Concept of Routing
in Network Layer
1
Network Layer II (routing)
Routing Styles:
Static vs. Dynamic Routing
Routing Protocols/Algorithms
 Routing Table
 Routing Information Protocol (RIP) & Distance Vector Routing
(DVR)
 Open Shortest Path First (OSPF) & Link State Routing (LSR)
 Dijkstra’s “Shortest Path” Algorithm
 Border Gateway Protocol (BGP) and Path Vector Routing (PVR)
2
Routing Protocol & Routing Algorithm
A Routing Protocol is a combination of rules and
procedures that lets routers in an internet inform
each other of changes. It allows routers to share
whatever they know about the internet or their
neighbourhood.
A Routing Algorithm is that part of network layer
software responsible fro deciding which output
line and incoming packet should be transmitted
on.
3
Routing
a) Routing requires a host or a router to have a routing table.
b) Usually when a host has a packet to send or when a router has
received a packet to be forwarded, it looks at this table to find the
route to the final destination.
c) However, this simple solution is impossible in today’s Internet
world because the number of entries in the routing table makes the
table lookups inefficient.
d) Need to make the size of table manageable and handles issues such
security at the same time. The key question is how to design the
routing table.
e) Next-hop routing, Network-specific routing, host specific routing
f) Static versus Dynamic Routing
g) Routing Protocols: RIP, OSPF, BGP
h) Routing Algorithms: DVR, LSR, PVR
4
Next-hop routing
Next-hop routing holds only the information that leads to the
next hop instead of complete route.
5
Network-specific & host-specific routing
The destination host address
is given in the routing table;
to have greater control over
routing.
Instead of having an entry for every host
connected to the same network, only one entry
is needed to defined the address of the network
itself. All host connected to the same network
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as one single entity.
Default routing
R1 is used to route packets to hosts
connected to N2.
However, R2 is used to as default to
route other packets to the rest of
Internet without listing all the
networks involved
Only one default routing is allowed
with network address 0.0.0.0
7
General Routing Table
Flags
U
G
H
D
M
The router is up and running.
The destination is in another network.
Host-specific address.
Added by redirection.
Modified by redirection.
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Routing table
a) Generally, a routing table needs a minimum of 4
columns: mask, destination network address, next hop
address and interface.
b) When a packet arrives, the router applies the mask to the
destination address it receives (one-by-one until a match
is found) in order to find the corresponding destination
network address.
c) So, the mask serves as essential tool to match destination
address in routing table and the address it receives.
d) If found, the packet is sent out from the corresponding
interface in the table. If not found, the packet is delivered
to the default interface which carries the packet to
default router.
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Configuration for routing example
Mask
Dest.
255.0.0.0
111.0.0.0
--
m0
255.255.255.224
193.14.5.160
-
m2
255.255.255.224
193.14.5.192
-
m1
255.255.255.255
194.17.21.16
111.20.18.14
m0
Networkspecific
255.255.255.0
192.16.7.0
111.15.17.32
m0
255.255.255.0
194.17.21.0
111.20.18.14
m0
Default
0.0.0.0
0.0.0.0
111.30.31.18
m0
Standard
delivery
Host-specific
Next Hop
I.
