esis.4

早期freebsd实现

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.\"     @(#)esis.4	8.2 (Berkeley) 11/30/93
.\"
.Dd November 30, 1993
.Dt ESIS 4
.Os
.Sh NAME
.Nm es-is
.Nd End System to Intermediate System Routing Protocol
.Sh SYNOPSIS
.Sy pseudo-device
.Nm ether
.Sh DESCRIPTION
The
.Nm ES-IS
routing protocol is used to dynamically map between
.Tn ISO NSAP
addresses and
.Tn 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
.Tn NPDU Ns s
to a particular destination.
.Pp
The mapping between
.Tn NSAP
addresses and
.Tn SNPA
addresses is accomplished by
transmitting hello
.Tn PDU Ns s
between the cooperating Systems. These
.Tn PDU Ns s
are transmitted whenever the
.Em configuration
timer expires.
When a hello
.Tn PDU
is received, the
.Tn SNPA
address that it conveys is stored in the routing table for as long as the
.Em holding time
in the
.Tn PDU
suggests. The default
.Em holding time
(120 seconds) placed in the hello
.Tn PDU ,
the configuration timer value,
and the system type (End System or Intermediate System) may be changed by
issuing an
.Dv SIOCSSTYPE
.Xr ioctl 2 ,
which is defined in
.Pa /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
.Tn SNPA
of any known Intermediate System is returned. If an Intermediate
System is not known, then the
.Em all end systems
multicast address
is returned. It is assumed that the intended recipient of the NPDU will
immediately transmit a hello
.Tn PDU
back to the originator of the
.Tn NPDU .
.Pp
If an
.Tn NPDU
is forwarded by the End System, a redirect
.Tn PDU
will not be
generated.
However, redirect
.Tn PDU Ns s
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
.Tn NSAP
address contained in the redirect
.Tn 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
.Tn 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
.Tn NSAP Ns s
that have been configured using the ifconfig command, and no others.
It is possible to have more than one
.Tn NSAP
assigned to a given interface,
and it is also possible to have the same
.Tn NSAP
assigned to multiple
interfaces.
However, any
.Tn NSAP
containing an NSEL that is consistent with the
nsellength option (default one) of any interface will be accepted as
an
.Tn 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
.Tn NPDU
is forwarded out on the same interface that the
.Tn NPDU
arrived upon,
a redirect
.Tn PDU
is generated.
.Sh MANUAL ROUTING TABLE MODIFICATION
.Pp
To facilitate communications with systems which do not use
.Nm ES-IS,
one may add a route whose destination is a sockaddr_iso containing
the
.Tn NSAP
in question, and the gateway being a link-level sockaddr,
either by writing a special purpose program, or using the
.Xr route 8
command e.g.:
.Bd -literal
route add -iface -osi 49.0.4.8.0.2b.b.83.bf \
	-link qe0:8.0.2b.b.83.bf
.Ed
.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
.Tn IS ,
using the route command in a similar way.
There, the destination address should be
.Dq default
(spelled 
out literally as 7
.Tn ASCII
characters), and the gateway should be
once again be a link-level sockaddr specifying the
.Tn SNPA
of the
.Tn IS .
.Sh SEE ALSO
.Xr un 4 ,
.Xr iso 4 ,
.Xr route 8 ,
.Xr ifconfig 8
.Rs
.%T "End system to Intermediate system routing exchange protocol for use in conjunction with the Protocol for providing the connectionless-mode network service"
.%R ISO
.%N 9542
.Re
.Sh BUGS
Redirect
.Tn PDU Ns s
do not contain options from the forwarded
.Tn NPDU
which generated
the redirect. The multicast address used on the 802.3 network is taken from
the
.Tn 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
.Tn IS-IS
routing protocol.