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Understanding Differentiated Services (DiffServ) and Its Implementation in Networking

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This document explores the Differentiated Services (DiffServ) model as defined by IETF standards, particularly RFC 2474 and RFC 2475. It covers the significance of the DS CodePoint (DSCP) in packet prioritization, including the IPv4 Type of Service and IPv6 Traffic Class fields. The structure of the DSCP byte—comprised of bits for priority levels and reserved bits—is explained. The document also outlines essential DiffServ policies such as Expedited Forwarding (EF) and Assured Forwarding (AF), highlighting the requirements for effective implementation across network devices while noting the limitations.

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Understanding Differentiated Services (DiffServ) and Its Implementation in Networking

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  1. L3 QoS Standards CIT 443

  2. Differentiated Services • DiffServ • IETF Standard • RFC 2474 & 2475

  3. DiffServ • Priority is set in DS CodePoint (DSCP) byte of the IP Header • IPv4 Type of Service field • IPv6 Traffic Class field • 8 bits – hence the DSCP byte • Two bits are reserved • Six bits (64 values) are used for priority levels • 32 are reserved for testing

  4. DiffServ Image from Cisco Systems Inc: http://www.cisco.com/en/US/technologies/tk543/tk766/technologies_white_paper09186a00800a3e2f.html

  5. DiffServ Image from Cisco Systems Inc: http://www.cisco.com/en/US/technologies/tk543/tk766/technologies_white_paper09186a00800a3e2f.html

  6. DiffServ • Each network device: • Must support DiffServ • Must agree on the treatment for each CodePoint value (policy) • Queues and forwards based on ToS priority

  7. DiffServ • Per hop forwarding policies • Expedited Forwarding (EF) RFC 3246 • Maximum priority • Packets matching the EF CodePoint are immediately forwarded • Egress rate exceeds the ingress rate on these packets • Must be kept to a small fraction of overall traffic • Assured Forwarding (AF) • Four priorities • Gold, Silver, Bronze, Best Available

  8. DiffServ • Limitations • Requires advance provisioning and bandwidth reservation • All network devices must agree on the prioritization scheme • Including the source, destination, and all intermediate networks

  9. For Next Time • Find and document the table of all DiffServ Code Points (DSCP) • Identify which DSCP values would be applicable to the service(s) you are deploying in lab *Be sure to include all necessary references

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