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Network Layer (Part 7)
Computer Networks
Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & Informatics
Institut Teknologi Bandung
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UNICAST ROUTING PROTOCOLS(RIP, OSPF, AND BGP)
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INTRODUCTION
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Cost or Metric
A router is usually attached to several networkswhen it receives a packet, to which network
should it pass the packet?
The decision is based on optimization: Which ofthe available pathways is the optimum pathway?
What is the definition of the term optimum?
One approach is to assign a cost for passing
through a networkWe call this cost a metric High cost can be thought of as something bad;
low cost can be thought of something good
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Static versus Dynamic Routing Tables
A static table is one with manual entries
A dynamic table, on the other hand, is one
that is updated automatically when there is
a change somewhere in the internet
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Routing Protocol
A routing protocol is a combination of rules and
procedures that lets routers in the internet inform
each other of changes
It allows routers to share whatever they know
about the internet or their neighborhood The routing protocols also include procedures
for combining information received from other
routers
Routing protocols can be either an interior
protocol or an exterior protocol
An interior protocol handles intradomain routing
an exterior protocol handles interdomain routing 6
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INTRA- AND INTER-DOMAINROUTING
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a group of networks and routers under the authority of a single administration
intra-domain routing
inter-domain routing
AS Numbers is
assigned for each AS
Ex: ITBs ASN is
4796
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DISTANCE VECTOR ROUTING
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This method sees an AS, with all routersand networks, as a graph, a set of nodes
and lines (edges) connecting the nodes
A router normally be represented by a nodeA network be represented by a link
connecting two nodes
The graph theory used Bellman-Ford (alsocalled Ford-Fulkerson) algorithm to find
the shortest path between nodes in a
graph given the distance between nodes11
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Bellman-Ford Algorithm
It looks circular
To solve the problem, we use iteration to create a shortest distance table(vector) for each node using the following steps:
1. The shortest distance and the cost between a node and itself is initialized to 0.
2. The shortest distance between a node and any other node is set to infinity. The cost between a
node and any other node should be given (can be infinity if the nodes are not connected)
3. The algorithm repeat as shown in Figure 11.4 until there is no more change in the shortest
distance vector.
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Distance Vector Routing Algorithm
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Count to Infinity
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In distance vector routing, any decrease in cost(good news) propagates quickly, but any
increase in cost (bad news) propagates slowly
For a routing protocol to work properly, if a link is
broken (cost becomes infinity), every other
router should be aware of it immediately, but in
distance vector routing, this takes some time.
The problem is referred to as count to infinity takes several updates before the cost for a
broken link is recorded as infinity by all routers.
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Example of count to infinity
Two-Node Loop
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Another example24
A converged network
Source
http://technet.microsoft.com/en-s/library/cc940478.aspx
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The solutions
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Defining Infinity
The first obvioussolution to count to
infinity is to redefine
infinity to a smaller
number
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Most implementations of the Distance VectorProtocol define 16as infinity
However, this means that distance vector cannot be
used in large systems The size of the network, in
each direction, can not exceed 15 hops
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Split Horizon
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Split horizon helps reduce convergence time by not
allowing routers to advertise networks in the
direction from which those networks were learned
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Source
http://technet.microsoft.com/en-s/library/cc940478.aspx
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Split Horizon and Poison Reverse
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Iit announces all networks. However, those networks learned in a
given direction are announced with a hop count of 16, indicating
that the network is unreachable It avoids the Distance Vector
Protocol deleting the route because it has no news about it during a
certain time duration (timer)
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Three-Node Instability
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RIPRouting Information Protocol
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RIP implements distance vector routing directly with someconsiderations
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RIP Message Format
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Requests and Responses
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Request
A request message is sent by a router thathas just come up or by a router that has
some time-out entries.
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Response
A response can be : Solicited: sent only in answer to a request.
It contains information about the destination
specified in the corresponding request Unsolicited: is sent periodically
every 30 seconds or
when there is a change in the routing table
The response is sometimes called an
update packet
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Timers in RIP
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controls the
advertising of
regular update
messages
Governs the validity of a
route. If the timer is
reached, route is
declared unreachable,
but does not immediatelypurge, instead, it
continues to advertise
the route with a metric
value of 16.
At the same time when a
route declared unreachable,
the garbage collection timer
is set to 120 s for that route.
When the count reacheszero, the route is purged
from the table.
