Datagram Delivery Protocol

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

The Datagram Delivery Protocol is the communication protocol within AppleTalk. DDP is similar 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 a network layer.

Addressing

DDP addresses consist of a 2-byte network number ( ie in the range 0 ... 65535 ) and a 1- byte node ID (ie, in the range 0 ... 255). The addressing of individual nodes is negotiated completely dynamic. An existing on the local network segment router serves as the contracting unit for the network number ( s).

In the first version of the AppleTalk protocol suite, a segment could be granted a network number exactly when a router was present; or the number 0 without a router. Addressing was thus allows only by the node ID. The IDs 0 and 255 have special meanings, so 254 knots were addressed in a network segment. This mode is referred to as Phase 1 or nonextended network. A further limitation was that each network segment only one zone could be assigned.

These limitations meant on the LocalTalk originally intended as a single transmission medium is no essential restriction, since a company with more than 32 nodes by increasing data collisions allows no useful data throughput more.

To circumvent these limitations, AppleTalk Phase 2 was created. Here is a complete range of network numbers assignable ( Cable Range) per segment. The node IDs 0, 254 and 255 are reserved, so the maximum number of nodes per segment from the number of assigned network numbers, multiplied with 253 In Phase2 networks several zones can be defined per network segment. Zones can also segment boundaries have the same name.

Packet types

DDP has two types of packages:

• Short header for communication in nonextended, Phase1 networks (5- byte header, 587 bytes of user data ),
• Extended header for communication in extended, Phase2 networks (13 - byte header, 587 bytes of user data ).

May optionally be used in networks nonextended extended header.

The AppleTalk protocol stack

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