ESIS(4) BSD Kernel Interfaces Manual ESIS(4)NAMEesis — 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 Interme‐
diate 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 particu‐
lar destination.
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 con‐
veys 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 Sys‐
tem 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 con‐
figured 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 Intermediate
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 pro‐
cessing consists of adding an entry in the routing table. If the redi‐
rect 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 mul‐
tiple interfaces. However, any NSAP containing an NSEL that is consis‐
tent 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) com‐
mand 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 ALSOiso(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 gener‐
ated 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 implemen‐
tation of the IS-IS routing protocol.
NBS is now known as the National Institute for Standards and Technology
(NIST).
BSD November 30, 1993 BSD
