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Network Working Group ANSI X3S3.3 86-80
Request for Comments: 994 ISO TC97/SC6/N 3998
March 1986
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: Final Text of DIS 8473, Protocol for Providing the Connectionless-
mode Network Service
Source: DIS 8473 Editor
ISO 8473 [Page 1]
RFC 994 December 1986
Contents
1 Scope and Field of Application 6
2 References 7
SECTION ONE. GENERAL 9
3 Definitions 9
3.1 Reference Model Definitions . . . . . . . . . . . . . . . . . 9
3.2 Service Conventions Definitions . . . . . . . . . . . . . . . 9
3.3 Network Layer Architecture Definitions . . . . . . . . . . . . 9
3.4 Network Layer Addressing Definitions . . . . . . . . . . . . . 10
3.5 Additional Definitions . . . . . . . . . . . . . . . . . . . . 10
4 Symbols and Abbreviations 11
4.1 Data Units . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2 Protocol Data Units . . . . . . . . . . . . . . . . . . . . . 11
4.3 Protocol Data Unit Fields . . . . . . . . . . . . . . . . . . 11
4.4 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.5 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . 11
5 Overview of the Protocol 12
5.1 Internal Organization of the Network Layer . . . . . . . . . . 12
5.2 Subsets of the Protocol . . . . . . . . . . . . . . . . . . . 12
5.3 Addresses and Titles . . . . . . . . . . . . . . . . . . . . . 13
5.3.1 Addresses . . . . . . . . . . . . . . . . . . . . . . 13
5.3.2 Network-entity Titles . . . . . . . . . . . . . . . . 13
5.4 Service Provided by the Network Layer . . . . . . . . . . . . 14
5.5 Underlying Service Assumed by the Protocol . . . . . . . . . . 14
5.5.1 Subnetwork Points of Attachment . . . . . . . . . . . 15
5.5.2 Subnetwork Quality of Service . . . . . . . . . . . . 15
5.5.3 Subnetwork User Data . . . . . . . . . . . . . . . . 16
5.5.4 Subnetwork Dependent Convergence Functions . . . . . . 16
5.6 Service Assumed from Local Environment . . . . . . . . . . . . 16
SECTION TWO. SPECIFICATION OF THE PROTOCOL 18
6 Protocol Functions 18
6.1 PDU Composition Function . . . . . . . . . . . . . . . . . . . 18
6.2 PDU Decomposition Function . . . . . . . . . . . . . . . . . . 19
6.3 Header Format Analysis Function . . . . . . . . . . . . . . . 19
ISO 8473 [Page 2]
RFC 994 December 1986
6.4 PDU Lifetime Control Function . . . . . . . . . . . . . . . . 20
6.5 Route PDU Function . . . . . . . . . . . . . . . . . . . . . . 20
6.6 Forward PDU Function . . . . . . . . . . . . . . . . . . . . . 21
6.7 Segmentation Function . . . . . . . . . . . . . . . . . . . . 21
6.8 Reassembly Function . . . . . . . . . . . . . . . . . . . . . 22
6.9 Discard PDU Function . . . . . . . . . . . . . . . . . . . . . 23
6.10 Error Reporting Function . . . . . . . . . . . . . . . . . . . 24
6.10.1 Overview . . . . . . . . . . . . . . . . . . . . . . . 24
6.10.2 Requirements . . . . . . . . . . . . . . . . . . . . . 25
6.10.3 Processing of Error Reports . . . . . . . . . . . . . 25
6.10.4 Relationship of Data PDU Options to Error Reports . . 26
6.11 PDU Header Error Detection . . . . . . . . . . . . . . . . . . 27
6.12 Padding Function . . . . . . . . . . . . . . . . . . . . . . . 28
6.13 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
6.14 Source Routing Function . . . . . . . . . . . . . . . . . . . 28
6.15 Record Route Function . . . . . . . . . . . . . . . . . . . . 29
6.16 Quality of Service Maintenance Function . . . . . . . . . . . 30
6.17 Priority Function . . . . . . . . . . . . . . . . . . . . . . 31
6.18 Congestion Notification Function . . . . . . . . . . . . . . . 31
6.19 Classification of Functions . . . . . . . . . . . . . . . . . 31
7 Structure and Encoding of PDUs 33
7.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 33
7.2 Fixed Part . . . . . . . . . . . . . . . . . . . . . . . . . . 34
7.2.1 General . . . . . . . . . . . . . . . . . . . . . . . 34
7.2.2 Network Layer Protocol Identifier . . . . . . . . . . 34
7.2.3 Length Indicator . . . . . . . . . . . . . . . . . . 35
7.2.4 Version/Protocol Identifier Extension . . . . . . . . 35
7.2.5 PDU Lifetime . . . . . . . . . . . . . . . . . . . . 35
7.2.6 Flags . . . . . . . . . . . . . . . . . . . . . . . . 35
7.2.6.1 Segmentation Permitted . . . . . . . . . . . 35
7.2.6.2 More Segments . . . . . . . . . . . . . . . 35
7.2.6.3 Error Report . . . . . . . . . . . . . . . 36
7.2.7 Type Code . . . . . . . . . . . . . . . . . . . . . . 36
7.2.8 PDU Segment Length . . . . . . . . . . . . . . . . . 36
7.2.9 PDU Checksum . . . . . . . . . . . . . . . . . . . . 36
7.3 Address Part . . . . . . . . . . . . . . . . . . . . . . . . 37
7.3.1 General . . . . . . . . . . . . . . . . . . . . . . . 37
7.3.1.1 Destination and Source Addresses . . . . . . 37
7.4 Segmentation Part . . . . . . . . . . . . . . . . . . . . . . 38
7.4.1 Data Unit Identifier . . . . . . . . . . . . . . . . . 38
7.4.2 Segment Offset . . . . . . . . . . . . . . . . . . . . 38
7.4.3 PDU Total Length . . . . . . . . . . . . . . . . . . . 39
7.5 Options Part . . . . . . . . . . . . . . . . . . . . . . . . 39
7.5.1 General . . . . . . . . . . . . . . . . . . . . . . . 39
7.5.2 Padding . . . . . . . . . . . . . . . . . . . . . . . 40
7.5.3 Security . . . . . . . . . . . . . . . . . . . . . . . 40
7.5.3.1 Source Address Specific . . . . . . . . . . 41
7.5.3.2 Destination Address Specific . . . . . . . . 41
7.5.3.3 Globally Unique Security . . . . . . . . . . 41
7.5.4 Source Routing . . . . . . . . . . . . . . . . . . . 41
ISO 8473 [Page 3]
RFC 994 December 1986
7.5.5 Recording of Route . . . . . . . . . . . . . . . . . . 42
7.5.6 Quality of Service Maintenance . . . . . . . . . . . . 43
7.5.6.1 Source Address Specific . . . . . . . . . . 43
7.5.6.2 Destination Address Specific . . . . . . . . 43
7.5.6.3 Globally Unique QoS . . . . . . . . . . . . 43
7.5.7 Priority . . . . . . . . . . . . . . . . . . . . . . 44
7.6 Data Part . . . . . . . . . . . . . . . . . . . . . . . . . . 45
7.7 Data (DT) PDU . . . . . . . . . . . . . . . . . . . . . . . . 46
7.7.1 Structure . . . . . . . . . . . . . . . . . . . . . . 46
7.7.1.1 Fixed Part . . . . . . . . . . . . . . . . . . . . . 47
7.7.1.2 Addresses . . . . . . . . . . . . . . . . . . . . . 47
7.7.1.3 Segmentation . . . . . . . . . . . . . . . . . . . . 47
7.7.1.4 Options . . . . . . . . . . . . . . . . . . . . . . 47
7.7.1.5 Data . . . . . . . . . . . . . . . . . . . . . . . 47
7.8 Inactive Network Layer Protocol . . . . . . . . . . . . . . . 47
7.8.1 Network Layer Protocol Id . . . . . . . . . . . . . . 47
7.8.2 Data Field . . . . . . . . . . . . . . . . . . . . . 47
7.9 Error Report PDU (ER) . . . . . . . . . . . . . . . . . . . . 48
7.9.1 Structure . . . . . . . . . . . . . . . . . . . . . . 48
7.9.1.1 Fixed Part . . . . . . . . . . . . . . . . . 49
7.9.1.2 Addresses . . . . . . . . . . . . . . . . . 49
7.9.1.3 Options . . . . . . . . . . . . . . . . . . 49
7.9.1.4 Reason for Discard . . . . . . . . . . . . . 50
7.9.1.5 Error Report Data Field . . . . . . . . . . 51
8 Conformance 51
8.1 Provision of Functions for Conformance . . . . . . . . . . . . 51
List of Tables
1 Service Primitives for Underlying Service . . . . . . . . . . . . 14
2 Service Primitives for Underlying Service . . . . . . . . . . . . 14
3 Timer Primitives . . . . . . . . . . . . . . . . . . . . . . . . 14
4 Categorization of Protocol Functions . . . . . . . . . . . . . . . 32
5 Valid PDU Types . . . . . . . . . . . . . . . . . . . . . . . . . 36
6 Encoding of Option Parameters . . . . . . . . . . . . . . . . . . 39
7 Reason for Discard . . . . . . . . . . . . . . . . . . . . . . . . 50
8 Categorization of Functions . . . . . . . . . . . . . . . . . . . 52
List of Figures
1 Interrelationship of Standards . . . . . . . . . . . . . . . . . 6
2 PDU Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 PDU Header -- Fixed Part . . . . . . . . . . . . . . . . . . . . . 34
4 PDU Header -- Address Part . . . . . . . . . . . . . . . . . . . 37
5 Address Parameters . . . . . . . . . . . . . . . . . . . . . . . . 38
6 PDU Header -- Segmentation Part . . . . . . . . . . . . . . . . . 38
7 PDU Header -- Options Part . . . . . . . . . . . . . . . . . . . . 39
8 PDU Header -- Data Field . . . . . . . . . . . . . . . . . . . . 45
ISO 8473 [Page 4]
RFC 994 December 1986
9 DT PDU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
10 Inactive Network Layer Protocol . . . . . . . . . . . . . . . . . 47
11 Error Report PDU . . . . . . . . . . . . . . . . . . . . . . . . . 48
ISO 8473 [Page 5]
RFC 994 December 1986
0 Introduction
This Protocol Standard 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 interconnection.
