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Chapter 2
Protocol Attributes
is a Layer 2 heading that is prepended to the DDP header. We start from the
beginning of the DDP header in this discussion.
Long DDP Header - Datagram Delivery Protocol
Unused: %00
Hop Count: %0000
Datagram Length: 21
DDP Checksum: 0x0000
Dest. Network: 107
Source Network: 101
Dest Node: 1
Source Node: 217
Dest. Socket: 6 Zone Information
Source Socket: 245
DDP Type: 3 ATP
As you can see, the DDP header starts with an unused field. The next field
is four bits and is used for the hop count. The count is incremented as the
packet transits additional routers.
Following the hop count, you see the Datagram Length field. This field
is 10 bits long and it measures the length of the datagram in bytes. After the
datagram length field, we move onto the DDP Checksum field. The DDP
checksum
field is a full 16 bits, and is used to aid in CRC calculation.
The next four fields are used only in the long (extended) DDP header.
They represent destination and source information, each according to the
description. Following the extended information, you see the DSN and SSN.
These fields are used to define destination and source socket numbers. The
allowable socket numbers are RTMP (value = 1), names information (value
= 2), echoer (value = 4), and zone information (value = 6).
There are seven different DDP types that can be specified by the DDP Type
field using eight bits. These types include: RTMP response or data (value =
1), NBP (value = 2), ATP (value = 3), AEP (value = 4), RTMP requests (value
= 5), ZIP (value = 6), and ADSP (value = 7). The actual DDP datagram fol-
lows the DDP type.
Routing Table Maintenance Protocol
Routing Table Maintenance Protocol (RTMP) is responsible for AppleTalk
routing tables and their information. This is AppleTalk's proprietary
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