Subject: [manet] Souce address based default forwarding

From: "HyunWook Cha" <[email-masked]>
Date: Wed, 24 Sep 2003 14:08:07 +0900

Hi, Richard

I add further comments to previous ones.
I marked previous comments with "[]".
Please see additional comments.

> > Right. For source address based forwarding, additional search
> > procedures are needed in intermediate nodes. 1. lookup of 
> route table
> > for destination (/128) 2. search corresponding gateway with prefix
> > (/64) of source ip in gateway cache
> > 3. lookup of route table for the gateway found in step 2 (/128) 
> 
> Step 2 can be avoided if the routing table has a default route entry 
> for each known prefix.  Thus, step 2 is performed when the route is 
> created, not for each packet. The following procedure is then taken 
> for each packet:
>   1. lookup of route table for destination (host route)
>   2. if no host route, use the default route corresponding
>      to the source address prefix.

[Ok. You mean two route entries (host route and prefix route) per one
gateway are created and managed. ]
Your idea violates semantic of current route table because default route
for a given prefix should be looked up for source ip.
In my step 2, search of gateway cache is deployed for this reason. 
What is your opinion? 

You mentioned that if a gateway with the same prefix as the destination
is known, it sends the packet toward that gateway.
This works naturally with your scheme since this follows longest prefix
match.
However, is it safe with ingress filtering? 
I think that this forwarding policy is useful in reactive routing
protocols because gateway can notifiy sender 
that destination is reachable not through gateway but directly when
gateway has route entry for destination.
In Wakikawa's draft, some redirect message is introduced for this
purpose.
I think prefix route entry is useful but we should treat it specially if
normal forwarding using it is not safe with ingress filtering. 

Regards,
Hyunwook Cha

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