The Part 1 of ths Netlink socket structures discussion can be found in the below link.

The Netlink Socket Structures – Part 1

The netlink message is sent using a netlink message. The format of the netlink message is shown below

Fig Courtesy: https://people.redhat.com/nhorman/papers/netlink.pdf

The nlmsghdr is the netlink message header and is given below

Fig Courtesy: https://people.redhat.com/nhorman/papers/netlink.pdf

The parameters are described below

• nlmsg_len – length of the packet including the header
• nlmsg_type – The type of the message. Some message types are given below
  • NLMSG_NOOP – message is to be ignored
  • NLMSG_ERROR – error message. This is part of a structure and is provided below

Fig Courtesy: https://linux.die.net/man/7/netlink

• • NLMSG_DONE – sent in a multi-part netlink message signalling the end 
• nlmsg_flags – this is a set of logically OR’ed bit-mask which determines what action needs to be performed on the message. Some examples are provided below. For further message flags – refer  (1) in the reference section
  • NLM_F_REQUEST – Request message
  • NLM_F_ACK – Ack to a previous packet
  • NLM_F_ECHO – associated packet is echoed back to user space process
  • NLM_F_MULTI – message is a multi-part message
  • NLM_F_MATCH – This flag indicates only a subset of data
    should be returned on a protocol specific GET request 
  • NLM_F_ROOT – Used With various data retrieval (GET) operations
    for various netlink protocols, request messages with this flag
    set indicate that an entire table of data should be returned rather
    than a single entry

and various other messages.

• nlmsg_seq – a unique sequence number for the message
• nlms_pid – a unique number associated with the process – can be the pid of the process using getpid () API

Reference:

1. https://people.redhat.com/nhorman/papers/netlink.pdf
2. https://www.infradead.org/~tgr/libnl/doc/core.html#:~:text=Message%20Format,by%20a%20stream%20of%20attributes.
3. https://linux.die.net/man/7/netlink
4. https://man7.org/linux/man-pages/man7/netlink.7.html

Netlink Message Macros