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<main id="server-traffic-shaping">
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<h1><a href="#server-traffic-shaping">Server traffic shaping</a></h1>
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<p>This article covers implementing server traffic shaping on Linux with CAKE. The aim
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is to provide fair usage of bandwidth between clients and consistently low latency
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for dedicated and virtual servers provided by companies like OVH and others.</p>
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<p>Traffic shaping is generally discussed in the context of a router shaping traffic
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for a local network with assorted clients connected. It also has a lot to offer on a
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server where you don't control the network. If you control your own infrastructure
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from the server to the ISP, you probably want to do this on the routers instead.</p>
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<p>This article was motivated by the serious lack of up-to-date information on this
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topic elsewhere. It's very easy to implement on modern Linux kernels and the results
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are impressive from extremely simple test cases to heavily loaded servers.</p>
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<section id="problem">
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<h2><a href="#problem">Problem</a></h2>
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<p>A server will generally be provisioned with a specific amount of bandwidth
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enforced by a router in close proximity. This router acts as the bottleneck and
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ends up being in charge of most of the queuing and congestion decisions. Unless
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that's under your control, the best you can hope for is that the router is
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configured to use <code>fq_codel</code> as the queuing discipline (qdisc) to
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provide fair queuing between streams and low latency by preventing a substantial
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backlog of data.</p>
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<p>Unfortunately, the Linux kernel still defaults to <code>pfifo_fast</code>
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instead of the much saner <code>fq_codel</code> algorithm. This is changed by a
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configuration file shipped with systemd, so <em>most</em> distributions using
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systemd as init end up with a sane default. Debian removes that configuration and
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doesn't set a sane default itself, and is widely used. Many server providers like
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OVH do not appear to use consistently use modern queue disciplines like
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<code>fq_codel</code> within their networks, particularly at artificial
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bottlenecks implementing rate limiting based on product tiers.</p>
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<p>If the bottleneck doesn't use fair queuing, division of bandwidth across
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streams is very arbitrary and latency suffers under congestion. These issues are
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often referred to as bufferbloat, and <code>fq_codel</code> is quite good at
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resolving it.</p>
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<p>The <code>fq_codel</code> algorithm is far from perfect. It has issues with
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hash collisions and more importantly only does fair queuing between streams.
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Buffer bloat also isn't the only relevant issue. Clients with multiple connections
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receive more bandwidth and a client can open a large number of connections to
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maximize their bandwidth usage at the expense of others. Fair queuing is important
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beyond as a solution to bufferbloat and there's more to fair queuing than doing it
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only based on streams.</p>
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<p>Traditionally, web browsers open a bunch of HTTP/1.1 connections to each server
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which ends up giving them an unfair amount of bandwidth. HTTP/2 is much friendlier
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since it uses a single connection to each server for the entire browser. Download
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managers take this to the extreme and intentionally use many connections to bypass
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server limits and game the division of resources between clients.</p>
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</section>
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<section id="solution">
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<h2><a href="#solution">Solution</a></h2>
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<p>Linux 4.19 and later makes it easy to solve all of these problems. The CAKE
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queuing discipline provides sophisticated fair queuing based on destination and
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source addresses with finer-grained fairness for individual streams.</p>
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<p> Unfortunately, simply enabling it as your queuing discipline isn't enough
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since it's highly unlikely that your server is the network bottleneck. You need to
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configure it with a bandwidth limit based on the provisioned bandwidth to move the
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bottleneck under your control where you can control how traffic is queued.</p>
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</section>
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<section id="results">
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<h2><a href="#results">Results</a></h2>
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<p>We've used an 100mbit OVH server for as a test platform for a case where
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clients can easily max out the server bandwidth on their own. As a very simple
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example, consider 2 clients with more than 100mbit of bandwidth each downloading a
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large file. These are (rounded) real world results with CAKE:</p>
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<ul>
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<li>client A with 1 connection gets 50mbit</li>
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<li>client B with 10 connections gets 5mbit each adding up to 50mbit</li>
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</ul>
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<p>CAKE with <code>flows</code> instead of the default <code>triple-isolate</code> to
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mimic <code>fq_codel</code> at a bottleneck:</p>
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<ul>
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<li>client A with 1 connection gets 9mbit</li>
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<li>client B with 10 connections gets 9mbit each adding up to 90mbit</li>
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</ul>
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<p>The situation without traffic shaping is a mess. Latency takes a serious hit
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that's very noticeable via SSH. Bandwidth is consistently allocated very unevenly
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and ends up fluctuating substantially between test runs. The connections tend to
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settle near a rate, often significantly lower or higher than the fair 9mbit
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amount. It's generally something like this, but the range varies a lot:</p>
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<ul>
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<li>client A with 1 connection gets ~6mbit to ~14mbit</li>
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<li>client B with 10 connections gets ~6mbit to ~14mbit each adding up to ~86mbit
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to ~94mbit</li>
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</ul>
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<p>CAKE continues working as expected with a far higher number of connections. It
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technically has a higher CPU cost than <code>fq_codel</code>, but that's much more
