linux-kernel - Re: Inconsistency in packet drop due to MTU (eth vs veth)

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Message-ID: <159f4b9e4ab.12b79bda8122843.8859202944513905410@gmail.com>
Date:   Tue, 31 Jan 2017 14:32:20 +0100
From:   Fredrik Markstrom <fredrik.markstrom@...il.com>
To:     "Eric Dumazet" <eric.dumazet@...il.com>
Cc:     "netdev" <netdev@...r.kernel.org>, "Evan Jones" <ej@...njones.ca>,
        "David S. Miller" <davem@...emloft.net>,
        "Cong Wang" <cwang@...pensource.com>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Vijay Pandurangan" <vijayp@...ayp.ca>,
        "linux-kernel" <linux-kernel@...r.kernel.org>,
        "ebiederm" <ebiederm@...stanetworks.com>
Subject: Re: Inconsistency in packet drop due to MTU (eth vs veth)

 ---- On Thu, 19 Jan 2017 19:53:47 +0100 Eric Dumazet <eric.dumazet@...il.com> wrote ---- 
 > On Thu, 2017-01-19 at 17:41 +0100, Fredrik Markstrom wrote: 
 > > Hello, 
 > >  
 > > I've noticed an inconsistency between how physical ethernet and veth handles mtu. 
 > >  
 > > If I setup two physical interfaces (directly connected) with different mtu:s, only the size of the outgoing packets are limited by the mtu. But with veth a packet is dropped if the mtu of the receiving interface is smaller then the packet size.  
 > >  
 > > This seems inconsistent to me, but maybe there is a reason for it ?  
 > >  
 > > Can someone confirm if it's a deliberate inconsistency or just a side effect of using dev_forward_skb() ? 
 >  
 > It looks this was added in commit 
 > 38d408152a86598a50680a82fe3353b506630409 
 > ("veth: Allow setting the L3 MTU") 
 >  
 > But what was really needed here was a way to change MRU :( 

Ok, do we consider this correct and/or something we need to be backwards compatible with ? Is it insane to believe that we can fix this "inconsistency" by removing the check ?

The commit message reads "For consistency I drop packets on the receive side when they are larger than the MTU", do we know what it's supposed
to be consistent with or is that lost in history ?

/Fredrik

 >  
 >  
 >  
 >