
AppleTalk Overview
117366-A Rev. A 1-9
Figure 1-4. DDP Packet
When the Bay Networks AppleTalk router receives a packet, it checks to see if the
packet’s destination network number is the local network. If it is, the router passes
it down to the data link layer that forwards the packet toward the destination node.
If the destination network number is a different network, the router refers to its
routing tables to determine the next hop on the shortest path to the destination.
Finally, the router increments the hop count by one, and forwards the packet
toward the next hop. The router’s routing tables are maintained using the Routing
Table Maintenance Protocol.
Sockets
0 = Invalid
1 = RTMP
2 = NIS
4 = AEP
6 = ZIS
3, 5, $07 - $3F reserved
$40 - $7F experimental
$80 - $FE dynamic
DDP Types
0 = In valid
1 = R TMP
2 = NBP
3 = A TP
4 = AEP
5 = R TMP Req
6 = ZIP
7 = ADSP
$08 - $0F reserved
Special Addresses
0000.FF = network-wide broadcast
nnnn.
FF = broadcast to net
nnnn
nnnn
.00 = any router on net
nnnn
nnnn
.FE = reserved
net FF00 - FFFE = startup range
DDP Header
DDP Type
Datagram Data
(0 to 586 bytes)
Destination Node ID
Datagram Length
00
Source Node ID
DDP Checksum
Destination Network
Number
Source Network
Number
Destination Socket No.
Source Socket No.
Hop Count
APT004A
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