The sender/trapper protocol is documented, but can't be found in the most logical place: under Appendixes/Protocols. The protocol is described under Appendixes/Items/Passive and active agent checks.

Writing custom senders is probably a common task. It'd be convenient if the protocol had it's own documentation page, written with coders in mind. Even just a stub page linking to the available documentation would be a start.

• https://www.zabbix.com/documentation/3.4/manual/appendix/protocols
• https://www.zabbix.com/documentation/3.4/manual/appendix/items/activepassive