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The Evolution of Layered Protocol Stacks Leads to an Hourglass-Shaped Architecture

The Evolution of Layered Protocol Stacks Leads to an Hourglass-Shaped Architecture. Saamer Akhshabi Constantine Dovrolis Georgia Institute of Technology s.akhshabi,constantine@gatech.edu. My co-author, Saamer Akhshabi ( 2 nd year PhD student, he could not travel to Toronto).

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The Evolution of Layered Protocol Stacks Leads to an Hourglass-Shaped Architecture

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  1. The Evolution of Layered Protocol Stacks Leads to an Hourglass-Shaped Architecture SaamerAkhshabi Constantine Dovrolis Georgia Institute of Technology s.akhshabi,constantine@gatech.edu

  2. My co-author, SaamerAkhshabi(2nd yearPhD student, he could not travel to Toronto)

  3. Outline • Motivation • Model description • Results • Concluding remarks

  4. Where does this work come from? Firefox Silverlight Thunderbird MPlayer … Modeling study of the Internet protocol stack (architecture) and its evolution HTTP SMTP RTP … TCP UDP IPv4 PPP Ethernet … OpticalFiber TwistedPair CoaxialCable …

  5. Why is it an hourglass? Firefox Silverlight Thunderbird MPlayer … HTTP SMTP RTP … Why ? -Random? -Designed? -Emergence? TCP UDP IPv4 PPP Ethernet … OpticalFiber TwistedPair CoaxialCable …

  6. What happens at the “waist” compared to other layers? Firefox Silverlight Thunderbird MPlayer … Frequent innovations HTTP SMTP RTP … TCP UDP Conserved (“ossified”) IPv4 PPP Ethernet … Frequent innovations OpticalFiber TwistedPair CoaxialCable …

  7. How can a new protocol survive at the waist? Firefox Silverlight Thunderbird MPlayer … HTTP SMTP RTP … TCP UDP ATM SNA X.25 IPv6 IPv4 PPP Ethernet … OpticalFiber TwistedPair CoaxialCable …

  8. What about “Future Internet” those architectures? NDN XIA MpbilityFirst Nebula • Will these architectures also evolve to an hourglass in few years? • How to make them more “evolvable”? • So that they can better accommodate innovation? • So that no single protocol at the waist “kills” all competitors ?

  9. Outline • Motivation • Model: EvoArch • Results • Conclusions

  10. Two Disclaimers • EvoArch is only an abstraction of protocol stacks • EvoArch does not capture many practical aspects and protocol-specific or layer-specific semantics • EvoArch is certainly not the only model, or “the correct model”, for the emergence of hourglass-shaped network architectures • Models should be judged based on their assumptions, parsimony and predictions

  11. Model description L Protocols as nodes … Protocol dependencies as edges 4 Products:P(u) 3 u Layer of u: l(u) Substrates:S(u) 2 1 Layered acyclic network Every layer provides a service

  12. The value of a protocol • The value of a protocol depends on the value of its products • Protocols with valuable products are more valuable • example TCP, HTTP 1 1 1 1 1 1 1 1 3 2 5 5

  13. The generality of a layer As we go higher in the stack: • Protocols become less general – they offer more specialized services • The probability that a protocol is used by next-layer’s protocols decreases Firefox Silverlight Thunderbird MPlayer HTTP SMTP RTP TCP UDP IPv4 PPP Ethernet OpticalFiber TwistedPair CoaxialCable

  14. Generality as a probability • We introduce a parameter called generality vector s • s(l) : probability that new node at layer l+1 chooses each node at layer l as substrate • s(l) decreases as we go higher in protocol stack s(L-1) = 0.1 s(3) = 0.5 s(1) = 0.9

  15. Competition between protocols • Two protocols at the same layer compete if they offer similar services • i.e., if they have large overlap in their products • HTTP competes with FTP due to several overlapping products • TCP does not compete with UDP because they haveminimal service overlap HTTP FTP TCP UDP

  16. Modeling competition • Let C(u) be set of competitors of u • Node w competes with u if • c: competition threshold • If c = 3/5 • u competes with q and w • q does not compete with w q u w

  17. When does a protocol “die”? • Protocols can become extinct due to competition with other protocols • For example, HTTP services cover the set of services provided by FTP • Competition from HTTP has led to FTP’s demise HTTP FTP

