ISO/IEC 8885:1993

INTERNATIONAL lSO/IEC
STANDARD 8885
Third edition
1993-l 2-l 5
Information technology -
Telecommunications and information
exchange between systems - High-level
data link control (HDLC) procedures -
General purpose XID frame information
field content and format
Technologies de I’informa tion - T6l6communications et &change
d ‘informa tions en tre sys t&mes - Pro&dures de commande de liaison de
don&es 9 haut niveau (HDLC) - Format et contenu du champ
d’in formation de la trame XID pour application g6n6rale
Reference number
lSO/IEC 8885:1993(E)

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ISOhEC 8885:1993(E)
Page
Contents
. . .
111
Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1v
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
scope .
2
.....................................................................................
Normative references
2
Definitions .
........................................................... 3
XID frame information field structure
5
...........................................................
XID f!rame information field encoding
7
..........
Definition and encoding of data link layer subfield parameter fields.
12
...................................................
Single-frame exchange negotiation process
13
........................................................
Frame check sequence negotiation rules
Annex
15
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A Illustrative examples of FCS negotiation
o ISO/IEC 1993
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ISO/IEC Copyright Office l Case postale 56 l CH-1211 Geneve 20 l Switzerland
Printed in Switzerland
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0 ISO/IEC
ISO/IEC 8885:1993(E)
Foreword
IS0 (the International Organization for Standardization) and IEC (the Inter-
national Electrotechnical Commission) form the specialized system for worldwide
standardization. National bodies that are members of IS0 or IEC participate in
the development of International Standards through technical committees estab-
lished by the respective organization to deal with particular fields of technical
activity. IS0 and IEC technical committees collaborate in fields of mutual inter-
est. Other international organizations, governmental and non-governmental, in
liaison with IS0 and IEC, also take part in the work.
In the field of information technology, IS0 and IEC have established a joint
technical committee, ISO/IEC JTC 1. Draft International Standards adopted by
the joint technical committee are circulated to national bodies for voting. Publi-
cation as an International Standard requires approval by at least 75 % of the na-
tional bodies casting a vote.
International Standard ISO/IEC 8885 was prepared by Joint Technical Committee
ISO/IEC JTC 1, Information technology, Subcommittee SC 6, Telecommuni-
cations and information exchange between systems.
This third edition cancels and replaces the second edition (ISO/IEC 8885: 1991),
and incorporates ISO/IEC 8885 amendments 3, 4 and 5.
Annex A of this International Standard is for information only.
. . .
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o ISO/IEC
ISO/IEC 8885:1993(E)
Introduction
High-level data link control (HDLC) procedures define the exchange identification
(XID) command/response frame as an optional function for exchange of data link
information. This International Standard defines the content and format for the
general purpose XID fkme information field.
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INTERNATIONAL STANDARD @ ISO/IEC ISO/IEC 8885 : 1993 (E)
Information technology - Telecommunications and
information exchan een systems -
High-level data lin ontrol (HDLC) procedures -
General purpose frame information field content
and format
1 scope
The principal use of the XID frame is to exchange data link information between two or more HDLC stations. For the
mational Standard, data link information shall include any and all essential operational
purpose of this Inte
characteristics such as identification, authentication and/or selection of optional functions and facilities concerning
each station. This International Standard defines a single-exchange negotiation procedure for establishing operational
characteristics when either one or more stations are capable of providing multiple selections.
This International Standard provides a means for interchanging the necessary information to establish, at a minimum, a
data link connection between two correspondents wishing to communicate. It describes the general purpose XID frame
information field content and format.
This International Standard defines encoding for information related to the basic HDLC standards only. Mechanisms
are provided to permit the general-purpose XID frame information field to be used to negotiate private parameters in a
single XID exchange simultaneously with negotiation of the defined basic parameters.
This International Standard does not limit or restrict the use of the XID frame information field from defining other
standard formats for use in specific applications.
