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Date:   Fri, 11 Jan 2019 09:09:04 -0800
From:   Peter Oskolkov <>
To:     Timothy Winters <>
Cc:     Michal Kubecek <>, netdev <>,
        Eric Dumazet <>,
        Eric Dumazet <>,
        Florian Westphal <>,
        Tom Herbert <>,
        David Miller <>
Subject: Re: [PATCH v3 23/30] ipv6: defrag: drop non-last frags smaller than
 min mtu

On Fri, Jan 11, 2019 at 6:54 AM Timothy Winters <> wrote:
> Hi Michal and Eric,
> Thanks for the clarification.   I'm thinking about creating a draft to say no fragments less then 640 unless it's the last fragment.   Does that work for your code going forward?

I will prepare a patchset to convert IPv6 defrag queue to rbtree+list,
similarly to how IPv4 defrag queue currently works. Just in case it is
decided to go this route. I don't think having an
arbitrary/non-standard size cap (640) is a good approach.


> ~Tim
> On Fri, Jan 11, 2019 at 9:21 AM Michal Kubecek <> wrote:
>> On Friday, 11 January 2019 14:26 Timothy Winters wrote:
>> > Hi Eric,
>> >
>> > So I understand correctly the attack that you are trying to prevent is
>> > many small fragments from different IPs?
>> >
>> > The 6MAN working group has had some discussion about this topic, if
>> > you want read some IPv6 networking prospectives.
>> >
>> >
>> > dXdk9MN4dP-uiiag
>> I haven't read all mails in that discussion but most seem to be missing
>> the point. The problem is not memory consumption, we have a (runtime
>> configurable) limit for that. The problem is that by sending many small
>> (8 bytes) fragments of a large (up to 64 KB) packet but never finishing
>> it, an attacker can force receiving host into using quite a lot of CPU
>> time just by looking up the fragment queue of the partially reassembled
>> packet.
>> Currently, IPv6 reassembly uses a simple linear list which is fine with
>> 1280 byte long fragments (up to ~50 of them) but not with 8 byte long
>> ones (there could be as many as ~8000). The IPv4 reassembly code (where
>> we cannot assume minimal size of non-last fragment) switched to rbtree
>> (with logarithmic lookup time) exactly for this reason.
>> > What about lowering the value of accepted fragments?  to something
>> > like 1280/2?
>> That would be probably sufficient to mitigate the DoS attacks.
>> Michal Kubecek

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