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<center><h2><b>ProFTPD: KeepAlives</b></h2></center>
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<p>
<b>Overview of KeepAlive</b><br>
Any discussion of so-called "keep-alive" functionality must start by answering
the question: "What is does 'keep-alive' mean?" As one specification
succintly states:
<pre>
A "keep-alive" mechanism periodically probes the other end of a connection
when the connection is otherwise idle, even when there is no data to be sent.
</pre>
Keepalive mechanisms appear in many different protocols, under various names.
Protocols may be layered over the top of another protocol; for example, HTTPS
consists of HTTP layered over SSL/TLS, itself layered over TCP/IP. Each
protocol layer may have its own form of such keepalive functionality.
<p><a name="TCPKeepAlive"></a>
<b>TCP KeepAlive</b><br>
For TCP, the definitive specification for keepalive functionality is
<a href="http://www.faqs.org/rfcs/rfc1122.html">RFC 1122</a>, Section
4.2.3.6. While RFC 1122 includes a good discussion of why TCP keepalives are
meant to be off by default, in practice TCP keepalives <b>do</b> have value,
especially when dealing with network equipment (such as routers, firewalls,
NATs) between the client and the server; such equipment might terminate the
connection when the connection has been idle for too long. The use of TCP
keepalives can help to prevent such network equipment from breaking the
connection needlessly.
<p>
<b><i>How TCP KeepAlives Works</i></b><br>
OK, so you want to use the TCP keepalive functionality in your program. The
question is "How exactly does the TCP keepalive feature work?" Good question.
Answering this requires three different numeric values: the <i>idle time</i>,
the number of probes to send (the probe <i>count</i>), and the <i>interval
time</i> between each probe. Remember, though, that the
<code>SO_KEEPALIVE</code> TCP socket option <b>must</b> be enabled on the socket
in order for the TCP keepalive feature to be used.
<p>
First, let's assume that you have created a TCP connection, and have transferred
data back and forth on that connection. All of the data have been transferred,
but you have not <i>closed</i> the connection, so now it is idle. How long
does that connection sit idle, with no data transferred <i>at the TCP layer</i>,
before one end of the connection or the other starts to wonder whether the
connection is still alive? This amount of time where the connection sits idle
is the TCP keepalive <i>idle time</i>; the default <i>idle time</i> is two
hours (per RFC 1122).
<p>
Our TCP connection has been sitting idle now for amount of time given by the
<i>idle time</i> value; what happens then? At this point, the end of the TCP
connection with TCP keepalive enabled sends out a "probe". This probe is
just a small TCP packet which requires a response from the other side. Once
the probe has been sent, the amount of time given by the <i>interval time</i>
value passes. If we hear nothing back from the remote peer within the
<i>interval time</i>, we send another probe. This process repeats until either
<i>a)</i> we receive a response back from the peer, or <i>b)</i> the probe
<i>count</i> value has been reached.
<p>
Let us assume that our TCP connection was idle for so long that TCP keepalive
probes were sent, and still no response was received. What happens then? At
this point, the connection is broken. When the programs at either end of
the connection next try to read or write data on that connection, the read/write
attempts will fail.
<p>
<b><i>When To Use TCP KeepAlive</i></b><br>
When the TCP client is connected directly to the TCP server, it usually does
not matter whether one end or the other uses TCP keepalive. As long as
<i>one</i> of them does, a broken connection can be detected.
<pre>
client <-------------------------------> server
</pre>
However, if the TCP client connects to the TCP server via
proxies/routers/firewalls/NAT, the picture changes. When this happens (and
it is the common scenario), then <b>both</b> sides may need to use TCP
keepalive to learn when their side of the proxied connection is broken:
<pre>
client <-----------> NAT <-------------> server
</pre>
In this situation, there are actually <em>two</em> different TCP connections
involved: between the client and the NAT, and between the NAT and the server.
Each TCP connection may break independently of the other, which is why both
ends of the connection (client and server) may need to use TCP keepalives.
Use of TCP keepalives <i>also</i> helps here because when the
router/firewall/NAT receives the TCP keepalive probe, it <i>may</i> (depending
on the network equipment in question) cause the router to reset any timers that
were about to close the TCP connections on either side.
<p>
<b><i>Why are TCP KeepAlives Useful for FTP?</i></b><br>
"This is all very fascinating", you say, "but what does it have to do with
<code>proftpd</code> and FTP?" If you have ever had an FTP download (or upload)
take a <i>very long time</i>, only to have that transfer timed out in the
middle, then TCP keepalives may prevent the timeout.
