Re: SFQ hash question

[Julien Vehent <julien@linuxwall.info>]

On Fri, 11 Sep 2009 15:50:45 +0200, Julien Vehent <julien@linuxwall.info>
wrote:
> Hi there,
> 
> Since LARTC is down, I believe the best place to ask traffic control
> questions is here.
> I hope you guys won't mind...
> 
> I was reading the code of SFQ, and since I'm no C expert I don't
> understand exactly on what the hash is computed:
> 
> 
> -------
> static unsigned sfq_hash(struct sfq_sched_data *q, struct sk_buff *skb)
> {
> 	u32 h, h2;
> 
> 	switch (skb->protocol) {
> 	case htons(ETH_P_IP):
> 	{
> 		const struct iphdr *iph = ip_hdr(skb);
> 		h = iph->daddr;
> 		h2 = iph->saddr ^ iph->protocol;
> 		if (!(iph->frag_off&htons(IP_MF|IP_OFFSET)) &&
> 		    (iph->protocol == IPPROTO_TCP ||
> 		     iph->protocol == IPPROTO_UDP ||
> 		     iph->protocol == IPPROTO_UDPLITE ||
> 		     iph->protocol == IPPROTO_SCTP ||
> 		     iph->protocol == IPPROTO_DCCP ||
> 		     iph->protocol == IPPROTO_ESP))
> 			h2 ^= *(((u32*)iph) + iph->ihl);
> 		break;
> 	}
> ------
> 
> OK for "h" and h2" before the "if", but what does this "if
> (!(iph->frag_off&htons(IP_MF|IP_OFFSET))" really does?
> 
> And, also, I'm not sure about this: "h2 ^= *(((u32*)iph) + iph->ihl);"
> 
> Does it XOR h2 with the first 32 bits of the L4 protocol header ?
> 

Sorry for the noise... just read the man page and it's actually explained
there.

-----
On enqueueing, each packet is assigned to a hash bucket, based on
(i)   Source address
(ii)  Destination address
(iii) Source port
If these are available. SFQ knows about ipv4 and ipv6 and also UDP, TCP
and ESP. Packets with other protocols are hashed based on the 32bits
representation of their destination and the socket they belong to. A flow
corresponds mostly to a TCP/IP connection.
-----


> 
> Thanks,
> Julien
> 
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