Logon (MsgType A)
Logon is the FIX message with MsgType A, defined in FIX 4.4 and FIX 4.2. It is a session-level (admin) message: it manages the connection itself rather than carrying a business event. Its body has 13 top-level members once the standard header and trailer are counted.
A is Logon, but a is a different message entirely — QuoteStatusRequest. Twenty-three FIX message types differ from another only by letter case.A real Logon
| Tag | Field | Wire value | Meaning |
|---|---|---|---|
| 8 | BeginString | FIX.4.4 | |
| 9 | BodyLength | 74 | |
| 35 | MsgType | A | Logon |
| 49 | SenderCompID | BOARTEAM | |
| 56 | TargetCompID | COUNTERPARTY | |
| 34 | MsgSeqNum | 2 | |
| 52 | SendingTime | 20240101-12:00:00.000 | |
| 98 | EncryptMethod | 0 | None / other |
| 108 | HeartBtInt | 1 | |
| 10 | CheckSum | 168 |
Shown with | separators for readability. On the wire FIX uses SOH (0x01), an invisible control byte — and because BodyLength and CheckSum are computed over the actual bytes, the two forms have different checksums. Both are valid and both decode here.
Generated from the FIX 4.4 dictionary and verified at build time: it parses and validates with no issues. Open the decoder to try your own message.
Message body — FIX 4.4
Components are listed by name and link to their own page rather than being expanded inline; repeating groups are expanded, including where they nest.
- —Standard Message HeadercomponentRequired
- 98EncryptMethodRequired
- 108HeartBtIntRequired
- 95RawDataLengthConditional
- 96RawDataConditional
- 141ResetSeqNumFlagOptional
- 789NextExpectedMsgSeqNumOptional
- 383MaxMessageSizeOptional
- 384NoMsgTypesrepeating groupOptional
- 372RefMsgTypeConditional
- 385MsgDirectionConditional
- 464TestMessageIndicatorOptional
- 553UsernameOptional
- 554PasswordOptional
- —Standard Message TrailercomponentRequired
Message body — FIX 4.2
Components are listed by name and link to their own page rather than being expanded inline; repeating groups are expanded, including where they nest.
- —Standard Message HeadercomponentRequired
- 98EncryptMethodRequired
- 108HeartBtIntRequired
- 95RawDataLengthOptional
- 96RawDataOptional
- 141ResetSeqNumFlagOptional
- 383MaxMessageSizeOptional
- 384NoMsgTypesrepeating groupOptional
- 372RefMsgTypeOptional
- 385MsgDirectionOptional
- —Standard Message TrailercomponentRequired
What changed between the dialects
Present in FIX 4.4 only (4)
tag 789, tag 464, tag 553, tag 554
Handle Logon in TypeScript
import { createFixEngine } from "@boarteam/fix";import { dictionary } from "@boarteam/fix-dict-fix44";const fix = createFixEngine(dictionary);const { message, issues } = fix.parse(raw);if (message.msgType === "A") { message.name; // "Logon" // fields are keyed by tag; repeating groups live on message.groups}// Problems come back as data, never as exceptions:issues; // FixIssue[]fix.validate(message); // dictionary-level problemsDecode this in your own code
The same engine that produced the decoded example above is an Apache-2.0 npm package with zero runtime dependencies. It runs in Node and in the browser, and parse() returns problems as data instead of throwing.
npm i @boarteam/fix @boarteam/fix-dict-fix44