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Resend Request logic is the FIX session mechanism by which 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. The counterparty resends the requested message range sequentially until both systems are back in sequence.   

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• A newly detected message sequence gap must be distinguished from a continuation of a gap and a new gap versus a duplicate Resend Request must be issued accordingly.
• When a message sequence number gap is newly detected (the sequence number received is greater than expected), issue a new Resend Request on its own sequence number. The requested sequence number range must be the next expected sequence number to infinity.
• For each subsequent message that constitutes a continuation of the gap, issue a duplicate Resend Request. This message must be sent on the same message sequence number of the original Resend Request, tag 43-PossDupFlag must be set to 'Y' and tag 122-OrigSendingTime must not be sent.
• The FIX Engine should detect receipt of the first message sent in response to a previous Resend Request and discontinue issuing the Resend Requests marked PossDup.
• The FIX Engine should issue a new Resend Request, if needed, only after the receipt of the counterparty's response has been completed.
• For resend requests from the Client System to CME Globex, both basic and enhanced resend logic are supported.

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Resend behavior from CME Globex to Client System interleaves real-time messages with resend responses. This model ensures real-time information is delivered to the customer as quickly as possible. Customers must be able to receive out-of-sequence messages during a Resend Response without triggering another Resend Request.

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