Note that in NetworkManager API (D-Bus, libnm, and nmcli),
the features are called "feature-xyz". The "feature-" prefix
is used, because NMSettingEthtool possibly will gain support
for options that are not only -K|--offload|--features, for
example -C|--coalesce.
The "xzy" suffix is either how ethtool utility calls the feature
("tso", "rx"). Or, if ethtool utility specifies no alias for that
feature, it's the name from kernel's ETH_SS_FEATURES ("tx-tcp6-segmentation").
If possible, we prefer ethtool utility's naming.
Also note, how the features "feature-sg", "feature-tso", and
"feature-tx" actually refer to multiple underlying kernel features
at once. This too follows what ethtool utility does.
The functionality is not yet implemented server-side.
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