Financial services — Financial information eXchange session layer — Part 1: FIX tagvalue encoding

This document provides the normative specification of the FIX tagvalue encoding, which is one of the possible syntaxes for FIX messages.

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Financial services — Financial
information eXchange session layer —
Part 1:
FIX tagvalue encoding
Reference number
ISO 3531-1:2022(E)
© ISO 2022

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ISO 3531-1:2022(E)
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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  © ISO 2022 – All rights reserved

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ISO 3531-1:2022(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 FIX tagvalue message syntax . 2
4.1 Character encoding . 2
4.2 Field syntax . 2
4.2.1 Tag (field identifier) . 2
4.2.2 Tag delimiter . 2
4.2.3 Field value . 2
4.2.4 Field delimiter . 2
4.2.5 Well-formed field. 2
4.2.6 Example of a FIX tagvalue message . 3
4.3 Message structure . 3
4.3.1 General . 3
4.3.2 Message type . 3
4.3.3 Field presence . 3
4.3.4 Field sequence . . 3
4.3.5 Message delimiter . 4
4.3.6 Components . 4
4.3.7 Groups and repeating groups . . 4
4.3.8 Encoded data fields . 6
5 Standard header and trailer .7
5.1 General . 7
5.2 Standard header . 7
5.2.1 General . 7
5.2.2 Body length calculation . 7
5.2.3 Standard header definition . 7
5.3 Standard trailer . 8
5.3.1 Standard trailer definition . 8
5.3.2 Checksum . 8
6 FIX tagvalue datatypes.8
6.1 General . 8
6.2 Value space . 8
6.3 Lexical space . 8
6.3.1 General . 8
6.3.2 Character encoding . 9
6.3.3 Lexical encoding for FIX datatypes . 9
6.3.4 XML data .15
7 Code sets .15
7.1 General . 15
7.2 Underlying value type .15
7.2.1 General .15
7.2.2 Internal code sets .15
7.2.3 External code sets .15
Annex A (informative) Checksum calculation .16
Bibliography .17
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ISO 3531-1:2022(E)
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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committee has been established has the right to be represented on that committee. International
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
This document was prepared by FIX Trading Community (as FIX Session Layer Technical Specification)
and drafted in accordance with its editorial rules. It was assigned to Technical Committee ISO/TC 68,
Financial services, Subcommittee SC 9, Information exchange for financial services, and adopted under
the “fast-track procedure”.
A list of all parts in the ISO 3531 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at
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ISO 3531-1:2022(E)
FIX session protocol was written to be independent of any specific communications protocol (e.g. X.25,
async, TCP/IP) or physical medium (e.g. copper, fibre, satellite) chosen for electronic data delivery. It
offers a reliable stream where a message is delivered once and in order. The FIX session layer is designed
to survive and resume operation in the event of the loss of transport level connections caused by any
type of failure, including network outage, application failure or computer hardware failures.
The session layer is concerned with the ordered delivery of data while the application level defines
business-related data content. This document focuses on the ordered delivery of data using the “FIX
session protocol”.
The FIX session protocol is implemented using the FIX tagvalue encoding syntax for the standard
header, standard trailer and the session level messages which make up the FIX session protocol. It is
possible to send messages encoded using other FIX-defined encodings (e.g. FIXML, SBE, JSON, GPB,
ASN.1) or other non-FIX-defined encodings (e.g. XML, FpML, ISO 20022 XML, JSON).
The Financial Information eXchange tagvalue encoding is the original encoding used for FIX messages.
The tagvalue encoding is the encoding used by the FIX session layer; it corresponds to the Presentation
Layer of the ISO Open Systems Interconnection model. The encoding uses an integer number known as
a tag to identify the field, followed by the “=” character (hexadecimal 0x3D), then the value of that field
encoded in the ISO/IEC 8859-1 character set. Each tagvalue pair is separated by the Start of Heading
control character (hexadecimal value 0x01), which is defined by ISO/IEC 6429. The tagvalue
encoding also supports the encoding of binary and multibyte character data in certain encoded data
fields that are preceded by a Length field.
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Financial services — Financial information eXchange
session layer —
Part 1:
FIX tagvalue encoding
1 Scope
This document provides the normative specification of the FIX tagvalue encoding, which is one of the
possible syntaxes for FIX messages.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC 11404, Information technology — General-Purpose Datatypes (GPD)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO/IEC 11404 and the following
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
field presence
existence or use of a field within a message
Note 1 to entry: FIX specifications and rules of engagement based on FIX should refer to a field as being present
in a message.
component presence
existence or use of a component within a message
Note 1 to entry: FIX specifications and rules of engagement based on FIX should refer to a component as being
present in a message.
repeating group instance
specific record of the group within a repeating group
Note 1 to entry: Records are defined in ISO/IEC 11404.
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ISO 3531-1:2022(E)
character digit
character representation of a number, 0 to 9, in the character set used for encoding
