Subject: Re: [Mip6] DHAAD and source address selection

From: Alexandru Petrescu <[email-masked]>
Date: Wed, 17 Oct 2007 12:18:43 +0200

Nicolas Dichtel wrote:
> Hi,
> 
> I've a question about source address selection in HAAD Reply message.
> 
> Here is the topology:
> 
>                        MN1X
>                         |
>         --------+-------+--------------- Link1X
>                 |
>                 R1
>                 |
> --------+-------+----------------------- Link1
>         |
>        RUT
>         |
> - - - - + - - - - - - - - - - - - - - -  Link0

I'd also say that RUT (your HA) should send the DHAAD Reply having the
src field as its address on Link0.

I think the choice on Link0 or Link1 address may have implications on
how secure DHAAD works. The address in src of Reply being different than
the address on the Reply payload may be not good for IPsec SA.

Alex

> RUT is a Home Agent and the Home Link is Link0. Let's call RUT(Link1)
> the unicast address sets of the interface connected to Link1 and
> RUT(Link0) the unicast address sets to the interface connected to Link0.
> 
> MN1X sends a HAAD Request message to its home agent (RUT). The 
> destination address of this message is an anycast address. When RUT
> sends the HAAD Reply message, which source address must be
> used ? RUT(Link1) or RUT(Link0) ?
> 
> RFC3775 Section 10.5 just says:
>                                                "A home agent receiving a
>    Home Agent Address Discovery Request message that serves this subnet
>    SHOULD return an ICMP Home Agent Address Discovery Reply message to
>    the mobile node with the Source Address of the Reply packet set to
>    one of the global unicast addresses of the home agent."
> 
> Following RFC3484, I think that rules #5 (Prefer outgoing interface) in
> Section 4 is reached. But, there is some cases, like for ICMP ECHO Reply
> (RFC 4443 Section 4.2), where source address must be the destination
> address of the request message. In our case, the destination address is
> an anycast address (assigned to the interface attached to Link0) and,
> following the RFC2526 Section 1, the packet is delivered to RUT(Link0):
>   "The anycast address may be assigned to one or more network interfaces
>    (typically on different nodes), with the network delivering each
>    packet addressed to this address to the "nearest" interface based on
>    the notion of "distance" determined by the routing protocols in use."
> 
> So, it's not clear which source address must/can be used. For example,
> the TAHI tests suite expects the use of RUT(Link0).
> 
> Regards,
> Nicolas
> 
> _______________________________________________
> Mip6 mailing list
> [email-masked]
> https://www1.ietf.org/mailman/listinfo/mip6
> 

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---

From: Vijay Devarapalli <[email-masked]>
Date: Tue, 16 Oct 2007 14:06:53 -0700

It should be the unicast source address of the RUT on Link0.

But moving forward, when we revise RFC 3775, I think the anycast
address should also be allowed to be used as the source address
on the DHAAD reply message. There used to be a restriction earlier
that an anycast address shouldn't be used as the source address,
but was removed recently. I forget which spec.

Vijay

---

From: Brian Haley <[email-masked]>
Date: Wed, 17 Oct 2007 14:42:52 -0400

Hi,

> I've a question about source address selection in HAAD Reply message.
...
> RUT is a Home Agent and the Home Link is Link0.

This is important.

> destination address of this message is an anycast address.

This anycast address is on Link0, the Home Link, so you should reply 
with an address from that link - the one you're using as the Home Agent 
address.  The global address on Link1 is not a Home Agent address.

> When RUT

Other parts of the RFC do state this more clearly, for example Section 
10.6.3 on sending Advertisements:

    o  The Source Address in the packet's IPv6 header MUST be set to the
       home agent's IP address to which the mobile node addressed its
       current home registration or its default global home agent address
       if no binding exists.

Basically, it should use an address in the same prefix with which it's 
offering home agent services for - the one that would be included in the 
DHAAD reply if it included its own.

 > If the HAAD is sent with RUT(Link1), is it an error ? or is it
 > compliant with the specification ?

Well, a lot of us helped define the correct behavior for the TAHI tests 
years ago during Connectathon testing, and our implementations behave 
this way.  If you choose to do it differently there's no guarantee 
you'll be inter-operable with all the MNs out there, and you won't be 
able to get the IPv6 Ready MIPv6 Logo.

This is at least something that should be addressed in the issues list 
that Alex is keeping track of.

-Brian