Network Working Group ANSI X3S3.3 86-118
Request for Comments: 995 ISO TC97/SC6/N 4053
I S O
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
ORGANISATION INTERNATIONALE DE NORMALISATION
| ISO/TC 97/SC 6 |
| TELECOMMUNICATIONS AND INFORMATION |
| EXCHANGE BETWEEN SYSTEMS |
| Secretariat: USA (ANSI) |
Title: End System to Intermediate System Routing Exchange Protocol
for use in conjunction with ISO 8473
|This document is a progression of SC6/N3862, edited to incorporate member |
|body comments and discussion at the Florence meeting of SC6/WG2. Pursuant |
|to Recommendation 5 of that meeting, comments from member bodies on this |
|revision text are requested for discussion at the Tokyo meeting of SC6 |
|and WGs. |
This Protocol is one of a set of International Standards produced to
facilitate the interconnection of open systems. The set of standards
covers the services and protocols required to achieve such intercon-
This Protocol is positioned with respect to other related standards
by the layers defined in the Reference Model for Open System Inter-
connection (ISO 7498) and by the structure defined in the Internal
Organization of the Network Layer (DIS 8648). In particular, it is a
protocol of the Network Layer. This protocol permits End Systems and
Intermediate Systems to exchange configuration and routing informa-
tion to facilitate the operation of the routing and relaying func-
tions of the Network Layer.
The aspects of Network Layer routing that are concerned with communi-
cation between end systems and intermediate systems on the same sub-
network are to a great extent separable from the aspects that are
concerned with communication among the intermediate systems that con-
nect multiple subnetworks. This protocol addresses only the former
aspects. It will be significantly enhanced by the cooperative opera-
tion of an additional protocol that provides for the exchange of
routing information among intermediate systems, but is useful whether
or not such an additional protocol is available.
This protocol provides solutions for the following practical problems:
1. How do end systems discover the existence and reachability of
intermediate systems that can route NPDUs to destinations on
subnetworks other than the one(s) to which the end system is
2. How do end systems discover the existence and reachability of
other end systems on the same subnetwork (when direct
examination of the destination NSAP address does not provide
information about the destination subnetwork)?
3. How do intermediate systems discover the existence and
reachability of end systems on each of the subnetworks to
which they are directly connected?
4. How do end systems decide which intermediate system to use
to forward NPDUs to a particular destination when more than one
intermediate system is accessible?
The protocol assumes that:
1. Routing to a specified subnetwork point of attachment address
(SNPA) on the same subnetwork is carried out satisfactorily by
the subnetwork itself.
2. The subnetwork is not, however, capable of routing on a global
basis using the NSAP address alone to achieve communication
with a requested destination.
Consequently, it is not possible to use Application Layer
communication to carry out the functions of this protocol.
The protocol is connectionless, and is designed to:
1. minimize the amount of a priori state information needed by
end systems before they can begin to communicate with other
2. minimize the amount of memory needed to store routing
information in end systems; and
3. minimize the computational complexity of end system routing
The protocol is also designed to operate in close conjunction with
the Protocol for the Provision of the Connectionless-mode Network
Service (ISO 8473). Since routing styles are usually closely related
to communication styles, the information that this protocol provides
to end systems and intermediate systems may or may not be appropriate
information for supporting routing functions when a Network Layer
protocol other than ISO 8473 is used.
2 Scope and Field of Application
This International Standard specifies a protocol which is used by
Network Layer entities operating ISO 8473 in End Systems and Inter-
mediate Systems (referred to herein as ES and IS respectively) to
maintain routing information. The Protocol herein described relies
upon the provision of a connectionless-mode underlying service.
This Standard specifies:
a) procedures for the transmission of configuration and routing
information between network entities residing in End Systems
and network entities residing in Intermediate Systems;
b) the encoding of the protocol data units used for the transmission
of the configuration and routing information;
c) procedures for the correct interpretation of protocol control
d) the functional requirements for implementations claiming
conformance to this Standard.
The procedures are defined in terms of:
a) the interactions between End System and Intermediate System
network entities through the exchange of protocol data units;
b) the interactions between a network entity and an underlying
service provider through the exchange of subnetwork service
This protocol does not specify any protocol elements or algorithms for
facilitating routing and relaying among Intermediate Systems. Such
functions are intentionally beyond the scope of this protocol.
ISO 7498 Information Processing Systems --- Open Systems Intercon-
nection - Basic Reference Model
DIS 7498/DAD1 Information Processing Systems --- Open Systems Intercon-
nection - Addendum to ISO 7498 Covering Connectionless-
ISO 8348 Information Processing Systems --- Telecommunications and
Information Exchange between Systems - Network Service
ISO 8348/AD1 Information Processing Systems --- Telecommunications and
Information Exchange between Systems - Addendum to the
Network Service Definition Covering Connectionless-mode
ISO 8348/AD2 Information Processing Systems --- Telecommunications and
Information Exchange between Systems - Addendum to the
Network Service Definition Covering Network Layer Address-
ISO 8473 Information Processing Systems --- Telecommunications and
Information Exchange between Systems - Protocol for Pro-
viding the Connectionless Network Service
DIS 8648 Information Processing Systems --- Telecommunications and
Information Exchange between Systems - Internal Organiza-
tion of the Network Layer
SECTION ONE. GENERAL
4.1 Reference Model Definitions
This document makes use of the following concepts defined in ISO 7498:
(a) Network layer
(b) Network service access point
(c) Network service access point address
(d) Network entity
(f) Network protocol
(g) Network relay
(h) Network protocol data unit
4.2 Network Layer Architecture Definitions
This document makes use of the following concepts from DIS 8648, Internal
Organization of the Network Layer:
(b) End System
(c) Intermediate System
(d) Subnetwork Service
(e) Subnetwork Access Protocol
(f) Subnetwork Independent Convergence Protocol
4.3 Network Layer Addressing Definitions
This document makes use of the following concepts from DIS 8348/DAD2,
Addendum to the Network Service Definition Covering Network Layer Ad-
(a) Subnetwork address
(b) Subnetwork point of attachment
4.4 Local Area Network Definitions
This document makes use of the following concepts from DIS 8802, Local
(a) multicast address
(b) broadcast medium
4.5 Additional Definitions
For the purposes of this document, the following definitions apply:
Configuration: The collection of End and Intermediate Systems
attached to a single subnetwork, defined in terms of the
system types, NSAP addresses present, Network Entities
present, and the correspondence between systems and SNPA
Network Entity Title: An identifier for a network entity which
has the same abstract syntax as an NSAP address, and which
can be used to unambiguously identify a network entity in
an End or Intermediate System.
