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Date: Fri, 27 Apr 2007 13:32:51 -0700 From: Rick Jones <rick.jones2@...com> To: Bryan Lawver <lawver1@...l.gov> Cc: "Michael S. Tsirkin" <mst@....mellanox.co.il>, general@...ts.openfabrics.org, Linux Network Development list <netdev@...r.kernel.org> Subject: Re: [ofa-general] Re: IPoIB forwarding 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 <lawver1@...l.gov>: >> > 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 > > > _______________________________________________ > general mailing list > general@...ts.openfabrics.org > http://lists.openfabrics.org/cgi-bin/mailman/listinfo/general > > To unsubscribe, please visit > http://openib.org/mailman/listinfo/openib-general - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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