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354 lines
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<div id="main">
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<h1 class="title">libtorrent manual</h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>Arvid Norberg, <a class="last reference external" href="mailto:arvid@rasterbar.com">arvid@rasterbar.com</a></td></tr>
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<tr><th class="docinfo-name">Version:</th>
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<td>1.0.0</td></tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first">Table of contents</p>
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<ul class="simple">
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<li><a class="reference internal" href="#utp" id="id1">uTP</a><ul>
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<li><a class="reference internal" href="#rationale" id="id2">rationale</a></li>
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<li><a class="reference internal" href="#tcp" id="id3">TCP</a></li>
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<li><a class="reference internal" href="#ledbat-congestion-controller" id="id4">LEDBAT congestion controller</a></li>
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<li><a class="reference internal" href="#one-way-delays" id="id5">one way delays</a></li>
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<li><a class="reference internal" href="#path-mtu-discovery" id="id6">Path MTU discovery</a></li>
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<li><a class="reference internal" href="#clock-drift" id="id7">clock drift</a></li>
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<li><a class="reference internal" href="#features" id="id8">features</a></li>
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</ul>
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</li>
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</ul>
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</div>
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<div class="section" id="utp">
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<h1>uTP</h1>
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<p>uTP (uTorrent transport protocol) is a transport protocol which uses one-way
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delay measurements for its congestion controller. This article is about uTP
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in general and specifically about libtorrent's implementation of it.</p>
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<div class="section" id="rationale">
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<h2>rationale</h2>
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<p>One of the most common problems users are experiencing using bittorrent is
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that their internet "stops working". This can be caused by a number of things,
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for example:</p>
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<ol class="arabic simple">
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<li>a home router that crashes or slows down when its NAT pin-hole
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table overflows, triggered by DHT or simply many TCP connections.</li>
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<li>a home router that crashes or slows down by UDP traffic (caused by
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the DHT)</li>
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<li>a home DSL or cable modem having its send buffer filled up by outgoing
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data, and the buffer fits seconds worth of bytes. This adds seconds
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of delay on interactive traffic. For a web site that needs 10 round
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trips to load this may mean 10s of seconds of delay to load compared
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to without bittorrent. Skype or other delay sensitive applications
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would be affected even more.</li>
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</ol>
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<p>This document will cover (3).</p>
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<p>Typically this is solved by asking the user to enter a number of bytes
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that the client is allowed to send per second (i.e. setting an upload
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rate limit). The common recommendation is to set this limit to 80% of the
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uplink's capacity. This is to leave some headroom for things like TCP
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ACKs as well as the user's interactive use of the connection such as
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browsing the web or checking email.</p>
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<p>There are two major drawbacks with this technique:</p>
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<ol class="arabic simple">
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<li>The user needs to actively make this setting (very few protocols
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require the user to provide this sort of information). This also
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means the user needs to figure out what its up-link capacity is.
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This is unfortunately a number that many ISPs are not advertizing
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(because it's often much lower than the download capacity) which
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might make it hard to find.</li>
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<li>The 20% headroom is wasted most of the time. Whenever the user
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is not using the internet connection for anything, those extra 20%
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could have been used by bittorrent to upload, but they're already
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allocated for interactive traffic. On top of that, 20% of the up-link
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is often not enough to give a good and responsive browsing experience.</li>
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</ol>
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<p>The ideal bandwidth allocation would be to use 100% for bittorrent when
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there is no interactive cross traffic, and 100% for interactive traffic
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whenever there is any. This would not waste any bandwidth while the user
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is idling, and it would make for a much better experience when the user
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is using the internet connection for other things.</p>
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<p>This is what uTP does.</p>
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</div>
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<div class="section" id="tcp">
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<h2>TCP</h2>
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<p>The reason TCP will fill the send buffer, and cause the delay on all traffic,
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is because its congestion control is <em>only</em> based on packet loss (and timeout).</p>
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<p>Since the modem is buffering, packets won't get dropped until the entire queue
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is full, and no more packets will fit. The packets will be dropped, TCP will
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detect this within an RTT or so. When TCP notices a packet loss, it will slow
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down its send rate and the queue will start to drain again. However, TCP will
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immediately start to ramp up its send rate again until the buffer is full and
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it detects packet loss again.</p>
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<p>TCP is designed to fully utilize the link capacity, without causing congestion.
