Inside Macintosh: Networking /

Chapter 7 - Datagram Delivery Protocol (DDP)

This chapter describes how you can use the Datagram Delivery Protocol (DDP) to send data to and receive it from another socket across an AppleTalk internet. To use DDP, you send and receive data as discrete packets, each packet carrying its own addressing information. DDP does not allow you to set up a connection between two sockets, nor does DDP ensure that data is delivered error free as do some of the AppleTalk protocols that are built on top of it.

You should use DDP if your application does not require reliable delivery of data and you do not want to incur the additional processing associated with the use of a protocol that entails setting up and breaking down a connection. Because it is connectionless
and does not include reliability services, DDP offers faster performance than do the higher-level protocols that add these services. Applications such as diagnostic tools that retransmit packets at regular intervals to estimate averages or 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 may traverse a single high-speed EtherTalk network or they may wind across multiple intermediate data links such as LocalTalk or TokenTalk, which are connected
by routers. During the course of this process, some packet loss can occur, for example,
as a result of collisions. If you do not plan on implementing recovery from packet loss
in your application, but your application requires it, you should consider using an AppleTalk transport 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 packet retransmission mechanisms.

This chapter describes how to

• open and close sockets for sending and receiving DDP packets
• prepare the data and addressing information for each packet that you want to send
• write a socket listener that receives packets addressed to the DDP socket associated with your application
• measure packet-delivery performance

This chapter includes a sample socket listener that you can use as a model for your own socket listener or modify to fit your application's requirements.

For an overview of DDP and how it fits within the AppleTalk protocol stack, read the chapter "Introduction to AppleTalk" in this book, which also introduces and defines some of the terminology used in this chapter.

For an explanation of the DDP specification, see Inside AppleTalk, second edition.

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Chapter Contents

About DDP

About Sockets and Socket Listeners

Assigning Socket Numbers

DDP Client Protocol Types

Obtaining Data From the Network

Using DDP

Sending and Receiving Data: An Overview

Opening a Socket

Sending Data

Receiving Data

Creating a DDP Write-Data Structure

Using Registers and Packet Headers

How the .MPP Driver Calls Your Socket Listener

The DDP Packet and Frame Headers

The MPW Equates

Reading an Incoming Packet

Using Checksums

A Sample Socket Listener

Socket Listener Queues and Buffers

Setting Up the Socket Listener

Initializing the Socket Listener

Processing a Packet

Testing for Available Packets

Measuring Packet-Delivery Performance

DDP Reference

Data Structures

The Write-Data Structure

The Address Block Record

MPP Parameter Block

Routines

Opening and Closing DDP Sockets

Sending DDP Datagrams

Summary of DDP

Pascal Summary

Constants

Data Types

The Write-Data Structure

The Address Block Record

MPP Parameter Block

Routines

Opening and Closing DDP Sockets

Sending DDP Datagrams

C Summary

Constants

Data Types

The Write-Data Structure

The Address Block Record

MPP Parameter Block

Routines

Opening and Closing DDP Sockets

Sending DDP Datagrams

Assembly-Language Summary

Constants

Data Structures

MPP Parameter Block Common Fields for DDP Routines

OpenSkt Parameter Variant

CloseSkt Parameter Variant

WriteDDP Parameter Variant

Result Codes