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Transcript of COS 420
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COS 420
Day 3
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Agenda Assignment Due Jan 29, 2003
Next Class Individual Projects assigned
Today
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Individualized project Will be a research project & paper
~ 20 page paper MLA Format
10 Min Presentation You can pick any topic that one of the
IETF working groups is developing http://www.ietf.org/html.charters/wg-dir.html Past history, Current state, upcoming
developments Due Date
Papers due March 15 Presentations will be on March 22
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CLASSFUL INTERNET ADDRESSES Definitions
Name Identifies what an entity is Often textual (e.g., ASCII)
Address Identifies where an entity is located Often binary and usually compact Sometimes called locator
Route Identifies how to get to the object May be distributed
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Internet Protocol Address(IP Address) Analogous to hardware address Unique value assigned as unicast
address to each host on Internet Used by Internet applications
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IP Address Details 32-bit binary value Unique value assigned to each
host in Internet Values chosen to make routing
efficient
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IP Address Division Address divided into two parts
Prefix (network ID) identifies network to which host attaches
Suffix (host ID) identifies host on that network
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Classful Addressing Original IP scheme Explains many design decisions New schemes are backward
compatible
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Desirable Properties Of AnInternet Addressing Scheme Compact (as small as possible) Universal (big enough) Works with all network hardware Supports efficient decision making
Test whether a destination can be reached directly
Decide which router to use for indirect delivery Choose next router along a path to the
destination
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Division Of Internet AddressInto Prefix And Suffix How should division be made?
Large prefix, small suffix means many possible networks, but each is limited in size
Large suffix, small prefix means each network can be large, but there can only be a few networks
Original Internet address scheme designed to accommodate both possibilities Known as classful addressing
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Original IPv4 Address Classes
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Important Property Classful addresses are self-
identifying Consequences
Can determine boundary between prefix and suffix from the address itself
No additional state needed to store boundary information
Both hosts and routers benefit
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Endpoint Identification Because IP addresses encode both
a network and a host on that network, they do not specify an individual computer, but a connection to a network.
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IP Address Conventions When used to refer to a network
Host field contains all 0 bits Broadcast on the local wire
Network and host fields both contain all 1 bits Directed broadcast: broadcast on specific
(possibly remote) network Host field contains all 1 bits Nonstandard form: host field contains all 0 bits
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Assignment Of IP Addresses All hosts on same network assigned
same address prefix Prefixes assigned by central authority Obtained from ISP
Each host on a network has a unique suffix Assigned locally Local administrator must ensure uniqueness
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Advantages Of Classful Addressing Computationally efficient
First bits specify size of prefix / suffix Allows mixtures of large and small
networks
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Directed Broadcast IP addresses can be used to specify
a directed broadcast in which a packet is sent to all computers on a network; such addresses map to hardware broadcast, if available. By convention, a directed broadcast address has a valid netid and has a hostid with all bits set to 1.
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Limited Broadcast All 1’s Broadcast limited to local network
only (no forwarding) Useful for bootstrapping
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All Zeros IP Address Can only appear as source address Used during bootstrap before
computer knows its address Means ‘‘this’’ computer
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Internet Multicast IP allows Internet multicast, but no
Internet-wide multicast delivery system currently in place
Class D addresses reserved for multicast Each address corresponds to group of
participating computers IP multicast uses hardware multicast
when available More later in the course
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Consequences Of IP Addressing If a host computer moves from one
network to another, its IP address must change
For a multi-homed host (with two or more addresses), the path taken by packets depends on the address used
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Multi-Homed Hosts And Reliability
Knowing that B is multi-homed increases reliability
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Dotted Decimal Notation Syntactic form for expressing 32-
bit address Used throughout the Internet and
associated literature Represents each octet in decimal
separated by periods (dots)
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Example Of Dotted DecimalNotation A 32-bit number in binary
10000000 00001010 00000010 00000011
The same 32-bit number expressed in dotted decimal notation 128 . 10 . 2 . 3
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Loopback Address Used for testing Refers to local computer (never
sent to Internet) Address is 127.0.0.1
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Classful Address Ranges
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Summary Of Address Conventions
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Example Of IP Addressing Assume an organization has three
networks Organization obtains three
prefixes, one per network Host address must begin with
network prefix
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Illustration Of IP Addressing
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Summary IP address
32 bits long Prefix identifies network Suffix identifies host
Classful addressing uses first few bits of address to determine boundary between prefix and suffix Special forms of addresses handle
Limited broadcast Directed broadcast Network identification This host Loopback
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COS 420
DAY 4
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Agenda Assignment 1 Due Assignment 2 will be posted
over the weekend Grading for Individual Projects ARP and RARP
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Grading for Individualized Projects Rubric
