Subject: [mobile-ip] some questions about MIPv6 spec

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Wed, 13 Mar 2002 16:35:41 +0900

Hi all,

I have two questions about MIPv6 spec.

I know the spec will soon be revised.  But, since there have been no
discussions/clarifications in the mailing list about the following
topics before, let me ask them.

1. The meaning of the 'routing prefixes'
2. The cases that we cannot use the HAO.

1. The meaning of the 'routing prefixes'

In section 9.1 'Primary Care-of Address Registration', there are
following sentences.

   If the `S' bit field in the Binding Update is zero, The Home Agent
   creates or updates Binding Cache entries for each of possible
   several home addresses.  The set of such home addresses is formed
   by replacing the routing prefix for the given home address with
   all other routing prefixes that are supported by the home agent
   processing the Binding Update.  The Home Agent creates such a
   separate primary care-of address registration for each such home
   address.

When `S' bit is zero, the HA creates the BCEs for the MN with all
'routing prefixes' it has.  But I couldn't find the definition of the
'routing prefix'.

What is the definition?

a) The prefixes advertised by the routers on the home network.
	(In this case, all the addresses derived from the prefixes
	must be the home addresses.)
b) The prefixes advertised by the home agents on the home network.
	(In this case, all the addresses derived from the prefixes
	advertised by the home agents must be the home addresses)
c) administratively selected prefixes to be the home prefix.
d) other.

2. The cases that we cannot use the HAO.

In section 10.1 'Sending Packets While Away from Home', the spec says
that the MN SHOULD use the HAO to provide the transparent access to
transport-level connections.  The exception is short term
communications.  In short term communications, we MAY use COA.

However, I think using the HAO is harmful in some cases (at lease one
case.  I will explain bellow) and must be noted in the spec as 'MUST
NOT use HAO in some cases'.

Suppose the MN is foreign.

    +--+	    +--+
    |MN|P:A         |CN|Q:B	MN: a mobile node (P is home prefix)
    ++-+Q:A(COA)    +-++	CN: a CN that do not accept BU.
     |		      |
   --+----------------+-- Q::/64(or whatever)

In the above network, MN will learn the CN's lladdr for the address
Q::B by ND mechanism like bellow.

Neighbor	Linklayer Address
Q::B		(MAC addr of the CN)

Some time after, the MN may send a NS to check the reachability of the
Q::B.  In this case the dest address of the NS is Q::B and the source
address will be the home address because of the source address
selection algorithm.  Since the HAO is transparent to transport-layer
(such as ICMPv6), the CN will create the ND entry for P:A.

Neighbor	Linklayer Address
P::A		(MAC addr of the MN)

After the creation of the P:A entry, the CN sends packets to the MN
directly because the CN has a ND entry for P:A instead of sending to
the home agent of the MN.

Do the other protocols have same problem?

Best Regards,

---
Keiichi SHIMA
IIJ Research Laboratory  <[email-masked]>
KAME Project <[email-masked]>

---

From: Erik Nordmark <[email-masked]>
Date: Wed, 13 Mar 2002 14:18:06 +0100 (CET)

> 'routing prefixes' it has.  But I couldn't find the definition of the

I suspect it would be more clear if it said that the HA performs all
the functions of RFC 2462 on behalf of the mobile node.
That spec has the details of how the addresses are formed from
the prefixes advertised in router advertisements.

> selection algorithm.

Why will the source address be the home address?

>  Since the HAO is transparent to transport-layer

  Erik

---

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Thu, 14 Mar 2002 12:09:04 +0900

> Why will the source address be the home address?

When a node confirm the reachability of the Q::B (a global address in
this case), the node will sent a NS packet to the address (Q::B).  The
source address of the NS packet will be the home address of the
sending MN because of the latter half of the rule 4 of
draft-ietf-ipngwg-default-addr-select.

   Rule 4: Prefer home addresses. 
   If Source(DA) is simultaneously a home address and care-of address 
   and Source(DB) is not, then sort DA before DB. Similarly, if 
   Source(DB) is simultaneously a home address and care-of address and 
   Source(DA) is not, then sort DB before DA. 
   If Source(DA) is just a home address and Source(DB) is just a care-
   of address, then sort DA before DB. Similarly, if Source(DA) is just 
   a care-of address and Source(DB) is just a home address, then sort 
   DB before DA. 

Did I misread?

Best Regards,

---
Keiichi SHIMA

---

From: Keiichi SHIMA / 島慶一 <[email-masked]>
Date: Thu, 14 Mar 2002 13:40:50 +0900

Hi,

> the prefixes advertised in router advertisements.

OK.  Then, a HA must create all BCEs for all addresses which are
supposed to be assigned to a MN when it is on its home network.
I think I now have a clear definition.

Thank you.

---
Keiichi SHIMA