rfc1142.txt

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-Multiple Organisations: It allows for multiple route
ing and administrative domains through the provision 
of static routeing information at domain boundaries. 
(See clause 7.3 of ISO/TR 9575),
-Deliverability It accepts and delivers NPDUs ad
dressed to reachable destinations and rejects NPDUs 
addressed to destinations known to be unreachable. 
-Adaptability. It adapts to topological changes within 
the routeing domain, but not to traffic changes, except 
potentially as indicated by local queue lengths. It 
splits traffic load on multiple equivalent paths. (See 
clause 7.7 of ISO/TR 9575),
-Promptness. The period of adaptation to topological 
changes in the domain is a reasonable function of the 
domain diameter (that is, the maximum logical dis
 
tance between End Systems within the domain) and 
Data link speeds. (See clause 7.4 of ISO/TR 9575),
-Efficiency. It is both processing and memory effi
cient. It does not create excessive routeing traffic 
overhead. (See clause 7.4 of ISO/TR 9575),
-Robustness. It recovers from transient errors such as 
lost or temporarily incorrect routeing PDUs. It toler
ates imprecise parameter settings. (See clause 7.7 of 
ISO/TR 9575),
-Stability. It stabilises in finite time to good routes, 
provided no continuous topological changes or con
tinuous data base corruptions occur. 
-System Management control. System Management 
can control many routeing functions via parameter 
changes, and inspect parameters, counters, and routes. 
It will not, however, depend on system management 
action for correct behaviour. 
-Simplicity. It is sufficiently simple to permit perform
ance tuning and failure isolation. 
-Maintainability. It provides mechanisms to detect, 
isolate, and repair most common errors that may affect 
the routeing computation and data bases. (See clause 
7.8 of ISO/TR 9575),
-Heterogeneity. It operates over a mixture of network 
and system types, communication technologies, and 
topologies. It is capable of running over a wide variety 
of subnetworks, including, but not limited to: ISO 
8802 LANs, ISO 8208 and X.25 subnetworks, PSTN 
networks, and the OSI Data Link Service. (See clause 
7.1 of ISO/TR 9575),
-Extensibility. It accommodates increased routeing 
functions, leaving earlier functions as a subset. 
-Evolution. It allows orderly transition from algorithm 
to algorithm without shutting down an entire domain.
-Deadlock Prevention. The congestion control compo
nent prevents buffer deadlock.
-Very Large Domains. With hierarchical routeing, and 
a very large address space, domains of essentially un
limited size can be supported. (See clause 7.2 of 
ISO/TR 9575),
-Area Partition Repair. It permits the utilisation of 
level 2 paths to repair areas which become partitioned 
due to failing level 1 links or ISs. (See clause 7.7 of 
ISO/TR 9575),
-Determinism. Routes are a function only of the physi
cal topology, and not of history. In other words, the 
same topology will always converge to the same set of 
routes. 
-Protection from Mis-delivery. The probability of  
mis-delivering a NPDU, i.e. delivering it to a Trans
port entity in the wrong End System, is extremely low. 
 
