Dear Stig,
Stig Venaas wrote:
So far two principal solutions have been suggested, RAs and DHCP. If
people want to work on solutions we could possibly look into both of
these.
Some issues have already been mentioned on this list. Another issue
which was brought up in dhc wg, is that the policy is a host global
config, not per interface. This might be an issue when you have
multiple interfaces. This needs to be considered for both RAs and
DHCP.
It's unclear at the moment how a DHCP server on one link is able
to describe how to use addresses available on another interface
or link.
Particularly, it's not clear how the DHCP server can have authority
to modify the previously advertised address state from another link,
where it isn't responsible for configuration.
Given that the policy is based on per interface available addresses,
and the host will need to make a combination of these policies anyway,
there's little to be gained by assuming the DHCP server is all-knowing
about foreign addressing policies.
So I don't see the "per-host" argument as convincing at all.
I have two problems with RAs. One is that all hosts on the link will
get the same policy. The other is that I'm worried the policies may
get large, and I'm not sure if it's a good idea to send relatively
large RAs regularly to all the nodes on a link.
The Cost in RA is actually completely subsumed by the existing
advertisement of Prefix Information Options, which may be
augmented (without size modification) by identifying the
preference of the particular prefix, for source address
selection policies.
It is worth noting, that in the DHC proposal, 24 bits of data:
(label, precedence, zone-index) are added which aren't present
in PIOs.
There's an unused 32 octet field available (and another 5 unused
bits for flags) in each PIO, which are currently unused.
So there's no on-the-medium additional cost.
So the only issue is: Does it make sense to use RAs for distributing
the default source address preferences to hosts?
While it may seem to be a good idea to have different preferences
proposed for each host, it's not actually very useful.
Hosts can ignore or override the preferences, which is why
the policy is described as a default policy in the DHC draft.
Considering that the advertised Valid and Preferred Lifetime values
for prefixes are per prefix and not per-host, if we're providing
routability and stability information to hosts, then the information
should likewise be per prefix (not per host).
In that case, I guess that PIOs (in RAs) are a good place for them,
just like the Valid and Preferred Lifetimes.
If the mechanism is being used for some stealthy control of which hosts
should use which addresses on the link, I'm against it (since it's
unenforceable anyway). I'm also against it if the policy is aimed
at being used as a load sharing mechanism (since there are better ways
of achieving this).
Greg
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