Computer Networks Network layer Network Layer q Design

Computer Networks Network layer

Network Layer q Design issues q General principles q Routing q Prevention policies q Congestion q Internetworking q Handling virtual circuit subnets q General techniques q Jitter control q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004 2

Congestion? q Too many packets in (a part of) the subnet! Network layer -- May 2004

Congestion? q Too many packets in (a part of) the subnet! q caused by o the same output line is used by many input lines o mismatch between system parts • slow processor • low bandwidth lines q tends to feed upon itself q Congestion control <> flow control o Congestion: global issue o Flow control: point-to-point Network layer -- May 2004

Congestion: principles Control theory viewpoint open loop closed loop q How? Good design o Make sure the problem does not occur q Tools o Decide when to accept traffic o Decide when to discard packets and which ones o Make scheduling decisions in the subnet q Monitor: where and when congestion? o % packets discarded o average queue length o number of packets that time out o average packet delay q Pass collected info to places where actions can be taken = source of traffic o explicit <> implicit feedback o explicit: (extra) packet, flags (in other packets), probe packets q Adjust system operation o Increase resources: bandwidth o Decrease load: deny, degrade service Network layer -- May 2004 5

Network Layer q Design issues q General principles q Routing q Prevention policies q Congestion q Internetworking q Handling virtual circuit subnets q General techniques q Jitter control q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004 6

Congestion: prevention q Minimize congestion Layer Transport Policies q Retransmission policy q Out-of-order caching policy q Acknowledgement policy q Flow control policy q Timeout determination Network q Virtual circuits <> datagrams in subnet q Packet queueing and service policy q Packet discard policy q Routing algorithm q Packet lifetime management Data link q See transport layer Network layer -- May 2004

Network Layer q Design issues q General principles q Routing q Prevention policies q Congestion q Internetworking q Handling virtual circuit subnets q General techniques q Jitter control q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004 8

Virtual circuit subnets q Dynamic approach: act when needed q Admission control o No new virtual circuits when congestion is signalled o e. g. telephone network q Route new virtual circuits around problem areas Network layer -- May 2004

Virtual circuit subnets q Dynamic approach q Admission control q Route new virtual circuits around problem areas q Negotiation when virtual circuit is set up o About kind of traffic + service desired o Resource reservation in subnet • Line capacity • Buffers in routers No congestion Unused resources Network layer -- May 2004

Network Layer q Design issues q General principles q Routing q Prevention policies q Congestion q Internetworking q Handling virtual circuit subnets q General techniques q Jitter control q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004 11

General techniques q Approaches: reduce traffic by o Requesting senders local measurements + info packets to senders + hope for …. o Throwing away packets local measurements + local actions to reduce load Network layer -- May 2004

Source based approach q Basic algorithm o Router monitors utilisation of output lines • u recent utilisation: 0 u 1 f a • good estimate of u unew = a uold + (1 – a ) f o In case of overload: Instantaneous line utilisation constant unew > threshold • Output line enters warning state • Some action is taken: – Warning bit – Choke packets – Hop-by-hop choke packets Network layer -- May 2004

Source based approach q Warning bit o Output line in warning state • Warning bit set in header • Destination copies bit into next ack • Source cuts back traffic o Algorithm at source • As long as warning bits arrive: reduce traffic • Less warning bits: increase traffic o Problems • voluntary action of host! • correct source selected? o Used in • Dec. Net • Frame relay Network layer -- May 2004

Source based approach q Choke packet o In case of overload: router sends choke packet to host causing the overload o Host 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 o Problems: • voluntary action of host! • correct host selected? Network layer -- May 2004

Source based approach q Choke packets: o Example showing slow reaction o Solution: Hop-by-Hop choke packets Network layer -- May 2004

Source based approach q Hop-by-Hop choke packets o Have choke packet take effect at every hop o Problem: more buffers needed in routers Network layer -- May 2004 17

Load shedding q Throw away packets that cannot be handled!! q Packet selection? o Random o Based on application • File transfer: discard new packet • Multimedia: discard old packet o Let sender indicate importance of packets • Low, high priority • Incentive to mark a packet with low priority – Price – Allow hosts to exceed agreed upon limits q Random early detection … Network layer -- May 2004

Load shedding q Throw away packets that cannot be handled!! q Packet selection? q Random early detection o Discard packets before all buffer space is exhausted o Routers maintain running average of queue lengths o Select at random a packet o Inform source? • Send choke packet? more load!! • No reporting o When does it work? • Source slows down when packets are lost Network layer -- May 2004

Network Layer q Design issues q General principles q Routing q Prevention policies q Congestion q Internetworking q Handling virtual circuit subnets q General techniques q Jitter control q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004 20

Congestion: jitter control q Important for audio and video applications? o not delay o variance of delay Network layer -- May 2004

Congestion: jitter control q Jitter = variation in packet delay q Compute feasible mean value for delay o compute expected transit time for each hop o router checks to see if packet is • behind • ahead schedule o behind: forward packet asap o ahead: hold back packet to get it on schedule again q Buffering? Depends on characteristics: o Video on demand: ok o Videoconferencing: nok Network layer -- May 2004

Network Layer q Design issues q Routing q Congestion q Internetworking q Internet Protocols q Multimedia or Qo. S Network layer -- May 2004

Computer Networks Network layer