Resend Request allows a receiving application to ask for the retransmission of previously sent messages. The requested messages are specified with a range of sequence numbers, defined by BeginSeqNo (Tag 7) and EndSeqNo (Tag 16). To request a single message, set BeginSeqNo equal to EndSeqNo. Setting EndSeqNo to zero (0) indicates that the range ends with the message of the highest sequence number (i.e., the last message sent).

Resend Request messages are commonly encountered under the following conditions:

• A sequence number gap is detected.
• A message has been lost by a receiving application.
• As part of the re-initialization process for an out-of-sequence condition.

A proper response to a Resend Request includes all application messages (except for MarketData Snapshot and Security Definition) but excludes administrative messages (except for Reject). The resending application may choose not to resend certain messages (e.g., NewOrder) if it deems them risky or if they fall outside an acceptable time window. All resent messages must be labeled as Possible Duplicates (i.e., Tag 43=Y). The T4 FIX API will honor all Possible Duplicates received from the client. If gaps exist in the resend message lot, Sequence Reset messages may be received to fill these gaps. This is common when administrative messages are excluded from the resend lot.