Subject: Re: A question about Source Address of IP Field of ND Packets(RFC2461)

From: Tim Enos <[email-masked]>
Date: Tue, 26 Sep 2006 17:29:26 -0500 (CDT)

>From: Su Thunder <[email-masked]>
>Date: 2006/09/26 Tue AM 10:53:59 CDT
>To: [email-masked]
>Subject: A question about Source Address of IP Field of ND Packets(RFC2461)

>The Source Address of Router Solicitation Message is specified in section 
>4.1 as follows:
>   IP Fields:
>
>      Source Address
>                     An IP address assigned to the sending interface, or
>                     the unspecified address if no address is assigned
>                     to the sending interface.
>My question is, why not specify whether the Source Address is link-local 
>address or other type when it is not the unspecified address?And when to 
>use which type?Or it must be link-local address as the one of the Router 
>Advertisement Message?
>The same problem occurs in the same field of Neighbor Solicitation 
>Message,Neighbor Advertisement Message.

Unless there is no address assigned to a sending interface (in which case the SA would be ::), I can't think of a reason for any ND message to require the SA be anything but link-local in scope. 

As a general rule (and perhaps going beyond the intended scope of the original e-mail), RFC 3484 section 5 provides guidance for source address selection. Further, section 10.1 of that same spec provides what are IMO good examples thereof.

Best Regards,

Tim Enos
Rom 8:28

>
>
>
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From: Elwyn Davies <[email-masked]>
Date: Wed, 27 Sep 2006 00:44:03 +0100

> Unless there is no address assigned to a sending interface (in which case the SA would be ::), I can't think of a reason for any ND message to require the SA be anything but link-local in scope. 
For neighbour solicitations:
Please read section 7.2.2 of RFC2461 which explains when it is desirable 
to use another (generally a global scope address) of the sending 
interfaces' assigned addresses rather than the link local address.  It 
helps get the neighbour caches at both ends set up and the association 
of link layer and IP addresses determined in one round trip.BTW this 
sometimes means that NS/ND packets contravene the usual rules on 
source/destination addresses matching - see the end of section 2.3 in 
the updated version of RFC2461 (draft-ietf-ipv6-2461bis-08.txt).

For router solicitations:
The usual case is that a node sending a router solicitation will only 
have link local addresses at the stage in configuration when the 
soliictation is sent, so it doesn't have a choice.  As with neighbout 
solitctation using a real address rather tha the unspecified address 
allows address resolution to be piggybacked onto the single round trip.

Regards,
Elwyn