Computer Networks and Internets, 5e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 18
WAN Technologies
and
Dynamic Routing
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Topics Covered
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18.1 Introduction
18.2 Large Spans and Wide Area Networks
18.3 Traditional WAN Architecture
18.4 Forming a WAN
18.5 Store and Forward Paradigm
18.6 Addressing in a WAN
18.7 Next-Hop Forwarding
18.8 Source Independence
18.9 Dynamic Routing Updates in a WAN
18.10 Default Routes
18.11 Forwarding Table Computation
18.12 Distributed Route Computation
18.13 Shortest Path Computation in a Graph
18.14 Routing Problems
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18.1 Introduction
• This chapter
– considers the structure of a network that spans an arbitrarily large
area
– describes the basic components used to build a packet switching
system
– explains the fundamental concept of routing
– presents the two basic routing algorithms
– explains the advantages of each algorithm
– extends the discussion of routing to the Internet
– presents routing protocols that use the algorithms described here
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18.2 Large Spans and Wide Area
Networks
• Networking technologies can be classified according to the
distance spanned:
–
–
–
–
PAN spans a region near an individual
LAN spans a building or campus
MAN spans a large metropolitan area
WAN spans multiple cities or countries
• Consider a company that uses a satellite bridge to connect
LANs at two sites
– Should the network be classified as a WAN or as an extended LAN?
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18.2 Large Spans and Wide Area
Networks
• The key issue that separates WAN technologies from LAN
technologies is scalability
• A WAN must be able to grow as needed to connect many
sites
– spread across large geographic distances
• A technology is not classified as a WAN unless it can deliver
reasonable performance for a large scale network
– A WAN does not merely connect to many computers at many sites
• it must provide sufficient capacity to permit all computers to communicate
• Thus, a satellite bridge that connects a pair of PCs and
printers is merely an extended LAN
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18.3 Traditional WAN Architecture
• WAN designers chose to create a special-purpose hardware
device that could be placed at each site
• A packet switch provides
– local connections for computers at the site
– as well as connections for data circuits that lead to other sites
• A packet switch consists of a small computer system
– with a processor, memory, and I / O devices used to send and
receive packets
• Early packet switches were constructed from conventional
computers
– the packet switches used in the highest-speed WANs require
special-purpose hardware
• Figure 18.1 illustrates the internal architecture
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18.3 Traditional WAN Architecture
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18.3 Traditional WAN Architecture
• Since the advent of LAN technology, most WANs separate a
packet switch into two parts:
– a Layer 2 switch that connects local computers
– a router that connects to other sites
• Part 4 of the text
– discusses Internet routers in detail
– and explains how the concepts covered here apply to the Internet
• Communication with local computers can be separated from
transmission across a WAN
• Figure 18.2 illustrates the separation
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18.3 Traditional WAN Architecture
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18.4 Forming a WAN
• A WAN can be formed by interconnecting a set of sites
• The exact details of the interconnections depend on
– the data rate needed
– the distance spanned
– and the delay
• Many WANs use leased data circuits
– However, other forms are also available
• such as microwave and satellite channels
• A network designer must choose a topology
– For a given set of sites, many topologies are possible
• Figure 18.3 illustrates a possible way to interconnect
– a WAN does not need to be symmetric in the interconnections
among packet switches
– the capacity of each connection can be chosen to accommodate the
expected traffic and provide redundancy in case of failure
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18.4 Forming a WAN
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18.5 Store and Forward Paradigm
• The goal of a WAN is to allow as many computers as possible to
send packets simultaneously
• The fundamental paradigm used to achieve simultaneous
transmission is known as store and forward
• To perform store and forward processing
– a packet switch buffers packets in memory
• The store operation occurs when a packet arrives:
– I / O hardware in the switch places a copy of the packet in memory
• The forward operation occurs once a packet has arrived and
is waiting in memory. The processor
– examines the packet
– determines its destination
– and sends the packet over the I / O interface that leads to the
destination
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18.6 Addressing in a WAN
• From the view of an attached computer
– a traditional WAN network operates similar to a LAN
– Each WAN technology defines the exact frame format a computer
uses when sending and receiving data
– Each computer connected to a WAN is assigned an address
• WANs addresses follow a key concept that is used in the
Internet: hierarchical addressing
– Hierarchical addressing divides each address into two parts:
(site, computer at the site)
– In practice, instead of a identifying a site, each packet switch is
assigned a unique number
• first part of an address identifies a packet switch
• second part identifies a specific computer
• Figure 18.4 shows two-part hierarchical addresses assigned
to computers connected to a pair of packet switches
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18.6 Addressing in a WAN
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18.6 Addressing in a WAN
• Figure 18.4 shows each address as a pair of decimal integers
– A computer connected to port 6 on packet switch 2 is assigned address
[2, 6]
• In practice, an address is represented as a single binary value
with some bits of the binary value
– used to represent a packet switch
– and others used to identify a computer
• In Part 4 of the text, we’ll show that each Internet address
consists of a binary number, where
– a prefix of the bits identify a specific network in the Internet
– the remainder of the bits identify a computer attached to the network
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18.7 Next-Hop Forwarding
• What is the importance of hierarchical addressing?
