netdev - Re: [RFC] vxlan: use ether header as fallback hash

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Date:	Wed, 03 Oct 2012 22:06:34 -0700 (PDT)
From:	Stephen Hemminger <stephen.hemminger@...tta.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jesse Gross <jesse@...ira.com>, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [RFC] vxlan: use ether header as fallback hash


> On Wed, 2012-10-03 at 21:39 -0700, Stephen Hemminger wrote:
> > VXLAN bases source UDP port based on flow to help the
> > receiver to be able to load balance based on outer header
> > contents.
> > 
> > This patches changes the algorithm to better handle packets
> > that can not be categorized by the rxhash() function.
> > It adds a fallback to use jhash on the Ether header.
> > 
> > It also fixes a bug where the old code could assign 0 as a port
> > value.
> > 
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> > 
> > ---
> > RFC for now, compile tested only
> > 
> > --- a/drivers/net/vxlan.c	2012-10-03 21:25:43.747968165 -0700
> > +++ b/drivers/net/vxlan.c	2012-10-03 21:36:10.213805422 -0700
> > @@ -622,12 +622,30 @@ static inline u8 vxlan_ecn_encap(u8 tos,
> >  	return INET_ECN_encapsulate(tos, inner);
> >  }
> >  
> 
> #include <linux/jhash.h>
> 
> > +/* Compute hash to use for source port
> > + *   first choice to use L4 flow hash since it will spread
> > + *     better and maybe available from hardware
> > + *   secondary choice is to use jhash on the Ethernet header
> > + * Always returns non-zero value
> > + */
> > +static u16 vxlan_flow_hash(struct sk_buff *skb)
> > +{
> > +	u16 hash = skb_get_rxhash(skb);
> 
> skb_get_rxhash(skb) returns an u32, that could have low order 16bits
> set
> to 0.
> 
> So I would use u32 hash = skb_get_rxhash(skb);


Turns out that 0 is a valid source port if no reply is requested
(per RFC and wikipedia)
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