The scheduling rates are affected by the offset, as
well as the statistics (accounting) associated with the queue. Packet byte offset does
not affect port-level and service-level statistics. It only affects the queue statistics.
When a QoS policy configured with packet byte offset is applied to a SAP or network
interface, all the octet counters and statistics operate and report based on the new
adjusted value. If configured, per-SAP aggregate shapers and per-customer
aggregate shapers also operate on the adjusted packet sizes. The only exceptions
to this rule are port shapers. The egress port shapers do not take the adjusted packet
size into account, except on the 8-port Ethernet Adapter card, version 2, but operate
only on the final packet size.
Table 17 shows PBO support on the 7705 SAR.
Note: The fabric shaper, in general, does not take the adjusted packet size into account,
except on the 8-port Ethernet Adapter card, version 2.
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