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>3. Traditional Elements of Traffic Control</H1
><P
> </P
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-shaping"
></A
>3.1. Shaping</H2
><P
> Shapers delay packets to meet a desired rate.
</P
><P
> Shaping is the mechanism by which packets are delayed before
transmission in an output queue to meet a desired output rate. This is
one of the most common desires of users seeking bandwidth control
solutions. The act of delaying a packet as part of a traffic control
solution makes every shaping mechanism into a non-work-conserving
mechanism, meaning roughly: "Work is required in order to delay
packets."
</P
><P
> Viewed in reverse, a non-work-conserving queuing mechanism is performing
a shaping function. A work-conserving queuing mechanism (see
<A
HREF="classful-qdiscs.html#qc-prio"
>PRIO</A
>) would not be capable of delaying a packet.
</P
><P
> Shapers attempt to limit or ration traffic to meet but not exceed a
configured rate (frequently measured in packets per second or bits/bytes
per second). As a side effect, shapers can smooth out bursty traffic
<A
NAME="AEN271"
HREF="#FTN.AEN271"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>.
One of the advantages of shaping bandwidth is the ability to control
latency of packets. The underlying mechanism for shaping to a rate is
typically a token and bucket mechanism. See also
<A
HREF="overview.html#o-tokens"
>Section 2.7</A
> for further detail on tokens and buckets.
</P
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><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-scheduling"
></A
>3.2. Scheduling</H2
><P
> Schedulers arrange and/or rearrange packets for output.
</P
><P
> Scheduling is the mechanism by which packets are arranged (or
rearranged) between input and output of a particular queue. The
overwhelmingly most common scheduler is the FIFO (first-in first-out)
scheduler. From a larger perspective, any set of traffic control
mechanisms on an output queue can be regarded as a scheduler, because
packets are arranged for output.
</P
><P
> Other generic scheduling mechanisms attempt to compensate for various
networking conditions. A fair queuing algorithm (see <A
HREF="classless-qdiscs.html#qs-sfq"
>SFQ</A
>)
attempts to prevent any single client or flow from dominating the
network usage. A round-robin algorithm (see WRR) gives each
flow or client a turn to dequeue packets. Other sophisticated
scheduling algorithms attempt to prevent backbone overload (see
<A
HREF="classless-qdiscs.html#qs-gred"
>GRED</A
>) or refine other scheduling mechanisms (see
<A
HREF="classless-qdiscs.html#qs-esfq"
>ESFQ</A
>).
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-classifying"
></A
>3.3. Classifying</H2
><P
> Classifiers sort or separate traffic into queues.
</P
><P
> Classifying is the mechanism by which packets are separated for
different treatment, possibly different output queues. During the
process of accepting, routing and transmitting a packet, a networking
device can classify the packet a number of different ways.
Classification can include
<A
HREF="elements.html#e-marking"
>marking</A
> the packet, which usually
happens on the boundary of a network under a single administrative
control or classification can occur on each hop individually.
</P
><P
> The Linux model (see
<A
HREF="components.html#c-filter"
>Section 4.3</A
>) allows for a packet to cascade across a
series of classifiers in a traffic control structure and to be
classified in conjunction with
<A
HREF="elements.html#e-policing"
>policers</A
> (see also
<A
HREF="components.html#c-police"
>Section 4.5</A
>).
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-policing"
></A
>3.4. Policing</H2
><P
> Policers measure and limit traffic in a particular queue.
</P
><P
> Policing, as an element of traffic control, is simply
a mechanism by which traffic can be limited. Policing is most
frequently used on the network border to ensure that a peer is not
consuming more than its allocated bandwidth. A policer will accept
traffic to a certain rate, and then perform an action on traffic
exceeding this rate. A rather harsh solution is to
<A
HREF="elements.html#e-dropping"
>drop</A
> the traffic, although the
traffic could be
<A
HREF="elements.html#e-classifying"
>reclassified</A
> instead of being
dropped.
</P
><P
> A policer is a yes/no question about the rate at which traffic is
entering a queue. If the packet is about to enter a queue below a given
rate, take one action (allow the enqueuing). If the packet is about to
enter a queue above a given rate, take another action. Although the
policer uses a token bucket mechanism internally, it does not have the
capability to delay a packet as a <A
HREF="elements.html#e-shaping"
>shaping</A
> mechanism does.
</P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-dropping"
></A
>3.5. Dropping</H2
><P
> Dropping discards an entire packet, flow or classification.
</P
><P
> Dropping a packet is a mechanism by which a packet is discarded.
</P
><P
> </P
></DIV
><DIV
CLASS="section"
><H2
CLASS="section"
><A
NAME="e-marking"
></A
>3.6. Marking</H2
><P
> Marking is a mechanism by which the packet is altered.
</P
><DIV
CLASS="note"
><P
></P
><TABLE
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><TD
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HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>This is not <TT
CLASS="constant"
>fwmark</TT
>. The <B
CLASS="command"
>iptables</B
>target <TT
CLASS="constant"
>MARK</TT
>and the
<B
CLASS="command"
>ipchains</B
><TT
CLASS="option"
>--mark</TT
>are used to modify packet metadata, not the packet
itself.
</TD
></TR
></TABLE
></DIV
><P
> Traffic control marking mechanisms install a DSCP on the packet
itself, which is then used and respected by other routers inside an
administrative domain (usually for DiffServ).
</P
></DIV
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><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
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><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
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><P
> This smoothing effect is not always desirable, hence the HTB
parameters burst and cburst.
</P
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