The following are examples of potential uses of the XID command/response frame interchange:
a) Identification of the calling and called stations when using circuit switched networks (including switched
network backup applications).
b) Identification of stations operating on non-switched networks requiring identification at start-up.
c) The XID command frame with an individual, group or all-station address may be used to solicit XID response
frame(s) from other station(s) on the data link, prior to or following data link establishment.
d) Negotiation of the Frame Check Sequence (FCS) to be used for subsequent information interchange, by
stations that support both 16-bit FCS and 32&t FCS capabilities.
e) Convey higher layer information that may be required prior to data link establishment.
f) Transmission of an XID response frame at any respond opportunity to request an XID exchange to modify
some of the operational parameters (for example, window size) following data link establishment.
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@ ISO/IEC
ISO/IEC 8885 : 1993 (E)
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
International Standard. At the time of publication, the editions indicated were valid. All standards are subject to
revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of
applying the most recent editions of the standards indicated below. Members of IEC and IS0 maintain registers of
currently valid International Standards.
IS0 2382-9 : 1984, Data processing - Vocabulary - Part 09 : Data communication.
ISO/IEC 3309 : 1993, Information technology - Telecommunications and information exchange between systems -
High-level data link control (HDLC) procedures - Frame structure.
ISO/IEC 4335 : 1993, Information technology - Telecommunications and information exchange between systems -
High-level data link control (HDLC) procedures - Elements of procedures.
IS0 7478 : 1987, Information processing systems - Data communication - Multilink procedures.
IS0 7498: 1984 Information processing systems - Open Systems Irtterconnectioh - Basic I?eference Model.
IS0 8471 : 1987, Information processing systems - Data communication - High-level data link control balunced
classes of procedures - Data link layer address resolutionlnegotiation in switched emironments.
3 Definitions
For the purpose of this International Standard the definitions given in IS0 7498 : 1984, ISO/IEC 4335 : 1993 and IS0
7478 : 1987 as well as the following definitions apply:
.
the data link layer identity of each data
31 address resolution/negotiation . Procedure for exchanging/determining
link layer entity.
3.2 basic HDLC standards : ISO/IEC 3309, ISO/IEC 4335, IS0 7478, ISO/IEC 7809, and IS0 8471, defining
respectively the frame structure, elements of procedures, multilink procedures, and classes of procedures of HDLC,
and the address resolution procedures for switched connections.
3.3 data link connection : See IS0 7498 : 1984.
3.4 format identifier : See ISO/IEC 4335 : 1993.
3.5 group identifier : Classifier of data link layer characteristics or parameters function example,
resolution, parameter negotiation, user data).
3.6 HDLC-based protocol : A protocol which is a subset of the elements and classes of procedure and optional
functions defined in the basic HDLC standards, and adopted as a standard by IS0 or a recognized international
standards body (e.g., CCIIT).
3.7 layer parameter : The specification of data link layer characteris tics and parameters, and their values, available
or chosen.
3.8 private parameter : An implementation-specific data link layer parameter not defined in the basic HDLC
standards.
negotiation procedure : The initiating station indicates its “menu” of capabilities in its
3.9 single-exchange
menu in its response frame.
command frame, and the responding station indicates its choices from the
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0 ISO/IEC ISO/IEC 8885 : 1993 (E)
3.10 unique identifier : A unique bit/character sequence (for example, global telephone number, station
identification, or equivalent) associated with each station.
3.11 user data : The information obtained f&m or delivered to the user of the data link layer.
4 XID frame information field structure
The general purpose XID frame information field general structure is shown in figure 1.
