Subject: [mobile-ip] Questions on Multiple Home Agents

From: Hongbo Shi <[email-masked]>
Date: Tue, 1 Jul 2003 13:35:29 +0900

Hi, dear all. I have some questions on the multiple home agents.
1) If the draft allow a mobile node to have several home agents on the 
same link at the same time?
2) if the draft is thinking about to let a mobile node have several 
home agents on different link?

If the draft hasn't thought about the multiple home agents on the same 
link at the same time.
Following are the reasons why we need multiple home agents on the same 
link:
1) one home agent can be another home agent's backup.
2) when a home agent is down, the other working home agent can send 
warning message to the mobile node.
      Due to current mobile ipv6 solution, when the mobile node will 
decide to find a new home agent? When the mobile node
      will consider that its home agent is down? Are there any nodes 
which possible to send warning message to the mobile node
      that its home agent is down?
3) Though 2 home agents on the same home link, but it is possible that 
they connected to the different network on the other side.
      Packets sent from those correspondent nodes might use different 
routes to the different home agents.
      Is there any reason to limit the various routes?

Correct me, if I am wrong.
Best Regards.

Hongbo Shi

---

From: Jari Arkko <[email-masked]>
Date: Tue, 01 Jul 2003 22:04:56 +0300

> same link at the same time?

Yes, but this is intended as a backup mechanism.

> 2) if the draft is thinking about to let a mobile node have several home 
> agents on different link?

Yes, this is always allowed.

--Jari

---

From: Hongbo Shi <[email-masked]>
Date: Wed, 2 Jul 2003 07:41:26 +0900

On Wednesday, July 2, 2003, at 04:04  AM, Jari Arkko wrote:

>> 1) If the draft allow a mobile node to have several home agents on 
>> the same link at the same time?
>

    Right, the main reason is to backup.
    I didn't find the mechanism on current draft....

    I think the draft need to write the mechanism more clearly, like a 
mechanism that the mobile node's
    another home agent will inform the mobile node that one of its home 
agent is down and ask it to use
    the DHAAD to get a new one.

    Maybe it is just regarded as a backup mechanism,  it is very 
important to Mobile IPv6.
    Because of route optimization, a mobile node doesn't need to 
communicate with its home agent
    in Mobile IPv6 as frequently as in Mobile IPv4.

    If there is anything wrong with the home agent, a correspondent node 
with binding cache won't
    know the problem, because of the binding cache or won't be able to 
inform the problem to the
    mobile node because there is no home agent which is able to tunnel 
the packet. Other home
    agents won't be able to inform the news to the mobile node either. 
Then the remaining node is
    the mobile node itself. When the mobile node will think to 
communicate with its home agent?
    It will be the time to renew the binding update cache.

> Yes, this is always allowed.

  I didn't find the detail mechanism. I have following questions.

  Then you meant that in current draft-ietf-mobileip-ipv6-23, a mobile 
node can have several home agents
  with its different home addresses and a current care-of address at the 
same time, am i right?

  When a mobile node is on one of its home links, will the mobile node 
send binding update to the other
  home agents on its different home links? Or if not, how the other home 
agents know which home link
  the mobile node is current attached?

  Best Regards.

Hongbo Shi

---

From: Brian Haley <[email-masked]>
Date: Wed, 02 Jul 2003 10:16:03 -0400

Hi Hongbo,

>    It will be the time to renew the binding update cache.

The next time the MN sends a BU to it's HA and doesn't get an answer (in 
MAX_BINDACK_TIMEOUT/32 seconds), it will do DHAAD (if it hadn't already done so 
to get it's original HA), and choose the next HA in the list.  This is described 
in Section 11.7.1.  It is not mandatory to have >1 HA on the home link either, 
that is up to the network owner - if they think it's important then they'll do it.

There is one other problem you're overlooking - how would other HAs on the link 
know this MN had a binding?  They can't snoop on the BU since it's IPsec 
protected, and the HA doesn't broadcast onto the home link that it has a new 
binding.

Maybe there's something here for a new draft?  I would not suggest anything like 
this go into the base draft.

>  the mobile node is current attached?

The MN could have many HAs on one link, or on many links if it's multi-homed 
(but that gets complicated), the draft does not limit this in any way.  I would 
assume a MN would tell all its HAs when it is not at home, but I'm sure 
implementations will have knobs to configure such behavior.

-Brian

---

From: Hongbo Shi <[email-masked]>
Date: Thu, 3 Jul 2003 07:28:22 +0900

Hi, Brian,

On Wednesday, July 2, 2003, at 11:16  PM, Brian Haley wrote:

>>    another home agent will inform the mobile node that one of its 
>> home agent is down and ask it to use
>>    If there is anything wrong with the home agent, a correspondent 
>> node with binding cache won't
> The next time the MN sends a BU to it's HA and doesn't get an answer 
> (in MAX_BINDACK_TIMEOUT/32 seconds), it will do DHAAD (if it hadn't 
> already done so to get it's original HA), and choose the next HA in 
> the list.  This is described in Section 11.7.1.  It is not mandatory 
> to have >1 HA on the home link either, that is up to the network owner 
> - if they think it's important then they'll do it.

　Though DHAAD will cost a few seconds, the real problem is a mobile 
node won't contact with its home agent when it thinks
     its binding on its home agent is still available. If a mobile 
node's home agent is just down when the mobile node is away from
     home and the binding cache lifetime on its home agent is still 
remaining a lot, like a few minutes, the binding cache lifetime
     will be the blank time.  So if the cache lifetime is set to 420 
seconds, the worst case will be a mobile node lose all its new coming
     CNs for 420 seconds.
        　
> There is one other problem you're overlooking - how would other HAs on 
> the link know this MN had a binding?  They can't snoop on the BU since 
> it's IPsec protected, and the HA doesn't broadcast onto the home link 
> that it has a new binding.

   I don't think home agents need to care if there is a binding coming 
from a mobile node or if other home agents have the
   same binding. All they need to know is who is managing the same 
mobile node as itself.
   # This kind of information can be included in binding update messages 
sent by the mobile node.

   In order to make the Home Agent List,  home agents use IPv6 Neighbor 
Unreachability Detection to check each other
   if they are still reachable. They can catch the information faster 
than the mobile node. When a home agent detected that
   another home agent who manages the same mobile node as itself is 
unreachability, the working home agent will send
   warning message to a mobile node.

> Maybe there's something here for a new draft?  I would not suggest 
> anything like this go into the base draft.

    I would like submitting a draft, if the working group think it is 
better.

>>  with its different home addresses and a current care-of address at 
>> the same time, am i right?
>>  home agents on its different home links? Or if not, how the other 
>> home agents know which home link
> The MN could have many HAs on one link, or on many links if it's 
> multi-homed (but that gets complicated), the draft does not limit this 
> in any way.  I would assume a MN would tell all its HAs when it is not 
> at home, but I'm sure implementations will have knobs to configure 
> such behavior.

    A multi-homed mobile node is allowed to have multiple home agents, 
then how about those mobile nodes who are not multi-homed.
    If a mobile node can has its home agents on other links, I think 
that is the first step for a mobile node to move its home link.
    # I am now writing the program to try to realize above behavior.

  Best Regards.

Hongbo Shi