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Quality of Service
The Flow Label and the Priority fields
in the IPv6 header are used by a host to identify packets that need
special handling by IPv6 routers, such as non-default quality of
service or "real-time" service. This capability is important in
that it needs to support applications that require some degree of
consistent throughput, delay, and jitter.
Flow Labels
The 24-bit Flow Label field in the IPv6 header is a
sequence of packets sent from a particular source to a particular
destination for which the source desires special handling by the
intervening routers. The nature of the handling can be used by a
control protocol, or by information within the flow's packets
themselves, such as a hop-by-hop option. It is identified by the
combination of a source address and a non-zero flow label. It is
assigned by the flow's source node, and must be chosen between 1 to
FFFFFF hex. All packets belonging t the same flow must be sent
with the same source address, same destination, and same non-zero flow
label. If any of those packets include a Hop-by -Hop Options
header, then they all must be originated from the same header
contents. If a violation is detected, it should be reported to
the source by an ICMP Parameter Problem message, Code 0, pointing to
the high-order octet of the Flow Label field. Also, routers are
free to to set up flow handling state for any flow, even when no
explicit flow establishment information has been provided to them. The
router may choose to use the flow label plus the source address as a
cache key to remember the process of hopping interfaces.
Note: hosts or routers that do not support the field are required to set the
field to zero when originating, forwarding a packet, and ignore the
field when receiving a packet.
Priority
The 4-bit Priority field in the IPv6 header enables a
source to identify the desired delivery priority of its packet,
relative to other packets from the same source. For
congestion-controlled traffic, the values of 0-7 are used; for
non-congestion-controlled traffic, values from 8-15 are used.
For details of each value, go to
Header Format. |
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