Real-Time Guarantees. Traffic Characteristics. Flow Control

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Regina James
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1 Real-Tme Guarantees Requrements on RT communcaton protocols: delay (response s) small jtter small throughput hgh error detecton at recever (and sender) small error detecton latency no thrashng under peak load lmted/no packet loss Characterstcs of traffc: messages - packets (cells, frames,...) packet nter-arrval s Qualty-of-Servce (QoS) parameters ntegraton of real- and non-real- traffc buffer requrements header overheads and per-packet processng overheads Traffc Characterstcs Traffc characterstcs of real- sources constant bt rate: ar traffc control varable bt rate: voce traffc perodc wth varable packet sze: vdeo data P P P constant bt rate t varable bt rate t t perodc wth varable packet sze Flow Control Control of the nformaton flow between sender and recever, such that sender does not outpace recever. Recever should determne speed of communcaton. Implct flow control: a-pror establshed send rate (wll not be exceeded) communcaton can be un-drectonal error detecton s responsblty of recever dffuson-based protocols rely on mplct flow control Explct flow control: sender sends message and wats for explct acknowledgment sender s n the sphere of control of the recever,.e., the recever can slow down sender ( back pressure flow control) error detecton s responsblty of sender mssng ACK: message (or ACK) has been lost or recever s late or has faled 1

2 Swtch Schedulng Work-conservng (greedy) vs. non-work-conservng (non-greedy) mechansms. Rate-allocatng dscplnes: Allow packets to be served at hgher rates than the guaranteed rate. Rate-controlled dscplnes: Ensures each connecton the guaranteed rate, but does not allow packets to be served above guaranteed rate. Prorty-based schedulng: far queung earlest due date (Delay-EDD and Jtter-EDD) Weghted Round-Robn schedulng: WRR Weghted Far Queueng Packet queues w 1 w 2 R w n Each flow gven a weght (mportance) w WFQ guarantees a mnmum servce rate to flow r = R * w / (w 1 + w w n ) Imples solaton among flows (one cannot mess up another) Flud Flow w 1 water ppes w 2 w 3 water buckets t 2 t 1 w 1 w 2 w 3 2

3 Bt-by-Bt Weghted Round Robn w bt-by-bt round robn each connecton s gven a weght each queue served n FIFO order Flud Flow System: Example 1 (w 1 = 1) 100 Kbps (w 2 = 1) Packet Sze (bts) Packet nter-arrval (ms) Arrval Rate (Kbps) (arrval traffc) (arrval traffc) Servce n flud flow (ms) Flud Flow System: Example 2 Red flow has packets backlogged between 0 and 10 Backlogged flow flow s queue not empty Other flows have packets contnuously backlogged All packets have the same sze flows weghts lnk

4 Real System Packet transmsson cannot be preempted. Soluton: serve packets n the order n whch they would have fnshed beng transmtted n the flud flow Servce n flud flow Select the frst packet that fnshes n the flud flow Packet Example 2 Servce n flud flow (ms) Select the frst packet that fnshes n the flud flow Packet Implementaton Challenge Need to compute the fnsh of a packet n the flud flow but the fnsh may change as new packets arrve! Need to update the fnsh s of all packets that are n servce n the flud flow when a new packet arrves But ths s very expensve; a hgh speed router may need to handle hundred of thousands of flows!

5 Example Four flows, each wth weght 1 Flow 3 Flow ε Fnsh s computed at Fnsh s re-computed at ε Vrtual Tme Key Observaton: whle the fnsh s of packets may change when a new packet arrves, the order n whch packets fnsh doesn t! Only the order s mportant for schedulng Soluton: nstead of the packet fnsh mantan the round # when a packet fnshes (vrtual fnshng ) Vrtual fnshng doesn t change when a packet arrves System vrtual V(t) ndex of the round n the bt-bybt round robn scheme Vrtual Tme Flow 3 Flow ε Suppose each packet s 1000 bts, so t takes 1000 rounds to fnsh So, packets of F1, F2, F3 fnsh at vrtual 1000 When packet F arrves at vrtual 1 (after one round), the vrtual fnsh of packet F s 1001 But the vrtual fnsh of packet F1,2,3 remans 1000 Fnshng order s preserved

6 System Vrtual Tme (Round #): V(t) V(t) ncreases nversely proportonally to the sum of the weghts of the backlogged flows Snce round # ncreases slower when there are more flows to vst each round. (w1 = 1) (w2 = 1) V(t) C C/2 Schedulng wth WFQ When frst packet arrves n empty queue, scheduler computes fnsh number and commences transmsson. When scheduler busy, each new packet on dle connecton receves a fnsh number whch s nserted nto SFN queue. When transmsson of packet of connecton completes, packet s removed from and entry contanng fnsh number of ths packet s removed from SFN queue. Next packet s chosen: f s stll backlogged, the scheduler computes the fnsh number of ts new ready packet and nserts ths number nto SFN queue. the head packet n the SFN queue s chosen. Far Queueng Implementaton Defne k F - vrtual fnshng of packet k of flow k a - arrval of packet k of flow k L - length of packet k of flow w weght of flow The fnshng of packet k+1 of flow s F k + 1 = max( V ( a k + 1 ), F k k + 1 ) + L / w Smallest fnshng frst schedulng polcy 6

CS 268: Lecture 8 Router Support for Congestion Control

CS 268: Lecture 8 Router Support for Congeston Control Ion Stoca Computer Scence Dvson Department of Electrcal Engneerng and Computer Scences Unversty of Calforna, Berkeley Berkeley, CA 9472-1776 Router