5 Symbols and Abbreviations
5.1 Data Units
PDU Protocol Data Unit
SNSDU Subnetwork Service Data Unit
5.2 Protocol Data Units
ESH PDU End System Hello Protocol Data Unit
ISH PDU Intermediate System Hello Protocol Data Unit
RD PDU Redirect Protocol Data Unit
5.3 Protocol Data Unit Fields
NPID Network Layer Protocol Identifier
LI Length Indicator
V/P Version/Protocol Identifier Extension
NETL Network entity Title Length
NET Network entity Title
DAL Destination Address Length
DA Destination Address
SAL Source Address Length
SA Source Address
BSNPAL SN Address Length of better route to destination
BSNPA SN Address of better route to destination
HT Holding timer
CT Configuration Timer
RT Redirect Timer
ES End System
IS Intermediate System
SNACP Subnetwork Access Protocol
SNICP Subnetwork Independent Convergence Protocol
6 Overview of the Protocol
6.1 Information Provided by the Protocol
This Protocol provides two types of information to Network entities
which support its operation:
a) Configuration Information, and
b) Route Redirection Information
Configuration Information permits End Systems to discover the ex-
istence and reachability of Intermediate Systems and permits Inter-
mediate Systems to discover the existence and reachability of End
Systems. This information allows ESs and ISs attached to the same
subnetwork to dynamically discover each other's existence and availa-
bility, thus eliminating the need for manual intervention at ESs and
ISs to establish the identity of Network entities that can be used to
Configuration Information also permits End Systems to obtain informa-
tion about each other in the absence of an available Intermediate
The term "configuration information" is not intended in the broad
sense of configuration as used in the context of OSI system
management. Rather, only the functions specifically defined herein
Route Redirection Information allows Intermediate Systems to inform
End Systems of (potentially) better paths to use when forwarding
NPDUs to a particular destination. A better path could either be
another IS on the same subnetwork as the ES, or the destination ES
itself, if it on the same subnetwork as the source ES. Allowing the
ISs to inform the ESs of routes minimizes the complexity of routing
decisions in End Systems and improves performance because the ESs may
make use of the better IS or local subnetwork access for subsequent
6.2 Subsets of the Protocol
A Network Entity may choose to support either the Configuration In-
formation, the Route Redirection Information, neither, or both. If
the Configuration Information is supported, it is not required that
it be employed over all subnetworks to which the Network entity is
The Source Address and Destination Address parameters referred to in
this International Standard are OSI Network Service Access Point Ad-
dresses. The syntax and semantics of an OSI Network Service Access
Point Address are described in a separate document, ISO 8348/DAD2,
Addendum to the Network Service Definition covering Network Layer Ad-
6.4 Underlying Service Assumed by the Protocol
The underlying service required to support this protocol is defined
by the primitives in Table 1.
| SN_UNITDATA .Request | SN_Destination_Address, |
| .Indication | SN_Source_Address, |
| | SN_Quality_of_Service, |
| | SN_Userdata |
Table 1: Service Primitives for Underlying Service
These service primitives are used to describe the abstract interface
which exists between the protocol machine and an underlying real
subnetwork or a Subnetwork Dependent Convergence Function which
operates over a real subnetwork or real data link to provide the
required underlying service.
6.4.1 Subnetwork Addresses
The source and destination addresses specify the points of attachment
to a public or private subnetwork(s) involved in the transmission
(known as Subnetwork Points of Attachment, or SNPAs).Subnetwork ad-
dresses are defined in the Service Definition of each individual sub-
network. This protocol is designed to take advantage of subnetworks
which support broadcast, multicast, or other forms of multi-
destination addressing for n-way transmission. It is assumed that the
SN_Destination_Address parameter may take on one of the following
multi-destination addresses in addition to a normal single destina-
All End System Network entities
All Intermediate System Network entities
Where a real subnetwork does not inherently support broadcast or oth-
er forms of transmission to multi-destination addresses, a conver-
gence function may be used to provide n-way transmission to these
When the SN_Destination_Address on the SN_UNITDATA.Request is a
multi-destination address, the SN_Destination_Address parameter in
the corresponding SN_UNITDATA.Indication shall be the same multi-
The syntax and semantics of subnetwork addresses, except for the pro-
perties described above, are not defined in this Protocol Standard.
6.4.2 Subnetwork User Data
The SN_Userdata is an ordered multiple of octets, and is transferred
transparently between the specified subnetwork points of attachment.
The underlying service is required to support a service data unit
size of at least that required to operate the Protocol for Providing
the Connectionless Network Service (ISO 8473).
6.5 Service Assumed from Local Environment
A timer service must be provided to allow the protocol entity to
There are three primitives associated with the S-TIMER service:
1. the S--TIMER Request,
2. the S--TIMER Response, and
3. the S--TIMER Cancel.
The S--TIMER Request primitive indicates to the local environment
that it should initiate a timer of the specified name and subscript
and maintain it for the duration specified by the time parameter.
The S--TIMER Response primitive is initiated by the local environment
to indicate that the delay requested by the corresponding S-TIMER Re-
quest primitive has elapsed.
The S--TIMER Cancel primitive is an indication to the local environ-
ment that the specified timer(s) should be canceled.If the subscript
parameter is not specified, then all timers with the specified name
are canceled; otherwise, the timer of the given name and subscript is
cancelled. If no timers correspond to the parameters specified, the
local environment takes no action.