This Protocol Standard is positioned with respect to other related
standards by the layers defined in the Reference Model for Open Sys-
tems Interconnection (ISO 7498). In particular, it is a protocol of
the Network Layer. This Protocol may be used between network-entities
in end systems or in Network Layer relay systems (or both). It pro-
vides the Connectionless-mode Network Service as defined in Addendum
1 to the Network Service Definition Covering Connectionless-mode
Transmission (ISO 8348/AD1).
The interrelationship of these standards is illustrated in Figure 1
below:
--------------------+--- ISO NETWORK SERVICE PROVIDER -----^-----------------
| |
| |
| |
PROTOCOL | REFERENCE TO AIMS -----------------+
|
SPECIFICATION | REFERENCE TO ASSUMPTIONS -----------+
| |
| |
| |
--------------------+---SUBNETWORK SERVICE DEFINITION(S)---v-----------------
Figure 1: Interrelationship of Standards
1 Scope and Field of Application
This International Standard specifies a protocol which is used to
provide the Connectionless-mode Network Service as described in Ad-
dendum 1 to the Network Service Definition Covering Connectionless-
mode Transmission. The protocol relies upon the provision of an
underlying connectionless-mode service by real subnetworks and/or
data links. The underlying connectionless-mode service assumed by the
protocol may be obtained either directly, from a connectionless-mode
real subnetwork, or indirectly, through the operation of an appropri-
ate Subnetwork Dependent Convergence Function (SNDCF) or Protocol
(SNDCP) over a connection-mode real subnetwork as described in ISO
8648, Internal Organization of the Network Layer.
ISO 8473 [Page 6]
RFC 994 December 1986
This Standard specifies:
a) procedures for the connectionless transmission of data and
control information from one network-entity to a peer
network-entity;
b) the encoding of the protocol data units (PDUs) used for the
transmission of data and control information, comprising a
variable-length protocol header format;
c) procedures for the correct interpretation of protocol control
information; and
d) the functional requirements for implementations claiming
conformance to the Standard.
The procedures are defined in terms of:
a) the interactions among peer network-entities through the
exchange of protocol data units;
b) the interactions between a network-entity and a Network Service
user through the exchange of Network Service primitives; and
c) the interactions between a network-entity and an underlying
service provider through the exchange of service primitives.
2 References
ISO 7498, Information Processing Systems --- Open Systems Intercon-
nection --- Basic Reference Model
DIS 7498/AD1, Information Processing Systems --- Open Systems In-
terconnection --- Addendum to ISO 7498 Covering Connectionless-mode
Transmission
ISO 8348, Information Processing Systems --- Telecommunications and
Information Exchange between Systems --- Network Service Definition
ISO 8348/AD1, Information Processing Systems --- Telecommunications
and Information Exchange between Systems --- Addendum to the Net-
work Service Definition Covering Connectionless-mode Transmission
ISO 8348/AD2, Information Processing Systems --- Telecommunications
and Information Exchange between Systems --- Addendum to the Net-
work Service Definition Covering Network Layer Addressing*
DIS 8648, Information Processing Systems --- Telecommunications and
Information Exchange between Systems --- Internal Organization of the
Network Layer
ISO 8473 [Page 7]
RFC 994 December 1986
ISO 8509, Technical Report --- OSI Service Conventions
ISO 9074, A Formal Description Technique based on an Extended State
Transition Model
________________________________
*At present, at the stage of Draft; publication anticipated in
due course.
ISO 8473 [Page 8]
RFC 994 December 1986
SECTION ONE. GENERAL
3 Definitions
3.1 Reference Model Definitions
This document makes use of the following concepts defined in ISO 7498:
(a) End system
(b) Network entity
(c) Network layer
(d) Network protocol
(e) Network protocol data unit
(f) Network relay
(g) Network service
(h) Network service access point
(i) Network service access point address
(j) Routing
(k) Service
(l) Service data unit
3.2 Service Conventions Definitions
This Protocol Standard makes use of the following terms from the OSI
Service Conventions Technical Report (ISO TR 8509):
(a) Service provider
(b) Service user
3.3 Network Layer Architecture Definitions
This Protocol Standard makes use of the following terms from the
Internal Organization of the Network Layer (ISO 8648):
(a) Intermediate system
(b) Relay system
(c) Subnetwork
ISO 8473 [Page 9]
RFC 994 December 1986
3.4 Network Layer Addressing Definitions
This Protocol Standard makes use of the following terms from ISO 8348/AD2,
Addendum to the Network Service Definition Covering Network Layer
addressing:
(a) Network addressing domain
(b) Network protocol address information
(c) Subnetwork point of attachment
3.5 Additional Definitions
For the purposes of this Protocol Standard, the following definitions
apply:
(a) derived PDU --- a protocol data unit whose fields are identical
to those of an initial PDU, except that it carries only a segment
of the user data from an N-UNITDATA request.
(b) initial PDU --- a protocol data unit carrying the whole of the
userq data from an N-UNITDATA request.
(c) local matter --- a decision made by a system concerning its
behavior in the Network Layer that is not prescribed or
constrained by this Protocol Standard.
(d) 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.
(e) reassembly --- the act of regenerating an initial PDU from two
or more derived PDUs.
(f) segment --- a distinct unit of data consisting of part or all
of the user data provided in the N-UNITDATA request and delivered
in the N-UNITDATA indication.
(g) segmentation --- the act of generating two or more derived PDUs
from an initial or derived PDU. The derived PDUs together carry
the entire user data of the initial or derived PDU from which they
were generated.
Note:
It is possible that such an initial PDU will never actually be
generated for a particular N-UNITDATA request, owing to the
immediate application of segmentation.
ISO 8473 [Page 10]
RFC 994 December 1986
4 Symbols and Abbreviations
4.1 Data Units
NSDU Network Service Data Unit
PDU Protocol Data Unit
SNSDU Subnetwork Service Data Unit
4.2 Protocol Data Units
DT PDU Data Protocol Data Unit
ER PDU Error Report Protocol Data Unit
4.3 Protocol Data Unit Fields
CS Checksum
DA Destination Address
DAL Destination Address Length
DUID Data Unit Identifier
E/R Error Report Flag
LI Length Indicator
LT Lifetime
MS More Segments Flag
NLPID Network Layer Protocol Identifier
SA Source Address
SAL Source Address Length
SL Segment Length
SO Segment Offset
SP Segmentation Permitted Flag
TL Total Lengt
TP Type
V/P Version/Protocol Identifier Extension
4.4 Parameters
DA Destination Address
QOS Quality of Service
SA Source Address
4.5 Miscellaneous
CLNP Connectionless-mode Network Protocol
NS Network Service
NPAI Network Protocol Address Information
NSAP Network Service Access Point
SDU Service Data Uni
SN Subnetwork
SNDCF Subnetwork Dependent Convergence Function
SNDCP Subnetwork Dependent Convergence Protocol
SNICP Subnetwork Independent Convergence Protocol
SNPA Subnetwork Point of Attachment
ISO 8473 [Page 11]
RFC 994 December 1986
5 Overview of the Protocol
5.1 Internal Organization of the Network Layer
The architectural organization of the Network Layer is described in a
separate document, Internal Organization of the Network Layer (ISO
8648). ISO 8648 identifies and categorizes the way in which functions
can be performed within the Network Layer by Network Layer protocols,
thus providing a uniform framework for describing how protocols
operating either individually or cooperatively in the Network Layer
can be used to provide the OSI Network Service. This protocol is
designed to be used in the context of the internetworking protocol
approach to the provision of the Connectionless-mode Network Service
defined in that Standard.
This protocol is intended for use in the Subnetwork Independent Con-
vergence Protocol (SNICP) role. A protocol which fulfills the SNICP
role operates to construct the OSI Network Service over a defined set
of underlying services, performing functions which are necessary to
support the uniform appearance of the OSI Connectionless-mode Network
Service over a homogeneous or heterogeneous set of interconnected
subnetworks. This protocol is defined to accommodate variability
where Subnetwork Dependent Convergence Protocols and/or Subnetwork
Access Protocols do not provide all of the functions necessary to
support the Connectionless-mode Network Service over all or part of
the path from one NSAP to another.
As described in ISO 8648, a protocol at the Network Layer may fulfill
different roles in different configurations. Although this protocol
is designed particularly to be suitable for a SNICP role in the con-
text of the internetworking protocol approach to the provision of the
Connectionless-mode Network Service, it may also be used to fulfill
other roles and may therefore be used in the context of other ap-
proaches to subnetwork interconnection.
The specification of this protocol begins with a definition of the
underlying service which it assumes. This service is made available
by the operation of other Network Layer protocols or through provi-
sion of the Data Link Service. The underlying service assumed by this
protocol is described in Clause 5.5.
5.2 Subsets of the Protocol
Two proper subsets of the full protocol are defined which permit the
use of known subnetwork characteristics and are therefore not subnet-
work independent.
The Inactive Network Layer protocol subset is a null-function subset
which can be used when it is known that the source and destination
end-systems are connected by a single subnetwork, and when none of
the functions performed by the full protocol is required to provide
ISO 8473 [Page 12]
RFC 994 December 1986
the Connectionless-mode Network Service between any pair of end-
systems.
The Non-segmenting protocol subset permits simplification of the
header where it is known that the source and destination end-systems
are connected by subnetworks whose service data unit sizes are
greater than or equal to a known bound which is large enough so that
segmentation is not required. This subset is selected by setting the
Segmentation Permitted flag to zero.
5.3 Addresses and Titles
The following Clauses describe the addresses and titles used by this
Protocol.
5.3.1 Addresses
The Source Address and Destination Address parameters referred to in
Clause 7.3 of this International Standard are OSI Network Service Ac-
cess Point Addresses. The syntax and semantics of an OSI Network
Service Access Point Address are described in a separate document,
ISO 8348/AD2, Addendum to the Network Service Definition Covering
Network Layer Addressing.
The encoding used by this protocol to convey NSAP Addresses shall be
the preferred binary encoding specified in ISO 8348/AD2; the entire
NSAP address, taken as a whole, is represented explicitly as a string
of binary octets. This string is conveyed in its entirety in the ad-
dress fields described in Clause 7.3. The rules governing the genera-
tion of the preferred binary encoding are described in ISO 8348/AD2.
