1. A system for backup replication, comprising: a data storage system comprising: a data differencer that determines difference data between a most recent backup file and a previous backup file, wherein the difference data comprises data present in the most recent backup file not present in the previous backup file; a data storage system interface that transmits an extent specification to a replica backup system, wherein the extent specification comprises a description of regions of unchanged data from the previous backup file to the most recent backup file, wherein the extent specification comprises file locations in the previous backup file, wherein the data storage system interface also transmits a checksum, wherein the checksum comprises a checksum for the most recent backup file; and a data sender that: transmits data segment fingerprints of the one or more data segments to the replica backup system using the data sender; receives one or more data segment requests from the replica backup system using the data storage system interface, wherein a data segment request of the one or more data segment requests comprises an indication that a segment from the transmitted data fingerprints is not previously stored on the replica backup system and is requested of the data storage system; and transmit the one or more data segments from the data storage system to the replica backup system through the data storage system interface, wherein the transmitted one or more data segments are combined with segments previously stored on the replica backup system into a replica backup file using the transmitted extent specification.