Computer%20Networks%20with%20Internet%20Technology%20William%20Stallings

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Computer Networks with Internet
TechnologyWilliam Stallings

• Chapter 11
• Interior Routing Protocols

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11.1 Internet Routing Principles

• Routing protocols essential to operation of an
  internet
• Routers forward IP datagrams from one router to
  another on path from source to destination
• Router must have idea of topology of internet
• Routing protocols provide this information

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Internet Routing Principles

• Routers receive and forward datagrams
• Make routing decisions based on knowledge of
  topology and conditions on internet
• Decisions based on some least cost criterion
  (chapter 14)

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Fixed Routing

• Single permanent route configured for each
  source-destination pair
• Routes fixed
• May change when topology changes
• Link cost not based on dynamic data
• Based on estimated traffic volumes or capacity of
  link

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Figure 11.1 A Configuration of Routers and
Networks
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Discussion of Example

• 5 networks, 8 routers
• Link cost for output side of each router for each
  network
• Next slide shows how fixed cost routing may be
  implemented
• Each router has routing table

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Routing Table

• One required for each router
• Entry for each network
• Not for each destination
• Routing only needs network portion
• Once datagram reaches router attached to
  destination network, that router can deliver to
  host
• IP address typically has network and host portion
• Each entry shows next node on route
• Not whole route

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Routing Tables in Hosts

• May also exist in hosts
• If attached to single network with single router
  then not needed
• All traffic must go through that router (called
  the gateway)
• If multiple routers attached to network, host
  needs table saying which to use

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Figure 11.2Example Routing Tables
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Adaptive Routing

• As conditions on internet changes, routes may
  change
• Failure
• Can route round problems
• Congestion
• Can route round congestion
• Avoid, or at least not add to further congestion

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Drawbacks of Adaptive Routing

• More complex routing decisions
• Router processing increases
• Depends on information collected in one place but
  used in another
• More information exchanged improves routing
  decisions but increases overhead
• May react too fast causing congestion through
  oscillation
• May react too slow, being irrelevant
• Can produce pathologies
• Fluttering
• Looping

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Fluttering

• Rapid oscillation in routing
• Due to router attempting load balancing or
  splitting
• Splitting traffic among a number of routes
• May result in successive packets bound for same
  destination taking very different routes (see
  next slide)

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Figure 11.3 Example of Fluttering
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Problems with Fluttering

• If in one direction only, route characteristics
  may differ in the two directions
• Including timing and error characteristics
• Confuses management and troubleshooting
  applications that measure these
• Difficulty estimating round trip times
• TCP packets arrive out of order
• Spurious retransmission
• Duplicate acknowledgements

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Looping

• Packet forwarded by router eventually returns to
  that router
• Algorithms designed to prevent looping
• May occur when changes in connectivity not
  propagated fast enough to all other routers

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Adaptive Routing Advantages

• Improve performance as seen by user
• Can aid congestion control
• Benefits depend on soundness of design
• Adaptive routing very complex
• Continual evolution of protocols

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Classification of Adaptive Routing Strategies

• Based on information sources
• Local
• E.g. route each datagram to network with shortest
  queue
• Balance loads on networks
• May not be heading in correct direction
• Include preferred direction
• Rarely used
• Adjacent nodes
• Distance vector algorithms
• All nodes
• Link-state algorithms
• Both need routing protocol to exchange information

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Autonomous Systems (AS)

• Group of routers exchanging information via
  common routing protocol
• Set of routers and networks managed by single
  organization
• Connected
• Except in time of failure

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Interior Routing Protocol (IRP)

• Passes routing information between routers within
  AS
• Does not need to be implemented outside AS
• Allows IRP to be tailored
• May be different algorithms and routing
  information in different connected AS
• Need minimum information from other connected AS
• At least one router in each AS must talk
• Use Exterior Routing Protocol (ERP)

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Exterior Routing Protocol (ERP)

• Pass less information than IRP
• Router in first system determines route to target
  AS
• Routers in target AS then co-operate to deliver
  datagram
• ERP does not deal with details within target AS

