Inside Macintosh: Networking With Open Transport / Part 2 - Open Transport Reference

Chapter 29 - TCP/IP Services Reference / Options

Protocol Levels

The protocol level specifies the protocol to which the option applies. You specify the protocol level in the level field of the TOption structure when you specify an option.

enum {
   INET_IP  = 0x0,   /* IP */
   INET_TCP = 0x06,  /* TCP */
   INET_UDP = 0x11   /* UDP */
};