> On Aug 7, 2025, at 6:35 AM, Gleb Smirnoff <gleb...@freebsd.org> wrote:
>
> Michael,
>
> On Fri, Aug 01, 2025 at 08:17:35AM +0000, Michael Tuexen wrote:
> M> When the SCTP, TCP, or UDP implementation send a packet, it does not
> M> compute the corresponding checksum but defers that. The network layer
> M> will determine whether the network interface selected for the packet
> M> has the requested capability and computes the checksum in software,
> M> if the selected network interface doesn't have the requested
> M> capability.
> M> Do this not only for packets being sent by the local SCTP, TCP,
> M> and UDP stack, but also when forwarding packets. Furthermore, when
> M> such packets are delivered to a local SCTP, TCP, or UDP stack, do not
> M> compute or validate the checksum, since such packets never have been on
> M> the wire.
> M> This allows to support checksum offloading also in the case of local
> M> virtual machines or jails.
> M> Support for epair, vtnet, and tap interfaces will be added in
> M> separate commits.
>
> Not a request for any action, but a general comment on the topic.
>
> Imagine we are developing an IP stack from scratch for modern computers. Most
> NICs do hardware checksumming, large fraction of installations run in
> containers/VMs and communicate via virtual interfaces. With modern reality we
> would probably do not do any checksumming in the stack at all, just completely
> ignore the checksum field. It would always be obligation of the NIC driver to
> care about the checksums. The existing software checksumming code would be
> just a library that drivers for legacy hardware call. Local traffic won't set
> and won't check checksums neither any related flags.
I have ever had the same idea, but that requires lots of modification to in-tree
drivers and I finally abort.
Think it twice, maybe a new driver flag say `IFF_CSUM_OFFLOAD` is needed to
indicate the driver is capable to delay calculating the checksums, and
eventually
all drivers are converted and tested.
>
> --
> Gleb Smirnoff
Best regards,
Zhenlei