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Figure 11.4 Application of Exterior and Interior
Routing Protocols
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Approaches to Routing Distance-vector

• Each node (router or host) exchange information
  with neighboring nodes
• Neighbors are both directly connected to same
  network
• First generation routing algorithm for ARPANET
• Node maintains vector of link costs for each
  directly attached network and distance and
  next-hop vectors for each destination
• Used by Routing Information Protocol (RIP)
• Requires transmission of lots of information by
  each router
• Distance vector to all neighbors
• Contains estimated path cost to all networks in
  configuration
• Changes take long time to propagate

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Approaches to Routing Link-state

• Designed to overcome drawbacks of distance-vector
• When router initialized, it determines link cost
  on each interface
• Advertises set of link costs to all other routers
  in topology
• Not just neighboring routers
• From then on, monitor link costs
• If significant change, router advertises new set
  of link costs
• Each router can construct topology of entire
  configuration
• Can calculate shortest path to each destination
  network
• Router constructs routing table, listing first
  hop to each destination
• Router does not use distributed routing algorithm
• Use any routing algorithm to determine shortest
  paths
• In practice, Dijkstra's algorithm
• Open shortest path first (OSPF) protocol uses
  link-state routing.
• Also second generation routing algorithm for
  ARPANET

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Exterior Router Protocols Path-vector

• Provide information about which networks can be
  reached by a given router and ASs crossed to get
  there
• Does not include distance or cost estimate
• Each block of information lists all ASs visited
  on this route
• Enables router to perform policy routing
• E.g. avoid path to avoid transiting particular AS
• E.g. link speed, capacity, tendency to become
  congested, and overall quality of operation,
  security
• E.g. minimizing number of transit Ass

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11.2 Least Cost Algorithms

• Least-cost criterion
• If minimize number of hops, link value 1
• Link value may be inversely proportional to
  capacity, proportional to current load, or some
  combination
• May differ in different two directions
• E.g. if cost equaled length of queue
• Cost of path between two nodes as sum of costs of
  links traversed
• For each pair of nodes, find least cost path 
• Two common algorithms
• Dijkstra's algorithm
• Bellman-Ford algorithm

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Dijkstra's Algorithm

• Find shortest paths from given node to all other
  nodes, by developing paths in order of increasing
  path length
• Proceeds in stages
• By kth stage, shortest paths to k nodes closest
  to (least cost away from) source have been
  determined
• T Set of nodes so far incorporated
• Stage (k 1), node not in T with shortest path
  from source added to T
• As each node added to T, path from source defined

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Dijkstra's Algorithm Formal (1)

• N set of nodes in the network
• s source node
• T set of nodes so far incorporated
• w(i, j) link cost from node i to node j
• w(i, i) 0
• w(i, j) 8 if nodes not directly connected
• w(i, j) ? 0 if nodes directly connected
• L(n) cost of least-cost path s to n currently
  known
• At termination, cost of least-cost path in graph
  from s to n

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Dijkstra's Algorithm Formal (2)

• Initialization
• T s
• L(n) w(s, n) for n ? s
• Get Next Node
• Find neighboring node not in T with least-cost
  path from s
• Incorporate node into T
• Also incorporate edge incident on that node and
  node in T that contributes to the path. This can
  be expressed as
• Find x Ï T such that
• Add x to T add to T the edge that is incident
  on x and that contributes the least cost
  component to L(x), that is, the last hop in the
  path.

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Dijkstra's Algorithm Formal (3)

• Update Least-Cost Paths
• L(n) minL(n), L(x) w(x, n) for all n Ï T
• If the latter term is the minimum, the path
  from s to n is now the path from s to x
  concatenated with the edge from x to n.
• The algorithm terminates when all nodes have been
  added to T

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Figure 11.6 Dijkstras Algorithm Applied to
Figure 11.1
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Example of Dijkstras Algorithm Applied to Figure
11.1
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Bellman-Ford Algorithm

• Find shortest paths from source node such that
  paths contain at most one link
• Find shortest paths such that paths have at most
  two links
• And so on

