Subject: [manet] Default route (internet route) for global connectivity

From: "Youngmin Kim" <[email-masked]>
Date: Mon, 27 Feb 2006 17:21:04 +0900

Hello.
I have had a question while I was reading draft-wakikawa-manet-globalv6-04.txt.
Manet node configures a routable IP address from the prefix of the
gateway and inserts the gateway address as an internet route. Next hop
for default route is the internet-gateway address.
As I know, next hop is one hop distance neighbor. Internet-gateway may
not be one hop neighbor from manet node. How can the internet-gateway
be set to next hop? You use a tunnel?

Thank you.

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From: Ryuji Wakikawa <[email-masked]>
Date: Mon, 27 Feb 2006 17:31:34 +0900

Hello Kim

> be set to next hop? You use a tunnel?

The route configuration is different on each OS.
I want to leave this as implementation specific.

What we wrote in the draft is that MN setup  both a default route and  
a host route to IGW.
1. :: -> IGW
2. IGW -> next hop
Then, a MN can recursively lookup the routing table.

I think how to treat a route inside OS is beyond specification.

regards
ryuji

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From: Christophe Jelger <[email-masked]>
Date: Mon, 27 Feb 2006 11:37:26 +0100

Hi Ryuji/all,

See my comments inline below.

regards,
Christophe

Ryuji Wakikawa wrote:
> What we wrote in the draft is that MN setup  both a default route and  a 
> host route to IGW.
> 1. :: -> IGW
> 2. IGW -> next hop

Well we've already discussed this a while ago and I tend to disagree 
here for a few reasons:

1. OS don't do dual-lookups, and having a solution that requires 
dual-lookups (or recursive as you call it) introduces a lot of hassles: 
changing kernel-code, patching, re-compiling, which slows down use and 
deployment (e.g. MIPv6).

2. Whether you use a tunnel (or a routing header) or rely on "routing 
coherence" imposes different requirements. The first solution is the 
easiest because you explicitly say via which gateway you want your 
packets to go through. The second solution requires a logical 
organization (e.g. prefix continuity) and it's more complex to setup but 
you don't need to introduce any forwarding indirection (which usually 
means changing or "hooking into" the kernel in order to setup the tunnel).

I think that leaving such details as "implementation specific" is risky 
because these details are exactly what makes MANETs different from 
traditional LANs. Especially the point 2. (above) is tricky because two 
implementations may end up using different schemes and may not 
interoperate. For having implemented various flavors of autoconf, I know 
it's not a simple issue that should be left aside. Other issues also 
arise: for example, once you have a default route how would your 
reactive protocol behave ? Always try to discover first and use the 
gateway if it fails ?

These kind of issues are exactly what we need to clarify.

Regards,
Christophe

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From: Fred Baker <[email-masked]>
Date: Mon, 27 Feb 2006 14:55:48 -0800

> gateway and inserts the gateway address as an internet route. Next  
> hop for default route is the internet-gateway address. As I know,  
> next hop is one hop distance neighbor. Internet-gateway may not be  
> one hop neighbor from manet node. How can the internet-gateway be  
> set to next hop? You use a tunnel?

I think there are several possible responses to this.

It turns out that there are next-hops and next-hops.

In the FIB, any routing protocol or combination of routing protocols  
boils everything down to a set of next-hop routers, often a unit set.  
In determining that, it applies the longest-match rule: if there are  
multiple prefixes in the FIB that could answer the question, the  
longest applicable prefix is deemed the definitive one. So if  
(proactive case) you have a default route to a given gateway and no  
more definitive prefix applies, you take that path, and the next-hop  
in the FIB will be the next router closer to the exit gateway.

In BGP, the routing protocol can also advertise what is called a  
"next hop", which is in fact potentially several router hops away.  
The point is that it is one BGP hop. What the BGP routers do with it  
is say "fine, my IGP is going to calculate a route to that router,  
and what I will store in the FIB as a next hop is the IGP routing  
neighbor next closer to the BGP next hop". One example of how that is  
used is in turning off ddos attacks - one configures every router in  
the network with a route like

	ip route 127.0.0.2 255.255.255.255 null 0

and allows BGP to now advertise a route to the address under attack  
has having the next hop 127.0.0.2. Voila, all BGP speakers  
(presumably every edge router in the network if nobody else) will  
immediately drop traffic to the indicated address, since it is being  
sent via a null interface.

 From my perspective, what I would expect a proactive protocol to do  
is a lot like what OSPF does in a stub area: advertise a default  
route from every area border router into the area. If it is an NSSA  
area (one that has multiple ABRs), the default route is sent as a  
type 7 route, and other ABRs receiving it know to not re-advertise it  
into the backbone. If there is a single ABR, it might simply be a  
type 1 route, or (I wish, as this would make the type 7 unnecessary)  
a type 3 summary. What I would expect a reactive protocol like AODV  
to do would be to get to the gateway and have the gateway respond to  
the request if it had a bona fide route to the address. One could  
imagine the gateway pinging the address and replying on receipt of  
the ping, or having an ACL that described routes it would reply on  
the behalf of.