-Availability. For domain topologies with cut set 
greater than one, no single point of failure will parti
tion the domain. (See clause 7.7 of ISO/TR 9575),
-Service Classes. The service classes of transit delay, 
expense22Expense is referred to as cost in ISO 8473. The latter term is
not used here because of possible confusion with the more general usage
of the term to 
indicate path cost according to any routeing metric.
, and residual error probability of ISO 8473 
are supported through the optional inclusion of multi
ple routeing metrics.
-Authentication. The protocol is capable of carrying 
information to be used for the authentication of Inter
mediate systems in order to increase the security and 
robustness of a routeing domain. The specific mecha
nism supported in this International Standard how
ever, only supports a weak form of authentication us
ing passwords, and thus is useful only for protection 
against accidental misconfiguration errors and does 
not protect against any serious security threat. In the 
future, the algorithms may be enhanced to provide 
stronger forms of authentication than can be provided 
with passwords without needing to change the PDU 
encoding or the protocol exchange machinery.
6.6.1 Non-Goals
The following are not within the design scope of the intra-
domain ISIS routeing protocol described in this Interna
tional Standard:
-Traffic adaptation. It does not automatically modify 
routes based on global traffic load.
-Source-destination routeing. It does not determine 
routes by source as well as destination.
-Guaranteed delivery. It  does not guarantee delivery 
of all offered NPDUs. 
-Level 2 Subdomain Partition Repair. It will not util
ise Level 1 paths to repair a level 2 subdomain parti
tion. For full logical connectivity to be available, a 
connected level 2 subdomain is required.
-Equal treatment for all ES Implementations. The 
End system poll function defined in 8.4.5 presumes 
that End systems have implemented the Suggested ES 
Configuration Timer option of ISO 9542. An End sys
tem which does not implement this option may experi
ence a temporary loss of connectivity following cer
tain types of topology changes on its local 
subnetwork.
6.7 Environmental Requirements
For correct operation of the protocol, certain guarantees are 
required from the local environment and the Data Link 
Layer. 
The required local environment guarantees are:
a)Resource allocation such that the certain minimum re
source guarantees can be met, including
 
1)memory (for code, data, and buffers)
2)processing;
See 12.2.5 for specific performance levels required for 
conformance
b)A quota of buffers sufficient to perform routeing func
tions;
c)Access to a timer or notification of specific timer expi
ration; and
d)A very low probability of corrupting data.
The required subnetwork guarantees for point-to-point links 
are: 
a)Provision that both source and destination systems 
complete start-up before PDU exchange can occur;
b)Detection of remote start-up;
c)Provision that no old PDUs be received after start-up 
is complete;
d)Provision that no PDUs transmitted after a particular 
startup is complete are delivered out of sequence;
e)Provision that failure to deliver a specific subnetwork 
SDU will result in the timely disconnection of the 
subnetwork connection in both directions and that this 
failure will be reported to both systems;  and
f)Reporting of other subnetwork failures and degraded 
subnetwork conditions.
The required subnetwork guarantees for broadcast links are:
a)Multicast capability, i.e., the ability to address a subset 
of all connected systems with a single PDU;
b)The following events are low probability, which 
means that they occur sufficiently rarely so as not to 
impact performance, on the order of once per  thou
sand PDUs
1)Routeing PDU non-sequentiality, 
2)Routeing PDU loss due to detected corruption; and
3)Receiver overrun;
c)The following events are very low probability, 
which means performance will be impacted unless 
they are extremely rare, on the order of less than one 
event per four years
1)Delivery of NPDUs with undetected data corrup
tion; and
2)Non-transitive connectivity, i.e. where system A 
can receive transmissions from systems B and C, 
but system B cannot receive transmissions from 
system C. 
 
The following services are assumed to be not available 
from broadcast links:
a)Reporting of failures and degraded subnetwork condi
tions that result in NPDU loss, for instance receiver 
failure. The routeing functions are designed to account 
for these failures.
6.8 Functional Organisation of 
Subnetwork Independent 
Components
The Subnetwork Independent Functions are broken down 
into more specific functional components. These are de
scribed briefly in this sub-clause and in detail in clause 7. 
This International Standard uses a functional decomposition 
adapted from the model of routeing presented in clause 5.1 
of ISO/TR 9575. The decomposition is not identical to that 
in ISO/TR 9575, since that model is more general and not 
specifically oriented toward a detailed description of intra-
domain routeing functions such as supplied by this proto
col.
 
