Inside Macintosh: Networking With Open Transport / Part 1 - Open Transport Essentials

Chapter 15 - Datagram Delivery Protocol (DDP)

This chapter describes the programming interface to Open Transport's implementation of the Datagram Delivery Protocol (DDP). It explains how you can use DDP to send and receive data across an AppleTalk internet. DDP is a connectionless transactionless service that you use to transmit data in discrete packets, each carrying its own addressing information. DDP is well suited to applications that do not require reliable delivery of data and that do not want to incur the additional processing associated with setting up and breaking down a connection. Because DDP is connectionless and does not include reliability services, it's faster than the higher-level protocols that add these services. Applications such as games that can tolerate packet loss are good candidates for the use of DDP.

A series of DDP packets transmitted over an AppleTalk internet from one node to another might incur some packet loss, for example, as a result of collisions. If your application requires a reliable service, and you do not want to implement it yourself, you should consider using a higher-level protocol such as the AppleTalk Data Stream Protocol (ADSP) or the AppleTalk Transaction Protocol (ATP). These protocols protect against packet loss and ensure reliability by using positive acknowledgment with mechanisms for retransmitting packets.

This chapter explains how you

• open and bind a DDP endpoint

• send and receive data using DDP

• set checksum options to verify that a packet has not been corrupted during transmission

• use echo packets to measure network performance

• use multinodes

This chapter begins with a description of DDP and the services that it provides under Open Transport. The section "Using General Open Transport Functions With DDP" then gives detailed information about how DDP client applications use the endpoint functions that Open Transport provides for connectionless transactionless protocols. For a more detailed explanation of endpoints and their functions, read the chapter "Endpoint" in this book.

For complete reference information about DDP options, see "DDP Reference". For an overview of DDP and how it fits within the AppleTalk protocol stack, read the chapter "Introduction to AppleTalk", which also introduces and defines some of the terminology used in this chapter. For more information about the AppleTalk address formats, see the chapter "AppleTalk Addressing". For a complete explanation of the DDP specification, see Inside AppleTalk, second edition.

---

Chapter Contents

About DDP

Using DDP

Binding a DDP Endpoint

Using the DDP Type Field to Filter Packet Delivery

Using the Self-Send and Checksum Options

Using Echo Packets

Working With Multinodes

The DDP Source Address Option

Using General Open Transport Functions With DDP