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of a concern for low end router hardware. It hardly matters on a server, even one
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that's under heavy CPU load. The improvement in user experience is substantial and
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it's very noticeable in web page load speeds when a server is under load.</p>
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</section>
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<section id="implementation">
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<h2><a href="#implementation">Implementation</a></h2>
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<p>For a server with 2000mbit of bandwidth provisioned, you could start by trying
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it with 99.75% of the provisioned bandwidth:</p>
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<pre>tc qdisc replace dev eth0 root cake bandwidth 1995mbit besteffort</pre>
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<p>On a server, setting it to use 100% of the provisioned bandwidth may work fine
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in practice. Unlike a local network connected to a consumer ISP, you shouldn't
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need to sacrifice anywhere close to the typically recommended 5-10% of your
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bandwidth for traffic shaping.</p>
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<p>This also sets <code>besteffort</code> for the common case where the server
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doesn't have appropriate Quality of Service markings set up via Diffserv. Fair
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scheduling is already great at providing low latency by cycling through the hosts
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and streams without needing this kind of configuration. The defaults for Diffserv
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traffic classes like real-time video are set up to yield substantial bandwidth in
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exchange for lower latency. It's easy to set this up wrong and it usually won't
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make much sense on a server. You might want to set up marking low priority traffic
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like system updates, but it will already get a tiny share of the overall traffic
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on a loaded server due to fair scheduling between hosts and streams.</p>
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<p>You can use the <code>tc -s qdisc</code> command to monitor CAKE:</p>
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<pre>tc -s qdisc show dev eth0</pre>
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<p>If you want to keep an eye on how it changes over time:</p>
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<pre>watch -n 1 tc -s qdisc show dev eth0</pre>
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<p>This is very helpful for figuring out if you've successfully moved the
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bottleneck to the server. If the bandwidth is being fully used, it should
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consistently have a backlog of data where it's applying the queuing discipline.
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The backlog shouldn't be draining to near zero under full bandwidth usage as that
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indicates the bottleneck is the server application itself or a different network
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bottleneck.</p>
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<p>If you use systemd-network, you can add a CAKE configuration section to the
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network configuration file instead of manually running the <code>tc</code> command
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with a <code>Type=oneshot</code> service on boot:</p>
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<pre>[CAKE]
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Bandwidth=1995M
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PriorityQueueingPreset=besteffort</pre>
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</section>
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<section id="quicker-backpressure-propagation">
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<h2><a href="#quicker-backpressure-propagation">Quicker backpressure propagation</a></h2>
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<p>The Linux kernel can be tuned to more quickly propagate TCP backpressure up to
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applications while still maximizing bandwidth usage. This is incredibly useful for
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interactive applications aiming to send the freshest possible copy of data and for
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protocols like HTTP/2 multiplexing streams/messages with different priorities over
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the same TCP connection. This can also substantially reduce memory usage for TCP
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by reducing buffer sizes closer to the optimal amount for maximizing bandwidth
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use without wasting memory. The downside to quicker backpressure propagation is
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increased CPU usage from additional system calls and context switches.</p>
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<p>The Linux kernel automatically adjusts the size of the write queue to maximize
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bandwidth usage. The write queue is divided into unacknowledged bytes (TCP window
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size) and unsent bytes. As acknowledgements of transmitted data are received, it
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frees up space for the application to queue more data. The queue of unsent bytes
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provides the leeway needed to wake the application and obtain more data. This can
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be reduced using <code>net.ipv4.tcp_notsent_lowat</code> to reduce the default and
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the <code>TCP_NOTSENT_LOWAT</code> socket option to override it per-socket.</p>
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<p>A reasonable choice for internet-based workloads concerned about latency and
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particularly prioritization within TCP connections but unwilling to sacrifice
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throughput is 128kiB. To configure this, set the following in
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<code>/etc/sysctl.d/local.conf</code> or another sysctl configuration file and
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load it with <code>sysctl --system</code>:</p>
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<pre>net.ipv4.tcp_notsent_lowat = 131072</pre>
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<p>Using values as low as 16384 can make sense to further improve latency and
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prioritization. However, it's more likely to negatively impact throughput and will
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further increase CPU usage. Use at least 128k or the default of not limiting the
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automatic unsent buffer size unless you're going to do substantial testing to make
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sure there's not a negative impact for the workload.</p>
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<p>If you decide to use <code>tcp_notsent_lowat</code>, be aware that newer Linux
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kernels (Linux 5.0+ with a further improvement for Linux 5.10+) are recommended to
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substantially reduce system calls / context switches by not triggering the
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application to provide more data until over half the unsent byte buffer is
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empty.</p>
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</section>
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<section id="future">
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<h2><a href="#future">Future</a></h2>
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<p>Ideally, data centers would deploy CAKE throughout their networks with the
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default <code>triple-isolate</code> flow isolation. This may mean they need to use
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more powerful hardware for routing. If the natural bottlenecks used CAKE, setting
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up traffic shaping on the server wouldn't be necessary. This doesn't seem likely
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any time soon. Deploying <code>fq_codel</code> is much more realistic and tackles
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buffer bloat but not the issue of fairness between hosts rather than only
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streams.</p>
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