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This document details the client interface to tZERO via FIX 4.4 protocol. While the majority of the tZERO FIX 4.4 specification follows the FIX 4.4 protocol as outlined at fixprotocol.org, all customizations are noted here. Where no comments exist in this document, apply the comments and values from the FIX 4.4 document as found on fixprotocol.org.
Communications Protocol
tZERO supports TCP/IP only
Connection and Logon
tZERO can act as either the initiator or the acceptor of the FIX session, depending on the customer's preference.
Encryption
tZERO does not support encryption of FIX messages.
Authentication
Session Level – CompIDs must match as agreed upon Application Layer – SenderSubID and Account (if provided) must be valid
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| 8 | BeginString | Y | Value is always "FIX.4.4" (Must be first field in message) |
| 9 | BodyLength | Y | (Must be second field in message) |
| 35 | MsgType | Y | (Must be third field in message) |
| 49 | SenderCompID | Y | Assigned by tZERO |
| 56 | TargetCompID | Y | Assigned by tZERO |
| 34 | MsgSeqNum | Y | |
| 57 | TargetSubID | N | Assigned by tZERO |
| 97 | PossResend | N | Indicates that message may contain information that has been sent under another sequence number. Value Description Y Possible resend N Original transmission Required when message may be duplicate of another message sent under a different sequence number. |
| 52 | SendingTime | Y | Time of message transmission (always expressed in UTC, also known as "GMT"). |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| 10 | Checksum | Y | (Must be last field in message) |
Session-level messages used to manage the FIX connection lifecycle.
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 0 | |
| 112 | TestReqID | N | Required when Heartbeat is the result of a Test Request message. The value must then be the TestReqID of the causal Test Request message. |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = A | |
| 98 | EncryptMethod | Y | Encryption method 0 (none) is currently supported. |
| 108 | HeartBtInt | Y | The initiator of the session dictates the Heartbeat Interval (in seconds). The acceptor must comply. |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 1 | |
| 112 | TestReqID | Y | |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 2 | |
| 7 | BeginSeqNo | Y | |
| 16 | EndSeqNo | Y | |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 3 | |
| 45 | RefSeqNum | Y | |
| 58 | Text | N | Explanation for reject, when possible |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 4 | |
| 123 | GapFillFlag | N | |
| 36 | NewSeqNo | Y | |
| Trailer | Y |
| Tag # | Field Name | Req'd | Comments |
|---|---|---|---|
| Header | Y | MsgType = 5 | |
| 58 | Text | N | |
| Trailer | Y |
tZERO Technologies · FIX 4.4 DROPCOPY Specification · December 2025