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Figure 11.7 Bellman-Ford Algorithm Applied to
Figure 11.1
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Bellman-Ford Algorithm Formal (1)

• s source node
• w(i, j) link cost from node i to node j
• w(i, i) 0
• w(i, j) ? if nodes are directly connected
• w(i, j) ? 0 if nodes directly connected
• h maximum number of links in path at current
  stage
• Lh(n) cost of least-cost path from s to n such
  that no more than h links

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Bellman-Ford Algorithm Formal (2)

• Initialization
• L0(n) ?, for all n ? s
• Lh(s) 0, for all h
• Update
• For each successive h ? 0
• For each n ? s, compute
• Connect n with predecessor node j that achieves
  minimum
• Eliminate any connection of n with different
  predecessor node formed during an earlier
  iteration
• Path from s to n terminates with link from j to n

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Example of Bellman-Ford Algorithm Applied to
Figure 11.1
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Comparison of Algorithms

• Bellman-Ford
• Link cost to all neighboring nodes to node n
  i.e., w(j, n) plus total path cost to those
  neighboring nodes from a particular source node s
  i.e., Lh(j)
• Each node can maintain set of costs and
  associated paths for every other node and
  exchange information with direct neighbors
• Each node can use Bellman-Ford based only on
  information from neighbors and knowledge of its
  link costs
• Dijkstra
• Each node must know link costs of all links
• Information must be exchanged with all other
  nodes
• Both converge under static conditions to same
  solution
• If costs change algorithm will attempt to catch
  up
• If cost depends on traffic
• Depends on routes chosen
• then feedback condition exists
• Instabilities may result

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11.3Distance Vector Routing RIP

• Each node exchange information with neighbors
• Directly connected by same network
• Each node maintains three vectors
• Link cost
• Distance vector
• Next hop vector
• Every 30 seconds, exchange distance vector with
  neighbors
• Use this to update distance and next hop vector

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Figure 11.8 Distance Vector Algorithm Applied to
Figure 11.1
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Distributed Bellman-Ford

• RIP is a distributed version of Bellman-Ford
• Original routing algorithm in ARPANET
• Each simultaneous exchange of vectors between
  routers is equivalent to one iteration of step 2
• In fact, asynchronous exchange used
• At start-up, get vectors from neighbors
• Gives initial routing
• By own timer, update every 30 seconds
• Changes are propagated across network
• Routing converges within finite time
• Proportional to number of routers

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RIP Details Incremental Update

• Updates do not arrive from neighbors within small
  time window
• RIP packets use UDP
• Tables updated after receipt of individual
  distance vector
• Add any new destination network
• Replace existing routes with small delay ones
• If update from router R, update all routes using
  R as next hop

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RIP Details Topology Change

• If no updates received from a router within 180
  seconds, mark route invalid
• Invalid timer 180 sec
• Assumes router crash or network connection
  unstable
• Set distance value to infinity
• Actually 16

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Counting to Infinity Problem (1)

• Slow convergence may cause
• All link costs 1
• B has distance to network 5 as 2, next hop D
• A C have distance 3
• and next hop B

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Counting to Infinity Problem (2)

• Suppose router D fails
• B determines network 5 no longer reachable via D
• Sets distance to 4 based on report from A or C
• At next update, B tells A and C this
• A and C receive this and increment their network
  5 distance to 5
• 4 from B plus 1 to reach B
• B receives distance count 5 and assumes
• network 5 is 6 away
• Repeat until reach infinity (16)
• Takes 8 to 16 minutes to resolve

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Split Horizon

• Counting to infinity problem caused by
  misunderstanding between B and A, and B and C
• Each thinks it can reach network 5 via the other
• Split Horizon rule says do not send information
  about a route back in the direction it came from
• Router sending information is nearer destination
  than you
• That is, A should not tell B the distance to
  network 5.
• Erroneous route now eliminated within time out
  period (180 seconds)

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Poisoned Reverse

• Send updates with hop count of 16 to neighbors
  for route learned from those neighbors
• If two routers have routes pointing at each other
  advertising reverse route with metric 16 breaks
  loop immediately
• B tells A and C distance to network 5 is 16

