@node Sync
-@unnumbered Sync protocol
+@cindex sync protocol
+@cindex online protocol
+@cindex synchronization
+@unnumbered Synchronization protocol
-So-called sync protocol is used in current TCP daemon's implementation.
-It is aimed to be very simple and effective. It is used over reliable
-transport like TCP connections.
+So-called synchronization protocol (SP) is used in current TCP daemon's
+implementation. It is used for synchronizing @ref{Spool, spool}
+directory contents between two nodes.
-It must be effective both on single-duplex and full-duplex links (for
-example satellites have very high throughput, but high-delay links).
-Acknowledging of each received packet, like
+@cindex XMODEM
+It is aimed to be very simple and effective. It uses reliable transport
+like TCP connections. It must be effective both on single-duplex and
+full-duplex links: for example satellites have very high throughput but
+high-delay links, so acknowledging of each received packet, like
@url{https://en.wikipedia.org/wiki/XMODEM, XMODEM} does, causes
unacceptable performance degradation.
-TODO
+@vindex NNCPDEADLINE
+Internally it uses various timeouts and deadlines. One of them used
+extensively is 10 seconds default deadline timeout. You can override it
+with @env{$NNCPDEADLINE} environment variable, that could be useful with
+very high delay links.
-http://noiseprotocol.org/noise.html#interactive-patterns
+@cindex Noise-IK
+SP works on top of
+@url{http://noiseprotocol.org/noise.html#interactive-patterns,
+@code{Noise_IK_25519_ChaChaPoly_BLAKE2b}} protocol. Each Noise packet
+is sent inside an @url{https://tools.ietf.org/html/rfc4506, XDR} envelope:
+
+@verbatim
++-----------------+
+| MAGIC | PAYLOAD |
++-----------------+
+@end verbatim
+
+@multitable @columnfractions 0.2 0.3 0.5
+@headitem @tab XDR type @tab Value
+@item Magic number @tab
+ 8-byte, fixed length opaque data @tab
+ @verb{|N N C P S 0x00 0x00 0x01|}
+@item Payload @tab
+ variable length opaque data @tab
+ Noise packet itself
+@end multitable
+
+Peers static keys are specified as @ref{Configuration, @emph{noisepub}}
+configuration entry.
+
+Payload inside Noise packets has maximum size of @emph{64 KiB - 256 B =
+65280 B}. It is sent immediately in the first message by each side. The
+very first payload (that is carried inside handshake messages) is always
+padded to the maximum size with @emph{HALT} packets (read below), for
+hiding actual number of @emph{INFO} packets (number of files available
+for transmission).
+
+Each SP payload is a concatenation of SP packets. Each packet has
+XDR-encoded header and then corresponding XDR-encoded body. Header is
+just an unsigned integer telling what body structure follows.
+
+@table @emph
+
+@cindex HALT payload
+@item HALT
+ Stop file transmission, empty sending queue on the remote side.
+ Actually @emph{HALT} packet does not have any body, only the header
+ with the type. It is also used in the first payload for padding to
+ the maximum size.
+
+@verbatim
++------+
+| HALT |
++------+
+@end verbatim
+
+@cindex PING payload
+@item PING
+ Dummy packet only used for determining workability of the connection.
+
+@verbatim
++------+
+| PING |
++------+
+@end verbatim
+
+@cindex INFO payload
+@item INFO
+ Information about the file we have for transmission.
+
+@verbatim
++------+--------------------+
+| INFO | NICE | SIZE | HASH |
++------+--------------------+
+@end verbatim
+
+ @multitable @columnfractions 0.2 0.3 0.5
+ @headitem @tab XDR type @tab Value
+ @item Niceness @tab
+ unsigned integer @tab
+ 1-255, file niceness level
+ @item Size @tab
+ unsigned hyper integer @tab
+ File size
+ @item Hash @tab
+ 32-byte, fixed length opaque data @tab
+ Unique file identifier, its checksum
+ @end multitable
+
+@cindex FREQ payload
+@item FREQ
+ File transmission request. Ask remote side to queue the file for
+ transmission.