The parameters of the S--TIMER service primitives are specified in
| | |
| S--TIMER .Request | S-Time, |
| | S-Name, |
| | S-Subscript |
| | |
| .Response | S-Name, |
| | S-Subscript |
Table 2: Timer Primitives
The time parameter indicates the time duration of the specified ti-
mer. An identifiying label is associated with a timer by means of
the name parameter.The subscript parameter specifies a value to dis-
tinguish timers with the same name. The name and subscript taken to-
gether constitute a unique reference to the timer.
Timers used in association with a specific protocol funtion are de-
fined under that protocol function.
This International Standard does not define specific values for the
timers.Any derivations described in this Standard are not mandatory.
Timer values should be chosen so that the requested Quality of
Service can be provided, given the known characteristics of the
6.6 Subnetwork Types
In order to evaluate the applicability of this protocol in particular
configurations of End Systems, Intermediate Systems and subnetworks,
three generic types of subnetwork are identified. These are:
1. the point-to-point subnetwork,
2. the broadcast subnetwork, and
3. the general topology subnetwork
These subnetwork types are discussed in the following clauses.
6.6.1 Point-to-Point Subnetworks
A point-to-point subnetwork supports exactly two systems. The two
systems may be either two End Systems, or an End System and a single
Intermediate System. A single point-to-point data link connecting two
Network Entities is an example of a point-to-point subnetwork.
Configuration Information on a point-to-point Subnetwork.On a point-
to-point subnetwork the Configuration Information of this protocol
informs the communicating Network entities of the following:
1. Whether the topology consists only of two End Systems, or
2. One of the two systems is a Intermediate System.
On a point-to-point subnetwork, if both systems are Intermediate Systems,
then this protocol is inapplicable to the situation, since a IS-to-IS
protocol should be employed instead. However, there is no reason why
the configuration information could not be employed in a IS-to-IS
environment to ascertain the topology and initiate operation of a
The Intermediate System is informed of the NSAP address(es) supported
by the Network entity in the End System. This permits reachability
information and routing metrics concerning these NSAPs to be dissem-
inated to other Intermediate Systems for the purpose of calculating
routes to/from this End System.
Route Redirection Information on a point-to-point Subnetwork. Route
Redirection Information is not employed on point-to-point subnetworks
because there are never any alternate routes.
6.6.2 Broadcast Subnetworks
A Broadcast subnetwork supports an arbitrary number of End Systems
and Intermediate Systems, and additionally is capable of transmitting
a single SNPDU to all or a subset of these systems in response to a
single SN_UNITDATA.Request.An example of a broadcast subnetwork is a
LAN (local area network) conforming to DIS8802/2, type 1 operation.
Configuration Information on a broadcast Subnetwork.On a broadcast
subnetwork the Configuration Information of this protocol is employed
to inform the communicating Network entities of the following:
1. End Systems are informed of the reachability, Network entity Title,
and SNPA address(es) of each active Intermediate System on the
2. Intermediate Systems are informed of the NSAP addresses supported
by each End System and the Subnetwork address of the ES. Once the
Intermediate System obtains this information, reachability
information and routing metrics concerning these NSAPs may be
disseminated to other ISs for the purpose of calculating routes
to/from each ES on the subnetwork.
3. In the absence of an available Intermediate System, End Systems may
query over a broadcast subnetwork to discover whether a particular
NSAP is reachable on the subnetwork, and if so, what SNPA address
to use to reach that NSAP.
Route Redirection Information on broadcast Subnetworks.Route Redirec-
tion Information may be employed on broadcast subnetworks to permit
Intermediate Systems to inform End Systems of superior routes to a
destination NSAP. The superior route might be another IS on the same
subnetwork as the ES, or it might be the destination ES itself, if it
is directly reachable on the same subnetwork as the source ES.
6.6.3 General Topology Subnetworks
A general topology subnetwork supports an arbitrary number of End
Systems and Intermediate Systems, but does not support a convenient
multidestination connectionless transmission facility as does a
broadcast subnetwork.An example of a general topology subnetwork is a
subnetwork employing X.25 or ISO 8208.
The crucial distinguishing characteristic between the broadcast
subnetwork and the general topology subnetwork is the "cost" of an
n-way transmission to a potentially large subset of the systems on
the subnetwork. On a general topology subnetwork, the cost is assumed
to be close to the cost of sending an individual PDU to each SNPA on
the subnetwork. Conversely, on a broadcast subnetwork the cost is
assumed to be close to the cost of sending a single PDU to one SNPA
on the subnetwork. Intermediate situations between these extremes
are of course possible. In such cases it would be possible to treat the
subnetwork as either in the broadcast or general topology categories.
Configuration Information on a general topology Subnetwork. On a
general topology subnetwork the Configuration Information is general-
ly not employed because this protocol can be very costly in the util-
ization (and charging for) subnetwork resources.
Route Redirection Information on a general topology Subnetwork.
Route Redirection Information may be employed on general topology
subnetworks to permit Intermediate Systems to inform End Systems of
superior routes to a destination NSAP. The superior route might be
another IS on the same subnetwork as the ES, or it might be the des-
tination ES itself, if it is directly reachable on the same subnet-
SECTION TWO. SPECIFICATION OF THE PROTOCOL
7 Protocol Functions
This section describes the functions performed as part of the Proto-
col. Not all of the functions must be performed by every implementa-
tion. Clause 7.12 specifies which functions may be omitted and the
correct behavior where requested functions are not implemented.
7.1 Protocol Timers
Many of the protocol functions are timer based. This means that they
are executed upon expiration of a timer rather than upon receipt of a
PDU or invocation of a service primitive. The two major types of ti-
mers employed by the protocol are the Configuration Timer (CT) and
the Holding Timer (HT).
7.1.1 Configuration Timer
The Configuration Timer is a local timer (i.e. maintained indepen-
dently by each system) which performs the Report Configuration func-
tion (see section 7.2). The timer determines how often a system re-
ports its availability to the other systems on the same subnetwork.
