Marking UDP/RTP packets for QoS Differentiation

Warning: This article assumes intimate knowledge with the iptables/firewall service available on the platform.  A mistake in configuration of iptables may impact service availability or access to the host.  Any configuration provided is intended to be an example.  Changes may be required to make it suitable.

RTP Packets can be marked when leaving the Media Broker; which can be inspected by the network infrastructure to enable QoS Policies to route packets appropriately. Packets which have been marked can be differentiated by network router; RTP traffic could be given a higher priority over packets which are not marked.

Iptables must be used to Mark Packets with a Differentiated Services Code Point (DSCP) value.

EF - Express Forwarding - will be used in the following examples.

More values are described here: Differentiated Services

Adding to an existing IP Tables Policy:

Run the commands:

iptables -t mangle -A OUTPUT -p udp -m udp --sport 16000 -j DSCP --set-dscp-class ef

iptables -t mangle -A OUTPUT -p udp -m udp --sport 17000:17099 -j DSCP --set-dscp-class ef

service iptables save

Inspecting the /etc/sysconfig/iptables file:

#The Mangle Table

*mangle
:PREROUTING ACCEPT [506:56861]
:INPUT ACCEPT [506:56861]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [472:54221]
:POSTROUTING ACCEPT [475:54522]
-A OUTPUT -p udp -m udp --sport 16000 -j DSCP --set-dscp-class ef
-A OUTPUT -p udp -m udp --sport 17000:17099 -j DSCP --set-dscp-class ef
COMMIT

Verifying the DSCP:

The following is an example Wireshark capture which shows the DSCP has been set on an output packet from Media Broker:

 

 

 

 

 

 

 

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