Notes on Networking

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A Big Picture of QoS Classification, Marking and Mapping

May
17

This document defines up to 13 classes of traffic. A brief table of these classes and their respective markings, mappings and different queuing systems is given.

qos

ToS Byte

P2 P1 P0 T2 T1 T0 CU1 CU0
  • IP precedence—three bits (P2 to P0)
  • Delay, Throughput and Reliability—three bits (T2 to T0)
  • CU (Currently Unused)—two bits(CU1-CU0)

DiffServ Field

DS5 DS4 DS3 DS2 DS1 DS0 ECN ECN
  • DSCP—six bits (DS5-DS0)
  • ECN—two bits

 

DS field in the IPv4/IPv6 headers replaced the IPv4 TOS field. (IETF RFC 2474)

Class Selectors in the DS field -> backward compatibility with the IP precedence in TOS field

PHBs -> define the packet-forwarding properties associated with a class of traffic

Expedited Forwarding (EF) PHB -> low-loss, low-latency traffic, low jitter, assured bandwidth

Assured Forwarding (AF) PHB -> gives assurance of delivery under prescribed conditions

Also visit: https://www.cisco.com/en/US/technologies/tk543/tk759/technologies_white_paper0900aecd80295a9b.pdf

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