Subject: IPv6 ND stateful address autoconfiguration

From: "Sandeep.P A" <[email-masked]>
Date: Thu, 06 Dec 2007 13:13:31 +0530

Hi,

Please go through the RFC excerpts below and answer the query in the end.

              RFC 4862

	========

	5.5.2. Absence of Router Advertisements

	Even if a link has no routers, the DHCPv6 service to obtain
addresses

	may still be available, and hosts may want to use the service. From

	the perspective of autoconfiguration, a link has no routers if no

	Router Advertisements are received after having sent a small number

	of Router Solicitations as described in [RFC4861].

	Note that it is possible that there is no router on the link in this

	sense, but there is a node that has the ability to forward packets.

	In this case, the forwarding node's address must be manually

	configured in hosts to be able to send packets off-link, since the

	only mechanism to configure the default router's address

	automatically is the one using Router Advertisements.

	RFC 4861

	=========

	6.3.7. Sending Router Solicitations

	...

	Once the host sends a Router Solicitation, and receives a valid

	Router Advertisement with a non-zero Router Lifetime, the host MUST

	desist from sending additional solicitations on that interface,
until

	the next time one of the above events occurs. Moreover, a host

	SHOULD send at least one solicitation in the case where an

	advertisement is received prior to having sent a solicitation.

	Responses to solicited advertisements may contain more information

	than unsolicited advertisements.

	If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives no

	Router Advertisements after having waited MAX_RTR_SOLICITATION_DELAY

	seconds after sending the last solicitation, the host concludes that

	there are no routers on the link for the purpose of [ADDRCONF].

	However, the host continues to receive and process Router

	Advertisements messages in the event that routers appear on the
link.

	Query :

	If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives
"no

	Router Advertisements" after having waited
MAX_RTR_SOLICITATION_DELAY

	Here "no Router Advertisements" Is it means, RA with Non-zero
lifetime or RA with zero lifetime ?

Love,
Sandeep
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---

From: Vlad Yasevich <[email-masked]>
Date: Thu, 06 Dec 2007 14:20:12 -0500

Sandeep.P A wrote:
> Hi,
> 
> * *
> 
> *Query :*
> 
> /If a host sends MAX_RTR_SOLICITATIONS solicitations, and receives "no/
> 
> /*Router Advertisements"* after having waited MAX_RTR_SOLICITATION_DELAY/
> 
> Here "no *Router Advertisements"** *Is it means, RA with Non-zero

Neither.  It simply means that a Router Advertisement has not been received.

-vlad

---

From: Sandeep P A <[email-masked]>
Date: Fri, 07 Dec 2007 14:44:47 +0530

Hi Vlad,

Thanks for immediate reply.
What I understood from it is given below. Please confirm the same.  

Router Discovery fails when no default router is found (NO VALID RA WITH
NON-ZERO LIFETIME IS RECEIVED).

Host assumes that there is no router in the network for the purpose of
autoconfiguration and tries to check for stateful ways to get an address for
itself When NO VALID RA IS RECEIVED.

Thanks & Regards,
Sandeep

-----Original Message-----
From: Vlad Yasevich [mailto:[email-masked]] 
Sent: Friday, December 07, 2007 00:50
To: Sandeep.P A
Cc: [email-masked]
Subject: Re: IPv6 ND stateful address autoconfiguration

-vlad

---

From: Vlad Yasevich <[email-masked]>
Date: Fri, 07 Dec 2007 10:02:37 -0500

Hi Sandeep

> NON-ZERO LIFETIME IS RECEIVED).

No, router discovery fails when you don't receive any RAs.

> itself When NO VALID RA IS RECEIVED.

Yes.

-vlad

---

From: Iljitsch van Beijnum <[email-masked]>
Date: Fri, 7 Dec 2007 17:36:00 -0800

> itself When NO VALID RA IS RECEIVED.

You may want to consider whether it's useful to do this.

DHCPv6 CAN give you an address (although unlike with DHCP for IPv4,  
not every DHCPv6 server is configured to do so) but it can't give you  
a subnet prefix length or a default gateway, so local communication  
will have to depend on the "on-link assumption" which is considered a  
security flaw by many and has the potential to cause timeouts when  
trying to communicate with IPv6 destinations. And non-local  
communication won't be possible.