> From: Stephen Hemminger [mailto:[email protected]] > Sent: Friday, 6 February 2026 02.49 > > Why are the header structures marked packed, that is bogus, BSD and > Linux don't do it.
They have been packed since the first public release in 2013 [1]. I guess it's because the IP and TCP headers contain 4-byte fields, which make those structures 4-byte aligned; but since the IP header follows a 14 byte Ethernet header (without the magic 2-byte pre-padding done by the kernel), the instances of the IP header are not 4-byte aligned, but 2-byte aligned. Marking them packed is a way of stripping the alignment. BTW, the IPv4 header was bumped (from no alignment) to 2-byte alignment with patch [2]. [1]: https://github.com/DPDK/dpdk/commit/af75078fece3615088e561357c1e97603e43a5fe#diff-620c2b2031359304a7f26328a52035c9f8ddf722b9280f957047dcb81467777f [2]: https://github.com/DPDK/dpdk/commit/c14fba68edfa4aeba7c0dfb5dbc3b4f23affbb81 > Windows probably does Yes, probably. The Microsoft compiler is more pedantic (leading to fewer bugs), and many of those structures should formally be packed (or more correctly: unaligned). > but Windows code seems to love packed even when > it is not necessary. I guess packing (without thinking about the need for it) has become a bad habit for some Windows programmers.