The shorter the Configuration Timer, the more quickly other systems
on the subnetwork will become aware when the reporting system becomes
available or unavailable. The increased responsiveness must be traded
off against increased use of resources in the subnetwork and in the
7.1.2 Holding Timer
The Holding Timer applies to both Configuration Information and Route
Redirection Information. The value of the Holding Timer is set by the
source of the information and transmitted in the appropriate PDU. The
recipient of the information is expected to retain the information no
longer than the Holding Timer. Old Configuration or Route Redirection
information must be discarded after the Holding Timer expires to en-
sure the correct operation of the protocol.
Further discussion of the rationale for these timers and guidelines
for their use may be found in annex 10.
7.2 Report Configuration Function
The Report Configuration Function is used by End Systems and Inter-
mediate Systems to inform each other of their reachability and
current subnetwork address. This function is invoked every time the
local Configuration Timer (CT) expires in an ES or IS. It is also in-
voked upon receipt of a Query Configuration PDU from another End Sys-
7.2.1 Report Configuration by End Systems
An End System constructs and transmits one ESH PDU (ESH stands for
"End System Hello") for each NSAP it serves, and issues one
SN_UNITDATA.- Request with the ESH PDU as the SNSDU on each subnet-
work to which it is attached.
The necessity to transmit a separate ESH PDU for each NSAP served by
the Network entity arises from the lack of a formalized relationship
between Network Entity Titles and NSAP addresses. If this relationship
could be constrained to require that all NSAP addresses be assigned as
leaf subdomains of a domain represented by the local Network entity's
Network entity Title, then a single ESH PDU could be transmitted
containing the ESs Network entity Title.The Network entity Title
would then imply which NSAPs might be present at that End system.
The Holding Timer (HT) field is set to approximately twice the ESs
Configuration Timer (CT) parameter. This variable is set to a value
large enough so that even if every other ESH PDU is discarded (due to
lack of resources), or otherwise lost in the subnetwork, the confi-
guration information will still be maintained. The value must be set
small enough so that Intermediate Systems can respond in a timely
fashion to End Systems becoming available or unavailable.
The SN_Destination_Address parameter is set to the group address that
indicates "All Intermediate System Network Entities". This ensures
that a single transmission on a broadcast subnetwork will reach all
of the active Intermediate Systems.
The actual value of the SN_Destination_Address used to mean "All
Intermediate System Network Entities" is subnetwork dependent and will
most likely vary from subnetwork to subnetwork. It would of course be
desirable that on widely-used subnetwork types (such as those based
on DIS 8802) that this value and the value of the "All End System
Network Entities" group address, be standardized.
7.2.2 Report Configuration by Intermediate Systems
An Intermediate System constructs a single ISH PDU (ISH stands for
"Intermediate System Hello") containing the ISs Network Entity Title
and issues one SN_UNITDATA.Request with the ISH PDU as the SNSDU on
each subnetwork to which it is attached.
The Holding Timer (HT) field is set to approximately twice the Inter-
mediate System's Configuration Timer (CT) parameter. This variable is
set to a value large enough so that even if every other ISH PDU is
discarded (due to lack of resources), or otherwise lost in the sub-
network, the configuration information will still be maintained.The
value must be set small enough so that End Systems will quickly cease
to use ISs that have failed, thus preventing "black holes" in the
The SN_Destination_Address parameter is set to the group address that
indicates "All End System Network Entities".This ensures that a sin-
gle transmission on a broadcast subnetwork will reach all of the ac-
tive End Systems.
7.3 Record Configuration Function
The Record Configuration function receives ESH or ISH PDUs, extracts
the configuration information, and adds or replaces the corresponding
configuration information in the local Network entity's Routing In-
formation base. If insufficient space is available to store new con-
figuration information, the PDU is discarded. No Error Report is gen-
The protocol is described such that End Systems receive and record
only ISH PDUs and Intermediate Systems receive and process only
ESH PDUs. If an ES so desires however, it may decide to process ESH
PDUs as well (on a broadcast network this is easily done by enabling
the appropriate group address). There is potentially some performance
improvement to be gained by doing this, at the expense of extra memory,
and possibly extra processing cycles in the End System.The
ES, by recording other ESs' Configuration information, may be able
to route NPDUs directly to ESs on the local subnetwork without first
being redirected by a Intermediate System.
Similarly, Intermediate Systems may choose to receive the ISH PDUs
of other ISs, allowing this protocol to be used as the initialization and
topology maintenance portion of a full IS-to-IS routing protocol.
Both of these possibilities are for further study.
7.4 Flush Old Configuration Function
The Flush Old Configuration Function is executed to remove Configura-
tion entries in the routing information base whose Holding Timer has
expired. When the Holding Time for an ES or IS expires, this func-
tion removes the corresponding entry from the routing information
base of the local Network Entity.
7.5 Query Configuration Function
The Query Configuration Function is performed under the following
1. The End System is attached to a broadcast subnetwork,
2. There is no Intermediate System currently reachable on the
subnetwork (i.e. no ISH PDUs have been received since the last
information was flushed by the Flush Old Configuration Function),
3. The Network Layer's Route PDU Function needs to obtain the SNPA
address to which to forward a PDU destined for a certain NSAP, and
4. The SNPA address cannot be obtained either by a local transformation
or a local table lookup.
Despite appearances, this is actually a quite common case, since it
is likely that there will be numerous isolated Local Area Networks
without Intermediate Systems to rely upon for obtaining routing
information (e.g.via the Request Redirect Function of this protocol).
Further, if the Intermediate System(s) are temporarily unavailable,
without this capability communication on the local subnetwork would
suffer unless manually-entered tables were present in each End System
or all NSAPs of the subnetwork had the subnetwork SNPA address
embedded in them.
The End System, when needing to route an NPDU to a destination NSAP
whose SNPA is unknown issues an SN_UNITDATA.Request with the NPDU as
the SN_Userdata.The SN_Destination_Address parameter is set to the
group address that indicates "All End System Network Entities".
