Based on the handling logic in src/server/listen.c, the directive "SendBufferSize" must be >= 512 bytes, or 0 for system default. However, in the document of (http://httpd.apache.org/docs/2.4/mod/mpm_common.html#sendbuffersize), no lower bound of this 512 bytes limit is mentioned. It is recommanded to add the lower bound in the doc.