© 2005 Cisco Systems, Inc. All rights reserved. IPTX v2.06-1 Introducing IP Quality of Service The Differentiated Services Model. - презентация

1 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Introducing IP Quality of Service The Differentiated Services Model

2 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Outline Overview Differentiated Services Model DSCP Encoding Per Hop Behaviors Backward Compatibility Using the Class Selector Mapping CoS to Network Layer QoS Summary

3 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Differentiated Services Model Differentiated Services model describes services associated with traffic classes Complex traffic classification and conditioning is performed at network edge resulting in a per- packet Differentiated Services Code Point (DSCP) No per-flow/per-application state in the core Core only performs simple per-hop behavior's on traffic aggregates The goal is scalability

4 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Differentiated Services Model (Cont.) Wide variety of services and provisioning policies Decouple service and application in use No application modification No hop-by-hop signaling Interoperability with non-DS-compliant nodes Incremental deployment

5 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v DSCP Encoding DS field: the IPv4 header ToS octet or the IPv6 Traffic Class octet when interpreted in conformance with the definition given in RFC2474 DiffServ Code Point (DSCP): the first six bits of the DS field, used to select a PHB (Per-Hop Behavior; forwarding and queuing method)

6 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Per-Hop Behavior DS Code point selects per-hop behavior (PHB) throughout the network Default PHB (FIFO, Tail Drop) Expedited Forwarding (EF) PHB Assured Forwarding (AF) PHB Class Selector (IP precedence) PHB

7 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Per-Hop Behavior (Cont.) Expedited Forwarding (EF) PHB: –Ensures a minimum departure rate –Guarantees bandwidth – the class is guaranteed an amount of bandwidth with prioritized forwarding –Polices bandwidth – the class is not allowed to exceed the guaranteed amount (excess traffic is dropped) DSCP value: ; looks like IP precedence 5 to non-DS compliant devices –Bits 5-7: 101 = 5 (Same three bits used for IP Precedence) –Bits 3-4: 11 = No drop probability –Bit 2: just 0

8 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Per-Hop Behavior (Cont.) Assured Forwarding (AF) PHB: –Guarantees bandwidth –Allows access to extra bandwidth if available Four standard classes (af1, af2, af3 and af4) DSCP value range: aaadd0 –aaa is a binary value of the class –dd is drop probability

9 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Per-Hop Behavior (Cont.) Each AF class uses three DSCP values Each AF class is independently forwarded with its guaranteed bandwidth Congestion Avoidance is used within each class to prevent congestion within the class

10 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Per-Hop Behavior (Cont.) A DS node must allocate a configurable, minimum amount of forwarding resources (buffer space and bandwidth) per AF class Excess resources may be allocated between non-idle classes. The manner must be specified Reordering of IP packets of the same flow is not allowed if they belong to the same AF class

11 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Backward Compatibility Using the Class Selector Class Selector xxx000 DSCP Compatibility with current IP precedence usage (RFC 1812) = maps IP precedence to DSCP Differentiates probability of timely forwarding (xyz000) >= (abc000) if xyz > abc –If a packet has DSCP = , then It has a greater probability of timely forwarding than a packet with DSCP =

12 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Mapping CoS to Network Layer QoS

13 © 2005 Cisco Systems, Inc. All rights reserved. IPTX v Summary The Differentiated Services model describes services associated with traffic classes. Complex traffic classification and conditioning is performed at network edge resulting in a per-packet Differentiated Services Code Point (DSCP). A per-hop behavior is an externally observable forwarding behavior applied at a DS-compliant node to a DS behavior aggregate. The Expedited Forwarding (EF) PHB guarantees and polices bandwidth while ensuring a minimum departure rate. The Assured Forwarding (AF) PHB guarantees bandwidth while providing four classes each having three DSCP values. The DSCP is backward compatible with IP Precedence (Class Selector Code point).

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