TOS byte

A well managed network is capable of differentiating between different network traffic. This means that a network may offer a better service to traffic types which require high reliability and throughput, with minimal delays. Similarly other less essential traffic can be downgraded in the service they receive, since they are likely to be more tolerant of network delays. Type of Service (ToS), also known as Differentiated Services (DS) is part of the Quality of Service (QoS) model. ToS can be used by components on the network to define how they should prioritize the packet they have received. ToS provides a mechanism to do this by providing the following sub-fields:

Precedence
Delay
Throughput
Reliability

All of these subfields are set to 'normal' by default.

By setting these individual sub-fields, packets can be separated into classes, each packet class can then be handled by the network in an appropriate manner (as defined by your network definitions). This means that each network component will attempt to satisfy, but not guarantee, the requirements specified by the ToS byte fields.

The ToS byte forms part of Internet Protocol (IP) packet header, it exists as a 8 bit field value. The relationship to sub-fields and bits is as follows:

Table 1. ToS byte sub-fields and bits.
This table describes for each bit position, the purpose, possible values and their meaning.
Bit Position	Purpose	Possible Values
Bits 0-2	Specifies precedence.	(0 = low, 7= high)
Bit 3	Specifies delay	(0 = normal delay, 1 = low delay)
Bit 4	Specifies throughput	(0 = normal throughput, 1 = high throughput)
Bit 5	Specifies reliability.	(0 = normal reliability, 1= high reliability)
Bits 6-7	Are currently reserved.

By default, all values are set to 0 (Normal). The Blueworx Voice Response VoIP feature allows users to set the ToS byte as a value between 0 - 64, representing bits 0 - 5 (inclusive), for all RTP media streams leaving all adapter cards.

Example

To turn on high Reliability, low Delay, normal Throughput and medium Precedence you would enter a value of 43 (out of 64). In the IP header, the field values would be set as follows:

This table describes the value to set for each bit to achieve a particular result in terms of Precedence, Delay, Throughput, and Reliability.

Bits	Value	Result
0	1	Medium Precedence
1	1
2	0
3	1	Low Delay
4	0	Normal Throughput
5	1	High Reliability
6	0	Currently Reserved
7	0	Currently Reserved

To set the ToS byte for the VoIP signalling, users must edit the AIX policyd and rsvpd daemons configuration files and make use of the AIX QoS commands mkqos and rmqos. For more details please refer to your AIX Systems Management Guide: Communications and Networks.

Note: ToS byte will only ever take effect if your network is enabled to respect ToS Byte settings.