iLink 3 uses Simple Binary Encoding (SBE) optimized for low latency of encoding and decoding while keeping bandwidth utilization reasonably small. All FIX semantics are supported. This encoding standard describes the wire protocol for iLink 3 messages and is complimentary to other FIX standards for session protocol and application-level behavior.

This topic describes implementation of SBE in submitting orders on CME Globex and receiving order entry responses.

Contents

Binary Type System

To support traditional FIX semantics, all documented field types are supported. The binary type system binds to native binary data types, and defines derived types as needed.

The binary type system has been enhanced to:

• Provide a means to specify precision of decimal numbers and timestamps, as well as valid ranges of numbers.
• Differentiate fixed-length character arrays from variable-length strings.
• Provide a consistent system of enumerations, Boolean switches, and multiple-choice fields.

Binary Encoding

Binary encoding provides direct data access without complex transformations or conditional logic by:

• Usage of native binary data types and simple types derived from native binaries, such as prices and timestamps.
• Fixed positions and fixed length fields, supporting direct access to data.

iLink 3 message schema supports the Simple Binary Encoding Specification - SBE version 1.0 release candidate 4

The default data ranges and null indicator are listed below for each integer encoding.

Encoding for null values

For charNULL primitive data type, null value is represented by:

For example:

  <enum name="CmtaGiveUpCD" encodingType="charNULL">

            <validValue name="GiveUp" description="Give Up">G</validValue>

            <validValue name="SGXoffset" description="SGX offset">S</validValue>

 </enum>

The null value of enumerations of data type char is represented by byte '0', not a character '0'.

Message Structure

This diagram shows the structure of an SBE encoded iLink 3 message.

Simple Open Framing Header (SOFH)

FIX Protocol Ltd. offers the Simple Open Framing Header standard for framing messages encoded with binary wire formats, such as Simple Binary Encoding. The framing header provides two features:

• An overall message length including headers to support framing.
• An identifier of the encoding used in the message payload. This supports selecting the correct decoder in the case where multiple message encodings are used on a session. It also aids tools such as protocol analyzers to identify message protocols contained in network packets.

The Simple Open Framing Header is four octets in length consisting of two fields, the Message_Length and Encoding_Type. The purpose of the Simple Open Framing Header will be to provide a simple mechanism to process messages from a stream and will always be encoded in little-endian byte order.

The Simple Open Framing Header is defined to contain the following information:

Message_Length Field

The Message_Length shall be defined to be the length in octets (i.e. bytes) of a message inclusive of the length of the Simple Open Framing Header.

The Message_Length field shall be the first field in the Simple Open Framing Header.

The Message_Length field shall be two octets in length (binary type uInt16), permitting a maximum message length of 2^16.

Encoding_Type Field

The Encoding_Type field shall be defined to be an integral enumeration.

The Encoding_Type field shall be the second field in the Simple Open Framing Header.

The values of encodingType used to indicate SBE payload is currently defined as: 

Please note that the encoding type is a reserved keyword represented as a hexadecimal digits in Little-Endian byte order.

This framing header differs from the official SOFH specification in these ways:

• Byte order is little-endian and not big-endian.
• Message length is 2 bytes and not 4 bytes.
• Encoding type is private user defined value outside designated range.

SBE Message Header

The purpose of the SBE header encoding header is to tell which message template was used to encode the message and to give information about the length of the message body to aid in decoding.

The fields of the SBE header are:

• Block length of the message root - the total space reserved for the root level of the message not counting any repeating groups or variable-length fields.
• Template ID - identifier of the message template.
• Schema ID - identifier of the message schema that contains the template.
• Schema version - the version of the message schema in which the message is defined.

SBE Message Body

The message body conveys the business information of the message and has the following attributes:

• Data only on the wire; without delimiters or metadata, such as tags.
• Access to data is positional, guided by a message schema that specifies a message type.

