FIX Tag 34 – MsgSeqNum

MsgSeqNum (Message Sequence Number) is an incremental sequence number for each message in a FIX session. It starts at 1 and increments by 1 for each message sent. Sequence numbers detect message loss, duplicates, and out-of-order delivery.

Correct Usage

Increment MsgSeqNum by 1 for each message sent. Persist sequence numbers to disk so they can be recovered after application restart. On session reset (ResetSeqNumFlag=Y), start sequence at 1. Both parties maintain separate sequence numbers (inbound and outbound).

Common Issues

1) Sequence gaps indicate message loss, 2) Duplicate sequence numbers suggest retransmission or bugs, 3) Sequence not persisted causes gaps after restart, 4) Sequence reset not synchronized between parties, 5) Out-of-order messages detected via sequence.

Example FIX Message

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