5.3.2 Network-entity Titles
A network-entity title is an identifier for a network-entity in an
endsystem or intermediate-system. Network-entity titles are allocated
from the same name space as NSAP addresses, and the determination of
whether an address is an NSAP address or a network-entity title
depends on the context in which the address is interpreted. The en-
tries in the Source Routing and Recording of Route parameters defined
in Clauses 7.5.4 and 7.5.5 are network-entity titles. The Source Ad-
dress and Destination Address parameters in the Error Report PDU de-
fined in Clause 7.9.1.2 are also network-entity titles.
The encoding used by this protocol to convey network-entity titles
shall also be the preferred binary encoding; again, the entire
network-entity title, taken as a whole, is represented explicitly as
a string of binary octets. This string is conveyed in its entirety
in the fields described in Clauses 7.5.4, 7.5.5, and 7.9.1.2.
ISO 8473 [Page 13]
RFC 994 December 1986
5.4 Service Provided by the Network Layer
The service provided by this protocol is the Connectionless-mode Net-
work Service described in ISO 8348/AD1, Addendum to the Network Ser-
vice Definition Covering Connectionless-mode Transmission. The Net-
work Service primitives provided are summarized in Table 1:
_____________________________________________________________
| PRIMITIVES PARAMETERS |
|____________________________________________________________ |
| N_UNITDATA .Request | N_Source_Address, |
| .Indication | N_Destination_Address, |
| | N_Quality_of_Service, |
| | N_Userdata |
|_________________________________|___________________________|
Table 1: Service Primitives for Underlying Service
The Addendum to the Network Service Definition Covering
Connectionless-mode Transmission (ISO 8348/AD1) states that the max-
imum size of a connectionless-mode Network-service-data-unit (NSDU)
is limited to 64512 octets.
5.5 Underlying Service Assumed by the Protocol
The underlying service required to support this protocol is defined
by the following primitives:
_____________________________________________________________
| PRIMITIVES PARAMETERS |
|____________________________________________________________ |
| SN_UNITDATA .Request | SN_Source_Address, |
| .Indication | SN_Destination_Address, |
| | SN_Quality_of_Service, |
| | SN_Userdata |
|_________________________________|___________________________|
Table 2: Service Primitives for Underlying Service
Note:
These service primitives are used to describe the abstract interface
which exists between the ISO 8473 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.
ISO 8473 [Page 14]
RFC 994 December 1986
5.5.1 Subnetwork Points of Attachment
The source and destination addresses specify the points of attachment
to a public or private subnetwork(s) involved in the transmission.
Subnetwork Point of Attachment addresses (SNPAs) are defined by each
individual subnetwork authority.
The syntax and semantics of SNPAs are not defined in this Standard.
5.5.2 Subnetwork Quality of Service
Subnetwork Quality of Service describes aspects of an underlying
connectionless-mode service which are attributable solely to the
underlying service.
Associated with each connectionless-mode transmission, certain meas-
ures of Quality of Service are requested when the primitive action is
initiated. These requested measures (or parameter values and op-
tions) are based on a priori knowledge of the service(s) made avail-
able to it by the subnetwork. Knowledge of the nature and type of
service available is typically obtained prior to an invocation of the
underlying connectionless-mode service.
The Quality of Service parameters identified for the underlying
connectionless-mode service may in some circumstances be directly
derivable from or mappable onto those identified in the
Connectionless-mode Network Service. The following parameters as de-
fined in ISO 8348/AD1, Addendum to the Network Service Definition
Covering Connectionlessmode Transmission, may be employed:
(a) transit delay;
(b) protection against unauthorized access;
(c) cost determinants;
(d) priority; and
(e) residual error probability.
Note:
For those subnetworks which do not inherently provide Quality of
Service as a parameter when the primitive action is initiated, it
is a local matter as to how the semantics of the service requested
might be preserved. In particular, there may be instances in which
the Quality of Service requested cannot be maintained. In such
circumstances, an attempt shall be made to deliver the protocol
data unit at whatever Quality of Service is available.
ISO 8473 [Page 15]
RFC 994 December 1986
5.5.3 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 assumed by the CLNP is required to support a
service data unit size of at least 512 octets.
If the minimum service data unit sizes supported by all of the sub-
networks involved in the transmission of a particular PDU are known
to be large enough that segmentation is not required, then the Non-
segmenting protocol subset may be used.
5.5.4 Subnetwork Dependent Convergence Functions
Subnetwork Dependent Convergence Functions may be performed to pro-
vide an underlying connectionless-mode service in the case where a
real subnetwork does not inherently provide the connectionless-mode
service assumed by the protocol. If a subnetwork inherently provides
a connection-mode service, a Subnetwork Dependent Convergence Func-
tion provides a mapping into the required underlying service. Sub-
network Dependent Convergence Functions may also be required in those
cases where functions assumed from the underlying service are not
performed. In some cases, this may require the operation of an ex-
plicit protocol (i.e., a protocol involving explicit exchanges of
protocol control information between peer network-entities) in the
Subnetwork Dependent Convergence Protocol (SNDCP) role. However,
there may also be cases where the functionality required to fulfill
the SNDCP role consists simply of a set of rules for manipulating the
underlying service.
5.6 Service Assumed from Local Environment
A timer service must be provided to allow the protocol entity to
schedule events.
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.
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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
Table 3.
__________________________________________________
| PRIMITIVES PARAMETERS |
|_________________________________________________|
| S--TIMER .Request | S-Time, |
| | S-Name, |
| | S-Subscript |
| | |
| .Response | S-Name, |
| | S-Subscript |
|___________________________|_____________________|
Table 3: 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.
Note:
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 underlying service.
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SECTION TWO. SPECIFICATION OF THE PROTOCOL
6 Protocol Functions
This Clause describes the functions performed as part of the Proto-
col.
Not all of the functions must be performed by every implementation.
Clause 6.17 specifies which functions may be omitted, and the correct
behavior when requested functions are not implemented.
6.1 PDU Composition Function
This function is responsible for the construction of a protocol data
unit according to the rules governing the encoding of PDUs given in
Clause 7. Protocol Control Information required for delivering the
data unit to its destination is determined from current state and lo-
cal information and from the parameters associated with the N-
UNITDATA Request.
Network Protocol Address Information (NPAI) for the Source Address
and Destination Address fields of the PDU header is derived from the
NS-Source-Address and NS-Destination-Address parameters. The NS-
Destination-Address and NS-Quality-of-Service parameters, together
with current state and local information, are used to determine which
optional functions are to be selected. User data passed from the Net-
work Service User (NS-Userdata) forms the Data field of the protocol
data unit.
During the composition of the protocol data unit, a Data Unit Iden-
tifier is assigned to distinguish this request to transmit NS-
Userdata to a particular destination NS User from other such re-
quests. The originator of the PDU must choose the Data Unit Identif-
ier so that it remains unique (for this Source and Destination ad-
dress pair) for the maximum lifetime of the Initial PDU in the net-
work; this rule applies for any PDUs derived from the Initial PDU as
a result of the application of the Segmentation Function (see Clause
6.7). Derived PDUs are considered to correspond to the same Initial
PDU, and hence the same N-UNITDATA Request, if they have the same
Source Address, Destination Address, and Data Unit Identifier.
The Data Unit Identifier is also available for ancillary functions
such as error reporting (see Clause 6.10).
The total length of the PDU in octets is determined by the originator
and placed in the Total Length field of the PDU header. This field is
not changed in any Derived PDU for the lifetime of the protocol data
unit.
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When the Non-segmenting protocol subset is employed, neither the To-
tal Length field nor the Data Unit Identifier field is present. The
rules governing the PDU composition function are modified in this
case as follows. During the composition of the protocol data unit,
the total length of the PDU in octets is determined by the originator
and placed in the Segment Length field of the PDU header. This field
is not changed for the lifetime of the PDU. No Data Unit Identifica-
tion is provided.
6.2 PDU Decomposition Function
This function is responsible for removing the Protocol Control Infor-
mation from the protocol data unit. During this process, information
pertinent to the generation of the N-UNITDATA Indication is deter-
mined as follows. The NS-Source-Address and NS-Destination-Address
parameters of the N-UNITDATA Indication are recovered from the NPAI
in the Source and Destination Address fields of the PDU header. The
data field of the PDU received is reserved until all segments of the
original service data unit have been received; collectively, these
form the NS-Userdata parameter of the N-UNITDATA Indication. Infor-
mation relating to the Quality of Service provided during the
transmission of the PDU is determined from the Quality of Service and
other information contained in the Options Part of the PDU header.
This information constitutes the NS-Quality-of-Service parameter of
the N-UNITDATA Indication.
6.3 Header Format Analysis Function
This function determines whether the full protocol described in this
Standard is employed, or one of the defined proper subsets thereof.
If the protocol data unit has a Network Layer Protocol Identifier in-
dicating that this is a standard version of the Protocol, this func-
tion determines whether a received PDU has reached its destination,
using the Destination Address provided in the PDU. If the Destination
Address provided in the PDU identifies an NSAP served by this
network-entity, then the PDU has reached its destination; if not, it
must be forwarded.
If the protocol data unit has a Network Layer Protocol Identifier in-
dicating that the Inactive Network Layer Protocol subset is in use,
then no further analysis of the PDU header is required. The network-
entity in this case determines that either the Subnetwork Point of
Attachment address encoded as network protocol address information in
the supporting subnetwork protocol corresponds directly to an NSAP
address serviced by this network-entity or that an error has oc-
curred. If the subnetwork protocol data unit has been delivered
correctly, then the PDU may be decomposed according to the procedures
described for that particular subnetwork protocol.
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6.4 PDU Lifetime Control Function
This function is used to enforce the maximum PDU lifetime. It is
closely associated with the Header Format Analysis function. This
function determines whether a PDU received may be forwarded or wheth-
er its assigned lifetime has expired, in which case it must be dis-
carded.
The operation of the PDU Lifetime Control function depends upon the
Lifetime field in the PDU header. This field contains, at any time,
the remaining lifetime of the PDU (represented in units of 500 mil-
liseconds). The Lifetime of the Initial PDU is determined by the ori-
ginating network-entity, and placed in the Lifetime field of the PDU.
When the Segmentation function is applied to a PDU, the value of the
Lifetime field of the Initial PDU is copied into all of the Derived
PDUs.
