Licklider Transmission Protocol
Licklider Transmission Protocol is a point to point protocol for use in deep space links. LTP is commonly seen as the standard underlying convergence layer protocol for the Bundle Protocols (RFC 5050), supporting a wide selection of networks. LTP is named in honor of American computer scientist Joseph Carl Robnett Licklider. Design. LTP is designed to run directly over a data link layer (e.g. the Advanced Orbiting Systems (AOS) Space Data Link Protocol). It can also run on the User Datagram Protocol (UDP), e.g. if used in a sensor network, in a private network (where bandwidth contestation  isn’t an issue), or for software debug purposes. Therefore, it does not address issues like routing or congestion control. It was originally designed to support “long-haul” transmission in an interplanetary network; its design is based on experiences with the CCSDS File Delivery Protocol (CFDP). Developed for space communication, its features are: The application divides its data to send into two parts (either part may have a size of 0): Protocol. The LTP engine stores data in a queue, checks whether the link towards the destination is available and if so, starts the transmission. If the link is not  available, the data remains in queue and transmission starts as soon as a so-called “link state cue” arrives, either from an underlying link layer or from a Management information base. Given red data to send, the last segment of the red data is marked with an “end of red data” flag and a timer is started for that segment. Absent green data to send, the “end of block” flag is set, otherwise the green data is transmitted and the last segment of this part gets marked with the “end of block” flag. The receiver issues a report segment for  the sender once it received the “end of red data” segment and all data is complete. Once the report segment has been transmitted, a timer is started. Upon arrival of the report segment, the sender cancels its timer, queues a report-acknowledgement for the receiver and informs the application that all red data has been successfully transmitted. Once the receiver receives the report-acknowledgement it cancels its timer and the transmission of the red data is completed. The transmission is completed if the “end of block” segment has arrived. Implementations. Reference implementations. The two reference implementations for LTP are: Licklider