wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

Quality of Service 19-7
Version Hd Len Type of service Total length
Identification
Flags Fragment offset
Time to live Protocol
Source IP address
Destination IP address
Header checksum
Options
Padding
20
bytes
Variable
length
Data portion
32 bits
FIGURE 19.1
ToS field in the IP header of any IP packet.
network devices before sending data. DiffServ reassigns bits in the type of service (ToS) (8 bits in ToS,
Figure 19.1) field of the IP header of any IP packet [20] and uses differentiated services code points
(DSCP) as the QoS descriptor value, which supports 64 possible classifications.
For a differentiated service, the network is trying to deliver a concrete kind of service based on the
QoS descriptor specified in each IP packet header on a per-hop basis rather than per-traffic flow as
IntServ does.
Usually, this service model is especially appropriate for aggregate flows; complex traffic classification
are performed at network edges resulting in a per-packet QoS handling.
The device, typically at the edge of the network, identifies packets based on the IP precedence (the
three first bits in the ToS field of the IP header) or on the DSCP fields in the ToS byte of the IP header
(Figure 19.2) that are going to be used in a per-hop behavior (PHB), packet by packet.
The six most significant bits of the ToS byte form the DiffServ field and the last two bits are used as
early congestion notification (ECN) bits. IP precedence uses three bits, while DSCP, really an extension
of IP precedence, uses six bits to select the PHB for the packet at each network node.
There are two typical ways for specifying this PHB: the assured forwarding (AF) PHB [21] and the
expedited forwarding (EF) PHB [22].
The AF group allows offering different levels of forwarding assurances for IP packets received from
a DiffServ domain. There are four AF classes, named AF1x through AF4x. Within each class, there are
three drop probabilities. Depending on the policy of the network, packets can be selected for a PHB
based on required throughput, delay, jitter or loss, or simply according to priority of access to network
services.
ToS byte in IP header
P2 P1 P0 T3 T2 T1 T0 CU
IP
precedence
DS5
DS4
DS3
DS2
DS1
DS0
ECN
ECN
DiffServ field
FIGURE 19.2
DiffServ field into the ToS byte of an IP packet.
© 2011 by Taylor and Francis Group, LLC