The Lifetime of the PDU is decremented by every network-entity which
processes the PDU. When a network-entity processes a PDU, it decre-
ments the PDU Lifetime by at least one. The value of the PDU Life-
time field shall be decremented by more than one if the sum of:
1. the transit delay in the underlying service from which the PDU
was received; and
2. the delay within the system processing the PDU
exceeds or is estimated to exceed 500 milliseconds. In this case,
the lifetime field should be decremented by one for each additional
500 milliseconds of delay. The determination of delay need not be
precise, but where a precise value cannot be ascertained, the value
used shall be an overestimate, not an underestimate.
If the Lifetime field reaches a value of zero before the PDU is
delivered to the destination, the PDU must be discarded. The Error
Reporting function shall be invoked as described in Clause 6.10, Er-
ror Reporting Function, and may result in the generation of an Error
Report PDU. It is a local matter whether the destination network-
entity performs the Lifetime Control function.
6.5 Route PDU Function
This function determines the network-entity to which a protocol data
unit should be forwarded and the underlying service that must be used
to reach that network-entity, using the Destination Address and the
total length of the PDU. Where segmentation is required, the Route
PDU function further determines over which underlying service Derived
PDUs/segments must be sent in order to reach that network-entity. The
results of the Route PDU function are passed to the Forward PDU func-
tion (along with the PDU itself) for further processing. Selection
of the underlying service that must be used to reach the "next" sys-
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tem in the route is initially influenced by the NS-Quality-of- Ser-
vice parameter of the N-UNITDATA Request, which specifies the QoS re-
quested by the sending NS User. Whether this QoS is to be provided
directly by the CLNP, through the selection of the Quality of Service
Maintenance parameter and other optional parameters, or through the
QoS facilities offered by each of the underlying services is deter-
mined prior to invocation of the Forward PDU function. Route selec-
tion by intermediate systems may subsequently be influenced by the
values of the Quality of Service Maintenance parameter (if present),
and other optional parameters (if present).
6.6 Forward PDU Function
This function issues an SN-UNITDATA Request primitive (see Clause
5.5), supplying the subnetwork or SNDCF identified by the Route PDU
function with the protocol data unit as user data to be transmitted,
the address information required by that subnetwork or SNDCF to iden-
tify the "next" system within the subnetwork-specific addressing
domain (this may be an intermediate-system or the destination end-
system), and Quality of Service constraints (if any) to be considered
in the processing of the user data.
When the PDU to be forwarded is longer than the maximum service data
user size provided by the underlying service, the Segmentation func-
tion is applied (See Clause 6.7, which follows).
6.7 Segmentation Function
Segmentation is performed when the size of the protocol data unit is
greater than the maximum service data unit size supported by the
underlying service to be used to transmit the PDU.
Segmentation consists of composing two or more new PDUs (Derived
PDUs) from the PDU received. The PDU received may be the Initial PDU,
or it may be a Derived PDU. All of the header information from the
PDU to be segmented, with the exception of the segment length and
checksum fields of the fixed part, and the segment offset of the seg-
mentation part, is duplicated in each Derived PDU, including all of
the address part, the data unit identifier and total length of the
segmentation part, and the options part (if present).
Note:
The rules for forwarding and segmentation guarantee that the
header length is the same for all segments (Derived PDUs) of
the Initial PDU, and is the same as the header length of the
Initial PDU. The size of a PDU header will not change due to
operation of any protocol function.
The user data encapsulated within the PDU received are divided such
that the Derived PDUs satisfy the size requirements of the user data
parameter field of the primitive used to access the underlying ser-
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vice.
Derived PDUs are identified as being from the same Initial PDU by
means of
(a) the source address,
(b) the destination address, and
(c) the data unit identifier.
Segmentation shall not result in the generation of a Derived PDU con-
taining less than eight (8) octets of user data.
The following fields of the PDU header are used in conjunction with
the Segmentation function:
(a) Segment Offset --- identifies, with respect to the start
of the Initial PDU, the octet at which the segment begins;
(b) Segment Length --- specifies the number of octets in the
Derived PDU, including both header and data;
(c) More Segments Flag --- is set to one if this Derived PDU
does not contain, as its final octet of user data, the final
octet of the Initial PDU; and
(d) Total Length --- specifies the entire length of the Initial
PDU, including both header and data.
Derived PDUs may be further segmented without constraining the rout-
ing of the individual Derived PDUs. The Segmentation Permitted flag
is set to one to indicate that segmentation is permitted. If the Ini-
tial PDU is not to be segmented at any point during its lifetime in
the network, the flag is set to zero by the source network-entity.
The setting of the Segmentation Permitted flag cannot be changed by
any other network-entity for the lifetime of the Initial PDU and any
Derived PDUs.
6.8 Reassembly Function
The Reassembly function reconstructs the Initial PDU from the Derived
PDUs generated by the operation of the Segmentation Function on the
Initial PDU (and, recursively, on subsequent Derived PDUs). A bound
on the time during which segments (Derived PDUs) of an Initial PDU
will be held at a reassembly point before being discarded is provid-
ed, so that reassembly resources may be released when it is no longer
expected that any outstanding segments of the Initial PDU will arrive
at the reassembly point. Upon reception of a Derived PDU, a reassem-
bly timer is initiated with a value which indicates the amount of
ISO 8473 [Page 22]
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time which must elapse before any outstanding segments of the Initial
PDU shall be assumed to be lost. When this timer expires, all seg-
ments (Derived PDUs) of the Initial PDU held at the reassembly point
are discarded, the resources allocated for those segments are freed,
and if selected, an Error Report is generated (see Clause 6.10).
While the exact relationship between reassembly lifetime and PDU
lifetime is a local matter, the Reassembly Function must preserve the
intent of the PDU lifetime. Consequently, the reassembly function
must discard PDUs whose lifetime would otherwise have expired had
they not been under the control of the reassembly function.
Note:
1. Methods of bounding reassembly lifetime are discussed in
Annex B.
2. The Segmentation and Reassembly functions are intended to
be used in such a way that the fewest possible segments are
generated at each segmentation point and reassembly takes
place at the final destination of a PDU. However, other
schemes which
(a) interact with the routing algorithm to favor paths on
which fewer segments are generated;
(b) generate more segments than absolutely required in
order to avoid additional segmentation at some subsequent
point; or
(c) allow partial or full reassembly at some intermediate
point along the route
are not precluded. The information necessary to enable the
use of one of these alternative strategies may be made
available through the operation of a Network Layer Management
function or by other means.
3. The originator of the Initial PDU determines the value of the
Segmentation Permitted flag in the Initial PDU and all Derived
PDUs (if any). Partial or full reassembly in an intermediate
system (Note 2 (c) above) cannot change this value in the
Initial PDU or any PDU derived from it, and cannot therefore
add or remove the segmentation part of the header.
6.9 Discard PDU Function
This function performs all of the actions necessary to free the
resources reserved by the network-entity when any of the following
situations is encountered (Note: the list is not exhaustive):
(a) A violation of protocol procedure has occurred.
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(b) A PDU is received whose checksum is inconsistent with its
contents.
(c) A PDU is received, but due to local congestion, it cannot be
processed.
(d) A PDU is received whose header cannot be analyzed.
(e) A PDU is received which cannot be segmented and cannot be
forwarded because its length exceeds the maximum service data
unit size supported by any underlying service available for
transmission of the PDU to the next network-entity on the
chosen route.
(f) A PDU is received whose destination address is unreachable or
unknown.
(g) Incorrect or invalid source routing was specified. This may
include a syntax error in the source routing field, an unknown
or unreachable address in the source routing field, or a path
which is not acceptable for other reasons.
(h) A PDU is received whose PDU lifetime has expired or whose
lifetime expires during reassembly.
(i) A PDU is received which contains an unsupported option.
6.10 Error Reporting Function
6.10.1 Overview
This function causes an attempt to return an Error Report PDU to the
source network-entity when a protocol data unit is discarded in ac-
cordance with Clause 6.9.
The Error Report PDU identifies the discarded PDU, specifies the type
of error detected, and identifies the location in the header of the
discarded PDU at which the error was detected. At least the entire
header of the Discarded PDU (and, at the discretion of the originator
of the Error Report PDU none, all, or part of the data field) is
placed in the data field of the Error Report PDU.
The originator of a Data PDU may control the generation of Error Re-
port PDUs. An Error Report flag in the original PDU is set by the
source network-entity to indicate that an Error Report PDU is to be
returned if the Initial PDU or any PDUs derived from it are discard-
ed; if the flag is not set, Error Reports are to be suppressed.
Note:
1. The suppression of Error Report PDUs is controlled by the
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originating network-entity and not by the NS User. Care
should be exercised by the originator with regard to
suppressing ER PDUs so that error reporting is not suppressed
for every PDU generated.
2. Non-receipt of an Error Report PDU does not imply correct
delivery of a PDU issued by a source network-entity.
6.10.2 Requirements
An Error Report PDU shall not be generated to report the discard of
an Error Report PDU.
An Error Report PDU shall not be generated to report the discard of a
Data PDU unless that PDU has the Error Report flag set to allow Error
Reports.
If a Data PDU is discarded, and the Error Report flag has been set to
allow Error Reports, an Error Report PDU shall be generated if the
reason for discard is one of the reasons for discard enumerated in
Clause 6.9, subject to the conditions described in Clause 6.10.4.
Note:
If a Data PDU with the E/R flag set to allow Error Reports is
discarded for any other reason, an ER PDU may be generated (as
an implementation option).
6.10.3 Processing of Error Reports
An Error Report PDU is composed from information contained in the
header of the discarded Data PDU to which the Error Report refers.
The contents of the Source Address field of the discarded Data PDU
are used as the Destination Address of the Error Report PDU. This
value, which in the context of the Data PDU was used as an NSAP Ad-
dress, is used in the context of the Error Report PDU as the
network-entity title of the network-entity that originated the Data
PDU. The network- entity title of the originator of the Error Report
PDU is conveyed in the Source Address field of the header of the Er-
ror Report PDU. The value of the Lifetime field is determined in ac-
cordance with Clause 6.4. Optional parameters are selected in accor-
dance with Clause 6.10.4.
