icmp rpf

Patrick W. Gilmore patrick at ianai.net
Mon Sep 25 01:40:46 UTC 2006

[Can we all have a moment of silence for a useful, interesting, and  
on-topic post?]

On Sep 24, 2006, at 5:59 PM, Mark Kent wrote:

> A smaller North American network provider, with a modest North
> American backbone, numbers their internal routers on public IP space
> that they do not announce to the world.
> One of the largest North American network providers filters/drops
> ICMP messages so that they only pass those with a source IP
> address that appears in their routing table.
> As a result, traceroutes from big.net into small.net have numerous
> hops that time out.
> Traceroutes from elsewhere that go into small.net but return on
> big.net also have numerous hops that time out.
> We do all still think that traceroute is important, don't we?
> If so, which of these two nets is unreasonable in their actions/ 
> policies?

Who said either was?

First: Your network, your rules.  Don't expect others to play by your  

But more importantly, there is nothing that says two perfectly  
reasonable, rational "rules" cannot create a problem when  
intersecting in interesting ways.

But if forced, I'd say Small.Net gets my vote for needing  
correction.  I see less "wrongness" in a networking running what is  
essentially loose RPF than a network who expects supposedly bogon- 
sourced packets to be forwarded.  (One could argue that non-announced  
space is bogus.)

Just remember, I would only say that if pushed.  Normally I would say  
neither is wrong.

> Please note that we're not talking about RFC1918 space, or reserved IP
> space of any kind.   Also, think about the scenario where some failure
> happens leaving big.net with an incomplete routing table, thus  
> breaking
> traceroute when it is perhaps most needed.

In such an instance, I would suggest Big.Net will have far, far  
larger problems than whether pings get returned from prefixes it  
can't reach anyway.


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