DiffServ

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DiffServ (short for Differentiated Services ) is a scheme for classification of IP packets . This classification can from network scheduler for prioritization of IP are used -Datenpaketen to a quality of service (English: Quality of Service , QoS ) guarantee. DiffServ was first described in 1998 in RFC  2474. In contrast to the IntServ method, the resources are requested for traffic classes and not for individual connections.

The original specification of IPv4 in RFC 791 defined a type of service byte (ToS byte) in the IP header, in bits 8 to 15. Three bits encoded a precedence between 0 and 7, three more bits marked packets for low latency , high throughput and high reliability. A later RFC gave one of the remaining bits the meaning "minimize financial costs".

DiffServ describes an alternative method to use the bits of the ToS byte: The original six bits encode a Differentiated Services Codepoint (DSCP) between 0 and 63. The remaining two bits are used in RFC 3168 for Explicit Congestion Notification (ECN).

DSCPs are only used for classification, not for prioritization. This means that a higher numerical value does not necessarily correspond to a preferred treatment. Instead, a DSCP refers to a forwarding behavior (English: Per-hop behavior , PHB). This specifies how exactly a package is handled. The intended architecture is to classify the packets at the boundaries of the network, for example on border routers , for example using port numbers . Routers within the network, on the other hand, only use the DS field to select a PHB.

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

To prevent misuse, it must be ensured that the DS field is actually set at the network boundaries. For example, an urgency class could be misused to give one's own service a higher priority than “average” services. In order to prevent this, so-called “trust boundaries” are defined at the transitions to active network components. At this point the DS fields of the packets are set, possibly simply to the default value 0. Such trust boundaries can be switch ports to which end devices of customers are connected or routers of service providers that connect customer networks.

See also

Web links

  • RFC 2474 - Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers
  • RFC 2475 - An Architecture for Differentiated Services
  • RFC 2597 - Assured Forwarding PHB Group
  • RFC 3168 - The Addition of Explicit Congestion Notification ( ECN ) to IP (Updates RFC 2474 )
  • RFC 3246 - An Expedited Forwarding PHB (Per-Hop Behavior) (Obsoletes RFC 2598 )
  • DSCP registry : List of all DSCPs officially assigned by the IANA .