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Understanding CoS: A Brief History and its Relevance in Network Convergence

This overview provides a historical perspective on Circuit-switched and Packet-switched networks, the emergence of CoS, and its importance in network convergence. Learn about CoS parameters, IP-based CoS advancements, and DiffServ architecture.

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Understanding CoS: A Brief History and its Relevance in Network Convergence

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  1. CoS Overview

  2. A Historic Perspective • Circuit-switched networks • Designed around service levels needed for telephony • Connection-oriented; one user per connection • Suitable for real-time, loss-sensitive applications • Overkill for most other telecommunications uses • Low (fixed) delay • Blocking of new connections during congestion • CoS is not required in the historic environment • The network is purpose-built to support application (telephony) requirements • CoS is inherently built-in

  3. Network Advances • Packet-switched networks • Developed to optimize efficiency for machine-to-machine communications • Based on statistical multiplexing • Not connection-oriented; multiple users share a connection • Unbounded delays and loss during congestion • CoS is still not applicable • The network is purpose-built to support application requirements • Applications do not require CoS

  4. Network Convergence Drives CoS • Convergence drives the need for CoS • Multiple applications supported over a common network infrastructure • Traffic from specific applications must be recognized and treated accordingly • Special handling is necessary to ensure that unique applications perform as expected in the face of congestion or queuing delays • User bandwidth usage must be controlled • IP is the convergence technology of choice Voice Video Data

  5. Definition of CoS Parameters • CoS parameters • Bandwidth: End-to-end information carrying capacity • Delay: End-to-end delay for information delivery • Delay variation (jitter) – Variation in end-to-end delays caused by packet queuing • Loss: Percentage of packets not delivered, usually related to congestion • Network CoS parameters affect a user’s perception of performance • An application can perform only as well as the underlying network that supports it

  6. A Brief History of IP CoS—Type-of-Service Field • Type-of-service byte in the IP header (circa 1981) • Defined in RFC 791 • 3-bit precedence field to prioritize discards • 3-bit ToS field to indicate whether delay, throughout, or reliability should be the primary metric • In practice, very little support exists for ToS-based routing • Industry supported precedence bits to minimize loss of network-control packets 0 1 2 3 4 5 6 7 MSB LSB Bits IP ToS RFC 791 IP Precedence D T R Reserved

  7. A Brief History of IP CoS—IntServ • Integrated Services (circa 1994): • IETF’s first attempt at extending IP for other than best-effort services • RSVP signaling used to describe specific CoS requirements to the network • Routers reserve resources across the network • Resembled a circuit-switched call setup • Never deployed • Scalability issues

  8. 0 1 2 3 4 5 6 7 DiffServ Emerges • DiffServ architecture (circa 1998): • Defined in RFCs 2474 and 2475 • Redefined the IPv4 ToS field to support a 6-bit DiffServ code point (DSCP) • DiffServ has no signaling component • Operates on hop-by-hop basis RFC 3168 MSB LSB Bits DiffServ RFC 2474 ----------------------------- DSCP ------------------------------ ECN

  9. DiffServ Terminology (1 of 2) • Key DiffServ terms: • DiffServ field • Original IPv4 ToS byte • DSCPs • Specific 6-bit value in the DS field • This is the CoS value for a packet • Behavior aggregate (BA) • Classification based on DSCP • Packets with a common DSCP belong to the same BA

  10. DiffServ Terminology (2 of 2) • Key DiffServ terms (contd.): • Per-hop behavior (PHB) • Forwarding treatment associated with a given BA • Packets with the same DSCP value have the same PHB • PHB group • A set of one or more PHBs with related forwarding behavior • Example: assured forwarding (AF) is a PHB group, consisting of PHBs AF1, AF2, AF3, and AF4

  11. CoS Domains • CoS domain • A contiguous collection of nodes under a common policy • Common set of PHBs • Edge devices define and apply CoS on traffic • Core devices efficiently forward traffic based on CoS markings Core or Internal Edge or Boundary Domain A Domain B CoS support might not exist across administrative boundaries.

