As per Relevance of the word intermediate, we have this rfc below:
Network Working Group J.
Request for Comments 1074 T.J. Watson Research
IBM
October 1988
The NSFNET Backbone SPF based Interior Gateway
Status of this
This memo is an implementation description of the standard ANSI IS-
and ISO ES-IS routing protocols within the NSFNET backbone network
Distribution of this memo is unlimited
I would like to express my thanks to Hans-Werner Braun (MERIT)
his contribution to this document
1.
This document provides an overview of the NSFNET Backbone
with specific emphasis on the intra-backbone routing
By the end of 1987, the American National Standardization
(ANSI) forwarded a specification for an Intermediate System
Intermediate System routing protocol to the
Standardization Organizations (ISO) for the adaptation as
international standard. This ANSI IS-IS protocol is used as
interior gateway protocol (IGP) of the NSFNET backbone.
here is an implementation description which also includes
definitions that were necessary for the integration into an
Protocol (IP) environment. Therefore, it should be viewed as
continuation of the specifications of the ANSI IS-IS protocol [1]
the ISO standard End System to Intermediate System (ES-IS)
[2]. While the ANSI IS-IS protocol suffices as an IGP,
methods are used to orchestrate routing between the backbone and
attached mid-level networks; most notably the Exterior
Protocol (EGP). Further information about the overall NSFNET
as well as some future aspects can be found in [3], [4], [5] and [6].
2. A brief overview of the NSFNET
The NSFNET backbone is a wide area network which currently
thirteen sites within the continental United States. All
are permanent point-to-point links at T1 speed (1.544Mbps). These T
links may contain multiple logical links at sub-T1 and up to the
T1 speed. The result is a hybrid circuit/packet switching
able to contain a connectivity-richer logical topology than
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RFC 1074 NSFNET Backbone SPF based IGP October 1988
underlying physical topology would allow by itself. Each site has
Nodal Switching Subsystem (NSS) which is responsible for
switching. Each NSS is a RISC technology based multiprocessor
using IBM RT/PC processors which operate a modified version of
4.3BSD kernel. For the purpose of routing, each NSS is considered
a single entity which has connections to both other NSS (via
logical network infrastructure) and to regional networks (via
area network attachments; typically an Ethernet).
The routing protocol which is used for the inter-NSS routing
the NSFNET backbone is an adaptation of the ANSI IS-IS
protocol [1]. The routing protocol which is used between
backbone and the attached mid-level networks is the Exterior
Protocol (EGP) [3]. The information exchange between the
and its connected EGP peers is subject to policy based
restrictions which are maintained in the Policy Based
Database [4,5].
3. An overview of the ANSI IS-IS routing
The ANSI IS-IS routing protocol specifies a two level
routing where Level 1 routing deals with routing within an area
while Level 2 routing deals with routing between different areas
This routing protocol belongs to a class of so called "Link State
protocols where each node maintains a complete topology of the
network. The route computation is based on a modified version
Dijkstra's Shortest Path First (SPF) algorithm
Both Level 1 and Level 2 routing use two types of Protocol Data
(PDU):
The Level 1 Router Link PDU lists IS neighbors. The Level 1
System PDU lists ES neighbors
The Level 2 Router Link PDU lists neighbor Level 2 routes.
