CDN server _ path selection - University of Southern California

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					     Tradeoffs in CDN Designs for
     Throughput Oriented Traffic

                    Minlan Yu
        University of Southern California



Joint work with Wenjie Jiang, Haoyuan Li, and Ion Stoica
                                                       1
      Throughput-Oriented Traffic
• Throughput-oriented traffic is growing in Internet
  – Cisco report predicts that 90% of the consumer traffic
    will be video by 2013 (E.g., NetFlix, Youtube)
  – Software, game, movie downloads
  – Most are delivered by content distribution networks
      Revisit CDN design choices for throughput-
      oriented traffic




                                                       2
 Where is the throughput bottleneck?




Client:         Network:               Server:
Computer/access Congestions at peering Not enough resource
link too slow   and upstream links     (CPU, power, bw)

                                                      3
Understanding Throughput Bottleneck
• Network bottlenecks are common
  – NetFlix sees reduced video rates due to low ISP capacity
  – Akamai reported bottlenecks at peering links
                        4
                      3.5
                                Degraded video performance
    Buffering ratio




                        3
                                caused by network congestion
                      2.5
                        2
                      1.5
                        1
                      0.5
                            0    2   4   6    8   10 12 14 16 18
                                     Concurrent views (K)
                                                                   4
    Nature of Bottleneck is Changing

• More throughput-oriented applications
  – Video traffic lasts longer and has higher volume


• More elephants step on each other in the future
  – Decreases the benefits of statistical multiplexing
  – Introduces more challenges in bandwidth provisioning



                                                       5
     Improving Network Throughput
• ISP-CDNs: multiple paths and better path selections
  – ISPs move up in the revenue chain to deliver content
     • ISP-CDNs such as AT&T and Verizon
  – Control both servers and the network
  – Better traffic engineering for CDN traffic
• Existing CDNs: Deploy servers at more locations and
  setting up more peering points

    Peering
    Question 1: What’s the throughput benefit of
    points
        more paths over more peering points?
                       ……                                  6
        Improving CDN Throughput
• Highly distributed approach (e.g., Akamai)
  – Many server locations, more high-throughput paths
  – Higher management, replication, bandwidth cost
• More centralized approach (e.g., Limelight)
  – A few large data centers with more peering points
  – Lower cost due to economy of scale
     More centralized              Highly distributed

 Question 2: How to compare more centralized vs.
 more distributed CDNs on throughput and cost?
          ……
   Modeling CDN Design Choices

• CDNs: Increase peering points at the edge

• ISPs: Improve path selection at the core




                                              8
            Increase Peering Points
• Modeling peering points (PPs)
  – Increase #PPs to study throughput effect
  – Pick PP locations from synthetic and real topologies


• Peering point selection
  – Maximize aggregate throughput
  – By assigning client locations to PPs
  … and splitting traffic to different PPs

                                                           9
           Improve Path Selection
• Today: No cooperation (1path)
  – ISPs: Shortest path routing (e.g., OSPF)
  – CDNs: Select peering points to maximize throughput


• Better contracts between ISPs and CDNs (n paths)
  – ISPs: Expose multiple shortest paths to CDNs (e.g.,MPLS)
  – CDNs: Select peering points and paths




                                                         10
          Improving Path Selection
• ISP-CDNs: Optimal throughput (mcf)
  – Joint traffic engineering and server selection
  – Reduced to multi-commodity flow problem


• Optimization formulation
  – Objectives: Max total throughput
  – Subject to: Client demands & Link capacity constraints
  – Variables: Peering point selection, traffic splitting on
    each paths (Flow_{path, pp, client})
                                                          11
                       An Example
Min-cut size
– improving path selection only approximates the min-cut size
– increasing #peering points essentially increases min-cut size
                Capacity =2   Capacity =1   Capacity =2




 • With PP2 and PP3, the maximum throughput of multiple paths is 4
   (min-cut size 4)
 • Increase to 4 PPs, the min-cut size now is 8                12
            Question 1:
What’s the benefit of path selection
  over peering point selection?



