About Quality of Service (Qos)
Digi TransPort User Guide
483
About Quality of Service (Qos)
The Quality of Service (QoS) functionality allows prioritizing different types of IP traffic. QoS is
generally used to ensure that low priority applications do not use most of the available
bandwidth to the detriment of those having a higher priority. For example, this might mean that
EPOS transactions carried out over XOT will be prioritized over HTTP-type traffic used for Internet
access. Without some form of QoS, all IP packets are treated as being equal, such as there is no
discrimination between applications.
The IP packet
Type of Service (TOS)
field is used to indicate how a packet should be prioritized.
Using the top 6 bits of the TOS field, a router that supports QoS will assign a Differentiated
Services Code Point (DSCP) code to the packet. This may take place within the router when it
receives the packet or another router closer to the packet source may have already assigned it.
Based on the DSCP code, the router will assign the packet to a priority queue. There are currently
four such queues for each PPP instance within the router. Each queue can be configured to
behave a particular way so that packets in that queue are prioritized for routing according to
predefined rules.
There are two principal ways in which priority may be implemented:
• A priority queue can be configured to allow packets to be routed at a specific data rate
(providing that queues of a higher priority are not already using the available bandwidth)
• Weighted Random Early Dropping (WRED) of packets can be used as queues become busy, in
an attempt to get the TCP socket generating the packets to back off its transmit timers. This
prevents the queue overflow, which would result in all subsequent packets being dropped.
QoS is a complex subject and can have a significant impact on the performance of the router. For
detailed background information on QoS, see the IETF document
RFC2472, Definition of the
Differentiated Services Field
.
In Digi TransPort routers, the classification of incoming IP packets for the purposes of QoS takes
place within the firewall. The firewall allows the system administrator to assign a DSCP code to a
packet with any combination of source/destination IP address/port and protocol. Details of how
this is done are given in the section on firewall scripts.
When the routing code within the router receives an incoming packet, it directs it to the interface
applicable to that packet at that time (this is the case whether or not QoS is being applied). Just
before the packet is sent to the interface, the QoS code intercepts the packet and assigns it to one
of the available priority queues (currently 10 per PPP instance) based on its DSCP value.
Each priority queue has a profile assigned to it. This profile specifies parameters such as the
minimum transmit rate to attempt, maximum queue length, and WRED parameters.
The packet is then processed by the queue management code, and either dropped or placed in
the queue for later transmission.
