Jakub Kicinski <kuba@xxxxxxxxxx> writes: > On Mon, 11 Dec 2023 16:40:34 +0000 Donald Hunter wrote: >> +Sub-messages >> +------------ >> + >> +Several raw netlink families such as rt_link and tc have type-specific >> +sub-messages. These sub-messages can appear as an attribute in a top-level or a >> +nested attribute space. >> + >> +A sub-message attribute uses the value of another attribute as a selector key to >> +choose the right sub-message format. For example if the following attribute has >> +already been decoded: > > We may want to explain why we call this thing "sub-message". How about: > > Several raw netlink families such as rt_link and tc use attribute > nesting as an abstraction to carry module specific information. > Conceptually it looks as follows:: > > [OUTER NEST OR MESSAGE LEVEL] > [GENERIC ATTR 1] > [GENERIC ATTR 2] > [GENERIC ATTR 3] > [GENERIC ATTR - wrapper] > [MODULE SPECIFIC ATTR 1] > [MODULE SPECIFIC ATTR 2] > > The GENERIC ATTRs at the outer level are defined in the core (or rt_link > or core TC), while specific drivers / TC classifiers, qdiscs etc. can > carry their own information wrapped in the "GENERIC ATTR - wrapper". > Even though the example above shows attributes nesting inside the wrapper, > the modules generally have full freedom of defining the format of the nest. > In practice payload of the wrapper attr has very similar characteristics > to a netlink message. It may contain a fixed header / structure, netlink > attributes, or both. Because of those shared characteristics we refer > to the payload of the wrapper attribute as a sub-message. I'll incorporate into next revision. Thanks! >> +A sub-message attribute uses the value of another attribute as a selector key to >> +choose the right sub-message format. For example if the following attribute has >> +already been decoded:
