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CSCI 233Internet Protocols
Class 3
Dave Roberts
FIRST…A LITTLE REVIEW
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Internet Protocol Principles • Good Citizen Principle• Scarcest Internet Resource
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Outline• Internet addresses• Mapping IP addresses to physical
addresses
INTERNET ADDRESSING
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The Internet• Is it a physical or virtual network?• It’s a virtual network, defined by
protocols that run on hosts and routers.
• Internet protocols make the Internet look like a world-wide uniform network, although it encompasses many networks that are very different from each other.
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Internet Addresses• Each host connection on the
Internet has a unique Internet address
• The addresses are designed to make forwarding of Internet packets simple
• An IP address has two parts: a prefix that identifies a network and a suffix that identifies a host on the network 7
Need for Control• To avoid conflicts in address use, some
sort of authority is needed• It makes sense to assign addresses in
blocks, not one at a time• ICANN (Internet Corporation for
Assigned names and Numbers) oversees IP address assignment
• Originally assigned in blocks of Class A, B and C addresses
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Question• How many Internet addresses can one
host have?• As many as it has network adapters
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IPv6 Addressing• Each address is 128 bits• Enough addresses for every
person on earth to have an internet with three times the addresses of the present Internet!
• 1024 addresses per square meter of the earth’s surface
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IPv6 Address Assignments
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IPv4 to IPv6 Transition
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IPv6 Address Split
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IP Addresses
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Functions of Internet Addresses
• Provide a unique identification for a particular interface between a device and the network so that a datagram can be delivered to the correct recipient
• Enable a path to be found across the Internet to reach the recipient, a process called routing
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IPv4 Address• 32-bit integer, unique for each
host on the network, used in all communication with the host
• <IP address> ::= <netid> <hostid>– Netid: identifier of a network– Hostid: identifier of a host on the
network
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Dotted Decimal Notation
32-bit Internet address
10000000 00001010 00000010 00011110
Is written
128.10.2.30
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Classes of IPv4 Addresses
“Classful” addresses—types A, B and C below
first 2 bits distinguish 3 primary classes
Design of these classes is for efficient routing
There have been other refinements—to discuss later
Class Determination Algorithm
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Address Class Characteristics
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Host Capacities
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IP Address Split
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Advantages of Classful Addressing
• Simplicity and clarity—addresses and their setup are very easy to understand
• Flexibility to accommodate different sizes of networks
• Ease of separating host address for routing
• Allows for reservation of some addresses for special purposes
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Classless IPv4 Addressing• Temporary addressing scheme
that does away with class A, B, C addresses
• Network prefix can be any specified length
• Forwarding techniques expanded to account for this: called Classless Inter-Domain Routing (CIDR)
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Special IP Addresses
ADDRESS RESOLUTION
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IP and MAC Addresses• IP address is used to send datagrams
across the Internet—that is, between networks, through routers
• MAC address is used to deliver a frame of data within a single network
• We send a datagram across the Internet with only an IP address
• To deliver to a device at the destination network, a MAC address must be used
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Local Delivery• The router at the destination
network has the job of delivering the packet to the appropriate host
• The router uses the local physical network to deliver to the local host
• The local physical (MAC) address must be used, not the IP address
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Address Resolution• The process of determining the
physical address that corresponds to an IP address is called address resolution
• Address resolution must occur at every network the packet encounters in its journey across the Internet
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Resolution by Direct Mapping
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HA = f(IA)
Resolution by Dynamic Binding
• ARP broadcasts a request packet• Host who has IP address in packet
replies with physical address
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ARP Caching• Broadcasting an inquiry is
expensive• Every host must have a cache of
recently acquired bindings• Results of ARP requests are
cached• Before sending request, the cache
is checked
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ARP Cache Timeout• Responsibility for cache
correctness is with the host maintaining the cache
• Timeout value is set, and addresses from cache are not used if timeout value is exceeded
• ARP performance is sensitive to the value of the timeout
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Soft State• ARP cache is an example of “soft
state”• Cache owner keeps record of
acquired results, avoids cost of future inquiries
• Cache is usually timed out to automatically remove stale values
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ARP Refinements• Every ARP request has binding of
source IP and MAC addresses• Since request is broadcast, all
machines can extract sender’s IP to MAC address mapping and cache it
• Most computers broadcast a gratuitous ARP request when they start up in case their mapping has changed 35
IPv6 ARP• Describe IPv6 ARP• There isn’t any!!!
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IPv4 ARP Message Format
Hardware type: 1 for EthernetProtocol type: 0800 for IP addressesXlen—length of physical and high-level addressesARP exchanges involve filling in missing addresses
IPv6 Neighbor Discovery• Neighbor is another computer on
the same network• NDP allows an IPv6 host to
discover all neighbors and routers upon startup
• Early binding avoids delays when packets are transmitted
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Conventions• 1’s refer to “all”• 0’s refer to “this”• Hostid=0 address refers to this
network• Hostid=1’s broadcast to all hosts
– Directed broadcast—netid of a specific network
– Local broadcast—32 1’s—used at startup
Address Resolution• Physical addresses are used by
hardware devices that can communicate directly (ie, MAC addresses on a LAN)
• IP addresses create a massive virtual network
• Network layer sends datagrams across the virtual network
• Data link layer sends frames between physical devices
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Need for Address Resolution
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ARP• Each host has an Internet address,
Ia• Each host also has a physical
address, Pa • How to route packet to physical
address, given its Internet address?
• Two instances– Sending packets to routers, which
have physical addresses– Sending packets to hosts, which have
physical addresses
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Direct Mapping Resolution• Choose a numbering scheme that
makes address resolution efficientPA = f(IA)
• If either P’s or I’s can be chosen, a correspondence can be established
• Alternatively, lists of P-A pairs can be stored
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Dynamic Binding Resolution
ARP—Address Resolution Protocol• host A broadcasts packet with
address IB• Asks host B to respond with PB
• B recognizes the packet, responds with PB
• A receives response, uses PB to send to B
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ARP Cache• Cache of recently-acquired
physical addresses is kept• ARP is used for packets after the
first in a transmission• ARP cache times out after an
interval• Example of “soft state”
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ARP Refinements• Sender includes its own I to P
binding in every ARP broadcast, so that receiving site, and others, can update ARP caches
• Receivers update I to P binding in ARP cache before processing ARP packet
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ARP Encapsulation
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Internet Protocol Principles • Use the Good Citizen Principle to
limit the impact of resource shortages
• Conserve the scarcest Internet resource
• Use caching to avoid repeated inquiries
Summary• IP Addressing• IP Address Resolution
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