<p>
Consider what happens for FTP transfers which take a long time (either due to
very large file(s) being transferred, or a slow connection): you have one TCP
connection for the control connection, and a <i>separate</i> TCP connection for
the data transfer conenction. All of the bytes are being transferred over the
data connection, so that data connection is certainly not idle -- but while the
data transfer is occurring, the <i>control</i> connection is idle! And let's
assume that your FTP connections are going through some NAT device in between
the client and the server. That NAT may not be very smart; it may not know
that the two different TCP connections of your FTP session are related to each
other; it only sees one idle TCP connection, and one busy TCP connection. If
that FTP control connection is idle for too long, then the NAT may close it
(in order to keep valuable space in its state tables available for TCP
connections that actually need to transfer bytes). (Some NATs have been known
to close TCP connections that have been idle for only 5 minutes.) The FTP
server sees that the FTP control connection is closed, and aborts the data
transfer. What a mess!
<p>
If either the FTP server or the FTP client had used TCP keepalives on the
control connection, then maybe that NAT would have seen the TCP keepalive
probes, and <b>not</b> closed the idle control connection. So how can we
make sure that either the client or the server has TCP keepalives enabled?
<p>
In <code>proftpd-1.3.5rc1</code> and later, ProFTPD's <a href="../modules/mod_core.html#SocketOptions"><code>SocketOptions</code></a> directive supports a
<em>keepalive</em> parameter for controlling whether the server uses TCP
keepalives, <i>e.g.</i>:
<pre>
# Disable use of TCP keepalives
SocketOptions keepalive off
# Enable use of TCP keepalives (this is the default)
SocketOptions keepalive on
</pre>
In addition, on some Unix platforms, the <code>SocketOptions</code>
directive's <em>keepalive</em> parameter can do finer-grained tuning of the
TCP keepalive values:
<pre>
# Enable use of TCP keepalives, with the given idle/count/interval values
SocketOptions keepalive 7200:9:75
</pre>
In general, though, you should use the system-wide defaults unless you are
running into data transfer timeout issues. If you <b>are</b> seeing timeouts,
try using the <em>keepalive</em> parameter of <code>SocketOptions</code> to
gradually reduce the idle timeout by small increments (<i>e.g.</i> 10-15
seconds), <i>then</i> if that does not help, increment the count by 1 at a
time (remember that each probe is more extra data transfer), <i>then</i> if that
still does not help, <b>increase</b> the interval time. Do <b>not reduce the
interval time</b>, since that is the amount of time that you should wait to
see if the other end responds, before sending another probe. Waiting less
time before the other end responds means a greater chance of killing your TCP
connection unnecessarily.
<p>
Not all TCP stacks let the application control the TCP keepalive timeout after
which the first probe will be sent, or the total number of probes sent, or how
much time between probes will be used. That is, many TCP stacks <i>only</i>
allow enabling/disabling of TCP keepalive. If TCP keepalive <i>is</i> enabled,
then the standard values of 2 hours for the idle timeout, a count of 9 probes,
with 75 seconds between probes, will be used.
<p>
Since many platforms do not allow fine-grained tuning of TCP keepalive values,
especially on a per-service basis, other means for checking whether the
connection is still alive must be used. And that leads us to application-level
keepalive mechanisms.
<p><a name="FTPKeepAlive">
<b>FTP KeepAlive</b><br>
If tuning TCP keepalives does not work to keep your long-lasting data transfers
from timing out, what can be done? Answer: use keepalive features at other
layers in the protocol stack. FTP has its own ideas for doing keepalive
checks, but they are not as elegant as that of TCP.
<p>
The main issue with FTP keepalives is that they are all initiated by the client.
FTP is a request/response model, and it does not allow for the FTP server to
send arbitrary unrequested data to the FTP client via the control connection.
Fortunately, there are many FTP clients which implement some sort of FTP
keepalive feature.
<p>
<b><i>How FTP KeepAlive Works</i></b><br>
Since the FTP server cannot do anything to test whether the FTP session is
alive, the FTP client must do the tests. The easiest way to test whether an
FTP session is alive is to send an FTP command. And fortunately,
<a href="http://www.faqs.org/rfcs/rfc959.html">RFC 959</a>, Section 4.1.3
defines the <code>NOOP</code> ("No Operation") command whose sole purpose is
to elicit the "OK" response from the FTP server. This makes the
<code>NOOP</code> command the ideal way to test whether the FTP server is
still alive and listening to the FTP client.
<p>
Some firewalls/routers know about this <code>NOOP</code> trick, though, and
may filter out/drop that FTP command. FTP clients, then, have been known to
resort to a number of other FTP commands for use as FTP keepalives, including:
<ul>
<li><code>NOOP</code>
<li><code>LIST</code>
<li><code>STAT</code>
<li><code>CWD</code>
<li><code>REST 0</code>
<li><code>PWD</code>
<li><code>TYPE A</code>
</ul>
Some FTP clients even choose a command at random from the above list, just to
keep any interfering router/firewall/NAT guessing!