Subsequently an ESH PDU may be received containing the NSAP address
along with the corresponding SNPA address (see clause 7.6). In such a
case the End System executes the Record Configuration function for
the NSAP, and therefore will be able to route subsequent PDUs to that
destination using the specified SNPA. If no ESH PDU is received, the
End System may declare the destination NSAP is not reachable. The
length of time to wait for a response before indicating a failure or
the possibility of repeating the process some number of times before
returning a failure are local matters and are not specified in this
7.6 Configuration Response Function
The Configuration Response function is performed when an End System
attached to a broadcast subnetwork receives an NPDU addressed to one
of its NSAPs, with the SN_Destination_Address from the
SN_UNITDATA.Indication set to the group address "All End System
Netowrk Entities". This occurs as a result of another ES having per-
formed the Query Configuration function described in clause 7.5.
The End System constructs an ESH PDU identical in content to the ESH
PDU constructed by the Report Configuration function (see clause
7.2.1) for the NSAP to which the received NPDU was addressed.It then
transmits the ESH PDU to the source of the original NPDU by issuing
an SN_UNITDATA.Request with the SN_Destination_Address set to the
value of the SN_Source_Address received in the SN_UNITDATA.Indication
with the original NPDU.
7.7 Request Redirect Function
The Request Redirect Function is present only in Intermediate Systems
and is closely coupled with the Routing and Relaying Functions of In-
termediate Systems. The Request Redirect Function is coupled with the
"Route PDU Function" described in clause 6.5 of ISO 8473. The Request
Redirect Function is performed after the Route PDU function has cal-
culated the next hop of the Data PDU's path.
When an NPDU is to be forwarded by a Intermediate System, the Request
Redirect Function first examines the SN_Source_Address associated
with the SN_UNITDATA.Indication which received the SNSDU (containing
this NPDU). If the SN_Source_Address is not from an End System on the
local subnetwork (determined by examining the Configuration informa-
tion obtained through the Record Configuration Function), then this
function does no further processing of the NPDU.
If the NPDU was received directly from an ES the output of the ISs
Routing and Relaying function for this NPDU is examined. This output
will contain, among other things, the following pieces of informa-
1. a local identifier for the subnetwork over which to forward the NPDU,
2. the Network entity title and subnetwork address of the IS to which to
forward the NPDU, or
3. the subnetwork address of the destination End System.
The Request Redirect function must now determine if the source ES
could have sent the NPDU directly to the Network entity the Inter-
mediate System is about to forward the PDU to. If any of the follow-
ing conditions hold, the source ESshould be informed of the "better"
path (by sending an RD PDU to the originating ES):
1. The next hop is to the destination system, and the destination is
directly reachable (at subnetwork address BSNPA) on the source ESs
2. The next hop is to a Intermediate System which is connected to the
same subnetwork as the ES.
If the better path exists, the IS first completes normal processing
of the received NPDU and forwards it.It then constructs a Redirect
PDU (RD PDU) containing the Destination Address of the original NPDU,
the subnetwork address of the better next hop (BSNPA), the Network
Entity Title of the IS to which the ES is being redirected (unless
the redirect is to the destination ES), a Holding Time (HT), QoS
Maintenance, Priority, and Security options that were present in the
Data NPDU (these are simply copied from the Data PDU). The HT is set
to the value of the local Redirect Timer (RT). See Annex A for a dis-
cussion of how to choose the value of RT. If there are insufficient
resources to both forward the original NPDU and to generate and send
an RD PDU, the original NPDU must be given preference. The Inter-
mediate System (assuming it has sufficient resources) then sends the
RD PDU to the source End System using the SN_Source_Address of the
received NPDU as the SN_Destination_Address for the SN_UNITDATA.-
7.8 Record Redirect Function
The Record Redirect Function is present only in End Systems. This
function is invoked whenever an RD PDU is received. It extracts the
redirect information and adds or replaces the corresponding redirec-
tion information in the local Network entity's Routing Information
base. The essential information is the redirection mapping from a
Destination Address to a subnetwork address, along with the Priority,
Security, and QoS Maintenance options and the Holding Time for which
this mapping is to be considered valid. If the Redirect was to anoth-
er Intermediate System, the Network Entity Title of the IS is record-
ed as well.
If insufficient memory is available to store new redirection information,
the RD PDU may be safely discarded since the original Intermediate
System will continue to forward PDUs on behalf of this Network entity
7.9 Refresh Redirect Function
The Refresh Redirect Function is present only in End Systems. This
function is invoked whenever an NPDU is received by a destination ES.
It is closely coupled with the function that processes received NPDUs
at a destination Network Entity.This is the "PDU Decomposition" func-
tion in ISO 8473. The purpose of this function is to increase the
longevity of a redirection without allowing an incorrect route to
persist indefinitely. The Source Address (SA), Priority, Security,
and QoS options are extracted and compared to any Destination Address
and QoS parameters being maintained in the Routing Information base
(such information would have been stored by the Record Redirect Func-
tion). If a corresponding entry is found, the previous hop of the PDU
is obtained from the SN_Source_Address parameter of the
SN_Unitdata.Indication primitive by which it was received. If this
address matches the next hop address stored with the redirection in-
formation, the remaining holding time for the redirection is reset to
the original holding timer that was obtained from the RD PDU.
The purpose of this function is to avoid timing out redirection entries
when the Network entity is receiving return traffic from the destination
via the same path over which it is currently sending traffic.This is
particularly useful when the destination system is on the same subnetwork
as the source, since after one redirect no IS need be involved in
the ES-to-ES traffic.
This function must operate in a very conservative fashion however,
to prevent the formation of black holes. The remaining holding time
should be refreshed only under the exact conditions specified above.
For a discussion of the issues surrounding the refresh of redirection
information, see Annex 10.
7.10 Flush Old Redirect Function
The Flush Old Redirect Function is executed to remove Configuration
entries in the routing information base whose Holding Timer has ex-
pired. When the Holding Time for an ES or IS expires, this function
removes the corresponding entry from the routing information base of
the local Network Entity.
7.11 PDU Header Error Detection
The PDU Header Error Detection function protects against failure of
Intermediate or End System Network entities due to the processing of
erroneous information in the PDU header.The function is realized by a
checksum computed on the entire PDU header. The checksum is verified
at each point at which the PDU is processed. If the checksum calcula-
tion fails, the PDU must be discarded.
