有没有办法在OS X上基于每个插槽禁用路径MTU发现？
This is how to set don't fragment bit on socket on Linux
但在OS X上无法识别IP_MTU_DISCOVER。
OS X上只有netinet/in.h的识别选项为：
/*
* Options for use with [gs]etsockopt at the IP level.
* First word of comment is data type; bool is stored in int.
*/
#define IP_OPTIONS 1 /* buf/ip_opts; set/get IP options */
#define IP_HDRINCL 2 /* int; header is included with data */
.....