First bit transmitted
12345678
Fl 0 1
Enaxiing of XID
User
Format Data link Data link
0 0 0
data
identifier layer
F A c Fcs F
hY=
subfield
subfield suMeld suMeld
11 XID information field
< z
XIDframe
Legmd:
F: Flag c: control
A: Address FCS: Frame Check Sequence
Figure 1 - General structure of the XID frame
The XID frame information field is composed of a number of subfields. These subfields are a Format Identifier (FI)
subfield, several data link layer subfields, and a user data subfield The amount of information (length) that can be
accommodated in the XID information field is limited only by the maximum length restrictions on the HDLC frame
information field.
4.1 Format identifier subfield
The format identifier (FI) subfield is defined in ISO/IEC 4335 : 1993. The list of standard format identifiers that are
registered is given in ISO/IEC TR 10178.
The FI subfield is a fixed length of one octet. It is encoded to have a capability of designating 128 different
standardized formats and 128 different user-defined formats.
This International Standard is concerned only with the general purpose format identifier, which defines the XID frame
information field content and format used by two correspondents wishing to communicate in an HDIC environment.
4.2 Data link layer subfields
The general structure of a data link layer subfield is illustrated in figure 2.
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ISO/IEC 8885 : 1993 (E)
Parameter field
Group Identifier (GI) Group Length (GL)
Data link layer subfield
Figure 2 - Data link layer subfield structure
The data link layer subfields specify various data link layer characteristics and parameters. The contents of these
subfields are generated by and consumed by the data link layer logic. The length of these subfields is limited only by
the maximum length restrictions on the HDLC frame information field, taking into account the lengths of the FI
subfield and the user data subfield.
NOTE - Actual system implementations may impose additional restrictions on the length of the XID frame.
In terms of figure 2, a data link layer subfield consists of
Group Identifier (1 octet),
Group Length (2 octets), and
Parameter field (n octets).
The Group Identifier (GI) identifies the function of that data link layer subfield. Four data link layer subfield identifiers
are defined:
Address resolution,
Parameter negotiation,
Multilink parameter negotiation, and
Private parameter negotiation.
The Parameter field consists of a series of Parameter Identifier (PI) (1 octet), Parameter Length (PL) (1 octet), and
Parameter Value (PV) (m octets) sets, one set for each defined data link layer subfield element. The structure of the
PI/PL/PV sets defined is detailed in clause 6.
A data link layer subfield, therefore, has the general organization depicted in figure 3.
.
GI GL PI PL PV me 0 PI PL
PV
c
Parameter field
Data link layer subfield
Figure 3 - A typical data link layer subfield
4.3 User data subfield
The user data subfield contains data link user information to be transferred during XID frame interchange. This data
link user information is transported transparently across the data link and passed to the user of the data link. The
amount of information (number of bits) that can be accommodated is limited only by the maximum length restrictions
on the HDIC frame information field, taking into account the lengths of the FI subfield and the data link layer
subfields.
NOTE - Actual system implementations may impose additional restrictions relative to octet orientation and/or the length of the
?CIDframe.
The user data subfield is composed of
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0 ISO/IEC ISO/IEC 8885 : 1993 (E)
User data identifier (1 octet), and
User data field (n bits).
The user data subfield, therefore, has the organization illustrated in figure 4.
User data identifier
I
Figure 4 - User data subfield
4.4 Summary
The entire XID frame information field has the general structure illustrated in figure 5.
User
User data
FI GI GL PI PL PV . . . PI PL PV . . . GI GL PI PL PV . PI PL pV
data
identifier
field
‘
.-~~~‘~~ _ “z,.lr- Userda~subfi~d~
<
XID information field
a
Figure 5 - XID information field structure
5 XID frame information field encoding
The general purpose format identifier subfield is always the first octet of the XID information field. The data link layer
subfields, if present, follow in ascending order according to their GI values. Except where noted, specific data link
layer subfields may appear only once in the standardized XID information field. The absence of a particular data link
layer subfield should be interpreted to mean that parameters within this subfield shall maintain their present values.
The user data subfield, if present, is always the last subfield of the XID information field.
5.1 Format identifier subfield encoding
The Format Identifier (FI) subfield is encoded as illustrated in figure 6.