The use of the Header Error Detection function is optional and is
selected by the originating Network Entity. If the function is not
used, the checksum field of the PDU header is set to zero.
If the function is selected by the originating Network Entity, the
value of the checksum field causes the following formulf to be satis-
(The Sum from i=1 to L of a(i)) (mod 255) = 0
(The Sum from i=1 to L of (L - i + 1) * a(i)) (mod 255) = 0
where L = the number of octets in the PDU header, and a(i) = the value of
the octet at position i. The first octet in the PDU header is considered to
occupy position i = 0.
When the function is in use, neither octet of the checksum field may be
set to zero.
7.12 Classification of Functions
Implementations do not have to support all of the functions described
in clause 7. Functions are divided into four categories:
Type A: These functions must be supported in all cases.
Type B: These functions must be supported by Systems which implement
the Configuration Information.
Type C: These functions must be supported by Systems which implement
the Redirect Information.
Type D: These functions are optional.
If a PDU is received which invokes an optional function that is not
implemented, that PDU is discarded.
Table 3 shows how the functions are divided into these four
categories, and to which type of system (ES, IS, or both) they apply.
| Function | Category | System Type |
| Report Configuration | B | ES,IS |
| Record Configuration | B | ES,IS |
| Configuration Response | A | ES |
| Flush Old Configuration | B | ES,IS |
| Request Redirect | C | IS |
| Query Configuration | B | ES |
| Record Redirect | C | ES |
| Refresh Redirect | D | ES |
| Flush Old Redirect | C | ES |
| PDU Header Error Detection | A | ES,IS |
Table 3: Categories of Protocol Functions
8 Structure and Encoding of PDUs
The encoding of the PDUs for this protocol is compatible with that
used in ISO 8473.
The method employed for describing the encoding of PDUs is provisional.
Member bodies are requested to comment on whether another
method (such as ASN.1 with an appropriate concrete syntax) would
All Protocol Data Units shall contain an integral number of
octets.The octets in a PDU are numbered starting from one (1) and in-
creasing in the order in which they are put into an SNSDU. The bits
in an octet are numbered from one (1) to eight (8), where bit one (1)
is the low-order bit. When consecutive octets are used to represent
a binary number, the lower octet number has the most significant
Any subnetwork supporting this protocol is required to state in its
specification the way octets are transferred, using the terms "most
significant bit" and "least significant bit". The PDUs of this proto-
col are defined using the terms "most significant bit" and "least
When the encoding of a PDU is represented using a diagram in this
section, the following representation is used:
a) octets are shown with the lowest numbered octet to the left,
higher number octets being further to the right;
b) within an octet, bits are shown with bit eight (8) to the left and
bit one (1) to the right.
PDUs shall contain, in the following order:
1. the fixed part;
2. the Network address part;
3. the Subnetwork address part, if present; and
4. the Options part, if present.
8.2 Fixed Part
The fixed part contains frequently occurring parameters including the
type code (ESH, ISH, or RD) of the protocol data unit.The length and
the structure of the fixed part are defined by the PDU code.
The fixed part has the following format:
| Network Layer Protocol Identifier | 1
| Length Indicator | 2
| Version/Protocol Id Extension | 3
| reserved (must be zero) | 4
| 0 |0 |0 | Type | 5
| Holding Time | 6,7
| Checksum | 8,9
Figure 1: PDU Header -- Fixed Part
8.2.2 Network Layer Protocol Identifier
The value of this field shall be 1000 0010.
The value 1000 0010 is provisional, pending resolution of the NLPID
issue in SC6.
This field identifies this Network Layer Protocol as ISO SC6/N4053,
End System to Intermediate System Routing Exchange Protocol for use in
conjunction with ISO 8473.
8.2.3 Length Indicator
The length is indicated by a binary number, with a maximum value of
254 (1111 1110).The length indicated is the length of the entire PDU
(which consists entirely of header, since this protocol does not car-
ry user data) in octets, as described in clause 8.1. The value 255
(1111 1111) is reserved for possible future extensions.
8.2.4 Version/Protocol Identifier Extension
The value of this field is binary 0000 0001. This identifies a stan-
dard version of ISO xxxx, End System to Intermediate System Routing
Exchange Protocol for use in conjunction with ISO 8473.
8.2.5 Type Code
The Type code field identifies the type of the protocol data unit.
Allowed values are given in table 4.
| | Bits 5 4 3 2 1 |
|ESH PDU | 0 0 0 1 0 |
|ISH PDU | 0 0 1 0 0 |
|RD PDU | 0 0 1 1 0 |
Table 4: Valid PDU Types
All other PDU type values are reserved.
8.2.6 Holding Time
The Holding Time field specifies for how long the receiving Network
entity should retain the configuration/routing information contained
in this PDU. The receiving Network entity should discard any infor-
mation obtained from this PDU from its internal state when the hold-
ing time expires. The Holding time field is encoded as an integral
number of micro-fortnights.
8.2.7 PDU Checksum
The checksum is computed on the entire PDU header. A checksum value
of zero is reserved to indicate that the checksum is to be ignored.
The operation of the PDU Header Error Detection function (Clause
7.11) ensures that the value zero does not represent a valid check-
sum. A non-zero value indicates that the checksum must be processed.
If the checksum calculation fails, the PDU must be discarded.
8.3 Network Address Part
Address parameters are distinguished by their location. The different
PDU types carry different address parameters however.The ESH PDU car-
ries a Source NSAP address (SA); the ISH PDU carries a Intermediate
System Network entity Title (NET); and the RD PDU carries a Destina-
tion NSAP address (DA), and possibly a Network Entity Title (NET).
8.3.2 NPAI (Network Protocol Address Information) Encoding
The Destination and Source Addresses are Network Service Access Point
addresses as defined in ISO 8348/AD2, Addendum to the Network Service
Definition Covering Network Layer addressing.The Network Entity Title
address parameter is defined in clause 4.5. The Destination Address,
Source Address, and Network Entity Title are encoded as NPAI using
the binary syntax defined in clause 8.3.1 of ISO 8348/AD2.
