>It is valid to say that a flow label can not be used in isolation, but I
>believe you are locking it too tightly to current implementations of QoS
>handling. If we consider that the flow label could be used as a modifier
>to a DSCP, flow lifetime is irrelevant, or more properly 'this packet'.
In this case, there would still be a (degenerate) flow-establishment
mechanism in use. The originating host, and routers in the path, would
need to know that the purpose of the flow label (for this traffic,
anyway) is to modify/augment the DSCP, and they would know that the
flow lifetime was for a single packet only.
In this case, the flow label values should be specified as part
of the DSCP modification/augmentation mechanism. No special treatment
of the flow label would be required by IPv6 nodes that were not using
that mechanism.
>One conceptual use of a DSCP/FL pair would be to manage congestion on a
>DSCP queue either dropping all packets of a single flow or ensuring the
>drops are spread across random flows. The choice would depend on the
>intended service level of the queue.
There is not way to identify "all packets of a single flow" without
having a set of fields that identify a unique flow (src addr, dest
addr and flow label?) PLUS the concept of a flow lifetime.
Otherwise, you may end up dropping packets that belong to a later
flow that just happens to share the same flow label.
>Hosts or routers that do not support the functions of the Flow Label
>field are required to set the field to zero when originating a packet,
>and ignore the field when receiving a packet. All routers are required
>to pass the field on unchanged when forwarding a packet.
Why would we want to restrict possible flow-management mechanisms in
this (somewhat arbitrary) way? I think that the question of whether
the flow label can be modified en route should be left to the
specific flow-management mechanism.
Margaret
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