netdev - IPv4 tunnels: why IP-IP and SIT enforce DF bit, but GRE does not?

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Message-ID: <56574F32.6070706@shemyak.com>
Date:	Thu, 26 Nov 2015 20:28:02 +0200
From:	Konstantin Shemyak <konstantin@...myak.com>
To:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: IPv4 tunnels: why IP-IP and SIT enforce DF bit, but GRE does not?

The kernel has taken the decision to always enforce DF bit on IPv4 
tunnels, which have fixed (not inherited) TTL (e.g. 
net/ipv4/ipip.c:ipip_tunnel_ioctl()). Commment by Alexey Kuznetsov in 
the head of ip_gre.c explains that the reason is attempting to avoid 
network loops.

But the commit c54419321455631 removed this enforcing from GRE tunnels,
not changing this behavior for IP-IP (net/ipv4/ipip.c) and SIT 
(net/ipv6/sit.c).

It can be discussed whether such enforcing of DF bit is exactly the
desired behavior, but shouldn't it at least be identical across IPv4 
tunnels?

Konstantin Shemyak
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