elaborate on fileserver transfer protocol

Rewritten-docs.md

@@ -936,20 +936,46 @@ However, all of the three options would have been possible to use.
 
 ### File transfers
 
-There has to be a way to access files stored on the fileserver from both workers
-and clients. We will present some of the possible options:
-
-@todo elaborate this stuff
-
-- HTTP(S)
-- FTP
-- SFTP
-- A network-shared file system (such as NFS)
-- A custom protocol over ZeroMQ
-
-We chose HTTPS because it is widely used and clients exist in all relevant
-environments. In addition, it is highly probable we will have to run an HTTP
-server, because it is intended for ReCodEx to have a web frontend.
+There has to be a way to access files stored on the fileserver from both worker
+and frontend server machines. The protocol used for this should handle large
+files efficiently and be resilient to network failures. Security features are
+not a primary concern, because all communication with the fileserver will happen
+in an internal network. However, a basic form of authentication can be useful to
+ensure correct configuration (if a development fileserver uses different
+credentials than production, production workers will not be able to use it by
+accident). Lastly, the protocol must have a client library for platforms
+(languages) used in the backend. We will present some of the possible options:
+
+- HTTP(S) -- a de-facto standard for web communication that has far more
+  features than just file transfers. Thanks to being used on the web, a large
+  effort has been put into the development of its servers. It supports
+  authentication and it can handle short-term network failures (thanks to being
+  built on TCP and supporting resuming interrupted transfers). We will use HTTP
+  for communication with clients, so there is no added cost in maintaining a
+  server. HTTP requests can be made using libcurl.
+- FTP -- an old protocol designed only for transferring files. It has all the
+  required features, but doesn't offer anything over HTTP. It is also supported
+  by libcurl.
+- SFTP -- a file transfer protocol most frequently used as a subsystem of the
+  SSH protocol implementations. It doesn't provide authentication, but it
+  supports working with large files and resuming failed transfers. The libcurl
+  library supports SFTP.
+- A network-shared file system (such as NFS) -- an obvious advantage of a
+  network-shared file system is that applications can work with remote files the
+  same way they would with local files. However, it brings an overhead for the
+  administrator, who has to configure access to this filesystem for every
+  machine that needs to access the storage. 
+- A custom protocol over ZeroMQ -- it is possible to design a custom file
+  transfer protocol that uses ZeroMQ for sending data, but it is not a trivial
+  task -- we would have to find a way to transfer large files efficiently, 
+  implement an acknowledgement scheme and support resuming transfers. Using
+  ZeroMQ as the underlying layer does not help a lot with this. The sole
+  advantage of this is that the backend components would not need another
+  library for communication.
+
+We chose HTTPS because it is widely used and client libraries exist in all
+relevant environments. In addition, it is highly probable we will have to run an
+HTTP server, because it is intended for ReCodEx to have a web frontend.
 
 ### Frontend - backend communication