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Whenever it sense congestion (through packet loss) it backs off. TCP is not
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designed to keep delays low. When you get the first packet loss (assuming the
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kind of queue described above, tail-queue) it is already too late. Your queue
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is full and you have the maximum amount of delay your modem can provide.</p>
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<p>TCP controls its send rate by limiting the number of bytes in-flight at any
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given time. This limit is called congestion window (<em>cwnd</em> for short). During
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steady state, the congestion window is constantly increasing linearly. Each
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packet that is successfully transferred will increase cwnd.</p>
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<pre class="literal-block">
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cwnd
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send_rate = ----
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RTT
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</pre>
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<p>Send rate is proportional to cwnd divided by RTT. A smaller cwnd will cause
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the send rate to be lower and a larger cwnd will cause the send rate to be
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higher.</p>
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<p>Using a congestion window instead of controlling the rate directly is simple
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because it also introduces an upper bound for memory usage for packets that
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haven't been ACKed yet and needs to be kept around.</p>
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<p>The behavior of TCP, where it bumps up against the ceiling, backs off and then
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starts increasing again until it hits the ceiling again, forms a saw tooth shape.
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If the modem wouldn't have any send buffer at all, a single TCP stream would
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not be able to fully utilize the link because of this behavior, since it would
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only fully utilize the link right before the packet loss and the back-off.</p>
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</div>
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<div class="section" id="ledbat-congestion-controller">
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<h2>LEDBAT congestion controller</h2>
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<p>The congestion controller in uTP is called <a class="reference external" href="https://datatracker.ietf.org/doc/draft-ietf-ledbat-congestion/">LEDBAT</a>, which also is an IETF working
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group attempting to standardize it. The congestion controller, on top of reacting
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to packet loss the same way TCP does, also reacts to changes in delays.</p>
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<p>For any uTP (or <a class="reference external" href="https://datatracker.ietf.org/doc/draft-ietf-ledbat-congestion/">LEDBAT</a>) implementation, there is a target delay. This is the
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amount of delay that is acceptable, and is in fact targeted for the connection.
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The target delay is defined to 25 ms in <a class="reference external" href="https://datatracker.ietf.org/doc/draft-ietf-ledbat-congestion/">LEDBAT</a>, uTorrent uses 100 ms and
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libtorrent uses 75 ms. Whenever a delay measurement is lower than the target,
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cwnd is increased proportional to (target_delay - delay). Whenever the measurement
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is higher than the target, cwnd is decreased proportional to (delay - target_delay).</p>
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<p>It can simply be expressed as:</p>
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<pre class="literal-block">
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cwnd += gain * (target_delay - delay)
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</pre>
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<a class="reference external image-reference" href="cwnd.png"><img alt="cwnd_thumb.png" class="align-right" src="cwnd_thumb.png" /></a>
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<p>Similarly to TCP, this is scaled so that the increase is evened out over one RTT.</p>
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<p>The linear controller will adjust the cwnd more for delays that are far off the
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target, and less for delays that are close to the target. This makes it converge
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at the target delay. Although, due to noise there is almost always some amount of
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oscillation. This oscillation is typically smaller than the saw tooth TCP forms.</p>
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<p>The figure to the right shows how (TCP) cross traffic causese uTP to essentially
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entirely stop sending anything. Its delay measurements are mostly well above the target
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during this time. The cross traffic is only a single TCP stream in this test.</p>
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<p>As soon as the cross traffic ceases, uTP will pick up its original send rate within
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a second.</p>
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<p>Since uTP constantly measures the delay, with every single packet, the reaction time
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to cross traffic causing delays is a single RTT (typically a fraction of a second).</p>
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</div>
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<div class="section" id="one-way-delays">
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<h2>one way delays</h2>
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<p>uTP measures the delay imposed on packets being sent to the other end
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of the connection. This measurement only includes buffering delay along
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the link, not propagation delay (the speed of light times distance) nor
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the routing delay (the time routers spend figuring out where to forward
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the packet). It does this by always comparing all measurements to a
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baseline measurement, to cancel out any fixed delay. By focusing on the
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variable delay along a link, it will specifically detect points where
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there might be congestion, since those points will have buffers.</p>
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<a class="reference external image-reference" href="delays.png"><img alt="delays_thumb.png" class="align-right" src="delays_thumb.png" /></a>
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<p>Delay on the return link is explicitly not included in the delay measurement.
