Information Technology Reference
In-Depth Information
Figure11-2
A DNA Phase IV Routing Layer Header
Field length,
in bytes
1
2
2
1
Routing
flags
Destination
node
Source
node
Nodes
traversed
The first field in a DNA Phase IV routing header is the routing flags field, which includes:
•
A return-to-sender bit that, if set, indicates that the packet is returning to the source.
•
A return-to-sender-request bit that, if set, indicates that request packets should be returned to the
source if they cannot be delivered to the destination.
•
An intraLAN bit, which is on by default. If the router detects that the two communicating end
systems are not on the same subnetwork, it turns the bit off.
•
Other bits that indicate header format, whether padding is being used, and other functions.
The destination node and source node fields identify the network addresses of the destination nodes and
the source node.
The nodes traversed field shows the number of nodes the packet has traversed on its way to the
destination. This field allows implementation of a maximum hop count so that obsolete packets can be
removed from the network.
DECnet identifies two types of nodes: end nodes and routing nodes. Both end nodes and routing nodes
can send and receive network information, but only routing nodes can provide routing services for other
DECnet nodes.
DECnet routing decisions are based on cost, an arbitrary measure assigned by network administrators to
be used in comparing various paths through an internetwork environment. Costs are typically based on
hop count, media bandwidth, or other measures. The lower the cost, the better the path. When network
faults occur, the DECnet Phase IV routing protocol uses cost values to recalculate the best paths to each
destination. Figure 11-3 illustrates the calculation of costs in a DECnet Phase IV routing environment.
