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Session Acceptor is the receiving party of the FIX session. It listens for the incoming connection on the pre-defined port. The Acceptor has a responsibility to perform first level authentication and formally declares the connection request "accepted" through transmission of an acknowledgment Logon message.

Conditionally required fields

Required fields are based on the presence or value of other fields.

Delimiter character

The non-printing, ASCII "SOH" (\u0001, hex: 0x01, referred to in this document as <SOH>), is used for field termination.

FIX Connection

It is comprised of three iterations: logon, message exchange, and logout.

FIX Dialect

A customised, specialised or extended implementation of the base FIX protocol standards.

FIX Session

It is comprised of one or more FIX Connections, which mean that a FIX Session spans multiple logins.


The Financial Information Exchange (FIX) Protocol is a message standard, that is developed to facilitate the electronic exchange of information related to securities transactions. It is intended for use between trading partners who have a wish to automate communications.


Session Initiator establishes the telecommunications link and initiates the session via the transmission of the initial Logon message.

MsgSeqNum field

Integer message sequence number, the tag number is 34.

Required Fields

Each message within the FIX protocol is comprised of the required, optional and conditionally required fields (the required fields are based on the presence or value of other fields). Systems should be designed to operate, when only the required and conditionally required fields are present.

SenderCompID field

Assigned value used to identify firm sending message, the tag number is 49.

TargetCompID field

Assigned value used to identify receiving firm, the tag number is 56.

User-defined fields

These fields are intended to be implemented between consenting trading partners on bi or mult-lateral basis. They should be used with caution to avoid the conflicts which may arise if multiple parties implement the same field tags in different contexts. The tag numbers 5000 to 9999 have been reserved for them. These tags can be registered/reserved via the FIX web site.

User-defined messages

Messages of this format is privately defined between the sender and receiver. A "U" as the first character in the MsgType field (i.e. U1, U2, etc) indicates such messages.