netdev - Re: [PATCH net] net: Clear local_df only if crossing namespace.

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Date:	Mon, 10 Feb 2014 13:00:14 -0800
From:	Pravin Shelar <pshelar@...ira.com>
To:	Pravin Shelar <pshelar@...ira.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"Templin, Fred L" <Fred.L.Templin@...ing.com>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Subject: Re: [PATCH net] net: Clear local_df only if crossing namespace.

On Fri, Feb 7, 2014 at 4:58 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> [Cc Nicolas]
>
> On Fri, Feb 07, 2014 at 02:49:20PM -0800, Pravin Shelar wrote:
>> On Fri, Feb 7, 2014 at 2:28 PM, Hannes Frederic Sowa
>> <hannes@...essinduktion.org> wrote:
>> > Hi!
>> >
>> > On Fri, Feb 07, 2014 at 02:12:38PM -0800, Pravin wrote:
>> >> --- a/net/core/skbuff.c
>> >> +++ b/net/core/skbuff.c
>> >> @@ -3905,12 +3905,13 @@ EXPORT_SYMBOL(skb_try_coalesce);
>> >>   */
>> >>  void skb_scrub_packet(struct sk_buff *skb, bool xnet)
>> >>  {
>> >> -     if (xnet)
>> >> +     if (xnet) {
>> >>               skb_orphan(skb);
>> >> +             skb->local_df = 0;
>> >> +     }
>> >>       skb->tstamp.tv64 = 0;
>> >>       skb->pkt_type = PACKET_HOST;
>> >>       skb->skb_iif = 0;
>> >> -     skb->local_df = 0;
>> >>       skb_dst_drop(skb);
>> >>       skb->mark = 0;
>> >>       secpath_reset(skb);
>> >
>> > I wonder if this should be the right behaviour for tunnels, which should just
>> > do fragmentation based on IP_DF, even if the packet originated locally from a
>> > socket which allowed local fragmentation (inet->pmtudisc < IP_PMTUDISC_DO).
>> >
>> This is not about tunneling, skb_scrub_packet() is generic function
>> which should not reset local_df on all packets.
>>
>> We can have separate discussion about use of local_df and tunneling in
>> another thread.
>
> This change only affects tunnel code as of current net branch, how do
> you not expect a discussion about that in this thread, I really wonder?
>
> May I know because of wich vport, vxlan or gre, you did this change?
>
It affects both gre and vxlan.

> I am feeling a bit uncomfortable handling remote and local packets that
> differently on lower tunnel output (local_df is mostly set on locally
> originating packets).

For ip traffic it make sense to turn on local_df only for local
traffic, since for remote case we can send icmp (frag-needed) back to
source. No such thing exist for OVS tunnels. ICMP packet are not
returned to source for the tunnels. That is why to be on safe side,
local_df is turned on for tunnels in OVS.

Thanks.
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