Subject: RE: Host Centric Multi-Homing

From: Cedric de Launois <[email-masked]>
Date: Mon, 02 Feb 2004 14:58:16 +0100

> I have already updated the draft with some of your comments, you can check
> it at the same location:
> 
> http://www.it.uc3m.es/marcelo/draft-huitema-multi6-hosts-02.txt

Thanks for the changes.

Here are some other comments. Yes, I like your draft ;)

6.4 Solution 4: Host based exit path selection + source address based
    routing

 "Required modifications: implementation of the path discovery
  mechanism"

 What is exactly the path discovery mechanism ? Is it the path failure
 detection described 5.1.2.2.2 ? You should perhaps refer to the
 appropriate section where the mechanism is described.

7. Proposed solution
7.1 Multihoming solutions for small sites

I don't see clearly which solution (from the ones you describe in
section 6) you recommend.
In fact, you are recommending 2 (optimized) solutions for small sites :

- Host based exit path selection + site exit discovery
- Host based exit path selection + source address based routing

Some mechanisms you discuss in section 7.1 apply to the first solution
while others apply to the second. This is a little bit confusing, and it
is difficult to have a coherent view of each solution you propose.

You should perhaps present the mechanisms separately for each solution.

BTW, it would be interested to know which solution (based on source
routing or based on site exit discovery) is preferred by you and by
people out of this wg. There is an important design choice to make here.

7.2. Multihoming solution for medium sites

Which solution from section 6 are you recommending ??
>From what you described, it seems that it is solution 5 (host
based exit path selection + site exit discovery). But this is 
contradicting since you said solution 5 is only suitable for small
sites.

I also deduce you are not recommending source based routing. Maybe
you should make this point more explicit.

-------------------------------
Purely editorial

1. Introduction

 "There are two basic forms of multihoming, multiple interface [...]"

  s/interface/interfaces/

3.2 Source address selection

 "the normal procedure is to select the address"

 s/address/destination address/

 "[...] and finally selects a source address"

 s/selects/select/

 "When the hosts has multiple addresses"

 s/hosts/host/

3.3 The site exit issue

 The example is more clear. Thanks.

 "ISP C, similarly, sees a packet whose match the prefix assigned by
  that ISP to Y."

 s/whose match/whose source address does not match/

4.1 Relaxing the source address checks

 "the provider has to be believe that the site will transmit the right
  prefixes"

 s/believe/believed/ ??

5.1.2 Internally initiated communications

 "to let the intra-site routing system to perform both tasks."

 s/to perform/perform/

 "The second possibility presented is to let the host to do both tasks"

 s/to do/do/

5.1.2.1 Exit path selection by the intra-site routing system

 "to let the intra-site routing system to perform"

 s/to perform/perform/

 "a single ehternet"

 s/ehternet/ethernet/

5.1.2.2 Host based exit path selection

 "a mechanism that to the host force the routing of packets"

 ?

7.1.2.1.1 Site exit redirection ICMP message

 "Site Exit Address: An IP address of the preferred exit router touse"

 "touse" ?

7.1.2.2.2.3 Presering established communications

 s/Presering/Preserving/

7.1.2.2.1 Direct link failures

 "be used as source address at anay given time"

 s/anay/any/

7.2.1 Reaction to topology changes

 "Each entry contain for each Destination Address"

   s/contain/contains/

 "If it does exists, the host obtains the source address [...]"

   s/exists/exist/

7.3 Multihoming solution for big sites

 "This mechanism is simpler becuase no ingress filtering"

   s/becuase/because/

Cedric

---

From: "marcelo bagnulo" <[email-masked]>
Date: Mon, 2 Feb 2004 18:50:55 +0100

Hi Cedric,

thank you very much for your feedback and your interest

>  detection described 5.1.2.2.2 ?

Yes

> You should perhaps refer to the

I will do that

> section 6) you recommend.

Yes, this is not clear

the proposed solution is a combination of some of the solutions

> - Host based exit path selection + source address based routing

This is correct.
the basic solution is the one based in source address routing and the site
exit discovery is an optimization

> You should perhaps present the mechanisms separately for each solution.

Yes, it is confusing, i agree

the point is that section 6 is an abstract analysis of the options, while
section 7 is an attempt of solution. The optimal solution is a combination
of multiple mecahnisms, as you have already pointed out.

But i will try to improve the readability of the doc.

> people out of this wg. There is an important design choice to make here.

IMHO the adoption of a multihomed solution can be proposed as a three step
process.

The different steps require different amount of modifications

1- the first step would be source address based routing restricted to the
site exit routers (a mesh of tunnels between the site exit routers). This si
simple, and required no upgrading (neither on hosts nor in routers)
This first step overcomes the site exit issue and allows regular (non
upgraded) IPv6 hosts to work propoerly in a multihomed site. The problem
here is that the routing of the packets will be suboptimal

2- the second step is to modify the hosts to make the site exit router
discovery. If so, upgraded hosts would obtain an optimized performance,
since they would directly send the packets to the appropriate exitr router.
This way we achieve optimal (internal) routing. The problem here is that we
still introduce overhead

3- The third step woulñd be based in source address based routing. This may
require a upgrade of the routers and perhaps routing protocol. At least it
requires to understand better what is needed to achieve this, and why people
don't like it

> Which solution from section 6 are you recommending ??

In this case the hybrid approach is proposed. Path availability is
discovered using BGP and the site exit routers inform the host which is the
correct isp (source address) that they should use (something like putting a
NAROS server in the exit router)

> you should make this point more explicit.

Well, you could use source address routing or let the host tunnel the packet
directly to the host. The three step roadmap would also fit here.

Ok, thanks

Regards, marcelo