<p>
Sadly, some FTP servers cannot handle receiving an FTP command on the control
connection while they are in the middle of transferring data on the data
connection (<code>proftpd</code> <b>can</b> handle this). But that may not
matter, for the purposes of FTP keepalives; all that matters is that at the
TCP level, the bytes were sent by the client and acknowledged by the server's
TCP stack.
<p>
<b><i>FTP Client-Specific KeepAlive Settings</i></b><br>
Not every FTP client supports the FTP keepalive functionality. If you want
to try out FTP clients which <i>do</i> support FTP keepalives, you might look
into the <code>ftp:nop-interval</code> setting for
<a href="http://lftp.yar.ru/lftp-man.html"><code>lftp</code></a>, or the
<code>control-timeout</code> setting for
<a href="http://www.ncftp.com/ncftp/doc/ncftp.html#sect5"><code>ncftp</code></a>.
<p><a name="SSHKeepAlive">
<b>SSH KeepAlive</b><br>
The SSH2 protocol (and SFTP, which runs over SSH2) is more complex than FTP,
and thus has <i>much</i> better support for application-level keepalive
functionality. Either end of an SSH2 connection can send messages at any time.
<p>
<b><i>How SSH KeepAlive Works</i></b><br>
The usual mechanism used by SSH2 implementations for implementing an SSH2-level
keepalive check is send either a <code>CHANNEL_REQUEST</code> or a
<code>GLOBAL_REQUEST</code> message for a known unsupported command, and to
request a response from the other side. It does not matter that the requested
command is unsupported; all that matters is that the response comes back,
signifying that the other end is still alive and listening. Both clients and
servers use this technique.
<p>
In the case of ProFTPD's <code>mod_sftp</code> module, the way to configure
SSH2 keepalives is the <a href="../contrib/mod_sftp.html#SFTPClientAlive"><code>SFTPClientAlive</code></a> directive. When configured, the
<code>mod_sftp</code> module sends <code>CHANNEL_REQUEST</code>/<code>GLOBAL_REQUEST</code> messages for "keepalive@proftpd.org" in order to solicit a response
from the connected client.
<p>
<b>KeepAlive in Other Protocols</b><br>
Many application protocols end up reinventing the keepalive feature in some
way, usually as a "ping/pong" mechanism where a "ping" is sent every so often
by one side, with a "pong" response expected from the other end of the
connection.
<p>
<b><i>HTTP</i></b><br>
Most HTTP connections have no need of a keepalive mechanism since HTTP
connections are usually short-lived, and since there are usually data flowing
in one direction or the other on the HTTP connection (thus an HTTP connection
is usually not idle for long enough time to warrant a keepalive feature).
HTTP long polling (<i>i.e.</i>
<a href="http://www.faqs.org/rfcs/rfc6202.html">RFC 6202</a>) is an exception;
and for HTTP long polling connections, use of TCP keepalives may be needed.
But the HTTP protocol itself does not specifically define a way for either
end to arbitrarily send data across the connection for the purpose of
determining whether the connection is still alive. (HTTP keepalive refers
to a different concept, <i>i.e.</i> that of telling the server to <b>not</b>
close the connection after sending its response so that the connection can
be reused, thus "kept alive".)
<p>
<b><i>LDAP</i></b><br>
For long-lived LDAP connections, keepalive functionality can be implemented
by using the Abandon operation, as described
<a href="http://stackoverflow.com/questions/313575/ldap-socket-keep-alive?rq=1">here</a>. The idea is to have the client send a request that it knows the
server will ignore/discard; the act of transmitting the request over the
connection acts to keep any intermediaries on the network (router/NAT/firewall)
from closing an "idle" connection prematurely.
<p>
<b><i>WebSocket</i></b><br>
The WebSocket protocol, defined in
<a href="http://www.faqs.org/rfcs/rfc6455.html">RFC 6455</a>, does have need
of a keepalive mechanism, since it establishes a long-lived connection.
Thus does the RFC define ping/pong messages; see Section 5.5.2
(<code>PING</code>), and Section 5.5.3 (<code>PONG</code>).