+
+@verbatim
++------+---------------+
+| FREQ | HASH | OFFSET |
++------+---------------+
+@end verbatim
+
+ @multitable @columnfractions 0.2 0.3 0.5
+ @headitem @tab XDR type @tab Value
+ @item Hash @tab
+ 32-byte, fixed length opaque data @tab
+ Unique file identifier, its checksum
+ @item Offset @tab
+ unsigned hyper integer @tab
+ Offset from which remote side must transmit the file
+ @end multitable
+
+@cindex FILE payload
+@item FILE
+ Chunk of file.
+
+@verbatim
++------+-------------------------+
+| FILE | HASH | OFFSET | PAYLOAD |
++------+-------------------------+
+@end verbatim
+
+ @multitable @columnfractions 0.2 0.3 0.5
+ @headitem @tab XDR type @tab Value
+ @item Hash @tab
+ 32-byte, fixed length opaque data @tab
+ Unique file identifier, its checksum
+ @item Offset @tab
+ unsigned hyper integer @tab
+ Offset from which transmission goes
+ @item Payload @tab
+ variable length opaque data @tab
+ Chunk of file itself
+ @end multitable
+
+@cindex DONE payload
+@item DONE
+ Signal remote side that we have successfully downloaded the file.
+
+@verbatim
++------+------+
+| DONE | HASH |
++------+------+
+@end verbatim
+
+ @multitable @columnfractions 0.2 0.3 0.5
+ @headitem @tab XDR type @tab Value
+ @item Hash @tab
+ 32-byte, fixed length opaque data @tab
+ Unique file identifier, its checksum
+ @end multitable
+
+@end table
+
+Typical peer's behaviour is following:
+
+@verbatiminclude sp.plantuml.txt
+
+@enumerate
+@item Perform @emph{Noise-IK} handshake:
+
+ @table @strong
+ @item Initiator
+ Collects all @emph{tx}-related files information and prepares
+ payload filled with @emph{INFO}s for including in the @strong{first}
+ handshake message.
+ @item Responder
+ After receiving the first handshake message, it gains remote
+ identity knowledge and similarly prepares the payload for including
+ in the @strong{second} handshake message.
+ @end table
+
+ All payloads are padded to maximal message size with @emph{HALT}s.
+
+@item If queued @emph{INFO}s are not sent completely in handshake
+payloads, then send all of remaining in the transport stage.
+
+@item When @emph{INFO} packet received:
+
+ @itemize
+ @item Check that it has an acceptable niceness level.
+ Ignore it if it is too nice.
+ @item If already downloaded file exists, then queue @emph{DONE}
+ sending.
+ @item If @file{seen/XXX} exists, then queue @emph{DONE} sending.
+ @item If @file{.part} exists, then queue @emph{FREQ} sending with
+ corresponding offset.
+ @end itemize
+
+@item When @emph{FREQ} packet received, insert it to current sending
+queue with niceness level sort: higher priority packets will be sent
+first. Sending queue contains files with offsets that are needed to be
+sent.
+
+@item While sending queue is not empty, send @emph{FILE} packets.
+@emph{FREQ} could contain offset equal to size -- anyway sent
+@emph{FILE} packet with an empty payload. @emph{FILE} sending is
+performed only if no other outgoing packets are queued: @emph{INFO}s
+have higher priority.
+
+@item When @emph{FILE} packet received, check if it is completely
+downloaded (comparing to @emph{INFO}'s packet size information). If so,
+then run background integrity checker on it. If check succeeds, then
+delete @file{.part} suffix from file's name and send @emph{DONE} packet.
+
+@item When @emph{DONE} packet received, delete corresponding file.
+
+@item When @emph{HALT} packet received, empty file sending queue.
+
+@item Each second, node checks: are there any new @emph{tx} packets
+appeared and queues corresponding @emph{INFO} packets.
+
+@item Each minute, if no packets were sent, node sends @emph{PING}
+packet.
+
+@item If no non-PING packets are sent and received during
+@ref{CfgOnlineDeadline, onlinedeadline} duration, then close the
+connection. There is no explicit indication that session is over.
+
+@item If no packets are received during two minutes (two PING timeouts),
+then close the connection.
+
+@end enumerate