Level 2 End System PDU lists address prefixes for systems
other Routing Domains
The ANSI IS-IS document separates subnetwork independent
from the subnetwork dependent functions. Subnetwork
functions include dissemination of Router Link and End System
PDU's and the Routing Algorithm. The subnetwork dependent
cover different types of subnets such as X.25, permanent point-to
point links and LANs
The IS-IS Protocol is designed to interoperate with the End System
Intermediate System (ES-IS) routing exchange protocol [2]. The ES-
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RFC 1074 NSFNET Backbone SPF based IGP October 1988
protocol is used to determine connectivity and network
addresses. This information is used to construct the Router
PDUs
4. How the ANSI IS-IS protocol is adapted for the NSFNET
The NSFNET backbone implements a subset of the ANSI IS-IS protocol
With respect to subnetwork independent functions, it only
Level 2 routing. With respect to subnetwork dependent functions,
only supports general topology subnetworks with permanent point-to
point links. Since the ANSI IS-IS protocol is designed for
Network Service Access Point (NSAP) addresses, there is a need
encapsulate IP addresses into NSAP addresses
For this, the Initial Domain Part (IDP) is unused. The
Specific Part (DSP) includes nine bytes which are partitioned
follows
2 bytes - administrative
2 bytes -
4 bytes - IP
1 byte -
In the ANSI IS-IS protocol, each router has its own identifier (ID
which is 6 bytes long. For the NSFNET implementation, the first 2
bytes of the ID are empty and the last four bytes include the
address of a particular router
The NSFNET backbone PDUs (both IS-IS and IS-ES) are transmitted as
protocol on top of IP, with "85" being the assigned protocol
for this purpose. The IS-IS PDUs are distinguished from the IS-
PDUs by the Protocol Discriminator Field within the PDUs. The
fragmentation/reassembly mechanism provides support for
of up to 64 kilobytes in a single IP packet. Within the backbone,
is highly unlikely that the size of IS-IS PDUs will exceed
limit. Therefore, no IS-IS fragmentation/reassembly is
for this environment. This is different from the ISO framework
the ISIS is located directly on top of the Data Link Layer
For the purpose of the NSFNET Backbone routing, each
System (AS) is treated as a separate Administrative Domain (AD).
list of administrative domains (as obtained via EGP and
through the Policy Based Routing Database) which are
directly to a particular NSS is distributed in the set of
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RFC 1074 NSFNET Backbone SPF based IGP October 1988
partitionAreaAddresses part of the Level 2 Router Links PDU.
area address is 5 bytes long and consists of 3 empty bytes (IDP
followed by 2 bytes of the Administrative Domain
The reachability information obtained from regional networks via
is distributed within the backbone by End System PDUs. In order
support multi-domain topologies, the ANSI IS-IS protocol allows for
set of Address Prefixes to be entered by the System Management at
boundary IS. In the NSFNET Backbone, these Address Prefixes
obtained via the Exterior Gateway Protocol. For each network
in EGP NR packets which is received from an EGP peer, the network
administrative domain number of the EGP peer are encapsulated
NSAP addresses (as described above). A complete NSAP address is
as an address prefix in the reachable address prefix neighbor part
the End System PDU. The cost field in the reachable address
neighbor part of the End System PDU is derived from the Policy
Routing Database maintained in each NSS
At each NSS, the reachability information obtained from other
(via their End System PDU's) is passed on to the mid-level
EGP peers, following the appropriate processing and
according to the Policy Based Routing Database
The Network Entity Title (NET) (which is used in the IS-ES protocol
is eleven bytes long and is constructed by first encapsulating an
address into a NSAP address, then taking the first 11 bytes of
address as a NET
5. Current timer
The following timer parameters are currently implemented
Hello Interval (IS-ES Hello): 10
Hold Time (ES-IS protocol): 40
Other timer parameters for the IS-IS protocol are taken from
section 6.3.7 of [1].
6.
[1] "Intermediate System to Intermediate System Intra-
Routing Exchange Protocol", ANSI X3S3.3/87-150R, 1987-10-29.
[2] "End System to Intermediate System Routing Exchange
for use in conjunction with the Protocol for providing
Connectionless-Mode Network Service (ISO8473)",
JTC1/SC6/N4802R, 1988-03-26.
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RFC 1074 NSFNET Backbone SPF based IGP October 1988
[3] Mills, D., "Exterior Gateway Formal Specification", RFC 904,
University of Delaware, April 1984.
[4] Rekhter, J., "EGP and Policy Based Routing in the New
Backbone", IBM, March 1988.
[5] Braun, H-W., "The NSFNET Routing Architecture", Merit
Network, University of Michigan, April 1988.
[6] Braun, H-W., "NSFNET Inter Autonomous System Routing",
Computer Network, University of Michigan, September 1988.
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if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
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