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

Chapter 30 - AppleTalk Reference / DDP Reference

Options

In order to use any option with DDP, you must indicate which protocol the option is intended for. To do this, you use a constant for the DDP protocol in the level field of the TOption structure when you specify an option.

#define ATK_DDP   'DDP '

DDP has one DDP-specific option, DDP_OPT_SRCADDR, that sets the source address for outgoing packets.

#define DDP_OPT_SRCADDR0x2101 

A multinode endpoint must use the DDP_OPT_SRCADDR option to specify the source address for outgoing packets on a per-packet basis. This option cannot be used with the OTOptionManagement function. The option's value must be a DDP address structure using the AF_ATALK_DDP address format. The source network number, node number, and source socket are taken from the DDP address.

This option is most often used in conjuction with a multinode endpoint, but it can also be used on normal endpoints.

DDP also allows you to use the generic Open Transport options OPT_SELFSEND and OPT_CHECKSUM, which are described in the chapter "Option Management" in this book.