                                   13
 Quantify the Benefits under Various Scenarios
• Network
  – Topologies: power-law, random, hierarchy, different link
    density, router-level ISP topo, AS-level Internet topo
  – Link capacity distribution: uniform, exp., pareto, higher
    inter-AS bandwidth
• CDN peering points
  – Map Akamai and Limelight server IP addresses to ASes
    (collected from PlanetLab measurement at Nov. 2010)
  – Randomly pick peering points for synthetic topologies
• Client demands
  – Session-level traces from Conviva collected between
    Dec. 2011 and April. 2012                           14
Multipath is better than Multiple Locations
    – Power law graph (500 nodes, 997 links)
    – Uniform link capacity distribution
    – 200 clients at random locations



      Multiple paths have little improvement
          over increasing peering points



                                               15
        Effect of Network Topology
 – Increasing peering points are better than multipath in
   most topologies
 – Except star-like topology with uniform link capacity
                                                                                                             58




• The throughput from
                                                                                          290 170136 28
                                                                                            229
                                                                   146             200 38         222 158169 3
                                                              13                 27 267 8
                                                                               40           90 245 131         64
                                                                                261133 175             201 67 193
                                                                                                          71 91 60
                                                                                                               223165
                                                           263
                                                                                                 47
                                                      10                  109230


  1path to mcf increases by
                                                                                                                                62
                                                           19                                                                     65                                21
                                                                                                                                                    227
                                                                                           283211 121
                                                                            117 122140
                                                                         48 312                       161                                               234
                                                                                          79        32 51 253
                                                                                                           258
                                                                                                                52 318
                                                                                                                 159                                     196
                                          23                                 300 83188                            302


  110% - 584%                                                 35 259
                                                               149 53             153       231                                       269                   255
                                                            74 309 268                    110 41 187
                                                                                                  151                                  303 297
                                        20               242 270 308   120                          100
                                                      147 220                   235                   185                                 11                  72
                                                         59     244     66                      319 106                                                      138
                                                    293 160 167 197                                                 42155                  128
                                     226                73                                111108                   208 81
                                                    198 246 95 93                                  191                                       317
                                                                                     114 209 80 156
                                                                                    69 189 145 150             232 49 249299



• The throughput from 10
                                                      135
                                                    311 284 216 31
                                                   82                                  301 294 154
                                                                                                        296     115 118 190                   37
                                     304               162248 26                                     103             184 102                  164
                                                    313                                 116                         105 152 87
                                                               94 291
                                                                                    277    325 183 86                                        171
                                      6             29 260 173 288                              104 264                       17
                                                    139                               233
                                                                                        292236143              326 239
                                                         78 305163 289             101              112            144     26514
                                                       213                                               207                                                 98
                                                                  217
                                                          57 287 166
                                                                          180      107 181                       322 275                                  212 75
                                                           168                       30 15   321323 179 295
                                                                                                   276                    113
                                                                    125


  PPs to 20 PPs increases by
                                                             2572 55                    278 315 282
                                                                                              88 205                 281 141 202                           274 36
                                              195                                         148                                 178
                                                                                                               182 204 206                               240 22
                                                                                    186
                                                                                      33                     279316 16
                                                                                                                237 320                                       224
                                          174                                                                                                 314        157
                                                                                                           298                                                247
                                                                  34                                                                256                 215 24
                                                                     89 127 12
                                                                           210
                                                                     280                                                                                    97


  337%
                                                                                                                                                      214
                                               134                             76 1306 68 285 25 51307
                                                                                                61                              126                       192
                                                                                                      119                     272                  45
                                               177                                                                                                       70
                                                                                        43324 85                            286                   56 199
                                 176                 262225                 203                                      241 63
                                                                                                                        238                   266
                                                                                                                                                    132
                                                       99 123                                                                              251 46
                           243                                228                                                                           4 250
                                                         124     54
                                                            252     219 172
                                                               254           221 96 92
                                                                  84 310                    77      307142
                                                                           9 137194 129 50       44
                                                                                     18

