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Date:   Tue, 30 Jun 2020 17:51:41 +0200
From:   Nicolas Dichtel <>
To:     Jakub Kicinski <>,
        Oliver Herms <>
Subject: Re: [PATCH v3] IPv4: Tunnel: Fix effective path mtu calculation

Le 30/06/2020 à 08:22, Jakub Kicinski a écrit :
> My understanding is that for a while now tunnels are not supposed to use
> dev->hard_header_len to reserve skb space, and use dev->needed_headroom, 
> instead. sit uses hard_header_len and doesn't even copy needed_headroom
> of the lower device.

I missed this. I was wondering why IPv6 tunnels uses hard_header_len, if there
was a "good" reason:

$ git grep "hard_header_len.*=" net/ipv6/
net/ipv6/ip6_tunnel.c:                  dev->hard_header_len =
tdev->hard_header_len + t_hlen;
net/ipv6/ip6_tunnel.c:  dev->hard_header_len = LL_MAX_HEADER + t_hlen;
net/ipv6/sit.c:         dev->hard_header_len = tdev->hard_header_len +
sizeof(struct iphdr);
net/ipv6/sit.c: dev->hard_header_len    = LL_MAX_HEADER + t_hlen;

A cleanup would be nice ;-)

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