Issue 2.0
5-24 Network Design Considerations
7.5 Diffserv (Differentiated Service)
Differentiated Services (Diffserv) uses the ToS field in an IP header. Diffserv is
now commonly used instead of IP Precedence (refer to 7.4 IP Precedence on
page 5-23) as it provides greater flexibility. This method uses six bits of the ToS
field to determine the priority – which provides up to 64 possible values. The
combination of binary digits is known as the Diffserv Codepoint (DSCP).
The example below shows an Ethernet Frame containing one RTP (speech)
packet. The IP Packet has the ToS field set to 101000 (binary) which is the
equivalent of Class Selector 5. The router(s) in this network should be
programmed to prioritize based on CS5.
Table 5-2 Diffserv Parameters
6 bits 2 bits
Differentiated Services Code Point ECN
(Not QoS related)
Figure 5-9 Ethernet Frame Example - Containing One RTP (Speech) Packet
Source Destination Protocol
172.16.0.21 172.16.0.101 RTP
Info
Payload type=ITU-T G.729, SSRC=732771006, Seq=30885, Time=20560
Frame 159 (65 bytes on wire, 65 bytes captured)
Arrival Time: Jan 18, 2005 13:55:44.834497000
Time delta from previous packet: 0.000445000 seconds
Time since reference or first frame: 2.901831000 seconds
Frame Number: 159
Packet Length: 65 bytes
Capture Length: 65 bytes
Ethernet II, Src: 00:60:b9:c1:ab:a3, Dst: 00:60:b9:c6:6e:45
Destination: 00:60:b9:c6:6e:45 (Nitsuko_c6:6e:45)
Source: 00:60:b9:c1:ab:a3 (Nitsuko_c1:ab:a3)
Type: IP (0x0800)
Internet Protocol, Src Addr: 172.16.0.21 (172.16.0.21), Dst Addr:
172.16.0.101 (172.16.0.101)
Version: 4
Header length: 20 bytes
Diff Services Field: 0xa0 (DSCP 0x28: Class Selector 5; ECN: 0x00)
1010 00.. = Diff Services Codepoint: Class Selector 5 (0x28)
.... ..0. = ECN-Capable Transport (ECT): 0
.... ...0 = ECN-CE: 0
To Next Page
Loading...
Need help?
General