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Example 1
Router R1 receives 500 packets for destination 192.16.7.14; the
algorithm applies the masks row by row to the destination address
until a match (with the value in the second column of Dest. in
table) is found:
Solution
Direct delivery
Rule of thumb: Apply the individual mask (from Routing
table) to the received destination address (row-by-row)
and see if its matches any of the DEST address stated in
its routing table. If match is found, then stop
192.16.7.14 & 255.0.0.0
 192.0.0.0
no match to 111.0.0.0
192.16.7.14 & 255.255.255.224  192.16.7.0 no match to 193.14.5.160
192.16.7.14 & 255.255.255.224  192.16.7.0
no match to 193.14.5.192
Host-specific
192.16.7.14 & 255.255.255.255 192.16.7.14 no match to 194.17.21.16
Network-specific
192.16.7.14 & 255.255.255.0
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192.16.7.0
match to 192.16.7.0
Example 2
Router R1 receives 100 packets for destination 193.14.5.176; the
algorithm applies the masks row by row to the destination address
until a match is found:
Solution
Direct delivery
193.14.5.176 & 255.0.0.0
 193.0.0.0
193.14.5.176 & 255.255.255.224 193.14.5.160
no match
match
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Example 3
Router R1 receives 20 packets for destination 200.34.12.34; the
algorithm applies the masks row by row to the destination address
until a match is found:
Solution
200.34.12.34 & 255.0.0.0
200.0.0.0
no match
200.34.12.34 & 255.255.255.224 200.34.12.32
no match
200.34.12.34 & 255.255.255.224 200.34.12.32
no match
200.34.12.34 & 255.255.255.255 200.34.12.34
no match
200.34.12.34 & 255.255.255.0  200.34.12.0
no match
200.34.12.34 & 255.255.255.0  200.34.12.0
no match
Default
200.34.12.34 & 0.0.0.0
 0.0.0.0.
match
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Example 4
Make the routing table for router R1 in figure below
Solution
Mask
Destination
Next Hop
I.
255.255.0.0
134.18.0.0
--
m0
255.255.0.0
129.8.0.0
222.13.16.40
m1
255.255.255.0
220.3.6.0
222.13.16.40
m1
0.0.0.0
0.0.0.0
134.18.5.2
m0
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Example 5
Make the routing table for router R1 in figure below
Solution
Subnet mask
I.
Destination
255.255.255.0
200.8.4.0
255.255.255.0
80.4.5.0
---201.4.10.3
or 200.8.4.12
255.255.255.0
80.4.6.0
201.4.10.3
or 200.4.8.12
0.0.0.0
0.0.0.0
Next Hop
m2
m1
or m2
m1
or m2
m0
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Note
In classless addressing, we need at least
four columns in a routing table.
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Routing Tables in IP with CIDR
(Classless InterDomain Routing)
Mask
Destination
Next Hop
/12
128.96.0.0
145.12.56.29
/17
128.125.0.0
153.202.12.128
/12
128.112.0.0
153.202.14.1
/26
128.105.14.64
153.2.45.101
/32
128.105.14.66
153.2.45.101
For each entry in the routing table:
MaskedAddress := EntryMask (bitAND) IPDatagramDestinationAddress;
if (MaskedAddress == EntryDestination)
Mark the entry;
Choose the marked entry with the longest Mask prefix.
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Example 7a
Make a routing table for router R1, using the
configuration in Figure below
m3
Solution
Routing table for router R1 in Figure above
The table is sorted from the longest mask to the shortest mask.
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Example 7b
Show the forwarding process if a packet arrives at
R1 with the destination address 180.70.65.140.
Solution
The router performs the following steps:
1. The first mask (/26) is applied to the destination address.
The result is 180.70.65.128, which does not match the
corresponding network address.
2. The second mask (/25) is applied to the destination
address. The result is 180.70.65.128, which matches the
corresponding network address. The next-hop address
and the interface number m0 are passed on for
further processing.
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Example 7c
Show the forwarding process if a packet arrives at
R1 with the destination address 201.4.22.35.
Solution
The router performs the following steps:
1. The first mask (/26) is applied to the destination address. The
result is 201.4.22.0, which does not match the corresponding
network address.
2. The second mask (/25) is applied to the destination address. The
result is 201.4.22.0, which does not match the corresponding
network address (row 2).
3. The third mask (/24) is applied to the destination address. The
result is 201.4.22.0, which matches the corresponding network
address..
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Example 7d
Show the forwarding process if a packet arrives at
R1 with the destination address 18.24.32.78.
