What is Redundant feed lines, snapshot recovery, retransmission request?

Redundant feed lines carry the same sequenced packets twice, on separate paths, so that a receiver can take each packet from whichever line delivers it first. Snapshot recovery rebuilds a book from a periodic snapshot of its state, tagged with the last incremental sequence number it includes, followed by the incremental messages after it. A retransmission request asks the venue’s recovery service for messages by sequence number.