Network Working Group ISO
Request for Comments: 926 December 1984
Protocol for Providing the Connectionless-Mode Network Services
(Informally - ISO IP)
ISO DIS 8473
Status of this Memo:
This document is distributed as an RFC for information only. It does
not specify a standard for the ARPA-Internet. Distribution of this
memo is unlimited.
Note:
This document has been prepared by retyping the text of ISO DIS 8473 of
May 1984, which is currently undergoing voting within ISO as a Draft
International Standard (DIS). Although this RFC has been reviewed
after typing, and is believed to be substantially correct, it is
possible that typographic errors not present in the ISO document have
been overlooked.
Alex McKenzie
BBNRFC 926 December 1984
RFC 926 December 1984
TABLE OF CONTENTS
1 SCOPE AND FIELD OF APPLICATION........................ 2
2 REFERENCES............................................ 3
3 DEFINITIONS........................................... 4
3.1 Reference Model Definitions......................... 4
3.2 Service Conventions Definitions..................... 4
3.3 Network Layer Architecture Definitions.............. 4
3.4 Network Layer Addressing Definitions................ 5
3.5 Additional Definitions.............................. 5
4 SYMBOLS AND ABBREVIATIONS............................. 7
4.1 Data Units.......................................... 7
4.2 Protocol Data Units................................. 7
4.3 Protocol Data Unit Fields........................... 7
4.4 Parameters.......................................... 8
4.5 Miscellaneous....................................... 8
5 OVERVIEW OF THE PROTOCOL.............................. 9
5.1 Internal Organization of the Network Layer.......... 9
5.2 Subsets of the Protocol............................. 9
5.3 Addressing......................................... 10
5.4 Service Provided by the Network Layer.............. 10
5.5 Service Assumed from the Subnetwork Service
Provider.............................................. 11
5.5.1 Subnetwork Addresses............................. 12
5.5.2 Subnetwork Quality of Service.................... 12
5.5.3 Subnetwork User Data............................. 13
5.5.4 Subnetwork Dependent Convergence Functions....... 13
5.6 Service Assumed from Local Evironment.............. 14
6 PROTOCOL FUNCTIONS................................... 16
6.1 PDU Composition Function........................... 16
6.2 PDU Decomposition Function......................... 17
6.3 Header Format Analysis Function.................... 17
6.4 PDU Lifetime Control Function...................... 18
6.5 Route PDU Function................................. 18
6.6 Forward PDU Function............................... 19
6.7 Segmentation Function.............................. 19
6.8 Reassembly Function................................ 20
6.9 Discard PDU Function............................... 21
ISO DIS 8473 (May 1984) [Page i]RFC 926 December 1984
6.10 Error Reporting Function.......................... 22
6.10.1 Overview........................................ 22
6.10.2 Requirements.................................... 23
6.10.3 Processing of Error Reports..................... 24
6.11 PDU Header Error Detection........................ 25
6.12 Padding Function.................................. 26
6.13 Security.......................................... 26
6.14 Source Routing Function........................... 27
6.15 Record Route Function............................. 28
6.16 Quality of Service Maintenance Function........... 29
6.17 Classification of Functions....................... 29
7 STRUCTURE AND ENCODING OF PDUS....................... 32
7.1 Structure.......................................... 32
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............................................ 36
7.2.6.1 Segmentation Permitted and More Segments Flags. 36
7.2.6.2 Error Report Flag.............................. 37
7.2.7 Type Code........................................ 37
7.2.8 PDU Segment Length............................... 37
7.2.9 PDUChecksum...................................... 38
7.3 Address Part....................................... 38
7.3.1 General.......................................... 38
7.3.1.1 Destination and Source Address Information... 39
7.4 Segmentation Part.................................. 40
7.4.1 Data Unit Identifier............................. 41
7.4.2 Segment Offset................................... 41
7.4.3 PDU Total Length................................. 41
7.5 Options Part....................................... 41
7.5.1 General.......................................... 41
7.5.2 Padding.......................................... 43
7.5.3 Security......................................... 43
7.5.4 Source Routing................................... 44
7.5.5 Recording of Route............................... 45
7.5.6 Quality of Service Maintenance................... 46
7.6 Priority........................................... 47
ISO DIS 8473 (May 1984) [Page ii]
RFC 926 December 1984
7.7 Data Part.......................................... 47
7.8 Data (DT) PDU...................................... 49
7.8.1 Structure........................................ 49
7.8.1.1 Fixed Part..................................... 50
7.8.1.2 Addresses...................................... 50
7.8.1.3 Segmentation................................... 50
7.8.1.4 Options........................................ 50
7.8.1.5 Data........................................... 50
7.9 Inactive Network Layer Protocol.................... 51
7.9.1 Network Layer Protocol Id........................ 51
7.9.2 Data Field....................................... 51
7.10 Error Report PDU (ER)............................. 52
7.10.1 Structure....................................... 52
7.10.1.1 Fixed Part.................................... 53
7.10.1.2 Addresses..................................... 53
7.10.1.3 Segmentation.................................. 53
7.10.1.4 Options....................................... 54
7.10.1.5 Reason for Discard............................ 54
7.10.1.6 Error Report Data Field....................... 55
8 FORMAL DESCRIPTION................................... 56
8.1 Values of the State Variable....................... 57
8.2 Atomic Events...................................... 57
8.2.1 N.UNITDATA_request and N.UNITDATA_indication..... 57
8.2.2 SN.UNITDATA_request and SN.UNITDATA_indication... 58
8.2.3 TIMER Atomic Events.............................. 59
8.3 Operation of the Finite State Automation........... 59
8.3.1 Type and Constant Definitions.................... 61
8.3.2 Interface Definitions............................ 65
8.3.3 Formal Machine Definition........................ 67
9 CONFORMANCE.......................................... 84
9.1 Provision of Functions for Conformance............. 84
ISO DIS 8473 (May 1984) [Page iii]
RFC 926 December 1984
ISO DIS 8473 (May 1984) [Page iv]
RFC 926 December 1984
INTRODUCTION
This Protocol is one of a set of International Standards produced to
facilitate the interconnection of open systems. The set of standards
covers the services and protocols required to achieve such
interconnection.
This Protocol Standard is positioned with respect to other related
standards by the layers defined in the Reference Model for Open Systems
Interconnection (ISO 7498). In particular, it is a protocol of the
Network Layer. The Protocol herein described is a Subnetwork
Independent Convergence Protocol combined with relay and routing
functions as described in the Internal Organization of the Network
Layer (ISO iiii). This Protocol provides the connectionless-mode
Network Service as defined in ISO 8348/DAD1, Addendum to the Network
Service Definition Covering Connectionless-mode Transmission, between
Network Service users and/or Network Layer relay systems.
The interrelationship of these standards is illustrated in Figure 0-1
below:
______________OSI Network Service Definition______________
| ^
|
| |
Protocol Reference to aims __________|
|
Specification | Reference to assumptions ___
|
| |
|
| |
|
| v
______________Subnetwork Service Definition(s) ___________
Figure 0-1. Interrelationship of Standards
ISO DIS 8473 (May 1984) [Page 1]RFC 926 December 1984
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 ISO
8348/DAD1, Addendum to the Network Service Definition Covering
Connectionless-mode Transmission. The protocol herein described relies
upon the provision of a connectionless-mode subnetwork service.
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 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 a subnetwork service
provider through the exchange of subnetwork service primitives.
ISO DIS 8473 (May 1984) [Page 2]RFC 926 December 1984
2 REFERENCES
ISO 7498 Information Processing Systems - Open Systems
Interconnection - Basic Reference Model
DP 8524 Information Processing Systems - Open Systems
Interconnection - Addendum to ISO 7498 Covering
Connectionless-Mode Transmission
DIS 8348 Information Processing Systems - Data Communications -
Network Service Definition
ISO 8348/DAD1 Information Processing Systems - Data Communications -
Addendum to the Network Service Definition Covering
Connectionless-Mode Transmission
ISO 8348/DAD2 Information Processing Systems - Data Communications -
Addendum to the Network Service Definition Covering
Network Layer Addressing
DP iiii Information Processing Systems - Data Communications -
Internal Organization of the Network Layer
DP 8509 Information Processing Systems - Open Systems
Interconnection - Service Conventions
ISO TC97/SC16 A Formal Description Technique based on an N1825
Extended State Transition Model
ISO DIS 8473 (May 1984) [Page 3]RFC 926 December 1984
SECTION ONE. GENERAL
3 DEFINITIONS
3.1 Reference Model Definitions
This document makes use of the following concepts defined in ISO 7498:
a) Network layer
b) Network service
c) Network service access point
d) network service access point address
e) Network entity
f) Routing
f) Service
h) Network protocol
i) Network relay
j) Network protocol data unit
k) End system
3.2 Service Conventions Definitions
This document makes use of the following concepts from the OSI Service
Conventions (ISO 8509):
l) Service user
m) Service provider
3.3 Network Layer Architecture Definitions
This document makes use of the following concepts from the Internal
Organization of the Network Layer (ISO iiii):
n) Subnetwork
ISO DIS 8473 (May 1984) [Page 4]
RFC 926 December 1984
o) Relay system
p) Intermediate system
q) Subnetwork service
3.4 Network Layer Addressing Definitions
This document makes use of the following concepts from DIS 8348/DAD2,
Addendum to the Network Service Definition Covering Network layer
addressing:
r) Network entity title
s) Network protocol address information
t) Subnetwork address
u) Domain
3.5 Additional Definitions
For the purposes of this document, the following definitions apply:
a) automaton - a machine designed to follow automatically a
predetermined sequence of operations or to respond
to encoded instructions.
b) local matter - a decision made by a system concerning its
behavior in the Network Layer that is not subject
to the requirements of this Protocol.
c) segment - part of the user data provided in the N_UNITDATA
request and delivered in the N_UNITDATA
indication.
d) initial PDU - a protocol data unit carrying the whole of the
user data from an N_UNITDATA request.
e) 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.
ISO DIS 8473 (May 1984) [Page 5]RFC 926 December 1984
f) 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.]
g) reassembly - the act of regenerating an initial PDU (in order
to issue an N_UNITDATA indication) from two or
more derived PDUs produced by segmentation.
ISO DIS 8473 (May 1984) [Page 6]RFC 926 December 1984
4 SYMBOLS AND ABBREVIATIONS
4.1 Data Units
PDU Protocol Data Unit
NSDU Network Service 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
NPID Network Layer Protocol Identifier
LI Length Indicator
V/P Version/protocol Identifier Extension
LT Lifetime
SP Segmentation Permitted Flag
MS More Segments Flag
E/R Error Report Flag
TP Type
SL Segment Length
CS Checksum
DAL Destination Address Length
DA Destination Address
SAL Source Address Length
SA Source Address
DUID Data Unit Identifier
SO Segment Offset
TL Total Length
ISO DIS 8473 (May 1984) [Page 7]
RFC 926 December 1984
4.4 Parameters
DA Destination Address
SA Source Address
QOS Quality of Service
4.5 Miscellaneous
SNICP Subnetwork Independent Convergence Protocol
SNDCP Subnetwork Dependent Convergence Protocol
SNAcP Subnetwork Access Protocol
SN Subnetwork
P Protocol
NSAP Network Service Access Point
SNSAP Subnetwork Service Access Point
NPAI Network Protocol Address Information
NS Network Service
ISO DIS 8473 (May 1984) [Page 8]
RFC 926 December 1984
5 OVERVIEW OF THE PROTOCOL
5.1 Internal Organization of the Network Layer
The architecture of the Network Layer is described in a separate
document, Internal Organization of the Network Layer (ISO iiii), in
which an OSI Network Layer structure is defined, and a structure to
classify protocols as an aid to the progression toward that structure
is presented. This protocol is designed to be used in the context of
the internetworking protocol approach defined in that document,
between Network Service users and/or Network Layer relay systems. As
described in the Internal Organization of the Network Layer, the
protocol herein described is a Subnetwork Independent Convergence
Protocol combined with relay and routing functions designed to allow
the incorporation of existing network standards within the OSI
framework.
A Subnetwork Independent Convergence Protocol is one which can be
defined on a subnetwork independent basis and which is necessary to
support the uniform appearance of the OSI Connectionless-mode Network
Service between Network Service users and/or Network Layer relay
systems over a set of interconnected homogeneous or heterogeneous
subnetworks. This protocol is defined in just such a subnetwork
independent way so as to minimize variability where subnetwork
dependent and/or subnetwork access protocols do not provide the OSI
Network Service.
The subnetwork service required from the lower sublayers by the
protocol described herein is identified in Section 5.5.
5.2 Subsets of the Protocol
Two proper subsets of the full protocol are also defined which permit
the use of known subnetwork characteristics, and are therefore not
subnetwork independent.
One protocol subset is for use where it is known that the source and
destination end-systems are connected by a single subnetwork. This is
known as the "Inactive Network Layer Protocol" subset. A second subset
permits simplification of the header where it is known that the source
and destination end-systems are connected by subnetworks whose
subnetwork service data unit (SNSDU) sizes are greater than or equal
to a known bound large enough for segmentation not to be required.
