Differentiated services

DiffServ ( Differentiated Services for short ) is a scheme for the classification of IP packets. This classification can be used by the network scheduler to prioritize IP packets to (English: Quality of Service, QoS) to ensure a quality of service. DiffServ has been described in RFC 2474 for the first time in 1998.

The original specification of IPv4 in RFC 791 defined a Type of Service byte ( ToS byte ) in the IP header. Three bits encoded a precedence 0-7, three more bits marked packets for low latency, high throughput and high reliability. A later RFC was one of the remaining bits of the meaning of " minimize financial costs."

DiffServ describes an alternative method to use the bits of the ToS byte: The original six bits ( now referred to as " DS- field") encoding a Differentiated Services Code Point (DSCP) from 0 to 63, the remaining two bits are described in RFC 3168 used for Explicit Congestion Notification (ECN ).

DSCPs purposes only classification, but not priorities. This means that a higher value may not necessarily be a preferred treatment. Instead refers a DSCP to a forwarding behavior (English: Per- hop behavior, PHB). This specifies what will be done with just one package. The proposed architecture is. At the frontiers of network, such as on border routers, to classify the packets, for example, based on port numbers However, routers within the network use only the DS field to select a PHB.

In several RFCs forwarding behaviors ( PHBs ) are defined, which are suitable for different types of network traffic and resource management. The corresponding DSCPs are managed and assigned by the IANA. The Class Selector PHB (RFC 2474) are backwards compatible with the eight precedence values ​​from RFC 791 and are representative of a priority-based resource allocation. Expedited Forwarding (EF, RFC 3246 ) describes a behavior for packets that require a low latency. Assured Forwarding (AF, RFC 2597 ) describes a PHB, divides the packets into four classes, each with three priorities. The default behavior with the code point 0 is called Best Effort (BE).

In order to prevent misuse, it must be ensured that the DS field is actually placed at the network boundaries. For example, a class of urgency could be abused to gain a higher priority than "average" services for their own service. To prevent this, to so-called active network components at the transitions " Trust Boundaries" are defined. At this point, the DS fields of packets are set, you may simply click on the default value of 0 Typically, the switch ports to which devices are connected to customers, such Trust Boundaries.