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Network layer -- May 20041 Network layer Computer Networks.

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Presentation on theme: "Network layer -- May 20041 Network layer Computer Networks."— Presentation transcript:

1 Network layer -- May Network layer Computer Networks

2 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS  General principles  Prevention policies  Handling virtual circuit subnets  General techniques  Jitter control

3 Network layer -- May Congestion?  Too many packets in (a part of) the subnet!

4 Network layer -- May Congestion?  Too many packets in (a part of) the subnet!  caused by othe same output line is used by many input lines omismatch between system parts slow processor low bandwidth lines  tends to feed upon itself  Congestion control <> flow control oCongestion: global issue oFlow control: point-to-point

5 Network layer -- May Congestion: principles  How? Good design oMake sure the problem does not occur  Tools oDecide when to accept traffic oDecide when to discard packets and which ones oMake scheduling decisions in the subnet  Monitor: where and when congestion? o% packets discarded oaverage queue length onumber of packets that time out oaverage packet delay  Pass collected info to places where actions can be taken = source of traffic oexplicit <> implicit feedback oexplicit: (extra) packet, flags (in other packets), probe packets  Adjust system operation oIncrease resources: bandwidth oDecrease load: deny, degrade service Control theory viewpoint open loopclosed loop

6 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS  General principles  Prevention policies  Handling virtual circuit subnets  General techniques  Jitter control

7 Network layer -- May Congestion: prevention  Minimize congestion LayerPolicies Transport  Retransmission policy  Out-of-order caching policy  Acknowledgement policy  Flow control policy  Timeout determination Network  Virtual circuits <> datagrams in subnet  Packet queueing and service policy  Packet discard policy  Routing algorithm  Packet lifetime management Data link  See transport layer

8 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS  General principles  Prevention policies  Handling virtual circuit subnets  General techniques  Jitter control

9 Network layer -- May Virtual circuit subnets  Dynamic approach: act when needed  Admission control oNo new virtual circuits when congestion is signalled oe.g. telephone network  Route new virtual circuits around problem areas

10 Network layer -- May Virtual circuit subnets  Dynamic approach  Admission control  Route new virtual circuits around problem areas  Negotiation when virtual circuit is set up oAbout kind of traffic + service desired oResource reservation in subnet Line capacity Buffers in routers  No congestion  Unused resources

11 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS  General principles  Prevention policies  Handling virtual circuit subnets  General techniques  Jitter control

12 Network layer -- May General techniques  Approaches: reduce traffic by oRequesting senders local measurements + info packets to senders + hope for …. oThrowing away packets local measurements + local actions to reduce load

13 Network layer -- May Source based approach  Basic algorithm oRouter monitors utilisation of output lines u recent utilisation: 0  u  1 good estimate of u u new = a  u old + (1 – a )  f oIn case of overload: u new > threshold Output line enters warning state Some action is taken: –Warning bit –Choke packets –Hop-by-hop choke packets fInstantaneous line utilisation aconstant

14 Network layer -- May Source based approach  Warning bit oOutput line in warning state Warning bit set in header Destination copies bit into next ack Source cuts back traffic oAlgorithm at source As long as warning bits arrive: reduce traffic Less warning bits: increase traffic oProblems voluntary action of host! correct source selected? oUsed in DecNet Frame relay

15 Network layer -- May Source based approach  Choke packet oIn case of overload: router sends choke packet to host causing the overload oHost receiving choke packet reduces traffic to the specified destination ignores choke packets for a fixed interval new choke packets during next listening interval? –Yes: reduce traffic –No: increase traffic oProblems: voluntary action of host! correct host selected?

16 Network layer -- May Source based approach  Choke packets: oExample showing slow reaction oSolution: Hop-by-Hop choke packets

17 Network layer -- May Source based approach  Hop-by-Hop choke packets oHave choke packet take effect at every hop oProblem: more buffers needed in routers

18 Network layer -- May Load shedding  Throw away packets that cannot be handled!!  Packet selection? oRandom oBased on application File transfer: discard new packet Multimedia: discard old packet oLet sender indicate importance of packets Low, high priority Incentive to mark a packet with low priority –Price –Allow hosts to exceed agreed upon limits  Random early detection …

19 Network layer -- May Load shedding  Throw away packets that cannot be handled!!  Packet selection?  Random early detection oDiscard packets before all buffer space is exhausted oRouters maintain running average of queue lengths oSelect at random a packet oInform source? Send choke packet?  more load!! No reporting oWhen does it work? Source slows down when packets are lost

20 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS  General principles  Prevention policies  Handling virtual circuit subnets  General techniques  Jitter control

21 Network layer -- May Congestion: jitter control  Important for audio and video applications? onot delay ovariance of delay

22 Network layer -- May Congestion: jitter control  Jitter = variation in packet delay  Compute feasible mean value for delay ocompute expected transit time for each hop orouter checks to see if packet is behind ahead schedule obehind: forward packet asap oahead: hold back packet to get it on schedule again  Buffering? Depends on characteristics: oVideo on demand: ok oVideoconferencing: nok

23 Network layer -- May Network Layer  Design issues  Routing  Congestion  Internetworking  Internet Protocols  Multimedia or QoS

24 Network layer -- May Network layer Computer Networks


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