Note 1 to entry: Characters 0x30 to 0x39 in the Latin alphabet No. 1 character set (ISO/IEC 8859-1).
4 FIX tagvalue message syntax
4.1 Character encoding
With the exception of datatype data, tagvalue encoding uses a single-byte character set. By default, the
encoding is ISO/IEC 8859-1, Latin alphabet No. 1.
By counterparty agreement, a different single-byte character set may be used.
Note that the Latin-1 alphabet is an 8-bit code but reserves two ranges for control codes. Message
structure is supplemented by ISO/IEC 6429 control character set C0.
4.2 Field syntax
4.2.1 Tag (field identifier)
Each field is uniquely identified by an integer, known as a tag. Tags must be unique among both session
and application message fields. (Fields in the standard header and standard trailer components are
shared by session and application messages.)
Tags are serialized according to the syntax of the TagNum datatype.
4.2.2 Tag delimiter
A tag is delimited from its field value by the equals sign (=), character value 61 (decimal).
4.2.3 Field value
Field values are serialized according to their FIX datatype syntax.
4.2.4 Field delimiter
All fields in a FIX message, including those of datatype data, must be terminated by a delimiter
character. The Start of Heading control character, value 0x01, referred to in this document as , is
used for field termination.
There must be no embedded characters within field values except for those of datatype data.
4.2.5 Well-formed field
A well-formed field has the form:
A field shall be considered malformed if any of the following occurs as a result of encoding:
• the tag is empty;
• the tag delimiter is missing;
• the value is empty;
• the value contains an character and the datatype of the field is not data or XMLdata;
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ISO 3531-1:2022(E)
• the datatype of the field is data and the field is not immediately preceded by its associated Length
4.2.6 Example of a FIX tagvalue message
The following is a FIX 4.2 NewOrderSingle(35=D) message in classic tagvalue pair format:
4.3 Message structure
4.3.1 General
A FIX message is a collection of fields that begins with the BeginString(8) field, followed by the
BodyLength(9) field, then the MsgType(35) field, and ends with the Checksum(10) field. The message is
identified by the value provided in the MsgType(35) field.
This clause summarizes general specifications for constructing messages in tagvalue syntax.
The general format of a message is a standard header followed by the message body fields and
terminated with a standard trailer.
Each message is constructed of a stream of tag=value fields with a field delimiter between fields in
the stream. Messages will be referenced as message_name(35=x) with x representing the message type;
fields will be referenced as field_name(tag).
4.3.2 Message type
The MsgType(35) field is used to identify the type of message encoded. The definition and scope of the
message type is provided by the encoder. For example, the FIX session layer standard defines a set of
messages to initiate and manage a FIX session. The FIX application layer standard (commonly referred
to as FIX Latest) defines additional message types for business level processing. There are no message
types or reserved values for message types defined at the encoding level.
4.3.3 Field presence
In a message definition, a field must be specified as either required, optional or conditionally required.
If it is conditionally required, the message specification must give a clear rule for when the field must
be present.
All fields present in an encoded message must have a value. Optional fields without values must be
omitted from the FIX message.
A tag (field) must appear at most once in a message, except when the tag appears within a repeating
A tag (field) must appear at most once per repeating group instance.
4.3.4 Field sequence
Except where noted, fields within a message can be defined in any sequence. (Relative position of a field
within a message is inconsequential.) The exceptions to this rule are:
• General message format is composed of the standard header, followed by the body, followed by the
standard trailer.
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ISO 3531-1:2022(E)
• The first three fields in the StandardHeader component must be BeginString(8), followed by
BodyLength(9), followed by MsgType(35), in that sequence.
• The last field in the standard trailer must be CheckSum(10).
• Within a repeating group, field sequence is strictly defined by a group definition.
4.3.5 Message delimiter
Messages are effectively delimited by the character at the end of the CheckSum(10) field.
All messages must begin with the BeginString(8) field and terminate with the CheckSum(10) field.
4.3.6 Components
Application level messages, representing the FIX application layer, can organize a collection of fields
into a set commonly referred to as a component or a submessage. These components can contain sub-
The FIX tagvalue encoding does not represent component boundaries in the encoding. Any component
boundary is lost during encoding. Further, FIX tagvalue encoding does not require the collection of
fields to be ordered and does not enforce component boundaries around the fields in the encoding.
4.3.7 Groups and repeating groups General
In ISO/IEC 11404, a FIX repeating group is an array of records. An instance of a repeated record is
called a repeating group instance in this document.
It is permissible for fields to be repeated within a repeating group. For example, the following represents
a repeating group with two repeating instances delimited by tag 372 (first field in the repeating group):
384=2372=6385=R372=7385=R Repeating group name
It is recommended that a repeating group be named XXXGrp, e.g. DividendPeriodGrp. NumInGroup field
