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.\"	@(#)esis.4	6.1 (Berkeley) 7/25/90
.\"
.TH TP 4 "July 25, 1990"
.UC 5
.SH NAME
es-is \- End System to Intermediate System Routing Protocol
.SH SYNOPSIS
.B "pseudo-device ether"
.SH DESCRIPTION
The ES-IS routing protocol is used to dynamically map between ISO NSAP
addresses and ISO SNPA addresses; to permit End and Intermediate Systems
to learn of each other's existence; and to allow Intermediate Systems
to inform End Systems of (potentially) better routes to use when 
forwarding NPDUs to a particular destination.
.PP
The mapping between NSAP addresses and SNPA addresses is accomplished by
transmitting hello PDUs between the cooperating Systems. These PDUs
are transmitted whenever the \fIconfiguration\fR 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
\fIholding time\fR in the PDU suggests. The default \fIholding time\fR
(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(), which is defined in /sys/netiso/iso_snpac.h.
.PP
The protocol behaves differently depending on whether the System is
configured as an End System or an Intermediate System.
.SH "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 \fIall end systems\fR 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.
.PP
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 may 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 sockaddr) as the
gateway.
.PP
If the System is configured as an End System, it will report all the
NSAPs that have been configured using the ifconfig 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.
.SH "INTERMEDIATE SYSTEM OPERATION"
When an interface requests a mapping for an address not in the routing table,
an error is returned.
.PP
When an NPDU is forwarded out on the same interface that the NPDU arrived upon,
a redirect PDU is generated.
.SH "MANUAL ROUTING TABLE MODIFICATION"
.PP
To facilitate communications with systems which do not use ES-IS,
one may add a route whose destination is a sockaddr_iso containing
the NSAP in question, and the gateway being a link-level sockaddr,
either by writing a special purpose program, or using the
.IR route (8)
command e.g.:
.nf

	route add -iface -osi 49.0.4.8.0.2b.b.83.bf -link qe0:8.0.2b.b.83.bf
.fi
.PP
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 sockaddr specifying the SNPA of the IS.
.SH SEE ALSO
un(4), iso(4F), route(8), ifconfig(8C)
.br
``End system to Intermediate system routing exchange protocol
for use in conjunction with the Protocol for providing the 
connectionless-mode network service'' (ISO 9542).
.SH BUGS
Redirect PDUs do not contain options from the forwarded NPDU which generated
the redirect. The multicast address used on the 802.3 network is taken from
the NBS December 1987 agreements. This multicast address is not compatible
with the 802.5 (Token Ring) multicast addresses format. Therefore, broadcast
addresses are used on the 802.5 subnetwork.
Researchers at the University of Wisconsin are constructing an implementation
of the IS-IS routing protocol.

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