This subset, selected by setting the "segmentation permitted" flag to
zero, is known as the "non-segmenting" protocol subset.
ISO DIS 8473 (May 1984) [Page 9]
RFC 926 December 1984
5.3 Addressing
The Source Address and Destination Address parameters referred to in
Section 7.3 of this International Standard are OSI Network Service
Access Point Addresses. The syntax and semantics of an OSI Network
Service Access Point Address, the syntax and encoding of the Network
Protocol Address Information employed by this Protocol, and the
relationship between the NSAP and the NPAI is described in a separate
document, ISO 8348/DAD2, Addendum to the Network Service Definition
covering Network Layer Addressing.
The syntax and semantics of the titles and addresses used for relaying
and routing are also described in ISO 8348/DAD2.
5.4 Service Provided by the Network Layer
The service provided by the protocol herein described is a
connectionless-mode Network Service. The connectionless-mode Network
Service is described in document ISO 8348/DAD1, Addendum to the
Network Service Definition Covering Connectionless-mode Transmission.
The Network Service primitives provided are summarized below:
ISO DIS 8473 (May 1984) [Page 10]
RFC 926 December 1984
Primitives Parameters
+--------------------------------------------------------+
| | |
| N_UNITDATA Request | NS_Destination_Address, |
| Indication | NS_Source_Address, |
| | NS_Quality_of_Service, |
| | NS_Userdata |
+--------------------------------------------------------+
Table 5-1. Network Service Primitives
The Addendum to the Network Service Definition Covering
Connectionless-mode Transmission (ISO 8348/DAD1) states that the
maximum size of a connectionless-mode Network-service-data-unit is
limited to 64512 octets.
5.5 Service Assumed from the Subnetwork Service provider
The subnetwork service required to support this protocol is defined as
comprising the following primitives:
Primitives Parameters
+--------------------------------------------------------+
| | |
| SN_UNITDATA Request | SN_Destination_Address, |
| Indication | SN_Source_Address, |
| | SN_Quality_of_Service, |
| | SN_Userdata |
+--------------------------------------------------------+
Table 5-2. Subnetwork Service Primitives
ISO DIS 8473 (May 1984) [Page 11]RFC 926 December 1984
5.5.1 Subnetwork Addresses
The source and destination addresses specify the points of attachment
to a public or private subnetwork(s) involved in the transmission.
Subnetwork addresses are defined in the Service Definition of each
individual subnetwork.
The syntax and semantics of subnetwork addresses are not defined in
this Protocol Standard.
5.5.2 Subnetwork Quality of Service
Subnetwork Quality of Service describes aspects of a subnetwork
connectionless-mode service which are attributable solely to the
subnetwork service provider.
Associated with each subnetwork connectionless-mode transmission,
certain measures of quality of service are requested when the
primitive action is initiated. These requested measures (or parameter
values and options) are based on a priori knowledge by the Network
Service provider of the service(s) made available to it by the
subnetwork. Knowledge of the nature and type of service available is
typically obtained prior to an invocation of the subnetwork
connectionless-mode service.
Note:
The quality of service parameters identified for the subnetwork
connectionless-mode service may in some circumstances be directly
derivable from or mappable onto those identified in the
connectionless-mode Network Service; e.g., the parameters
a) transit delay;
b) protection against unauthorized access;
c) cost determinants;
d) priority; and
e) residual error probability
as defined in ISO 8348/DAD1, Addendum to the Network Service
Definition Covering Connectionless-mode Transmission, may be
employed.
ISO DIS 8473 (May 1984) [Page 12]RFC 926 December 1984
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, the subnetwork service provider shall attempt to
deliver the protocol data unit at whatever Quality of Service is
available.
5.5.3 Subnetwork User Data
The SN_Userdata is an ordered multiple of octets, and is transferred
transparently between the specified subnetwork service access points.
The subnetwork service is required to support a subnetwork service
data unit size of at least the maximum size of the Data PDU header
plus one octet of NS-Userdata. This requires a minimum subnetwork
service data unit size of 256 octets.
Where the subnetwork service can support a subnetwork service data
unit (SNSDU) size greater than the size of the Data PDU header plus
one octet of NS_Userdata, the protocol may take advantage of this. In
particular, if all SNSDU sizes of the subnetworks involved 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
provide a connectionless-mode subnetwork service in the case where
subnetworks also provide a connection-oriented subnetwork service. If
a subnetwork provides a connection-oriented service, some subnetwork
dependent function is assumed to provide a mapping into the required
subnetwork service described in the preceding text.
A Subnetwork Dependent Convergence Protocol may also be employed in
those cases where functions assumed from the subnetwork service
provider are not performed.
ISO DIS 8473 (May 1984) [Page 13]RFC 926 December 1984
5.6 Service Assumed from Local Evironment
A timer service is 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
request primitive has elapsed.
The S_TIMER cancel primitive is an indication to the local environment
that the specified timer(s) should be cancelled. If the subscript
parameter is not specified, then all timers with the specified name
are cancelled; 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:
ISO DIS 8473 (May 1984) [Page 14]
RFC 926 December 1984
Primitives Parameters
+--------------------------------------------------------+
| | |
| S_TIMER Request | S_Time |
| | S_Name |
| | S_Subscript |
| | |
| S_TIMER Response | S_Name |
| Cancel | S_Subscript |
+--------------------------------------------------------+
Table 5-3. Timer Primitives
The time parameter indicates the time duration of the specified timer.
An identifying label is associated with a timer by means of the name
parameter. The subscript parameter specifies a value to distinguish
timers with the same name. The name and subscript taken together
constitute a unique reference to the timer.
ISO DIS 8473 (May 1984) [Page 15]RFC 926 December 1984
SECTION TWO. SPECIFICATION OF THE PROTOCOL
6 PROTOCOL FUNCTIONS
This section describes the functions performed as part of the Protocol.
Not all of the functions must be performed by every implementation.
Section 6.17 specifies which functions may be omitted and the correct
behavior where 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 of protocol given in Section 7. Protocol
Control Information required for delivering the data unit to its
destination is determined from current state information and from the
parameters provided with the N_UNITDATA Request; e.g., source and
destination addresses, QOS, etc. User data passed from the Network
Service user in the N_UNITDATA Request forms the Data field of the
protocol data unit.
During the composition of the protocol data unit, a Data Unit
Identifier is assigned to identify uniquely all segments of the
corresponding NS_Userdata. The "Reassemble PDU" function considers
PDUs to correspond to the same Initial PDU, and hence N_UNITDATA
request, if they have the same Source and Destination Addresses and
Data Unit Identifier.
The Data Unit Identifier is available for ancillary functions such as
error reporting. The originator of the PDU must choose the Data Unit
Identifier so that it remains unique (for this Source and Destination
Address pair) for the maximum lifetime of the PDU (or any Derived
PDUs) in the network.
ISO DIS 8473 (May 1984) [Page 16]
RFC 926 December 1984
During the composition of the PDU, a value of the total length of the
PDU 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.
Where the non-segmenting subset is employed, neither the Total Length
field nor the Data Unit Identifier field is present. During the
composition of the protocol data unit, a value of the total length of
the PDU 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.
6.2 PDU Decomposition Function
This function is responsible for removing the Protocol Control
Information from the protocol data unit. During this process,
information pertinent to the generation of the N_UNITDATA Indication
is retained. The data field of the PDU received is reserved until all
segments of the original service data unit have been received; this is
the NS_Userdata 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
indicating that this is a standard version of the Protocol, this
function determines whether a PDU received has reached its destination
using the destination address provided in the PDU is the same as the
one which addresses 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
indicating that the Inactive Network Layer Protocol subset is in use,
then no further analysis of the PDU header is required. The
ISO DIS 8473 (May 1984) [Page 17]
RFC 926 December 1984
network-entity in this case determines that either the network address
encoded in the network protocol address information of a supporting
subnetwork protocol corresponds to a network Service Access Point
address served by this network-entity, or that an error has occurred.
If the subnetwork PDU has been delivered correctly, then the protocol
data unit may be decomposed according to the procedure described for
that particular subnetwork protocol.
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 whether
its assigned lifetime has expired, in which case it must be discarded.
The operation of the 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
Milliseconds). The Lifetime of the Initial PDU is determined by the
originating network-entity, and placed in the Lifetime field of the
PDU.
6.5 Route PDU Function
This function determines the network-entity to which a protocol data
unit should be forwarded, using the destination NSAP address
parameters, Quality of Service parameter, and/or other parameters. It
determines the subnetwork which must be transited to reach that
network-entity. Where segmentation occurs, it further determines which
subnetwork(s) the segments may transit to reach that network-entity.
ISO DIS 8473 (May 1984) [Page 18]
RFC 926 December 1984
6.6 Forward PDU Function
This function issues a subnetwork service primitive (see Section 5.5)
supplying the subnetwork identified by the "Route PDU" function with
the protocol data unit as an SNSDU, and the address information
required by that subnetwork to identify the "next" intermediate-system
within the subnetwork-specific address domain.
When an Error Report PDU is to be forwarded, and is longer than the
maximum user data acceptable by the subnetwork, it shall be truncated
to the maximum acceptable length ad forwarded with no other change.
When a Data PDU is to be forwarded ad is longer than the maximum user
data acceptable by the subnetwork, the Segmentation function is
applied (See Section 6.7, which follows).
6.7 Segmentation Function
Segmentation is performed when the size of the protocol data unit is
greater than the maximum size of the user data parameter field of the
subnetwork service primitive.
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. The Protocol Control Information required to
identify, route, and forward a PDU is duplicated in each PDU derived
from the Initial PDU. The user data encapsulated within the PDU
received is divided such that the Derived PDUs satisfy the size
requirements of the user data parameter field of the subnetwork
service primitive.
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.
ISO DIS 8473 (May 1984) [Page 19]
RFC 926 December 1984
The following fields of the PDU header are used in conjunction with
the Segmentation function:
a) Segment Offset - identifies at which octet in the data field of
the Initial PDU the segment begins;
b) Segment Length - specifies the number of octets in the Derived
PDU, including both header and data;
c) More Segments Flag - 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 routing
of the individual Derived PDUs.
A Segmentation Permitted flag is set to one to indicate that
segmentation is permitted. If the Initial PDU is not to be segmented
at any point during its lifetime in the network, the flag is set to
zero.
When the "Segmentation Permitted" flag is set to zero, the non-
segmenting protocol subset is in use.
6.8 Reassembly Function
The Reassembly Function reconstructs the Initial PDU transmitted to
the destination network-entity from the Derived PDUs generated during
the lifetime of the Initial PDU.
A bound on the time during which segments (Derived PDUs) of an Initial
PDU will be held at a reassembly point is provided so that resources
may be released when it is no longer expected that any outstanding
segments of the Initial PDU will arrive at the reassembly point. When
such an event occurs, segments (Derived PDUs) of the Initial PDU held
at the reassembly point are discarded, the resources allocated for
those segments are freed,
ISO DIS 8473 (May 1984) [Page 20]RFC 926 December 1984
and if selected, an Error Report is generated.
Note:
The design of the Segmentation and Reassembly functions is intended
principally to be used such that reassembly takes place at the
destination. 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/full reassembly at some point along the route
where it is known that the subnetwork with the smallest PDU
size has been transited
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.
While the exact relationship between reassembly lifetime and PDU
lifetime is a local matter, the reassembly algorithm 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.
6.9 Discard PDU Function
This function performs all of the actions necessary to free the
resources reserved by the network-entity in any of the following
situations (Note: the list is not exhaustive):
a) A violation of protocol procedure has occurred.
b) A PDU is received whose checksum is inconsistent with its
contents.
ISO DIS 8473 (May 1984) [Page 21]RFC 926 December 1984
c) A PDU is received, but due to 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 subnetwork
service data unit size.
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, and 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 the 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 the return of an Error Report PDU to the source
network-entity when a protocol data unit is discarded. An "error
report flag" in the original PDU is set by the source network-entity
to indicate whether or not Error Report PDUs are to be returned.
The Error Report PDU identifies the discarded PDU, specifies the type
of error detected, and identifies the location at which the error was
detected. Part or all of the discarded PDU is included in the data
field of the Error Report PDU.
The address of the originator of the Data Protocol Data Unit is
ISO DIS 8473 (May 1984) [Page 22]RFC 926 December 1984
conveyed as both the destination address of the Error Report PDU as
well as the source address of the original Data PDU; the latter is
contained in the Data field of the Error Report PDU. The address of
the originator of the Error Report PDU is contained in the source
address field of the header of the Error Report PDU.
Note:
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 discarding
of a PDU that itself contains an Error Report.
An Error Report PDU shall not be generated upon discarding of a PDU
unless that PDU has the Error Report flag set to allow Error Reports.
If a Data PDU is discarded, and has the Error Report flag set to
allow Error Reports, an Error Report PDU shall be generated if the
reason for discard (See Section 6.9) is
a) destination address unreachable,
b) source routing failure,
c) unsupported options, or
d) protocol violation.
ISO DIS 8473 (May 1984) [Page 23]RFC 926 December 1984
Note:
It is intended that this list shall include all nontransient
reasons for discard; the list may therefore need to be amended or
extended in the light of any changes made in the definitions of
such reasons.