• When a packet arrives
– a switch must choose an outgoing path over which to forward it
• If a packet is destined
– for a local computer
• the switch sends the packet directly to the computer
– Otherwise
• the packet must be forwarded over to another switch
• To make the choice, a packet switch
– examines the destination address in the packet
– and extracts the packet switch number
• If the number in the destination address is identical to the packet switch's own
ID the packet is intended for a computer on the local packet switch
• Otherwise, the packet is intended for a computer on another switch
• Algorithm 18.1 explains the computation
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18.7 Next-Hop Forwarding
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18.7 Next-Hop Forwarding
• A packet switch does not need to keep complete information
about how to reach all possible computers
– nor does a switch need to compute the entire route a packet will follow
• Instead, a switch bases forwarding on packet switch IDs
– which means that a switch only needs to know which outgoing link
• A switch only needs to compute the next hop for a packet
• The process is called next-hop forwarding
– and is analogous to the way airlines list flights
• To make the computation efficient
– switches use table lookup
– that is, each packet switch contains a forwarding table
– such tables were originally called routing tables
• lists all possible packet switches and gives a next hop for each
• Figure 18.5 illustrates next-hop forwarding with a trivial example
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18.7 Next-Hop Forwarding
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18.7 Next-Hop Forwarding
• Using only one part of a two-part hierarchical address to
forward a packet has two practical consequences
– First, the computation time required to forward a packet is reduced
because the forwarding table can be organized as an array that uses
indexing instead of searching
– Second, the forwarding table contains one entry per packet switch
instead of one entry per destination computer
• The reduction in table size can be substantial, especially for a large WAN that
has many computers attached to each packet switch
• A two-part hierarchical addressing scheme allows packet
switches to use only the first part of the destination address
until the packet reaches the final switch
– Once the packet reaches the final switch
• the switch uses the second part of the address to choose a specific computer
– As Algorithm 18.1 describes
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18.8 Source Independence
• Next-hop forwarding does not depend on the packet's
original source or on the path the packet has taken before it
arrives at a particular packet switch
– Instead, the next hop to which a packet is sent depends only on the
packet's destination
– The concept is known as source independence
• Source independence allows the forwarding mechanism in a
computer network to be compact and efficient
– Because all packets follow the same path, only one table is required
– Because forwarding does not use source information, only the
destination address needs to be extracted from a packet
– A single mechanism handles forwarding uniformly packets that
originate on directly connected computers and packets that arrive
from other packet switches use the same mechanism
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18.9 Dynamic Routing Updates in a WAN
• Forwarding table must guarantee the following:
• Universal communication
– The forwarding table in each switch must contain a valid next-hop
route for each possible destination address
•
Optimal routes
– In a switch, the next-hop value in the forwarding table for a given
destination must point to the shortest path to the destination
• Network failures further complicate forwarding
– For example, if two paths exist to a given destination and one of the
paths becomes unavailable because hardware fails, forwarding