  12. Per-Hop Behavior (1 of 2) • PHB • A key component of DiffServ architecture • Describes how a node handles packets belonging to a specific behavior aggregate • PHBs are indexed by DSCPs • The default best-effort PHB is used for unmatched code points • PHB across the network • End-to-end flow characteristics can be predicted with consistent PHB support across a DiffServ domain

  13. Per-Hop Behavior (2 of 2) • PHB specifications identify recommended code points • Actual values are left to the DiffServ domain’s administration • Backward compatibility • RFC 791 defines the PHB for DSCPs coded with zeros in the least significant bits of the DS field (xxx000) • Grandfathered support for IP precedence handling • Referred to as class selector code points

  14. Standardized DiffServ PHBs (1 of 2) • Expedited forwarding (RFC 3246) • Designed to provide for low loss, low delay, and low jitter services • Example: Voice • Recommended code point: 101110 • Assured forwarding (RFC 2597) • Primarily concerned with controlling packet loss • Four classes: AF1, AF2, AF3, and AF4 • Each class supports three drop probabilities; for example, AF11 (low), AF12 (medium), and AF13 (high)

  15. Standardized DiffServ PHBs (2 of 2) • Class selector code points (RFC 2474) • Provide IP precedence compatibility • Typically used for network control traffic • Best effort is not specifically defined • Best effort is the default PHB

  16. Recommended DSCPs • IANA maintains a list of recommended DSCPs • Based on RFC recommendations for defined PHBs

  17. CoS Fields in Packet Headers—IPv4 • ToS/DiffServ field • RFC 791 • Up to 64 classes of service IHL Version Total Length Type of Service Flags Fragment Offset Identification Time to Live Protocol Header Checksum Source Address Destination Address Padding Options IP Precedence DSCP

  18. CoS Fields in Packet Headers—IPv6 • Traffic class field • RFC 2460 • Up to 64 classes of service Traffic Class Version Flow Label Hop Limit Next Header Payload Length Source Address (x 4) Destination Address (x 4) Traffic Class

  19. CoS Fields in Packet Headers—Ethernet • IEEE 802.1p • IEEE Std 802.1Q-2005 • Up to 8 classes of service VLAN Tag Dest Addr Src Addr FCS Type/Len Data TCI TPID PCP VLAN ID CFI Priority Code Point

  20. CoS Fields in Packet Headers—MPLS • Traffic class field • RFC 5462 • Was originally defined as EXP field in RFC 3032 • Up to 8 classes of service S TC TTL Label Formerly EXP

  21. Queue 2 (Class 2) Queue 3 (Class 3) Queue 1 (Class 1) Queue 0 (Class 0) Typical CoS Processing Stages Input Processing Classify Class 0 Class 1 To Fabric Ingress Policing or Rate Limit Class 2 Class 3 Class n Output Processing From Fabric Egress Header Rewrite Congestion Control (WRED) Scheduling andPrioritization The order of stages can vary slightly across Junos devices.

  22. Fabric RewriteMarker CoS Processing on Junos Devices Ingress Forwarding Policy Code Point (BA) Classifier Policing (Ingress) Multifield Classifier Egress Multifield Classifier Policing (Egress) Scheduler Shaper RED

  23. Traffic Classification Basic Service (BE) • Classifiers map traffic to a forwarding class at ingress • Can match on existing CoS values • BA classification • Can match on protocol, port, addresses, and so forth • Multifield classification • Support for IP precedence, DSCP (IPv4 and IPv6), MPLS EXP, and IEEE 802.1p Classifier Packet B Packet A Packet C Premium Service (EF) Control Traffic (NC) NC: Network control class

  24. Policing • Policing limits traffic volume and burstiness • Enforces and protects CoS SLAs • Excess traffic can be marked or discarded • Functions at ingress, egress, or both Ingress Interface Egress Interface InterfacePolicer InterfacePolicer Policer within MF Classifier MF: Multifield

  25. CoS and Forwarding Policy • Policy can select the forwarding next hop for traffic associated with a particular forwarding class • Facilitates CoS-based forwarding (CBF) BE packet BE packet EF packet EF packet CBF in place at R2 for the BE forwarding class

  26. Schedulers • Schedulers define the prioritization properties of forwarding classes (queues): • Transmission rate • Guaranteed and maximum rates • Queue priority • Support for five priority levels • Delay buffer • Storage space for traffic bursts • Congestion management and avoidance • Support for RED for equal, random dropping of traffic • Support for WRED for weighted, preferred dropping of traffic

  27. DSCP =0001001 DSCP =000000 Packet Packet Rewrite Markers Rewrite sets the packet’s DSCP coding based on the forwarding class The inbound classifier assigns a packet to forwarding class • The packet header rewrite sets CoS values for outbound traffic • Can be used by BA classification in downstream nodes • Support for IP precedence, DSCP (IPv4 and IPv6), MPLS EXP, and IEEE 802.1p

  28. CoS Is Unidirectional • CoS configuration is unidirectional • You must explicitly configure settings in both directions • Junos OS allows reuse of many CoS components Traffic flow C2 PE2 P1 fe-0/0/3 fe-1/1/1 fe-2/0/1 fe-2/0/2 fe-3/0/2 CoS Domain fe-1/0/0 PE1 P2 C1 fe-0/0/0 fe-1/0/2 fe-1/1/1 fe-0/0/2

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