BCP38 is hard, was TWC (AS11351) blocking all NTP?

Mark Andrews marka at isc.org
Tue Feb 4 23:46:47 UTC 2014

In message <52F17102.2000505 at alvarezp.ods.org>, Octavio Alvarez writes:
> On 04/02/14 14:18, John Levine wrote:
> > I was at a conference with people from some Very Large ISPs.  They
> > told me that many of their large customers absolutely will not let
> > them do BCP38 filtering.  ("If you don't want our business, we can
> > find someone else who does.")  The usual problem is that they have PA
> > space from two providers and for various reasons, not all of which
> > are stupid, traffic with provider A's addresses sometimes goes out
> > through provider B.  Adding to the excitement, some of these
> > customers are medium sized ISPs with multihomed customers of their
> > own.
> I haven't read it all, but section 3 says:
> > However, by restricting transit traffic which originates from a
> > downstream network to known, and intentionally advertised,
> > prefix(es), the problem of source address spoofing can be virtually
> > eliminated in this attack scenario.
> If ISP has customer A with multiple *known* valid networks --doesn't 
> matter if ISP allocated them to customer or not-- and ISP lets them all 
> out, but filters everything else, ISP is still complying with BCP 38.
> Here it's not a matter of blocking "just because". It's blocking unknown 
> addresses. It doesn't either mean that ISP should not open the filters 
> if a new prefix is requested by the customer.

Or downstream customers.

SIDR provides provides the crypotographic glue that can be used to
automate this.

The end customer has a CERT which authenticates their use of the
address.  The second ISP supplies a CERT which the end customer
signs saying they can source this range.  Repeat until you reach a
big enough ISP that you just have a agreement that no unverified
traffic will injected down the link.

These CERTS can then be used to build perfect input and output
filters.  Initially you may have to have manual inputs but with
increasing use of SIDR the amount of manual intervention will drop.

If you multi-home you need to have provable use of the address space.

This isn't significantly different to the regular use of SIDR.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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