Solution
This time all masks are applied, one by one, to the destination
address, but no matching network address is found. When it reaches
the end of the table, the module gives the default next-hop address
180.70.65.200 (because it could not find the match) . This is probably
an outgoing package that needs to be sent, via the default router, to
someplace else in the Internet.
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Routing/routers
a) An internet is a combination of networks connected by routers.
b) When a packet goes from a source to a destination, it will pass
through many routers until it reaches the router attached to
destination network.
c) A router consults a routing table when a packet is ready to be
forwarded. The routing table specifies the optimum path for the
packet and can be either static of dynamic. Dynamic routing is more
popular.
d) Static table does not change frequently. Dynamic table is updated
automatically when there is a change somewhere in the network; i.e
when a route is down or a better route has been created.
e) Routing protocols is a combination of rules/procedures that lets
routers in the internet inform one another when changes occur;
mostly based on sharing/combining information between routers at
different networks.
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Unicast Routing
a) Unicast = one source and one destination. (1-to-1 relationship).
b) In Unicast routing, when a router receives a packet, it forwards the
packet thru only one of its ports as defined in the routing table. The
router may discard the packet if it cannot find the destination address
c) Questions: In dynamic routing, how does the router decides to which
network should it pass the packet next? What routing algorithm is
the routing based on? The decision is based on optimisation: which
of the available pathways is the best/optimum path?
d) But how to measure? A metric is a cost assigned for passing thru a
network and the total metric of a particular route is equal to the sum
of the metrics of networks that comprise the route.
e) Simple protocols such as Routing Information Protocol (RIP), treat
all network equally; cost of passing each network is the same as one
hop count per network.
f) Other sophisticated protocols e.g. OSPF, based on services required
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and using different metrics: max throughput, minimum delay.
Routing Protocol:
Interior Vs Exterior
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Routing Architecture in the Internet
Fact:
Nobody owns the whole Internet.
However, parts of the Internet are owned and administered by
commercial and public organisations (such as ISPs, universities,
governmental offices, research institutes, companies etc.).
Idea:
Divide the Internet in Autonomous Systems (AS) that are
independently administered by individual organisations. Let each
administrative authority use its own routing protocol within the
AS. Let’s use one routing protocol to exchange routing
information among AS.
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Routing Architecture in the Internet
An AS is a group of networks and routers under the authority
of a single administrator.
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Static versus Dynamic Routing
A static routing table
contains information entered manually
Usually remained unchanged.
A dynamic routing table is updated
periodically or whenever necessarily
using one of the dynamic routing protocols
such as RIP, OSPF, or BGP.
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Routing Protocols: Interior vs Exterior
• Routing inside an AS is referred to as interior routing whereas routing between ASs
is referred to as exterior routing.
• Each AS can choose one or more interior routing protocols inside an AS.
• Only one exterior routing protocol is usually chosen to handle routing between ASs.
• To know the next ’path’ (or router) a packet should be pass-on, the decision is based
on some optimisation rule/protocol, e.g. using different assignment of the cost
(metric) for each passing through a network for different routing Protocol above. 28
Interior Routing Protocol 1:
Routing Information Protocol
(RIP)
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Distance Vector Routing (DVR)
a) 3 keys to understand how this algorithm works:
• Sharing knowledge about the entire AS. Each router
shares its knowledge about the entire AS with
neighbours. It sends whatever it has.
• Sharing only with immediate neighbours. Each router
sends whatever knowledge it has thru all its interface.
• Sharing at regular intervals. sends at fixed intervals,
e.g. every 30 sec.
b) Problems: Tedious comparing/updating process, slow
response to infinite loop problem, huge list to be
maintained!!
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Initialization of tables in distance vector routing (DVR)
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Updating in distance vector routing example: C to A
From C
From A
A to A via C: ACA = AC+ CA = 2+2
A to B via C: ACB = AC + CB = 2+4
A to D via C: ACD = AC + CD = 2+ inf.
