Making the field "immutable" by "default", but "mutable" when a router is so instructed by a flow setup and flow processing mechanism is a compromise that would satisfy all current and future possible cases.
Therefore I think last sentence of the first paragraph
All routers MUST pass the field on unchanged when forwarding a
packet.
should be something like:
All routers MUST leave the field unchanged, by default, when
forwarding a packet.
A specific flow setup/processing mechanism MAY give a "mutable"
character to the field,
by requesting routers, supporting the mechanism, to change the field
in certain ways.
Routers supporting such a mechanism MUST follow the steps indicated
by the flow
setup and flow processing mechanism.
Alex
Brian E Carpenter wrote:
>
> Taking Scott's suggestion, here's another try:
>
> I'd like to propose the following as the
> complete and total replacement of Section 6 of RFC 2460.
>
> The 20-bit Flow Label field in the IPv6 header MAY be set by a
> source to uniquely label sets of packets. Nodes that do not support
> the Flow Label field MUST set the field to zero when originating a
> packet, and MUST ignore the field when receiving a packet. All routers
> MUST pass the field on unchanged when forwarding a packet.
>
> This specification does not further define the meaning of the
> Flow Label.
>
> [and delete Appendix A, which is unhelpful.]
>
> Brian
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