should be changed to avoid the unavailable path
– A network manager cannot merely configure a forwarding table to
contain static values that do not change
– Instead, software running on the packet switches continually tests for
failures, and reconfigures the forwarding tables automatically
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18.9 Dynamic Routing Updates in a WAN
• We use the term routing software to describe software that
automatically reconfigures forwarding tables
• Route computation in a WAN is to think of a graph that
models the network
– software uses the graph to compute the shortest path to all possible
destinations
• Each node in the graph corresponds to a packet switch in
the network (individual computers are not part of the graph)
• If the network contains a direct connection between a pair of
packet switches
– the graph contains an edge or link between the corresponding nodes
• Figure 18.6 shows an example WAN and the corresponding
graph
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18.9 Dynamic Routing Updates in a WAN
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18.9 Dynamic Routing Updates in a WAN
• As Figure 18.6 shows, nodes in the graph are given a label
that is the same as the number assigned to the
corresponding packet switch
• A graph representation is useful in computing next-hop
forwarding
– because graph theory has been studied and efficient algorithms have
been developed
– a graph abstracts away details, allowing routing software to deal with
the essence of the problem
• When it computes next-hop forwarding for a graph
– a routing algorithm must identify a link
• Our examples will used the notation (k, j) to denote a link
from node k to node j
– When a routing algorithm runs on the graph in Figure 18.6b
– The algorithm produces output as shown in Figure 18.7
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18.9 Dynamic Routing Updates in a WAN
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18.10 Default Routes
• The forwarding table for node 1 in Figure 18.7 raises an
important point:
– a forwarding table may contain many entries that point to the same
next hop
• An examination of the WAN in Figure 18.6a reveals why all
remote entries contain the same next hop:
– the packet switch has only one connection to the network
– therefore, all outgoing traffic must be sent across the same
connection
– consequently, except for the entry that corresponds to the node
itself, all entries in node1's forwarding table have a next hop that
points to the link from node1 to node3
• In our trivial example, the list of duplicate entries in the
forwarding table is short
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18.10 Default Routes
• A large WAN may contains hundreds of duplicate entries
• Most WAN systems include a mechanism that can be used
to eliminate the common case of duplicate entries
• Default route is a mechanism that allows a single entry in a
forwarding table to replace a long list of entries that have the
same next-hop value
– Only one default entry is allowed in a forwarding table
– and the entry has lower priority than other entries
• If the forwarding mechanism does not find an explicit entry
for a given destination
– it uses the default
• Figure 18.8 shows the forwarding tables from Figure 18.7
revised to use a default route
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18.10 Default Routes
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18.10 Default Routes
• Default routing is optional
– a default entry is present only if more than one destination has the
same next-hop value
• For example, the forwarding table for node3 does not
contain a default route
– because each entry has a unique next hop
• However, the forwarding table for node1 benefits from a
default route
– because all remote destinations have the same next hop
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18.11 Forwarding Table Computation
• Basic approaches to construct forwarding tables?
