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CNPA B

CNPA B. Nasser S. Abouzakhar Queuing Disciplines Week 8 – Lecture 2 16 th November, 2009. Content. Introduction FIFO (first-in-first-out) FQ (fair queuing). Introduction.

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CNPA B

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  1. CNPA B Nasser S. Abouzakhar Queuing Disciplines Week 8 – Lecture 2 16th November, 2009

  2. Content • Introduction • FIFO (first-in-first-out) • FQ (fair queuing)

  3. Introduction • Each router must implement some queuing discipline that governs how packets are buffered while waiting to be transmitted. • The queuing algorithm can be thought of as: • allocating BW (which packets are transmitted), and • allocating buffer space (which packets get dropped). • It also affects the packets latency, by determining how long a packet waits to be transmitted. • The two common queuing algorithms are • FIFO and FQ (Fair Queuing) Source: Peterson & Davie, 2007 p 467

  4. FIFO • Also known as FCFS (first-come-first-served). • The first packet that arrives at a router is the first packet to be transmitted. • The router discards packets that arrive whilst the queue (buffer space) is full. This is sometimes called tail drop. (a) FIFO queuing (b) tail drop at a FIFO queue. Source: Peterson & Davie, 2007 p 468

  5. FIFO, cont. • FIFO is a scheduling discipline. • determines the order in which packet are transmitted. • Tail drop is a drop policy. • determines which packets get dropped. • FIFO with tail drop is the simplest of all queuing algorithms and the most widely used in Internet routers. Source: Peterson & Davie, 2007 p 469

  6. FIFO, cont. • Priority queuing is a simple variation on basic FIFO queuing. • Marks each packet with a priority. • For example, the IP Type of Service (TOS) field. • The routers then implements multiple FIFO queues, one for each priority class. • The router always transmits packets out of the highest-priority queue. • Limitations: • The high-priority queue can starve out all the other queues. Source: Peterson & Davie, 2007 p 469

  7. FIFO, cont. • The routers implement multiple FIFO queues, one for each class. • Packets with highest-priority are transmitted first. • However, the high-priority queue can dominate the front lines hence the lower-priority queues don’t get served. • Therefore, there should be certain limits on how much high-priority traffic is inserted in the queue. Source: Peterson & Davie, 2007 p 469

  8. Fair Queuing (FQ) • FIFO does not discriminate between different traffic sources. • FIFO does not separate packets according to the flow to which they belong. • FQ maintains a separate queue for each flow currently being handled by the router. • These queues are served by the router in a round-robin service. Source: Peterson & Davie, 2007 p 470

  9. FQ, cont. • When a flow sends packets too quickly, then its queue fills up. • When a queue reaches a certain length, any additional packets are discarded. • Therefore, a given source cannot increase its share of the network’s capacity at the expense of the other queues. Round-robin service of four flows at a router. Source: Peterson & Davie, 2007 p 470

  10. FQ, cont. • The packets being processed at a router may not have the same length. • To allocate the BW of outgoing link in a fair manner, packets length is taken into account. • Example: • If one router is managing 2 flows, one with 1000-byte packets and the other with 500-byte packets. • A round-robin servicing packets from each flow’s queue will give the 1st flow 2/3 of the link’s BW and the 2nd flow only 1/3 of its BW. Source: Peterson & Davie, 2007 p 471

  11. FQ, cont. • FQ determines when a given packet would finish being transmitted. • The finishing time is used to sequence the packets for transmission. • Let Pi denote the length of packet i in bits. • Let Si denote the time when the router starts to transmit packet i. • Pi is expressed in terms of clock ticks to transmit packet i. • Let Fi denote the time when the router finishes transmitting packet i. Source: Peterson & Davie, 2007 p 471

  12. FQ, cont. • Routers often have more than one active flow i.e. has data in the queue. • We calculate Fi for each packet that arrives using the above formula. • We treat all the Fi as timestamps, and the packet that has the lowest timestamp will be transmitted first. Source: Peterson & Davie, 2007 p 472

  13. FQ, cont. • It is possible that the router finished transmitting packet i – 1 long before i arrived. • This means that the round-robin mechanism could not send any packets from this flow during which the queue was empty. If so, then let Ai denote the time that packet i arrives at the router, thus Source: Peterson & Davie, 2007 p 472

  14. FQ, cont. • If there is more than one flow, we calculate Fi as timestamps for each flow. • The packet with the lowest timestamp is transmitted first. • Packets with earlier finishing times are sent first. • Sending of a packet already in progress is completed. Source: Peterson & Davie, 2007 p 472

  15. Weighted Fair Queuing (WFQ) • allows a weight to be assigned to each flow (queue). • specifies how many bits to send (BW) each time the router services that queue. • Example: a router has 3 flows (queues), one queue has a weight of 2, the second queue has a weight of 3, and the third queue has a weight of 1. Assuming that each flow always contains a packet waiting to be sent, what is the percentage of BW that is assigned to each flow? Source: Peterson & Davie, 2007 p 473

  16. WFQ, cont. • Solution • The first flow will get 1/3 of the available BW. • The second flow will get ½ of the available BW. • The third flow will get 1/6 of the available BW. Source: Peterson & Davie, 2007 p 473

  17. Example • Suppose a router has 3 input flows and one output. It receives the packets listed in Table 1 all at about the same time, in the order listed, during a period in which the output port is busy but all queues are otherwise empty. Give the order in which the packets are transmitted, assuming: • Fair queuing. • Weighted fair queuing with flow 1 having a weight of 2, flow 2 having twice as much share as flow 1, and flow 3 having 1.5 times as much share as flow 1. Note that ties are to be resolved in order flow 1, flow 2, flow 3. Source: Peterson & Davie, 2007 p 529

  18. Example, cont. Table 1 Source: Peterson & Davie, 2007 p 530

  19. Solution (a) Fi is the cumulative per-flow size. Consider Ai = 0 as all packets are received at about the same time so there is no waiting. Source: Peterson & Davie, 2007 p 737

  20. Solution, cont.

  21. Solution, cont. • So, packets are sent in increasing order of Fi: Packet 3, Packet 1, Packet 6, Packet 4, Packet 7, Packet 2, Packet 5, Packet 8.

  22. Solution, cont. (b) Flow 1 has a weight of 2, so Flow 2 has a weight of 4, so Flow 3 has a weight of 3, so Source: Peterson & Davie, 2007 p 737

  23. Solution, cont.

  24. Solution, cont. • So, packets are sent in increasing order of weighted Fi: • Packet 3, Packet 4, Packet 6, Packet 1, Packet 5, Packet 7, Packet 8, Packet 2.

  25. Reference • Computer Networks: A systems approach by Larry Peterson and Bruce Davie, published by Morgan Kaufmann (Fourth edition ISBN: 0 12 370548 7).

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