I'am sorry. Looks like having an mtu value multiply of 8 is a good practice.
But mtu value 1504 is also widely used in qinq linux interfaces. 2016-08-15 20:30 GMT+03:00 ????????? ??????? <kiselev99 at gmail.com>: > While playing with function rte_ipv4_fragment_packet I found that it > incorrectly fragments packets. > For example if the function takes 1200 bytes packet and mtu size 1000 it > will produces two fragments. And when those fragments are reassembled back > the resulting packet will be 4 bytes shorter than it should be. > > I played with linux ping program and it reports that a reply is truncated. > 1204 bytes from 192.168.125.1: icmp_seq=1 ttl=64 (truncated) > > Looking at the source of rte_ipv4_fragment_packet I discovered the cause > of the above behavior. > > Function makes the following assumption and the whole calculations are > bases on that assumption. > > /* Fragment size should be a multiply of 8. */ > IP_FRAG_ASSERT((frag_size & IPV4_HDR_FO_MASK) == 0); > > The problem is that this assert doesn?t make any sense. It's true that > fragment size should be a multiply of 8, but what this line real checks is > that > the size of mtu minus 20 bytes should be multiply of 8. In other words > it constrains the size of the mtu. So, if I take valid mtu value, say > 1504, > it will produce incorrect fragments when asserts are off. > > P.S. > I am using DPDK v 2.2.0 > > -- > -- > Kiselev Alexander > -- -- Kiselev Alexander
