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COMP30023 – Computer Systems
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© University of Melbourne 2022
Internet (Network) Layer –
Internet Control Protocols
–Routing vs forwarding
– Flooding
– Shortest path / Dijkstra / Link State
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• Internet Control Protocols
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• Protocols don’t really form a single stack
– BGP (network layer) uses TCP (transport layer) for updates
• A better model is to think of different “planes” of protocols
– Data plane (network layer: forwarding)
– Control plan (network layer: choosing routes)
– [Management plane (network layer: setting BGP policies)]
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Data plane vs control plane
• Protocols that are used at the internet layer to manage
functionality
– ICMP – Internet Control Message Protocol
– DHCP – Dynamic Host Configuration Protocol
– ARP – Address Resolution Protocol (not strictly internet layer)
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Internet Control Protocols
• You’ve probably all used ICMP messages (PING), but they
are not limited to just sending out echo requests
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Internet Control Message Protocol
Message Type Description
Destination Unreachable Packed could not be delivered
Time exceeded Time to live field hit 0
Parameter problem Invalid header field
Source quench Choke packet
Redirect Teach a router about geography
Echo and echo reply Check if a machine is alive
Timestamp request/reply Same as Echo, but with timestamp
Router advertisement/solicitation Find a nearby router
• Exploits the Time Exceeded message
– Recall TTL is hop count (routers the packet traverses)
– Traceroute sends out packets to the same destination, each with an
incremented TTL
– Counters will hit zero at successive routers, causing the router to
return a Time Exceeded message, revealing the IP address of the
– Sender can use this information to determine path and timings of
the route a packet will take
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Traceroute
• tracert on windows, traceroute on Un*x
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Traceroute example
tracert bbc.com
Tracing route to bbc.com [151.101.0.81]
over a maximum of 30 hops:
1 3 ms 2 ms 5 ms 10.128.0.1
2 4 ms * * 124.19.10.193
3 6 ms 2 ms 4 ms 59.154.142.234
4 11 ms 3 ms 8 ms 165.228.132.193
5 6 ms 5 ms 6 ms bundle-ether11.exi-core10.melbourne.telstra.net [203.50.11.113]
6 5 ms 4 ms 3 ms bundle-ether1.lon-edge901.melbourne.telstra.net [203.50.11.108]
7 5 ms 3 ms 8 ms fas2867546.lnk.telstra.net [110.145.207.178]
8 5 ms 8 ms 5 ms 151.101.0.81
Trace complete.
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Traceroute example (international)
Tracing route to ****************
over a maximum of 30 hops:
1 2 ms 1 ms 4 ms 10.128.0.1
2 4 ms * * 124.19.10.193
3 * * * Request timed out.
4 * * * Request timed out.
5 16 ms 18 ms 18 ms 59.154.18.26
6 16 ms 15 ms 16 ms 59.154.18.8
7 169 ms 169 ms 173 ms 203.208.174.49
8 164 ms 165 ms 165 ms las-b24-link.telia.net [213.248.81.165]
9 165 ms 165 ms 166 ms las-b21-link.telia.net [62.115.136.46]
10 167 ms 168 ms 169 ms be3052.ccr41.lax05.atlas.cogentco.com [154.54.10.9]
11 167 ms 173 ms 167 ms be3243.ccr41.lax01.atlas.cogentco.com [154.54.27.117]
12 183 ms 190 ms 180 ms be2931.ccr31.phx01.atlas.cogentco.com [154.54.44.85]
13 211 ms 201 ms 199 ms be2929.ccr21.elp01.atlas.cogentco.com [154.54.42.66]
14 200 ms 201 ms 199 ms be2927.ccr41.iah01.atlas.cogentco.com [154.54.29.221]
15 323 ms 324 ms 325 ms be2687.ccr41.atl01.atlas.cogentco.com [154.54.28.69]
16 341 ms 341 ms 341 ms be2112.ccr41.dca01.atlas.cogentco.com [154.54.7.157]
17 313 ms 310 ms 310 ms be2806.ccr41.jfk02.atlas.cogentco.com [154.54.40.105]
18 325 ms 325 ms 325 ms be2915.ccr21.alb02.atlas.cogentco.com [154.54.40.61]
19 327 ms 328 ms 327 ms be2088.ccr21.ymq01.atlas.cogentco.com [154.54.43.17]
20 310 ms 314 ms 319 ms be3042.ccr21.lpl01.atlas.cogentco.com [154.54.44.161]
21 339 ms 339 ms 343 ms be2391.ccr41.lon13.atlas.cogentco.com [154.54.39.149]
22 345 ms 343 ms 343 ms be12497.ccr41.par01.atlas.cogentco.com [154.54.56.130]