First bit transmitted
I High Order bit
General purpose XID information
01000001
field identifier
Figure 6 - Format identifier subfield encoding

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ISO/IEC 8885 : 1993 (E) 63 ISO/IEC
5.2 Data link layer subfield encoding
5.2.1 Group identifier encoding
Group Identifiers identify various functions that pertain to the data link layer. Figure 7 indicates the GI encodings for
the data link layer subfields covered in this International Standard.
First bit transmitted
. .
c High Order blt
1 0 0 0 0 0 1 0 Address resolution identifier
0 0 0 0 0 0 0 1 Parameter negotiation identifier
0 0 0 1 0 0 0 Multilink parameter negotiation identifier
Private parameter negotiation identifier
0 0 0 0 1 1
Figure 7 - Data link layer subfield encodings
NOTES
1 The parameter negotiation data link layer subfield and the private parameter negotiation data link layer subfield may each
appear more than once in an XID information field. This allows a station to convey multiple menus of supportable parameters
through a single XID frame exchange.
allones(llll 1lll)isnotusedasadatalinklayer subfield encoding. All other unused GI encodings are
2 The GI encoding of
reserved for future use.
5.2.2 Group length encodings
Group Length indicates the length of the associated Parameter field. This length is expressed as a two-octet binary
number representing the length of the associated Parameter field in octets. The high or&r bits of the length value are
in the first of the two octets.
NOTE - The Group Length value does not include the lengths of either itself or its associated Group Identifier.
that all parameters within the
A Group Length value of zero indicates that there is no associated Parameter field and
subfield specified by the associated Group Identifier should assume their default values.
5.23 Parameter field encoding
A Parameter field contains a series of Parameter Identifier (PI), Parameter Length (PL) and Parameter Value (PV) set
structures in that order. Each PI identifies a parameter, and is one octet in length. Each PL indicates the length of the
associated parameter value (PV), and is one octet in length. Each PV contains the parameter value, and is m octets in
length.
NOTE - The value of PL does not include the lengths of either itself or its associated PI.
A PL value of
The value of PL is expressed as a one-octet binary number representing the length of the PV in octets.
PV is absent, and that the parameter shall assume the default value.
zero indicates that the associated
A PI/PL/PV set may be omitted if it is not required for conveying information or if present values for the parameter are
to be used. A Parameter field containing a PI that is not specified in this International Standard is defined as invalid
and shall be ignored (except within the private negotiation subfield, in which PIs other than PI=0 may be defined by a
prior agreement between the stations). Except where noted, duplicate PIs should not be sent within the same data link
layer subfield. The behavior of the receiver upon receipt of duplicate PIs within the same data link layer subfield is not
defined in this International Standard.
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The encoding of each PI/PL/PV set is detailed in clause 6.
5.3 User data subfield encoding
53.1 User data identifier encoding
The user data identifier identifies the subfield as the user data subfield. Figure 8 provides its encoding.
. First bit transmitted
m Kgh Order bit
User data identifier
1 1 1 1 1 1 1 1
Figure 8 - User data subfield encoding
53.2 User data field encoding
The user data field is transported transparently by the data link and passed on to the user of the data link. The encoding
of the user data field is the responsibility of the data link user, and may be any format that is mutually agreed upon by
the data link users involved.
6 Definition and encoding of data link layer subfield parameter fields
The following is a list of parameter field elements that are defined for the address resolution, parameter negotiation,
multilink parameter negotiation, and private parameter negotiation data link layer subfields.
The following legend explains the symbols used in tables 1,2,3,4, and 5.
: Parameter Identifier, expressed as a decimal value.
PI
PL : Parameter Length in octets, expressed as a decimal value.
: Indicates the field is bit encoded. When this bit position is “l”, the feature is present or supported by
E
the sender. When the bit position is “O”, the feature is absent or not suppor
...