I have been observing large ethernet packets when generating TCP traffic over a local ethernet, up to a bit over 20000 bytes, even though the interface MTU is 1500 bytes. Furthermore, I tried to use setsockopt with TCP_MAXSEG to limit the TCP segment size further, to 1000 bytes, and that didn't have any effect. When bugreporting a related problem to the debian kernel maintainers, I was told that the behaviour may be linked to the use of TCP segmentation offload (see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=600286). Disabling TSO and GSO using ethtool solves both problems: Generated packets now are limited in size by both the interface MTU and the segment size set with setsockopt. (One annoying case is the atl1c driver, where at least the version I'm running, linux-2.6.32, driver 1.0.0.1-NAPI, responds to ethtool -K eth0 tso off with "Cannot set device tcp segmentation offload settings: Operation not supported", so I don't see any way to disable it). Before I try to write proper bug reports on specific network drivers (I have seen problems with several network drivers on different machines, unfortunately using different linux versions), I would like to know: 1. Is TCP_MAXSEG supposed to work at all with network drivers that do tcp segmentation offload? 2. If it is supposed to work, can someone give a rough sketch on how the per-socket segment size, set with setsockopt(... TCP_MAXSEG,...), is passed down to the driver and to the network hardware? I suspect it ought to be passed with each "pseudo-packet" to be transmitted. I have spent some time searching the documentation and the net for answers, without result, hence I'm posting to this list. I'm not subscribed, so please cc any replies. (Regarding packets larger than the interface MTU, that seems clearly buggy to me, and I think I already know enough to be able to file proper bug reports). Best regards, /Niels -- Niels Möller. PGP-encrypted email is preferred. Keyid C0B98E26. Internet email is subject to wholesale government surveillance. -- To unsubscribe from this list: send the line "unsubscribe linux-net" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