23 356 ms 361 ms 357 ms be2380.rcr21.lux01.atlas.cogentco.com [154.54.78.94]
24 322 ms 320 ms 324 ms be3456.nr51.b038844-0.lux01.atlas.cogentco.com [154.25.12.70]
• The internet layer requires each host/interface to have a
unique IP address
• We could manually configure each host (certain networks
– Difficult to administer, error prone
– Slow to respond to new devices
• DHCP is the automated way of handling IP address
allocation
• Security concerns – connecting any device will issue an IP
address (can apply restrictions)
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DHCP – Dynamic Host
Configuration Protocol
• Network has a DHCP server for issuing IP addresses
• Host sends out a DHCP DISCOVER packet
– Routers can be configured to relay these to the DHCP server if not
directly connected to the same network
• DHCP Server receives the request and responds with a DHCP
OFFER packet containing an available IP address
• IP addresses are typically issued on a lease – time after which
the IP address will be reclaimed by the server and re-issued
– Hosts can request a renewal before the lease expires
• Used to set a number of parameters
– Default gateway, DNS servers address, time servers
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• Leaves one obvious question…
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• If we don’t have an IP address how does the DHCP server know
where to send the response to?
– IP runs over another network – use that network’s address
• Ethernet / WiFi: use MAC (Media Access Control) Address
– Can think of it as a globally unique identifier for the interface
– Usually hard-coded by the manufacturer
– Between 48 and 64 bits long
• Addressing used at the Host-to-network/data link layer
– Sometimes called physical address, but not at physical layer
• (Not examinable)
– Why do we need a second addressing scheme?
– Read https://apenwarr.ca/log/?m=201708#10 for an interesting take on it
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MAC Address
https://apenwarr.ca/log/?m=201708#10
• ARP is the link between the internet layer and the
underlying network layer
– It translates an IP address into a MAC address
• Broadcasts an Ethernet (or WiFi, or…) packet asking who
owns the target IP address
• Broadcast arrives at every host on the network, the owner
will respond with its MAC address
• The low level sending is done via MAC addresses.
• This protocol is run a lot, even to find out how to
communicate with the nearest router
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Address Resolution Protocol
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ARP in Action
• Incredibly simple
• Not particularly efficient
• Security nightmare
– No authentication
– Caching of responses, even when not directly requested
– ARP spoofing is the gateway attack for most man-in-the-middle
– Provides a way of intercepting and spoofing ARP messages to
associate the attackers MAC address with another hosts IP address
(i.e. default gateway, DNS server, website)
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• We have spoken a lot about internetworking without
looking at the global scale of the internet.
• The global communication we have come to take for
granted are dependent on a network of undersea cables
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Internetworking
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https://www.submarinecablemap.com/
And finally…
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• The slides were prepared by based on
those of , based on material developed
previously by: , , ,
• Some of the images included in the notes were supplied as
part of the teaching resources accompanying the text books
listed in lecture 1.
• Textbook Reference: Ch 5.6, end of 4.3.3, 6.4.1
Acknowledgement
© University of Melbourne 2022
Internet (Network) Layer – Internet Control Protocols
Data plane vs control plane
Internet Control Protocols
Internet Control Message Protocol
Traceroute
Traceroute example
Traceroute example (international)
DHCP – Dynamic Host Configuration Protocol
MAC Address
Address Resolution Protocol
ARP in Action
Internetworking
And finally…
Acknowledgement
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