Application Level Routing: What can it teach transport level routing Midnight Sun Routing Workshop J

1
Application Level RoutingWhat can it teach
transportlevel routing?Midnight Sun Routing
Workshop June 2002
Elwyn Davies18 June 2002
2
Agenda

• What does Application Level Routing do?
• First generation Napster and Gnutella
• Scalability fixes Distributed Hash table
  algorithms
• Uses of DHT
• Open Questions for DHT
• What can transport level routing learn?

3
Why do we wantApplication Layer Routing?

• Driven by finding objects and services on a
  chaotic network
• Data (content) and services are mobile
• Data and services need to be widely replicated,
  giving
• Availability
• Durability
• Locality (I.e. ability to exploit local resources
  rather than remote ones)
• Want to be able to route queries (operations)
  to objects using location independent names
• Keep the number of overlay hops in scale with
  network size

4
Challenges in the Wide-area

• Trends
• Exponential growth in CPU capacity and storage
• Networks expanding in reach and bandwidth
• Can applications leverage new resources?
• Scalability increasing users, requests, traffic
• Resilience more components gt inversely low MTBF
• Management intermittent resource availability gt
  complex management scheme
• Proposal(s) Distributed Object Location
  Routing infrastructures to solve these issues and
  pass benefits onto applications (DOLR)

5
Requirements for a DOLR Infrastructure

• Deterministic Location
• Objects should be located if they exist in the
  network
• Routing Locality
• Routes should have low stretch (ratio of actual
  to minimal distance that query travels from
  origin to object), not just a small number of
  application-level hops.
• Sending queries to nearest copy across shortest
  distance path possible is ideal.
• Minimality and Load Balance
• Infrastructure must not place undue stress on any
  of its components
• Implies minimal storage and balanced
  computational load
• Dynamic Membership
• Infrastructure must adapt to arriving and
  departing nodes whilst maintaining previous
  properties
• Self-organising

Not a bad set of basic requirements for transport
routing !
6
First Generation Solutions

• Domain Name System
• Issues Root servers, Not self-organising
• Web Search Engines
• Issues Non-deterministic, Centralised
• P2P File Sharing Systems
• Napster Not fully decentralised (central
  directory)
• Gnutella Broadcast based protocol limits
  scalability/resource hungry
• FreeNet Not guaranteed to find existing object

All these systems make tradeoffs
betweendecentralised operation,efficient
bandwidth usage anddeterministic operation
7
DHT to the Rescue

• Most current research to improve scalability of
  P2P systems and DOLR in particular focusses on
  Distributed Hash Table (DHT) functionality
• Trying to scale
• CPU load
• Local storage
• Network Bandwidth
• Typical scalabilities for a network of n nodes
• Routing hops O(log n)
• Space O(n log n)

8
Basics of DHT

• Take a key as input Route a message to node
  responsible for key in response
• Keys are strings of digits of a given length
• Nodes have identifiers
• Taken from same space as keys (ie same number of
  digits)
• Each node maintains a routing table of a small
  subset of nodes in the system
• When a node receives a query for a key for which
  it is NOT responsible, it routes the query to the
  neighbour node which makes the most progress
  towards resolving the query
• Notion of progress differs from scheme to scheme
• Generally defined as a metric between identifier
  of current node and queried key
• Involves some sense of locality

9
Example Overlay Routing Networks

• CAN Ratnasamy et al, (ACIRI, UCB)
• Uses d-dimensional coordinate space to implement
  distributed hash table
• Route to neighbour closest to destination
  coordinate in this space
• Chord Stoica, Morris, Karger et al, (MIT, UCB)
• Linear namespace modelled as a circular address
  space
• Finger table entries point to logarithmic of
  increasingly remote nodes
• Pastry Rowston and Druschel (Microsoft / Rice)
• Hypercube routing similar to Plaxton
• Objetcs replicated to servers by name

• Fast insertion deletion
• Constant sized routing state
• Unconstrained of hops
• Overlay distance not proportional to physical
  distance
• Simplicity in algorithms
• Fast fault-recovery
• Log2(n) hops and routing state
• Overlay distance not proportional to physical
  distance
• Fast fault recovery
• Log(n) hops and routing state
• Data replication required for fault tolerance

Plaxton et al, 1997, Accessing nearby copies
of replicated objects in a distributed
environment is seminal work on DHT
10
Example applications of DOLR/DHT

• Distibuted file systems
• OceanStore
• Application-layer multicast
• Event notification services
• Scribe Publish-subscribe system
• Chat services

11
Issues State-Efficiency Tradeoff

• Can we achieve O(log n) pathlengths (or better)
  with O(1) neighbours?
• What suffers to make this better?

12
Issues Resilience to failures

• Static resilience How badly is routing affected
  in the interval between a node failing and the
  other nodes getting round to fixing up the
  problem with new neighbours?
• Can one characterize the static resilience of an
  algorithm?
• What makes for good static resilience?
• Does the use of special neighbours really
  help?
• How long does it take to recover? At what cost?
• Are there design rules for the net to improve
  recovery performance?

13
Issues Routing Hotspots

• Do routing hotspots exist?
• If so, how does one deal with them?

14
Issues Incorporating Geography

• If you use proximity, can you characterise the
  effectiveness of the results?
• Does knowing about close neighbours help?
• Does choosing identifiers in a geographically
  informed manner help? Can one choose identifiers
  in a d-dimensional key space that will acurately
  capture the geographical layout of nodes?
• Do existing local (topology) techniques actually
  achieve much of the benefit of geography?
• What tradeoffs have to be made to incorporate
  geography?

15
Issues Heterogeneity

• Can the routing algorithms exploit heterogeneity?
• One technique is cloning so that capable nodes
  look like multiple nodes.

16
Pointers for further reading

• CAN
• Chord http//pdos.lcs.mit.edu/chord
• Pastry http//www.research.microsoft.com/7Eantr/
  pastry
• Survey papers
• Ratnasamy et al, Routing Algorithms for DHTs
  Some Open Questions, ICSI/CS, UCB
• Hildrum et al, Distributed Object Location in a
  Dynamic Network, Submitted for SPAA02, August
  10-13, 2002, Winnipeg, Manitoba, Canada

17
(No Transcript)