The address information is of variable length. Each address parameter
is encoded as follows:
| Octet | Address parameter Length Indicator |
| n | (e.g., 'm') |
| Octets | |
| n + 1 | Address Parameter Value |
| thru | |
| n + m | |
Figure 2: Address Parameters
8.3.3 Source Address Parameter for ESH PDU
The Source Address is the NSAP address of an NSAP served by the Net-
work entity sending the ESH PDU. It is encoded in the ESH PDU as fol-
|Source Address Length Indicator (SAL) | 10
| | 11
: Source Address (SA) :
| | m - 1
Figure 3: ESH PDU - Network Address Part
8.3.4 Network Entity Title Parameter for ISH PDU
The Network entity Title parameter is the Network Entity Title of the
Intermediate System sending the ISH PDU. It is encoded in the ISH PDU
|Network Entity Title Length Indicator (NETL) | 10
| | 11
: Network Entity Title (NET) :
| | m - 1
Figure 4: ISH PDU - Network Address Part
8.3.5 Destination Address Parameter for RD PDU
The Destination Address is the NSAP address of a destination associ-
ated with some NPDU being forwarded by the Intermediate System send-
ing the RD PDU. It is encoded in the RD PDU as follows:
|Destination Address Length Indicator (DAL) | 10
| | 11
: Destination Address (DA) :
| | m - 1
Figure 5: RD PDU - Network Address Part
8.4 Subnetwork Address Part
The Subnetwork Address Part is present only in RD PDUs.It is used to
indicate the subnetwork address of another Network entity on the same
subnetwork as the End System (and Intermediate System) which may be a
better path to the destination specified in the Network Address Part.
The Subnetwork Address parameter is encoded in the same manner as the
Network Address parameters.
8.4.1 Subnetwork Address Parameter for RD PDU
The Subnetwork Address Parameter is encoded in the RD PDU as fol-
|Subnetwork Address Length Indicator (BSNPAL) | m
| | m + 1
: Subnetwork Address (BSNPA) :
| | n - 1
Figure 6: ESH PDU - Address Part
8.5 Options Part
The options part is used to convey optional parameters. The options
of the PDU header is illustrated below:
| | p
: Options :
| | q
Figure 7: All PDUs - Options Part
If the options part is present, it may contain one or more parame-
ters. The number of parameters that may be contained in the options
part is constrained by the length of the options part, which is
determined by the following formula:
PDU Header Length - (length of fixed part + length of address
part + length of segmentation part),
and by the length of the individual optional parameters.
Parameters defined in the options part may appear in any order. Du-
plication of options is not permitted.Receipt of a PDU with an option
duplicated must be treated as a protocol error.
The encoding of parameters contained within the options part of the
PDU header is illustrated below in figure 8.
| n | Parameter Code |
| n + 1 | Parameter Length |
| n + 2 | |
| to | Parameter Value |
| n + m + 1 | |
Figure 8: Encoding of Option Parameters
The parameter code field is coded in binary and, without extensions,
provides a maximum of 255 different parameters. No parameter codes
use bits 8 and 7 with the value 00, so the actual maximum number of
parameters is lower. A parameter code of 255 (binary 1111 1111) is
reserved for possible future extensions.
The parameter length field indicates the length, in octets, of the
parameter value field.The length is indicated by a positive binary
number, m, with a theoretical maximum value of 254. the practical
maximum value of m is lower. For example, in the case of a single
parameter contained within the options part, two octets are required
for the parameter code and the parameter length indicators. Thus, the
value of m is limited to:
m = 252-(length of fixed part +length of address part
+length of segmentation part )
For each succeeding parameter the maximum value of m decreases. The
parameter value field contains the value of the parameter identified
in the parameter code field.
The following parameters are permitted in the options part.
The Security parameter conveys information about the security re-
quested in the Data PDU that caused the containing RD PDU to be gen-
erated. This parameter has the same encoding and semantics as the
Security parameter in ISO 8473.
Parameter Code: 1100 0101
Parameter Length: variable
Parameter Value: See Section 7.5.3 of ISO 8473
8.5.3 Quality of Service Maintenance
The Quality of Service parameter conveys information about the quali-
ty of service requested in the Data PDU that caused the containing RD
PDU to be generated.
This parameter has the same encoding and semantics as the QoS Mainte-
nance parameter in ISO 8473.
Parameter Code: 1100 0011
Parameter Length: variable
Parameter Value: See Section 7.5.6 of ISO 8473
The Priority parameter conveys information about the priority re-
quested in the Data PDU that caused the containing RD PDU to be gen-
This parameter has the same encoding and semantics as the Priority
parameter in ISO 8473.
Parameter Code: 1100 1101
Parameter Length: one octet
Parameter Value: See Section 7.5.7 of ISO 8473
ANNEX A. SUPPORTING TECHNICAL MATERIAL
A.1 Use of Timers
This protocol makes extensive use of timers to ensure the timeliness
and accuracy of information disseminated using the Configuration and
Route Redirection functions.This section discusses the rationale for
using these timers and provides some background for how they operate.
Systems using this protocol learn about other systems exclusively by
receiving PDUs sent by those systems. In a connectionless environ-
ment, a system must periodically receive updated information to en-
sure that the information it previously received is still correct.
For example, if a system on a subnetwork becomes unavailable (either
it has ceased operating, or its SNPA becomes inoperative) the only
way another system can detect this fact is by the absence of
transmissions from that system. If information were retained in the
absence of new PDUs being received, configuration and/or routing in-
formation would inevitably become incorrect. The Holding Timers
specified by this protocol guarantee that old information will not be
A useful way of thinking of the configuration and route redirection
information is as a cache maintained by each system. The cache is
periodically flushed to ensure that only up-to-date information is
stored.Unlike most caches, however, the time to retain information is
not a purely local matter. Rather, information is held for a period
of time specified by the source of the information. Some examples
will help clarify this operation.
