es-is - End System to Intermediate System Routing Protocol
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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 forward-
ing NPDUs to a particular destination.
The mapping between NSAP addresses and SNPA addresses is
accomplished by transmitting hello PDUs between the
cooperating Systems. These PDUs are transmitted whenever the
_
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\bn 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 _
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\be in the PDU suggests.
The default _
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\be (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
The protocol behaves differently depending on whether the
System is configured as an End System or an Intermediate
E
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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
_
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\bs 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 Intermedi-
ate 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.
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 pos-
sible 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.
I
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\bON
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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.
M
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\bIF
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\bCA
\bAT
\bTI
\bIO
\bON
\bN
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 gate-
way being a link-level sockaddr, either by writing a special
purpose program, or using the _
\br_
\bo_
\bu_
\bt_
\be(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 sin-
gle network interface which does not support multicast
reception, it is necessary to manually configure the loca-
tion 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 gate-
way should be once again be a link-level sockaddr specifying
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un(4), iso(4F), route(8), ifconfig(8C)
``End system to Intermediate system routing exchange proto-
col for use in conjunction with the Protocol for providing
the connectionless-mode network service'' (ISO 9542).
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 agree-
ments. 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.
projects / unix-history / blob