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Figure 11.9 RIP Packet Format (v1)
Command 1 request, 2 response Address
Family identifier IP, IPX,
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RIP v2
Route Tag 0 or AS
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(No Transcript)
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RIP Packet Format Notes

• Command 1request 2reply
• Updates are replies whether asked for or not
• Initializing node broadcasts request
• Requests are replied to immediately
• Version 1 or 2
• Address family 2 for IP
• IP address non-zero network portion, zero host
  portion
• Identifies particular network
• Metric
• Path distance from this router to network
• Typically 1, so metric is hop count

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RIP Limitations

• Destinations with metric more than 15 are
  unreachable
• If larger metric allowed, convergence becomes
  lengthy
• Simple metric leads to sub-optimal routing tables
• Packets sent over slower links
• Accept RIP updates from any device
• Misconfigured device can disrupt entire
  configuration

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11.4Link-State Protocol OSPF

• RIP limited in large internets
• Open Shortest Path First (OSPF)
• OSPF preferred interior routing protocol for
  TCP/IP based internets
• Link state routing used

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Link State Routing

• When initialized, router determines link cost on
  each interface
• Router advertises these costs to all other
  routers in topology
• Router monitors its costs
• When changes occurs, costs are re-advertised
• Each router constructs topology and calculates
  shortest path to each destination network
• Not distributed version of routing algorithm
• Can use any algorithm
• Dijkstra

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Flooding

• Packet sent by source router to every neighbor
• Incoming packet resent to all outgoing links
  except source link
• Duplicate packets already transmitted are
  discarded
• Prevent incessant retransmission
• All possible routes tried so packet will get
  through if route exists
• Highly robust
• At least one packet follows minimum delay route
• Reach all routers quickly
• All nodes connected to source are visited
• All routers get information to build routing
  table
• High traffic load

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Figure 11.10 Flooding Example
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OSPF Overview

• Router maintains descriptions of state of local
  links
• Transmits updated state information to all
  routers it knows about
• Router receiving update must acknowledge
• Lots of traffic generated
• Each router maintains database
• Directed graph

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Router Database Graph

• Vertices
• Router
• Network
• Transit
• Stub
• Edges
• Connecting two routers
• Connecting router to network
• Built using link state information from other
  routers

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Figure 11.11 Sample Autonomous System
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Figure 11.12 Directed Graph of Autonomous System
of Figure 19.7
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Link Costs

• Cost of each hop in each direction is called
  routing metric
• OSPF provides flexible metric scheme based on
  type of service (TOS)
• Normal (TOS) 0
• Minimize monetary cost (TOS 2)
• Maximize reliability (TOS 4)
• Maximize throughput (TOS 8)
• Minimize delay (TOS 16)
• Each router generates 5 spanning trees (and 5
  routing tables)

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Figure 11.13 The SPF Tree for Router R6
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Areas

• Make large internets more manageable
• Configure as backbone and multiple areas
• Area Collection of contiguous networks and
  hosts plus routers connected to any included
  network
• Backbone contiguous collection of networks not
  contained in any area, their attached routers and
  routers belonging to multiple areas

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Operation of Areas

• Each area runs a separate copy of the link state
  algorithm
• Topological database and graph of just that area
• Link state information broadcast to other routers
  in area
• Reduces traffic
• Intra-area routing relies solely on local link
  state information

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Inter-Area Routing

• Path consists of three legs
• Within source area
• Intra-area
• Through backbone
• Has properties of an area
• Uses link state routing algorithm for inter-area
  routing
• Within destination area
• Intra-area

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Figure 11.14OSPF Packet Header
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Packet Format Notes

• Version number 2 is current
• Type one of 5, see next slide
• Packet length in octets including header
• Router id this packets source, 32 bit
• Area id Area to which source router belongs
• Authentication type null, simple password or
  encryption
• Authentication data used by authentication
  procedure

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OSPF Packet Types

• Hello used in neighbor discovery
• Database description Defines set of link state
  information present in each routers database
• Link state request
• Link state update
• Link state acknowledgement