The mapping between
NSAP
addresses and
SNPA
addresses is accomplished by transmitting
"hello
Protocol Data Units
(PDUs)
between the cooperating Systems.
These
PDUs
are transmitted whenever the
configuration
timer expires.
When a
"hello
PDU
is received, the
SNPA
address that it conveys is stored in the routing table for as long as the
holding time
in the
PDU
suggests.
The default
holding time
(120 seconds) placed in the
"hello
PDU,
the configuration timer value, and the system type (End System or
Intermediate System) may be changed by issuing an
SIOCSSTYPE
ioctl(2),
which is defined in
<sys/netiso/iso_snpac.h
>.
The protocol behaves differently depending on whether the System is configured as an End System or an Intermediate System.
If an NPDU is forwarded by the End System, a redirect PDU will not be generated. However, redirect PDUs received will be processed. This processing consists of adding an entry in the routing table. If the redirect is towards an Intermediate System, then an entry is made in the routing table as well. The entry in the routing table will mark the NSAP address contained in the redirect PDU as the gateway for the destination system (if an NET is supplied), or will create a route with the NSAP address as the destination and the SNPA address (embodied as a link-level struct sockaddr) as the gateway.
If the System is configured as an End System, it will report all the NSAPs that have been configured using the ifconfig(8) command, and no others. It is possible to have more than one NSAP assigned to a given interface, and it is also possible to have the same NSAP assigned to multiple interfaces. However, any NSAP containing an NSEL that is consistent with the nsellength option (default one) of any interface will be accepted as an NSAP for this System.
When an NPDU is forwarded out on the same interface that the NPDU arrived upon, a redirect PDU is generated.
route add -iface -osi 49.0.4.8.0.2b.b.83.bf -link qe0:8.0.2b.b.83.bf
If the System is configured as an End System and has a single network interface which does not support multicast reception, it is necessary to manually configure the location of an IS, using the route command in a similar way. There, the destination address should be ``default'' (spelled out literally as 7 ASCII characters), and the gateway should be once again be a link-level struct sockaddr specifying the SNPA of the IS.
Researchers at the University of Wisconsin are constructing an implementation of the IS-IS routing protocol.
NBS is now known as the National Institute for Standards and Technology (NIST).