Segmentation of Error Report PDUs is not permitted; hence, no Segmen-
tation Part is present. The total length of the ER PDU in octets is
placed in the Segment Length field of the ER PDU header. This field
is not changed during the lifetime of the ER PDU. If the originator
of the ER PDU determines that the size of the ER PDU exceeds the max-
imum service data unit size of the underlying service, the ER PDU
shall be truncated to the maximum service data unit size (see Clause
5.5.3) and forwarded with no other change. Error Report PDUs are
routed and forwarded by intermediate-system network-entities in the
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same way as Data PDUs.
Note:
The requirement that the underlying service assumed by the CLNP
must be capable of supporting a service data unit size of at least
512 octets guarantees that the entire header of the discarded Data
PDU can be conveyed in the data field of any ER PDU.
When an ER PDU is decomposed upon reaching its destination, informa-
tion that may be used to interpret and act upon the Error Report is
obtained as follows. The network-entity title recovered from the NPAI
in the Source Address field of the ER PDU header is used to identify
the network-entity which generated the Error Report. The reason for
generating the Error Report is extracted from the Options Part of the
PDU header. The entire header of the discarded Data PDU (and part or
all of the original user data) is extracted from the data field of
the ER PDU to assist in determining the nature of the error.
6.10.4 Relationship of Data PDU Options to Error Reports
The generation of an Error Report is affected by options that are
present in the corresponding Data PDU. The presence of options in the
original Data PDU that are not supported by the system which has dis-
carded that PDU may cause the suppression of an Error Report even if
the original Data PDU indicated that an Error Report should be gen-
erated in the event of a discard.
The processing of an Error Report is also affected by options which
are present in the corresponding Data PDU. In particular, options
selected for the original Data PDU affect which options are included
in the corresponding Error Report PDU. The selection of options for
an Error Report PDU is governed by the following requirements:
(a) If the Priority Option or the QoS Maintenance Option is selected
in the original Data PDU, and the system generating the Error
Report PDU supports the option, then the Error Report PDU shall
specify the option.
(b) If the Security Option is selected in the Data PDU, and the system
generating the Error Report supports this option, then the Error
Report PDU shall specify the option using the value that was
specified in the original Data PDU. If the system does not support
the Security Option, an Error Report must not be generated for
a Data PDU that selects the Security Option.
(c) If the Complete Source Route Option is selected in the original
Data PDU, and the system generating the Error Report PDU supports
this option, then the error Report shall specify the Complete Source
Route option. The Source Route parameter value is obtained by
extracting from the original Data PDU that portion of the complete
source route that has already been traversed, and reversing the
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order of network-entity titles which comprise the list.
If the system does not support the Complete Source Route Option,
an Error Report must not be generated for a Data PDU that selects
the Complete Source Route option.
(d) The Padding, Partial Source Routing, and Record Route Options,
if supported, may be specified in the Error Report PDU.
Note:
The values of the optional parameters in (d) above may be
derived as a local matter, or they may be based upon the
corresponding values in the original Data PDU.
6.11 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 veri-
fied at each point at which the PDU header is processed. If the
checksum calculation fails, the PDU must be discarded. If PDU header
fields are modified (for example, due to operation of the lifetime
function), then the checksum is modified so that the checksum remains
valid.
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 formulae to be sa-
tisfied:
(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.
Note:
1. To ensure that inadvertent modification of a header while a
PDU is being processed by an intermediate system (for
example, due to a memory fault) may still be detected by the
PDU Header Error function, an intermediate system network-
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entity must not recompute the checksum for the entire header,
even if fields are modified.
2. Annex C contains descriptions of algorithms which may be
used to calculate the correct value of the checksum field
when the PDU is created, and to update the value of the
checksum field when the header is modified.
6.12 Padding Function
The padding function is provided to allow space to be reserved in the
PDU header which is not used to support any other function. Octet
alignment must be maintained.
Note:
An example of the use of this function is to cause the data field
of a PDU to begin on a convenient boundary for the originating
network-entity, such as a computer word boundary.
6.13 Security
The provision of protection services (e.g., data origin authentica-
tion, data confidentiality, and data integrity of a single
connectionless-mode NSDU) is performed by the Security Function.
The Security Function is related to the Protection from Unauthorized
Access Quality of Service parameter described in ISO 8348/AD1, Adden-
dum to the Network Service Definition Covering Connectionless-mode
Transmission. The function is realized through selection of the secu-
rity parameter in the options part of the PDU header.
This Standard does not specify the way in which protection services
are to be provided; it only provides for the encoding of security in-
formation in the PDU header. To facilitate interoperation between
end-systems and network relay-systems by avoiding different interpre-
tations of the same encoding, a means to distinguish user-defined
security encodings from standardized security encodings is described
in Clause 7.5.3.
Note:
As an implementation consideration, data origin authentication
may be provided through the use of a cryptographically generated
or enciphered checksum (unique from the PDU Header Error Detection
mechanism); data confidentiality and data integrity may be
provided via route control mechanisms.
6.14 Source Routing Function
The Source Routing function allows the originator to specify the path
a generated PDU must take. Source routing may only be selected by the
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originator of a PDU. Source Routing is accomplished using a list of
network-entity titles held in a parameter within the options part of
the PDU header. The length of this parameter is determined by the
originating network-entity, and does not change as the PDU traverses
the network.
The Source Route parameter includes information used by the originat-
ing end-system when determining the initial route of the PDU. Only
the titles of intermediate system network-entities are included in
the list; the network-entity title of the destination of the PDU is
not included in the list.
Associated with the list of network-entity titles is an indicator
which identifies the next entry in the list to be used; this indica-
tor is advanced by the receiver of the PDU when the next title in the
list matches its own. The indicator is updated as the PDU is forward-
ed so as to identify the appropriate entry at each stage of relaying.
Two forms of the Source Routing function are provided. The first
form, referred to as Complete Source Routing, requires that the
specified path must be taken; that is, only those systems identified
in the list may be visited by the PDU while en route to the destina-
tion, and each system must be visited in the order specified. If the
specified path cannot be taken, the PDU must be discarded. Clause
6.10 describes the circumstances in which an attempt shall be made to
inform the originator of the discard using the Error Reporting func-
tion.
The second form is referred to as Partial Source Routing. Again, each
system identified in the list must be visited in the order specified
while en route to the destination. However, with this form of source
routing the PDU may take any path necessary to arrive at the next in-
termediate system in the list, which may include visiting intermedi-
ate systems that are not identified in the list. The PDU will not be
discarded (for source routing related reasons) unless one of the sys-
tems specified cannot be reached by any available route.
6.15 Record Route Function
The Record Route function records the path(s) taken by a PDU as it
traverses a series of intermediate systems. A recorded route consists
of a list of network-entity titles held in a parameter within the op-
tions part of the PDU header. The length of this parameter is deter-
mined by the originating network-entity, and does not change as the
PDU traverses the network.
The list is constructed as the PDU is forwarded along a path towards
its destination. Only the titles of intermediate system network-
entities are included in the recorded route. The network-entity title
of the originator of the PDU is not recorded in the list.
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When an intermediate system network-entity processes a PDU containing
the Record Route parameter, the system adds its own networkentity ti-
tle at the end of the list of recorded network-entity titles. An in-
dicator is maintained to identify the next available octet to be used
for recording of route. This indicator is updated as entries are ad-
ded to the list as follows. The length of the entry to be added to
the list is added to the value of the next available octet indicator,
and this sum is compared with the length of the Record Route parame-
ter. If the addition of the entry to the list would exceed the size
of the parameter, the next available octet indicator is set to indi-
cate that route recording has been terminated. The network-entity ti-
tle is not added to the list. The PDU may still be forwarded to its
final destination, without further addition of network-entity titles.
If the addition of the entry would not exceed the size of the Record
Route parameter, the next available octet indicator is updated with
the new value, and the network-entity title is added to the head of
the list after the other entries have been moved.
Two forms of the Record Route function are provided. The first form
is referred to as Complete Route Recording. It requires that the
list of network-entity titles be a complete and accurate record of
all intermediate systems visited by a PDU (including Derived PDUs),
except when a shortage of space in the record route option field
causes termination of recording of route, as described above. When
Complete Route Recording is selected, PDU reassembly at intermediate
systems is performed only when the Derived PDUs that are reassembled
all took the same route; otherwise, the PDU is discarded, and if
selected, an Error Report is generated (see Clause 6.10).
The second form is referred to as Partial Route Recording. It also
requires a record of intermediate systems visited by a PDU. When Par-
tial Route Recording is selected, PDU reassembly at intermediate sys-
tems is always permitted. When reassembly is performed at an inter-
mediate system, the route recorded in any of the Derived PDUs may be
placed in the PDU resulting from the reassembly.
Note:
The Record Route function is intended to be used in the diagnosis
of subnetwork problems and/or to provide a return path that could
be used as a source route in a subsequent PDU.
6.16 Quality of Service Maintenance Function
The Quality of Service Maintenance function provides information to
network-entities in intermediate systems which may be used to make
routing decisions where such decisions affect the overall QoS provid-
ed to NS users. This information is conveyed to intermediate system
network- entities in a parameter in the options part of the PDU
header.
ISO 8473 [Page 30]
RFC 994 December 1986
In those instances where the QoS requested cannot be maintained, in-
termediate system network-entities shall attempt to deliver the PDU
at a QoS different from the QoS requested. Intermediate system
network-entities do not necessarily provide a notification of failure
to meet the requested Quality of Service.
6.17 Priority Function
The Priority function allows a PDU with a numerically higher priority
value to be processed preferentially with respect to other PDUs with
numerically lower priority values. The function is realized through
selection of a parameter in the options part of the PDU header.
The lowest priority value is zero; a source network-entity that does
not support the Priority function must set the Priority value to
zero. The Priority function provides a means whereby the resources
of end and intermediate system network-entities, such as outgoing
transmission queues and buffers, can be used preferentially to pro-
cess higher-priority PDUs ahead of lower-priority PDUs. The specific
action taken by an individual network-entity to support the Priority
function is a local matter.
6.18 Congestion Notification Function
To allow NS Users to take appropriate action when congestion is ex-
perienced within the NS provider, intermediate systems may inform the
destination network-entity of congestion through the use of a flag in
the QoS Maintenance parameter in the options part of the PDU header.
The value of this flag is initially set to zero (0) by the originator
of the PDU and may be set to one (1) by any intermediate system which
processes the PDU to indicate that it is experiencing congestion. The
criteria for determining when this action is to be taken are a local
matter.
