Asterisk Documentation 1.4 ip-tos.txt

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IP Type of Service settings for VoIP channels

Asterisk can set the Type of Service (TOS) byte on outgoing IP packets
for various protocols.  The TOS byte is used by the network to provide
some level of Quality of Service (QoS) even if the network is
congested with other traffic. 

In sip.conf, there are three parameters that control the TOS settings:
"tos_sip", "tos_audio", and "tos_video".  tos_sip controls what TOS SIP call
signalling packets are set to.  tos_audio controls what TOS RTP audio
packets are set to.  tos_video controls what TOS RTP video packets are
set to.  
There is a "tos" parameter that is supported for backwards
compatibility.  The tos parameter should be avoided in sip.conf
because it sets all three tos settings in sip.conf to the same value.

* IAX2
In iax.conf, there is a "tos" parameter that sets the global default TOS
for IAX packets generated by chan_iax2.  Since IAX connections combine
signalling, audio, and video into one UDP stream, it is not possible
to set the TOS separately for the different types of traffic.

In iaxprov.conf, there is a "tos" parameter that tells the IAXy what TOS
to set on packets it generates.  As with the parameter in iax.conf,
IAX packets generated by an IAXy cannot have different TOS settings
based upon the type of packet.  However different IAXy devices can
have different TOS settings.

The allowable values for any of the tos* parameters are:
CS0, CS1, CS2, CS3, CS4, CS5, CS6, CS7, AF11, AF12, AF13,
AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, AF43 and
ef (expedited forwarding),

The tos* parameters also take numeric values.

The lowdelay, throughput, reliability, mincost, and none values are
deprecated because they set the IP TOS using the outdated "IP
precedence" model as defined in RFC 791 and RFC 1349. They still
work in this version of Asterisk, but will be removed in future releases.

Configuration	Parameter	Recommended
File				Setting
sip.conf	tos_sip		cs3
sip.conf	tos_audio	ef
sip.conf	tos_video	af41
iax.conf	tos		ef
iaxprov.conf	tos		ef

RFC 2474 - "Definition of the Differentiated Services Field
(DS field) in the IPv4 and IPv6 Headers", Nichols, K., et al,
December 1998.

IANA Assignments, DSCP registry
Differentiated Services Field Codepoints

To get the most out of setting the TOS on packets generated by
Asterisk, you will need to ensure that your network handles packets
with a TOS properly.  For Cisco devices, see the previously mentioned
"Enterprise QoS Solution Reference Network Design Guide".  For Linux
systems see the "Linux Advanced Routing & Traffic Control HOWTO" at

For more information on Quality of
Service for VoIP networks see the "Enterprise QoS Solution Reference
Network Design Guide" version 3.3 from Cisco at:


Created by: josiahbryan, Last modification: Sun 25 of Jul, 2010 (08:40 UTC)
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