If a Data PDU with the Error Report flag set to allow Error Reports
is discarded for any other reason, an Error Report PDU may be
generated (as an implementation option).
6.10.3 Processing of Error Reports
Error Report PDUs are forwarded by intermediate network-entities in
the same way as Data PDUs. It is possible that an Error Report PDU
may be longer than the maximum user data size of a subnetwork that
must be traversed to reach the origin of the discarded PDU. In this
case, the Forward PDU function shall truncate the PDU to the maximum
size acceptable.
The entire header of the discarded data unit shall be included in the
data field of the Error Report PDU. Some or all of the data field of
the discarded data unit may also be included.
Note:
Since the suppression of Error Report PDUs is controlled by the
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.
ISO DIS 8473 (May 1984) [Page 24]RFC 926 December 1984
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 PDU header. The checksum is verified at each
point at which the PDU header is processed. If PDU header fields are
modified (for example, due to lifetime function), then the checksum is
modified so that the checksum remains valid.
An intermediate system network-entity must not recompute the checksum
for the entire header, even if fields are modified.
Note:
This is 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.
The use of this 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
satisfied:
L
(SUM) a = 0 (modulo 255)
i
i=1
L
(SUM) (L-i+1) a = 0 (modulo 255)
i
i=1
Where L = the number of octets in the PDU header, and
a = value of octet at position i.
i
ISO DIS 8473 (May 1984) [Page 25]RFC 926 December 1984
When the function is in use, neither octet of the checksum field may
be set to zero.
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 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
An issue related to the quality of the network service is the
protection of information flowing between transport-entities. A system
may wish to control the distribution of secure data by assigning
levels of security to PDUs. As a local consideration, the Network
Service user could be authenticated to ascertain whether the user has
permission to engage in communication at a particular security level
before sending the PDU. While no protocol exchange is required in the
authentication process, the optional security parameter in the options
part of the PDU header may be employed to convey the particular
security level between peer network-entities.
The syntax and semantics of the security parameter are not specified
by this Standard. The security parameter is related to the "protection
from unauthorized access" Quality of service parameter described in
ISO 8348/DAD1, Addendum to the Network Service Definition Covering
Connectionless-mode Transmission. However, to facilitate
interoperation between end-systems and relay-systems by avoiding
different interpretations of the same encoding, a mechanism is
provided to distinguish user-defined security encoding from
standardized security encoding.
ISO DIS 8473 (May 1984) [Page 26]RFC 926 December 1984
6.14 Source Routing Function
The Source Routing function allows the originator to specify the path
a generated PDU must take. Source routing can only be selected by the
originator of a PDU. Source Routing is accomplished using a list of
intermediate system addresses (or titles, see Section 5.3 and 5.5.1)
held in a parameter within the options part of the PDU Header. The
size of the option field is determined by the originating
network-entity. The length of this option does not change as the PDU
traverses the network. Associated with this list is an indicator which
identifies the next entry in the list to be used; this indicator is
advanced by the receiver of the PDU when the next address matches its
own address. The indicator is updated as the PDU is forwarded so as to
identify the appropriate entry at each stage of relaying.
Two forms of the source routing option are provided. The first form,
referred to as complete source routing, requires that the specified
path must be taken; if the specified path cannot be taken, the PDU
must be discarded. The source may be informed of the discard using the
Error Reporting function described in Section 6.10.
The second form is referred to as partial source routing. Again, each
address in the list must be visited in the order specified while on
route to the destination. However, with this form of source routing
the PDU may take any path necessary to arrive at the next address in
the list. The PDU will not be discarded (for source routing related
causes) unless one of the addresses specified cannot be reached by any
available route.
ISO DIS 8473 (May 1984) [Page 27]
RFC 926 December 1984
6.15 Record Route Function
The Record Route function permits the exact recording of the paths
taken by a PDU as it traverses a series of interconnected subnetworks.
A recorded route is composed of a list of intermediate system
addresses held in a parameter within the options part of the PDU
header. The size of the option field is determined by the originating
network-entity. The length of this option does not change as the PDU
traverses the network.
The list is constructed as the PDU traverses a set of interconnected
subnetworks. Only intermediate system addresses are included in the
recorded route. The address of the originator of the PDU is not
recorded in the list. When an intermediate system network-entity
processes a PDU containing the record route parameter, the system
inserts its own address (or titles, see Sections 5.3 or 5.5.1) into
the list of recorded addresses.
The record route option contains an indicator which identifies the
next available octet to be used for recording of route. This
identifier is updated as entries are added to the list. If the
addition of the current address to the list would exceed the size of
the option field, the indicator is set to show that recording of route
has terminated. The PDU may still be forwarded to its final
destination, without further addition of intermediate system
addresses.
Note:
The Record Route function is principally intended to be used in the
diagnosis of network problems. Its mechanism has been designed on
this basis, and may provide a return path.
ISO DIS 8473 (May 1984) [Page 28]RFC 926 December 1984
6.16 Quality of Service Maintenance Function
In order to support the Quality of Service requested by Network
Service users, the Protocol may need to make QOS information available
at intermediate systems. This information may be used by network
entities in intermediate systems to make routing decisions where such
decisions affect the overall QOS provided to NS users.
In those instances where the QOS indicated cannot be maintained, the
NS provider will attempt to deliver the PDU at a QOS less than that
indicated. The NS provider will not necessarily provide a notification
of failure to meet the indicated quality of service.
6.17 Classification of Functions
Implementations do not have to support all of the functions described
in Section 6. Functions are divided into three categories:
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 by a PDU, then the PDU shall be
discarded, and an Error Report PDU shall be generated and
forwarded to the originating network-entity, providing that
the Error Report flag is set.
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 by a PDU, then the function is not
performed and the PDU is processed exactly as though the
function was not selected. The protocol data unit shall not
be discarded.
Table 6-1 shows how the functions are divided into these three
categories:
ISO DIS 8473 (May 1984) [Page 29]RFC 926 December 1984
+---------------------------------------------------+
| Function | Type |
|--------------------------------|------------------|
| | |
| PDU Composition | 1 |
| PDU Decomposition | 1 |
| Header Format Analysis | 1 |
| PDU Lifetime Control | 1 |
| Route PDU | 1 |
| Forward PDU | 1 |
| Segment PDU | 1 |
| Reassemble PDU | 1 |
| Discard PDU | 1 |
| Error Reporting | 1 (note 1) |
| PDU Header Error Detection | 1 (note 1) |
| Padding | 1 (notes 1 2) |
| Security | 2 |
| Complete Source Routing | 2 |
| Partial Source Routing | 3 |
| Priority | 3 |
| Record Route | 3 |
| Quality of Service Maintenance | 3 |
+---------------------------------------------------+
Table 6-1. Categorization of Protocol Functions
ISO DIS 8473 (May 1984) [Page 30]RFC 926 December 1984
Notes:
1) While the Padding, Error Reporting, and Header Error Detection
functions must be provided, they are provided only when selected
by the sending Network Service user.
2) The correct treatment of the Padding function involves no
processing. Therefore, this could equally be described as a Type
3 function.
3) 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 and should not be the cause of the discarding of a PDU
when not supported.
ISO DIS 8473 (May 1984) [Page 31]RFC 926 December 1984
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 put into an SNSDU. The bits in an octet
are numbered from one (1) to eight (8), where bit one (1) is the
low-order bit.
When consecutive octets are used to represent a binary number, the
lower octet number has the most significant value.
Any subnetwork supporting this protocol is required to state in its
specification the way octets are transferred, using the terms "most
significant bit" and "least significant bit." The PDUs of this
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
section, the following representation is used:
a) octets are shown with the lowest numbered octet to the left,
higher number octets being further to the right;
b) within an octet, bits are shown with bit eight (8) to the left
and bit one (1) to the right.
PDUs shall contain, in the following order:
1) the header, comprising:
a) the fixed part;
b) the address part;
c) the segmentation part, if present;
d) the options part, if present
and
ISO DIS 8473 (May 1984) [Page 32]RFC 926 December 1984
2) the data field, if present.
This structure is illustrated below:
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 7-1. PDU Structure
ISO DIS 8473 (May 1984) [Page 33]RFC 926 December 1984
7.2 Fixed Part
7.2.1 General
The fixed part contains frequently occuring parameters 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:
Octet
+------------------------------------+
| Network Layer Protocol Identifier | 1
|------------------------------------|
| Length Indicator | 2
|------------------------------------|
| Version/Protocol Id Extension | 3
|------------------------------------|
| Lifetime | 4
|------------------------------------|
|S |M |E/R| Type | 5
| P| S| | |
|------------------------------------|
| Segment Length | 6,7
|------------------------------------|
| Checksum | 8,9
+------------------------------------+
Figure 7-2. PDU Header--Fixed Part
7.2.2 Network Layer Protocol Identifier
The value of this field shall be binary 1000 0001. This field
identifies this Network Layer Protocol as ISO 8473, Protocol for
Providing the Connectionless-mode Network Service.
ISO DIS 8473 (May 1984) [Page 34]RFC 926 December 1984
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 Section 7.1, Structure. The value 255 (1111
1111) is reserved for possible future extensions.
Note:
The rules for forwarding and segmentation ensure 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.
7.2.4 Version/Protocol Identifier Extension
The value of this field is binary 0000 0001. This Identifies a
standard version of ISO 8473, Protocol for Providing the
Connectionless-mode Network Service.
7.2.5 PDU Lifetime
The Lifetime field is encoded as a binary number representing the
remaining lifetime of the PDU, in units of 500 milliseconds.
The Lifetime field is set by the originating network-entity, and is
decremented by every network-entity which processes the PDU. The PDU
shall be discarded if the value of the field reaches zero.
When a network-entity processes a PDU, it decrements the Lifetime by
at least one. The Lifetime shall be decremented by more than one if
the sum of:
1) the transit delay in the subnetwork from which the PDU was
received; and
ISO DIS 8473 (May 1984) [Page 35]RFC 926 December 1984
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 error exists the value used shall be an
overestimate, not an underestimate.
If the Lifetime reaches a value of zero before the PDU is delivered
to the destination, the PDU shall be discarded. The Error Reporting
function shall be invoked, as described in Section 6.10, Error
Reporting Function, and may result in the generation of an ER PDU. It
is a local matter whether the destination network-entity performs the
Lifetime Control function.
When the Segmentation function is applied to a PDU, the Lifetime
field is copied into all of the Derived PDUs.
7.2.6 Flags
7.2.6.1 Segmentation Permitted and More Segments Flags
The Segmentation Permitted flag determines whether segmentation is
permitted. A value of one indicates that segmentation is permitted.
A value of zero indicates that the non-segmenting protocol subset is
employed. Where this is the case, the segmentation part of the PDU
header is not present, and the Segment Length field serves as the
Total Length field.
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) then
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 Segmentation Permitted flag is not set to one (1).
ISO DIS 8473 (May 1984) [Page 36]RFC 926 December 1984
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.
7.2.6.2 Error Report Flag
When the Error Report flag is set to one, the rules in Section 6.10
are used to determine whether to generate an Error Report PDU upon
discard of the PDU.
When the Error Report flag is set to zero, discard of the 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 7-1:
Bits 5 4 3 2 1
+-----------------------------+
| DT PDU | 1 1 1 0 0 |
|-----------------------------|
| ER PDU | 0 0 0 0 1 |
+-----------------------------+
Table 7-1. Valid PDU Types
7.2.8 PDU Segment Length
The Segment Length field specifies the entire length of the PDU
segment including both header and data, if present. When the full
protocol is employed and a PDU is not segmented, then the value of
this field is identical to the value of the Total Length field
located in the Segmentation Part of the header.
ISO DIS 8473 (May 1984) [Page 37]RFC 926 December 1984
When the Non-segmenting protocol subset is employed, no segmentation
part is present in the header. In this subset, the Segment Length
field serves as the Total Length field of the header (see Section
7.4.3).
7.2.9 PDU Checksum
The checksum is computed on the entire PDU header. This includes the
segmentation and options parts, if present. A checksum value of zero
is reserved to indicate that the checksum is to be ignored. The
operation of the PDU Header Error Detection function ensures that the
value zero does not represent a valid checksum. A non-zero value
indicates that the checksum must be processed or the PDU must be
discarded.
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
illustrated below:
ISO DIS 8473 (May 1984) [Page 38]
RFC 926 December 1984
Octet
+--------------------------------------+
| |
| Destination Address Length Indicator | 10
| |
|--------------------------------------|
| | 11
| Destination Address |
| | m-1
|--------------------------------------|
| |
| Source Address Length Indicator | m
| |
|--------------------------------------|
| | m+1
| Source Address |
| | n-1
+--------------------------------------+
Figure 7-3. PDU header--Address Part
7.3.1.1 Destination and Source Address Information
The Destination and Source addresses are Network Service Access
Point addresses as defined in ISO 8348/DAD2, Addendum to the Network
Service Definition Covering Network Layer Addressing.
The Destination and Source Address information is of variable
length.
The Destination Address Length Indicator field specifies the length
of the Destination Address in number of 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 number of octets. The Source Address
Length Indicator field follows the Destination Address field. The
Source Address field follows the Source Address Length Indicator
field.
