Δίκτυα Η/Υ ΙΙ Έλεγχος Συμφόρησης Congestion Control

Δίκτυα Η/Υ ΙΙ 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

Δίκτυα Η/Υ ΙΙ Τα αίτια •Bursty traffic (When, How fast, How much) •Processing power of the gateways •Buffering capacities of the gateways

Δίκτυα Η/Υ ΙΙ Τα αποτελέσματα

Δίκτυα Η/Υ ΙΙ 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)

Δίκτυα Η/Υ ΙΙ Congestion control taxonomy Open loop system Closed loop system Source basedDestination based Explicit f/bImplicit f/b Congestion Control mechanisms PersistentResponsive Local Global

Δίκτυα Η/Υ ΙΙ Πολιτικές ελέγχου της συμφόρησης στα διάφορα επίπεδα

Δίκτυα Η/Υ ΙΙ 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.

Δίκτυα Η/Υ ΙΙ 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

Δίκτυα Η/Υ ΙΙ Ο μηχανισμός Leaky bucket

Δίκτυα Η/Υ ΙΙ 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.

Δίκτυα Η/Υ ΙΙ • Bursty traffic gets shaped into a constant rate one. • Retransmissions are internal to host and do not congest the network.

Δίκτυα Η/Υ ΙΙ 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

Δίκτυα Η/Υ ΙΙ Token bucket mechanism

Δίκτυα Η/Υ ΙΙ 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.

Δίκτυα Η/Υ ΙΙ 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 + r S = 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

Δίκτυα Η/Υ ΙΙ Traffic patterns from a token and leaky bucket

Δίκτυα Η/Υ ΙΙ 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

Δίκτυα Η/Υ ΙΙ 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.

Δίκτυα Η/Υ ΙΙ 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.