[iproute PATCH 12/12] doc: Add my article about tc, filters and actions

Phil Sutter
2016-03-04T12:13:09Z

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-PSFILES=ip-cref.ps ip-tunnels.ps api-ip6-flowlabels.ps ss.ps nstat.ps arpd.ps rtstat.ps
+PSFILES=ip-cref.ps ip-tunnels.ps api-ip6-flowlabels.ps ss.ps nstat.ps arpd.ps rtstat.ps tc-filters.ps
# tc-cref.ps
# api-rtnl.tex api-pmtudisc.tex api-news.tex
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+\documentclass[12pt,twoside]{article}
+
+\usepackage[hidelinks]{hyperref} % \url
+\usepackage{booktabs} % nicer tabulars
+\usepackage{fancyvrb}
+\usepackage{fullpage}
+\usepackage{float}
+
+\newcommand{\iface}{\textit}
+\newcommand{\cmd}{\texttt}
+\newcommand{\man}{\textit}
+\newcommand{\qdisc}{\texttt}
+\newcommand{\filter}{\texttt}
+
+\begin{document}
+\title{QoS in Linux with TC and Filters}
+\author{Phil Sutter (phil@nwl.cc)}
+\date{January 2016}
+\maketitle
+
+TC, the Traffic Control utility, has been there for a very long time - forever
+in my humble perception. It is still (and has ever been if I'm not mistaken) the
+only tool to configure QoS in Linux.
+
+Standard practice when transmitting packets over a medium which may block (due
+to congestion, e.g.) is to use a queue which temporarily holds these packets. In
+Linux, this queueing approach is where QoS happens: A Queueing Discipline
+(qdisc) holds multiple packet queues with different priorities for dequeueing to
+the network driver. The classification (i.e. deciding which queue a packet
+should go into) is typically done based on Type Of Service (IPv4) or Traffic
+Class (IPv6) header fields but depending on qdisc implementation, might be
+controlled by the user as well.
+
+Qdiscs come in two flavors, classful or classless. While classless qdiscs are
+not as flexible as classful ones, they also require much less customizing. Often
+it is enough to just attach them to an interface, without exact knowledge of
+what is done internally. Classful qdiscs are the exact opposite: flexible in
+application, they are often not even usable without insightful configuration.
+
+As the name implies, classful qdiscs provide configurable classes to sort
+traffic into. In it's basic form, this is not much different than, say, the
+classless \qdisc{pfifo\_fast} which holds three queues and classifies per
+packet upon priority field. Though typically classes go beyond that by
+supporting nesting and additional characteristics like e.g. maximum traffic
+rate or quantum.
+
+When it comes to controlling the classification process, filters come into play.
+They attach to the parent of a set of classes (i.e. either the qdisc itself or
+a parent class) and specify how a packet (or it's associated flow) has to look
+like in order to suit a given class. To overcome this simplification, it is
+possible to attach multiple filters to the same parent, which then consults each
+of them in row until the first one accepts the packet.
+
+Before getting into detail about what filters there are and how to use them, a
+simple setup of a qdisc with classes is necessary:
+\begin{figure}[H]
+\begin{Verbatim}
+ .-------------------------------------------------------.
+ | |
+ | HTB |
+ | |
+ | .----------------------------------------------------.|
+ | | ||
+ | | Class 1:1 ||
+ | | ||
+ | | .---------------..---------------..---------------.||
+ | | | || || |||
+ | | | Class 1:10 || Class 1:20 || Class 1:30 |||
+ | | | || || |||
+ | | | .------------.|| .------------.|| .------------.|||
+ | | | | ||| | ||| | ||||
+ | | | | fq_codel ||| | fq_codel ||| | fq_codel ||||
+ | | | | ||| | ||| | ||||
+ | | | '------------'|| '------------'|| '------------'|||
+ | | '---------------''---------------''---------------'||
+ | '----------------------------------------------------'|
+ '-------------------------------------------------------'
+\end{Verbatim}
+\end{figure}
+\noindent
+The following commands establish the basic setup shown:
+\begin{Verbatim}
+(1) # tc qdisc replace dev eth0 root handle 1: htb default 30
+(2) # tc class add dev eth0 parent 1: classid 1:1 htb rate 95mbit
+(3) # alias tclass='tc class add dev eth0 parent 1:1'
+(4) # tclass classid 1:10 htb rate 1mbit ceil 20mbit prio 1
+(4) # tclass classid 1:20 htb rate 90mbit ceil 95mbit prio 2
+(4) # tclass classid 1:30 htb rate 1mbit ceil 95mbit prio 3
+(5) # tc qdisc add dev eth0 parent 1:10 fq_codel
+(5) # tc qdisc add dev eth0 parent 1:20 fq_codel
+(5) # tc qdisc add dev eth0 parent 1:30 fq_codel
+\end{Verbatim}
+A little explanation for the unfamiliar reader:
+\begin{enumerate}
+\item Replace the root qdisc of \iface{eth0} by an instance of \qdisc{HTB}.
