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
•
•
•
•
Dr. Clincy
Routing conceptually
How routers work
Routing Protocols versus Routing Algorithms
Unicast Routing and Multicast Routing
1
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)
Dr. Clincy
2
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
Dr. Clincy
3
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
Dr. Clincy
4
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.
Dr. Clincy
5
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)
Dr. Clincy
6
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
Dr. Clincy
7
Configuration for routing for R1, Classful Case
Dr. Clincy
8
Simplified forwarding module in classful
address with subnetting
Recall for the Classful case, subnetting is done within the
organization
Dr. Clincy
9
Configuration for the Classful and Subnetting Case
Doesn’t know
what network
is connected
to router here
Dr. Clincy
10
Simplified forwarding module in classless
address
Dr. Clincy
11
Routing Table for R1 in the Illustrated Configuration –
Classless Case
Dr. Clincy
12
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
Dr. Clincy
13
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
Dr. Clincy
14
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
Dr. Clincy
15
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
Dr. Clincy
16
A routing example
Router R1 receives 500 packets for destination 192.16.7.14
- how does Router R1 uses it’s routing table ???
Dr. Clincy
17
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
Dr. Clincy
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
18
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
Dr. Clincy
Network-specific
192.16.7.14 & 255.255.255.0
192.16.7.0 match
19
Example 2
Make the routing table for router R1 in the Figure
U
UG
Dr. Clincy
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
20
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
Dr. Clincy
21
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
Dr. Clincy
This is where delay is incurred
Performs layer 1 and 2 functions:
bits to signal, packet encapsulated
into frame, error control overhead
added
22
Crossbar Switching Fabric
Cross Point
Dr. Clincy
23
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
Dr. Clincy
24