Scaling the Network
Chapters 3-4 Part 2
Networking
CS 3470, Section 1
Network Layer Functions
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Addressing
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Globally unique address for each routable device
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Assigned by network operator
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Need to map to MAC address
Forwarding
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Logical address, unlike MAC address
From input port to appropriate output port in a router
Routing
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Which path to use to forward packets from src to dest
2
Today
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Subnetting
Forwarding with CIDR
ARP and DHCP Protocols
3
Subnetting
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Add another level to address/routing hierarchy: subnet
Subnet masks define variable partition of host part of
class A and B addresses
Subnets visible only within site
Subnetting
R1 Routing Table
Subnetting
Forwarding Algorithm
D = destination IP address
for each entry < SubnetNum, SubnetMask, NextHop>
D1 = SubnetMask & D
if D1 = SubnetNum
if NextHop is an interface
deliver datagram directly to destination
else
deliver datagram to NextHop (a router)
Longest-Prefix Match
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Suppose two network IDs exist:
1) 128.186.0.0/16
2) 128.186.134.0/24
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Suppose you have destination IP of
128.186.134.100
Both subnet mask & IP of 1 and 2 will yield
match – what to do?
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Longest-prefix match – route to network with
the most matching host bits.
7
Subnetting
Notes
 Would use a default router if nothing matches
 Not necessary for all ones in subnet mask to be
contiguous
 Can put multiple subnets on one physical network
 Subnets not visible from the rest of the Internet
Routing with CIDR
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Choices: A, B, C, D, E, F
9
Address Resolution
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IP address is virtual
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Not understood by underlying physical networks
IP packets need to be transmitted by the
underlying physical network
Address resolution
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Translating IP address to physical address
Address Resolution Protocol (ARP)
10
ARP
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A router has to know where to deliver
packets on the local network
ARP is used to discover MAC addresses
based on IP addresses
arp who-has 192.168.10.1 tell node31.ceee.lab
arp reply 192.168.10.1 is-at 00:60:08:ce:9d:3b
arp who-has node31.ceee.lab tell 192.168.10.254
arp reply node31.ceee.lab is-at 00:02:3f:b4:cd:87
ARP Cache
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Each computer maintains a cache table
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IP address  hardware address mapping
Only about computers on the same network
Try out “/usr/sbin/arp –a” command
Exchanges ARP messages
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To resolve IP addresses with unknown hardware
addresses
Encapsulated in DLL frame (e.g., Ethernet data
frame)
12
ARP Protocol
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When a node sends an IP packet
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Look up destination address in the ARP table
If not found
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To another node on the same physical network
Broadcast a request to the local network
Whose IP address is this?
What info should the request message
contain?
13
ARP Response
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The target node responds to sender (unicast?)
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On receiving the response
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Requester updates its table
Other nodes upon receiving the request
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With its physical address
Adds the requester into its ARP table (why?)
Refresh the requester entry if already there
No action otherwise (why?)
Table entries deleted if not refreshed for a while
14
ARP Example
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ARP broadcast by W requesting hardware
address of Y
15
Dynamic Host Configuration Protocol (DHCP)
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DHCP server is responsible for providing
configuration information to hosts
There is at least one DHCP server for an
administrative domain
DHCP server maintains a pool of available
addresses
DHCP
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State Protocol
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DHCPDISCOVER (client)
DHCPOFFER (server)
DHCPREQUEST (client)
DHCPACK (server)
DHCPNAK (server)
DHCPINFORM (client)
DHCP
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Newly booted or attached host sends
DHCPDISCOVER message to a special IP address
(255.255.255.255)
DHCP relay agent unicasts the message to DHCP
server and waits for the response
DHCP
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IP leases are valid for a predefined period of
time (T1)
Leases are renewed at T1/2
Leases are released if they have not been
renewed at the expiration of the lease time
Internet Control Message Protocol (ICMP)
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Defines a collection of error messages that are sent
back to the source host whenever a router or host is
unable to process an IP datagram successfully
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ICMP-Redirect
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Destination host unreachable due to link /node failure
Reassembly process failed
TTL had reached 0 (so datagrams don't cycle forever)
IP header checksum failed
From router to a source host
With a better route information
The ping application is a very common ICMP-messagegenerator