Note:
Congestion typically corresponds to inavailability of buffer space
to maintain output queues. An appropriate policy for indicating
congestion may be based upon the depth of the output queue selected
for a PDU (according to its destination address or other routing
information). When the depth of a particular output queue exceeds
a certain proportion of the depth of that queue, an intermediate
system will start to discard PDUs. The intermediate system will set
the Congestion Experienced flag in the next PDU to be forwarded
and may continue to do so until the condition is alleviated.
6.19 Classification of Functions
Implementations are not required to support all of the functions
described in Clauses 6.1 through 6.18. Functions are divided into
three categories:
ISO 8473 [Page 31]
RFC 994 December 1986
Type 1: These functions must be supported.
Type 2: These functions may or may not be supported.
If an implementation does not support a Type 2 function, and the
function is selected in a PDU, then that PDU must be discarded,
and an Error Report PDU must be generated and forwarded to the
originating network-entity, providing that the Error Report flag is
set and the conditions of Clause 6.10.4 are satisfied.
Type 3: These functions may or may not be supported.
If an implementation does not support a Type 3 function, and the
function is selected in a PDU, then the function is not performed,
and the PDU is processed exactly as though the function had not
been selected. The protocol data unit shall not be discarded for
this reason.
Table 4 shows how the functions are divided into these three categories:
_____________________________________________________________________________
| | FULL | NON | INACTIVE |
| FUNCTION | PROTOCOL | SEGMENTING | SUBSET |
| | | SUBSET | |
|________________________________|_____________|_________________|____________|
|PDU Composition | 1 | 1 | 1 |
|PDU Composition | 1 | 1 | 1 |
|Header Format Analysis | 1 | 1 | 1 |
|PDU Lifetime Control | 1 | 1 | N/A |
|Route PDU | 1 | 1 | N/A |
|Forward PDU | 1 | 1 | N/A |
|Segment PDU | 1 | N/A | N/A |
|Reassemble PDU | 1 | N/A | N/A |
|Discard PDU | 1 | 1 | N/A |
|Error Reporting (Note 1) | 1 | 1 | N/A |
|Header Error Detection (Note 1) | 1 | 1 | N/A |
|Security | 1 | 2 | N/A |
|Complete Source Routing | 1 | 2 | N/A |
|Complete Route Recording | 2 | 2 | N/A |
|Partial Source Routing | 3 | 3 | N/A |
|Partial Route Recording | 3 | 3 | N/A |
|Priority | 3 | 3 | N/A |
|QoS Maintenance | 3 | 3 | N/A |
|Congestion Notification | 3 | 3 | N/A |
|Padding | 3 | 3 | N/A |
|________________________________|_____________|_________________|____________|
Table 4: Categorization of Protocol Functions
ISO 8473 [Page 32]
RFC 994 December 1986
Note:
1. While the Error Reporting and Header Error Detection functions
must be provided, they are provided only when selected
by the sending Network Service user.
2. The rationale for the inclusion of type 3 functions is that in
the case of some functions it is more important to forward
the PDUs between intermediate systems or deliver them to
an end-system than it is to support the functions. Type 3
functions should be used in those cases where they are of an
advisory nature; they cannot cause a PDU to be discarded
when they are not supported.
7 Structure and Encoding of PDUs
7.1 Structure
All Protocol Data Units shall contain an integral number of octets.
The octets in a PDU are numbered starting from one (1) and increasing
in the order in which they are submitted to the underlying service.
The bits in an octet are numbered from one (1) to eight (8), where
bit one (1) is the low-order (least significant) bit.
When consecutive octets are used to represent a binary number, the
lower octet number has the most significant value.
Any implementation supporting this protocol is required to state in
its specification the way in which octets are transferred, using the
terms "most significant bit" and "least significant bit". The PDUs of
this protocol are defined using the terms "most significant bit" and
"least significant bit".
Note:
When the encoding of a PDU is represented using a diagram in this
Clause 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 address part;
3. the segmentation part, if present;
4. the Options part, if present;
and the data field, if present. This structure is illustrated in Figure 2:
ISO 8473 [Page 33]
RFC 994 December 1986
7.2 Fixed Part
7.2.1 General
The fixed part of the PDU header contains frequently occurring param-
eters including the type code (DT or ER) 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:
Part Described in
___________________________________
| Fixed Part | Section 7.2
|_________________________________|
| Address Part | Section 7.3
|_________________________________|
| Segmentation Part | Section 7.4
|_________________________________|
| Options Part | Section 7.5
|_________________________________|
| Data | Section 7.6
|_________________________________|
Figure 2: PDU Structure
Octet
________________________________________
| Network Layer Protocol Identifier | 1
|______________________________________|
| Length Indicator | 2
|______________________________________|
| Version/Protocol Id Extension | 3
|______________________________________|
| Lifetime | 4
|______________________________________|
| SP vline M S vline e/R | Type | 5
|______________________________________|
| Segment Length | 6,7
|______________________________________|
| Checksum | 8,9
|______________________________________|
Figure 3: PDU Header -- Fixed Part
7.2.2 Network Layer Protocol Identifier
The value of this field is set to binary 1000 0001 to identify this
Network Layer protocol as ISO 8473, Protocol for Providing the
Connectionless- mode Network Service. The value of this field is set
ISO 8473 [Page 34]
RFC 994 December 1986
to binary 0000 0000 to identify the Inactive Network Layer protocol
subset.
7.2.3 Length Indicator
The length is indicated by a binary number, with a maximum value of
254 (1111 1110). The length indicated is the length in octets of the
header, as described in Clause 7.1. The value 255 (1111 1111) is
reserved for possible future extensions.
Note:
The rules for forwarding and segmentation guarantee that the header
length is the same for all segments (Derived PDUs) of the Initial
PDU, and is the same as the header length of the Initial PDU.
The size of a PDU header will not change due to operation of any
protocol function.
7.2.4 Version/Protocol Identifier Extension
The value of this field is binary 0000 0001, which identifies the
standard Version 1 of ISO 8473, Protocol for Providing the
Connectionless-mode Network Service.
7.2.5 PDU Lifetime
The PDU Lifetime field is encoded as a binary number representing the
remaining lifetime of the PDU, in units of 500 milliseconds.
7.2.6 Flags
7.2.6.1 Segmentation Permitted
The Segmentation Permitted flag indicates whether segmentation is
permitted. Its value is determined by the originator of the PDU and
cannot be changed by any other network-entity for the lifetime of the
Initial PDU and any Derived PDUs.
A value of one (1) indicates that segmentation is permitted. A value
of zero (0) indicates that the non-segmenting protocol subset is em-
ployed. When the value of zero is selected, the segmentation part of
the PDU header is not present, and the Segment Length field serves as
the Total Length field (see Clause 7.2.8).
7.2.6.2 More Segments
The More Segments flag indicates whether the data segment in this PDU
contains (as its last octet) the last octet of the User Data in the
NSDU. When the More Segments flag is set to one (1), segmentation
has taken place and the last octet of the NSDU is not contained in
this PDU. The More Segments flag cannot be set to one (1) if the Seg-
mentation Permitted flag is not set to one (1).
ISO 8473 [Page 35]
RFC 994 December 1986
When the More Segments flag is set to zero (0), the last octet of the
Data Part of the PDU is the last octet of the NSDU.
7.2.6.3 Error Report
When the Error Report flag is set to one, the rules in Clause 6.10
are used to determine whether to generate an Error Report PDU if it
is necessary to discard this Data PDU.
When the Error Report flag is set to zero, discard of the Data PDU
will not cause the generation of an Error Report PDU.
7.2.7 Type Code
The Type code field identifies the type of the protocol data unit.
Allowed values are given in Table 5:
__________________________________________________
| | Bits 5 4 3 2 1 |
|_________|______________________________________|
| DT PDU | 1 1 1 0 0 |
|_________|______________________________________|
| ER PDU | 0 0 0 0 1 |
|_________|______________________________________|
Table 5: Valid PDU Types
7.2.8 PDU Segment Length
The Segment Length field specifies the entire length, in octets, of
the Derived PDU, including both header and data (if present). When
the full protocol is employed and a PDU is not segmented, the value
of this field is identical to the value of the Total Length field lo-
cated in the Segmentation Part of the header.
When the non-segmenting protocol subset is employed, no segmentation
part is present in the header. In this subset, the Segment Length
field specifies the entire length of the Initial PDU, including both
header and data (if present). The Segment Length field is not changed
for the lifetime of the PDU.
7.2.9 PDU Checksum
The checksum is computed on the entire PDU header. For the Data PDU,
this includes the segmentation and options parts (if present). For
the Error Report PDU, this includes the reason for discard field as
well.
A checksum value of zero is reserved to indicate that the checksum is
to be ignored. The operation of the PDU Header Error Detection func-
tion (Clause 6.11) ensures that the value zero does not represent a
ISO 8473 [Page 36]
RFC 994 December 1986
valid checksum. A non-zero value indicates that the checksum must be
processed. If the checksum calculation fails, the PDU must be dis-
carded.
7.3 Address Part
7.3.1 General
Address parameters are distinguished by their location, immediately
following the fixed part of the PDU header. The address part is il-
lustrated Figure 4:
Octet
____________________________________________
| Destination Address Length Indicator | 10
|___________________________________________|
| | 11
: Destination Address :
| | m - 1
|___________________________________________|
| Source Address Length Indicator | m
|___________________________________________|
| | m + 1
: Source Address :
| | n - 1
|___________________________________________|
Figure 4: PDU Header -- Address Part
7.3.1.1 Destination and Source Addresses
The Destination and Source addresses used by this protocol are Net-
work Service Access Point addresses as defined in ISO 8348/AD2, Ad-
dendum to the Network Service Definition Covering Network Layer Ad-
dressing.
The Destination and Source Addresses are variable length. The Desti-
nation and Source Address fields are encoded as Network Protocol Ad-
dress Information using the Preferred Binary Encoding defined in
Clause 8.3.1 of ISO 8348/AD2.
The Destination Address Length Indicator field specifies the length
of the Destination Address in octets. The Destination Address field
follows the Destination Address Length Indicator field.
The Source Address Length Indicator field specifies the length of the
Source Address in octets. The Source Address Length Indicator field
follows the Destination Address field. The Source Address field fol-
lows the Source Address Length Indicator field.
