netdev - RE: LRO ip

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Date:	Tue, 5 Feb 2008 08:56:17 +0000
From:	Kostya B <bkostya@...mail.com>
To:	Jan-Bernd Themann <ossthema@...ibm.com>
CC:	<netdev@...r.kernel.org>
Subject: RE: LRO ip_summed

1. Let's assume a driver which has a HW csum capability, however it has a privilege to assign any value for ip_summed.
For example the driver assigns "unnecessary" for most of the packets, however it fails to verify the packets less than 96 bytes ("checksum_none").
Now, the LRO will aggregate all of them and assign the value "used for all aggregated packets" - that is wrong, 

Of course, it is possible to avoid that situation in driver, however [proposal] from the point of view of the *generic* LRO it could be better to check each newly aggregated skb whether it has the same ip_summed as the first one (preserve coherency). Thus, there is no need to keep ip_summed field in the  struct net_lro_mgr.

2. Now, when a skb could not be aggregated (modes: aggregating SKBs) I think where's no need to override its original ip_summed (see "out" label of __lro_proc_skb()) The comment says  "Original SKB has to be posted to stack". I would be wrong, but I don't get the reason to possible change of ip_summed.

Regards,
- Kostya

----------------------------------------
> From: ossthema@...ibm.com
> To: bkostya@...mail.com
> Subject: Re: LRO ip_summed
> Date: Mon, 4 Feb 2008 10:14:23 +0100
> CC: netdev@...r.kernel.org
> 
> On Sunday 03 February 2008 10:48, Kostya B wrote:
>> 
>> Hi,
>> 
>> The mail is related to the way LRO manipulates the ip_summed value. Could anybody (author) explain why to overwrite the original value of skb->ip_summed, when it's processing by __lro_proc_skb ?
>> E.g. in out: label
>> 
>> Why not to preserve the coherency of csum status of each incoming to LRO packet, the same way the IP defragmentation does? That means - all skb in descriptor have the same value of ip_summed.
>> 
> 
> Hi,
> 
> I'm not sure if I understand your proposal correctly.
> Currently the driver decides via the struct net_lro_mgr
> a) which ip_summed value will be used for all aggregated packets
> b) which one to use for those packets that can not be aggregated
> 
> to a) this one should always be the same. The driver knows whether its HW is
>       capable of performing ip checksum checking for this kind of traffic (TCP)
> 
> There are two modes: aggregating SKBs or aggregating fragments. 
> When fragments are aggregated, there is no SKB with a filled ip_summed available.
> 
> Please outline which parts of which mode you suggest to change.
> 
> Regards,
> Jan-Bernd
> 
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