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COS 420 . Day 3. Agenda. Assignment Due Jan 29, 2003 Next Class Individual Projects assigned Today. 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

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COS 420

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Cos 420

COS 420

Day 3


Agenda

Agenda

  • Assignment Due Jan 29, 2003

    • Next Class

  • Individual Projects assigned Today


Individualized project

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


Classful internet addresses

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


Internet protocol address ip address

Internet Protocol Address(IP Address)

  • Analogous to hardware address

  • Unique value assigned as unicast address to each host on Internet

  • Used by Internet applications


Ip address details

IP Address Details

  • 32-bit binary value

  • Unique value assigned to each host in Internet

  • Values chosen to make routing efficient


Ip address division

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


Classful addressing

Classful Addressing

  • Original IP scheme

  • Explains many design decisions

  • New schemes are backward compatible


Desirable properties of an internet addressing scheme

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


Division of internet address into prefix and suffix

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


Original ipv4 address classes

Original IPv4 Address Classes


Important property

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


Endpoint identification

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.


Ip address conventions

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


Assignment of ip addresses

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


Advantages of classful addressing

Advantages Of Classful Addressing

  • Computationally efficient

    • First bits specify size of prefix / suffix

  • Allows mixtures of large and small networks


Directed broadcast

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.


Limited broadcast

Limited Broadcast

  • All 1’s

  • Broadcast limited to local network only (no forwarding)

  • Useful for bootstrapping


All zeros ip address

All Zeros IP Address

  • Can only appear as source address

  • Used during bootstrap before computer knows its address

  • Means ‘‘this’’ computer


Internet multicast

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


Consequences of ip addressing

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


Multi homed hosts and reliability

Multi-Homed Hosts And Reliability

  • Knowing that B is multi-homed increases reliability


Dotted decimal notation

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)


Example of dotted decimal notation

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


Loopback address

Loopback Address

  • Used for testing

  • Refers to local computer (never sent to Internet)

  • Address is 127.0.0.1


Classful address ranges

Classful Address Ranges


Summary of address conventions

Summary Of Address Conventions


Example of ip addressing

Example Of IP Addressing

  • Assume an organization has three networks

  • Organization obtains three prefixes, one per network

  • Host address must begin with network prefix


Illustration of ip addressing

Illustration Of IP Addressing


Summary

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


Part v

PART V

MAPPING INTERNET ADDRESSES

TO PHYSICAL ADDRESSES

(ARP)


Motivation

Motivation

  • Must use hardware (physical) addresses to communicate over network

  • Applications only use Internet addresses


Example

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


Consequence

Consequence

  • Protocol software needs a mechanism that maps an IP address to equivalent hardware address

  • Known as address resolution problem


Address resolution

Address Resolution

  • Performed at each step along path through Internet

  • Two basic algorithms

    • Direct mapping

    • Dynamic binding

  • Choice depends on type of hardware


Direct mapping

Direct Mapping

  • Easy to understand

  • Efficient

  • Only works when hardware address is small

  • Technique: assign computer an IP address that encodes the hardware address


Example of direct mapping

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


Dynamic binding

Dynamic Binding

  • Needed when hardware addresses are large (e.g., Ethernet)

  • 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


Internet address resolution protocol arp

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


Cos 420

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


Illustration of arp request and reply messages

Illustration Of ARPRequest And Reply Messages


Arp packet format when used with ethernet

ARP Packet Format WhenUsed With Ethernet


Observations about packet format

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


Retention of bindings

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


Arp caching

ARP Caching

  • ARP table is a cache

  • Entries time out and are removed

  • Avoids stale bindings

  • Typical timeout: 20 minutes


Algorithm for processing arp requests

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


Algorithm features

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


Conceptual purpose of arp

Conceptual Purpose Of ARP

  • Isolates hardware address at low level

  • Allows application programs to use IP addresses


Arp encapsulation

ARP Encapsulation

  • ARP message travels in data portion of network frame

  • We say ARP message is encapsulated


Illustration of arp encapsulation

Illustration Of ARP Encapsulation


Ethernet encapsulation

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


Summary1

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


Part vi

PART VI

DETERMINING AN INTERNET

ADDRESS AT STARTUP (RARP)


Ip address assignment

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


Reverse address resolution protocol rarp

Reverse Address ResolutionProtocol (RARP)

  • Old protocol

  • Designed for diskless computer

  • Obtains an IP address

  • Adapted from ARP

  • Broadcasts request to server

  • Waits for response


Ethernet encapsulation1

Ethernet Encapsulation

  • RARP message carried in data portion of Ethernet frame

  • Ethernet type 0x0835 assigned to RARP


Illustration of packet flow

Illustration Of Packet Flow

  • In (a) client broadcasts a request

  • In (b) one or more servers respond


Client identification

Client Identification

  • Computer must identify itself

  • RARP uses network hardware address as unique ID

    • Only works on network with permanent address (e.g., Ethernet)


Modern bootstrap

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


For next week

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


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