It may also be called the DS field or DSF. The 8-bit field is sometimes called the ToS or TOS field.Differentiated Services Code Point (DSCP) is a 6-bit value within an 8-bit field in the IP header.There are other models that can be used too – for example IntServ and RSVP. DiffServ is a broad term covering the architecture and mechanisms of this particular approach to QoS.Let’s try to unpick the various points of confusion as briefly as possible: ![]() You don’t want to have to read two articles in Wikipedia and a couple of RFC’s just to be able to set that one value. As a VoIP engineer, you probably just want to know what value you should set for one parameter on the VoIP equipment’s setup menus. While the basic concepts are straightforward, the practical implementation – especially the terminology and labelling associated with the Differentiated Services Field – is remarkably confusing. Differentiated Services and the DSCP value The above diagram is copied directly from RFC 3168 which describes ECN. The 8-bit DS field comprises the first 6 bits as a DSCP value and the last 2 bits which are used for “Explicit Congestion Notification” (ECN) data: ![]() The name “ToS” continues to be used today, even though the original interpretation given in RFC 791 has been superseded by the newer Differentiated Services model and the name “DS field” should be used instead. In 19, new standards were published (see RFC 2474, RFC 2475 and RFC 3168) which redefined the 8-bit ToS field to allow its use for the so-called “Differentiated Services”. The last two bits were reserved for future use. Three of the eight bits were set aside for an “IP Precedence” value (very similar to the PCP priority value described in part 1) and a further three bits were designated as facility flags to indicate if a packet had preference for low delay, high throughput or high reliability. Its purpose was to provide storage for QoS parameters to allow network equipment to prioritise certain traffic at times of high load. The original design for the 元 Internet Protocol (see RFC 791) defined a standard structure for Internet Datagram Headers that included an 8 bit “Type of Service” (ToS) field. The ToS field and how it has changed over time However, in practice you are unlikely to be able to guarantee that routing equipment outside your control will respect the QoS tagging that has been applied to the IP packets. Theoretically, 元 QoS prioritisation could be applied all the way through to the remote end-point in a VoIP connection. These 元 mechanisms have the potential to operate over the Internet thereby giving them a wider reach than L2 (which normally only operates within the scope of a LAN or private network). The following diagram illustrates how the different mechanisms exist at different layers in the OSI model:Īs the above diagram shows, the QoS mechanisms that operate at 元 are DiffServ, DSCP and ToS. The L2 priority value is stored in the 3-bit PCP field of the Ethernet frame and this field is part of an optional 4-byte section loosely referred to as the “VLAN tag”.Ĭompletely separate to this, it is also possible to tag IP packets within Layer 3 in a way that allows routers and switches to recognise that some packets should be given a higher priority than others. I explained in part 1 how Layer 2 data can be tagged with a priority value (sometimes called CoS). We will briefly look at the history and structure of the ToS and DSCP fields and their place within the DiffServ packet prioritisation model. In this second part, I will explore the Layer 3 parameters and offer practical suggestions for the values that should be assigned to them. ![]() In part 1, we examined the Layer 2 QoS settings available on most VoIP equipment.
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