A to E via C: ACD = AC + CE = 2+4
A to C via C: ACB = AC + CC = 2+0
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Final Distance vector routing tables
33
DVR example from Tenenbaum (with estimated delay given
slightly different)
Neighbour routers
JA,8
JB, JAB, 8+12
JC, JIC, 10+18
JD, JHD, 12+8
JE, JIE. 10+7
JF, JIF, 10+20
JG, JHG, 12+6
JH, 12
JI. 10
JJ, 0
JK, 6
Each router maintain a table (a vector)
giving the best known metric (or delay)
to each destination and which line to
use. These tables are then updated by
exchanging information with the
neighbours (direct link, 1 hop)
JL, JKL, 6+9
(a) A subnet. (b) Input from A, I, H, K, and the new routing table for J.
1st DRAWBACK: VERY SLOW!!!
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Routing Information Protocol (RIP)
a) RIP is based on distance vector routing, which uses the BellmanFord algorithm for calculating the routing table.
b) RIP treats all network equals; the cost of passing thru a network is
the same: one hop count per network.
c) Each router/node maintains a vector (table) of minimum distances
to every node. (the least-cost route btw any nodes is the route
with the minimum number of hop-count).
d) The hop-count is the number of networks that a packet encounters
to reach its destination. Path costs are based on number of hops.
e) In distance vector routing, each router periodically shares its
knowledge about the entire internet with its neighbour.
f) Each router keeps a routing table that has one entry for each
destination network of which the router is aware.
g) The entry consists of Destination Network Address/id, HopCount and Next-Router.
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Example of Initial routing tables (RIP)
in a small autonomous system
36
Example of Final routing tables
37
Example of a domain using RIP
38
Infinite loop problem
39
Infinite loop problem in DVR
A initially down; hence
A initially up then down
The count-to-infinity problem!
Good news (a) travels faster than bad news (b)
React rapidly to good news but slowly to bad news
Although it will eventual converge to correct answer, they adapt slowly,
they must be told to change. Convergence to the correct answer is slow.
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Interior Routing Protocol 2:
Open Shortest Path First
Protocol (OSPF)
41
Open Shortest Path First (OSPF)
a) OSPF uses link state routing to update the routing table in an area;
OSPF divides an AS into different areas (depending on their
type).
b) Unlike RIP, OSPF treats the entire network within differently with
different philosophy; depending on the types, cost (metric) and
condition of each link: to define the ‘state’ of a link.
c) OSPF allows the administrator to (only) assign a cost for passing
through a network based on the type of service required. e.g.
minimum delay, maximum throughput. (but not stating exact path)
d) Each router should have the exact topology of the AS network(a
picture of entire AS network) at every moment. The topology is a
graph consisting of nodes and edges.
e) Each router needs to advertise to the neighbourhood of every
other routers involved in an Area. (flood)
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Open Shortest Path First (OSPF)
Areas in an Autonomous System
(AS>Areas)
OSPF divides an AS into areas. An area is a collection of network, hosts and routers
all contained within an AS. Routers inside an area flood the area with routing info. At
the border of an Area, special routers called Area Border routers summarize the info.
about the area and send it to other area. Among the areas inside an AS is a special area
called the Backbone connecting all areas through Backbone routers and serves as a
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primary area to the outside (other ASs) via the AS Boundary router.
Link State Routing (LSR)
a) Like RIP, in link state routing, each router also shares its knowledge
about its neighbourhood with every routers in the area.
b) However, in LSR, the link-state packet (LSP) defines the best known
network topology (of an area) is sent to every routers (of other area)
after it is constructed locally. Whereas RIP slowly converge to final
routing list based information received from immediate neighbours.
c) 3 keys to understand how this algorithm works:
• Sharing knowledge about the neighbourhood. Each router sends the state of
its neighbourhood to every other router in the area.
• Sharing with every other routers. Thru process of flooding. each router sends
the state of its neighbourhood thr all its output ports and each neighbour sends
to every other neighbours and so on until all routers received same full
information eventually.
