netdev - Re: [PATCH 2/2] ipvs: Extend MTU check to account for IPv6 NAT defrag changes

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Message-ID: <alpine.LFD.2.00.1208281235080.1693@ja.ssi.bg>
Date:	Tue, 28 Aug 2012 12:47:40 +0300 (EEST)
From:	Julian Anastasov <ja@....bg>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
cc:	netdev@...r.kernel.org, Patrick McHardy <kaber@...sh.net>,
	lvs-devel@...r.kernel.org, Simon Horman <horms@...ge.net.au>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Hans Schillstrom <hans@...illstrom.com>,
	Wensong Zhang <wensong@...ux-vs.org>,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 2/2] ipvs: Extend MTU check to account for IPv6 NAT defrag
 changes


	Hello,

On Tue, 28 Aug 2012, Jesper Dangaard Brouer wrote:

> On Mon, 2012-08-27 at 18:20 +0300, Julian Anastasov wrote:
> > 
> > > @@ -956,8 +963,11 @@ ip_vs_tunnel_xmit_v6(struct sk_buff *skb, struct ip_vs_conn *cp,
> > >  		skb_dst(skb)->ops->update_pmtu(skb_dst(skb), NULL, skb, mtu);
> > >  
> > >  	/* MTU checking: Special for tunnel mode */
> > > -	if (mtu < ntohs(old_iph->payload_len) + sizeof(struct ipv6hdr) &&
> 
> I guess:
>  ntohs(old_iph->payload_len) + sizeof(struct ipv6hdr)
> Is the same as:
>  skb->len

	I think so. You can think of this in different way:
all transmitters are called from same place, there is no
difference in the packets we see. When we can use
__mtu_check_toobig_v6 for other methods relying on skb->len
being correct, we can do the same for tunnels, only that
tunnels have lower MTU, that is the only difference.

> > > -	    !skb_is_gso(skb)) {
> > > +	if ((!IP6CB(skb)->frag_max_size &&
> > > +	     (mtu < ntohs(old_iph->payload_len) + sizeof(struct ipv6hdr) &&
> > > +	      !skb_is_gso(skb)))
> > > +	    || IP6CB(skb)->frag_max_size + sizeof(struct ipv6hdr) > mtu) {
> 
> > 
> > 	mtu is already reduced with the new outer header size,
> > may be we can just call __mtu_check_toobig_v6 with mtu?
> 
> To Julian, is the extra sizeof(struct ipv6hdr) addition to
> frag_max_size, wrong? (as the mtu is already reduced)

	Yes, sizeof(struct ipv6hdr) is needed only together
with payload_len because payload_len does not include the
first header.

> If above statements hold, I think we can simply use
> __mtu_check_toobig_v6() also for the tunnel case :-)

	Yep

> --Jesper

Regards

--
Julian Anastasov <ja@....bg>
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