netdev - Re: [PATCH net-next] inet: defrag: drop non-last frags smaller than min mtu

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Message-ID: <4b249fb2-d826-dbea-cd97-55cb336e230f@gmail.com>
Date:   Thu, 2 Aug 2018 16:54:22 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Cc:     Peter Oskolkov <posk@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] inet: defrag: drop non-last frags smaller than
 min mtu



On 08/02/2018 04:43 PM, Florian Westphal wrote:
> don't bother with pathological cases, they only waste cycles.
> IPv6 requires a minimum MTU of 1280 so we should never see fragments
> smaller than this (except last frag).
> 
> For IPv4, in practice, we could probably also adopt a higher limit,
> but for now use ipv4 min mtu (68).

...

> +	if (-skb_network_offset(skb) + skb->len < IPV4_MIN_MTU &&
> +	    ip_hdr(skb)->frag_off & htons(IP_MF))
> +		goto drop;
> +
>

I am not totally sure this is legit for IPv4.

Some intermediate nodes can try to be smart and could decide to further split fragments.

I am pretty sure I have seen this behavior on some radio environments :/

Eventually we could add a sysctl to allow an admin to set the threshold ?