[lwip-users] IPv6 MTU vs IPv4 MTU

[Raphael Zulliger]

Hi

The following findings and statements are mostly based on observation 
and I'm not an IP guru, so take the following with a grain of salt.

Having, among others, IPv6 and LWIP_ND6_ALLOW_RA_UPDATES enabled, an 
"IPv6 router advertisement" that specifies an MTU causes LWIP to update 
its internal MTU size to the advertised one. Therefore, any forthcoming 
packet will be treated by this new MTU. The problem with this (IMO) is 
that this mistakenly not only happens for IPv6 packets but also for 
forthcoming IPv4 packets.

IMHO, the MTUs for both versions of IP should be treated separately, at 
least this is what my Ubuntu 16.04/64bit seems to do: It uses 1500 Bytes 
MTU for IPv4 (as reported by 'ifconfig') and uses 1480, as advertised by 
our router (reported by 'traceroute6 
xxx:xxx:xxxx::0204:3dff:fe00:2864'). Moreover, Wireshark actually 
doubles that, as the IPv4 packets sent from Linux to the LWIP embedded 
device use up to 1500 payload, where as the responses from the LWIP 
embedded device occupy at most 1480 Bytes.

As for now, I've patched my (not 100% up to date) version of LWIP with 
the attached patch, whereas both MTU's are initially set to 1500.

What do you think? Have I found a bug and shall I open a bug report or 
am I wrong?

Thanks,
Raphael

mtu.patch
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