• Sharing when there is a change. Each router share its state of its neighbour
only when there is a change; contrasting DVR results in lower traffic.
d) From the received LSPs and knowledge of entire topology, a router
44
can then calculate the shortest path between itself and each network.
Types of links
When the link between two routers is broken, the administrator may create a virtual
link between them using longer path that probably goes through several routers
45
Link State Advertisement (LSA)
To share information about the neighbourhood, each entity
distribute link state advertisements (LSAs).
5 Types of LSAs
Info. exchange
within inside an Area
Info exchange
between different
Areas inside an AS
Info exchange
outside across
different AS
Info exchange
to external
internet
46
Router link
A router link advertisement defines the links of a true router.
A true router uses this advertisement to announce information about all
its links and what is at the other side of the link (neighbour).
47
Network link
A network link advertisement defines the links of a network.
A designated router on behalf of the transient network distributes this
types of LSA packet. The packet announces the existence of all the
routers connected to the network.
48
Summary link to network
area border
router R2
area border
router R1
Backbone network
Router and network link advertisements flood each area with info about the router
links and network links within/inside an area. But a router must also know about the
networks outside its area, and the area border routers can provide this information.
An area border router is active in more than one area. It receives router link and
network link advertisements and creates a routing table for each area.
49
Summary link to AS boundary router
The previous advertisement lets every router know the cost to reach all networks
within/inside an AS. But what about the network outside the AS? If a router inside an
area wants to send a packet outside the autonomous system, it should first know the
route to an AS boundary router; the summary link to AS boundary router provides this
information. The border routers can then flood their areas with this information.
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External link
Although the previous advertisement lets each router know the route to different AS
boundary router, this information is not enough. A router inside an AS also wants to
know which networks are available outside the AS; i.e. the external internet.
The external link advertisement provide this information. The AS boundary router
floods the AS with cost of each network outside the AS, using a routing table created
by an exterior routing table protocol. Each advertisement announces one single
51
network. If there is more than one network. Separate announcements are made.
Example
In the figure below, which router(s) sends out
router link LSAs? and which router(s) sends out
network link LSAs?
Solution
All routers advertise router link LSAs.
R1 has two links, Net1 and Net2.
R2 has one link, Net2 in this AS.
R3 has two links, Net2 and Net3.
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Solution Continue
All three network must advertise network link LSAs:
Advertisement for Net1 is done by R1 because it is the only router
and therefore the designated router.
Advertisement for Net2 can be done by either R1, R2, or R3,
depending on which one is chosen as the designated router.
Advertisement for Net3 is done by R3 because it is the only router
and therefore the designated router.
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In OSPF, all routers have
the same Link State database.
• Every router in an area receives the router link and network link
LSAs and form a link state database.
• Every router in the same area has the same link state database.
• A link state database is a tabular representation of the topology of
the internet inside an area. It shows the relationship between each
router and its neighbors including the metrics used.
• To calculate its next-route in the routing table, each router applies
the Dijkstra algorithm to its state database, to find the shortest path
between 2 points on a network, using a graph (nodes and edges).
• The algorithm divides the nodes into two sets: tentative and
permanent. It chooses nodes, makes them tentative, examines them,54
and if they pass the criteria, makes permanent.
Graph representation of AS:
nodes and edges
(a) An autonomous system. (b) A graph representation of (a).
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Shortest Path Search
Dijkstra’s Algorithm
1. Start with the local node (router): the root of the tree.
2. Assign a cost of 0 to this node and make it the first permanent node.
3. Examine each neighbour node of the node that was the last
permanent node.
4. Assign a cumulative cost to each node and make it tentative.
5. Among the list of tentative nodes
a. Find the node with the smallest cumulative cost and make it permanent.
b. If a node can be reached from more than one direction
i. Select the direction with the shortest cumulative cost.