ISO DIS 8473 (May 1984) [Page 39]RFC 926 December 1984
Each address parameter is encoded as follows:
Bits 8 7 6 5 4 3 2 1
+---------------------------------------------+
| Octet | Address parameter Length Indicator |
| n | (e.g., 'm') |
|---------------------------------------------|
| Octets | |
| n+1 | Address Parameter Value |
| thru | |
| n+m | |
+---------------------------------------------+
Table 7-2. 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 below, must be present:
Octet
+------------------------+
| Data Unit Identifier | n,n+1
|------------------------|
| Segment Offset | n+2,n+3
|------------------------|
| Total Length | n+4,n+5
+------------------------+
Figure 7-4. PDU Header--Segmentation Part
Where the "Segmentation Permitted" flag is set to zero, the
nonsegmenting protocol subset is in use.
ISO DIS 8473 (May 1984) [Page 40]RFC 926 December 1984
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
reassembled by the destination network-entity. The Data Unit
Identifier size is two octets.
7.4.2 Segment Offset
For each segment the Segment Offset field specifies the relative
position of the segment in the data part of the Initial PDU with
respect to the start of the data field. The offset is measured in
units of octets. The offset of the first segment is zero.
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 in
any segment (Derived PDU) for the lifetime of the PDU.
7.5 Options Part
7.5.1 General
The options part is used to convey optional parameters. If the
options part is present, it contains one or more parameters. The
number of parameters that may be contained in the options part is
indicated by the length of the options part which is:
PDU Header Length - (length of fixed part +
length of address part +
length of segmentation part).
ISO DIS 8473 (May 1984) [Page 41]RFC 926 December 1984
The options part of the PDU header is illustrated below:
Octet
+--------------------+
| | n+6
| Options |
| | p
+--------------------+
Figure 7-5. PDU Header--Options Part
Each parameter contained within the options part of the PDU header is
encoded as follows:
BITS 8 7 6 5 4 3 2 1
+------------------------------------------+
| Octets | |
| n | Parameter Code |
|------------------------------------------|
| n+1 | Parameter Length (e.g., 'm') |
|------------------------------------------|
| n+2 | Parameter Value |
| n+m+1 | |
+------------------------------------------+
Table 7-3. Encoding of Parameters
The parameter code field is coded in binary and, without extensions,
provides a maximum number of 255 different parameters. However, as
noted below, bits 8 and 7 cannot take every possible value, so the
practical maximum number of different parameters is less. A parameter
code of 255 (binary 1111 1111) is reserved for possible extensions of
the parameter code.
The parameter length field indicates the length, in octets, of the
parameter value field. The length is indicated by a binary number,
'm', with a theoretical maximum value of 255. 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 indication itself. Thus, the
value of 'm' is limited to:
ISO DIS 8473 (May 1984) [Page 42]RFC 926 December 1984
253 - (length of fixed part +
length of address part +
length of segmentation part).
For each succeeding parameter the maximum value of 'm' decreases.
The parameter value field contains the value of the parameter
identified in the parameter code field.
No parameter codes use bits 8 and 7 with the value 00.
Implementations shall accept the parameters defined in the options
part in any order. Duplication of options (where detected) is not
permitted. Receipt of a PDU with an option duplicated should be
treated as a protocol error. The rules governing the treatment of
protocol errors are described in Section 6.10, Error Reporting
Function.
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
convenient size (See Section 6.12).
Parameter Code: 1100 1100
Parameter Length: variable
Parameter Value: any value is allowed
7.5.3 Security
This parameter is user defined.
Parameter Code: 1100 0101
Parameter Length: variable
Parameter Value:
High order bit of first octet is Security Domain bit, S, to be
interpreted as follows:
ISO DIS 8473 (May 1984) [Page 43]RFC 926 December 1984
S=0
+---------------------------
| S | User Defined ----
+------------------------
S=1
+---------------------------
| S | CODE | ORGANIZATION ----
+------------------------
where
CODE = This field contains a geographic or non-geographic code to
which the option applies.
ORGANIZATION = This is a further subdivision of the CODE field
and is determined by an administrator of the
geographic or non-geographic domain identified by
the value of CODE.
7.5.4 Source Routing
The source routing parameter specifies, either completely or
partially, the route to be taken from Source Network Address to
Destination Network Address.
Parameter Code: 1100 1000
Parameter Length: variable
Parameter Value: 2 octet control information
succeeded by a concatenation
of ordered address fields
(ordered from source to destination)
ISO DIS 8473 (May 1984) [Page 44]RFC 926 December 1984
The first octet of the parameter value is the type code. This has the
following significance.
0000 0001 complete source routing
0000 0000 partial source routing
<all other values reserved>
The second octet indicates the octet offset of the next address to be
processed in the list. A value of three (3) indicates that the next
address begins immediately after this control octet. Successive
octets are indicated by correspondingly larger values of this
indicator.
The third octet begins the intermediate-system address list. The
address list consists of variable length address fields. The first
octet of each address field identifies the length of the address
which comprises the remainder of the address field.
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: two octets control information
succeeded by a concatenation of
ordered addresses
The first octet is used to indicate that the recording of route has
been terminated owing to lack of space in the option. It has the
following significance:
0000 0000 Recording of Route still in progress
1111 1111 Recording of Route terminated
<all other values reserved>
ISO DIS 8473 (May 1984) [Page 45]RFC 926 December 1984
The second octet identifies the next octet which may be used to
record an address. It is encoded relative to the start of the
parameter, such that a value of three (3) indicates that the octet
after this one is the next to be used.
The third octet begins the address list. The address list consists of
variable length address fields. The first octet of each address field
identifies the length of the address which comprises the remainder of
the field. Address fields are always added to the beginning of the
address list; i.e., the most recently added field will begin in the
third octet of the parameter value.
7.5.6 Quality of Service Maintenance
The Quality of Service parameter conveys information about the
quality of service requested by the originating Network Service user.
At intermediate systems, Network Layer relay entities 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 Section 6.16).
Parameter Code: 1100 0011
Parameter Length: one octet
Parameter Value: Bit 8: transit delay vs. cost
Bit 7: residual error probability vs.
transit delay
Bit 6: residual error probability vs.
cost
Bits 5 thru 0 are not specified.
Bit 8 is set to one indicates that where possible, routing decision
should favor low transit delay over low cost. A value of 0 indicates
that routing decisions should favor low cost over low transit delay.
ISO DIS 8473 (May 1984) [Page 46]RFC 926 December 1984
Bit 7 set to one indicates 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 6 is set to one indicates that where possible, routing decisions
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.
7.6 Priority
The priority parameter carries the relative priority of the protocol
data unit. Intermediate systems that support this option should make
use of this information in routing and in ordering PDUs for
transmission.
Parameter Code: 1100 1100
Parameter Length: one octet
Parameter Value: 0000 0000 - Normal (Default)
thru
0000 1111 - Highest
The values 0000 0001 through 0000 1111 are to be used for higher
priority protocol data units. If an intermediate system does not
support this option then all PDUs shall be treated as if the field had
the value 0000 0000.
7.7 Data Part
The Data part of the PDU is structured as an ordered multiple of
octets, 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 below:
ISO DIS 8473 (May 1984) [Page 47]RFC 926 December 1984
Octet
+------------------+
| | p+1
| Data |
| | z
+------------------+
Figure 7-6. PDU header--Data Field
ISO DIS 8473 (May 1984) [Page 48]RFC 926 December 1984
7.8 Data (DT) PDU
7.8.1 Structure
The DT PDU has the following format:
Octet
+--------------------------------------+
| Network Layer Protocol Identifier | 1
|--------------------------------------|
| Length Indicator | 2
|--------------------------------------|
| Version/Protocol Id Extension | 3
|--------------------------------------|
| Lifetime | 4
|--------------------------------------|
|SP|MS|E/R| Type | 5
|--------------------------------------|
| Segment Length | 6,7
|--------------------------------------|
| Checksum | 8,9
|--------------------------------------|
| Destination Address Length Indicator | 10
|--------------------------------------|
| Destination Address | 11 through m-1
|--------------------------------------|
| Source Address Length Indicator | m
|--------------------------------------|
| Source Address | m+1 through n-1
|--------------------------------------|
| Data Unit Identifier | n,n+1
|--------------------------------------|
| Segment Offset | n+2,n+3
|--------------------------------------|
| Total Length | n+4,n+5
|--------------------------------------|
| Options | n+6 through p
|--------------------------------------|
| Data | p+1 through z
+--------------------------------------+
Figure 7-7. PDU Header Format
ISO DIS 8473 (May 1984) [Page 49]RFC 926 December 1984
7.8.1.1 Fixed Part
1) Network Layer Protocol Identifier See Section 7.2.2.
2) Length Indicator See Section 7.2.3.
3) Version/Protocol Id Extension See Section 7.2.4.
4) Lifetime See Section 7.2.5.
5) SP, MS, E/R See Section 7.2.6.
6) Type Code See Section 7.2.7.
7) Segment Length See Section 7.2.8.
8) Checksum See Section 7.2.9.
7.8.1.2 Addresses
See Section 7.3.
7.8.1.3 Segmentation
See Section 7.4.
7.8.1.4 Options
See Section 7.5.
7.8.1.5 Data
See Section 7.7.
ISO DIS 8473 (May 1984) [Page 50]RFC 926 December 1984
7.9 Inactive Network Layer Protocol
Octet
+-----------------------------+
| Network Layer Protocol Id | 1
|-----------------------------|
| Data | 2 through n
+-----------------------------+
Figure 7-9. Inactive Network Layer Protocol
7.9.1 Network Layer Protocol Id
The value of the Network Layer Protocol Identifier field is binary
zero (0000 0000).
7.9.2 Data Field
See Section 7.7.
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.
ISO DIS 8473 (May 1984) [Page 51]RFC 926 December 1984
7.10 Error Report PDU (ER)
7.10.1 Structure
Octet
+--------------------------------------+
| Network Layer Protocol Identifier | 1
|--------------------------------------|
| Length Indicator | 2
|--------------------------------------|
| Version/Protocol Id Extension | 3
|--------------------------------------|
| Lifetime | 4
|--------------------------------------|
|SP|MS|E/R| Type | 5
|--------------------------------------|
| Segment Length | 6,7
|--------------------------------------|
| Checksum | 8,9
|--------------------------------------|
| Destination Address Length Indicator | 10
|--------------------------------------|
| Destination Address | 10 through m-1
|--------------------------------------|
| Source Address Length Indicator | m
|--------------------------------------|
| Source Address | m+1 through n-1
|--------------------------------------|
| Data Unit Identifier | n,n+1
|--------------------------------------|
| Segment Offset | n+2,n+3
|--------------------------------------|
| Total Length | n+4,n+5
|--------------------------------------|
| Options | n+6 through p-1
|--------------------------------------|
| Reason for Discard | p through q-1
|--------------------------------------|
| Error Report Data Field | z
+--------------------------------------+
Figure 7-10. Error Report PDU
ISO DIS 8473 (May 1984) [Page 52]RFC 926 December 1984
7.10.1.1 Fixed Part
The fixed part of the Error Report Protocol Data Unit is set as
though this is a new (Initial) PDU. Thus, references are provided to
precious sections describing the composition of the fields
comprising the fixed part:
1) Network Layer Protocol Identifier See Section 7.2.2.
2) Length Indicator See Section 7.2.3.
3) Version/Protocol Id Extension See Section 7.2.4.
4) Lifetime See Section 7.2.5.
5) SP, MS, E/R See Section 7.2.6.
6) Type Code See Section 7.2.7.
7) Segment Length See Section 7.2.8.
8) Checksum See Section 7.2.9.
7.10.1.2 Addresses
See Section 7.3.
The Destination Address specifies the original source of the
discarded PDU. The Source Address specifies the intermediate system
or end system network-entity initiating the Error Report PDU.
7.10.1.3 Segmentation
See Section 7.4.
ISO DIS 8473 (May 1984) [Page 53]RFC 926 December 1984
7.10.1.4 Options
See Section 7.5.
7.10.1.5 Reason for Discard
This parameter is only valid for the Error Report PDU. It provides a
report on the discarded protocol data unit.
Parameter Code:
1100 0001
Parameter Length:
two octets
type of error encoded in binary:
0000 0000: Reason not specified.
0000 0001: Protocol Procedure Error.
other than below:
0000 0010: Incorrect checksum.
0000 0011: PDU discarded due to congestion.
0000 0100: Header syntax error (header cannot
be parsed).
0000 0101: Segmentation is needed but is not
permitted.
1000 xxxx: Addressing Error:
0000 0000: Destination Address
Unreachable.
1000 0001: Destination Address
Unknown.
1001 xxxx: Source Routing Error:
1001 0000: Unspecified Source
Routing error.
1001 0001: Syntax error in Source
Routing field.
1001 0010: Unknown Address in
Source Routing field.
1001 0011: Path not acceptable.
ISO DIS 8473 (May 1984) [Page 54]RFC 926 December 1984
1010 xxxx: Lifetime Expiration:
1010 0000: Lifetime expired while
data unit in transit.
1010 0001: Lifetime expired
during reassembly.