ISO 8473 [Page 37]
RFC 994 December 1986
Each address parameter is encoded as illustrated in Table 5:
______________________________________________
| Octet | Address parameter Length Indicator |
| n | (e.g., 'm') |
|________|____________________________________|
| Octets | |
| n + 1 | Address Parameter Value |
| thru | |
| n + m | |
|________|____________________________________|
Figure 5: Address Parameters
7.4 Segmentation Part
If the Segmentation Permitted Flag in the Fixed Part of the PDU
Header (Octet 4, Bit 8) is set to one, the segmentation part of the
header, illustrated in Figure 6, must be present:
If the Segmentation Permitted flag is set to zero, the non-segmenting
protocol subset is in use.
Octet
________________________
| Data Unit Identifier | n, n + 1
|______________________|
| Segment Offset | n + 2, n + 3
|______________________|
| Total Length | n + 4, n + 5
|______________________|
Figure 6: PDU Header -- Segmentation Part
7.4.1 Data Unit Identifier
The Data Unit Identifier identifies an Initial PDU (and hence, its
Derived PDUs) so that a segmented data unit may be correctly reassem-
bled. The Data Unit Identifier size is two octets.
7.4.2 Segment Offset
For each Derived PDU, the Segment Offset field specifies the relative
position of the segment contained in the data field of the Derived
PDU with respect to the start of the data field of the Initial PDU.
The offset is measured in units of octets. The offset of the first
segment (and hence, the Initial PDU) is zero; an unsegmented (Initial
PDU) has a segment offset value of zero (0). The value of this field
shall be a multiple of eight 8).
ISO 8473 [Page 38]
RFC 994 December 1986
7.4.3 PDU Total Length
The Total Length field specifies the entire length of the Initial
PDU, including both the header and data. This field is not changed
for the lifetime of the Initial PDU (and hence, its Derived PDUs).
7.5 Options Part
7.5.1 General
The options part is used to convey optional parameters. The options
part of the PDU header is illustrated below:
Octet
___________________________________________________
| | n + 6
: Options :
| | p
|__________________________________________________|
Figure 7: PDU Header -- 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 Protocol Data
Unit with an option duplicated should be treated as a protocol error.
The rules governing the treatment of protocol errors are described in
Clause 6.10, Error Reporting Function.
The encoding of parameters contained within the options part of the
PDU header is illustrated in Table 6:
Octets
___________________________________________
| n | Parameter Code |
|____________|____________________________|
| n + 1 | Parameter Length (e.g.m) |
|____________|____________________________|
| n + 2 | |
| to | Parameter Value |
| n + m + 1 | |
|____________|____________________________|
ISO 8473 [Page 39]
RFC 994 December 1986
Table 6: Encoding of 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 seg-
mentation 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.
7.5.2 Padding
The padding parameter is used to lengthen the PDU header to a con-
venient size (See Clause 6.12).
Parameter Code: 1100 1100
Parameter Length: variable
Parameter Value: any value is allowed
7.5.3 Security
This parameter allows a unique and unambiguous security level to be
assigned to a protocol data unit.
Parameter Code: 1100 0101
Parameter Length: variable
Parameter Value: The high order two bits of the first octet
specify the Security Format Code, where:
Security Type of Security Field:
Format Code
ISO 8473 [Page 40]
RFC 994 December 1986
00 Reserved
01 Source Address Specific
10 Destination Address Specific
11 Globally Unique
The rest of the first octet is reserved and must be zero. The
remainder of the Parameter Value field specifies the security
level as described in the following Clauses.
7.5.3.1 Source Address Specific
The Security Format Code value of binary "01" indicates that the
remaining octets of the parameter value field specify a security lev-
el which is unique and unambiguous in the context of the security
classification system employed by the authority responsible for as-
signing the source NSAP Address.
7.5.3.2 Destination Address Specific
The Security Format Code value of binary "10" indicates that the
remaining octets of the parameter value field specify a security lev-
el which is unique and unambiguous in the context of the security
classification system employed by the authority responsible for as-
signing the destination NSAP Address.
7.5.3.3 Globally Unique Security
The Security Format Code value of binary "11" indicates that the
remaining octets of the parameter value field specify a globally
unique and unambiguous security level. This security classification
system is not specified in this Standard.
7.5.4 Source Routing
The source routing parameter specifies, either completely or partial-
ly, the route to be taken from Source Network Address to Destination
Network Address.
Parameter Code: 1100 0101
Parameter Length: variable
Parameter Value: 2 octets of control information succeeded by a
concatenation of ordered network-entity title entries (ordered
from source to destination)
The first octet of the parameter value is the type code, and has the
following significance:
0000 0000 partial source routing
0000 0001 complete source routing
<all other values reserved>
ISO 8473 [Page 41]
RFC 994 December 1986
The second octet indicates the octet offset of the next network-
entity title entry to be processed in the list. It is relative to
the start of the parameter, such that a value of three (3) indicates
that the next network-entity title entry begins immediately after
this control octet. Successive octets are indicated by corresponding-
ly larger values of this indicator.
The third octet begins the network-entity title list. The list con-
sists of variable length network-entity title entries. The first oc-
tet of entry identifies the length of the network-entity title which
comprises the re- mainder of the entry.
7.5.5 Recording of Route
The recording of route parameter identifies the route of intermediate
systems traversed by the PDU.
Parameter Code: 1100 1011
Parameter Length: variable
Parameter Value: 2 octets of control information succeeded by a
con catenation of ordered network-entity title entries (ordered
from destination to source)
The first octet of the parameter value is the type code, and has the
following significance:
0000 0000 Partial Recording of Route in progress
0000 0001 Complete Recording of Route in progress
<all other values reserved>
The second octet identifies the first octet not currently used for a
recorded network-entity title, and therefore also the end of the
list. It is encoded relative to the start of the parameter value,
such that a value of three (3) indicates that no network-entity ti-
tles have yet been recorded. A value of all ones is used to indicate
that route recording has been terminated.
The third octet begins the network-entity title list. The list con-
sists of variable length network-entity title entries. The first oc-
tet of each entry specifies the length of the network-entity title
comprising the remainder of the entry. Network-entity title entries
are always added to the beginning of the list; that is, the most re-
cently added entry will begin in the third octet of the parameter
value.
Note:
The length of the Record Route parameter is determined by the
originator of the PDU and is not changed during the lifetime of
the PDU; hence, the operation of the Record Route function does
ISO 8473 [Page 42]
RFC 994 December 1986
not affect the length of the header.
7.5.6 Quality of Service Maintenance
The Quality of Service parameter conveys information about the quali-
ty of service requested by the originating Network Service user.
Network-entities in intermediate systems may (but are not required
to) make use of this information as an aid in selecting a route when
more than one route satisfying other routing criteria is available
and the available routes are known to differ with respect to Quality
of Service see Clause 6.16).
Parameter Code: 1100 0011
Parameter Length: variable
Parameter Value: The high order two bits of the first octet
specify the QoS Format Code, where:
QoS Format Type of QoS
Code Field
00 Reserved
01 Source Address Specific
10 Destination Address Specific
11 Globally Unique
The rest of the first octet is reserved and must be zero. The
remainder of the Parameter Value field specifies the QoS as described
in the following Clauses.
7.5.6.1 Source Address Specific
The QoS Format Code value of binary "01" indicates that the remaining
octets of the parameter value field specify a QoS which is unique and
unambiguous in the context of the QoS Maintenance system employed by
the authority responsible for assigning the source NSAP Address.
7.5.6.2 Destination Address Specific
The QoS Format Code value of binary "10" indicates that the remaining
octets of the parameter value field specify a QoS which is unique and
unambiguous in the context of the QoS Maintenance system employed by
the authority responsible for assigning the destination NSAP Address.
7.5.6.3 Globally Unique QoS
The QoS Format Code value of binary "11" indicates that the remainder
of the parameter value field specifies a globally unique QoS Mainte-
nance field. When the globally unique QoS Maintenance function is em-
ployed, the parameter value field must have a total length of one oc-
tet, which is assigned the following values:
Bits 8 and 7: QoS Format Code of binary "11"
ISO 8473 [Page 43]
RFC 994 December 1986
Bit 6: Reserved
Bit 5: sequencing vs. transit delay
Bit 4: congestion experienced
Bit 3: transit delay vs. cost
Bit 2: residual error probability vs. transit delay
Bit 1: residual error probability vs. cost
Bit 5 is set to one to indicate that, where possible, routing deci-
sions should favor sending all PDUs to the specified destination NSAP
address over a single path (in order to maintain sequence) over
minimizing transit delay. A value of zero (0) indicates that, where
possible, routing decisions should favor low transit delay over se-
quence preservation.
Bit 4 is set to zero by the network-entity which originates the pro-
tocol data unit. It is set to one by an intermiediate system to indi-
cate that this PDU has visited a congested intermediate system, and
appropriate action should be taken by the destination network-entity.
Once the congestion experienced bit is set by an intermediate system,
it may not be reset by any intermediate system traversed by the PDU
further along the path towards the destination.
Bit 3 is set to one to indicate that, where possible, routing deci-
sions should favor low transit delay over low cost. A value of 0 in-
dicates that routing decisions should favor low cost over low transit
delay.
Bit 2 set to one to indicate that, where possible, routing decisions
should favor low residual error probability over low transit delay.
A value of zero indicates that routing decisions should favor low
transit delay over low residual error probability.
Bit 1 is set to one to indicate that, where possible, routing deci-
sions should favor low residual error probability over low cost. A
value of 0 indicates that routing decisions should favor low cost
over low residual error probability.
7.5.7 Priority
The value of the Priority parameter indicates the relative priority
of the protocol data unit. Intermediate systems that support this
option shall make use of this information in routing and in ordering
PDUs for transmission.
Parameter Code: 1100 1101
Parameter Length: one octet
Parameter Value: 0000 0000 - Normal (Default) through
0000 1110 - Highest
<all other values reserved>
ISO 8473 [Page 44]
RFC 994 December 1986
The values 0000 0001 through 0000 1110 are to be used for higher
priority protocol data units. If an intermediate system does not sup-
port this option, all PDUs shall be treated as if the field had the
value 0000 0000.
