Transcript System Models for Distributed and Cloud Computing

Network Layer – Subnetting and Control
Protocols
Dr. Sanjay P. Ahuja, Ph.D.
Fidelity National Financial Distinguished Professor of CIS
School of Computing, UNF
Example of Subnetting
128.64.2.253
128.64.1.254
Lan 1
R1
128.64.2.254
Lan 2
128.64.1.1
H1
M,N
128.64.2.2
H2
Lan 1
Subnet # 128.64.1.0
Subnet Mask 255.255.255.0
Lan 2
Subnet # 128.64.2.0
Subnet Mask 255.255.255.0
Lan 3
Subnet # 128.64.3.0
Subnet Mask 255.255.255.0
R2
128.64.3.254
Lan 3
128.64.3.2
H3
Example of Subnetting
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The bitwise AND of a host IP address and the subnet mask provides
the subnet # of the host and of all other hosts on the same subnet.
Say H1 wants to send a packet to H2. H1 does a bitwise AND between
its own subnet mask and the destination IP address.
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255.255.255.0 AND 128.64.2.2 = 128.64.2.0 = 128.64.1.0
This result does not match the subnet # for H1 and so H1 knows H2 is
on a different subnet.
To support subnetting, the routing table of a router holds entries of
the form <subnet #, subnet mask, next hop>
Router R1’s table:
Subnet #
Subnet Mask
Next Hop
128.64.1.0
255.255.255.0
Interface 0
128.64.2.0
255.255.255.0
Interface 1
128.64.3.0
255.255.255.0
R2
Internet Control Protocols in
the Network Layer - ICMP
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Internet Control Message Protocol (ICMP) is a companion
protocol to IP.
ICMP 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.
Examples of error messages:
Destination Unreachable: Could be due to link failure or router unable to
locate destination
Time Exceeded: When TTL decrements to 0.
ICMP – Redirect: Tells source host that there is a better route to the
destination.
ECHO REQUEST: To see if a destination is alive and reachable.
ECHO REPY
Source Quench: Used to throttle hosts that were sending too many packets.
Internet Control Protocols in
the Network Layer - ARP
Address Resolution Protocol (ARP)
An IP address (4-bytes) needs to be mapped into a link-level address (e.g. a 6bytes Ethernet or MAC address) before a packet encapsulated in a frame can be
delivered to a host on a network (LAN) or to a router on a LAN that will forward
it.
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A source host on a LAN broadcasts an ARP REQUEST packet with the IP
address of the destination host (“Who owns this IP address?”) and the
destination host replies with an ARP REPLY packet containing its MAC
address. The source host caches the destination MAC address for future use.
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Then the source host sends the MAC/Ethernet frame containing the IP
packet to the destination host.
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To allow mappings to change (e.g. if an Ethernet NIC card breaks or is
replaced) entries in the ARP cache are timed out every 15 minutes.
Internet Control Protocols in the Network
Layer – RARP, BOOTP, DHCP
Reverse Address Resolution Protocol (RARP)
This answers the reverse question, i.e. given an Ethernet address, what is the
corresponding IP address. Used when booting a diskless workstation on the
LAN.
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Upon boot-up, RARP allows a workstation to broadcast its Ethernet address
and ask if any one knows its IP address (“Does anyone know my IP address?”)
The RARP server responds with the IP address of the workstation.
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A disadvantage of RARP is that it uses a destination address of all 1s (limited
broadcasting) to reach the RARP server. However, such broadcasts are not
forwarded by routers, so a RARP server is needed on each LAN in an
organization.
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Once a workstation has its IP address it can contact the file server for the
image of the OS, file server’s IP address, default router’s IP address. Each of
these requests is a separate request-reply. This is inefficient.
Internet Control Protocols in the Network
Layer – RARP, BOOTP, DHCP
Boot Protocol (BOOTP)
Uses UDP. It does a 255.255.255.255 limited broadcast which is forwarded over
routers. Only one BOOTP server is needed for all the LANs in an organization.
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To improve on efficiency, BOOTP server responds with the workstation’s IP
address, default router’s IP address, file server’s IP address, and filename of
the OS image in one reply packet.
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BOOTP does not automatically assign IP addresses to a client workstation.
The MAC address – IP address mapping table is still required to be
configured and maintained manually at the BOOTP server.
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So when a new host is added to a LAN, it cannot use BOOTP until a network
admin has assigned it an IP address and entered its Ethernet address and IP
address into the BOOTP configuration table manually.
Internet Control Protocols in the Network
Layer – RARP, BOOTP, DHCP
Dynamic Host Configuration Protocol (DHCP)
An IP address can be automatically assigned to a client host from a pool of IP
addresses. IP addresses assigned automatically are leased for a fixed amount of
time.
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DHCP allows both manual IP address and automatic assignment and has
replaced both RARP and BOOTP.
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The DHCP server need not be on the same LAN as the requesting client host.
Since the DHCP server may not be reachable by broadcasting, a DHCP relay
agent is needed on each LAN.