Presented by: Peng Wang EE Department University of Delaware A Probabilistic Approach for Achieving Fair Bandwidth Allocation in CSFQ

Background Why need fairness? –Ill-behaved flows consume most of the resource –Well-behaved flows are starved out Three approaches for fairness –Stateful solution: (Fair Queueing) Each router maintains per-state information and operates on per-state basis Good performance Scalability problem –Stateless solution: (CHOKe) No per-flow information Approximately fairness –Partial State solution: (CSFQ, RAINBOW, SRED) Partial State information Approximately fairness (Generally better than stateless solution) Complexity between stateful solution and stateless solution

Fair Queueing Disadvantage: Need to perform packet classification and maintain state and buffers on per-flow basis and perform operations on per-flow basis

The CSFQ (core stateless FQ) Approach Goal: Achieve approximately fair bandwidth allocation Differentiate between Edge router and Core router Edge router maintains per-flow state Core router is stateless

Assume that flow i has arrival rate r i (t) and the fair rate is α(t). If r i (t) < α(t), all of its traffic is forwarded. If r i (t) > α(t), then a fraction (r i (t) - α(t))/ r i (t) will be dropped; each packet of the flow is dropped with probability (1- α(t)/r i (t)). CSFQ In an island of routers, edge routers measure per-flow rate and label the packets with these measures. Routers drop packets probabilistically based on the per- flow state in the packet header and fair share estimation

CSFQ The problem now becomes how to calculate the flow rate r i (t) values and the fair rate a(t), without keeping per flow state in the core routers. Flow rates r i (t), are calculated at edge routers which keep per flow state and then insert the rate value inside the packet header of packets belonging to that flow. Estimation of flow arrival rates: R new = (1-e -T/K )*l/T + e -T/K *R old where T = packet interarrival time l = packet size K = constant

CSFQ: Estimate fair rate (Heuristic) To estimate the fair rate α (t), an iterative procedure is used: routers meausre aggregate arrival rate A and the aggregate accepted rate F. (arrival packets dropped packets accepted packets). Based on these, the fair rate a is computed periodically as: - Uncongested: A< C at all times during a time interval of length Kc. then a is set to the maximum r i (t) during Kc - Congested: A > C at all times during a time interval of length Kc. then a new = a old *C/F - Normal: others NormalUncongCong A<C during Kc Update α and return immediately: α=max(p.label) during Kc A>C during Kc Update α and return immediately: α= α*C/F Fig: FSM for fair share estimation in CSFQ

Zombie List –A list of M recently seen packets –Longer memory than the buffer alone P i : Probability that arrival packet belongs to flow i –Assume P i doesn’t change in a limited time interval Hit SRED: Some Definitions Zombie List n incoming packets

SRED: Estimate fair share Symmetric case: N flows, pi = 1/N –ΣPi 2 = 1/N (exact estimate)  m/n=1/N  N=n/m Asymmetric case: N ≈ 1/ ΣPi 2 = n/m good estimation Zombie List n incoming packets Pi Pi: random select a packet Hit prob for flow i:Pi 2 Hit prob: ΣPi 2 # of hits: m = n ΣPi 2 ΣPi 2 =m/n 1/N=<ΣPi 2 < 1 n: sample size After n packets arrives, the estimation of fair share is updated. Fair Share = C/N where N ≈ n/m

Simulations – Single Congested Link (ALL UDP) Mbps UDP Flows

ALL UDPs: Fair Share Estimation Ns-2.1b7aNs-2.27

ALL UDPs: Throughput of each flow Ns-2.1b7aNs-2.27

Simulations – Single Congested Link (ONE UDP) Mbps TCP Flows UDP Flow UDP flows at 10Mbps 10Mbps

ONE UDP: Fair Share Estimation Ns-2.1b7aNs-2.27 congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F= congested--->α= , C/F=

ONE UDP: Throughput of each flow Ns-2.1b7aNs-2.27

Further work: Make clear the confusion of NS2 version Optimize my fair share estimation Flows with different RTT Multiple congested links Web traffic THANKS !!!