By way of introduction, I have a preference for standards text to be as short
as possible, because the more we say, the more likely we are to be wrong
(especially when speculating about future router design). So here is my
next proposal for the text about routers setting the flow label. As always,
comments welcome (suggested text especially welcome...):
A node that forwards a flow whose flow label value in arriving
packets is zero MAY change the flow label value. In that case, it is
RECOMMENDED that the forwarding node sets the flow label field for a
flow to a uniformly distributed value as just described for source
nodes.
o The same considerations apply as to source hosts setting the flow
label; in particular, the preferred case is that a flow is defined
by the 5-tuple. However, there are cases in which the complete
5-tuple for all packets is not readily available to a forwarding
node, in particular for fragmented packets. In such cases a flow
can be defined by fewer IPv6 header fields, typically using only
the 2-tuple {dest addr, source addr}. There are alternative
approaches that implementers could choose, such as:
* A forwarding node might use the 5-tuple to define a flow
whenever possible, but use the 2-tuple when the complete
5-tuple is not available. In this case, unfragmented and
fragmented packets belonging to the same transport session
would receive different flow label values, altering the effect
of subsequent load distribution based on the flow label.
* A forwarding node might use the 2-tuple to define a flow in all
cases. In this case, subsequent load distribution would be
based only on IP addresses.
o This option, if implemented, would presumably be of value in
first-hop or ingress routers. It might place a considerable per-
packet processing load on them, even if they adopted a stateless
method of flow identification and label assignment. Also, it
should not interfere with host-to-router load sharing [RFC4311].
Therefore, it needs to be under the control of network managers,
to avoid unwanted processing load and any other undesirable
effects. For this reason it MUST be a configurable option,
disabled by default.
The preceding rules taken together allow a given network to include
routers that set flow labels on behalf of hosts that do not do so.
The complications described explain why the principal recommendation
is that the source hosts should set the label.
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