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Inside Macintosh: Networking /

Chapter 7 - Datagram Delivery Protocol (DDP) / About DDP

DDP Client Protocol Types

AppleTalk allows for the implementation of up to 254 parallel protocols that are clients of DDP. The DDP protocol type field, which is the last field of the DDP packet header, specifies the type of protocol that the packet is intended for. Figure 7-6 on page 7-15 shows the 1-byte DDP protocol type field of the DDP packet header.

The socket listener for a single socket can receive packets whose protocol type fields contain different values. It is the responsibility of your socket-client application to define its own protocol types. Your socket-client application can define more than one DDP protocol type and receive packets for any of the protocol types it handles, sorting them by reading the value of the DDP protocol type field.

For example, if you are implementing a server, you might define one protocol type
for data and another for attention messages, and have separate routines to handle
the different packet types. You fill in the DDP protocol type field when you build the contents of a DDP packet to be sent to another socket.

For more information on how to specify a protocol type for a DDP client application
and the range of valid values for the DDP protocol type field, see Appendix C in Inside AppleTalk, second edition.