Lets get rid of HIERARCHY ... Make way for DHT networks !!!

Distributed Compact Routing

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• Routes over arbitrary names rather than location-dependent addresses (hierarchy independent)
• Disco: Distributed Compact Routing
  • Flexible mobility
  • Flexible multi-homing
  • Easier management

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Why Disco

For general networks, the common scaling technique is hierarchy: routing is performed over high-level aggregate units until it reaches the destination's unit, at which point routing proceeds at a finer granularity. For example, the Internet routes at the level of IP prefixes to a destination domain, and then over an intra-domain routing protocol to a subnet.

Problem with Hierarchy

• First, it can have arbitrarily high stretch, defined as the ratio of route length to the shortest path length. Simultaneously guaranteeing scalability and low stretch on arbitrary networks is a nontrivial problem which no deployed routing protocols achieve.
• Location dependent addressing complicating mobility, management and multi-homing

SOLUTION

• Scalable, low stretch routing on flat names

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Requirements

• Guaranteed Scalability
• Guaranteed Low Stretch
• Flat Names

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Components of Disco

(i) Name Dependent Distributed Compact Routing Protocol : NDDisco

(ii)Distributed Name Resolution Module

(iii) Distributed Location Database to achieve name independence \

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Component 01 : Name Dependent Distributed Compact Routing Protocol (NDDisco)

Name Dependent : Assume the source knows the destination's current address, as opposed to just its name. NDDisco is based on a centralized algorithm.

Components of NDDisco

• LANDMARKS
• VICINITY
• ADDRESSING
• ROUTING
• SHORT CUTTING HEURISTICS

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01.LANDMARKS

• A landmark is a node to which all nodes know shortest paths. Landmarks will allow us to construct end-to-end routes of the form s --> 'l' --> t.
• Landmarsk give us source an approximate way to locate destination 't'.
• Landmarks are selected uniform-randomly by having each node decide locally and independently whether to become a landmark.
• Specically, each node picks a random number p uniform in [0; 1], and decides to become a landmark if p <sqrt(log n)=n.
• Thus, the expected number of landmarks is (n*sqrt((log n)/n))= sqrt(n log n) and by a Chernoff bound, there will be theta(nlog n)(w.h.p)
• Node v flips its landmark status only if n has changed by atleast a factor of 2 since the last flip.

if s and t are close , this will lead to poor approximation

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02.VICINITY

• Each node 'v' learns shortest path to every node in its vicinity V(v).
• theta(sqrt(nlogn)) nodes closest to 'v'

These sizes ensure that each node has a landmark within its vicinity w.h.p., which is necessary for the stretch guarantee

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03. ADDRESSING

• The address of node v is the identi er of its closest landmark lv, paired with the necessary information to forward along lv--> v.
• Addresses are location-dependent, of course, but they are only used internally by the protocol
• Dynamically updated to reflect topology changes

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Distributed database which maps node names to their addresses

The database requires the key properties that

• any necessary query can be accomplished with low stretch,
• the O~(n) per-node state bound is not violated, and
• maintaining the state in the face of network dynamics requires little messaging

Design a random overlay network organized around these groups which can efficiently distribute flat-name routing state

Adopt the idea of color-grouping of nodes from , but with sloppy grouping and routing schemes which are more amenable to a distributed setting.

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SUMMARY

To build state

• Pick random neighbors in G(t) to build overlay
• Gossip in overlay to send address to n^{1/2 nodes}

To route from s to t

• Check V(s). If t isn’t there, then...
• w = node in both V(t) and G(t)
• Route to w and from there to t via NDDisco: provable stretch ≤ 7
• Subsequent packets follow NDDisco: stretch ≤ 3