Request for comment -- BCP38

Jay R. Ashworth jra at
Sun Oct 2 01:25:31 UTC 2016

----- Original Message -----
> From: "Laszlo Hanyecz" <laszlo at>

>> If you have links from both ISP A and ISP B and decide to send traffic
>> out ISP A's link sourced from addresses ISP B allocated to you, ISP A
>> *should* drop that traffic on the floor.  There is no automated or
>> scalable way for ISP A to distinguish this "legitimate" use from
>> spoofing; unless you consider it scalable for ISP A to maintain
>> thousands if not more "exception" ACLs to uRPF and BCP38 egress
>> filters to cover all of the cases of customers X, Y, and Z sourcing
>> traffic into ISP A's network using IPs allocated to them by other ISPs?
> This is a legitimate and interesting use case that is broken by BCP38.
> The effectiveness of BCP38 at reducing abuse is dubious, but the
> benefits of asymmetric routing are well understood.  Why should everyone
> have to go out of their way to break this.. it works fine if you just
> don't mess with it.

Let me see if I have your argument straight:

In order to enable an "interesting" use case that is used by maybe well under 
1% of end nodes not in PI address space, we should decide *not* to do 
something which makes it much easier to protect attack targets against
well over 75% of incoming attacks of ridiculous (>OC-12) bandwidth?

Is that what you're advocating?


-- jra
Jay R. Ashworth                  Baylink                       jra at
Designer                     The Things I Think                       RFC 2100
Ashworth & Associates          2000 Land Rover DII
St Petersburg FL USA      BCP38: Ask For It By Name!           +1 727 647 1274

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