Datagram Delivery Protocol

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Datagram Delivery Protocol (DDP) is a term from computer science .

The Datagram Delivery Protocol is the data transfer protocol within AppleTalk . The DDP is comparable to the Internet Protocol (IP).

Name Binding Protocol (NBP), Routing Table Maintenance Protocol and Zone Information Protocol (ZIP) use DDP (Datagram Delivery Protocol).

The protocol is part of the network layer .

addressing

DDP addresses consist of a 2-byte network number ( i.e. in the range 0… 65535) and a 1-byte node ID (i.e. in the range 0… 255). The addressing of the individual nodes is negotiated completely dynamically. A router in the local network segment serves as an allocation unit for the network number (s).

In the first version of the AppleTalk protocol family, a segment could be assigned exactly one network number if a router was present; or the number 0 without router. Addressing was only made possible by the node ID. The IDs 0 and 255 have special meanings, so that 254 nodes could be addressed in a network segment. This mode is called phase 1 or nonextended network. Another limitation was that only one zone could be assigned per network segment .

These limitations did not mean any significant restrictions on LocalTalk, which was originally intended to be the only transmission medium , since operation with more than 32 nodes no longer enables usable data throughput due to the increasing data collisions .

AppleTalk Phase2 was created to circumvent these limitations. A whole range of network numbers can be assigned to each segment ( cable range ). Node IDs 0, 254 and 255 are reserved, so the maximum number of nodes per segment results from the number of assigned network numbers multiplied by 253. In phase 2 networks, several zones can be defined per network segment. Zones can also have the same name across segment boundaries.

Package types

DDP knows two types of packages:

  • Short header , for communication in nonextended , phase 1 networks (5-byte header, 587 bytes of user data ),
  • Extended header for communication in extended , phase 2 networks (13-byte header, 587 bytes of user data).

Optionally, extended headers can also be used in nonextended networks.

The AppleTalk protocol stack

The AppleTalk protocols can be divided into several layers that form a protocol stack . The protocols can be classified in the ISO-OSI reference model as follows :

OSI layer AppleTalk protocol stack
7th AFP PAP
6th
5 ZIP ASP ADSP
4th          ATP AEP NBP RTMP
3 DDP
2 LLAP ELAP TLAP FDDI ← AARP
1 LocalTalk Ethernet
driver 		Token Ring
Driver 		FDDI
driver

literature

  • Gursharan S. Sidhu, Richard F. Andrews, Alan B. Oppenheimer: Inside AppleTalk, Second Edition . Ed .: Apple Computer, Inc. 2nd edition. Addison-Wesley Publishing Company, Inc., 1990, ISBN 0-201-55021-0 (English).
  • Apple Computer Inc .: Inside Macintosh: Networking , 2nd, Addison-Wesley, 1994 , Chapter 1 - Introduction to AppleTalk (online version)