RE: [NDP] Router autoconfiguration with RS/RA

["Hemant Singh (shemant)" <[e-mail]>]

Silviu,
 
A router can receive an RA on the router's upstream and use this RA to
autoconfigure the ipv6 address on interface(s) of the router. Such a
router interface configuration is no different from how a host interface
statelessly autoconfigures as per ND RFC 4861 and 4862. However, ND
RFC's do not mandate what does a router implementation do for sending
RA, configuring network prefixes in the router downstream direction -
these are conceptual variables that a router vendor is left to do what
they want to do.
 
As to answering your question which was:
 
"Why wouldn't a router be authorized to send Router Sollicitation
messages?"
 
here is my reply.
 
As far as the interface on the router has no RA configured, and the
interface is configuring an IPv6 address using stateless
autoconfiguration or even manual configuration, this interface is OK to
send an RS in the router downstream. However, soon as any RA
configuration for router downstream is configured on the network
interface, then ND prohibits a router to send any RS. 
 
Furthermore, I totally agree with Remi on his reply to this question of
yours:
 
"The same question for autoconfiguring the prefix it advertises on its
subnets."
 
You cannot mix router upstream and downstream operations in random
fashion. IPv6 stateless autoconfiguration does not support prefix and
router configuration of an upstream router. One should be careful
discussing router downstream vs. router upstream directions for address
configuration, routing configuration, and IPv6 ND RA configuration.
 
Hemant

________________________________

From: [e-mail] [mailto:[e-mail]] On Behalf Of
Silviu VLASCEANU
Sent: Friday, June 06, 2008 8:29 AM
To: [e-mail]
Subject: [NDP] Router autoconfiguration with RS/RA


Hello,

I have been trying to figure out a response for the following questions,
but I have only suppositions and I haven't found (yet) a document that
accurately talks about. So I am asking here.

Why wouldn't a router be authorized to send Router Sollicitation
messages?
Moreover, why couldn't a router autoconfigure its egress interface based
on Router Advertisements received on this interface? The same question
for autoconfiguring the prefix it advertises on its subnets.

The only answer that comes in my mind is because an attack over these
messages could render not only a host unreachable, but maybe a whole
subnet. But apart this, is there really any other reason for not
allowing this?

Thank you in advance for the answers.

Best regards,
-- 
Silviu

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