• Static routing
– A program computes and installs routes when a packet switch boots
– The routes do not change
•
Dynamic routing
– A program builds an initial forwarding table when a switch boots
– Program then alters the table as conditions in the network change
• Each approach has advantages and disadvantages
– Advantages of static routing are simplicity and low overhead
– Disadvantage is inflexibility
• static routes cannot be changed when communication is disrupted
• Large networks are designed with redundant connections to
handle occasional hardware failures
– most WANs use a form of dynamic routing
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18.12 Distributed Route Computation
• Algorithm 18.2 shows how a forwarding table can be
computed
– after information about a network is encoded in a graph
• In practice, WANs need to perform distributed route
computation
– Instead of a centralized program computing all shortest paths
• each packet switch must compute its own forwarding table locally
– All packet switches must participate in distributed route computation
• There are two general forms:
– Link-State Routing (LSR), which uses Dijkstra's algorithm
– Distance-Vector Routing (DVR), which uses another approach
•
The next sections describe each of the two approaches
– Chapter 27 explains how each approach is used to control routes in
the Internet
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18.12 Distributed Route Computation
•
18.12.1 Link-State Routing (LSR)
Link-state routing or Link-status routing
– the approach also known as Shortest Path First (SPF) routing
– Dijkstra algorithm used it to characterize the way it works
• actually all routing algorithms find shortest paths
• To use LSR, packet switches periodically send messages
across the network that carry the status of a link
– For example, packet switches 5 and 9 measure the link between
them and send a status message
• such as “the link between 5 and 9 is up”
– Each status message is broadcast to all switches
• Every switch collects incoming status messages
– and uses them to build a graph of the network
• Each switch then uses Algorithm 18.2 to produce a
forwarding table by choosing itself as the source
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Algorithm 18.2
A version of Dijkstra’s
algorithm that computes R,
a nexthop forwarding table,
and D, the distance to each
node from the specified
source node
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18.12 Distributed Route Computation
18.12.1 Link-State Routing (LSR)
• An LSR algorithm can adapt to hardware failures
• If a link between packet switches fails
– the attached packet switches will detect the failure
– and broadcast a status message that specifies the link is down
• All packet switches receive the broadcast
– change their copy of the graph to reflect the change in the link's
status
– and re-compute shortest paths
• Similarly, when a link becomes available again
– the packet switches connected to the link detect that it is working
– and start sending status messages that report its availability
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18.12 Distributed Route Computation
18.12.2 Distance Vector Routing (DVR)
• As with LSR, each link in the network is assigned a weight
• The distance to a destination between two packet switches
is defined to be the sum of weights along the path between
the two
• Like LSR, DVR arranges for packet switches to exchange
messages periodically
• In DVR, a switch sends a complete list of destinations and
the current cost of reaching each
• When it sends a DVR message
– a switch is sending a series of individual statements, of the form:
“I can reach destination X, and its current distance from me is Y”
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18.12 Distributed Route Computation
18.12.2 Distance Vector Routing (DVR)
• DVR messages are not broadcast
– Each switch periodically sends a DVR message to its neighbors
• Each message contains pairs of (destination, distance)
• Each packet switch must keep a list of possible destinations
– along with the current distance to the destination and the next hop to
use
– the list of destinations and the next hop for each can be found in the
forwarding table
• DVR software can be considered as maintaining an
extension to the forwarding table
– that stores a distance for each destination
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18.12 Distributed Route Computation
18.12.2 Distance Vector Routing (DVR)
• When a message arrives at a switch from neighbor N
– the switch examines each item in the message
– and changes its forwarding table if the neighbor has a shorter path to
some destination than the path currently being used
• Example:
– if neighbor N advertises a path to destination D as having cost 5 and
the current path through neighbor K has cost 100
• the current next hop for D will be replaced by N
• and the cost to reach D will be 5 plus the cost to reach N
• Algorithm 18.3 specifies how routes are updated when using
the distance-vector approach
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Algorithm 18.3
Distance-vector algorithm
for route computation
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18.13 Shortest Path Computation in a
Graph
• Once a graph has been created that corresponds to a
network
– software uses a method known as Dijkstra's Algorithm
• To find the shortest path from a source node to each of the
other nodes in the graph:
– a next-hop forwarding table is constructed during the computation of
shortest paths
– The algorithm must be run once for each node in the graph
– That is, to compute the forwarding table for packet switch P
• the node that corresponds to P is designated as the source node
• and the algorithm is run
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18.13 Shortest Path Computation in a
Graph
•
Dijkstra's algorithm is popular
– because it can be used with various definitions of shortest path