SBE has two features to control alignment of message elements:

• The length of a block is controlled with the blockLength attribute of a message or group. When applied to a message, it controls the length of the root level of the message, prior to any repeating groups or variable-length data. When applied to a group, it controls the length of each entry of the repeating group. The blockLength attribute is not required. By default, the length of a block is the sum of its field lengths. When specified, it must be at least that much, but can be greater. When greater than the field lengths, the extra space resides at the end of the block.
• The position of an individual field can be controlled with the offset attribute. Since individual field alignment is not considered crucial, this field attribute is optional. When not specified, the field is packed to the previous field without padding.

SBE Message Example

The following is an example of the schema used to interpret the iLink 3 New Order - Single message:

This standard describes how fields are encoded and the general structure of messages. The content of a message type is specified by a message schema. A message schema tells which fields belong to a message and their location within a message. Additionally, the metadata describes valid value ranges and information that need not be sent on the wire, such as constant values. For iLink 3 Simple Binary Encoding, message schemas are expressed as an XML template.Message Schema.

A FIX template corresponds to a FIX message type, and uniquely identifies an ordered collection of fields. The template also includes syntax indicating the type of field and transfer decoding to apply. A template is communicated between CME Group and client systems in XML syntax using the Simple Binary Encoding XML schema. The XML format is human- and machine-readable and can be used for authoring and storing FIX templates.

Each template has a unique Template ID that describes the format of the binary encoded message. A Template ID is present in every message to provide a reference to the correct template. The Template ID is unique and is included in every message header, allowing the client system to apply the correct schema to the message upon receiving it.

New Order Single Message

8000FECA740002020800000000E876481700000001000000CBA70D000101000000437563756D626572000000000000000000000000595A3733340000000000000000000000000000007B00000000000000DE02000000000000D7096EAB9B27BB15FFFFFFFFFFFFFF7F4D696E736B0000000000000000FFFF3200000000FFFFFF

The following is the translation of the New Order Single message:

Schema Distribution

iLink 3 schema file will be available for download via the SFTP site (sftpng.cmegroup.com), CME Globex network direct connection. This SFTP site contains the schema files for all environments. The SFTP site is a secure site that requires a user name and password for access.

Information applies as follows in the table:

• Environment - specific environment (i.e., Certification, New Release, Production).

  The AutoCert+ tool will indicate which environment you need to connect to for certification. For additional information on AutoCert+ access, refer to the AutoCert+ Access Guide.

• Service - the Schema services.

• SFTP Site - address of SFTP site.
• User Name - identifies the user name.
• Password - identifies the password.
• Directory Location - identifies the directory.
• Client System Update Schedule - Client systems should download updates according to the schedule specified.

In addition to the generic User Name/Password, client systems can connect using the same credential currently used for CME SFTP site. Additional information pertaining to the CME Secure SFTP site is available in CME Clearing Advisory Notice 15-105.

Schema Versioning

The schema.xml file is versioned each time an update is made. All elements in a message schema are of the same version. The first version of a schema is version zero, and the version number is incremented each time a schema is changed.

iLink 3 message schema is backward compatible. Customers can send messages using the preceding version of a schema, however CME Globex will only send messages using the latest version of a schema. 

Schema Header 

The following is the layout of the SBE Schema header:

<ns2:messageSchema id="8" xsi:schemaLocation="http://www.fixtradingcommunity.org/pg/file/fplpo/read/1196759/simple-binary-encoding-rc2xsd SimpleBinary-RC2.xsd" byteOrder="littleEndian"

description="20180225"

semanticVersion="FIX5.0"

version="0"

package="iLinkBinary"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ns2="http://www.fixprotocol.org/ns/simple/1.0">

Schema Format

The schema file is constructed of several sections including the following:

Primitive Type Encodings - Valid encoding specifications for each of the integer primitive types.

Simple Encodings - Definition of specific values.

Composite Encodings - Specifies the range of valid values for a decimal field.

Enumerations - Conveys a single choice of mutually exclusive valid values. 

