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
bryan
At 11:06 AM 4/26/2007, Michael S. Tsirkin wrote:
> Quoting Bryan Lawver <[EMAIL PROTECTED]>:
> 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.
--
MST
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