+ Specifying the handle is necessary so it can be referenced in consecutive
+ calls to \cmd{tc}. The default class for unclassified traffic is set to
+ 30.
+\item Create a single top-level class with handle 1:1 which limits the total
+ bandwidth allowed to 95mbit/s. It is assumed that \iface{eth0} is a 100mbit/s link,
+ staying a little below that helps to keep the main point of enqueueing in
+ the qdisc layer instead of the interface hardware queue or at another
+ bottleneck in the network.
+\item Define an alias for the common part of the remaining three calls in order
+ to improve readability. This means all remaining classes are attached to the
+ common parent class from (2).
+\item Create three child classes for different uses: Class 1:10 has highest
+ priority but is tightly limited in bandwidth - fine for interactive
+ connections. Class 1:20 has mid priority and high guaranteed bandwidth, for
+ high priority bulk traffic. Finally, there's the default class 1:30 with
+ lowest priority, low guaranteed bandwidth and the ability to use the full
+ link in case it's unused otherwise. This should be fine for uninteresting
+ traffic not explicitly taken care of.
+\item Attach a leaf qdisc to each of the child classes created in (4). Since
+ \qdisc{HTB} by default attaches \qdisc{pfifo} as leaf qdisc, this step is optional. Still,
+ the fairness between different flows provided by the classless \qdisc{fq\_codel} is
+ worth the effort.
+\end{enumerate}
+More information about the qdiscs and fine-tuning parameters can be found in
+\man{tc-htb(8)} and \man{tc-fq\_codel(8)}.
+
+Without any additional setup done, now all traffic leaving \iface{eth0} is shaped to
+95mbit/s and directed through class 1:30. This can be verified by looking at the
+\texttt{Sent} field of the class statistics printed via \cmd{tc -s class show dev eth0}:
+Only the root class 1:1 and it's child 1:30 should show any traffic.
+
+
+\section*{Finally time to start filtering!}
+
+Let's begin with a simple one, i.e. reestablishing what \qdisc{pfifo\_fast} did
+automatically based on TOS/Priority field. Linux internally translates the
+header field into the priority field of struct skbuff, which
+\qdisc{pfifo\_fast} uses for
+classification. \man{tc-prio(8)} contains a table listing the priority (and
+ultimately, \qdisc{pfifo\_fast} queue index) each TOS value is being translated into.
+Here is a shorter version:
+\begin{center}
+\begin{tabular}{lll}
+TOS Values & Linux Priority (Number) & Queue Index \\
+\midrule
+0x0 - 0x6 & Best Effort (0) & 1 \\
+0x8 - 0xe & Bulk (2) & 2 \\
+0x10 - 0x16 & Interactive (6) & 0 \\
+0x18 - 0x1e & Interactive Bulk (4) & 1 \\
+\end{tabular}
+\end{center}
+Using the \filter{basic} filter, it is possible to match packets based on that skbuff
+field, which has the added benefit of being IP version agnostic. Since the
+\qdisc{HTB} setup above defaults to class ID 1:30, the Bulk priority can be
+ignored. The \filter{basic} filter allows to combine matches, therefore we get along
+with only two filters:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: basic \
+ match 'meta(priority eq 6)' classid 1:10
+# tc filter add dev eth0 parent 1: basic \
+ match 'meta(priority eq 0)' \
+ or 'meta(priority eq 4)' classid 1:20
+\end{Verbatim}
+A detailed description of the \filter{basic} filter and the ematch syntax it uses can be
+found in \man{tc-basic(8)} and \man{tc-ematch(8)}.