6. Repeat steps 3 to 5 until every node becomes permanent.
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Dijkstra algorithm
57
Shortest Path Search
The steps used in computing the shortest path from A to D.
The arrows indicate the working node – permanent label.
The cost can relates to
delay
Start search and
compare with
tentative label
Mark permanent
when shortest
node found
Once permanent
never changed
Tentative label
change
Tentative node can
always be search
and relabelled
58
The label on each node can be TENTATIVE or PERMANENT
Example of formation of shortest path tree
59
Example of an internet
Graphical representation of an internet
8
5
2
2
0
0
0
5
4
4
2
60
Shortest path calculation
8
2
0
5
2
0
0
4
5
4
2
61
Shortest path calculation
14
8
2
5
2
0
0
0
4
5
4
2
62
Shortest path calculation
8
2
5
2
0
0
0
4
5
4
2
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Link state routing table for router A
Each router uses the shortest-path tree
method to construct its routing table.
Network
Cost
Next router
N1
5
N2
7
C
N3
10
D
N4
11
B
N5
15
D
Other infor
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Link State Routing (LSR)
Idea Behind LSR: Each router must do the following:
1.
2.
3.
4.
5.
Discover its neighbors, learn their network address. (Send HELLO packet)
Measure the delay or cost to each of its neighbors. (Send ECHO packet)
Building Link State packet telling all it has just learned. (ACK flag)
Distribute/Send this packet to all other routers (Flooding – SEND flag).
Compute the shortest path to every other router. (Dijkstra’s algorithm)
In effect the complete topology and all the delays are experimentally
measured and distributed to every router, then Dijkstra’s algorithm can
be run to find the shortest path to every other router.
HELLO packet can be acknowledged by a reply to signal its present.
ECHO packet requires instant response to know the round-trip-time.
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OSPF Packets
• Based on Link State Routing
• OSPF messages are transported directly
in IP packets
• OSPF standard supports novel concepts
such as type of service routing, load
balancing and authentication
66
Building Link State Packets (LSP)
(a) A subnet.
(b) The link state packets for this subnet.
Building the link state packets is easy. The difficult part is to determine
when to build them.
One possibility is to build them periodically at regular intervals.
The other is to build when some significant even occurs; i.e. a line or
neighbour going down or coming back up again.
67
Distributing the Link State Packets (LSP)
Arriving packets
Above is the packet buffer at router B. Routers A, F, C are directly
connected to B. Each row corresponds to a recently-arrived, but as
yet not fully processed LSP. The table shows where the packet
originated, sequence number, age and the data.
The process of distributing the LSP is called Flooding
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**?
1.
2.
LSR potential problems
If a router ever crashes, it will lose track of its
sequence number and starts from 0 again; next
arriving good packet will be rejected as duplicate.
If a sequence number is ever corrupted.
Solution
The solution to all above is to introduce an ‘Age’ field for each packet
after the sequence number and decrement it once per second. When the
Age hits zero, the information from that router is discarded. The Age
field is also decremented by each router during initial flooding process,
to make sure no packets can get lose and live for indefinite period of
time
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Compute the shortest path
•
•
•
•
•
When a router has accumulate a full set of link state packets,
it can build the entire subnet graph because every link is
represented.
Every link is in fact, represented twice, once for each
direction. E.g AB – 4, BA – 4.
The two values can be averaged or used separately
Next, Dijkstra’s algorithm can be run locally to find the
shortest path to all possible destinations. The results can be
installed in the routing tables and normal operation resumed.
For a subnet with n routers, each of which has k neighbours,
therefore memory required to store the input data is
proportional to k*n.
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Exterior Routing Protocol:
Border Gateway Protocol
(BGP)
71
BGP & Path Vector Routing (PVR)
a) Border Gateway Protocol (BGP) is an inter-domain or interautonomous system routing protocol: routing between different ASs.
b) BGP uses path vector routing to update the routing table in an area.
c) DVR and LSR are not suitable candidates for inter-AS routing :
• DVR: there are occasions in which the route with the smallest hop count is not
the preferred route; non-secure path although the shortest route taken.