  18. Modeling protocol deaths • A node u dies if its value is significantly less than the value of its strongest (i.e., maximum value) competitor. • z: mortality parameter

  19. Cascade deaths • u is w’scompetitor • Suppose that w dies due to competition with u (r=3/7) • If a node w dies, its products also die if their only substrate is w. This can lead to cascade deaths. 1 1 1 1 1 1 1 2 1 1 2 1 2 4 7 3 q u w

  20. Protocol births • Basic birth process • Number of new nodes at given time is a small fraction of total number of nodes in network at that time. • New nodes assigned randomly to layers • Death-regulated birth process • The birth rate at a layer is regulated by the death rate in that layer • Discussed later

  21. Summary of EvoArch • Discrete-time model • Time advances in rounds • Each round includes • birth of new nodes • competition among nodes at the same layer • potentially, death of some nodes • Key parameters • Generality vector s • Competition threshold c • Mortality parameter z

  22. Outline • Motivation • Model Description • Results • Emergence of hourglass structures • Controlling the location/width of the waist • Evolutionary kernels • Protocol differences • Conclusions

  23. Hourglass shape • The network forms an hourglass structure over time • The waist usually occurs at layer 5 or 6. • L = 10 • c = 3/5 • z = 1 • s(l) = 1-l/L

  24. Hourglass Resemblance Metric • w(l) : width of layer l • Minimum occurs at layer b • X = {w(l), l = 1, . . . b} • Y = {w(l), l = b, . . .L} • Mann-Kendall statistic for monotonic trend on the sequences X and Y: coefficients τX and τY • H = (τY – τX)/2 • H=1 when widths first decrease and then increase (monotonically) • W(L) • … • w(b+1) • Layer number • w(b) • … • w(2) • w(1) • Width

  25. Robustness • High hourglass scores under a wide range of parameters

  26. Why does EvoArch generatehourglass-shaped networks? Small generality Low competition (local) Few deaths Generality close to 50% Few protocols with many products Most other protocols die Large generality High competition Protocols have similar products -similar values Few deaths

  27. How can we get a wider waist? • As γ increases • Location of the waist moves to higher layers • Width of waist increases • γ is the layer atwhich the generality is 50% • s(l) • 0.5 • γ • Layer number

  28. Evolutionary kernels

  29. How can a kernel die? • Normalized value of a node: value divided by maximum possible value at that round • If several nodes appear at the next higher layer, and • kernel fails to quickly acquire those new possible products, • someone else may do so..

  30. Death-regulated birth process? • What if the birth probability in a layer is regulated by the death probability in that layer? • It becomes practically impossible to replace kernels

  31. What if protocols differ in term of a “quality factor”? • The “quality factor” can be interpreted broadly • Performance, • Extent of deployment, • Reliability or security, • Incremental improvements, • etc

  32. Effects of quality factor • We still get an hourglass • Slower network growth • Lower part of hourglass is smaller in size • only high quality nodes survive at the lower part • Kernels are often NOT the highest quality protocols

  33. Outline • Motivation • Model Description • Results • Concluding remarks

  34. What does this mean for the Internet architecture? • New way to think about (and teach) Internet’s hourglass architecture • New way to think about “ossification” of protocols at the waist • Parameterized modelfor TCP/IP stack: • Two protocols compete when their service overlap is more than 70% • A protocol survives only if its value is more than 90% of its strongest competitor’s value • Death-regulated births

  35. What does this mean for IPv4 vs IPv6? • IPv6 has same products but lower extent of deployment (i.e., lower “quality factor”) • IPv6 would find it easier to compete w IPv4 if: • It had some distinct products that IPv4 does not have • Unfortunately, it only offers more addresses • IPv6 would face easier adoption if it was not presented as “IPv4 replacement” but as “the second network-layer protocol”

  36. What does this mean for future Internet architectures? • Hourglass structures should be expected if these new architectures evolve/compete • Designers should strive for wider waist • More diverse waist -> more evolvable architecture • EvoArch: as the waist moves higher, it also becomes wider • How to push the waist to a higher layer? • See highly relevant paper: • L. Popa, A. Ghodsi, and I. Stoica. HTTP as the Narrow Waist of the Future Internet. In ACM SIGCOMMHotNets, 2010

  37. From Networking to Network Science • Hourglass effect in development of embryos • Hourglass effect in organization structures • Hourglass effect in innate immune system

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