QoS: IP Precedence and DSCP

IP Headers have a 1 byte field called Type of service (ToS) byte.  This field is intended to mark a packet for treatment with QoS tools.  The ToS byte itself is subdivided with a high order 3 bits defined as the IP Precedence field (IPP).


bits 3 – 6 of the ToS byte included flags that were toggled on or off to imply a service level.  The final 7th bit, was not defined for use.  The flags weren’t used often so essentially the 3bit IPP field in the ToS byte was primarily used.

Various RFCs came along and defined the Differentiated Services (DiffServ).  DiffServe required more than three bits to mark pakets, so it redefined the use of the ToS byte by utilizing the lower order 3 bits and referred the IP Precedence field and lower order 3bits field as the Differentiated services code Point (DSCP) field.

Later RFC3168 defined the low order 2 bits of the DS field for use with the QoS Explicit Congestion Notification (ECN) feature.

RFC 3246 defined specific dscp values with an implied meaning for example a DSCP value of 46 was given the name Expedited Forwarding (EF).  According to that RFC packets marked as EF should be given queue prference so they experience minimal latency, but they should be policed so they  don’t over congest a link preventing anything else through.  These suggested settings and the associated QoS behavior are called Per-Hop Behaviors (PHB) by DiffServ.



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