In tagvalue encoding, repeating group instances are preceded by a count of the number of instances to
follow. The count is serialized as a FIX field with a value of datatype NumInGroup, commonly referred to
as a NumInGroup field.
It is recommended that NumInGroup fields be named NoXXX, e.g. NoContraBrokers(382). Field sequence within a repeating group
• The NumInGroup field [e.g. NoTradingSessions(386), NoAllocs(78)], which specifies the number of
repeating group instances, occurs once for a repeating group and must immediately precede the
repeating group instances.
• Fields within repeating groups must be specified in the order that the fields are specified in the
message definition.
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ISO 3531-1:2022(E) Field presence within a repeating group
• The NumInGroup field is required and must be larger than zero if the repeating group is required, or
if the repeating group is optional and the message contains one or more instances for that repeating
• If a repeating group field is specified as required, then it must appear in every instance of that
repeating group.
• If a repeating group is used in a message, its first field (after the NumInGroup field) must be
populated in each instance of the repeating group. This allows implementations of the protocol to
use the first field as the indicator for the start of a new instance within the repeating group.
• The first field listed after the NumInGroup field may be a component or nested repeating group. In
this case, the first field is defined as the first field of the component or the NumInGroup field of the
nested repeating group. The component or nested repeating group becomes necessary for every
instance of the outer repeating group.
• The presence of optional or conditionally required fields may vary across repeating group instances. Nested repeating groups
Repeating groups may be nested within another repeating group. Multiple levels of nesting are allowed.
In an encoded message, nested repeating groups are serialized as a depth-first tree traversal. That is, all
instances of a nested group of the first top-level group instance are encoded before the second instance
of the top-level group, and so forth.
Nesting level Tag Field name Notes
Start Level 1 453 NoPartyIDs This repeating group is the Parties component in the FIX
448 > PartyID Must always be the first field in the repeating group, and must
be provided if NoPartyIDs(453) > 0.
447 > PartyIDSource Required if NoPartyIDs(453) > 0.
452 > PartyRole Required if NoPartyIDs(453) > 0.
2376 > PartyRoleQualifier Optional; not required for each repeating group instance.
Start Level 2 802 > NoPartySubIDs This nested repeating group is the PtysSubGrp component in
the FIX Standard.
523 > > PartySubID Required if NoPartySubIDs(802) > 0.
803 > > PartySubIDType Required if NoPartySubIDs(802) > 0.
End Level 2
End Level 1 Nested repeating group example
The following is an example of a Parties repeating group with three instances, two of which contain
nested PtysSubGrp repeating groups. This example also demonstrates that repeating group instances
may be heterogeneous, meaning that the fields present in an instance can vary across instances.
  PartyIDSource(447)=B     (Bank Identifier Code (BIC) ISO 9362)
  PartyRole(452)=1       (Executing Firm)
     PartySubIDType(803)=10 (Securities account number)
  PartyIDSource(447)=H     (CSD Participant Number)
  PartyRole(452)=83      (Clearing Account)
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ISO 3531-1:2022(E)
  PartyIDSource(447)=B     (Bank Identifier Code (BIC) ISO 9362)
  PartyRole(452)=4       (Clearing Firm)
  PartyRoleQualifier(2376)=23 (Firm or legal entity)
     PartySubIDType(803)=10 (Securities account number)
This example is encoded in FIX tagvalue format as follows:
4.3.8 Encoded data fields General
Tagvalue encoding provides features for embedding fields in any IANA-registered encoding, possibly
using multibyte character sets. Such fields must be preceded by an associated Length field that specifies
the number of octets in the encoded data field. MessageEncoding field
MessageEncoding(347) is a field in the StandardHeader component that gives the name of an encoding
used in a message. Examples of using encoded data fields for Japanese language support
Example 1 — Specify the English value as Issuer plus Japanese character set as EncodedIssuer(349)
Tag Field name Value
…Other standard header fields
347 MessageEncoding Shift_JIS
…Other standard header fields
…Other message body fields
106 Issuer HITACHI
348 EncodedIssuerLen 10
349 EncodedIssuer 日立製作所
…Other message body fields
Example 2 — Specify the English value as Issuer(106) plus Japanese character set as EncodedIssuer(349).
Specify the English value as Text(58) plus Japanese character set as EncodedText(357).
Tag Field name Value
…Other standard header fields
347 MessageEncoding Shift_JIS
…Other standard header fields
…Other message body fields
106 Issuer HITACHI
348 EncodedIssuerLen 10
349 EncodedIssuer 日立製作所
…Other message body fields
58 Text This is a test
356 EncodedTextLen 17
357 EncodedText これはテストです
…Other message body fields
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ISO 3531-1:2022(E) Precaution when using multibyte encodings
FIX tagvalue encoding processors must use the Length field associated with the encoded data field
when parsing encoded data fields to avoid field truncation and subsequent decoding errors. There is
the possibility that one of the octets in a multibyte encoded data field contains the 0x01 value, which
can be interpreted by message parsers as the field delimiter if the associated Length field is not
5 Standard header and trailer
5.1 General
Fields specified in the standard header and trailer serve as delimiters of

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