• LSR: internet is too big for this routing method to require each router to have a
huge link state database. Taking very long time to calculate the routing table.
d) PVR defines the exact paths as an ordered list of ASs that a packet
should travel thru to reach the destination (besides having the
destination network and next router info.) in its routing table.
e) Security and Political issues involved: more desired to avoid ‘unsaved’
paths/routes/ASs than to take a shorter route.
f) The AS boundary router that participate in PVR advertise the routes of
the networks in their own AS to neighbour AS boundary routers.
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g) Solve the count-to-infinity problem
Path vector packets
•Each AS has its ‘speaker’ router/node that acts on behalves of the AS. Only
speaker router can communicate with other speaker routers.
•R1 send a path vector message advertising its reachability of N1. R2 receives
the message, updates its routing table and after adding its AS to the path and
inserting itself as next router, send message to R3. R3 receives the message,
updates its routing table, make changes and sends the message to R4.
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BGP – the Exterior Gateway Routing Protocol
PVR
After all paths are in, router F
examines them to see which
is the best & quickly drop I & E
as they pass thru itself.
•Instead of periodically advertise to its neighbours the cost to each destination,
each BGP router tells its neighbour the exact path it is using. e.g. F receives
information from its neighbour routers to reach D.
•Can solve count-infinity problem: suppose G is down; then IFGCD and EFGCD
routes are discarded since G’s state will be know immediately render BCD as74
only choice.
Path Vector Routing Policy
a) Policy routing can be easily implemented through path vector routing.
b) When a router receives a message from its neighbour, the speaker
node or AS boundary router can check the path with its approved list
of ASs.
c) If one of the ASs listed in the path is against its policy, the router can
ignore that path entirely and that destination.
d) For any unapproved paths, the router does not update its routing table
with this path, and it does not send the PV message to its neighbours.
e) This means that the routing table in path vector routing are not based
on the smallest hop count (as in distance vector routing) or the
minimum delay metric (as in open shortest path first routing); they are
based on the policy imposed on the router by the administrator.
f) The path was presented as a list of ASs, but is in fact, a list of
attributes. Each attributes gives some information about the path. The
list of attributes helps the receiving router make a better decision
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when applying its policy. (Well-known & Optional)
Types of BGP messages
a) Open: To create a relationship, a router running BGP opens a
connection with a neighbouring AS and sends an open message. If the
neighbour accepted, it responds with a Keep-alive message to
establish relationship between the two routers.
b) Update: The heart of BGP protocol used by router to withdraw
destination that have been advertised previously, announce a route to a
new destination or do both. (Withdraw several but advertise only one).
c) Notification: sent by a router whenever an error condition is detected76
or router wants to close the connection (down).
Initial routing tables in path vector routing
Stabilized tables for three autonomous systems
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Big picture: RIP/OSPF/BGP
The relation between ASs, backbones, and areas.
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Too much to take in!!!
- Brain Freeze?
Due to time limitation and the vast-scope of the network layer, we
will NOT go into any more detail about Multicast.
Interested students can find more detail in “Internetworking with
TCP/IP vol.1” by Douglas Comer, Section-10.22 and Forouzan’s
Chapter-21.
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Further Reading
1- “Computer Networks”, Andrew Tanenbaum, 4th Ed. to learn more
about the generic network layer.
2- “Internetworking with TCP/IP vol.1”, Douglas Comer, 4th Ed.,
provides a detailed and comprehensive presentation of TCP/IP.
3- “Data Communications and Networking”, Behrouz Forouzan, 4th
Ed., when you get confused and wonder if there’s a simpler
explanation of all these issues.
Copyright Information : Some figures used in this presentation have been
either directly copied or adapted from several books.
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