The Internet: Packet Switching and Other Big Ideas

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The InternetPacket Switching and Other Big Ideas

• Ian Foster

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RecallThe Internet
1969 4 nodes
2004 100s of millions
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RecallBig Ideas Underlying the Internet

• Packet switching
• Flexible, robust, efficient (in the network)
• Enabled by smart terminals
• End-to-end arguments in system design
• E.g., reliable in-order delivery via TCP
• Rough consensus and running code
• As a means of creating and evolving a complex
  artifact

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(No Transcript)
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Or?

• Metcalfes Law
• Utility of a network of N entities scales as
  number of potential connections, i.e., as N2
• Reeds Law
• Utility scales as the number of potential
  subgroups, i.e., as 2N

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Or?

• The Internet isn't complicated
• The Internet isn't a thing. It's an agreement.
• The Internet is stupid.
• Adding value to the Internet lowers its value.
• All the Internet's value grows on its edges.
• The Internets three virtues   a. No one owns
  it   b. Everyone can use it   c. Anyone can
  improve it

http//www.worldofends.com/
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Or? (Bradners 10 Critical Choices)

• 1) Make it all work on top of existing networks
• 2) Use packets, not circuits.
• 3) Create a (decentralized) routing'' function
• 4) Split Transmission Control Protocol (TCP)
  Internet Protocol (IP)
• 5) ARPA funds UC Berkeley to put TCP/IP into Unix
• 6) CSNET, an early network used by universities,
  connects with the ARPANET
• 7) NSF requires users of the NSFNET to use TCP/IP
• 8) International telecommunications standards
  bodies reject TCP/IP, then create a separate
  standard called OSI
• 9) NSF creates an Acceptable Use Policy''
  restricting NSFNET use to noncommercial
  activities
• 10) Once things start to build, government stays
  mostly out of the way

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Or? (Tim OReilly)

• I really believe we really are moving to a very,
  very different computing paradigm where
  applications actually live on the network. I
  mean, where exactly does Google live? It lives
  obviously on Google's bank of servers, but it
  also lives in a PC-based application.

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DatagramDiagram
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Scalable Infrastructure

• Scaling the Internet reliably and efficiently to
  hundreds of millions of nodes
• Routing
• Naming
• Email
• Search
• Concepts
• Hierarchy (abstraction)
• Soft state
• Big central systems! (e.g., Google)

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Routing in Packet Switched Network

• Complex, crucial aspect of packet-switched
  networks
• Characteristics required
• Correctness
• Simplicity
• Robustness
• Stability
• Fairness
• Optimality
• Efficiency

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Routing

• End systems and routers maintain routing tables,
  which indicate next router to which datagram
  should be sent
• Static
• May contain alternative routes
• Dynamic
• Flexible response to congestion and errors

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Network Information Source and Update Timing

• Routing decisions usually based on knowledge of
  network (not always)
• Distributed routing
• Nodes use local knowledge
• May collect info from adjacent nodes
• May collect info from all nodes on a potential
  route
• Central routing
• Collect info from all nodes
• Update timing
• When is network info held by nodes updated
• Fixed - never updated
• Adaptive - regular updates

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Routing Strategies

• Fixed
• Flooding
• Random
• Adaptive

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Fixed Routing

• Single permanent route for each source to
  destination pair
• Determine routes using a least cost algorithm
• Route fixed, at least until a change in network
  topology

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Sample AS
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Directed Graph of AS
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Performance Criteria

• Used for selection of route
• Minimum hop
• Least cost

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Fixed RoutingTables
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Flooding

• No network info required
• Packet sent by node to every neighbor
• Incoming packets retransmitted on every link
  except incoming link
• Eventually a number of copies will arrive at
  destination
• Each packet is uniquely numbered so duplicates
  can be discarded
• Nodes can remember packets already forwarded to
  keep network load in bounds
• Can include a hop count in packets

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Flooding Example
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Properties of Flooding

• All possible routes are tried
• Very robust
• At least one packet will have taken minimum hop
  count route
• Can be used to set up virtual circuit
• All nodes are visited
• Useful to distribute information (e.g. routing)

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AdaptiveUnstructured Multicast
Application overlay
D
E
C
A
B
D
Base overlay
E
C
A
B
D
Physical topology
E
C
A
B
UMM A dynamically adaptive, unstructured
multicast overlay M. Ripeanu et al.
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Random Routing

• Node selects one outgoing path for retransmission
  of incoming packet
• Selection can be random or round robin
• Can select outgoing path based on probability
  calculation
• No network info needed
• Route is typically not least cost nor minimum hop

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Adaptive Routing

• Used by almost all packet switching networks
• Routing decisions change as conditions on the
  network change
• Failure
• Congestion
• Requires info about network
• Decisions more complex
• Tradeoff between quality of network info and
  overhead
• Reacting too quickly can cause oscillation
• Too slowly may not be relevant

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Adaptive Routing - Advantages

• Improved performance
• Aid congestion control
• Complex system
• May not realize theoretical benefits

