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Date:	Fri, 27 Apr 2007 15:26:23 -0700
From:	Bryan Lawver <>
To:	Rick Jones <>
Cc:	"Michael S. Tsirkin" <>,,
	Linux Network Development list <>
Subject: Re: [ofa-general] Re: IPoIB forwarding

I hit the IP NIC over the head with a hammer and turned off all offload 
features and I no longer get the super jumbo packet and I have symmetric 
performance.  This NIC supported "ethtool -K ethx tso/tx/rx/sg on/off" and 
I am not sure at this time which one I needed to whack but all off solved 
the problem.

Thanks for listening and re enforcing my search process.


At 01:32 PM 4/27/2007, Rick Jones wrote:
>Bryan Lawver wrote:
>>Your right about the ipoib module not combining packets (I believed you 
>>without checking) but I did never the less.  The ipoib_start_xmit routine 
>>is definitely handed a "double packet"  which means that the IP NIC 
>>driver or the kernel is combining two packets into a single super jumbo 
>>packet.  This issue is irrespective of the IP MTU setting because I have 
>>set all interfaces to 9000k yet  ipoib accepts and forwards this 17964 
>>packet to the next IB node and onto the TCP stack where it is never 
>>acknowledged.  This may not have come up in prior testing because I am 
>>using some of the fastest IP NICs which have no trouble keeping up with 
>>or exceeding the bandwidth of the IB side.  This issue arises exactly 
>>every 8 packets...(ring buffer overrun??)
>>I will be at Sonoma for the next few days as many on this list will be.
>Some NICs (esp 10G) support large receive offload - they coalesce TCP 
>segments from the wire/fiber into larger ones they pass up the 
>stack.  Perhaps that is happening here?
>I'm going to go out a bit on a limb, cross the streams, and include 
>netdev, because I suspect that if a system is acting as an IP router, one 
>doesn't want large receive offload enabled.  That may need some discussion 
>in netdev - it may then require some changes to default settings or some 
>documentation enhancements.  That or I'll learn that the stack is already 
>dealing with the issue...
>rick jones
>>At 11:06 AM 4/26/2007, Michael S. Tsirkin wrote:
>>> > Quoting Bryan Lawver <>:
>>> > Subject: Re: IPoIB forwarding
>>> >
>>> > Here's a tcpdump of the same sequence.  The TCP MSS is 8960 and it 
>>> appears
>>> > that two payloads are queued at ipoib which combines them into a single
>>> > 17920 payload with assumingly correct IP header (40) and IB header
>>> > (4).  The application or TCP stack does not acknowledge this double 
>>> packet
>>> > ie. it does not ACK until each of the 8960 packets are resent
>>> > individually.  Being an IB newbie, I am guessing this combining is
>>> > allowable but may violate TCP protocol.
>>>IPoIB does nothing like this - it's just a network device so
>>>it sends all packets out as is.
>>general mailing list
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