The Financial Information eXchange session protocol is used to provide reliable and recoverable messaging for electronic trading. The protocol is intended for use by asset managers, trading firms, brokerages, trading venues, clearing houses, custodians, depositories, asset servicers, among others involved in the trading life cycle activities of a wide range of financial instruments. The FIX session layer functionality is a realization of the ISO/IEC 7498-1:1994 Open System Interconnection basic reference model level 5 session layer.

A FIX session is a bidirectional stream of ordered messages between two peers within a continuous sequence number series beginning with 1. A single FIX session can exist across multiple sequential (not concurrent) FIX connections, which means that peers may intentionally or unintentionally connect and disconnect multiple times while maintaining a single FIX session. The FIX session can be thought of as a bi-directional durable session sharing characteristics of the guaranteed delivery and durable subscriber enterprise integration architecture patterns.

A FIX connection consists of three parts: logon process, message exchange (inclusive of resynchronization of state), and possible logout process over a transport layer connection. A FIX connection may be concluded by the unrecoverable loss of the transport layer, a system failure, or an application failure.

With the release of FIX Protocol Version 5.0 the session layer specification was separated from the application layer specification (aka Transport Independence). “FIXT” was the label used to identify the separate session layer specification and to identify the FIX Session version being used. The FIXT session protocol is now fully incorporated into the the FIX Session Protocol as the FIXT Session Profile.

Documents

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FIX Session Layer - Technical Standard 1.09 MB 13 downloads June 23, 2020

Technical specification document - June 2020 ...
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FIX Session Layer - Test Cases 84.64 KB 10 downloads June 23, 2020

Set of mandatory and optional compliance tests. V1.0 - June 2020 ...
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    FIX Transport (FIXT) V1.1 - Technical Standard 580.00 KB 705 downloads July 23, 2017

    Technical specification document. ...