ESIS(4) 						   BSD Kernel Interfaces Manual 						   ESIS(4)

NAME

     esis -- End System to Intermediate System Routing Protocol

SYNOPSIS

     #include <sys/types.h>
     #include <netiso/esis.h>

DESCRIPTION

     The ES-IS routing protocol is used to dynamically map between ISO NSAP addresses and ISO SNPA addresses; to permit End Systems (ES) and
     Intermediate Systems (IS) to learn of each other's existence; and to allow Intermediate Systems to inform End Systems of (potentially) better
     routes to use when forwarding Network Protocol Data Units (NPDUs) to a particular destination.

     The mapping between NSAP addresses and SNPA addresses is accomplished by transmitting "hello" Protocol Data Units (PDUs) between the cooper-
     ating 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.

END SYSTEM OPERATION

     When an interface requests a mapping for an address not in the cache, the SNPA of any known Intermediate System is returned.  If an Interme-
     diate System is not known, then the all end systems multicast address is returned.  It is assumed that the intended recipient of the NPDU
     will immediately transmit a "hello" PDU back to the originator of the NPDU.

     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 desti-
     nation 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.

INTERMEDIATE SYSTEM OPERATION

     When an interface requests a mapping for an address not in the routing table, an error is returned.

     When an NPDU is forwarded out on the same interface that the NPDU arrived upon, a redirect PDU is generated.

MANUAL ROUTING TABLE MODIFICATION

     To facilitate communications with systems which do not use ES-IS, one may add a route whose destination is a struct sockaddr_iso containing
     the NSAP in question, and the gateway being a link-level struct sockaddr, either by writing a special purpose program, or using the route(8)
     command e.g.:

     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.

SEE ALSO

     iso(4), ifconfig(8), route(8)

     End system to Intermediate system routing exchange protocol for use in conjunction with the Protocol for providing the connectionless-mode
     network service, ISO, 9542.

BUGS

     Redirect PDUs do not contain options from the forwarded NPDU which generated the redirect.  The multicast address used on the IEEE 802.3
     (Ethernet) network is taken from the National Bureau of Standards (NBS) December 1987 agreements.	This multicast address is not compatible
     with the IEEE 802.5 (Token Ring) multicast addresses format.  Therefore, broadcast addresses are used on the IEEE 802.5 subnetwork.

     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).

BSD
								 November 30, 1993							       BSD