Έλεγχος Συμφόρησης

1. Έλεγχος Συμφόρησης Congestion Control

2. Congestion Control Συμφόρηση (congestion) είναι η κατάσταση εκείνη κατά την οποία το φορτίο σε μια τηλεπικοινωνιακή ζεύξη ή ένα υποδίκτυο, ξεπερνά τη χωρητικότητά της και οδηγεί σε επιδείνωση της επίδοσης της Congestion control Flow control

3. Τα αίτια • Bursty traffic (When , How fast , How much) • Processing power of the gateways • Buffering capacities of the gateways

4. Τα αποτελέσματα

5. Μεγέθη από τα οποία αντιλαμβανόμαστε τη συμφόρηση • Το ποσοστό των απολεσθέντων πακέτων (υπερχείλιση buffer κλπ) • Μέση καθυστέρηση (average delay) • Τυπική απόκλιση της καθυστέρησης (Standard deviation of the delay) • Ο αριθμός των επανεκπομπών (retransmissions)

6. Congestion Control mechanisms Open loop system Closed loop system Source based Destination based Explicit f/b Implicit f/b Persistent Responsive Local Global Congestion control taxonomy

7. Πολιτικές ελέγχου της συμφόρησης στα διάφορα επίπεδα

8. Αλγόριθμοι ελέγχου συμφόρησης • Open loop • Rely upon good design to prevent congestion. • No corrections once system is in place. • Closed loop • Monitor network for congestion. • Relay information to places where action can be taken. • Perform corrections.

9. Πολιτικές αποφυγής της συμφόρησης • Πολιτικές επανεκπομπής στα datalink και transport • Out of order caching at datalink and transport • Acknowledgement policy datalink and transport • Αλγόριθμοι δρομολόγησης • Διαχείριση του χρόνου ζωής των πακέτων

10. Ο μηχανισμός Leaky bucket

11. Leaky bucket • Open loop algorithm • Prevents congestion with traffic shaping. • Source incorporates finite internal queue into which all outgoing traffic is made to wait. • Queue is processed at constant rate thereby • If queue length exceeded packets are dropped.

12. Bursty traffic gets shaped into a constant rate one. • Retransmissions are internal to host and do not • congest the network.

13. Παράδειγμα Leaky bucket Given M(prod) = 25 MB/sec M(cons) = 2 MB/sec B(prod)= 1 MB every 40 msec C = 1 MB r = 2 MB/sec M – Peak transmission rate B – Burst rate C – Capacity of the leaky bucket r –Output rate of the leaky bucket

14. Token bucket mechanism

15. Token bucket • Leaky bucket ‘forces’ rigid output pattern. • Token bucket also has queue but this time it’s of tokens generated periodically. • Every packet has to acquire and destroy token in order to get transmitted. • Tokens get dropped if queue full. • Intermittent bursts can get the required capacity.

16. Παράδειγμα Token bucket Given M(prod) = 25 MB/sec M(cons) = 2 MB/sec B(prod)= 1 MB every 40 msec C = 250 KB/sec r = 2 MB/sec C + rS = MS = Total data transmitted M – Peak transmission rate B – Burst rate C – Capacity of the token bucket r –Token arrival rate S – Duration of regulated traffic

17. Traffic patterns from a token and leaky bucket

18. Choke packets • Closed loop strategy. • Router that notices congestion sends back choke packets to source specifying destination. • Source ‘can’ reduce traffic sent to that destination. • Optimistic. • Comes in two flavors • End to end • Hop by hop

19. Fair queuing • Multiple queues maintained on various output lines for different hosts. • Queues processed in round robin fashion. • Sources causing congestion will get drops whereas honest ones will not be affected. • Not all sources should have equal priority and the bandwidth percentage will really depend on what service the host has to offer. • Weighted fair queueing.

20. Load shedding • In face of congestion router drops packets from queue. • Packets to be dropped can be selected based on application they belong to rather than just random drops. • Different applications will need differently aged packets to be dropped. Milk and wine. • Intelligent policies need QoS parameters in network layer implementations.