1011 xxxx: PDU discarded due to unsupported
option:
1011 0000: unsupported option not
specified.
1011 0001: unsupported padding
option.
1011 0010: unsupported security
option.
1011 0011: unsupported source
routing option.
1011 0100: unsupported recording
of route option.
1011 0101: unsupported QoS
Maintenance option.
The second octet contains a pointer to the field in the associated
discarded PDU which caused the error. If no one particular field
can be associated with the error, then this field contains the
value of zero.
7.10.1.6 Error Report Data Field
This field provides all or a portion of the discarded PDU. The
octets comprising this field contain the rejected or discarded PDU
up to and including the octet which caused the rejection/discard.
ISO DIS 8473 (May 1984) [Page 55]RFC 926 December 1984
8 FORMAL DESCRIPTION
The operation of the protocol is modelled as a finite state automaton
governed by a state variable with three values. The behavior of the
automaton is defined with respect to individual independent Protocol
Data Units. A transition of the automaton is prompted by the occurrence
of an atomic event at one of three interfaces:
1) an interface to the Transport Layer, defined by the service
primitives of the Addendum to the Network Service Definition
Covering Connectionless-mode Transmission;
2) an interface to the subnetwork service provider, defined by the
SN_UNITDATA primitive of Section 5.5 of this Standard;
3) an interface to an implementation-dependent timer function defined
by the TIMER primitives described in Section 5.6 of this Standard.
In addition, a transition of the automaton may be prompted by the
occurrence of a condition of the automaton.
The atomic events are defined in Section 8.2. The occurrence of an
atomic event is not in itself sufficient to cause a transition to take
place; other conditions, called "enabling conditions" may also have to
be met before a particular transition can take place. Enabling
conditions are boolean expressions that depend on the values of
parameters associated with the corresponding atomic event (that is, the
parameters of some primitive), and on the values of locally maintained
variables.
More than one enabling condition -- and therefore, more than one
possible transition -- may be associated with a single atomic event. In
every such case, the enabling conditions are mutually exclusive, so
that for any given combination of atomic event and parameter values,
only one state transition can take place.
Associated with each transition is an action, or "output." Actions
consist of changes to the values of local variables and the sequential
performance of zero or more functions. The operation of the finite
state automaton is completely specified in Section 8.3 by defining the
action associated with every possible transition.
ISO DIS 8473 (May 1984) [Page 56]RFC 926 December 1984
8.1 Values of the State Variable
The protocol state variable has three values:
1) INITIAL The automaton is created in the INITIAL state. No
transition may carry the automaton into the INITIAL
state.
2) REASSEMBLING The automaton is in the REASSEMBLING state for the
period in which it is assembling PDU segments into a
complete PDU.
3) CLOSED The final state of the automaton is the CLOSED
state. When the automaton enters the CLOSED state
it ceases to exist.
8.2 Atomic Events
An atomic event is the transfer of a unit of information across an
interface. The description of an atomic event specifies a primitive
(such as an N_UNITDATA.Request), and the service boundary at which it
is invoked (such as the Network Service boundary). The direction of
information flow across the boundary is implied by the definition of
each of the primitives.
8.2.1 N.UNITDATA_request and N.UNITDATA_indication
The N.UNITDATA_request and N.UNITDATA_indication atomic events occur
at the Network Service boundary. They are defined by the Addendum to
the Network Service Definition Covering Connectionless Data
Transmission (ISO 8348/DAD1).
ISO DIS 8473 (May 1984) [Page 57]RFC 926 December 1984
N.UNITDATA_request (NS Source_Address,
NS_Destination_Address,
NS_Quality_of_Service,
NS_Userdata)
N.UNITDATA_indication (NS_Source_Address,
NS_Destination_Address,
NS_Quality_of_Service, NS_Userdata)
The parameters of the N.UNITDATA_request and
N.UNITDATA_indication are collectively referred to as Network
Service Data Unit (NSDUs).
8.2.2 SN.UNITDATA_request and SN.UNITDATA_indication
The SN.UNITDATA_request and SN.UNITDATA_indication atomic events
occur at the interface between the Protocol described herein and a
subnetwork service provider. They are defined in Section 5.5 of this
Standard.
SN.UNITDATA_request (SN_Source_Address,
SN_Destination_Address,
SN_Quality_of_Service,
SN_Userdata)
SN.UNITDATA_indication (SN_Source_Address,
SN_Destination_Address,
SN_Quality_of_Service,
SN_Userdata)
The parameters of the SN_UNITDATA request and SN_UNITDATA Indication
are collectively referred to as Subnetwork Service Data Units
(SNSDUs).
The value of the SN_Userdata parameter may represent an Initial PDU
or a Derived PDU.
ISO DIS 8473 (May 1984) [Page 58]RFC 926 December 1984
8.2.3 TIMER Atomic Events
The TIMER atomic events occur at the interface between the Protocol
described herein and its local environment. They are defined in
Section 5.6 of this Standard.
S.TIMER_request (Time,
Name,
Subscript)
S.TIMER_cancel (Name
Subscript)
S.TIMER_response (Name,
Subscript)
8.3 Operation of the Finite State Automation
The operation of the automaton is defined by use of the formal
description technique and notation specified in ISO/TC97/SC16 N1347.
This technique is based on an extended finite state transition model
and the Pascal programming language. The technique makes use of strong
variable typing to reduce ambiguity in interpretation of the
specification.
This specification formally specifies an abstract machine which
provides a single instance of the Connectionless-Mode Network Service
by use of the Protocol For Providing the Connectionless-Mode Network
Service. It should be emphasized that this formal specification does
not in any way constrain the internal operation or design of any
actual implementation. For example, it is not required that the
program segments contained in the state transitions will actually
appear as part of an actual implementation. A formal protocol
specification is useful in that it goes as far as possible to
eliminate any degree of ambiguity or vagueness in the specification of
a protocol standard.
The formal specification contained here specifies the behavior of a
single finite-state machine, which provides the protocol
ISO DIS 8473 (May 1984) [Page 59]RFC 926 December 1984
behavior corresponding to a single independent service request. It is
expected that any actual implementation will be able to handle
behavior corresponding to many simultaneous finite state machines.
ISO DIS 8473 (May 1984) [Page 60]
RFC 926 December 1984
8.3.1 Type and Constant Definitions
const
ZERO = 0;
max_user_data = 64512;
type
NSAP_addr_type = ...;
{ NSAP_addr_type defines the data type for NSAP addresses, as
passed across the Network Service Boundary. }
NPAI_addr_type = ...;
{ NPAI_addr_type defines the data type for the addresses carried in
PDUs. }
SN_addr_type = ...;
{ SN_addr_type defines the data type for addresses in the
underlying service used by this protocol. }
quality_of_service_type = ...;
{ Quality_of_service_type defines the data type for the QOS
parameter passed across the Network Service boundary. }
SN_QOS_type = ...;
{ SN_QOS_type defines the data type for the QOS parameter, if any,
passed to the underlying service used by this protocol. }
data_type = ...;
{ Data_type defines the data type for user data. Conceptually this
is equivalent to a variable length binary string. }
buffer_type = ...;
{ Buffer_type defines the data type for the memory resources used
in sending and receiving of user data. This provides capabilities
required for segmentation and reassembly. }
ISO DIS 8473 (May 1984) [Page 61]RFC 926 December 1984
timer_name_type = (lifetime_timer);
timer_data_type = ...;
network_layer_protocol_id_type = (ISO_8473_protocol_id);
version_id_type = (version1);
pdu_tp_type = (DT, ER);
options_type = ...;
{ Options_type defines the data type used to store the options part
of the PDU header. }
subnet_id_type = ...;
{ The subnet_id_type defines the data type used to locally identify
a particular underlying service used by this protocol. In general
there may be multiple underlying subnetwork (or data link)
services. }
error_type = (NO_ERROR,
TOO_MUCH_USER_DATA,
PROTOCOL_PROCEDURE_ERROR,
INCORRECT_CHECKSUM, CONGESTION,
SYNTAX_ERROR,
SEG_NEEDED_AND_NOT_PERMITTED,
DESTINATION_UNREACHABLE,
DESTINATION_UNKNOWN,
UNSPECIFIED_SRC_ROUTING_ERROR,
SYNTAX_ERROR_IN_SRC_ROUTING,
UNKNOWN_ADDRESS_IN_SRC_ROUTING,
PATH_NOT_ACCEPTABLE_IN_SRC_ROUTING,
LIFETIME_EXPIRED_IN_TRANSIT,
LIFETIME_EXPIRED_IN_REASSEMBLY,
UNSUPPORTED_OPTION_NOT_SPECIFIED,
UNSUPPORTED_PADDING_OPTION,
UNSUPPORTED_SECURITY_OPTION,
UNSUPPORTED_SRC_ROUTING_OPTION,
UNSUPPORTED_RECORDING_OF_ROUTE_OPTION,
UNSUPPORTED_QOS_MAINTENANCE_OPTION);
ISO DIS 8473 (May 1984) [Page 62]RFC 926 December 1984
nsdu_type = record
da : NSAP_addr_type;
sa : NSAP_addr_type;
qos : quality_of_service_type;
data : data_type;
end;
pdu_type = record
nlp_id : network_layer_protocol_id_type;
hli : integer;
vp_id : version_id_type; lifetime : integer;
sp : boolean;
ms : boolean;
er_flag : boolean;
pdu_tp : pdu_tp_type;
seg_len : integer;
checksum : integer;
da_len : integer;
da : NPAI_addr_type;
sa_len : integer;
sa : NPAI_addr_type;
du_id : optional integer;
so : optional integer;
tot_len : optional integer;
{ du_id, so, and tot_len are present
only if sp has the value TRUE. }
options : options_type;
data : data_type;
end;
ISO DIS 8473 (May 1984) [Page 63]RFC 926 December 1984
route_result_type =
record
subnet_id : subnet_id_type;
sn_da : SN_addr_type;
sn_sa : SN_addr_type;
segment_size : integer;
end;
ISO DIS 8473 (May 1984) [Page 64]RFC 926 December 1984
8.3.2 Interface Definitions
channel Network_access_point (User, Provider);
by User:
UNITDATA_request
(NS_Destination_address : NSAP_addr_type;
NS_Source_address : NSAP_addr_type;
NS_Quality_of_Service : quality_of_service_type;
NS_Userdata : data_type);
by Provider:
UNITDATA_indication
(NS_Destination_address : NSAP_addr_type;
NS_Source_address : NSAP_addr_type;
NS_Quality_of_Service : quality_of_service_type;
NS_Userdata : data_type);
channel Subnetwork_access_point (User, Provider);
by User:
UNITDATA_request
(SN_Destination_address : SN_addr_type;
SN_Source_address : SN_addr_type;
SN_Quality_of_Service : SN_QOS_type;
SN_Userdata : pdu_type);
by Provider:
UNITDATA_indication
(SN_Destination_address : SN_addr_type;
SN_Source_address : SN_addr_type;
SN_Quality_of_Service : SN_QOS_type;
SN_Userdata : pdu_type);
channel System_access_point (User, Provider);
by User:
TIMER_request
(Time : integer;
Name : timer_name_type;
Subscript : integer);
ISO DIS 8473 (May 1984) [Page 65]RFC 926 December 1984
TIMER_cancel
(Name : timer_name_type;
Subscript : integer);
by Provider:
TIMER_indication
(Name : timer_name_type;
Subscript : integer);
ISO DIS 8473 (May 1984) [Page 66]RFC 926 December 1984
8.3.3 Formal Machine Definition
module Connectionless_Network_Protocol_Machine
(N: Network_access_point (Provider) common queue;
SN: array [subnet_id_type] of Subnetwork_access_point
(User) common queue;
S: System_access_point (User) individual queue );
var
nsdu : nsdu_type;
pdu : pdu_type;
rcv_buf : buffer_type;
state : (INITIAL, REASSEMBLING, CLOSED);
ISO DIS 8473 (May 1984) [Page 67]RFC 926 December 1984
procedure send_error_report (error : error_type;
pdu : pdu_type);
var
er_pdu : pdu_type;
begin
if (pdu.er_flag) then
begin
er_pdu.nlp_id := ISO_8473_protocol_id;
er_pdu.vp_id := version1;
er_pdu.lifetime := get_er_lifetime(pdu.sa);
er_pdu.sp := get_er_seg_per(pdu);
er_pdu.ms := FALSE;
er_pdu.er_flag := FALSE;
er_pdu.pdu_tp := ER;
er_pdu.da_len := pdu.sa_len;
er_pdu.da := pdu.sa;
er_pdu.sa_len := get_local_NPAI_addr_len;
er_pdu.sa := get_local_NPAI_addr;
er_pdu.options := get_er_options
(error,
er_pdu.da,
pdu.options);
er_pdu.hli := get_header_length
(er_pdu.da_len, er_pdu.sa_len,
er_pdu.sp,
er_pdu.options);
er_pdu.data := get_er_data_field(error, pdu);
if (er_pdu.sp) then
begin
er_pdu.du_id :=
get_data_unit_id(er_pdu.da);
er_pdu.so := ZERO;
er_pdu.tot_len := er_pdu.hli +
size(er_pdu.data);
end;
ISO DIS 8473 (May 1984) [Page 68]RFC 926 December 1984
if (NPAI_addr_local(er_pdu.da))
then
post_error_report(er_pdu)
else
send_pdu(er_pdu);
end;
end;
ISO DIS 8473 (May 1984) [Page 69]RFC 926 December 1984
procedure send_pdu (pdu : pdu_type);
var
rte_result : route_result_type;
error_code : error_type;
send_buf : buffer_type;
data_maxsize : integer;
more_seg : boolean;
sn_qos : SN_QOS_type;
begin
send_buf := make_buffer(pdu.data);
more_seg := pdu.ms;
repeat
begin
error_code := check_parameters
(pdu.hli,
pdu.sp,
pdu.da,
pdu.options,
size(pdu.data));
if (error_code = NO_ERROR) then
begin
rte_result := route(pdu.hli,
pdu.sp,
pdu.da,
pdu.options,
size(pdu.data));
data_maxsize := rte_result.segment_size -
pdu.hli;
pdu.data := extract(send_buf,
data_maxsize);
pdu.seg_len := pdu.hli + size(pdu.data);
if (size(send_buf) = ZERO) then
pdu.ms := more_seg
else
pdu.ms := TRUE;
ISO DIS 8473 (May 1984) [Page 70]RFC 926 December 1984
pdu.checksum := get_checksum(pdu);
sn_qos := get_sn_qos
(rte_result.subnet_id,
pdu.options);
out SN[rte_result.subnet_id].UNITDATA_request
(rte_result.sn_da,
