System information

Troubleshooting DECnet 11-253
The Transport Layer
Figure 11-5 DECnet Level 1 and Level 2 Routers
End systems send routing requests to a designated Level 1 router. The Level 1 router with the highest
priority is elected to be the designated router. If two routers have the same priority, the one with the
larger node number becomes the designated router. A router’s priority can be manually configured
to force it to become the designated router.
As shown in Figure 11-5, multiple Level 2 routers can exist in any area. When a Level 1 router
wishes to send a packet outside its area, it forwards the packet to a Level 2 router in the same area.
In some cases, the Level 2 router may not have the optimal path to the destination, but the mesh
network configuration offers a degree of fault tolerance not provided by the simple assignment of
one Level 2 router per area.
The Transport Layer
The DNA transport layer is implemented by a variety of transports, both proprietary and standard.
OSI transports TP0, TP2, and TP4 are supported.
Digital’s own Network Services Protocol (NSP) is functionally similar to TP4 in that it offers
connection-oriented, flow-controlled service with message fragmentation and reassembly. Two
subchannels are supported—one for normal data and one for expedited data and flow control
information. Two flow control types are supported—a simple start/stop mechanism where the
receiver tells the sender when to terminate and resume data transmission and a more complex flow
End
system
End
system
Level 1
router
Level 2
router
Level 2
router
Level 2
router
Level 1
router
Level 1
router
Level 1
router
Level 2
router
Area 10
Area 5