Paolo, Thats a fundamental H/W limitation of the current FPGA design. Long story short, it would have been inefficient to support an MTU slightly larger than a 2^N size. It’s also a pretty conservative number in terms of a an MTU that works for 99% of Hosts and network gear. -Ian
> On Aug 3, 2017, at 6:32 AM, Paolo Palana via USRP-users > <usrp-users@lists.ettus.com> wrote: > > Hello to all the mailing list. > > I'm playing around with my X310, as far as I can understand the maximum > ethernet frame size supported by this device is 8000. > (in fact in x300_impl.hpp there is this constant static const size_t > X300_10GE_DATA_FRAME_MAX_SIZE = 8000;). > > Because I'm curious I made an attempt and modified this value to 9710 (the > maximum MTU size for my linux box) and I put some cout inside the function > determine_max_frame_size (in x300_impl.cpp). This function seems to negotiate > the MTU between the X310 and the host. > > What I observerd is that if I keep X300_10GE_DATA_FRAME_MAX_SIZE=8000 > determine_max_frame_size negotiate the rigth MTU > > recv_test_frame_size = 6004 recv_size = 6004 > recv_test_frame_size = 7004 recv_size = 7004 > recv_test_frame_size = 7504 recv_size = 7504 > recv_test_frame_size = 7752 recv_size = 7752 > recv_test_frame_size = 7876 recv_size = 7876 > recv_test_frame_size = 7940 recv_size = 7940 > recv_test_frame_size = 7972 recv_size = 7972 > recv_test_frame_size = 7988 recv_size = 7988 > recv_test_frame_size = 7996 recv_size = 7996 > recv_test_frame_size = 8000 recv_size = 8000 > > while if I set X300_10GE_DATA_FRAME_MAX_SIZE=9710 when > determine_max_frame_size try to send more than 8912 bytes, happens like a > brick on the X310, > no more ethernet traffic is possible (even ICMP). This sounds really like > somewhere there is a buffer of 8192 bytes in length. > > I made also an attempt to modify the MTU parameter on the noc_shell and > axi_wrapper modules without results. > > Now I perfectly understand that I want to use the device, let me say, out of > specification, but I want to ask if there is way to have a MTU larger than > 8000. > > Thank you in advance for your attention. > > Best > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
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