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This is because in a peer-to-peer application, the other end is likely to also
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be connected via a modem, with the same send buffer restrictions as we assume
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for the sending side. The other end having its send queue full is not an indication
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of congestion on the path going the other way.</p>
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<p>In order to measure one way delays for packets, we cannot rely on clocks being
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synchronized, especially not at the microsecond level. Instead, the actual time
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it takes for a packet to arrive at the destination is not measured, only the changes
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in the transit time is measured.</p>
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<p>Each packet that is sent includes a time stamp of the current time, in microseconds,
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of the sending machine. The receiving machine calculates the difference between its
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own timestamp and the one in the packet and sends this back in the ACK. This difference,
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since it is in microseconds, will essentially be a random 32 bit number. However,
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the difference will stay somewhat similar over time. Any changes in this difference
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indicates that packets are either going through faster or slower.</p>
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<p>In order to measure the one-way buffering delay, a base delay is established. The
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base delay is the lowest ever seen value of the time stamp difference. Each delay
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sample we receive back, is compared against the base delay and the delay is the
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difference.</p>
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<p>This is the delay that's fed into the congestion controller.</p>
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<p>A histogram of typical delay measurements is shown to the right. This is from
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a transfer between a cable modem connection and a DSL connection.</p>
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<p>The details of the delay measurements are slightly more complicated since the
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values needs to be able to wrap (cross the 2^32 boundry and start over at 0).</p>
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</div>
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<div class="section" id="path-mtu-discovery">
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<h2>Path MTU discovery</h2>
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<p>MTU is short for <em>Maximum Transfer Unit</em> and describes the largest packet size that
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can be sent over a link. Any datagrams which size exceeds this limit will either
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be <em>fragmented</em> or dropped. A fragmented datagram means that the payload is split up
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in multiple packets, each with its own individual packet header.</p>
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<p>There are several reasons to avoid sending datagrams that get fragmented:</p>
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<ol class="arabic simple">
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<li>A fragmented datagram is more likely to be lost. If any fragment is lost,
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the whole datagram is dropped.</li>
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<li>Bandwidth is likely to be wasted. If the datagram size is not divisible
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by the MTU the last packet will not contain as much payload as it could, and the
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payload over protocol header ratio decreases.</li>
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<li>It's expensive to fragment datagrams. Few routers are optimized to handle large
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numbers of fragmented packets. Datagrams that have to fragment are likely to
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be delayed significantly, and contribute to more CPU being used on routers.
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Typically fragmentation (and other advanced IP features) are implemented in
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software (slow) and not hardware (fast).</li>
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</ol>
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<p>The path MTU is the lowest MTU of any link along a path from two endpoints on the
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internet. The MTU bottleneck isn't necessarily at one of the endpoints, but can
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be anywhere in between.</p>
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<p>The most common MTU is 1500 bytes, which is the largest packet size for ethernet
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networks. Many home DSL connections, however, tunnel IP through PPPoE (Point to
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Point Protocol over Ethernet. Yes, that is the old dial-up modem protocol). This
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protocol uses up 8 bytes per packet for its own header.</p>
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<p>If the user happens to be on an internet connection over a VPN, it will add another
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layer, with its own packet headers.</p>
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<p>In short; if you would pick the largest possible packet size on an ethernet network,
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1472, and stick with it, you would be quite likely to generate fragments for a lot
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of connections. The fragments that will be created will be very small and especially
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inflate the overhead waste.</p>
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<p>The other approach of picking a very conservative packet size, that would be very
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unlikely to get fragmented has the following drawbacks:</p>
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<ol class="arabic simple">
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<li>People on good, normal, networks will be penalized with a small packet size.
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Both in terms of router load but also bandwidth waste.</li>
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<li>Software routers are typically not limited by the number of bytes they can route,
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but the number of packets. Small packets means more of them, and more load on
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software routers.</li>
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</ol>
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<p>The solution to the problem of finding the optimal packet size, is to dynamically
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adjust the packet size and search for the largest size that can make it through
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without being fragmented along the path.</p>
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<p>To help do this, you can set the DF bit (Don't Fragment) in your Datagrams. This
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asks routers that otherwise would fragment packets to instead drop them, and send
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back an ICMP message reporting the MTU of the link the packet couldn't fit. With
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this message, it's very simple to discover the path MTU. You simply mark your packets
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not to be fragmented, and change your packet size whenever you receive the ICMP
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packet-too-big message.</p>
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<p>Unfortunately it's not quite that simple. There are a significant number of firewalls
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in the wild blocking all ICMP messages. This means we can't rely on them, we also have
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to guess that a packet was dropped because of its size. This is done by only marking
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certain packets with DF, and if all other packets go through, except for the MTU probes,
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we know that we need to lower our packet sizes.</p>
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<p>If we set up bounds for the path MTU (say the minimum internet MTU, 576 and ethernet's 1500),
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we can do a binary search for the MTU. This would let us find it in just a few round-trips.</p>
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<p>On top of this, libtorrent has an optimization where it figures out which interface a
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uTP connection will be sent over, and initialize the MTU ceiling to that interface's MTU.
