The CurrCon Java Applet displays prices on this
web page converted with today’s exchange rates into your local international currency,
e.g. Euros, US dollars, Canadian dollars, British Pounds, Indian Rupees…
CurrCon requires an up-to-date browser
and Java version 1.7 or later, preferably 1.8.0_20.
If you can’t see the prices in your local currency,
Troubleshoot. Use Chrome for best results.
FTP (File Transfer Protocol). A protocol
built on top of TCP/IP (Transmission Control Protocol/Internet Protocol)
that lets you send or receive a file over the Internet. The url for a file has this
FTP works better
than HTTP (Hypertext Transfer Protocol)
because it can pick up where it left off if the connection is broken.
theory has this feature as well, but it is more frequently implemented in
FTP. If you
simply want to download or copy a file, you can use HTTP
which is faster and simpler.
Normally you use PASV (Passive ftp file transfer) mode, the newer style of FTP
where the client sets up both the uplink and downlink. This is more secure than the
old style when the server set up the downlink. Further, it is easier to tunnel
through firewalls with PASV. You would not use it if you have some very old
server that does not support it. Active connections are
also slightly faster.
Typically FTP gives you access to different files than
does, e.g. the logs of website activity. You can typically both upload and
download. HTTP upload is fairly rare and usually set up to handle
only one file at a time.
With FTP you can upload, download, delete, rename, make a
directory, get a directory listing or abort the transfer in progress. In contrast,
with HTTP, all you can do in download a file.
If you are setting up an FTP
server, you will have to tell your firewall/router to forward ports 20 and 21 for
and 2000..2010 for PASV to your FTP
If your FTP session aborts, the server may lock the file you
were working until the session times out. The server can’t tell the session
was aborted. Until the timeout expires, you won’t be able to update or rename
that file in a new session.
Jon Postel and Joyce Reynolds devised the FTP
standard,RFC 959, back in 1985.
Like all RFC (Request For Comment) s, it has not been updated since. Mr. Postel has
died. Follow on RFCs (Request For Comments)
include RFC 2228
(internationalisation), RFC 2773
(encryption) and RFC 3659FEAT (ftp Feature enumeration).
There is now a secure FTP
protocol, SFTP (Secure File Transfer Protocol)
covered in RFC 2228
and RFC 4217. There is
also FTPS (File Transfer Protocol over Secure Sockets Layer) (FTP
over SSL (Secure Sockets Layer)), and SFTP
SSH (Secure Shell)), and
over TLS (Transport Layer Security) that encrypt the transmissions for privacy.
There is nothing in the FTP
protocol to find out what time the server thinks it is, or what TimeZone it is
using. You can kludge by uploading a tiny dummy file, then do a directory list on
it to find out what time the server stamped it with — which will be not the
file’s original time, but the time it arrived at the server. This tells you
what time the server is using accurate to a second or two. Typically
just stomps uploaded files with the server time they were uploaded.
FTP does not
preserve the last modified time of the files. FTP
software must adjust for the difference between local time and server time to
figure out which files have changed since they were last uploaded. In a sane
world, both FTP client and server would exchanged timestamps in
UTC (Coordinated Universal Time/Temps Universel Coordonné).
When the server returns a directory list, the FTP
protocol does not specify which file time to use, client, server or
UTC, daylight saving or standard, current
DST (Daylight Saving Time) or historic DST.
When you upload a file, the FTP
protocol does not specify what time the server should date the files with, client
upload time, server upload time, client last modified time…
There are a large number of parameters to configure such as:
whether server supports FEAT (feature detect) whether server supports RESUME,
time zone of the server, name conversion to lower case, port,
URL (Uniform Resource Locator), default directory, login in, password, account,
whether you are permitted multiple connections etc. The transfer won’t work
if you don’t match the server capabilities/expectations.
If you upload a file while someone on the web is downloading it to read, the
upload will fail. With a busy webserver you need to upload the files to a temporary
name, then wait for a lull in the traffic to delete the old file and rename the
temporary. The FTP protocol does nothing to help you with this.
Uploads are not atomic. If for example you upload a file A that links to file
B, and file A is uploaded first there will be a dangling link. Ideally all the
uploads should be done, then then once they have all successfully completed, reveal
all the updates and adds at once.
uploads a file at a time. If you have many small files, there is considerable time
overhead in the handshaking. Ideally they should be uploaded in compressed large
chunks to reduce the handshaking overhead.
There are more FTP packages than you can shake a stick at. Here are a
few. To find even more, search Google for FTP upload
automation These usually preposterously overpriced. They are typically not
smart enough to keep track of what they have already uploaded to avoid re-uploading
or having to download directory information. They are not clever enough to bundle the
upload into a compressed archive. They are not smart enough to retry files that are
busy being downloaded. They are not smart enough to do atomic uploads (the uploads
are not visible until all have been uploaded). We need a much more efficient,
reliable way to upload files to websites.
FTP is a bit
long in the tooth. It is being replaced by VPN
where you just copy files via an encrypted link to the server as if it were another
drive on your desktop.
AutoMate starts at
They refuse to tell you the price until you
have evaluated the product.
Ports, PASV and Firewalls
FTP is an odd
protocol in that a session requires two TCP/IP
socket connections. It starts out with a control channel then later adds a data
channel. In an Active connection, it works like this:
The client connects using a random port number on the client to port
21 on the server to create the control connection.
The client sends a port command over the control channel containing a random
free port to use for the data connection.
The server connects back to the client to socket number given in the port
command via port 20 on the server.
This third step is fire-wall unfriendly. Firewalls don’t trust servers out
on the net connecting in. They want all connections to be created by the client. So
FTP was modified
to make it more firewall-friendly, with PASV
connections. In that case, the client sets up both connections. The only catch is not
all servers and clients support it. In that case,you have to tell the firewall the
incoming connection is ok.
If your software is written in Java, and you are running it from a jar, the
firewall considers the generic java.exe to be the agent
trying to trying to tunnel through the firewall. On my machine there are ten
different java.exes. Make sure you tell the firewall which
one you are using to run your program. I found it easier to compile my app with Jet
so that I had a unique *.exe for the firewall to
FTP is long in
the tooth, so it will likely be completely replaced rather than patchwork upgraded.
Here are some things that will need to be fixed.
Run the FTP server and the protocol on
UTC with millisecond or nanosecond timestamps.
When you upload a file, preserve its timestamp accurately rather than using the
servers current local time.
Provide a filesystem like-way to access FTP
so that the remote files look just like local files.
Provide batch atomicity so that uploaded files logically appear all at once on
the server at the end of the batch.
It should upload at least two files at time without special request on the part
of the client.