+
+Obviously, this first example cries for optimization. A simple one would be to
+just change the default class from 1:30 to 1:20, so filters are only needed for
+Bulk and Interactive priorities:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: basic \
+ match 'meta(priority eq 6)' classid 1:10
+# tc filter add dev eth0 parent 1: basic \
+ match 'meta(priority eq 2)' classid 1:20
+\end{Verbatim}
+Given that class IDs are random, choosing them wisely allows for a direct
+mapping. So first, recreate the qdisc and classes configuration:
+\begin{Verbatim}
+# tc qdisc replace dev eth0 root handle 1: htb default 10
+# tc class add dev eth0 parent 1: classid 1:1 htb rate 95mbit
+# alias tclass='tc class add dev eth0 parent 1:1'
+# tclass classid 1:16 htb rate 1mbit ceil 20mbit prio 1
+# tclass classid 1:10 htb rate 90mbit ceil 95mbit prio 2
+# tclass classid 1:12 htb rate 1mbit ceil 95mbit prio 3
+# tc qdisc add dev eth0 parent 1:16 fq_codel
+# tc qdisc add dev eth0 parent 1:10 fq_codel
+# tc qdisc add dev eth0 parent 1:12 fq_codel
+\end{Verbatim}
+This is basically identical to above, but with changed leaf class IDs and the
+second priority class being the default. Using the \filter{flow} filter with it's \texttt{map}
+functionality, a single filter command is enough:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: handle 0x1337 flow \
+ map key priority baseclass 1:10
+\end{Verbatim}
+The \filter{flow} filter now uses the priority value to construct a destination class ID
+by adding it to the value of \texttt{baseclass}. While this works for priority values of
+0, 2 and 6, it will result in non-existent class ID 1:14 for Interactive Bulk
+traffic. In that case, the \qdisc{HTB} default applies so that traffic goes into class
+ID 1:10 just as intended. Please note that specifying a handle is a mandatory
+requirement by the \filter{flow} filter, although I didn't see where one would use that
+later. For more information about \filter{flow}, see \man{tc-flow(8)}.
+
+While \filter{flow} and \filter{basic} filters are relatively easy to apply and understand, they
+are as well quite limited to their intended purpose. A more flexible option is
+the \filter{u32} filter, which allows to match on arbitrary parts of the packet data -
+yet only on that, not any meta data associated to it by the kernel (with the
+exception of firewall mark value). So in order to continue this little
+exercise with \filter{u32}, we have to base classification directly upon the actual TOS
+value. An intuitive attempt might look like this:
+\begin{Verbatim}
+# alias tcfilter='tc filter add dev eth0 parent 1:'
+# tcfilter u32 match ip dsfield 0x10 0x1e classid 1:16
+# tcfilter u32 match ip dsfield 0x12 0x1e classid 1:16
+# tcfilter u32 match ip dsfield 0x14 0x1e classid 1:16
+# tcfilter u32 match ip dsfield 0x16 0x1e classid 1:16
+# tcfilter u32 match ip dsfield 0x8 0x1e classid 1:12
+# tcfilter u32 match ip dsfield 0xa 0x1e classid 1:12
+# tcfilter u32 match ip dsfield 0xc 0x1e classid 1:12
+# tcfilter u32 match ip dsfield 0xe 0x1e classid 1:12
+\end{Verbatim}
+The obvious drawback here is the amount of filters needed. And without the
+default class, eight more filters would be necessary. This also has performance
+implications: A packet with TOS value 0xe will be checked eight times in total
+in order to determine it's destination class. While there's not much to be done
+about the number of filters, at least the performance problem can be eliminated
+by using \filter{u32}'s hash table support:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: prio 99 handle 1: u32 divisor 16
+\end{Verbatim}
+This creates a hash table with 16 buckets. The table size is arbitrary, but not
+random: Since the first bit of the TOS field is not interesting, it can be
+ignored and therefore the range of values to consider is just [0;15], i.e. a
+number of 16 different values. The next step is to populate the hash table:
+\begin{Verbatim}
+# alias tcfilter='tc filter add dev eth0 parent 1: prio 99'
+# tcfilter u32 match u8 0 0 ht 1:0: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:1: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:2: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:3: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:4: classid 1:12
+# tcfilter u32 match u8 0 0 ht 1:5: classid 1:12
+# tcfilter u32 match u8 0 0 ht 1:6: classid 1:12
+# tcfilter u32 match u8 0 0 ht 1:7: classid 1:12