<p>
<b>Additional Reading</b></br>
<ul>
<li><a name="[1]"><a href="http://www.gnugk.org/keepalive.html">http://www.gnugk.org/keepalive.html</a>
<li><a name="[2]"><a href="http://www.starquest.com/Supportdocs/techStarLicense/SL002_TCPKeepAlive.shtml">http://www.starquest.com/Supportdocs/techStarLicense/SL002_TCPKeepAlive.shtml</a>
<li><a name="[3]"><a href="http://rolande.wordpress.com/2010/12/30/performance-tuning-the-network-stack-on-mac-osx-10-6/">http://rolande.wordpress.com/2010/12/30/performance-tuning-the-network-stack-on-mac-osx-10-6/</a>
<li><a name="[4]"><a href="http://stackoverflow.com/questions/1480236/does-a-tcp-socket-connection-have-a-keep-alive">http://stackoverflow.com/questions/1480236/does-a-tcp-socket-connection-have-a-keep-alive</a>
<li><a name="[5]"><a href="http://www.wftpserver.com/kb_keepalive.htm">http://www.wftpserver.com/kb_keepalive.htm</a>
<li><a name="[6]"><a href="http://serverfault.com/questions/84310/how-to-prevent-tcp-connection-timeout-when-ftping-large-file">http://serverfault.com/questions/84310/how-to-prevent-tcp-connection-timeout-when-ftping-large-file</a>
<li><a name="[7]"><a href="http://stackoverflow.com/questions/5533185/keeping-ftp-control-connection-alive">http://stackoverflow.com/questions/5533185/keeping-ftp-control-connection-alive</a>
</ul>
<p><a name="FAQ">
<b>Frequently Asked Questions</b><br>
<p>
<font color=red>Question</font>: When should I use application-level keepalives like FTP or SSH2 keepalives instead of TCP keepalives? Or can I use them all
at the same time?<br>
<font color=blue>Answer</font>: There is no issue with using the different
types of keepalives at the same time; remember that each type of keepalive
functions at a different protocol layer.
<p>
That said, if you have to choose, I would recommend using the application-level
keepalive feature when you can, as opposed to TCP keepalives. Why? First,
you probably have more control over when the application-level keepalive
feature is used than if you used TCP keepalives (<i>e.g.</i> you can use the
application-level keepalive check more frequently than the TCP keepalive
feature would detect a dead connection).
<p>
Second, TCP keepalives may <b>not</b> cause a NAT to keep the TCP connection
on each side open, but an application-level keepalive should do so, since
the application-level keepalive data <b>must</b> traverse both connections:
<pre>
<------------ SSH2 REQUEST ----------->
-------------- FTP NOOP -------------->
client <---------------> NAT <---------------> server
<-- TCP probe --> <-- TCP probe -->
</pre>
<p>
<font color=red>Question</font>: Why should I use <code>SocketOptions</code> to
configure <code>proftpd</code>'s TCP keepalive settings, as opposed to
changing some of the sysctls on my machine that apply to TCP KeepAlives?<br>
<font color=blue>Answer</font>: Using the <code>SocketOptions</code> directive
means that your TCP keepalive tunings will only affect the FTP/SFTP connections
to <code>proftpd</code>, instead of applying to <b>all</b> TCP connections to
your machine. FTP/SFTP sessions, being long-lived, probably have different
keepalive timing needs than from other TCP connections, so it is best to tune
their settings separately, without impacting all connections to that machine.
<p>
<font color=red>Question</font>: If TCP keepalives are so useful, why not
tune them to be quite short? Why does RFC 1122 recommend a default of
<em>two</em> hours before checking if the peer is still there?<br>
<font color=blue>Answer</font>: There are a couple of reasons why you might
<b>not</b> want to tune your TCP keepalive settings to be shorter.
<p>
First, keep in mind that TCP was designed to keep the TCP level connection
"alive" even though various parts of the underlying hardware, such as
routers, firewalls, etc might crash and be rebooted in the middle of things.
This is why TCP retransmits lost packets, routes around unresponsive
hops, etc. If, then, your TCP keepalive settings are aggressively short,
your TCP connection may be shut down (due to not responding to TCP keepalive
probes in time) because of a crashed network component. The recommended
default of two hours allows for such network route changes <i>without losing
the TCP connection</i>.
<p>
Second, every TCP keepalive probe counts as more data transferred over your
network link. Some links may have exorbitantly high rates for transferred
bytes (think satellite links), so on such links, keeping the number of bytes
transferred down amounts to cost savings. Tuning TCP keepalive to use shorter
times on such links means <i>more</i> data transferred, thus higher costs.
On other links, where data transfer rates are cheap, the additional bytes
transferred due to shorter TCP keepalive settings may be neglible.
<p>
Last, many people argue that use of TCP keepalives may consume unnecessary
bandwidth. The question becomes "If no one is using the connection, who
cares if the connection is still good?" (To be fair, this argument makes more
sense when the connected peers are <b>not</b> separated by proxies, gateways,
and NATs.)
<p>
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