Medias Mediatrix 1204, Справочное руководство

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Chapter 6 - Web Interface – Advanced Quality of Service (QoS)
96 Mediatrix 1204
DiffServ Configuration
Differentiated Services (DiffServ, or DS) is a protocol for specifying and controlling network traffic by class so
that certain types of traffic – for example, voice traffic, which requires a relatively uninterrupted flow of data,
might get precedence over other kinds of traffic.
DiffServ replaces the first bits in the ToS byte with a differentiated services code point (DSCP). It uses the
existing IPv4 Type of Service octet.
X
To set the DiffServ configuration:
1. In the DiffServ section of the QoS page, set the following DiffServ fields:
• Voice DiffServ Value:
• Voice Band Data DiffServ Value
• T.38 DiffServ Value
• Signalling DiffServ Value
These fields are 1 octet scalar ranging from 0 to 255. The DSCP default value should be 101110.
This results in the DS field value of 10111000 (184d).
This default value would result in a value of “101” precedence bits, low delay, high throughput, and
normal reliability in the legacy IP networks (RFC 791, RFC 1812). Network managers of legacy IP
networks could use the above-mentioned values to define filters on their routers to take advantage
of priority queuing. The default value is based on the Expedited Forwarding PHB (RFC 2598)
recommendation.
Standards Supported RFC 2475 – An Architecture for Differentiated Services
What are Differentiated Services?
Differentiated Services avoids simple priority tagging and depends on more complex policy or rule
statements to determine how to forward a given network packet. An analogy is made to travel services, in
which a person can choose among different modes of travel – train, bus, airplane – degree of comfort,
the number of stops on the route, standby status, the time of day or period of year for the trip, and so
forth.
For a given set of packet travel rules, a packet is given one of 64 possible forwarding behaviors – known
as per hop behaviors (PHBs). A six-bit field, known as the Differentiated Services Code Point (DSCP), in
the Internet Protocol header specifies the per hop behavior for a given flow of packets. The DS field
structure is presented below:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| DSCP | CU |
+---+---+---+---+---+---+---+---+
MSB LSB
• DSCP : Differentiated Services CodePoint.
• CU : Currently Unused. The CU bits should always be set to 0.
For both signalling and media packets, the DSCP field is configurable independently. The entire DS field
(TOS byte) is currently configurable.
Note:
RFC 3168 now defines the state in which to set the two least significant bits in the TOS byte. On the
other hand, this RFC only applies to TCP transmissions and the bits are thus set to “0” in the Mediatrix 1204.
This has the following effects:
•
The TOS values for UDP packets are the same as in the MIB.
•
The TOS values for TCP packets are equal to the closest multiple of 4 value that is not greater than the
value in the MIB.