[1] Modern data networks carry many different types of services, including voice, video, streaming music, web pages and email.
Many of the proposed QoS mechanisms that allowed these services to co-exist were both complex and failed to scale to meet the demands of the public Internet.
DiffServ relies on a mechanism to classify and mark packets as belonging to a specific class.
DiffServ-aware routers implement per-hop behaviors (PHBs), which define the packet-forwarding properties associated with a class of traffic.
Core routers simply apply PHB treatment to packets based on their markings.
For tight control over volumes and type of traffic in a given class, a network operator may choose not to honor markings at the ingress to the DiffServ domain.
Traffic that exceeds the subscription rate faces a higher probability of being dropped if congestion occurs.
The combination of classes and drop precedence yields twelve separate DSCP encodings from AF11 through AF43 (see table).
Re-marking a packet is sometimes used to increase its drop precedence if a stream's bandwidth exceeds a certain threshold.
DF= Default Forwarding Prior to DiffServ, IPv4 networks could use the IP precedence field in the TOS byte of the IPv4 header to mark priority traffic.
In order to maintain backward compatibility with network devices that still use the Precedence field, DiffServ defines the Class Selector PHB.
This means that in the core of the Internet, routers are unhindered by the complexities of collecting payment or enforcing agreements.
That is, in contrast to IntServ, DiffServ requires no advance setup, no reservation, and no time-consuming end-to-end negotiation for each flow.
The details of how individual routers deal with the DS field are configuration specific, therefore it is difficult to predict end-to-end behavior.
From a commercial viewpoint, this means that it is impossible to sell different classes of end-to-end connectivity to end users, as one provider's Gold packet may be another's Bronze.
DiffServ or any other IP-based QoS marking does not ensure the quality of the service or a specified service-level agreement (SLA).
It is up to all the service providers and their routers in the path to ensure that their policies will take care of the packets in an appropriate fashion.