Paper Quality of research 30% Quality of Paper 30% Adherence to MLA format 10%
Presentation Apparent mastery of material 10% Value of PowerPoint 10% Oral effectiveness10%
Grading will be done by both students and instructor 50% of Grade is average of your fellow students assessment 50% of Grade is instructor’s assessment Both use the same rubric for grading Students will only see final total
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PART V
MAPPING INTERNET ADDRESSES
TO PHYSICAL ADDRESSES(ARP)
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Motivation Must use hardware (physical)
addresses to communicate over network
Applications only use Internet addresses
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Example Computers A and B on same
network Application on A generates packet
for application on B Protocol software on A must use
B’s hardware address when sending a packet
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Consequence Protocol software needs a
mechanism that maps an IP address to equivalent hardware address
Known as address resolution problem
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Address Resolution Performed at each step along path
through Internet Two basic algorithms
Direct mapping Dynamic binding
Choice depends on type of hardware
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Direct Mapping Easy to understand Efficient Only works when hardware
address is small (smaller than Host Portion of Internet Address)
Technique: assign computer an IP address that encodes the hardware address
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Example Of Direct Mapping Hardware: proNet ring network Hardware address: 8 bits Assume IP address 192.5.48.0 (24-bit
prefix) Assign computer with hardware address K
an IP address 192.5.48.K Resolving an IP address means extracting
the hardware address from low-order 8 bits
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Dynamic Binding Needed when hardware addresses are large
(e.g., Ethernet) 48 bits MAC to 32 bit IP
Allows computer A to find computer B’s hardware address
A starts with B’s IP address A knows B is on the local network
Technique: broadcast query and obtain response
Note: dynamic binding only used across one network at a time
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Internet Address Resolution Protocol (ARP) Standard for dynamic address
resolution in the Internet Requires hardware broadcast Intended for LAN Important idea: ARP only used to
map addresses within a single physical network, never across multiple networks
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ARP Machine A broadcasts ARP request with
B’s IP address All machines on local net receive
broadcast Machine B replies with its physical
address Machine A adds B’s address information
to its table Machine A delivers packet directly to B
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Illustration Of ARPRequest And Reply Messages
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ARP Packet Format WhenUsed With Ethernet
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Observations About Packet Format General: can be used with
Arbitrary hardware address Arbitrary protocol address (not just IP)
Variable length fields (depends on type of addresses)
Length fields allow parsing of packet by computer that does not understand the two address types
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Retention Of Bindings Cannot afford to send ARP request
for each packet Solution
Maintain a table of bindings Effect
Use ARP one time, place results in table, and then send many packets
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ARP Caching ARP table is a cache Entries time out and are removed Avoids stale bindings Typical timeout: 20 minutes
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Algorithm For ProcessingARP Requests Extract sender’s pair, (IA, EA) and
update local ARP table if it exists If this is a request and the target is
‘‘me’’ Add sender’s pair to ARP table if not present Fill in target hardware address Exchange sender and target entries Set operation to reply Send reply back to requester
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Algorithm Features If A ARPs B, B keeps A’s
information B will probably send a packet to A
soon If A ARPs B, other machines do not
keep A’s information Avoids clogging ARP caches
needlessly
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Conceptual Purpose Of ARP Isolates hardware address at low
level Allows application programs to use
IP addresses
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ARP Encapsulation ARP message travels in data
portion of network frame We say ARP message is
encapsulated
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Illustration Of ARP Encapsulation
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Ethernet Encapsulation ARP message placed in frame data
area Data area padded with zeroes if
ARP message is shorter than minimum Ethernet frame Ethernet type 0x0806 used for ARP
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Arp at WORK
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Summary Computer’s IP address independent of computer’s
hardware address Applications use IP addresses Hardware only understands hardware addresses Must map from IP address to hardware address
fortransmission Two types
Direct mapping Dynamic mapping
Address Resolution Protocol (ARP) used for dynamic address mapping
Important for Ethernet Sender broadcasts ARP request, and target sends ARP
reply ARP bindings are cached
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PART VI
DETERMINING AN INTERNETADDRESS AT STARTUP (RARP)
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IP Address Assignment For conventional computer
IP address stored on disk OS obtains address by reading from
file at startup For diskless computer
IP address obtained from server
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Reverse Address ResolutionProtocol (RARP) Old protocol Designed for diskless computer Obtains an IP address Adapted from ARP Broadcasts request to server Waits for response
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Ethernet Encapsulation RARP message carried in data
portion of Ethernet frame Ethernet type 0x0835 assigned to
RARP
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Illustration Of Packet Flow
In (a) client broadcasts a request In (b) one or more servers respond
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Client Identification Computer must identify itself RARP uses network hardware
address as unique ID Only works on network with
permanent address (e.g., Ethernet)
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Modern Bootstrap Except for a few special cases,
RARP has largely been replaced by DHCP
We will postpone further discussion of bootstrapping until later in the course when we can consider DHCP
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For next week Assignment #2 will be posted We begin a more in depth look at
IP IP Architecture IP routing IP error and control messaging