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always including the context tag
In his review Iljitsch raises the following issue:
my comments are online:
please comment on the issue related to if we should support other
mechanisms for demux other than the context tag contained in the shim
1. Always including the context tag
As I've said many times, it's a very bad idea to unnecessarily
well, i think having an additional header is the clean way to do this,
because there is an additional layer that is being inserted and the
layer needs to perform its signalling and this is achieved using a
Not only is this a bad thing in and of itself, but it also doesn't
help convince people to forego IPv4+BGP in favor of IPv6+shim.
Now if we only include the shim header containing the context tag
after a failure, this could be a reasonable tradeoff. However, I'm
quite sure that if and when shim6 is adopted, some people will use it
to manage their traffic engineering by doing a locator change
immediately and proceed in a shimmed state for the rest of the
Earlier, I proposed to include a mandatory way for a host to instruct
its peer to not include the shim header, pending mechanisms to do
demultiplexing without the context tag. After reading the draft, I
realize that this imposes some extra ICMP demultiplexing difficulties
depending on which fields do you use for the demux, i guess... i mean
if you use the flow label, there wouldn't be additional problems, i
So if the wg doesn't want to include this mandatory option, I suggest
something different: since the traffic engineering mechanisms aren't
really developed anyway and this problem is related to that, simply
remove the locator preferences mechanism so that the context tag
suppression can be included when proper traffic engineering is added.
i don't think this is such a good idea...
i mean i think locator preferneces are useful, i don't see why we
should remove them from the spec...
Also see draft-van-beijnum-shim6-suppress-header-00.txt (do it while
you can, it expires next week...), this outlines a more fully formed
mechanism to suppress the context tag header.
I would very much like to see a consensus call on this subject.