Financial Information EXchange - Technical Specification

Technical Specification

The FIX protocol is a technical specification for electronic communication of trade-related messages. It is a self-describing protocol in many ways similar to other self-describing protocols such as the newer XML; XML representation of Business content of FIX messages is known as FIXML but FIXML is not widely deployed.

FIX Session is layered on TCP (Transmission Control Protocol). FIX is more popular than FIXML in Securities Trading because FIXML messages are much larger in size due to XML tags.

FIX messages are formed from a number of fields, each field is a tag value pairing that is separated from the next field by a delimiter SOH (0x01). The TAG is a string representation of an integer that indicates the meaning of the field. The value is an array of bytes that hold a specific meaning for the particular TAG (e.g. TAG 48 is securityID, a string that identifies the security; TAG 22 is IDSource, an integer that indicates the identifier class being used). The values may be in plain text or encoded as pure binary (in which case the value is preceded by a length field). The FIX protocol defines meanings for most TAGs, but leaves a range of TAGs reserved for private use between consenting parties.

The FIX protocol also defines sets of fields that make a particular message, within the set of fields some will be mandatory and others optional. The ordering of fields within the message is generally unimportant, however repeating groups are preceded by a count and encrypted fields are preceded by their length. The message is broken into three distinct sections: the head, body and tail. Fields must remain within the correct section and within each section the position may be important as fields can act as delimiters that stop one message from running into the next - the final field in any FIX message is TAG 10 (checksum).

There are two main groups of messages - admin and application. The admin messages handle the basics of a FIX session. They allow for a session to be started and terminated and for recovery of missed messages. The application messages deal with the sending and receiving of trade-related information such as an order request or information on the current state and subsequent execution of that order.