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Using IPv6 with prefixes shorter than a /64 on a LAN



On Sun, Jan 30, 2011 at 6:24 PM, Fernando Gont <fernando at gont.com.ar> wrote:
> Hi, Matthew,
>
> On 30/01/2011 08:17 p.m., Matthew Petach wrote:
>>>> The problem I see is the opening of a new, simple, DoS/DDoS scenario.
>>>> By repetitively sweeping a targets /64 you can cause EVERYTHING in
>>>> that /64 to stop working by overflowing the ND/ND cache, depending on
>>>> the specific ND cache implementation and how big it is/etc.
>>>
>>> That depends on the ND implementation being broken enough by not
>>> limiting the number of neighbor cache entries that are in the INCOMPLETE
>>> state. (I'm not saying those broken implementations don't exist, though).
>>
>> Even without completely overflowing the ND cache, informal lab testing
>> shows that a single laptop on a well-connected network link can send
>> sufficient packets at a very-large-scale backbone router's connected /64
>> subnet to keep the router CPU at 90%, sustained, for as long as you'd
>> like. ?So, while it's not a direct denial of service (the network keeps
>> functioning, albeit under considerable pain), it's enough to impact the
>> ability of the network to react to other dynamic loads. ?:/
>
> This is very interesting data. Are you talking about Ciscos? Any
> specific model?

Uh, I've gotten into some trouble in the past for mentioning router
vendors by name before in public forums, so I'm going to avoid
public mention of names; but it seems that others in this thread
are able to speak up with specific details, if that helps answer
your question in a slightly more roundabout way.  ^_^;

> I guess that a possible mitigation technique (implementation-based)
> would be to limit the number of ongoing addresses in address resolution.
> (i.e., once you have X ongoing ND resolutions, the router should not be
> engaged in ND for other addresses) -- note that addresses that the
> router had already resolved in the past would not suffer from this
> penalty, as their corresponding entries would be in states other than
> INCOMPLETE.
>
> Thoughts?
>
> Thanks,

That's been one of the areas that's ripe for development, yes; have the
control plane take some preferential actions to avoid harming established
connectivity under stressful circumstances like that; potentially taking
steps to avoid aging out older, potentially still valid entries if there may
not be sufficient resources to safely re-learn them, for example.

Matt