+# tcfilter u32 match u8 0 0 ht 1:8: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:9: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:a: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:b: classid 1:16
+# tcfilter u32 match u8 0 0 ht 1:c: classid 1:10
+# tcfilter u32 match u8 0 0 ht 1:d: classid 1:10
+# tcfilter u32 match u8 0 0 ht 1:e: classid 1:10
+# tcfilter u32 match u8 0 0 ht 1:f: classid 1:10
+\end{Verbatim}
+The parameter \texttt{ht} denotes the hash table and bucket the filter should be added
+to. Since the first TOS bit is ignored, it's value has to be divided by two in
+order to get to the bucket it maps to. E.g. a TOS value of 0x10 will therefore
+map to bucket 0x8. For the sake of completeness, all possible values are mapped
+and therefore a configurable default class is not required. Note that the used
+match expression is not necessary, but mandatory. Therefore anything that
+matches any packet will suffice. Finally, a filter which links to the defined
+hash table is needed:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: prio 1 protocol ip u32 \
+ link 1: hashkey mask 0x001e0000 match u8 0 0
+\end{Verbatim}
+Here again, the actual match statement is not necessary, but syntactically
+required. All the magic lies within the \texttt{hashkey} parameter, which defines which
+part of the packet should be used directly as hash key. Here's a drawing of the
+first four bytes of the IPv4 header, with the area selected by \texttt{hashkey mask}
+highlighted:
+\begin{figure}[H]
+\begin{Verbatim}
+ 0 1 2 3
+ .-----------------------------------------------------------------.
+ | | | ######## | | |
+ | Version| IHL | #DSCP### | ECN| Total Length |
+ | | | ######## | | |
+ '-----------------------------------------------------------------'
+\end{Verbatim}
+\end{figure}
+\noindent
+This may look confusing at first, but keep in mind that bit- as well as
+byte-ordering here is LSB while the mask value is written in MSB we humans use.
+Therefore reading the mask is done like so, starting from left:
+\begin{enumerate}
+\item Skip the first byte (which contains Version and IHL fields).
+\item Skip the lowest bit of the second byte (0x1e is even).
+\item Mark the four following bits (0x1e is 11110 in binary).
+\item Skip the remaining three bits of the second byte as well as the remaining two
+ bytes.
+\end{enumerate}
+Before doing the lookup, the kernel right-shifts the masked value by the amount
+of zero-bits in \texttt{mask}, which implicitly also does the division by two which the
+hash table depends on. With this setup, every packet has to pass exactly two
+filters to be classified. Note that this filter is limited to IPv4 packets: Due
+to the related Traffic Class field being at a different offset in the packet, it
+would not work for IPv6. To use the same setup for IPv6 as well, a second
+entry-level filter is necessary:
+\begin{Verbatim}
+# tc filter add dev eth0 parent 1: prio 2 protocol ipv6 u32 \
+ link 1: hashkey mask 0x01e00000 match u8 0 0
+\end{Verbatim}
+For illustration purposes, here again is a drawing of the first four bytes of
+the IPv6 header, again with masked area highlighted:
+\begin{figure}[H]
+\begin{Verbatim}
+ 0 1 2 3
+ .-----------------------------------------------------------------.
+ | | ######## | |
+ | Version| #Traffic Class| Flow Label |
+ | | ######## | |
+ '-----------------------------------------------------------------'
+\end{Verbatim}
+\end{figure}
+\noindent
+Reading the mask value is analogous to IPv4 with the added complexity that
+Traffic Class spans over two bytes. Yet, for comparison there's a simple trick:
+IPv6 has the interesting field shifted by four bits to the left, and the new
+mask's value is shifted by the same amount. For further information about
+\filter{u32} and what can be done with it, consult it's man page
+\man{tc-u32(8)}.
+
+Of course, the kernel provides many more filters than just \filter{basic},
+\filter{flow} and \filter{u32} which have been presented above. As of now, the
+remaining ones are:
+\begin{description}
+\item[bpf]
+ Filtering using Berkeley Packet Filter programs. The program's return
+ code determines the packet's destination class ID.
+
+\item[cgroup]
+ Filter packets based on control groups. This is only useful for packets
+ originating from the local host, as control groups only exist in that
+ scope.
+
+\item[flower]
+ An extended variant of the flow filter.
