Scheduling for QoS Management

Engineering Internet QoS2 Outline  What is Queue Management and Scheduling?  Goals of scheduling  Fairness (Conservation Law/Max-min fair share)  Various scheduling techniques  Research directions in scheduling

Engineering Internet QoS3 What is scheduling?  Packets from multiple flows compete for same outgoing link.  Which packets should be given preference?  How many packets should be transmitted from a flow?  Simple solution: First come best served  Complex solution: Provide QoS guarantees.

Engineering Internet QoS4 Scheduling Goals  Sharing bandwidth  Fairness to competing flows  Meeting bandwidth guarantees (max and min)  Meeting loss guarantees (multiple level)  Meeting delay guarantees (multiple level)  Reducing delay variations

Engineering Internet QoS5 Conservation law  Sum of the mean queuing delays received by the set of multiplexed connections, weighted by their share of link’s load is independent of the scheduling discipline – Kleinrock

Engineering Internet QoS6 Conservation Law Contd

Engineering Internet QoS7 Max-min fair share  Allocates the smallest of all demands from all flows  Distribute remaining resources equally competing of the flows  Guarantees fairness

Engineering Internet QoS8 Scheduling Disciplines  First come first serve (FCFS)  Priority (PQ)  Round Robin (RR)/Weighed round robin  Deficit round robin (DRR)  Weighted fair queuing (WFQ)  Class based queuing (CBQ)

Engineering Internet QoS9 First Come First Serve  Packets enqueued into a common buffer  Server serves packet from front of queue  No fair sharing of bandwidth  No flow isolation  No priority or QoS guarantee

Engineering Internet QoS10 FCFS example Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS11 Priority Queuing  Multiple queues with priority 0 to n-1  Priority 0 served first  Priority i served only if 0 to i-1 empty  Highest priority – lowest delay/loss, highest bandwidth  Possible starvation of lower class

Engineering Internet QoS12 Priority Queue example Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS13 Generalized processor sharing  Ideal work conserving scheme  Flows kept in separate queue  Serve infinitesimal amount of data from each queue  Serve all active queues in finite time  Weight can be associated with each queue  Achieves max-min fair share

Engineering Internet QoS14 GPS Continued  In GPS terminology, a connection is called backlogged when it has data present in queue.  Lets assume that there are K flows to be served by a server implementing GPS with weights w(1),.. w(k) Service rate of i th flow in interval [τ, t] is represented as R(i, τ,t). For any backlogged flow i in interval [τ,t] and for another flow j, the following equation holds:

Engineering Internet QoS15 Round Robin  Flows kept in separate queue  Serve one packet from each active queue  Fair share but no bandwidth guarantee  What if packet size variable

Engineering Internet QoS16 Weighted Round Robin  Allows variable length packet  Serves n packet from a queue  n adjusted to specific fraction of link share  Fairness problem at small time scale  Needs to know packet size a priori  Assume 3 ATM sources (small cell size) with weights 0.75, 1.0 and 1.5. If these weights are normalised to integer values, each source will be served 3, 4 and 6 cells in each round.

Engineering Internet QoS17 Deficit Round Robin  No need to know packet size a priori  Initially serves each queue quantum worth of bits  If packet less than or equal to quantum, serve it  Else increment deficit_counter by quantum  If no more outstanding packet, reset deficit_counter (Why?)  Set quantum to minimum MTU of all incoming links  Fairness problem at smaller time scale

Engineering Internet QoS18 DRR Example 1 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS19 DRR Example 2 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS20 DRR Example 3 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS21 DRR Example 4 Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS22 Weighted Fair Queuing  Packets tagged with a value identifying the time last bit of packet should be transmitted using GPS simulation  Packet with lowest tag value transmitted by scheduler  Uses complex finish time calculation  Hard to implement with variable packet size  QoS guarantees possible (gets bandwidth in proportion of weight)

Engineering Internet QoS23 WFQ Delay bounds  Delay can be bounded if flows can be policed (token bucket)  Flows regulated by token bucket are put in different queues  Each queue has assigned weight  With token bucket policing, assume that intially the token bucket is full and a brust of b i packets arrive for a flow of class i. Last packet to complete service will suffer a maximum delay of d max given by equation

Engineering Internet QoS24 WFQ Delay with Token bucket Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS25 Finish Time Calculation  Following equation shows the finish time calcuation where R(t) is called round number. P c m is the time required to transmit m th packet from c th connection and w(c) is the weight of connection c.

Engineering Internet QoS26 Round Number  This is the number a bit-by-bit round robing scheduler (in place of GPS’s non-implementable infinitesimal data) has completed at a given time. The round number is a variable that depends on number of active queues to be served (inversely proportional to the active queue number). The more queues to serve, the longer a round will take to complete (example and figure in section of text)

Engineering Internet QoS27 Virtual Clock  Also known as Fair Queuing  WFQ finish time calculation is very complex  Virtual clock replaces round time with real time as per the following equation (here, A m is the real-arrival time of packet m):

Engineering Internet QoS28 VC Example Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS29 Class Based Queuing Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS30 CBQ Contd Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA.

Engineering Internet QoS31 Scheduling Research Directions  Worst-case fair weighted Fair queuing (WF 2 Q)  Self clocked fair queuing (SCFQ)  Start time fair queuing (SFQ)  Core state fair queuing (CSFQ)  Score others