The Sequence Reset message has two modes: Gap Fill mode and Reset mode.
Gap Fill mode
Gap Fill mode is used in response to a Resend Request when one or more messages must be skipped over for the following reasons:
Gap Fill mode is indicated by GapFillFlag (tag 123) field = "Y".
If the GapFillFlag field is present (and equal to "Y"), the MsgSeqNum should conform to standard message sequencing rules (i.e. the MsgSeqNum of the Sequence Reset GapFill mode message should represent the beginning MsgSeqNum in the GapFill range because the remote side is expecting that next message sequence number).
Reset mode involves specifying an arbitrarily higher new sequence number to be expected by the receiver of the Sequence Reset-Reset message, and is used to reestablish a FIX session after an unrecoverable application failure.
Reset mode is indicated by the GapFillFlag (tag 123) field = "N" or if the field is omitted.
If the GapFillFlag field is not present (or set to N), it can be assumed that the purpose of the Sequence Reset message is to recover from an out-of-sequence condition. In Sequence Reset - Reset mode, the MsgSeqNum in the header should be ignored (i.e. the receipt of a Sequence Reset - Reset mode message with an out of sequence MsgSeqNum should not generate resend requests). Sequence Reset - Reset should NOT be used as a normal response to a Resend Request (use Sequence Reset - Gap Fill mode). The Sequence Reset - Reset should ONLY be used to recover from a disaster situation which cannot be recovered via the use of Sequence Reset - Gap Fill. Note that the use of Sequence Reset - Reset may result in the possibility of lost messages.
Rules for processing all Sequence Reset messages
The sending application will initiate the Sequence Reset. The message in all situations specifies NewSeqNo to reset to as the value of the next sequence number to be expected by the message receipient immediately following the messages and/or sequence numbers being skipped.
The Sequence Reset can only increase the sequence number. If a sequence reset is received attempting to decrease the next expected sequence number the message should be rejected and treated as a serious error. It is possible to have multiple Resend Requests issued in a row (e.g. 5 to 10 followed by 5 to 11). If sequence number 8, 10, and 11 represent application messages while the 5-7 and 9 represent administrative messages, the series of messages as result of the Resend Request may appear as Sequence Reset-GapFill mode with NewSeqNo of 8, message 8, Sequence Reset-GapFill with NewSeqNo of 10, and message 10. This could then followed by Sequence Reset-GapFill with NewSeqNo of 8, message 8, Sequence Reset-GapFill with NewSeqNo of 10, message 10, and message 11. One must be careful to ignore the duplicate Sequence Reset-GapFill mode which is attempting to lower the next expected sequence number. This can be detected by checking to see if its MsgSeqNum is less than expected. If so, the Sequence Reset-GapFill mode is a duplicate and should be discarded.
The '^' character is used to represent SOH character.