Financial Information eXchange (FIX) Protocol

FIX defines a session layer (message framing, sequencing, resend handling) and an application layer (a large, versioned dictionary of tagged fields, e.g. 55=Symbol, and messages built from ordered fields/repeating groups/components). It was originally exchanged as a tag=value wire format (8=FIX.4.4|9…|35=W|…|10=…=, pipe/SOH delimited); later versions can also be carried over FIXML (an XML encoding of the same fields) or the newer Simple Binary Encoding (SBE) for low-latency use. Governance is committee-based (the FIX Trading Community, a not-for-profit industry body), and the specification is open and free to implement without a license fee — in this respect FIX resembles MDDL far more than it resembles RIC or the Bloomberg ticker.

• FIX 2.7–4.2 (1992–2000): early releases, incrementally adding message coverage; 4.2 became a long-lived baseline for order/exec flow.
• FIX 4.3 introduced the dedicated Market Data message family (MarketDataRequest=/=MarketDataSnapshotFullRefresh=/ =MarketDataIncrementalRefresh=/=MarketDataRequestReject) as a first-class part of the dictionary.
• FIX 4.4 (2003) is, per the retrieved dictionary, the version that remained most widely deployed for market data and order flow for many years; it is the baseline most third-party FIX engines (including QuickFIX, this document's source for the retrieved dictionaries) ship as their default.
• FIX 5.0 (2006) and FIX 5.0 Service Pack 1/2 (2009/2011) split the session and application layers apart (allowing FIX's application messages to run over transports other than the classic FIX session protocol) and added MarketDataStatisticsRequest/Report and MarketDataReport to the Market Data family.
• FIX Latest (from 2019): the dictionary is no longer released as a single numbered version; instead it is maintained continuously and extended via versioned "extension packs" documented through the Orchestra machine-readable format and browsable at fiximate.fixtrading.org.

• MarketDataRequest (35=V): a subscriber's request for a snapshot and/or ongoing updates for one or more instruments, specifying which =MDEntryType=s it wants (bid, offer, trade, …) via a repeating group.
• MarketDataSnapshotFullRefresh (35=W): a full snapshot of the requested market data for one instrument — the Instrument component (symbology) plus an MDFullGrp repeating group of NoMDEntries entries, each an =MDEntryType=/=MDEntryPx=/ =MDEntrySize=/=MDEntryTime=+ tuple.
• MarketDataIncrementalRefresh (35=X): incremental updates to a previously established view, via an MDIncGrp repeating group whose entries carry an MDUpdateAction (NEW=/=CHANGE=/=DELETE) alongside the same entry fields as the full refresh.
• MarketDataRequestReject (35=Y): rejects a MarketDataRequest (e.g. unknown symbol, unsupported MDEntryType), with a MDReqRejReason code.
• MarketDataStatisticsRequest=/=Report and MarketDataReport (FIX 5.0 SP2+): later additions covering statistical market data and a more general market data reporting message.

MDEntryType (tag 269) is itself a controlled vocabulary spanning far more than a simple bid/offer: BID, OFFER, TRADE, INDEX_VALUE, OPENING_PRICE, CLOSING_PRICE, SETTLEMENT_PRICE, TRADING_SESSION_HIGH/LOW_PRICE, TRADING_SESSION_VWAP_PRICE, IMBALANCE, TRADE_VOLUME, OPEN_INTEREST, and others — a single field distinguishing many of the same "what kind of market data point is this" cases MDDL's per-domain properties and ORE's metric segment each handle differently.

Rather than defining its own competing short-string notation, FIX identifies an instrument via the Instrument component's Symbol=/ =SecurityID fields tagged with a SecurityIDSource (tag 22) enum naming which external scheme the accompanying code belongs to. Per the retrieved FIX 4.4 dictionary, the enum includes (among others): CUSIP, SEDOL, ISIN_NUMBER, RIC_CODE, ISO_CURRENCY_CODE, ISO_COUNTRY_CODE, EXCHANGE_SYMBOL, BLOOMBERG_SYMBOL, and ISDA_FP_ML_SPECIFICATION. This is architecturally the same move MDDL makes with its instrumentIdentifier/codeType — RIC and the Bloomberg symbol are both explicitly named, first-class members of FIX's own symbology vocabulary, not competitors to it.

FIX remains the dominant open standard for pre-trade, trade, and post-trade messaging across the industry and continues to be actively maintained by the FIX Trading Community as "FIX Latest" — unlike MDDL, it was never wound down or contributed elsewhere; if anything, per Market Data Definition Language (MDDL), MDDL's own vocabulary was reportedly folded into FIX (and ISO 20022) after FISD stopped releasing new MDDL versions.

The authoritative, browsable specification is published by the FIX Trading Community at fiximate.fixtrading.org (the "FIX Repository"). Unlike mddl.org, this is a live, actively maintained site, but it does not offer a single downloadable machine-readable dictionary file the way mddl.org's Downloads page did — its content is served through a browsing UI. The XML data dictionaries retrieved for this document were instead sourced from the QuickFIX open-source FIX engine project, which redistributes the same field/message definitions as machine-readable XML under a permissive license; see external/fix/methodology.txt for the exact retrieval steps and external/fix/manifest.json for per-file source URLs and checksums. Files retrieved:

• FIX44.xml — the FIX 4.4 data dictionary (historically dominant for market data and order flow).
• FIX50SP2.xml — the FIX 5.0 Service Pack 2 data dictionary (the last numbered release before "FIX Latest").
• LICENSE — the QuickFIX Software License governing this redistributed copy.