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+.. SPDX-License-Identifier: BSD-3-Clause
+
+=================================================================
+Netlink specification support for legacy Generic Netlink families
+=================================================================
+
+This document describes the many additional quirks and properties
+required to describe older Generic Netlink families which form
+the ``genetlink-legacy`` protocol level.
+
+Specification
+=============
+
+Attribute type nests
+--------------------
+
+New Netlink families should use ``multi-attr`` to define arrays.
+Older families (e.g. ``genetlink`` control family) attempted to
+define array types reusing attribute type to carry information.
+
+For reference the ``multi-attr`` array may look like this::
+
+  [ARRAY-ATTR]
+    [INDEX (optionally)]
+    [MEMBER1]
+    [MEMBER2]
+  [SOME-OTHER-ATTR]
+  [ARRAY-ATTR]
+    [INDEX (optionally)]
+    [MEMBER1]
+    [MEMBER2]
+
+where ``ARRAY-ATTR`` is the array entry type.
+
+array-nest
+~~~~~~~~~~
+
+``array-nest`` creates the following structure::
+
+  [SOME-OTHER-ATTR]
+  [ARRAY-ATTR]
+    [ENTRY]
+      [MEMBER1]
+      [MEMBER2]
+    [ENTRY]
+      [MEMBER1]
+      [MEMBER2]
+
+It wraps the entire array in an extra attribute (hence limiting its size
+to 64kB). The ``ENTRY`` nests are special and have the index of the entry
+as their type instead of normal attribute type.
+
+type-value
+~~~~~~~~~~
+
+``type-value`` is a construct which uses attribute types to carry
+information about a single object (often used when array is dumped
+entry-by-entry).
+
+``type-value`` can have multiple levels of nesting, for example
+genetlink's policy dumps create the following structures::
+
+  [POLICY-IDX]
+    [ATTR-IDX]
+      [POLICY-INFO-ATTR1]
+      [POLICY-INFO-ATTR2]
+
+Where the first level of nest has the policy index as it's attribute
+type, it contains a single nest which has the attribute index as its
+type. Inside the attr-index nest are the policy attributes. Modern
+Netlink families should have instead defined this as a flat structure,
+the nesting serves no good purpose here.
+
+Operations
+==========
+
+Enum (message ID) model
+-----------------------
+
+unified
+~~~~~~~
+
+Modern families use the ``unified`` message ID model, which uses
+a single enumeration for all messages within family. Requests and
+responses share the same message ID. Notifications have separate
+IDs from the same space. For example given the following list
+of operations:
+
+.. code-block:: yaml
+
+  -
+    name: a
+    value: 1
+    do: ...
+  -
+    name: b
+    do: ...
+  -
+    name: c
+    value: 4
+    notify: a
+  -
+    name: d
+    do: ...
+
+Requests and responses for operation ``a`` will have the ID of 1,
+the requests and responses of ``b`` - 2 (since there is no explicit
+``value`` it's previous operation ``+ 1``). Notification ``c`` will
+use the ID of 4, operation ``d`` 5 etc.
+
+directional
+~~~~~~~~~~~
+
+The ``directional`` model splits the ID assignment by the direction of
+the message. Messages from and to the kernel can't be confused with
+each other so this conserves the ID space (at the cost of making
+the programming more cumbersome).
+
+In this case ``value`` attribute should be specified in the ``request``
+``reply`` sections of the operations (if an operation has both ``do``
+and ``dump`` the IDs are shared, ``value`` should be set in ``do``).
+For notifications the ``value`` is provided at the op level but it
+only allocates a ``reply`` (i.e. a "from-kernel" ID). Let's look
+at an example:
+
+.. code-block:: yaml
+
+  -
+    name: a
+    do:
+      request:
+        value: 2
+        attributes: ...
+      reply:
+        value: 1
+        attributes: ...
+  -
+    name: b
+    notify: a
+  -
+    name: c
+    notify: a
+    value: 7
+  -
+    name: d
+    do: ...
+
+In this case ``a`` will use 2 when sending the message to the kernel
+and expects message with ID 1 in response. Notification ``b`` allocates
+a "from-kernel" ID which is 2. ``c`` allocates "from-kernel" ID of 7.
+If operation ``d`` does not set ``values`` explicitly in the spec
+it will be allocated 3 for the request (``a`` is the previous operation
+with a request section and the value of 2) and 8 for response (``c`` is
+the previous operation in the "from-kernel" direction).
+
+Other quirks
+============
+
+Structures
+----------
+
+Legacy families can define C structures both to be used as the contents of
+an attribute and as a fixed message header. Structures are defined in
+``definitions``  and referenced in operations or attributes.
+
+members
+~~~~~~~
+
+ - ``name`` - The attribute name of the struct member
+ - ``type`` - One of the scalar types ``u8``, ``u16``, ``u32``, ``u64``, ``s8``,
+   ``s16``, ``s32``, ``s64``, ``string`` or ``binary``.
+ - ``byte-order`` - ``big-endian`` or ``little-endian``
+ - ``doc``, ``enum``, ``enum-as-flags``, ``display-hint`` - Same as for
+   :ref:`attribute definitions <attribute_properties>`
+
+Note that structures defined in YAML are implicitly packed according to C
+conventions. For example, the following struct is 4 bytes, not 6 bytes:
+
+.. code-block:: c
+
+  struct {
+          u8 a;
+          u16 b;
+          u8 c;
+  }
+
+Any padding must be explicitly added and C-like languages should infer the
+need for explicit padding from whether the members are naturally aligned.
+
+Here is the struct definition from above, declared in YAML:
+
+.. code-block:: yaml
+
+  definitions:
+    -
+      name: message-header
+      type: struct
+      members:
+        -
+          name: a
+          type: u8
+        -
+          name: b
+          type: u16
+        -
+          name: c
+          type: u8
+
+Fixed Headers
+~~~~~~~~~~~~~
+
+Fixed message headers can be added to operations using ``fixed-header``.
+The default ``fixed-header`` can be set in ``operations`` and it can be set
+or overridden for each operation.
+
+.. code-block:: yaml
+
+  operations:
+    fixed-header: message-header
+    list:
+      -
+        name: get
+        fixed-header: custom-header
+        attribute-set: message-attrs
+
+Attributes
+~~~~~~~~~~
+
+A ``binary`` attribute can be interpreted as a C structure using a
+``struct`` property with the name of the structure definition. The
+``struct`` property implies ``sub-type: struct`` so it is not necessary to
+specify a sub-type.
+
+.. code-block:: yaml
+
+  attribute-sets:
+    -
+      name: stats-attrs
+      attributes:
+        -
+          name: stats
+          type: binary
+          struct: vport-stats
+
+C Arrays
+--------
+
+Legacy families also use ``binary`` attributes to encapsulate C arrays. The
+``sub-type`` is used to identify the type of scalar to extract.
+
+.. code-block:: yaml
+
+  attributes:
+    -
+      name: ports
+      type: binary
+      sub-type: u32
+
+Multi-message DO
+----------------
+
+New Netlink families should never respond to a DO operation with multiple
+replies, with ``NLM_F_MULTI`` set. Use a filtered dump instead.
+
+At the spec level we can define a ``dumps`` property for the ``do``,
+perhaps with values of ``combine`` and ``multi-object`` depending
+on how the parsing should be implemented (parse into a single reply
+vs list of objects i.e. pretty much a dump).