A.1.1 Example of Holding Time for Route Redirection
Route Redirection Information is obtained by an End System through
the Request Redirect function (see clause 7.7).It is quite possible
that a Intermediate System might redirect an End System to another IS
which has recently become unavailable (this might happen if the IS-
to-IS routing algorithm is still converging following a configuration
change). If the Holding Timer were not present, or was set very long
by the sending IS, an End System would have been redirected into a
Black Hole from which none of its Data PDUs would ever emerge. The
length of the Holding Timer on Redirects specifies, in essence, the
length of time black holes are permitted to exist.
On the other hand, setting the Holding Timer on Route Redirects very
short to minimize the effect of black holes has other undesirable
consequences.First, for each PDU that causes a redirect, an addition-
al PDU beside the original Data PDU must be composed and transmitted;
this increases overhead. Second, each time a "working" redirect's
Holding Timer expires, the redirected End System will revert to a
poorer route for at least one PDU.
A.1.2 Example of Holding Timer for Configuration Information
A similar type of problem can occur with respect to Configuration in-
formation. If the Holding Time of a ISH PDU (see clause 7.2.2) is set
very long, and the only Intermediate System (which has been sending
this Configuration Information) on the subnetwork becomes unavail-
able, a subnetwork-wide black hole can form. During this time, End
Systems on the subnetwork may not be able to communicate with each
other because they presume that a Intermediate System is operating
which will forward their Data PDUs to destination ESs on the local
subnetwork and return RD PDUs.Once the Holding Time expires, the ESs
will realize that no IS is available and will take their only
recourse, which is to send their traffic directly on the local sub-
Given the types of problems that can occur, it is important that
responsibility for incorrect information can be unambiguously as-
signed to the source of the information. For this reason all Holding
Timers are calculated by the source of the Configuration or Route
Redirection information and communicated explicitly to each recipient
in the appropriate PDU.
A.2 Refresh and timeout of Redirection information
The protocol allows End Systems to refresh redirection information
without first allowing the holding time to expire and being redirect-
ed by a Intermediate System for a second (or subsequent) time. Such
schemes are prevalent in connectionless subnetworks and are often
called "reverse path information", "previous hop cache" or something
Refreshing the redirection information has obvious performance bene-
fits, but can be dangerous if not handled in a very conservative
fashion. In order for a redirection to be safely refreshed, all of
the following conditions must hold:
1. The source address of the received PDU must be exactly the same
as the destination address specified in a prior RD PDU (this
defines a "match" on the redirection information). Making
assumptions about the equivalence of abbreviated addresses,
group addresses, or similar "special" addresses is dangerous
since routing for these addresses cannot be assumed to be
2. The Quality of Service parameters of the received PDU must be
exactly the same as the QoS parameters specified in the matching
(by destination address) redirection entry.Again, there is no
guarantee that PDUs with different QoS parameters will be routed
the same way. It is quite possible that the redirected path is
even a black hole for certain values of the QoS parameters (the
security field is a good example).
3. The "previous hop" of the received Data PDU must match the "next
hop" stored in the redirection information. Specifically, the
SN_Source_Address of the SN_UNITDATA.Indication which received the
PDU must match exactly the SN_Destination_Address specified in the
redirect to be used for sending traffic via the SN_UNITDATA.Request
primitive. This comparison ensures that redirects are refreshed only
when the reverse traffic is being received from the same IS (or
destination ES) as the forward traffic is being sent through (or
to). This check make certain that redirects are not refreshed for
just on the basis of traffic being received from the destination.
It is quite possible that the traffic is simply indicating that the
forward path in use is not working!
Note that these conditions still allow refresh in the most useful and
common cases where either the destination is another ES on the same
subnetwork as the source ES, or the redirection is to a IS which is
passing traffic to/from the destination in both directions (i.e. the
path is symmetric).
A.3 System Initialization Considerations
This protocol is designed to make the exchange of information as free
as possible from dependencies between the two types of systems.
therefore, it is not possible for an End System to request all Inter-
mediate Systems on a subnetwork to report their configuration, nor is
it possible for an Intermediate System to request all End Systems on
a subnetwork to report their configuration.
In certain operating environments a constraint may be imposed than an
ES, upon becoming operational, must discover the existence of an IS
as soon as possible.The converse relationship also holds if it is
necessary for an IS to discover the existence of End Systems as soon
as possible. In both cases the availability of this information is
normally determined by the Configuration Timer of the system for
which the knowledge is desired. there is therefore a tradeoff between
the overhead associated with performing the Report and Record Confi-
guration functions and the timely availability of the configuration
information. Decreasing the Configuration Timer increases the availa-
bility at the expense of an increase in overhead.
The following solution is recommended for addressing the constraint
described above. When the Record Configuration function is invoked in
either an End System or an Intermediate System, the function will
determine if the received configuration information was previously
unknown.If this is the case, then the Report Configuration function
may be invoked before the expiration of the system's Configuration
Timer. The Hello PDU generated by the Report Configuration function
is then sent only to the Network Entity whose configuration was pre-
viously unknown. Thus when an ES or IS first becomes operational it
immediately reports its configuration. As soon as systems of the oth-
er type discover the new network entity, they will make their own
configuration known to this entity.
The additional overhead incurred by this solution is minimal. Also,
since the discovery of new configurations is made timely by this ap-
proach the Configuration Timer period can be increased in order to
decrease the overhead of the configuration functions, provided that
other factors not discussed here are accounted for by the longer time
period.One caveat is that the first Hello PDU generated by a system
may be lost during transmission. To solve this problem one or more
additional PDUs may be transmitted at short time intervals during
this initialization period.
Note that this solution may be implemented in ISs only, in ESs only,
or in both Intermediate and End Systems.This decision is purely a lo-
cal matter and may be alterable through System Management.
A.4 Optimizations for Flushing Redirects
An ES will attempt to forward NPDUs through an IS to which it has
been redirected until the Holding Timer specified in the RD PDU has
expired, even if that IS is no longer reachable. Under certain cir-
cumstances, it is possible to do better and recognize the existence
of a black hole sooner. In particular, if the ES expects to hear ISH
PDUs from the IS to which it has been redirected, and the Holding Ti-
mer for that IS expires, all knowledge of the IS may be forgotten by
the ES. This includes any redirects, which may be flushed (see the
Flush Old Redirect function) even though their timeouts have not ex-