Schema:

• Message Description 
  • ID - The Schema ID used identify the Template ID
  • Field Description - Field name
  • Name - FIX tag name
  • Semantic Type - FIX Message Type value for the message
  • Block Length - The length of the root level or the repeating group of the message in bytes of the field
• Field Descriptions 
  • ID - FIX tag
  • Field Description - Field name
  • Name - FIX tag name
  • Semantic Type - FIX semantic data type
  • Type - Binary encoding type
  • Offset - The number of bytes from the start of the message body or group to the first byte of the field
• Repeating Group Definitions 
  • ID - FIX Tag
  • Field Description - Field Name
  • Name - FIX tag name
  • Block Length - the length of the root level or the repeating group of the message in bytes
  • Dimension Type - Dimensions of the repeating group 

Template Extension

In the event that a message template is updated, the change can be made as an appended extension, which requires no immediate action on the part of the client, or a non-appended extension, which requires implementation of the new template.

Template extensions can be applied to the body and/or repeating groups of an SBE message. When a template is extended as an appended extension, customers can choose to continue using the prior template version or process the new data with the new schema version.

Customers who support template extension should verify any SBE schema update in New Release prior to a Production schema launch to ensure compatibility.  

For the body and repeating groups of SBE messages, appended template extension provides the following benefits:

• Templates are backward compatible
• Template ID changes are less frequent
• Expedited development time

In the event of an appended template extension, the following conditions will apply:

• Tags added to the body of the message will be added to the end of the body, before the repeating groups. In this scenario, the TemplateID will not change, but the schema version will change.
• The BlockLength field in the Binary Header or Group will increase in value to include the length of the new tag.
• Data fields can also be appended at the end of the repeating groups without updating the TemplateID, although the contents of the template and the schema version number will change. Each repeating group will include:
• the new length of each entry in the repeating group
• the number of entries in the repeating group

Example: Template Before Appended Extension

For the example below, the schema version is 1.

<ns2:message id="99" name="ExampleTemplate99" description="ExampleTemplate" blockLength="9" semanticType="X" >

<field name="Field1" id="9980" type="uInt64” description="ExampleField1" offset="0" semanticType="int" />

<field name="Field2" id="9981" type=" Field2Type" description="ExampleField2" offset="8" semanticType="char" />

<group name="NoMDEntries"  id="268" description="Number of entries in iLink message" blockLength="5" dimensionType="groupSize">

<field name="GroupField1" id="9990" type="Int32" description="ExampleGroupField1" offset="0" semanticType="int" />

<field name=" GroupField2" id="9991" type="GroupField2Type" description="ExampleGroupField2" offset="4" semanticType="char" />

</group>

</ns2:message>

Example: Template After Appended Extension

In the example below “ExampleField3” has been appended to the template 99 header. Additionally, the following updates were made to the schema:

• The schema version increases from 1 to 2.
• The header blockLength increases from 9 to 17.  Eight bytes are added to the blockLength due to Field3 having a type uInt64.

When client systems implement template extension in a decoder, the new field can be ignored and the location of the group can be determined within the message by leveraging the updated binary message header blockLength.    

<ns2:message id="99" name="ExampleTemplate99" description="ExampleTemplate" blockLength="17" semanticType="X" >

<field name="Field1" id="9980" type="uInt64” description="ExampleField1" offset="0" semanticType="int" />

<field name="Field2" id="9981" type=" Field2Type" description="ExampleField2" offset="8" semanticType="char" />

<field name="Field3" description="ExampleField3" id="9982" semanticType="int" offset="9" sinceVersion="2"  type="uInt64"/>

<group name="NoMDEntries"  id="268" description="Number of entries in iLink message" blockLength="5" dimensionType="groupSize">

<field name="GroupField1" id="9990" type="Int32" description="ExampleGroupField1" offset="0" semanticType="int" />

<field name=" GroupField2" id="9991" type="GroupField2Type" description="ExampleGroupField2" offset="4" semanticType="char" />

</group>

</ns2:message>

For a non-appended template extension, the template is not backward-compatible and the new template must be implemented. Examples of a non-appended extension include:

• fields inserted or removed from the middle of a block (message root or repeating group entry)
• encoding changes to existing fields, i.e. different data type or different size integer
• repeating group removed or inserted in the middle of a message

For both appended and non-appended message changes, customers will be notified of the schema file version change to indicate new information in the schema file.