                                                        271

                                                                                                         273
                                                                                 39            218


                                                                                                                                                                         16
  Path selection not useful under Flash Crowd
   – Conviva traces during normal and flash crowd periods
   – Path selection has little benefits under normal traffic
   – Path selection is worse than only peering point
     selection
                       Relative scaling ratio   1.4
                                                1.2                   flash crowd
                                                                           normal
Thpt (Path + peering                              1
point selection)                                0.8
Thpt (Peering point                             0.6
selection)                                      0.4
                                                0.2
                                                  0
                                                      5min 10min30min 1hour 2hour
                                                                                    17
                                                         Path selection interval
More peering points always better than more paths
      with long-tail Distribution of Contents
– Long-tail content distribution trace from Conviva
– With fewer replications, the throughput benefit of
  multipath increases
   • Without replication the content delivery is closer to the single-
     source traffic
                                    8
            Normalized Throughput




                                    7   100PP,1path
                                           10PP,mcf
                                    6    10PP,1path
                                    5
                                    4
                                    3
                                    2
                                    1
                                    0
                                         0.1    1      2    10     20
                                                                        18
                                           Duplication Threshold (%)
Takeaway 1:

CDNs only need to control the edge of the Internet to
improve the throughput.

ISP-CDNs don’t get significant benefits from controlling
the network over CDNs



                                                           19
            Question 2:
How to compare throughput and cost
              between
more centralized vs more dist. CDNs?



                                  20
       Throughput Comparison of CDNs
– Assume a fixed aggregate peering bandwidth per CDN
– A more distributed CDN achieves better throughput than
  more centralized one
                         200
                             peering bw
                                     2-3        Distributed
        Throughput (K)




                         150
                                     4-5
                                   6-10
                         100        >10

                          50                         Centralized
                           0
                               0   50 100 150 200 250 300 350 400
                                                                    21
                                           #peering links
              CDN Operation Cost
• Management cost
  – At each location: electricity, cooling, equip maintenance,
    and human resources
• Content replication cost
  – Storage cost to replicate popular content
  – Bandwidth cost to redirect traffic for rare content
• Bandwidth cost
  – CDNs often pay ISPs for the bandwidth they use at the
    peering points based on mutually-agreed billing model
                                                          22
                            Different Cost Functions
• Cost as a function of bandwidth at a location
  – Different functions: polynomial, linear, log, exp
  – Model how fast the unit cost drops with throughput
  – In practice: a linear combination of different functions
                                     1
         Unit price per bandwidth




                                                             Polynomial
                                                             Linear
                                    0.8
                                                             Log
                                                             Exponential
                                    0.6

                                    0.4

                                    0.2
                                       0   20    40    60      80      100
                                                Throughput                   23
                                       Polynomial Cost
• Dist. CDN is more expensive than Centralized one
  – Limelight has larger throughput at each location and
    thus better scalability gains
  – Same observation holds across various operational cost
    functions and their combinations
     Unit price per bandwidth




                                 0.5       Distributed
                                0.45
                                 0.4
                                0.35         2-3
                                             4-5
                                 0.3        6-10              Centralized
                                             >10
                                0.25
                                       0     20    40    60    80   100 120 140
                                                                                  24
                                                   Throughput (K)
Takeaway 2:

More distributed CDNs achieve higher throughput than
more centralized CDNs, but…

… are more expensive for same throughput




                                                       25
                     Conclusion
• A simple model to quantify CDN design choices
  – Increasing the number of peering points
  – Improving path selection
  – More distributed vs more centralized design
• Optimizations at the edge is enough for CDNs
  – Multipath has little benefit over increasing # locations
    and choosing different peering links
  – There’s a tradeoff of throughput and cost among CDNs

                                                          26
 Thanks!


Questions?


             27

				
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posted:4/17/2013
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