Re: [PATCH v6 0/4] af_packet correctness, performance, cksum

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 6 Feb 2026 15:36:37 +0100
Morten Brørup <mb@smartsharesystems.com> wrote:

> > From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> > 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.
> 

Making structure packed (in the past) made code slower on some architectures because
it required generating multiple load/store operations.