@node UsecaseSatelliteLinks
@cindex satellite link
@section Satellite links

Satellite links have @strong{very} high delays together with high
bandwidths. You can send several megabits of data per second, but they
will reach the remote side only after half a second!
Most file sharing protocols like
@url{https://en.wikipedia.org/wiki/Files_transferred_over_shell_protocol, FISH},
@url{https://en.wikipedia.org/wiki/FTP, FTP},
@url{https://en.wikipedia.org/wiki/Secure_copy, scp},
@url{https://en.wikipedia.org/wiki/XMODEM, XMODEM}
will perform very badly because of round-trips quantity. Each file
transmission explicitly generates request and acknowledgement packets
that are send over the link. Remote side won't do anything until it
receives them. Moreover not all protocols allow duplex data
transmission (when both sides are sending data simultaneously).

NNCP's @ref{Sync, synchronization protocol} (SP) tries to mitigate all
that issues by reducing number of round-trips, number of packets passing
through. All file lists, file download requests are grouped together
(pipelined) in one huge packet. Only transmission halt and successful
file download acknowledgements are sent explicitly. SP could be asked
only either to upload or download packets for our node. SP could ignore
files with low priority. Full files listing is passing even during the
handshake procedure.