– In particular, the algorithm does not require edges in the graph to
represent geographic distance. Instead, the algorithm
• allows each edge to be assigned a nonnegative value called a weight
• defines the distance between two nodes to be the sum of the weights along
a path between the nodes
• Figure 18.9 illustrates the concept of weights
– by showing an example graph with an integer weight assigned to
each edge
– and a least-weight path between two nodes in the graph
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18.13 Shortest Path Computation in a
Graph
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18.13 Shortest Path Computation in a
Graph
• Dijkstra's algorithm maintains a set of nodes, S
– for which the minimum distance and next hop have not been computed
• The set is initialized to all nodes except the source
• The algorithm then iterates until set S is empty
• On each iteration
– the algorithm removes a node from S that has the least distance from
the source
– As it deletes node u, the algorithm examines the current distance from
the source to each of the neighbors of u that remains in the set
– If a path from the source through u to the neighbor has less weight than
the current path, the algorithm updates the distance to the neighbor
• After all nodes have been removed from S
– the algorithm will have computed the minimum distance to each node
and a correct next-hop forwarding table for all possible paths
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18.13 Shortest Path Computation in a Graph
• Dijkstra's algorithm needs 3 data structures to store:
– the current distance to each node
– the next hop for the shortest path
– and information about the remaining set of nodes
• Nodes can be numbered from 1 to n as Figure 18.9
– which makes the implementation efficient
• because a node number can be used as an index into a data structure
• The algorithm can use two arrays, D and R
– each indexed by the node number
– The ith entry in array D stores a current value of the minimum
distance from the source to node i
– The ith entry in array R stores the next hop used to reach node i
along the path being computed
– The set S can be maintained as a doubly linked list of node numbers
• which facilitates searching the entire set or deleting an entry
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18.13 Shortest Path Computation in a Graph
• Algorithm 18.2 uses weight (i, j) as a function
– that returns the weight of the edge from node i to node j
• Function weight is assumed to return a reserved value
infinity if no edge exists from node i to node j
– In practice, any value can be used to represent infinity provided the
value is larger than the sum of weights along any path in the graph
– One way to generate a value infinity consists of adding one to the
sum of all weights on all edges
• Allowing arbitrary weights to be assigned to edges of a
graph means that
– one algorithm can be used with different measures of distance
– For example, some WAN technologies measure distance by
counting the number of packet switches along a path
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18.13 Shortest Path Computation in a Graph
• To use the algorithm for such technologies
– each edge in the graph is assigned a weight of 1
• In other WAN technologies, weights are assigned to reflect
the capacity of the underlying connections
– A network manager can assign weights to links to control routing
– For example, consider a case where two separate paths exist
between a pair of packet switches with
• one path designated to be the primary path
• and the other designated to be a backup path
• To enforce such a policy
– a network manager can assign the primary link a low weight and the
other link a high weight
– Routing software will configure forwarding tables to use the path with
low weight
• unless the path is not available
• in which case routing software will select the alternative path
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18.14 Routing Problems
• In theory, either LSR or DVR routing will compute shortest
paths
• Furthermore, each approach will eventually converge
– meaning that the forwarding tables in all packet switches agree
• However, problems do occur
– For example, if LSR messages are lost, two packet switches can
disagree about the shortest path
• DVR problems can be more severe
– because a link failure can cause two or more packet switches to
create a routing loop
• in which each packet switch thinks the next packet switch in the set is the
shortest path to a particular destination
• As a result, a packet can circulate among the switches indefinitely
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18.14 Routing Problems
• One of the primary reasons DVR protocols exhibit problems comes from
backwash
– (i.e., a packet switch receives information that it sent)
• For example, suppose a switch tells its neighbors
“I can reach destination D at cost 3”
– If the connection leading to destination D fails
• the switch will remove the entry for D from its forwarding table
(or mark the entry invalid)
– But the switch has told neighbors that a route exists
• Imagine that just after the link fails
– one of the neighbors sends a DVR message that specifies
“I can reach destination D at cost 4”
• Unfortunately
– the message will be believed
– and a routing loop will be created
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18.14 Routing Problems
• Most practical routing mechanisms contain constraints and
heuristics to prevent problems like routing loops
• For example, DVR schemes employ split horizon
– which specifies that a switch does not send information back to its
origin
• Furthermore, most practical routing systems introduce
hysteresis
– that prevents the software from making many changes in a short
time
• However, in a large network where many links fail and
recover frequently, routing problems can occur
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