7.6 Data Part
The Data part of the PDU is structured as an ordered multiple of oc-
tets, which is identical to the same ordered multiple of octets
specified for the NS-Userdata parameter of the N-UNITDATA Request and
Indication primitives. The data field is illustrated in Figure 8:
Octet
___________________________________________________
| | p + 1
: Data :
| | z
|__________________________________________________|
Figure 8: PDU Header -- Data Field
ISO 8473 [Page 45]
RFC 994 December 1986
7.7 Data (DT) PDU
7.7.1 Structure
The DT PDU has the following format:
__________________________________________
| Network Layer Protocol Identifier | 1
|________________________________________|
| Length Indicator | 2
|________________________________________|
| Version/Protocol Id Extension | 3
|________________________________________|
| Lifetime | 4
|________________________________________|
| S P vline M S vline e/R | Type | 5
|____________________________|___________|
| Segment Length | 6,7
|________________________________________|
| Checksum | 8,9
|________________________________________|
| Destination Address Length Indicator | 10
|________________________________________|
| | 11
: Destination Address :
|________________________________________| m - 1
| Source Address Length Indicator | m
|________________________________________|
| | m + 1
: Source Address :
| | n - 1
|________________________________________|
| Data Unit Identifier | n, n + 1
|________________________________________|
| Segment Offset | n + 2, n + 3
|________________________________________|
| Total Length | n + 4, n + 5
|________________________________________|
| | n + 6
| Options |
| | p
|________________________________________|
| | p + 1
| Data |
| | z
|________________________________________|
Figure 9: DT PDU
ISO 8473 [Page 46]
RFC 994 December 1986
7.7.1.1 Fixed Part
1) Network Layer Protocol Identifier See Clause 7.2.2
2) Length Indicator See Clause 7.2.3
3) Version/Protocol Id Extension See Clause 7.2.4
4) Lifetime See Clause 7.2.5
5) SP, MS, E/R See Clause 7.2.6
6) Type Code See Clause 7.2.7
7) Segment Length See Clause 7.2.8
8) Checksum See Clause 7.2.9
7.7.1.2 Addresses
See Clause 7.3.
7.7.1.3 Segmentation
See Clause 7.4.
7.7.1.4 Options
See Clause 7.5.
7.7.1.5 Data
See Clause 7.7.
7.8 Inactive Network Layer Protocol
Octet
____________________________________
|Network Layer Protocol Identifier | 1
|__________________________________|
| | 2
| Data |
| | 2 - n
|__________________________________|
Figure 10: Inactive Network Layer Protocol
7.8.1 Network Layer Protocol Id
The value of the Network Layer Protocol Identifier field is binary
zero (0000 0000).
7.8.2 Data Field
The length of the NS-Userdata parameter is constrained to be less
than or equal to the value of the length of the SN-Userdata parameter
minus one (see Clause 7.7).
ISO 8473 [Page 47]
RFC 994 December 1986
7.9 Error Report PDU (ER)
7.9.1 Structure
The ER PDU has the following format:
Octet
______________________________________________
| Network Layer Protocol Identifier | 1
|____________________________________________|
| Length Indicator | 2
|____________________________________________|
| Version/Protocol Id Extension | 3
|____________________________________________|
| Lifetime | 4
|____________________________________________|
| SP= 0 vline MS= 0 vline Reserved | Type | 5
|_____________________________________|______|
| Segment Length | 6,7
|____________________________________________|
| Checksum | 8,9
|____________________________________________|
| Destination Address Length Indicator | 10
|____________________________________________|
| | 11
: Destination Address :
| | m - 1
|____________________________________________|
| Source Address Length Indicator | m
|____________________________________________|
| | m + 1
: Source Address :
| | n - 1
|____________________________________________|
| | n
| Options |
| | p - 1
|____________________________________________|
| | p
| Reason for Discard |
| | q - 1
|____________________________________________|
| | q
| Error Report Data Field |
| | z
|____________________________________________|
Figure 11: Error Report PDU
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RFC 994 December 1986
7.9.1.1 Fixed Part
The fixed part of the Error Report Protocol Data Unit is composed in
the same way as a new (Initial) Data PDU. References are provided to
previous Clauses describing the encoding of the fields comprising the
fixed part:
1) Network Layer Protocol Identifier See Clause 7.2.2
2) Length Indicator See Clause 7.2.3
3) Version/Protocol Id Extension See Clause 7.2.4
4) Lifetime See Clause 7.2.5
5) SP, MS, E/R Always set to zero,
(See Clause 6.10)
6) Type Code See Clause 7.2.7
7) Segment Length See Clause 7.2.8
8) Checksum See Clause 7.2.9
7.9.1.2 Addresses
See Clause 7.3.
The Destination Address specifies the network-entity title of the origi-
nator of the discarded PDU. The Source Address specifies the title of the
intermediate-system or end-system network-entity initiating the Error
Report PDU.
7.9.1.3 Options
See Clause 7.5.
ISO 8473 [Page 49]
RFC 994 December 1986
7.9.1.4 Reason for Discard
This parameter is valid only for the Error Report PDU.
Parameter Code: 1100 0001
Parameter Length: two octets
Parameter Value: type of error encoded in binary. Values are listed
in Table 7:
_______________________________________________________________________________
| Parameter Value | Class of | Meaning |
| Octet 1 Octet 2| Error | |
|__________________|_____________|_____________________________________________|
| 0000 0000 | | Reason not specified |
| 0001 | | Protocol Procedure Error |
| 0010 | | Incorrect Checksum |
| 0011 | General | PDU Discarded due to Congestion |
| 0100 | | Header Syntax Error (cannot be parsed) |
| 0101 | | Segmentation needed but not permitted |
| 0110 | | Incomplete PDU Received |
| 0111 | | Duplicate Option |
|__________________|_____________|_____________________________________________|
| 1000 0000 | Address | Destination Address Unreachable |
| 0001 | | Destination Address Unknown |
|__________________|_____________|_____________________________________________|
| 1001 0000 | | Unspecified Source Routing Error |
| 0001 | Source | Syntax Error in Source Routing Field |
| 0010 | Routing | Unknown Address in Source Routing Field |
| 0011 | | Path not Acceptable |
|__________________|_____________|_____________________________________________|
| 1010 0000 | Lifetime | Lifetime Expired while Data Unit in Transit |
| 0001 | | Lifetime Expired during Reassembly |
|__________________|_____________|_____________________________________________|
| 1011 0000 | | Unsupported Option not Specified |
| 0001 | PDU | Unsupported Protocol Version |
| 0010 | Discarded | Unsupported Security Option |
| 0011 | | Unsupported Source Routing Option |
| 0100 | | Unsupported Recording of Route Option |
|__________________|_____________|_____________________________________________|
| 1100 0000 | Reassembly | Reassembly interference |
|__________________|_____________|_____________________________________________|
Table 7: Reasons for Discard
The first octet of the parameter value contains an error type code.
If the error in the discarded Data PDU can be localized to a particu-
lar field, the number of the first octet of that field is stored in
the second octet of the reason for discard parameter field. If the
error cannot be localized to a particular field, or if the error is a
checksum error, then the value zero is stored in the second octet of
the reason for discard parameter field.
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RFC 994 December 1986
7.9.1.5 Error Report Data Field
This field contains the entire header of the discarded Data PDU, and
may contain some or all of the data field of the discarded PDU.
8 Conformance
For conformance to this International Standard, the ability to ori-
ginate, manipulate, and receive PDUs in accordance with the full pro-
tocol (as opposed to the non-segmenting or Inactive Network Layer
Protocol subsets) is required.
Additionally, conformance to the Standard requires provision of the
protocol functions described in Clause 6. Provision of the optional
functions described in Clause 6.18 and enumerated in Table 9-1 must
meet the requirements described therein. Exceptions to this require-
ment are described in Clause 8.1 below.
Additionally, conformance to the Standard requires adherence to the
structure and encoding of PDUs of Clause 7.
If and only if the above requirements are met is there conformance to
this International Standard.
8.1 Provision of Functions for Conformance
The following table categorizes the functions in Clause 6 with
respect to the type of system providing the function:
Note:
1. The support of the PDU Composition and Forward PDU functions
is necessary for the generation of Error Report PDUs.
2. The Segment PDU function is in general mandatory for an
intermediate system. However, a system which is to be
connected only to subnetworks all offering the same maximum
SDU size (such as identical Local Area Networks) will not
need to perform this function and therefore does not need to
implement it.
If this function is not implemented, this shall be stated
as part of the specification of the implementation.
3. The correct treatment of the padding function requires no
processing. A conforming implementation shall support the
function, to the extent of ignoring this parameter wherever
it may appear.
4. This function may or may not be supported. If an
implementation does not support this function, and the
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RFC 994 December 1986
function is selected in a PDU, then the PDU shall be discarded,
and an ER PDU shall be generated and forwarded to the
originating network-entity if the Error Report flag is set
and the conditions of Clause 6.10.4 are satisfied.
5. This function may or may not be supported. If an implementation
does not support this function, and the function is selected
in a PDU, then the function is not performed and the PDU is
processed exactly as though the function had not been
selected. The PDU shall not be discarded for this reason.
___________________________________________________________________
| Function | Send | Forward | Receive |
|____________________________|____________|___________|___________|
| PDU Composition | M | (Note 1) | (Note 1) |
| PDU Decomposition | M | - | M |
| Header Format Analysis | - | M | M |
| PDU Lifetime Control | | M | I |
| Route PDU | - | M | - |
| Forward PDU | M | M | (Note 1) |
| Segment PDU | M | (Note 2) | - |
| Reassemble PDU | - | I | M |
| Discard PDU | - | M | M |
| Error Reporting | M | M | M |
| Header Error Detection | (Note 3) | M | M |
| | | | |
| Security | - | (Note 3) | (Note 4) |
| Complete Source Routing | - | (Note 4) | - |
| Complete Route Recording | - | (Note 4) | - |
| Partial Source Routing | - | (Note 5) | - |
| Partial Route Recording | - | (Note 5) | - |
| Priority | - | (Note 5) | - |
| QoS Maintenance | - | (Note 5) | - |
| Congestion Notification | - | (Note 5) | - |
| Padding | - | (Note 5) | (Note 3) |
|____________________________|____________|___________|___________|
Table 8: Categorization of Functions
Key:
M: Mandatory Function; this function must be implemented.
-: Not applicable.
I: Implementation option, as described in the text.
NOTE: See notes above
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