1. Lecture 31-32 Network layer -- May 2004 1 Congestion control Algorithms

2. Congestion Control Algorithms Network layer -- May 2004 2 Design issues Routing Congestion Internetworking Internet Protocols General principles of congestion control Prevention policies Handling virtual circuit subnets General techniques Load shedding Jitter control

3. Congestion? Network layer -- May 2004 3 Too many packets in (a part of) the subnet!

4. Congestion? Network layer -- May 2004 4 Too many packets in (a part of) the subnet! caused by  the same output line is used by many input lines  mismatch between system parts  slow processor  low bandwidth lines tends to feed upon itself Congestion control <> flow control  Congestion: global issue ( subnet is able to carry the traffic)  Flow control: point-to-point ( control the traffic between a given sender and reciever)

5. Congestion: principles Network layer -- May 2004 5 How? Good design  Make sure the problem does not occur Tools  Decide when to accept traffic  Decide when to discard packets and which ones  Make scheduling decisions in the subnet They make decision without considering current state of network Monitor: where and when congestion?  % packets discarded  average queue length  number of packets that time out  average packet delay Rising no. Indicate growing congestion Pass collected info to places where actions can be taken = source of traffic  explicit <> implicit feedback  explicit: (extra) packet, flags (in other packets), probe packets ( ex. Radio stations have flying halicopters) Adjust system operation  Increase resources: bandwidth  Decrease load: deny, degrade service Control theory viewpoint: dividing all the solutions to two groups open loopclosed loop: feedback loop

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

7. Congestion: prevention 7 open loop solutions: Minimize congestion, they try to achieve there goals by using appropriate policies at various levels LayerPolicies Transport  Retransmission policy  Out-of-order caching policy  Acknowledgement policy  Flow control policy  Timeout determination ( transit time over the network is hard to predict) Network  Virtual circuits <> datagrams in subnet( many cog. Control algo work only with VC)  Packet queueing and service policy ( 1 Q / input/output line and round robin)  Packet discard policy  Routing algorithm ( spreading traffic over all lines)  Packet lifetime management Data link  Retransmission policy( Go back N will put heavy load than Selective Reject)  Out-of-order caching policy ( Selective repeat is better )  Acknowledgement policy( piggyback onto reverse traffic )  Flow control policy ( small window reduce traffic and thus congestion )

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

9. Virtual circuit subnets 9 Dynamic approach: act when needed, closed loop solutions Admission control  No new virtual circuits when congestion is signalled  e.g. telephone network : when a switch gets overloaded, it also practice admission control by not giving dial tones. Route new virtual circuits around problem areas

10. Virtual circuit subnets 10 Dynamic approach Admission control Route new virtual circuits around problem areas Negotiation when virtual circuit is set up  About kind of traffic + service desired  Resource reservation in subnet  Line capacity  Buffers in routers  No congestion  Unused resources

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

12. General techniques ( can be used in Datagram network and Virtual Circuit as well) Network layer -- May 2004 12 Approaches: reduce traffic by  Requesting senders local measurements + info packets to senders + hope for ….  Throwing away packets local measurements + local actions to reduce load

13. Source based approach Network layer -- May 2004 13 Basic algorithm  Router monitors utilisation of output lines  u recent utilisation: 0  u  1  good estimate of u u new = a  u old + (1 – a )  f  In 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. Source based approach Network layer -- May 2004 14 Warning bit  Output line in warning state  Warning bit set in packet header  Destination copies bit into next ack  Source cuts back traffic  Algorithm at source  As long as router is in warning state, warning bits arrive at source: reduce traffic  Less warning bits: increase traffic  Problems  voluntary action of host!  correct source selected?  Used in  DecNet  Frame relay

15. Source based approach 15 Choke packet: Tell host directly to slow down  In case of overload: router sends choke packet to host causing the overload  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  Problems:  voluntary action of host!  correct host selected?

16. Source based approach Network layer -- May 2004 16 Choke packets:  Example showing slow reaction  Solution: Hop-by-Hop choke packets

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

18. Load shedding 18 It is a fancy way of saying that ::::Throw away packets that cannot be handled!! Packet selection?  Random  Based on application  File transfer: discard new packet( out of order problem - wine)  Multimedia: discard old packet ( milk )  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 Random early detection …

19. Load shedding 19 Throw away packets that cannot be handled!! Packet selection? Random early detection  Discard packets before all buffer space is exhausted( response to lost packet is for source to slow down- ex- TCP for wired network)  Routers maintain running average of queue lengths ( to determine when to start discarding )  Select at random a packet  Inform source?  Send choke packet?  more load!!  No reporting  When does it work?  Source slows down when packets are lost This approach can not be used in wireless networks

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

21. Congestion: jitter control 21 Important for audio and video applications?  not delay  variance of delay

22. Congestion: jitter control Network layer -- May 2004 22 Jitter = variation in packet delay Compute feasible mean value for delay  compute expected transit time for each hop  router checks to see if packet is  behind  ahead schedule  behind: forward packet asap  ahead: hold back packet to get it on schedule again Buffering? Depends on characteristics:  Video on demand: ok  Videoconferencing: nok

23. Network Layer Network layer -- May 2004 23 Design issues Routing Congestion Internetworking : Next Lecture