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Communication Networks

Communication Networks. Recitation 10 QoS. QoS. network provides application with level of performance needed for application to function. Quality of Service: What is it?. Multimedia applications: network audio and video. Traffic Shaping The Leaky Bucket Algorithm.

sebastian-rowland
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Communication Networks

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  1. Communication Networks Recitation 10 QoS Comnet 2010

  2. QoS network provides application with level of performance needed for application to function. Quality of Service: What is it? Multimedia applications: network audio and video Comnet 2010

  3. Traffic Shaping The Leaky Bucket Algorithm (a) A leaky bucket with water. (b) a leaky bucket with packets. Comnet 2010

  4. Leaky Bucket example • A source generates data in terms of bursts: 3 MB bursts lasting 2 msec once every 100 msec. • The network offers a bandwidth of 60 MB/sec. • The leaky bucket has a capacity of 4 MB. How does the output look like? • Input:0-2 msec: 1500 MB/sec; 100-102 msec: 1500 MB/sec; 200-202 msec: 1500 MB/sec; … • Output: 0-50 msec: 60 MB/sec; 100-150 msec: 60 MB/sec; …. Comnet 2010

  5. Leaky Bucket CNTD. • What should be the capacity of the leaky bucket to avoid loss? • During the burst, data inflow is at the rate of 1.5 MB/msec and the outflow is at the rate of 0.06 MB/msec. • So accumulation is at the rate of 1.44 MB/msec. So at the end of 2 msec, there will be an accumulation of 2.88 MB. This is the minimum leaky bucket capacity to avoid buffer overflow and hence data loss. Comnet 2010

  6. The Token Bucket Algorithm 5-34 (a) Before. (b) After. Token bucket allows some burstiness (up to the number of token the bucket can hold) Comnet 2010

  7. Token Bucket – simple example • 2 tokens of size 100 bytes added each second to the token bucket of capacity 500 bytes • Avg. rate = 200 bytes/sec, burst size = 500 bytes • Packets bigger than 500 bytes will never be sent • Peak rate is unbounded – i.e., 500 bytes of burst can be transmitted arbitrarily fast Comnet 2010

  8. Token Bucket example • Bucket capacity: 1 MB • Token arrival rate: 2 MB/sec • Network capacity: 10 MB/sec • Application produces 0.5 MB burst every 250 msec For 3 seconds • The bucket is full of tokens Comnet 2010

  9. Token Bucket example CNTD. • Initially, output can be at the rate of 10 MB/s. But how long can the bucket sustain this? • First, 1MB can be sent • From then on, for X seconds, the token input rate is 2MB/s, the traffic rate is 10MB/s • 1 + 2X = 10X  8X = 1  X = 1/8 sec =125 ms • The bucket can transmit 1.25 MB in this time > 0.5MB the application produces • Output: 0-50 ms: 10 MB/s 50-250 ms: None Comnet 2010

  10. Token Bucket example CNTD. • At the end of this period, the amount of tokens in the bucket is: • 1MB+250ms*2MB/s-0.5MB=1MB • So the bucket is full again! • Repeat for 3 seconds Comnet 2010

  11. Minimum Bucket size and Token Rate • Discarding Bucket (Policing) • Bucket Size ≥ 0.5MB • Token Rate ≥ 0.5MB/250ms = 2MB/s • Queueing Bucket (Shaping) • How will the traffic look with Bucket Size = 200K? • 0.2+2X=10X  X=0.2/8=0.025s=25ms • 0-25ms : 10 MB/s = 0.25MB. 0.25MB left • 0.25MB/(2MB/s) = 125ms • 25-150ms: 2MB/s • 150-250ms: None • Tokens after: 100ms*2MB/s=0.2MB Comnet 2010

  12. (σ,ρ) Model • Over an interval of length t the number of packets/bits that are admitted is less than or equal to (σ+ρt). • Composing flows (σ1,ρ1) & (σ2,ρ2) • Resulting flow (σ1+ σ2,ρ1+ρ2) • What does a router need to support streams: (σ1,ρ1) … (σk,ρk) • Buffer size B > Σσi • Rate R > Σ ρi • Admission Control (at the router) • Can support (σk,ρk) if • Enough buffers and bandwidth • R > Σ ρi and B > Σ σi Comnet 2010

  13. (σ,ρ) Model example • The line from the previous question has router with 4MB of buffers. How many flows of the above kind can it accept? • σ = 0.5MB, ρ = 0.5MB/250ms = 2MB/s • For n flows, we require 0.5n MB buffers, 2n MB/s rate  n = 5. • Each line will be served with a 0.5MB:2MB/s token bucket Comnet 2010

  14. Random Early Detection (RED) Basic premise: router should signal congestion when the queue first starts building up (by dropping a packet) but router should give flows time to reduce their sending rates before dropping more packets Note: when RED is coupled with ECN, the router can simply mark a packet instead of dropping it Therefore, packet drops should be: early: don’t wait for queue to overflow random: don’t drop all packets in burst, but space them Comnet 2010 QoS

  15. RED FIFO scheduling Buffer management: Probabilistically discard packets Probability is computed as a function of average queue length (why average?) Discard Probability 1 0 Average Queue Length queue_len min_th max_th Comnet 2010

  16. RED (cont’d) Discard Discard Probability (P) 1 0 queue_len min_th max_th Average Queue Length Enqueue Discard/Enqueue probabilistically Comnet 2010

  17. RED (cont’d) Setting the discard probability P: Discard Probability max_P 1 P 0 Average Queue Length queue_len min_th max_th avg_len Comnet 2010

  18. Average vs Instantaneous Queue Comnet 2010

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