On Fri, Apr 8, 2016 at 7:04 PM, Alexander Duyck <alexander.du...@gmail.com> wrote: > On Fri, Apr 8, 2016 at 2:40 PM, Jesse Gross <je...@kernel.org> wrote: >> Maybe I missed it but I didn't see any checks for the DF bit being set >> when we transmit a packet with NETIF_F_TSO_MANGLEID. Even if I am >> comfortable mangling my IDs in the DF case, I don't think this would >> ever extend to non-DF packets. In the documentation you noted that it >> is the driver's responsibility to do this check but I couldn't find it >> in either ixgbe or igb. It would also be nice if the core stack could >> enforce it somehow as well rather than each driver. > > Yeah I had glossed over that in the igb and ixgbe patches. A check is > only really needed for the incrementing to non-incrementing case and I > wasn't sure how common it was to have TCP with an IP header that > didn't set the DF bit. In the case of the outer headers igb and ixgbe > will increment the IP ID always so we don't have to worry about if DF > is set of not there. For the inner headers I had fudged it a bit and > didn't add the validation. If needed I can see about adding that > shortly.
TCP without the DF bit set is not the default but it is possible (it can be enabled by setting /proc/sys/net/ipv4/ip_no_pmtu_disc). I also did a quick check of some Internet services and at least some of them seem to return TCP without DF, so it's not too rare.
