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Header and data length are present in response and request messages between Zabbix components. It is required to determine the end of message.
<HEADER> - "ZBXD\x01" (5 bytes) <DATALEN> - data length (8 bytes). 1 will be formatted as 01/00/00/00/00/00/00/00 (eight bytes, 64 bit number in little-endian format)
To not exhaust memory (potentially) Zabbix protocol is limited to accept only 128MB in one connection.