Netmap Checksum Offloading

[Jim Thompson]

We've focused on just the IP header checksum, but it's possible to add
L4 checksum offload as well.

I asked Luigi why he hadn't included checksum offload (with a library
in software for devices that don't offer a hw offload), and his answer
was that when he wrote netmap, he wanted a fast path to the wire.

On Tue, Jun 14, 2016 at 10:08 AM, Dominik Schoeffmann
<schoeffm at in.tum.de> wrote:
> Dear Netmap Developers,
>
> during the course of my bachelor's thesis, I modified a packet generator
> called MoonGen [1] in order to utilize netmap.
> One key component was to flexibly offload checksums for different kinds
> of packets (IPv4, UDP, TCP).
> The ixgbe netmap patch was modified [2] in order to construct context
> descriptors and suitable data descriptors. This is implemented in less
> than 250 LoC (including pseudo-header calculations).
> The man page states, that checksum offloading is available via ethtool,
> although a solution inside the netmap API might be a cleaner way for
> applications to actually use these features.
> Attached is a graph showing the performance implication of using
> offloading in the current implementation.
> As can be seen, offloading has only a minor impact.
> When regarding this data (and comparing it to other frameworks), please
> keep in mind, that internally a lot of per-packet effort is needed due
> to the software architecture of the packet generator.
>
> The question being:
> Would it not make sense to include checksum offloading inside of netmap
> in order to accomodate applications operating on layer 3 and above?
> As these programs need to calculate the checksums in software, it would
> be just as fast to move these calculations to the kernel for NICs
> without checksum offloading support (and the kernel would act as a library).
> The problem which currently is imposed by the fact, that netmap exports
> the complete ring, is that context descriptors disrupt the data
> descriptors, which is unpleasant for the application.
> But you may find the data interesting nevertheless.
>
>
> Best Regards,
> Dominik Schoeffmann
>
>
>
> [1] https://github.com/dschoeffm/MoonGen/tree/netmap
> [2] https://github.com/dschoeffm/netmap/tree/mg-chksum-offloading
>