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145 lines
5.4 KiB
145 lines
5.4 KiB
\subsubsection{Introduction}
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Asterisk supports different QoS settings at the application level for various
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protocols on both signaling and media. The Type of Service (TOS) byte can be
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set on outgoing IP packets for various protocols. The TOS byte is used by the
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network to provide some level of Quality of Service (QoS) even if the network
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is congested with other traffic.
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Asterisk running on Linux can also set 802.1p CoS marks in VLAN packets for the
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VoIP protocols it uses. This is useful when working in a switched environment.
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In fact Asterisk only set priority for Linux socket. For mapping this priority
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and VLAN CoS mark you need to use this command:
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\begin{verbatim}
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vconfig set_egress_map [vlan-device] [skb-priority] [vlan-qos]
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\end{verbatim}
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The table below shows all VoIP channel drivers and other Asterisk modules that
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support QoS settings for network traffic. It also shows the type(s) of
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traffic for which each module can support setting QoS settings.
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\begin{table}[h!]
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\begin{center}
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\begin{tabular}{ | l || c | c | c | c | }
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\hline
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& Signaling & Audio & Video & Text \\ \hline \hline
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chan\_sip & + & + & + & + \\ \hline
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chan\_skinny & + & + & + & \\ \hline
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chan\_mgcp & + & + & & \\ \hline
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chan\_unistm & + & + & & \\ \hline
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chan\_h323 & & + & & \\ \hline
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chan\_iax2 & \multicolumn{4}{|c|}{+} \\
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\hline
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\end{tabular}
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\end{center}
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\caption{Channel Driver QoS Settings}
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\end{table}
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\begin{table}[h!]
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\begin{center}
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\begin{tabular}{ | l || c | c | c | c | }
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\hline
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& Signaling & Audio & Video & Text \\ \hline \hline
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dundi.conf & \multicolumn{4}{ | c | }{+ (tos setting)} \\ \hline
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iaxprov.conf & \multicolumn{4}{ | c | }{+ (tos setting)} \\ \hline
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\hline
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\end{tabular}
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\end{center}
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\caption{Other ToS Settings}
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\end{table}
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\subsubsection{IP TOS values}
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The allowable values for any of the tos* parameters are:
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CS0, CS1, CS2, CS3, CS4, CS5, CS6, CS7, AF11, AF12, AF13, AF21, AF22, AF23,
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AF31, AF32, AF33, AF41, AF42, AF43 and ef (expedited forwarding),
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The tos* parameters also take numeric values.
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Note that on a Linux system, Asterisk must be compiled with libcap in order to
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use the ef tos setting if Asterisk is not run as root.
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The lowdelay, throughput, reliability, mincost, and none values have been removed
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in current releases.
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\subsubsection{802.1p CoS values}
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Because 802.1p uses 3 bits of the VLAN header, this parameter can take integer
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values from 0 to 7.
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\subsubsection{Recommended values}
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The recommended values shown below are also included in sample configuration files:
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\begin{table}[h!]
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\begin{center}
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\begin{tabular}{ | l || l | l | }
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\hline
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& tos & cos \\ \hline \hline
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Signaling & cs3 & 3 \\ \hline
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Audio & ef & 5 \\ \hline
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Video & af41 & 4 \\ \hline
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Text & af41 & 3 \\ \hline
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Other & ef & \\
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\hline
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\end{tabular}
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\end{center}
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\caption{Recommended QoS Settings}
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\end{table}
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\subsubsection{IAX2}
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In iax.conf, there is a "tos" parameter that sets the global default TOS
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for IAX packets generated by chan\_iax2. Since IAX connections combine
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signalling, audio, and video into one UDP stream, it is not possible
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to set the TOS separately for the different types of traffic.
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In iaxprov.conf, there is a "tos" parameter that tells the IAXy what TOS
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to set on packets it generates. As with the parameter in iax.conf,
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IAX packets generated by an IAXy cannot have different TOS settings
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based upon the type of packet. However different IAXy devices can
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have different TOS settings.
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\subsubsection{SIP}
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In sip.conf, there are four parameters that control the TOS settings:
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"tos\_sip", "tos\_audio", "tos\_video" and "tos\_text". tos\_sip controls
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what TOS SIP call signaling packets are set to. tos\_audio, tos\_video
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and tos\_text control what TOS values are used for RTP audio, video, and text
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packets, respectively.
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There are four parameters to control 802.1p CoS: "cos\_sip", "cos\_audio",
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"cos\_video" and "cos\_text". The behavior of these parameters is the
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same as for the SIP TOS settings described above.
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\subsubsection{Other RTP channels}
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chan\_mgcp, chan\_h323, chan\_skinny and chan\_unistim also support TOS and
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CoS via setting tos and cos parameters in their corresponding configuration
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files. Naming style and behavior are the same as for chan\_sip.
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\subsubsection{Reference}
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IEEE 802.1Q Standard:
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\url{http://standards.ieee.org/getieee802/download/802.1Q-1998.pdf}
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Related protocols: IEEE 802.3, 802.2, 802.1D, 802.1Q
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RFC 2474 - "Definition of the Differentiated Services Field
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(DS field) in the IPv4 and IPv6 Headers", Nichols, K., et al,
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December 1998.
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IANA Assignments, DSCP registry
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Differentiated Services Field Codepoints
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\url{http://www.iana.org/assignments/dscp-registry}
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To get the most out of setting the TOS on packets generated by
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Asterisk, you will need to ensure that your network handles packets
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with a TOS properly. For Cisco devices, see the previously mentioned
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"Enterprise QoS Solution Reference Network Design Guide". For Linux
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systems see the "Linux Advanced Routing \& Traffic Control HOWTO" at
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\url{http://www.lartc.org/}.
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For more information on Quality of
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Service for VoIP networks see the "Enterprise QoS Solution Reference
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Network Design Guide" version 3.3 from Cisco at:
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\url{http://www.cisco.com/application/pdf/en/us/guest/netsol/ns432/c649/ccmigration\_09186a008049b062.pdf}
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