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Lecture #4
Chapter 6 Delivery & Forwarding of IP Packets
• Items you should understand by now – before routing
• Physical Addressing – with in the local network
• Network Addressing and subnetting – across
interconnected networks
• What is being routed across interconnected networks
– IP Datagram – Frame purpose ?
• Access Methods versus Routing versus Switching ??
• Go into Routing now
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Routing conceptually
How routers work
Routing Protocols versus Routing Algorithms
Unicast Routing and Multicast Routing
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ROUTING METHODS
There are various routing methods:
• Next-Hop Routing – table only holds the address of the next hop
(instead info regarding the entire route) – routing table for each host
• Network-Specific Routing – instead of an entry for each host (on the
same network), only one entry for the network is defined
• Host-Specific Routing – for a specific destination host, you might want
to control the exact route – in this case, the actual Rx is listed in the
routing table and the desired next hop is listed
• Default Routing – instead of listing all of the various networks in the
Internet, Tx host would use one entry called the Default (network
address 0.0.0.0)
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Next-hop routing
Next-Hop Routing – table only holds the address of the next hop
(instead info regarding the entire route)
Show more routers in better illustrating the routing table
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Network-specific routing
Network-Specific Routing – instead of an entry for each host
on the same physical network, only one entry for the network
is defined
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Host-specific Routing
Host-Specific Routing – for a specific destination host, you might want
to control the exact route – in this case, the actual Rx is listed in the
routing table and the desired next hop is listed
In this case, you want every packet traveling to Host B to traverse through R3.
For the other hosts on N2 and N3, the Network-specific routing approach is
used.
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Default Routing
•Default Routing – instead of listing all of the various networks in the
Internet, Tx host would use one entry called the Default (network
address 0.0.0.0)
In this case, R1 sends to a specific network however, R2 sends to the
remainder of the Internet (default)
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Simplified forwarding module in
classful address without subnetting
For the Classful case, per router, a table was needed for each
class – this made the searching simple
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Configuration for routing for R1, Classful Case
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Simplified forwarding module in classful
address with subnetting
Recall for the Classful case, subnetting is done within the
organization
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Configuration for the Classful and Subnetting Case
Doesn’t know
what network
is connected
to router here
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Simplified forwarding module in classless
address
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Routing Table for R1 in the Illustrated Configuration –
Classless Case
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Address aggregation
With the classless approach, routing tables increased – in
reducing the size of some tables, use a router to represent
multiple blocks – address aggregation
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STATIC VERSUS DYNAMIC ROUTING
• Host or router uses a routing table
• Table can be either static or dynamic in nature
• A static routing table contains information entered
manually.
• A dynamic routing table is updated periodically using
one of the dynamic routing protocols such as RIP, OSPF,
or BGP
• Regarding dynamic routing table: if fiber cut or router
failure, the tables are updated
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Router’s Table Logistics
When the router is looking for the route, it:
First check for direct delivery
Then host-specific delivery,
The network-specific delivery, and
Finally, default delivery
This order can be organized with in the routing table
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Routing Table
Mask: used to extract the net id of the Rx. For Host-Specific Routing - the mask is
255.255.255.255 and for Default Routing – the mask is 0.0.0.0.
Destination Address: either the destination host address or destination network address
Next-hop Address: next hop router address
Flags
U - The router is up and running. If router is down, the packet discarded
G - The destination is in another network. If G flag present, indirect delivery (if not, direct delivery)
H – If H flag present, destination field contains Host-specific address (if not present, network address)
D – If D flag present, routing info added to host routing table via ICMP’s redirection (cover later)
M - If M flag present, routing info was modified via ICMP’s redirection (cover later)
Reference count: # of users using this route at any moment
Use: # of packets transmitted through this router for the corresponding Rx
Interface: name of the interface
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A routing example
Router R1 receives 500 packets for destination 192.16.7.14
- how does Router R1 uses it’s routing table ???
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U case
UGH case
UG case
Mask
Dest.
Next Hop
255.0.0.0
111.0.0.0
255.255.255.224
193.14.5.160
-
m2
255.255.255.224
193.14.5.192
-
m1
--
I.
m0
--------------------------------------------------------------------------255.255.255.255
194.17.21.16
111.20.18.14
m0
---------------------------------------------------------------------------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
---------------------------------------------------------------------------0.0.0.0
the router
applies the
masks to the
destination
address until a
match with the
second column
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0.0.0.0
111.30.31.18
m0
Direct delivery
192.16.7.14 & 255.0.0.0
 192.0.0.0 no match
192.16.7.14 & 255.255.255.224  192.16.7.0 no match
192.16.7.14 & 255.255.255.224  192.16.7.0 no match
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U case
UGH case
UG case
Mask
Dest.
Next Hop
255.0.0.0
111.0.0.0
255.255.255.224
193.14.5.160
-
m2
255.255.255.224
193.14.5.192
-
m1
--
I.
m0
--------------------------------------------------------------------------255.255.255.255
194.17.21.16
111.20.18.14
m0
---------------------------------------------------------------------------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
---------------------------------------------------------------------------0.0.0.0
0.0.0.0
111.30.31.18
m0
Host-specific
192.16.7.14 & 255.255.255.255 192.16.7.14 no match
Router stops
when match is
made
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Network-specific
192.16.7.14 & 255.255.255.0
192.16.7.0 match
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Example 2
Make the routing table for router R1 in the Figure
U
UG
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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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STRUCTURE OF A ROUTER
We represent a router as a black box that accepts incoming
packets from one of the input ports (next hop), uses a routing table
to find the departing output port, and sends the packet from this
output port (interface).
The topics discussed in this section include:
Components
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Router components
Performs layer 1 and 2 functions:
signal to bits, packet decapsulated
from frame, error control performed
on bits, buffers packets before going
to the switching fabric
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This is where delay is incurred
Performs layer 1 and 2 functions:
bits to signal, packet encapsulated
into frame, error control overhead
added
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Crossbar Switching Fabric
Cross Point
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A banyan switch
Uses a binary string to
route across the switch
Example
Given a packet came in on port 1 and needed to go out of
port 6, the binary string of 110 will be used – explain this
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