Short Overview of Differentiated Services
نویسنده
چکیده
While Integrated Services provides per-flow guarantees, Differentiated Services (DiffServ) [1] follows the philosophy of mapping multiple flows into a few service levels—an approach sometimes referred to as Class of Service (CoS). DiffServ does not define any signaling mechanisms, but instead, packets are marked with a DiffServ Code Point (DSCP), which provides information about the QoS requested for a packet. The code point enables network routers to handle IP packets differently depending on the code point and hence their relative priority. DiffServ code points are located in the 8-bit Type of Service (TOS) field in the IP header. The TOS field used to define a specific forwarding treatment, which a packet could request from the network, for example, low delay, high reliability, or low cost. The old definition has been deprecated and DiffServ has adopted a new one for packet classification. The TOS field has been divided into a new 6bit DiffServ field and a 2-bit unused field. Each DiffServ code point (DSCP) is encoded into the low order six bits. The unused field is being allocated to the Explicit Congestion Notification (ECN) mechanisms [7]. Differentiated services can be constructed by a combination of: (i) marking packets with a DSCP at boundary nodes, (ii) using the DSCP or a multifield classification to determine how packets are forwarded by the nodes inside the domain, and (iii) conditioning the marked packets at boundary nodes.
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