Module 5.0: Internetworking

1
Module 5.0 Internetworking Network Layer

• Basic concepts
• Congestion Control
• Routing Protocols
• Flooding
• Source routing
• Distance vector routing
• Link state routing
• Routing for VC
• Routers vs. Switches

Chapters 19,20,21,22
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Concepts

• Internetworking refers to a collection of
  interconnected networks that function as a singe
  network.
• Internetwork is abbreviated as internet. However,
  Internet refers to the worlds largest
  internetwork.
• Network layer performs
• Provides services to Layer 4 (Transport)
  including fragmentation and re-assembly
• Performs congestion control
• End-to-end routing

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Congestion Control

• Congestion occurs when the number of packets
  being transmitted through the network approaches
  the packet handling capacity of the network
• Congestion control aims to keep number of packets
  below level at which performance falls off
  dramatically
• Data network is a network of queues
• Generally 80 utilization is critical
• Finite queues mean data may be lost
• Packets arriving are stored at input buffers
• Routing decision made
• Packet moves to output buffer
• Packets queued for output transmitted as fast as
  possible
• Statistical time division multiplexing
• If packets arrive too fast to be routed, or to be
  output, buffers will fill. And packets will be
  discarded

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Congestion Control

• When packets are discarded, the sources must
  retransmit these packets, in addition to a new
  packets. This will increase the load on the
  network and more buffers become saturated. Even
  if a packet makes it through, by that time, the
  upper layer times out. Under these
  circumstances, the throughput becomes zero.
• Not the same as flow control. Flow control
  applies only to point-to-point traffic. Flow
  control affects congestion. Congestion has a
  global scope.
• Mechanisms of congestion control
• Backpressue
• Send choke packet hop-by-hop
• Choke packet
• Send choke packet to source
• Implicit congestion signaling
• Source slows down if packets are discarded or
  delayed
• Explicit congestion signaling
• Binary based A bit set in a packet indicates
  congestion
• Credit based Indicates how many packets source
  may send
• Rate based Supply explicit data rate limit, e.g.
  ATM.

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Routing Protocols

• Routing protocols perform two primary functions
• Determine the best path
• Maintain a routing table
• Routing algorithms are used to calculate the
  least-cost path from source to destination.
• Common cost metrics
• Hops (the number of routers in the path)
• Propagation delay
• Bandwidth
• Time
• Channel utilization
• Two general algorithms
• Distance Vector Routing
• Link State Routing
• Other routing protocols
• Flooding
• Source Routing

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Flooding

• The principle says a router forward an incoming
  packet to all ports except the one the packet
  came through.
• Effective method
• At startup to build routing table
• Survivability (military networks).
• Can drag down the network
• TTL
• Each switch adds its id to packet before it
  floods it

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Source Routing

• Does not require intermediate node routing
• Sender must specify the entire route
• Sender uses router discovery at initialization.
  Intermediate nodes use flooding.
• At intermediate nodes, the header is examined,
  strips off the label identifying the node, and
  forward to the next node.

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Distance Vector Routing

• Each router periodically shares its knowledge
  about the entire network with its neighbors.
• Knowledge about the whole network Each router
  sends its accumulated knowledge about the entire
  network to its neighbors. This knowledge is
  sparse at the beginning.
• Routing only to neighbors Each router sends
  periodically knowledge to those routers that has
  directed links. Neighboring routers use this
  information to update their own knowledge.
• Information sharing at regular intervals Every
  30 seconds, Each router sends its knowledge to
  neighboring routers, regardless of any changes.
• Distance vector algorithm adapts to changes in
  network topology gradually as the information on
  the changes percolates through the network.

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Example
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Routing Table
Original routing tables. Next hop field is empty
initially.
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Updating routing table for router A
Final routing tables
12

• An Example of updating routing table
• Distance vector is based on Bellman-Ford
  algorithm to find the next hop that yields the
  shortest distance to destination.

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Link State Routing

• Each router shares its knowledge of its
  neighborhood with all routers in the
  internetwork.
• Knowledge about the neighborhood Router sends
  information about the neighborhood only, not the
  entire table.
• To all routers Such information propagates from
  router to router, using flooding. Eventually
  every router receives a copy of the same
  information.
• Information sharing when there is a change Each
  router sends out information about the neighbors
  when there is a change.
• In link state routing, eventually each router is
  able to construct a map of the entire network and
  from this map the best route is found.

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Link state packet (LSP)

• Cost is an outbound cost and set by the router.
• A router gets information about its neighbors by
  periodically sending them a short greeting
  message, thus determining the cost and Neighbor
  for the LSP.

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Link state database
Flooding of As LSP

• Because every router receives the same LSPs,
  every router builds the same database.
• This database is used to calculate the routing
  table, by applying Dijkstra algorithm.

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The Dijkastra Shortest Path First (SPF) Algorithm

• The algorithm uses the closest nodes concept
  and is based on the following principle
• Give a source node, n, the shortest path from
  n to the next closest node, s, either (a) is a
  path that directly connects n to s or (b)
  includes a path containing n and any of the
  previously found intermediate closest nodes plus
  a direct link from the last intermediate closest
  node of this path to s.

Routing table for router A
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Routing for VC

• Virtual Circuit network with 3 VCs
• A to B with VCI 1
• A to D with VCI 5
• C to B with VCI 6
• Using local VCI over global VCI
• Searching for available VCI is not simple

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• Routing tables for the previous network at
  intermediate nodes
• Used in ATM networks. ATM uses link-state
  routing to find the best route to construct its
  routing table.

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Most Popular Routing Protocols

• OSPF (Open Shortest Path First)
• Link-state, interior, 50 routers per area, about
  100 areas,
• RIP v1 (Routing Information Protocol, version 1)
• Distance-vector, interior, 15 hops
• RIP v2 (Routing Information Protocol, version 2)
• Distance-vector, interior, 15 hops
• BGP (Border Gateway Protocol)
• Path-vector, exterior, thousands of routers
• EIGRP (Enhanced Interior Gateway Routing
  Protocol)
• Advanced distance-vector, interior, thousands of
  routers
• IGRP (Interior Gateway Routing Protocol)
• Distance-vector, interior, 255 hops
• IS-ISx (Intermediate System to Intermediate
  System)
• Link-state, interior, thousands of routers
• Defined by IETF
• Defined by Cisco

20
Routers vs. Switches

• The primary difference is one semantics.
  Switches historically infer CO links routers use
  CL links. Traditionally, routers have performed
  router table lookups and packet forwarding in
  software.
• Layer-2 Switches start having routing
  functionality, and Layer-3 routers start having
  ASIC (Application Specific Integrated Circuit)
  switching technology for packets.
• Layers 2 and 3 are merging and it is becoming
  difficult to distinguish between switches and
  routers.
• Layer 3 or IP switching routing IP packets in
  ASIC, e.g, MPLS.
• Layer 4/7 switching is a new and emerging area,
  called information content switching.
• Layer 4 direct all traffic based on TCP
  destination port.
• All traffic with destination TCP port 80, is
  directed to a switch port where a web cache
  resides.
• Layer 7 direct traffic based on information used
  in the payload.
• Examine URL GET request. If request for image,
  direct it request to an optimized image server
  port.