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RIP Version 2
It supports
Classless Addressing
Authentication
Multicastinguses the all-router multicast
address to send the RIP messages only to
RIP routers in the network.42
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Encapsulation
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LINK STATE ROUTING
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Each node in the domain
has the entire topology
of the domainuse
Dijkstra algo r i thmto
build a routing table
The routing table for
each node is unique
because the calculations
are based on different
interpretations of the
topology
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The whole topology can be compiled from the partial knowledge of each
node (it knows the state (type, condition, and cost) of its links)
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BUILDING ROUTING TABLES
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Creation of Link State Packet (LSP)
A link state packet(LSP) carries (among othershuge information)
1. The node identity
2. The list of links
3. A sequence number4. Age
The first two are needed to make the topology
The third facilitates flooding and distinguishesnew LSPs from old ones
The fourth prevents old LSPs from remaining in
the domain for a long time49
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Flooding of LSPs
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Formation of Shortest Path Tree:
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Formation of Shortest Path Tree:Dijkstra Algorithm
After receiving all LSPs, each node willhave a copy of the whole topologynot
sufficient to find the shortest pathto every
other nodea shortest path treeisneeded
A shortest path treeis a tree in which the
path between the root and every othernode is the shortest
What we need for each node is a shortest
path tree with that node as the root52
Dijk l i h
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Dijkstra algorithm
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Calculation of Routing Table from
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g
Shortest Path Tree
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OSPFOpen Shortest Path First
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Area
OSPF divides an autonomous system into
areas
An area is a collection of networks, hosts,
and routers all contained within anautonomous system
All networks inside an area must be
connected
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Routers inside an area flood the area with
routing information
At the border of an area, special routers
called area border routerssummarize theinformation about the area and send it to
other areas
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All of the areas inside an autonomoussystem must be connected to a special
area called the backbone area
The backboneserves as aprimary areaand
the other areas as secondary areas
This does not mean that the routers
within areas cannot be connected to
each other
The routers inside the backbone are
called the backbone routers
backbone router can also be an
area border router
Each area has an area identification
The area identification of the
backboneis zero
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If, because of some problem, the
connectivity between a backbone and an
area is broken, a virtual linkbetween routers
must be created by the administration toallow continuity of the functions of the
backbone as the primary area
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M t i
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Metric
The OSPF protocol allows the
administrator to assign a cost, called the
metric, to each route
The metric can be based on a type ofservice (minimum delay, maximum
throughput, and so on)
As a matter of fact, a router can havemultiple routing tables, each based on a
different type of service64
T pes of Links
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Types of Links
In OSPF terminology, a connection is called a link
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Point to Point Link
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Point-to-Point Link
There is no need to assign a network address to
this type of link
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Transient Link
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Transient Link A transient link is a network with several routers
attached to it
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It is not efficienteach router needs
to advertise the neighborhood to four
other routers
It is not realisticthere is no single
network (link) between each pair of
routers (there is only one network
(not router)that serves as a crossroad
between all five routers)
To show that each router is connected to
every other router through one single
networkthe network itself is representedby a node
A network is not a machineit cannot
function as a router
So, one of the routers in the network takes
this responsibilityIt is assigned a dual
purposeit is a true routerand a
designated router
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While there is a metric from each node to the designated
router, there is no metric from the designated router to
any other nodeWe can only assign a cost to a packet
that is passing through the network (We cannot charge
for this twice)
When a packet enters a network, we assign a cost; when a
packet leaves the network to go to the router, there is no charge
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Stub Link
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Stub Link
A special case of the transient network
The link is only onedirectional, from therouter to the network
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Virtual Link
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Virtual Link
When the link between two routers is
broken, the administration may create a
virtual link between them using a longer path
that probably goes through several routers
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Graphical Representation
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Graphical Representation
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OSPF Packets
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OSPF Packets
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Common Header
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Common Header
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Network Link LSA
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Network Link LSA
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Summary Link to Network LSA
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Summary Link to Network LSA
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The summary link to network LSA is used by the area border router to
announce the existence of other networks outside the area
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Summary Link to AS Boundary Router LSA
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Summary Link to AS Boundary Router LSA
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External Link LSA
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External Link LSA
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Other Packets
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Other Packets
They are not used as LSAs, but areessential to the operation of OSPF
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Hello Message
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Hello Message
OSPF uses the hello message to create
neighborhood relationships and to test the
reachability of neighbors
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Database Description Message
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Database Description Message
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Link State Request Packet
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Link State Request Packet
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Encapsulation
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Encapsulation
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PATH VECTOR ROUTING
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Path vector routing is exterior routingprotocol proved to be useful for interdomain
or inter-AS routing
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Reachability
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Reachability
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Routing Tables
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Routing Tables
A path vector routing table for each router can
be created if ASs share their reachability list with
each other97
Loop Prevention
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Loop Prevention
The instability of distance vector routing andthe creation of loops can be avoided in path
vector routing. When a router receives a
reachability information, it checks to see if itsautonomous system is in the path list to any
destination. If it is, looping is involved and
that network-path pair is discarded.
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Aggregation
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Aggregation
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Policy Routing
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y g
When a router receives a message, it cancheck the path. If one of the autonomous
systems listed in the path is against its
policy, it can ignore that path and thatdestination. It does not update its routing
table with this path, and it does not send this
message to its neighbors
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BGPBorder Gateway ProtocolAn interdomain routing protocol
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Types of Autonomous Systems
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yp y
Stub AS
Multihomed AS
Transit AS
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Stub AS
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A stub AS has only one connection toanother AS
The hosts in the AS can send data traffic
to other Ass The hosts in the AS can receive data
coming from hosts in other Ass
Data traffic cannot pass through a stub AS A stub AS is either a source or a sink
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Multihomed AS
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A multihomed AS has more than oneconnection to other ASs, but it is still only
a source or sink for data traffic
It can receive data traffic from more thanone AS
It can send data traffic to more than one
AS, but there is no transient traffic
It does not allow data coming from one AS
and going to another AS to pass through
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Transit AS
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A transit AS is a multihomed AS that alsoallows transient traffic. Good examples of
transit ASs are national and international
ISPs (Internet backbones)
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Path Attributes
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Well-known attribute
One that every BGP router must recognize1. Well-known mandatory attribute
one that must appear in the description of a route
2. well-known discretionary attribute
one that must be recognized by each router, but is not required
to be included in every update message
Optional attribute
one that needs not be recognized by every router
1. optional transitive attribute
one that must be passed to the next router by the router that
has not implemented this attribute
2. optional nontransitive attribute
one that must be discarded if the receiving router has not implemented
it. 107
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A session is a connection that is established between two BGP
routers only for the sake of exchanging routing information.
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