DDP
The Datagram Delivery Protocol (DDP) provides a datagram delivery and routing service to higher layer protocols. DDP frame headers can use the long or short format. Short format DDP headers carry only the source and destination service socket numbers, while long format DDP headers also carry the source and destination network and node addresses needed for routing capability. Because AppleTalk Phase 2 does not use LLAP to identify source and destination nodes, it supports only the long format DDP header for Phase 2.
Short Format Frames
The following fields are present for short format DDP frames:
Destination socket
Destination service socket address used by the frame.
Source socket
Source service socket address used by the frame.
Length
Total length of the datagram.
DDP type
Code used to indicate the higher layer protocol used.
Long Format Frames
The following additional fields are present for long format DDP frames:
Destination
Destination network/node/socket. Destination network number, node address, and socket address used by the frame, displayed in the following format: NNNN.nn (ss), where NNNN is the network number, nn is the node address and ss is the socket address.
Source
Source network/node/socket. Source network number, node address, and socket address used by the frame, in the same format as D.
Checksum
Checksum of the entire datagram beginning after the checksum field. A checksum of zero implies that the checksum is not in use.
Hop count
Number of routers encountered by the frame. After 16 hops, the protocol discards the frame
Referenced By
ISO/OSI Reference Model | Internet protocol suite | Layer 2 | List of Internet topics | Network Layer | OSI Model | OSI reference model | OSI seven-layer model | OSI seven layer model | Open Systems Interconnection--Reference Model | TCP-IP | TCP/IP | TCP/IP Suite | TCP/IP stack | TCPIP | TLAs from AAA to DZZ
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