+
+\item[fw]
+ Matches on firewall mark values previously assigned to the packet by
+ netfilter (or a filter action, see below for details). This allows to
+ export the classification algorithm into netfilter, which is very
+ convenient if appropriate rules exist on the same system in there
+ already.
+
+\item[route]
+ Filter packets based on matching routing table entry. Basically
+ equivalent to the \texttt{fw} filter above, to make use of an already existing
+ extensive routing table setup.
+
+\item[rsvp, rsvp6]
+ Implementation of the Resource Reservation Protocol in Linux, to react
+ upon requests sent by an RSVP daemon.
+
+\item[tcindex]
+ Match packets based on tcindex value, which is usually set by the dsmark
+ qdisc. This is part of an approach to support Differentiated Services in
+ Linux, which is another topic on it's own.
+\end{description}
+
+
+\section*{Filter Actions}
+
+The tc filter framework provides the infrastructure to another extensible set of
+tools as well, namely tc actions. As the name suggests, they allow to do things
+with packets (or associated data). (The list of) Actions are part of a given
+filter. If it matches, each action it contains is executed in order before
+returning the classification result. Since the action has direct access to the
+latter, it is in theory possible for an action to react upon or even change the
+filtering result - as long as the packet matched, of course. Yet none of the
+currently in-tree actions make use of this.
+
+The Generic Actions framework originally evolved out of the filters' ability to
+police traffic to a given maximum bandwidth. One common use case for that is to
+limit ingress traffic, dropping packets which exceed the threshold. A classic
+setup example is like so:
+\begin{Verbatim}
+# tc qdisc add dev eth0 handle ffff: ingress
+# tc filter add dev eth0 parent ffff: u32 \
+ match u32 0 0
+ police rate 1mbit burst 100k
+\end{Verbatim}
+The ingress qdisc is not a real one, but merely a point of reference for filters
+to attach to which should get applied to incoming traffic. The \filter{u32} filter added
+above matches on any packet and therefore limits the total incoming bandwidth to
+1mbit/s, allowing bursts of up to 100kbytes. Using the new syntax, the filter
+command changes slightly:
+\begin{Verbatim}
+# tc filter add dev eth0 parent ffff: u32 \
+ match u32 0 0 \
+ action police rate 1mbit burst 100k
+\end{Verbatim}
+The important detail is that this syntax allows to define multiple actions.
+E.g. for testing purposes, it is possible to redirect exceeding traffic to the
+loopback interface instead of dropping it:
+\begin{Verbatim}
+# tc filter add dev eth0 parent ffff: u32 \
+ match u32 0 0 \
+ action police rate 1mbit burst 100k conform-exceed pipe \
+ action mirred egress redirect dev lo
+\end{Verbatim}
+The added parameter \texttt{conform-exceed pipe} tells the police action to allow for
+further actions to handle the exceeding packet.
+
+Apart from \texttt{police} and \texttt{mirred} actions, there are a few more. Here's a full
+list of the currently implemented ones:
+\begin{description}
+\item[bpf]
+ Apply a Berkeley Packet Filter program to the packet.
+
+\item[connmark]
+ Set the packet's firewall mark to that of it's connection. This works by
+ searching the conntrack table for a matching entry. If found, the mark
+ is restored.
+
+\item[csum]
+ Trigger recalculation of packet checksums. The supported protocols are:
+ IPv4, ICMP, IGMP, TCP, UDP and UDPLite.
+
+\item[ipt]
+ Pass the packet to an iptables target. This allows to use iptables
+ extensions directly instead of having to go the extra mile via setting
+ an arbitrary firewall mark and matching on that from within netfilter.
+
+\item[mirred]
+ Mirror or redirect packets. This is often combined with the ifb pseudo
+ device to share a common QoS setup between multiple interfaces or even
+ ingress traffic.
+
+\item[nat]
+ Perform stateless Native Address Translation. This is certainly not
+ complete and therefore inferior to NAT using iptables: Although the
+ kernel module decides between TCP, UDP and ICMP traffic, it does not
+ handle typical problematic protocols such as active FTP or SIP.
+
+\item[pedit]
+ Generic packet editing. This allows to alter arbitrary bytes of the
+ packet, either by specifying an offset into the packet or by naming a
+ packet header and field name to change. Currently, the latter is
+ implemented only for IPv4 yet.
+
+\item[police]
+ Apply a bandwidth rate limiting policy. Packets exceeding it are dropped
+ by default, but may optionally be handled differently.
