RFC 0926
Protocol for providing the connectionless mode network services
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 BBN
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
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
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
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
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.
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
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
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.
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.
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
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
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.
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:
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
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.
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.
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:
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.
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.
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
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.
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.
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,
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.
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
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.
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.
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
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.
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.
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.
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:
+---------------------------------------------------+
| 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
(next page on part 2)