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This means that a VPN tunnel would advertize its MTU as lower, and the uTP connection would
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immediately know to send smaller packets, no search required. It also has the side-effect
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of being able to use much larger packet sizes for non-ethernet interfaces or ethernet links
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with jumbo frames.</p>
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</div>
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<div class="section" id="clock-drift">
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<h2>clock drift</h2>
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<a class="reference external image-reference" href="our_delay_base.png"><img alt="our_delay_base_thumb.png" class="align-right" src="our_delay_base_thumb.png" /></a>
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<p>Clock drift is clocks progressing at different rates. It's different from clock
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skew which means clocks set to different values (but which may progress at the same
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rate).</p>
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<p>Any clock drift between the two machines involved in a uTP transfer will result
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in systematically inflated or deflated delay measurements.</p>
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<p>This can be solved by letting the base delay be the lowest seen sample in the last
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<em>n</em> minutes. This is a trade-off between seeing a single packet go straight through
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the queue, with no delay, and the amount of clock drift one can assume on normal computers.</p>
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<p>It turns out that it's fairly safe to assume that one of your packets will in fact go
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straight through without any significant delay, once every 20 minutes or so. However,
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the clock drift between normal computers can be as much as 17 ms in 10 minutes. 17 ms
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is quite significant, especially if your target delay is 25 ms (as in the <a class="reference external" href="https://datatracker.ietf.org/doc/draft-ietf-ledbat-congestion/">LEDBAT</a> spec).</p>
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<p>Clocks progresses at different rates depending on temperature. This means computers
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running hot are likely to have a clock drift compared to computers running cool.</p>
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<p>So, by updating the delay base periodically based on the lowest seen sample, you'll either
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end up changing it upwards (artificaially making the delay samples appear small) without
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the congestion or delay actually having changed, or you'll end up with a significant clock
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drift and have artificially low samples because of that.</p>
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<p>The solution to this problem is based on the fact that the clock drift is only a problem
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for one of the sides of the connection. Only when your delay measurements keep increasing
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is it a problem. If your delay measurements keep decreasing, the samples will simply push
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down the delay base along with it. With this in mind, we can simply keep track of the
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other end's delay measurements as well, applying the same logic to it. Whenever the
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other end's base delay is adjusted downwards, we adjust our base delay upwards by the same
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amount.</p>
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<p>This will accurately keep the base delay updated with the clock drift and improve
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the delay measurements. The figure on the right shows the absolute timestamp differences
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along with the base delay. The slope of the measurements is caused by clock drift.</p>
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<p>For more information on the clock drift compensation, see the slides from BitTorrent's
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presentation at <a class="reference external" href="http://www.usenix.org/event/iptps10/tech/slides/cohen.pdf">IPTPS10</a>.</p>
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</div>
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<div class="section" id="features">
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<h2>features</h2>
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<p>libtorrent's uTP implementation includes the following features:</p>
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<ul class="simple">
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<li>Path MTU discovery, including jumbo frames and detecting restricted
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MTU tunnels. Binary search packet sizes to find the largest non-fragmented.</li>
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<li>Selective ACK. The ability to acknowledge individual packets in the
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event of packet loss</li>
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<li>Fast resend. The first time a packet is lost, it's resent immediately.
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Triggered by duplicate ACKs.</li>
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<li>Nagle's algorithm. Minimize protocol overhead by attempting to lump
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full packets of payload together before sending a packet.</li>
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<li>Delayed ACKs to minimize protocol overhead.</li>
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<li>Microsecond resolution timestamps.</li>
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<li>Advertised receive window, to support download rate limiting.</li>
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<li>Correct handling of wrapping sequence numbers.</li>
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<li>Easy configuration of target-delay, gain-factor, timeouts, delayed-ack
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and socket buffers.</li>
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</ul>
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</div>
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</div>
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</div>
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<div id="footer">
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<span>Copyright © 2005-2013 Rasterbar Software.</span>
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</div>
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