rte_result.sn_sa,
sn_qos,
pdu);
pdu.so := pdu.so + data_maxsize;
end
else if (error_code = CONGESTION) then
begin
if (send_er_on_congestion (pdu)) then
send_error_report(CONGESTION, pdu);
end
else
send_error_report(error_code, pdu);
end;
until (size_buf(data_buf) = ZERO) or
(error_code <> NO_ERROR);
end;
ISO DIS 8473 (May 1984) [Page 71]RFC 926 December 1984
procedure allocate_reassembly_resources
(pdu_tot_len : integer);
primitive;
{ This procedure allocates resources required for reassembly of a
PDU of the specified total length. If this requires discarding of a
PDU in which the ER flag is set, then an error report is returned to
the source of the discarded data unit. }
function check_parameters
(hli : integer;
sp : boolean;
da : NPAI_addr_type;
options : options_type;
datalen : integer) : error_type;
primitive;
{ This function examines various parameters associated with a PDU,
to determine whether forwarding of the PDU can continue. If a
result of NO_ERROR is returned, then the primitive route can be
called to specify the route and segment size. Otherwise this
function specifies the reason that an error has occurred. }
function data_unit_complete
(buf : buffer_type) : boolean;
primitive;
{ This function returns a boolean value specifying whether the PDU
stored in the specified buffer has been completely received. }
ISO DIS 8473 (May 1984) [Page 72]RFC 926 December 1984
function elapsed_time : integer;
primitive;
{ This function returns an estimate of the time elapsed, in 500
microsecond increments, since the PDU was transmitted by the
previous peer network entity. This estimate includes both time
spent in transit, and any time to be spent in buffers within the
local system. Although this estimate need not be precise,
overestimates are preferable to underestimates, as underestimating
the time elapsed may defeat the intent of the lifetime function. }
procedure empty_buffer
(buf : buffer_type);
primitive;
{ This procedure empties the specified buffer. }
function extract
(buf : buffer_type;
amount : integer) : data_type;
primitive;
{ This function removes the specified amount of data from
the specified buffer, and returns this data as the function
value. }
procedure free_reassembly_resources;
primitive;
{ This procedure releases the resources that had been previously
allocated by the procedure allocate_reassembly_resources. }
function get_checksum
(pdu : pdu_type) : integer;
primitive;
{ This function returns the 16 bit integer value to be placed in the
checksum field of the PDU. If the checksum facility is not being
used, then this function returns the value zero. The algorithm for
producing a correct checksum value is specified in Annex A. }
function get_data_unit_id
(da : NPAI_addr_type) : integer;
primitive;
{ This function returns a data unit identifier which is unique for
the specified destination address. }
ISO DIS 8473 (May 1984) [Page 73]RFC 926 December 1984
function get_er_data_field
(error : error_type;
pdu : pdu_type) : data_type;
primitive;
{ This function returns the correct data field for an error report,
based on the information that the specified PDU is being discarded
due to the specified error. The data field of an error report must
include the header of the discarded PDU, and may optionally contain
additional user data. }
function get_er_flag
(nsdu : nsdu_type) : boolean;
primitive;
{ This function returns a boolean value to be used as the error
report flag in a PDU which transmits the specified nsdu. If the PDU
must be discarded at some future time, an error report can be
returned only if this value is set to TRUE. }
function get_er_lifetime
(da : NPAI_addr_type) : integer;
primitive;
{ This function returns the lifetime value to be used for an error
report being sent to the specified destination address. }
function get_er_options
(error : error_type;
da : NPAI_addr_type;
options : options_type) : options_type;
primitive;
{ This function returns the options field of an error report, based
on the reason for discard, and the destination address and options
field of the discarded PDU. The options field contains the reason
for discard option, and may contain other optional fields. }
ISO DIS 8473 (May 1984) [Page 74]RFC 926 December 1984
function get_er_seg_per
(pdu : pdu_type) : boolean;
primitive;
{ This function returns the boolean value which will be used for the
segmentation permitted flag of an error report. }
function get_header_len
(da_len : integer;
sa_len : integer;
sp : boolean;
options : options_type) : integer;
primitive;
{ This function returns the header length, in octets. This depends
upon the lengths of the source and destination addresses, whether
the segmentation part of the header is present, and the length of
the options part. }
function get_lifetime
(da : NSAP_addr_type;
qos : quality_of_service_type) : lifetime_type;
primitive;
{ This function returns the lifetime value to be used for a PDU,
based upon the destination address and requested quality of service.
}
function get_local_NPAI_addr : NPAI_addr_type;
primitive;
{ This functions returns the local address as used in the protocol
header. }
function get_local_NPAI_addr_len : integer;
primitive;
{ This functions returns the length of the local address as used in
the protocol header. }
ISO DIS 8473 (May 1984) [Page 75]RFC 926 December 1984
function get_NPAI
(addr : NSAP_addr_type) : NPAI_addr_type;
primitive;
{ This function returns the network address as used in the protocol
header, or "Network Protocol Addressing Information", corresponding
to the specified NSAP address. }
function get_NPAI_len
(addr : NSAP_addr_type) : integer;
primitive;
{ This function returns the length of the network address
corresponding to a specified NSAP address. }
function get_NSAP_addr
(addr : NPAI_addr_type;
len : integer) : NSAP_addr_type;
primitive;
{ This function returns the NSAP address corresponding to the
network protocol addressing information (as it appears in the
protocol header) of the specified length. }
function get_options
(da : NSAP_addr_type;
qos : quality_of_service_type) : options_type;
primitive;
{ This function returns the options field for a PDU, based on the
requested destination address and quality of service. }
function get_seg_permitted
(da : NSAP_addr_type;
qos : quality_of_service_type) : boolean;
primitive;
{ This function returns the boolean value to be used in the
segmentation permitted field of a PDU. This value may depend upon
the destination address, requested quality of service, and the
length of the user data. }
ISO DIS 8473 (May 1984) [Page 76]RFC 926 December 1984
function get_sn_qos
(subnet_id : subnet_id_type;
options : options_type) : SN_QOS_type;
primitive;
{ This function returns the quality of service to be used on the
specified subnetwork, in order to obtain the quality of service (if
any) and other parameters requested in the options part of the PDU.
}
function get_qos
(options : options_type) : quality_of_service_type;
primitive;
{ This function determines, to the extent possible, the quality of
service that was obtained for a particular PDU, based upon the
quality of service and other information contained in the options
part of the PDU header. }
function make_buffer
(data : data_type) : buffer_type;
primitive;
{ This function places the specified data in a newly created buffer.
The precise manner of handling buffers is implementation specific.
This newly created buffer is returned as the function value. }
procedure merge_seg
(buf : buffer_type;
so : integer;
data : data_type);
primitive;
{ This procedure merges the specified data into the specified
buffer, based on the specified segment offset of the data. }
function NPAI_addr_local
(addr : NPAI_addr_type) : boolean;
primitive;
{ This function returns the boolean value TRUE only if the specified
network protocol addressing information specifies a local address. }
ISO DIS 8473 (May 1984) [Page 77]RFC 926 December 1984
function NSAP_addr_local
(addr : NSAP_addr_type) : boolean;
primitive;
{ This function returns the boolean value TRUE only if the specified
NSAP address specifies a local address. }
procedure post_error_report
(er_pdu : pdu_type);
primitive;
{ This procedure posts the specified error report (ER) type PDU to
the appropriate local entity that handles error reports. }
function route
(hli : integer;
sp : boolean;
da : NPAI_addr_type;
options : options_type;
datalen : integer) : route_result_type;
primitive;
{ This function determines the route to be followed by a PDU
segment, as well as the segment size. Note that in general, the
segment size and route may be mutually dependent. This
determination is made on the basis of the header length, the
segmentation permitted flag, the destination address, several
parameters (such as source routing) contained in the options part of
the PDU header, and the length of data. This function returns a
structure that specifies the subnetwork on which the segment should
be transmitted, the source and destination addresses to be used on
the subnetwork, and the segment size. This routine may only be
called if the primitive function check_parameters has already
determined that an error will not occur. }
ISO DIS 8473 (May 1984) [Page 78]RFC 926 December 1984
function send_er_on_congestion
(pdu : pdu_type) : boolean;
primitive;
{ This function returns the boolean value true if an error report
should be sent when the indicated data unit is discarded due to
congestion. Note that if the value true is returned, then the
er_flag field of the discarded data unit must still be checked
before an error report can be sent. }
function size
(data : data_type) : integer;
primitive;
{ This function returns the length, in octets, of the specified
data. }
function size_buf
(buf : buffer_type) : integer;
primitive;
{ This function returns the length, in octets, of the data contained
in the specified buffer. }
initialize
begin
state to INITIAL;
end;
ISO DIS 8473 (May 1984) [Page 79]RFC 926 December 1984
trans (* begin transitions *)
from INITIAL to CLOSED
when N.UNITDATA_request
provided not NSAP_addr_local(NS_Destination_Address)
begin
nsdu.da := NS_Destination_Address;
nsdu.sa := NS_Source_Address;
nsdu.qos := NS_Quality_o _Service;
nsdu.data := NS_Userdata;
pdu.nlp_id := ISO_8473_protocol_id;
pdu.vp_id := version1;
pdu.lifetime := get_lifetime(nsdu.da, nsdu.qos);
pdu.sp := get_seg_permitted(nsdu.da, nsdu.qos);
pdu.ms := FALSE;
pdu.er_flag := get_er_flag(nsdu);
pdu.pdu_tp := DT;
pdu.da_len := get_NPAI_len(nsdu.da);
pdu.da := get_NPAI(nsdu.da);
pdu.sa_len := get_NPAI_len(nsdu.sa);
pdu.sa := get_NPAI(nsdu.sa);
pdu.options := get_options(nsdu.da, nsdu.qos);
pdu.data := nsdu.data;
pdu.hli := get_header_len(pdu.da_len,
pdu.sa_len,
pdu.sp,
pdu.options);
if (pdu.sp) then
begin
pdu.du_id := get_data_unit_id(pdu.da);
pdu.so := ZERO;
pdu.tot_len := pdu.hli + size(pdu.data);
end;
if (size(pdu.data) > max_user_data) then
send_error_report(TOO_MUCH_USER_DATA, pdu)
else
send_pdu(pdu);
end;
ISO DIS 8473 (May 1984) [Page 80]RFC 926 December 1984
from INITIAL to CLOSED
when N.UNITDATA_request
provided NSAP_addr_local(NS_Destination_Address)
begin
nsdu.da := NS_Destination_Address;
nsdu.sa := NS_Source_Address;
nsdu.qos := NS_Quality_of_Service;
nsdu.data := NS_Userdata;
out N.UNITDATA_indication
(nsdu.da, nsdu.sa, nsdu.qos, nsdu.data);
end;
from INITIAL to CLOSED
when SN[subnet_id].UNITDATA_indication
provided NPAI_addr_local(SN_Userdata.da) and
SN_Userdata.so = ZERO and
not SN_Userdata.ms
begin
pdu := SN_Userdata;
if (pdu.pdu_tp = DT) then
out N.UNITDATA_indication
(get_NSAP_addr(pdu.da_len, pdu.da),
get_NSAP_addr(pdu.sa_len, pdu.sa),
get_qos(pdu.options),
pdu.data)
else
post_error_report(pdu);
end;
ISO DIS 8473 (May 1984) [Page 81]RFC 926 December 1984
from INITIAL to REASSEMBLING
when SN[subnet_id].UNITDATA_indication
provided NPAI_addr_local(SN_Userdata.da) and
((SN_Userdata.so > ZERO) or (SN_Userdata.ms))
begin
pdu := SN_Userdata;
allocate_reassembly_resources(pdu.tot_len);
empty_buffer(rcv_buf);
merge_seg
(rcv_buf,
pdu.so,
pdu.data);
out S.TIMER_request
(pdu.lifetime,
lifetime_timer,
ZERO);
end;
from INITIAL to CLOSED
when SN[subnet_id].UNITDATA_indication
provided not NPAI_addr_local(SN_Userdata.da)
begin
pdu := SN_Userdata;
if (pdu.lifetime > elapsed_time) then
begin
pdu.lifetime := pdu.lifetime - elapsed_time;
send_pdu(pdu);
end
else
send_error_report(LIFETIME_EXPIRED, pdu);
end;
ISO DIS 8473 (May 1984) [Page 82]RFC 926 December 1984
from REASSEMBLING to REASSEMBLING
when SN[subnet_id].UNITDATA_indication
provided (SN_Userdata.du_id = pdu.du_id) and
(SN_Userdata.da_len = pdu.da_len) and
(SN_Userdata.da = pdu.da) and
(SN_Userdata.sa_len = pdu.sa_len) and
(SN_Userdata.sa = pdu.sa)
begin
merge_seg
(rcv_buf,
SN_Userdata.so,
SN_Userdata.data);
end;
from REASSEMBLING to CLOSED
provided data_unit_complete(rcv_buf)
no delay
begin
if (pdu.pdu_tp = DT) then
out N.UNITDATA_indication
(get_NSAP_addr(pdu.da_len, pdu.da),
get_NSAP_addr(pdu.sa_len, pdu.sa),
get_qos(pdu.options),
extract (rcv_buf, size_buf(rcv_buf)))
else
post_error_report(pdu);
out S.TIMER_cancel(lifetime_timer,ZERO);
free_reassembly_resources;
end;
from REASSEMBLING to CLOSED
when S.TIMER_indication
begin
send_error_report(LIFETIME_EXPIRED, pdu);
end;
ISO DIS 8473 (May 1984) [Page 83]RFC 926 December 1984
9 CONFORMANCE
For conformance to this International Standard, the ability to
originate, manipulate, and receive PDUs in accordance with the full
protocol (as opposed to the "non-segmenting" or "Inactive Network Layer
Protocol" subsets) is required.
Additionally, the provision of the optional functions described in
Section 6.17 and enumerated in Table 9-1 must meet the requirements
described therein.
Additionally, conformance to the Standard requires adherence to the
formal description of Section 8 and to the structure and encoding of
PDUs of Section 7.
If and only if the above requirements are met is there conformance to
this International Standard.
9.1 Provision of Functions for Conformance
The following table categorizes the functions in Section 6 with
respect to the type of system providing the function:
ISO DIS 8473 (May 1984) [Page 84]
RFC 926 December 1984
+---------------------------------------------------------+
| Function | Send | Forward | Receive |
|---------------------------------------------------------|
| PDU Composition | M | - | - |
| PDU Decomposition | M | - | M |
| Header Format Analysis | - | M | M |
| PDU Lifetime Control | - | M | I |
| Route PDU | - | M | - |
| Forward PDU | M | M | - |
| Segment PDU | M | (note 1)| - |
| Reassemble PDU | - | I | M |
| Discard PDU | - | M | M |
| Error Reporting | - | M | M |
| PDU Header Error Detection | M | M | M |
| Padding |(note 2)| (note 2)| (note 2)|
| Security | - | (note 3)| (note 3)|
| Complete Source Routing | - | (note 3)| - |
| Partial Source Routing | - | (note 4)| - |
| Record Route | - | (note 4)| - |
| QoS Maintenance | - | (note 4)| - |
+---------------------------------------------------------+
Table 9-1. Categorization of Functions
+---------------------------------------------------------+
| KEY: |
| M : Mandatory Function; must be implemented |
| - : Not applicable |
| I : Implementation option, as described in text |
+---------------------------------------------------------+
Notes:
1) 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 SNSDU 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.
ISO DIS 8473 (May 1984) [Page 85]RFC 926 December 1984
2) 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.
3) This function may or may not be supported. If an implementation
does not support this function, and the function is selected by 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.
4) This function may or may not be supported. If an implementation
does not support this function, and the function is selected by a
PDU, then the function is not provided and the PDU is processed
exactly as though the function was not selected. The PDU shall
not be discarded.
ISO DIS 8473 (May 1984) [Page 86]RFC 926 December 1984
ANNEXES
(These annexes are provided for information for implementors and are
not an integral part of the body of the Standard.)
ANNEX A. SUPPORTING TECHNICAL MATERIAL
A.1 Data Unit Lifetime
There are two primary purposes of providing a PDU lifetime capability
in the ISO 8473 Protocol. One purpose is to ensure against unlimited
looping of protocol data units. Although the routing algorithm should
ensure that it will be very rare for data to loop, the PDU lifetime
field provides additional assurance that loops will be limited in
extent.
The other important purpose of the lifetime capability is to provide
for a means by which the originating network entity can limit the
Maximum NSDU lifetime. ISO Transport Protocol Class 4 assumes that
there is a particular Maximum NSDU Lifetime in order to protect
against certain error states in the connection establishment and
termination phases. If a TPDU does not arrive within this time, then
there is no chance that it will ever arrive. It is necessary to make
this assumption, even if the Network Layer does not guarantee any
particular upper bound on NSDU lifetime. It is much easier for
Transport Protocol Class 4 to deal with occasional lost TPDUs than to
deal with occasional very late TPDUs. For this reason, it is
preferable to discard very late TPDUs than to deliver them. Note that
NSDU lifetime is not directly associated with the retransmission of
lost TPDUs, but relates to the problem of distinguishing old
(duplicate) TPDUs from new TPDUs.
Maximum NSDU Lifetime must be provided to transport protocol entity in
units of time; a transport entity cannot count "hops". Thus NSDU
lifetime must be calculated in units of time in order to be useful in
determining Transport timer values.
In the absence of any guaranteed bound, it is common to simply guess
some value which seems like a reasonable compromise. In essence one is
simply assuming that "surely no TPDU would ever take more than 'x'
seconds to traverse the network." This value is probably chosen by
observation of past performance, and may
ISO DIS 8473 (May 1984) [Page 87]RFC 926 December 1984
vary with source and destination.
Three possible ways to deal with the requirement for a limit on the
maximum NSDU lifetime are: (1) specify lifetime in units of time,
thereby requiring intermediate systems to decrement the lifetime field
by a value which is an upper bound on the time spent since the
previous intermediate system, and have the Network Layer discard
protocol data units whose lifetime has expired; (2) provide a
mechanism in the Transport Layer to recognize and discard old TPDUs;
or (3) ignore the problem, anticipating that the resulting
difficulties will be rare. Which solution should be followed depends
in part upon how difficult it is to implement solutions (1) and (2),
and how strong the transport requirement for a bounded time to live
really is.
There is a problem with solution (2) above, in that transport entities
are inherently transient. In case of a computer system outage or other
error, or in the case where one of the two endpoints of a connection
closes without waiting for a sufficient period of time (approximately
twice Maximum NSDU Lifetime), it is possible for the Transport Layer
to have no way to know whether a particular TPDU is old unless
globally synchronized clocks are used (which is unlikely). On the
other hand, it is expected that intermediate systems will be
comparatively stable. In addition, even if intermediate systems do
fail and resume processing without memory of the recent past, it will
still be possible (in most instances) for the intermediate system to
easily comply with lifetime in units of time, as discussed below.
It is not necessary for each intermediate system to subtract a precise
measure of the time that has passed since an NPDU (containing the TPDU
or a segment thereof) has left the previous intermediate system. It is
sufficient to subtract an upper bound on the time taken. In most
cases, an intermediate system may simply subtract a constant value
which depends upon the typical near-maximum delays that are
encountered in a specific subnetwork. It is only necessary to make an
accurate estimate on a per NPDU basis for those subnetworks which have
both a relatively large maximum delay, and a relatively large
variation in delay.
As an example, assume that a particular local area network has short
average delays, with overall delays generally in the 1 to 5
ISO DIS 8473 (May 1984) [Page 88]
RFC 926 December 1984
millisecond range and with occasional delays up to 20 milliseconds. In
this case, although the relative range in delays might be large (a
factor of 20), it would still not be necessary to measure the delay
for actual NPDUs. A constant value of 20 milliseconds (or more) can be
subtracted for all delays ranging from .5 seconds to .6 seconds (.5
seconds for the propagation delay, 0 to .1 seconds for queueing delay)
then the constant value .6 seconds could be used.
If a third subnetwork had normal delays ranging from .1 to 1 second,
but occasionally delivered an NPDU after a delay of 15 seconds, the
intermediate system attached to this subnetwork might be required to
determine how long it has actually take the PDU to transit the
subnetwork. In this last example, it is likely to be more useful to
have the intermediate systems determine when the delays are extreme ad
discard very old NPDUs, as occasional large delays are precisely what
causes the Transport Protocol the most trouble.
In addition to the time delay within each subnetwork, it is important
to consider the time delay within intermediate systems. It should be
relatively simple for those gateways which expect to hold on to some
data-units for significant periods of time to decrement the lifetime
appropriately.
Having observed that (i) the Transport Protocol requires Maximum NSDU
to be calculated in units of time; (ii) in the great majority of
cases, it is not difficult for intermediate systems to determine a
valid upper bound on subnetwork transit time; and (iii) those few
cases where the gateways must actually measure the time take by a NPDU
are precisely the cases where such measurement truly needs to be made,
it can be concluded that NSDU lifetime should in fact be measured in
units of time, and that intermediate systems should required to
decrement the lifetime field of the ISO 8473 Protocol by a value which
represents an upper bound on the time actually taken since the
lifetime field was last decremented.
A.2 Reassembly Lifetime Control
In order to ensure a bound on the lifetime of NSDUs, and to
effectively manage reassembly buffers in the Network Layer, the
Reassembly Function described in Section 6 must control the
ISO DIS 8473 (May 1984) [Page 89]
RFC 926 December 1984
lifetime of segments representing partially assembled PDUs. This annex
discusses methods of bounding reassembly lifetime and suggests some
implementation guidelines for the reassembly function.
When segments of a PDU arrive at a destination network-entity, they
are buffered until an entire PDU is received, assembled, and passed to
the PDU Decomposition Function. The connectionless Internetwork
Protocol does not guarantee the delivery of PDUs; hence, it is
possible for some segments of a PDU to be lost or delayed such that
the entire PDU cannot be assembled in a reasonable length of time. In
the case of loss of a PDU "segment", for example, this could be
forever. There are a number of possible schemes to prevent this:
a) Per-PDU reassembly timers,
b) Extension of the PDU Lifetime control function, and
c) Coupling of the Transport Retransmission timers.
Each of these methods is discussed in the subsections which follow.
A.2.1 Method (a)
assigns a "reassembly lifetime" to each PDU received and identified
by its Data-unit Identifier. This is a local, real time which is
assigned by the reassembly function and decremented while some, but
not all segments of the PDU are being buffered by the destination
network-entity. If the timer expires, all segments of the PDU are
discarded, thus freeing the reassembly buffers and preventing a "very
old" PDU from being confused with a newer one bearing the same
Data-unit Identifier. For this scheme to function properly, the
timers must be assigned in such a fashion as to prevent the
phenomenon of Reassembly Interference (discussed below). In
particular, the following guidelines should be followed:
1) The Reassembly Lifetime must be much less than the maximum PDU
lifetime of the network (to prevent the confusion of old and new
data-units).
ISO DIS 8473 (May 1984) [Page 90]RFC 926 December 1984
2) The lifetime should be less than the Transport protocol's
retransmission timers minus the average transit time of the
network. If this is not done, extra buffers are tied up holding
data which has already been retransmitted by the Transport
Protocol. (Note that an assumption has been made that such
timers are integral to the Transport Protocol, which in some
sense, dictates that retransmission functions must exist in the
Transport Protocol employed).
A.2.2 Method (b)
is feasible if the PDU lifetime control function operates based on
real or virtual time rather than hop-count. In this scheme, the
lifetime field of all PDU segments of a Data-unit continues to be
decremented by the reassembly function of the destination
network-entity as if the PdU were still in transit (in a sense, it
still is). When the lifetime of any segment of a partially
reassembled PDU expires, all segments of that PDU are discarded. This
scheme is attractive since the delivery behavior of the ISO 8473
Protocol would be identical for segmented and unsegmented PDUs.
A.2.3 Method (c)
couples the reassembly lifetime directly to the Transport Protocol's
retransmission timers, and requires that Transport Layer management
make known to Network Layer Management (and hence, the Reassembly
Function) the values of its retransmission timers for each source
from which it expects to be receiving traffic. When a PDU segment is
received from a source, the retransmission time minus the anticipated
transit time becomes the reassembly lifetime of that PDU. If this
timer expires before the entire PDU has been reassembled, all
segments of the PDU are discarded. This scheme is attractive since it
has a low probability of holding PDU segments that have already been
retransmitted by the source Transport-entity; it has, however, the
disadvantage of depending on reliable operation of the Transport
Protocol to work effectively. If the retransmission timers are not
set correctly, it is possible that all PDUs would be discarded too
soon, and the Transport Protocol would make no progress.
A.3 The Power of the Header Error Detection Function
ISO DIS 8473 (May 1984) [Page 91]RFC 926 December 1984
A.3.1 General
The form of the checksum used for PDU header error detection is such
that it is easily calculated in software or firmware using only two
additions per octet of header, yet it has an error detection power
approaching (but not quite equalling) that of techniques (such as
cyclic polynomial checks) which involve calculations that are much
more time- or space-consuming. This annex discusses the power of this
error detection function.
The checksum consists of two octets, either of which can assume any
value except zero. That is, 255 distinct values for each octet are
possible. The calculation of the two octets is such that the value of
either is independent of the value of the other, so the checksum has
a total of 255 x 255 = 65025 values. If one considers all ways in
which the PDU header might be corrupted as equally likely, then there
is only one chance in 65025 that the checksum will have the correct
value for any particular corruption. This corresponds to 0.0015 of
all possible errors.
The remainder of this annex considers particular classes of errors
that are likely to be encountered. The hope is that the error
detection function will be found to be more powerful, or at least no
less powerful, against these classes as compared to errors in
general.
A.3.2 Bit Alteration Errors
First considered are classes of errors in which bits are altered, but
no bits are inserted nor deleted. This section does not consider the
case where the checksum itself is erroneously set to be all zero;
this case is discussed in section A.3.4.
A burst error of length b is a corruption of the header in which all
of the altered bits (no more than b in number) are within a single
span of consecutively transmitted bits that is b bits long. Checksums
are usually expected to do well against burst errors of a length not
exceeding the number of bits in the header error detection parameter
(16 for the PDU header). The PDU header error detection parameter in
fact fails to detect only 0.000019 of all such errors, each distinct
burst error of length 16 or less being considered to be equally
likely. In particular,
ISO DIS 8473 (May 1984) [Page 92]
RFC 926 December 1984
it cannot detect an 8-bit burst in which an octet of zero is altered
to an octet of 255 (all bits = 1) or vice versa. Similarly, it fails
to detect the swapping of two adjacent octets only if one is zero and
the other is 255.
The PDU header error detection, as should be expected, detects all
errors involving only a single altered bit.
Undetected errors involving only two altered bits should occur only
if the two bits are widely separated (and even then only rarely). The
PDU header error detection detects all double bit errors for which
the spacing between the two altered bits is less than 2040 bits = 255
octets. Since this separation exceeds the maximum header length, all
double bit errors are detected.
The power to detect double bit errors is an advantage of the checksum
algorithm used for the protocol, versus a simple modulo 65536
summation of the header split into 16 bit fields. This simple
summation would not catch all such double bit errors. In fact, double
bit errors with a spacing as little as 16 bits apart could go
undetected.
A.3.3 Bit Insertion/Deletion Errors
Although errors involving the insertion or deletion of bits are in
general neither more nor less likely to go undetected than are all
other kinds of general errors, at least one class of such errors is
of special concern. If octets, all equal to either zero or 255, are
inserted at a point such that the simple sum CO in the running
calculation (described in Annex C) happens to equal zero, then the
error will go undetected. This is of concern primarily because there
are two points in the calculation for which this value for the sum is
not a rare happenstance, but is expected; namely, at the beginning
and the end. That is, if the header is preceded or followed by
inserted octets all equal to zero or 255 then no error is detected.
Both cases are examined separately.
Insertion of erroneous octets at the beginning of the header
completely misaligns the header fields, causing them to be
misinterpreted. In particular, the first inserted octet is
interpreted as the network layer protocol identifier, probably
eliminating any knowledge that the data unit is related to the
ISO DIS 8473 (May 1984) [Page 93]
RFC 926 December 1984
ISO 8473 Protocol, and thereby eliminating any attempt to perform the
checksum calculation or invoking a different form of checksum
calculation. An initial octet of zero is reserved for the Inactive
Network Layer Protocol. This is indeed a problem but not one which
can be ascribed to the form of checksum being used. Therefore, it is
not discussed further here.
Insertion of erroneous octets at the end of the header, in the
absence of other errors, is impossible because the length field
unequivocally defines where the header ends. Insertion or deletion of
octets at the end of the header requires an alteration in the value
of the octet defining the header length. Such an alteration implies
that the value of the calculated sum at the end of the header would
not be expected to have the dangerous value of zero and consequently
that the error is just as likely to be detected as is any error in
general.
Insertion of an erroneous octet in the middle of the header is
primarily of concern if the inserted octet has either the value zero
or 255, and if the variable CO happens to have the value zero at this
point. In most cases, this error will completely destroy the parsing
of the header, which will cause the data unit to e discarded. In
addition, in the absence of any other error, the last octet of the
header will be thought to be data. This in turn will cause the header
to end in the wrong place. In the case where the header otherwise can
parse correctly, the last field will be found to be missing. Even in
the case where necessary, the length field is the padding option, and
therefore not necessary, the length field for the padding function
will be inconsistent with the header length field, and therefore the
error can be detected.
A.3.4 Checksum Non-calculation Errors
Use of the header error detection function is optional. The choice of
not using it is indicated by a checksum parameter value of zero. This
creates the possibility that the two octets of the checksum parameter
(neither of which is generated as being zero) could both be altered
to zero. This would in effect be an error not detected by the
checksum since the check would not be made. One of three
possibilities exists:
1) A burst error of length sixteen (16) which sets the entire
ISO DIS 8473 (May 1984) [Page 94]RFC 926 December 1984
checksum to zero. Such an error could not be detected; however, it
requires a particular positioning of the burst within the
header. [A calculation of its effect on overall detectability of
burst errors depends upon the length of the header.]
2) All single bit errors are detected. Since both octets of the
checksum field must be non-zero when the checksum is being used,
no single bit error can set the checksum to zero.
3) Where each of the two octets of the checksum parameter has a
value that is a power of two, such that only one bit in each
equals one (1), then a zeroing of the checksum parameter could
result in an undetected double bit error. Furthermore, the two
altered bits have a separation of less than sixteen (16), and
could be consecutive. This is clearly a decline from the
complete detectability previously described.
Where a particular administration is highly concerned about the
possibility of accidental zeroing of the checksum among data units
within its domain, then the administration may impose the restriction
that all data units whose source or destination lie within its domain
must make use of the header error detection function. Any data units
which do not could be discarded, nor would they be allowed outside
the domain. This protects against errors that occur within the
domain, and would protect all data units whose source or destination
lies within the domain, even where the data path between all such
pairs crosses other domains (errors outside the protected domain
notwithstanding).
ISO DIS 8473 (May 1984) [Page 95]RFC 926 December 1984
ANNEX B. NETWORK MANAGEMENT
The following topics are considered to be major components of Network
Layer management:
A. Routing
Considered by many to be the most crucial element of Network Layer
management, since management of the Routing algorithms for networking
seem to be an absolutely necessary prerequisite to a practical
networking scheme.
Routing management consists of three parts; forwarding, decision, and
update. Management of forwarding is the process of interpreting the
Network Layer address to properly forward NSDUs on its next network
hop on a route through the network. Management of decision is the
process of choosing routes for either connections or NSDUs, depending
on whether the network is operating a connection-oriented or
connectionless protocol. The decision component will be driven by a
number of considerations, not the least of which are those associated
with Quality of Service. Management of update is the management
protocol(s) used to exchange information among
intermediate-systems/network- entities which is used in the decision
component to determine routes.
To what extent is it desirable and/or practical to pursue a single
OSI network routing algorithm and associated Management protocol(s)?
It is generally understood that it is impractical to expect ISO to
adopt a single global routing algorithm. On the other hand, it is
recognized that having no standard at all upon which to make routing
decisions effectively prevents an internetwork protocol from working
at all. One possible compromise would be to define the principles for
the behavior of an internetwork routing algorithm. A possible next
step would be to specify the types of information that must be
propagated among the intermediate-systems/network-entities via their
update procedures. The details of the updating protocol might then be
left to bilateral agreements among the cooperating administrations.
ISO DIS 8473 (May 1984) [Page 96]RFC 926 December 1984
B. Statistical Analysis
These management functions relate to the gathering and reporting of
information about the real-time behavior of the global network. They
consist of Data counts such as number of PDUs forwarded, entering
traffic, etc., and Event Counts such as topology changes, quality of
service changes, etc.
C. Network Control
These management functions are those related to the control of the
global network, and possibly could be performed by a Network Control
Center(s). The control functions needed are not al all clear. Neither
are the issues relating to what organization(s) is/are responsible
for the management of the environment. Should there be a Network
Control Center distinct from those provided by the subnetwork
administrations? What subnetwork management information is needed by
the network management components to perform their functions?
D. Directory Mapping Functions
Does the Network layer contain a Directory function as defined in the
Reference Model? Current opinion is that the Network Layer restricts
itself to the function of mapping NSAP addresses to routes.
E. Congestion Control
Does this come under the umbrella of Network Layer management? How?
F. Configuration Control
This is tightly associated with the concepts of Resource Management,
and is generally considered to be somehow concerned with the control
of the resources used in the management of the global network. The
resources which have to be managed are Bandwidth (use of subnetwork
resources), Processor (CPU), and Memory (buffers). Where is the
responsibility for resources assigned, and are they appropriate for
standardization? It appears that these
ISO DIS 8473 (May 1984) [Page 97]
RFC 926 December 1984
functions are tightly related to how one signals changes in Quality
of Service.
G. Accounting
What entities, administrations, etc., are responsible for network
accounting? How does this happen? What accounting information, if
any, is required from the subnetworks in order to charge for network
resources? Who is charged? To what degree is this to be standardized?
ISO DIS 8473 (May 1984) [Page 98]
RFC 926 December 1984
ANNEX C. ALGORITHMS FOR PDU HEADER ERROR DETECTION FUNCTION
This Annex describes algorithm which may be used to computer, check and
update the checksum field of the PDU Header in order to provide the PDU
Header Error Detection function described in Section 6.11.
C.1 Symbols used in algorithms
CO,C1 variables used in the algorithms
i number (i.e., position) of an octet within the header
n number (i.e., position) of the first octet of the checksum
parameter (n=8)
L length of the PDU header in octets
X value of octet one of the checksum parameter
Y value of octet two of the checksum parameter
a octet occupying position i of the PDU header
C.2 Arithmetic Conventions
Addition is performed in one of the two following modes:
a) modulo 255 arithmetic;
b) eight-bit one's complement arithmetic in which, if any of the
variables has the value minus zero (i.e., 255) it shall be
regarded as though it was plus zero (i.e., 0).
C.3 Algorithm for Generating Checksum Parameters
A: Construct the complete PDU header with the value of the checksum
parameter field set to zero;
B: Initialize C0 and C1 to zero;
C: Process each octet of the PDU header sequentially from i = 1 to L
by
a) adding the value of the octet to C0; then
b) adding the value of C0 to C1;
D: Calculate X = (L-8)C0 - C1 (modulo 255) and Y = (L-7) (-C0) + C1
(modulo 255)
ISO DIS 8473 (May 1984) [Page 99]RFC 926 December 1984
E: If X = 0, set X = 255;
F: If Y = 0, set Y = 255;
G: Place the values X and Y in octets 8 and 9 respectively.
C.4 Algorithm for Checking Checksum Parameters
A: If octets 8 and 9 of PDU header both contain 0 (all bits off),
then the checksum calculation has succeeded; otherwise initialize
C1 = 0, C0 - 0 and proceed;
B: process each octet of the PDU header sequentially from i = 1 to L
by
a) adding the value of the octet to C0; then
b) adding the value of C0 to C1;
C: If, when all the octets have been processed, C0 = C1 = 0 (modulo
255) then the checksum calculation has succeeded; otherwise, the
checksum calculation has failed.
C.5 Algorithm to adjust checksum parameter when an octet is altered
This algorithm adjusts the checksum when an octet (such as the
lifetime field) is altered. Suppose the value in octet k is changed by
Z = new_value - old_value.
If X and Y denote the checksum values held in octets n and n+1,
respectively, then adjust X and Y as follows:
If X = 0 and Y = 0 do nothing, else;
X := (k-n-1)Z + X (modulo 255) and
Y := (n-k)Z + Y (modulo 255).
If X is equal to zero, then set it to 255; and
similarly for Y.
For this Protocol, n = 8. If the octet being altered is the lifetime
field, k = 4. For the case where the lifetime is decreased by 1 unit
(Z = -1), the results simplify to
ISO DIS 8473 (May 1984) [Page 100]RFC 926 December 1984
X := X + 5 (modulo 255) and
Y := Y - 4 (modulo 255).
Note:
To derive this result, assume that when octet k has the value Z
added to it then X and Y have values ZX and ZY added to them. For
the checksum parameters to satisfy the conditions of Section 6.11
both before and after the values are added, the following is
required:
Z + ZX + ZY = 0 (modulo 255) and
(L-k+1)Z + (L-n+1)ZX + (L-n)ZY = 0 (modulo 255).
Solving these equations simultaneously yields ZX = (k-n-1)Z and ZY +
(m-k)Z.
ISO DIS 8473 (May 1984) [Page 101]