The functional decomposition is shown below in figure 2.
6.8.1 Routeing
The routeing processes are: 
-Decision Process 
-Update Process
NOTE  this comprises both the Information Collection 
and Information Distribution components identified in 
ISO/TR 9575.
-Forwarding Process 
-Receive Process
6.8.1.1 Decision Process
This process calculates routes to each destination in the do
main.  It is executed separately for level 1 and level 2 route
ing, and separately within each level for each of the route
ing metrics supported by the Intermediate system. It uses 
the Link State Database, which consists of information 
 
from the latest Link State PDUs from every other Interme
diate system in the area, to compute shortest paths from this 
IS to all other systems in the area  9in figure 2. The 
Link State Data Base is maintained by the Update Process.
Execution of the Decision Process results in the determina
tion of [circuit, neighbour] pairs (known as adjacencies), 
which are stored in the appropriate Forwarding Information 
base  10  and used by the Forwarding process as paths 
along which to forward NPDUs.
Several of the parameters in the routeing data base that the 
Decision Process uses are determined by the implementa
tion. These include:
-maximum number of Intermediate and End systems 
within the IS's area;
-maximum number of Intermediate and End system 
neighbours of the IS, etc., 
so that databases can be sized appropriately. Also parame
ters such as 
-routeing metrics for each circuit; and 
-timers 
can be adjusted for enhanced performance. The complete 
list of System Management set-able parameters is listed in 
clause 11. 
6.8.1.2 Update Process 
This process constructs, receives and propagates Link State 
PDUs. Each Link State PDU contains information about the 
identity and routeing metric values of the  adjacencies of 
the IS that originated the Link State PDU. 
The Update Process receives Link State and Sequence 
Numbers PDUs from the Receive Process  4in figure 
2. It places new routeing information in the routeing infor
mation base 6 and propagates routeing information to 
other Intermediate systems  7and 8 .
General characteristics of the Update Process are: 
-Link State PDUs are generated  as a result of topologi
cal changes, and also periodically. They may also be 
generated indirectly as a result of System Manage
ment actions (such as changing one of the routeing 
metrics for a circuit). 
-Level 1 Link State PDUs are propagated to all Inter
mediate systems within an area, but are not propa
gated out of an area. 
-Level 2 Link State PDUs are propagated to all Level 2 
Intermediate systems in the domain.
-Link State PDUs are not propagated outside of a do
main. 
 
-The update process, through a set of System Manage
ment parameters, enforces an upper bound on the 
amount of routeing traffic overhead it generates.
6.8.1.3 Forwarding Process
This process supplies and manages the buffers necessary to 
support NPDU relaying to all destinations. 
It receives, via the Receive Process, ISO 8473 PDUs to be 
forwarded  5 in figure 2.
It performs a lookup in the appropriate33The appropriate Forwarding
Database is selected by choosing a routeing metric based on fields in
the QoS Maintenance option of ISO 8473.
 Forwarding Data
base  11  to determine the possible output adjacencies 
to use for forwarding to a given destination, chooses one 
adjacency  12, generates error indications to ISO 8473 
 14 , and  signals ISO 9542 to issue Redirect PDUs  
13.
6.8.1.4 Receive Process
The Receive Process obtains its inputs from the following 
sources
-received PDUs with the NPID of Intra-Domain route
ing  2 in figure 2, 
-routeing information derived by the ESIS protocol 
from the receipt of ISO 9542 PDUs  1;  and
-ISO 8473 data PDUs handed to the routeing function 
by the ISO 8473 protocol machine  3. 
It then performs the appropriate actions, which may involve 
passing the PDU to some other function (e.g. to the For
warding Process for forwarding  5).
7 Subnetwork Independent 
Functions
This clause describes the algorithms and associated data
bases used by the routeing functions. The managed objects 
and attributes defined for System Management purposes are 
described in clause 11.
The following processes and data bases are used internally 
by the subnetwork independent functions. Following each 
process or data base title, in parentheses, is the type of sys
tems which must keep the database. The system types are 
L2 (level 2 Intermediate system), and L1 (level 1 Inter
mediate system). Note that a level 2 Intermediate system is 
also a level 1 Intermediate system in its home area, so it 
must keep level 1 databases as well as level 2 databases. 
 
Processes:
-Decision Process (L2, L1) 
-Update Process (L2, L1) 
-Forwarding Process (L2, L1) 
-Receive Process (L2, L1)
Databases: