y FIX needs session seq# over TCP seq#

My friend Alan said … Suppose your FIX process crashed or lost power, and reloads from disk the last sequence received. It would then receive a live seq # higher than expected. CME documentation states:

… a given system, upon detecting a higher than expected message sequence number from its counterparty, requests a range of ordered messages resent from the counterparty.

Major difference from TCP sequence number — FIX has no Ack. See Ack in FIX^TCP

— Sequence number reset policy:

After a logout, sequence numbers is supposed to reset to 1, but if connection is terminated ‘non-gracefully’ sequence numbers will continue when the session is restored. In fact a lot of service providers never reset sequence numbers during the day. There are also some, who reset sequence numbers once per week, regardless of logout.

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