Real-Time Applications. Delay-adaptive: applications that can adjust their playback point (delay or advance over time).

Transcription

1 Real-Time Applications Tolerant: can tolerate occasional loss of data. Intolerant: cannot tolerate such losses. Delay-adaptive: applications that can adjust their playback point (delay or advance over time). Rate-adaptive: can alter the bit rate depending on available bandwidth and BER. 84

2 Approaches to QoS Fine-grained: Provide QoS to individual applications or flows. Example: Integrated Services. Coarse-grained: Provide QoS to aggregated traffic. Example: Differentiated Services. 85

3 Integrated Services Specifications for a number of service classes: 1. Best effort service: default service in IP-based networks. 2. Guaranteed service: maximum packet delay never exceeds some spcified value. 3. Controlled load service: emulate lightly loaded network for applications using this service. 86

4 Integrated Services: Implementation Mechanism Flowspec: information provided to network about the packet flow. Admission control: decision regarding ability to support a flow. Resource reservation: mechanism to exchange information about requests for service, flowspecs, admission control decisions. Packet scheduling: managing packet queues and transmission schedules in switches and routers. 87

5 Flowspecs Tspec: traffic specifications. Example: token-bucket specification (rate + burst size). A flow can be described by many different token buckets. Goal: select the most informative specification to avoid overallocation of resources. Rspec: service requested from the network. 88

6 Admission Control Can a new flow s Tspec and Rspec be supported without violating the QoS requirements of existing flows? Policing: Performed on a per-packet basis. Does a flow conform to its specs? Take corrective action if a flow violates its Tspecs. mark some packets of the flow, marked packets first to be dropped during resource crunch. 89

7 Reservation Protocol: RSVP Maintain soft-state in routers. State information times out unless periodically refreshed. Supports unicast and multicast flows. Receiver-oriented: receivers keep track of their needs. 90

8 RSVP: Reservation Styles Wildcard-filter style: receiver indicating that it wants to receive from all upstream senders, with reserved bandwidth shared among all senders. Fixed-filter style: list of senders, and bandwidth for each sender explicitly specified. Shared-explicit style: requested reservation to be shared among the senders explicitly specified. Shared-explicit and wildcard filters appropriate for multicast session whose sources are unlikely to transmit simultaneously. 91

9 RSVP Operations PATH message sent from sender to receiver containing Tspec. Each router uses PATH message to determine the reverse path from receiver to sender. Receiver sends RESV message along the reverse path describing receiver s requirements. Router forwards RESV message upstream if it is able to satisfy the requirements. Otherwise: error message sent to requesting receiver. Receiver refreshes through RESV messages every 30 seconds. 92

10 RSVP: On Topology Changes PATH messages sent at least every 30 seconds. New PATH messages triggered when router discovers change in forwarding table. Receiver s next RESV message follows new path. Routers on the old path stop receiving RESV messages and withdraw reservations. 93

11 RSVP: For Multicast Traffic Let RESV message reach a multicast tree node where other receivers reservation already established: Reserved resoures upstream are adequate to support the receiver s needs: do not forward RESV message. Otherwise: forward RESV message upstream. Let multiple resources be part of multicast tree: Receiver calculates overall Tspec and requests this in RESV messages. 94

12 Packet Classification and Scheduling Packet classification: Based on source and destination address, protocol number, source and destination ports. Flowlabel field could possibly be used in IPv6 for this purpose. Packet classified as controlled load service, guaranteed service, etc. Packet scheduling: different queueing mechanisms used for this purpose. 95

13 Integrated Services: Scalability Issues Routers need to: 1. Maintain reservation state information for each flow. 2. Classify, police and queue packets of each flow. 3. Make admission control decisions for each new reservation request. Solution approach is not scalable due to additional responsibilities imposed on routers. 96

14 Architecture for Differentiated Services (RFC 2475) Service: Significant characteristics of packet transmission in one direction across a set of one or more paths within a network (throughput, delay, jitter, loss, etc.). Service differentiation: Desirable to accommodate heterogeneous application requirements, user expectations, and differentiated pricing of services. DS field: IPv4 ToS octet, or IPv6 Traffic class octet. 97

15 DS Architecture Model Traffic entering network is: classified and conditioned at network boundary, assigned to different behavior aggregates. Each behavior aggregate identified by DS codepoint. Within network core: packets forwarded according to per-hop behavior associated with the DS codepoint. 98

16 DS Domain Contiguous set of DS nodes with common policy. Under a single administration. Boundary nodes classify and condition ingress traffic. Interior nodes forward packets as per DS codepoint. 99

17 DS Region One or more contiguous DS domains. Can support differentiated services along paths spanning the member domains. Peering DS domains must establish service level agreements to specify transit traffic conditioning at their common boundaries. 100

18 Traffic Classification Behavior Aggregate (BA) Classifier: classifies packets based on DS codepoint only. Multi-Field (MF) Classifier: classifies packets on the value of a combination of packet header fields. 101

19 Traffic Profiles Temporal properties of traffic stream selected by a classifier. Codepoint X, use token-bucket r, b Provides rules to determine if packet in-profile or out-of-profile. Different conditioning actions may be applied to in-profile and out-of-profile packets. 102

20 Traffic Conditioners Meters: to measure traffic stream against a profile. Markers: set DS field of packet to a particular codepoint. Shapers: delay some or all packets of a traffic stream to bring it into compliance with a traffic profile. Droppers: discard some or all packets of a traffic stream to bring it into compliance with a traffic profile (policing) 103

21 Per-Hop Behaviors Two PHBs under discussion in the Diffserv working group: Expedited Forwarding PHB: departure rate of this class of traffic must never be below a configured rate. Assured Forwarding PHB: traffic divided into four classes, Each class is guaranteed some minimum bandwidth and buffering. Packets within a class are partitioned into three drop perference categories. During congestion, routers can drop packets based on their drop preference values. 104