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In order to provide Quality of Service, there should be a way to distinguish priority of each arriving packet according to DiffServ Architecture.
In IP packet header there is special group of 8 bits (from 8th to
15th respectively in header) dedicated for that purpose. They are called
Type of Service.
![precedence](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_vU9YgBfkJomuu4Uw2YzjIdMsqSZl5xkFzOVKMMEChtfoHTTeB-DyG6v53BymD3XR2t6Ct6_XRY0sETdHTjbO5HM0_GxKcxM7wcrlDT4iVOGAkPA6Y3WunyRpzHZ747H8vo4C7tQIAs=s0-d)
In Precedence could be one of fallowing values, which informs about packet type :
- 000 – routine
- 001 – priority
- 010 – immediate
- 011 – flash
- 100 – flash-override
- 101 – critical
- 110 – internetwork control
- 111 – network
Other bits inform that :
- 11 – packet is delay sensitive (D)
- 12 – packet wants high rate (T)
- 13 – packet wants high reliability (R)
- 14 – mainly unsued
- 15 – mainly unsued
In DiffServ architecture there was defined Differentiated Services Code Point in ToS field.
![dscp](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_s7Z3q-gGido8STIlDvZ2Z7megpwaanEG7fMG_ac8v1ftUZxyPfHHnQJAoka4waIAg5UmWf_U2Jmeg-HJSCGcQ2x5w4DlRInSFy2v_ewbWsXYKmJWrZf6l0zPShdjEwXhq2og=s0-d)
It allows to define 64 different classes. The mapping between IP Precendence and DSCP is as fallows:
![mapping](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_umD1hE2elK761WX00zsJp3CdtXOa5kVnbXVYlfoaxpu3a3z8-oSGW6YdBp6eo4objyrHTZoexJEdC_sbsjp-F8N68HY_5PESiGNMlRu3IcIzg7hXAavajtDj9XnYVZ=s0-d)
First 3 bits from DSCP agrees with IP Precedence.
Thanks to ToS and DSCP we know packets service requirements.
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