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Classification

• Based on information sources
• Local (isolated)
• Route to outgoing link with shortest queue
• Can include bias for each destination
• Rarely used - do not make use of easily available
  info
• Adjacent nodes
• All nodes

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Isolated Adaptive Routing
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ARPANET Routing StrategiesFirst Generation
(1969)

• Distributed adaptive
• Estimated delay as performance criterion
• Bellman-Ford algorithm
• Node exchanges delay vector with neighbors
• Update routing table based on incoming info
• Doesn't consider line speed, just queue length
• Queue length not a good measurement of delay
• Responds slowly to congestion

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ARPANET Routing Strategies2nd Generation (1979)

• Uses delay as performance criterion
• Delay measured directly
• Uses Dijkstras algorithm
• Good under light and medium loads
• Under heavy loads, little correlation between
  reported delays and those experienced

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ARPANET Routing Strategies3rd Generation (1987)

• Link cost calculations changed
• Measure average delay over last 10 seconds
• Normalize based on current value and previous
  results

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Least Cost Algorithms

• Basis for routing decisions
• Can minimize hop with each link cost 1
• Can have link value inversely proportional to
  capacity
• Given network of nodes connected by
  bi-directional links
• Each link has a cost in each direction
• Define cost of path between two nodes as sum of
  costs of links traversed
• For each pair of nodes, find a path with the
  least cost
• Link costs in different directions may be
  different
• E.g. length of packet queue

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Dijkstras Algorithm Definitions

• Find shortest paths from given source node to all
  other nodes, by developing paths in order of
  increasing path length
• N set of nodes in the network
• s source node
• T set of nodes so far incorporated by the
  algorithm
• w(i, j) link cost from node i to node j
• w(i, i) 0
• w(i, j) ? if the two nodes are not directly
  connected
• w(i, j) ? 0 if the two nodes are directly
  connected
• L(n) cost of least-cost path from node s to
  node n currently known
• At termination, L(n) is cost of least-cost path
  from s to n

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Dijkstras Algorithm Method

• Step 1 Initialization
• T s Set of nodes so far incorporated consists
  of only source node
• L(n) w(s, n) for n ? s
• Initial path costs to neighboring nodes are
  simply link costs
• Step 2 Get Next Node
• Find neighboring node not in T with least-cost
  path from s
• Incorporate node into T
• Also incorporate the edge that is incident on
  that node and a node in T that contributes to the
  path
• Step 3 Update Least-Cost Paths
• L(n) minL(n), L(x) w(x, n) for all n Ï T
• If latter term is minimum, path from s to n is
  path from s to x concatenated with edge from x to
  n
• Algorithm terminates when all nodes have been
  added to T

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Dijkstras Algorithm Notes

• At termination, value L(x) associated with each
  node x is cost (length) of least-cost path from s
  to x.
• In addition, T defines least-cost path from s to
  each other node
• One iteration of steps 2 and 3 adds one new node
  to T
• Defines least cost path from s to that node

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Example of Dijkstras Algorithm
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IRP and ERP
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Autonomous Systems (AS)

• Group of routers
• Exchange information
• Common routing protocol
• Set of routers and networks managed by single
  organization
• A connected network
• There is at least one route between any pair of
  nodes

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Interior Router Protocol (IRP)

• Passes routing information between routers within
  AS
• May be more than one AS in internet
• Routing algorithms and tables may differ between
  different AS
• Routers need some info about networks outside
  their AS
• Uses exterior router protocol (ERP)
• IRP needs detailed model
• ERP supports summary information on reachability

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Border Gateway Protocol

• Internet BGP routing tables number more than
  90,000 routes
• BGP uses many route parameters, called
  attributes, to define routing policies and
  maintain a stable routing environment

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BGP Routing Information Exchange

• Within AS, router builds topology picture using
  IGP
• Router issues Update message to other routers
  outside AS using BGP
• These routers exchange info with other routers in
  other AS
• Routers must then decide best routes

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Classless Interdomain Routing Used by BGP to
reduce table sizes

• E.g., an ISP owns the IP address block 195.10.x.x
  from the Class C address space
• This block consists of 256 Class C address
  blocks, 195.10.0.x through 195.10.255.x.
• ISP assigns Class C block to each customer
• Without CIDR, the ISP would advertise 256 Class C
  address blocks to its BGP peers
• With CIDR, BGP can supernet the address space and
  advertise one block, 195.10.x.x.
• This block is the same size as a traditional
  Class B address block (thus class distinctions
  are rendered obsolete)

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Subnets and Subnet Masks

• Allow arbitrary complexity of internetworked LANs
  within organization
• Insulate overall internet from growth of network
  numbers and routing complexity
• Site looks to rest of internet like single
  network
• Each LAN assigned subnet number
• Host portion of address partitioned into subnet
  number and host number
• Local routers route within subnetted network
• Subnet mask indicates which bits are subnet
  number and which are host number

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Routing Using Subnets
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Summary Routing

• Further reading William Stallings,Data and
  Computer Communications
• Routing Information Protocol http//www.faqs.org/
  rfcs/rfc1058.html