+
+\item[simple]
+ This is rather an example than real action. All it does is print a
+ user-defined string together with a packet counter. Useful maybe for
+ debugging when filter statistics are not available or too complicated.
+
+\item[skbedit]
+ Edit associated packet data, supports changing queue mapping, priority
+ field and firewall mark value.
+
+\item[vlan]
+ Add/remove a VLAN header to/from the packet. This might serve as
+ alternative to using 802.1Q pseudo-interfaces in combination with
+ routing rules when e.g. packets for a given destination need to be
+ encapsulated.
+\end{description}
+
+
+\section*{Intermediate Functional Block}
+
+The Intermediate Functional Block (\texttt{ifb}) pseudo network interface acts as a QoS
+concentrator for multiple different sources of traffic. Packets from or to other
+interfaces have to be redirected to it using the \texttt{mirred} action in order to be
+handled, regularly routed traffic will be dropped. This way, a single stack of
+qdiscs, classes and filters can be shared between multiple interfaces.
+
+Here's a simple example to feed incoming traffic from multiple interfaces
+through a Stochastic Fairness Queue (\qdisc{sfq}):
+\begin{Verbatim}
+(1) # modprobe ifb
+(2) # ip link set ifb0 up
+(3) # tc qdisc add dev ifb0 root sfq
+\end{Verbatim}
+The first step is to load the \texttt{ifb} kernel module (1). By default, this will
+create two ifb devices: \iface{ifb0} and \iface{ifb1}. After setting
+\iface{ifb0} up in (2), the root
+qdisc is replaced by \qdisc{sfq} in (3). Finally, one can start redirecting ingress
+traffic to \iface{ifb0}, e.g. from \iface{eth0}:
+\begin{Verbatim}
+# tc qdisc add dev eth0 handle ffff: ingress
+# tc filter add dev eth0 parent ffff: u32 \
+ match u32 0 0 \
+ action mirred egress redirect dev ifb0
+\end{Verbatim}
+The same can be done for other interfaces, just replacing \iface{eth0} in the two
+commands above. One thing to keep in mind here is the asymmetrical routing this
+creates within the host doing the QoS: Incoming packets enter the system via
+\iface{ifb0}, while corresponding replies leave directly via \iface{eth0}. This can be observed
+using \cmd{tcpdump} on \iface{ifb0}, which shows the input part of the traffic only. What's
+more confusing is that \cmd{tcpdump} on \iface{eth0} shows both incoming and outgoing traffic,
+but the redirection is still effective - a simple prove is setting
+\iface{ifb0} down,
+which will interrupt the communication. Obviously \cmd{tcpdump} catches the packets to
+dump before they enter the ingress qdisc, which is why it sees them while the
+kernel itself doesn't.
+
+
+\section*{Conclusion}
+
+My personal impression is that although the \cmd{tc} utility is an absolute
+necessity for anyone aiming at doing QoS in Linux professionally, there are way
+too many loose ends and trip wires present in it's environment. Contributing to
+this is the fact, that much of the non-essential functionality is redundantly
+available in netfilter. Another problem which adds weight to the first one is a
+general lack of documentation. Of course, there are many HOWTOs and guides in
+the internet, but since it's often not clear how up to date these are, I prefer
+the usual resources such as man or info pages. Surely nothing one couldn't fix
+in hindsight, but quality certainly suffers if the original author of the code
+does not or can not contribute to that.
+
+All that being said, once the steep learning curve has been mastered, the
+conglomerate of (classful) qdiscs, filters and actions provides a highly
+sophisticated and flexible infrastructure to perform QoS, which plays nicely
+along with routing and firewalling setups.
+
+
+\section*{Further Reading}
+
+A good starting point for novice users and experienced ones diving into unknown
+areas is the extensive HOWTO at \url{http://lartc.org}. The iproute2 package ships
+some examples (usually in /usr/share/doc/, depending on distribution) as well as
+man pages for \cmd{tc} in general, qdiscs and filters. The latter have been added
+just recently though, so if your distribution does not ship iproute2 version
+4.3.0 yet, these are not in there. Apart from that, the internet is a spring of
+HOWTOs and scripts people wrote - though these should be taken with a grain of
+salt: The complexity of the matter often leads to copying others' solutions
+without much validation, which allows for less optimal or even obsolete
+implementations to survive much longer than desired.
+
+\end{document}
--
2.7.2