W4140 Network Laboratory Lecture 3 Sept 18 - Fall 2006 Shlomo Hershkop Columbia University

W4140 Network Laboratory

Lecture 3Sept 18 - Fall 2006

Shlomo HershkopColumbia University

Announcements

Lab division I will be updating the webpage with lab groups, if everyone in the room would like

to move the lab around a little, that is ok Labs reports

are due generally when the next lab starts…contact me if you need more time. Single report per group, zipped and uploaded to TA through courseworks….include relevant information and name the files using some logical system. README should include everyone’s names and cunix ID

Lab pre-work/post courseworks will have the prelabs, which need to be completed BEFORE your

lab starts…individual work….post labs are to be submitted online (courseworks) by beginning of next lab (or earlier please)….

Lab 1 pre work was not collected, practice one….

reading list: lab 2 chapter 2 (see resources)

The Evolution of Internet

Introductory material.

An overview lecture that covers Internet related topics, including a definition of the Internet, an overview of its history and growth, and standardization and naming.

A Definition

On October 24, 1995, the FNC unanimously passed a resolution defining the term Internet. •RESOLUTION: The Federal Networking Council (FNC) agrees that the following language reflects our definition of the term "Internet". "Internet" refers to the global information system that --

•(i) is logically linked together by a globally unique address space based on the Internet Protocol (IP) or its subsequent extensions/follow-ons;

•(ii) is able to support communications using the Transmission Control Protocol/Internet Protocol (TCP/IP) suite or its subsequent extensions/follow-ons, and/or other IP-compatible protocols; and

•(iii) provides, uses or makes accessible, either publicly or privately, high level services layered on the communications and related infrastructure described herein.

Internet History

1961: Kleinrock - queueing theory shows effectiveness of packet-switching

1964: Baran - packet-switching in military nets

1967: ARPAnet conceived by Advanced Research Projects Agency

1969: first ARPAnet node operational

1972: ARPAnet demonstrated

publicly NCP (Network Control

Protocol) first host-host protocol

first e-mail program ARPAnet has 15 nodes

1961-1972: Early packet-switching principles

Internet History

1970: ALOHAnet satellite network in Hawaii

1973: Metcalfe’s PhD thesis proposes Ethernet

1974: Cerf and Kahn - architecture for interconnecting networks

late70’s: proprietary architectures: DECnet, SNA, XNA

late 70’s: switching fixed length packets (ATM precursor)

1979: ARPAnet has 200 nodes

Cerf and Kahn’s internetworking principles: minimalism, autonomy - no

internal changes required to interconnect networks

best effort service model stateless routers decentralized control

define today’s Internet architecture

1972-1980: Internetworking, new and proprietary nets

Internet History

Early 1990’s: ARPAnet decommissioned

1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)

early 1990s: Web hypertext [Bush 1945, Nelson

1960’s] HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990’s:

commercialization of the Web

Late 1990’s – 2000’s: more killer apps:

instant messaging, P2P file sharing

network security to forefront

est. 50 million host, 100 million+ users

backbone links running at Gbps

1990, 2000’s: commercialization, the Web, new apps

Applications of the Internet

Traditional core applications:EmailNewsRemote LoginFile Transfer

The killer application:World-Wide Web (WWW), P2P

Future applications:Videoconferencing and TelephonyMultimedia ServicesInternet Broadcast

Growth of the Internet

Source: Internet Software Consortium

Internet Infrastructure

local ISP

campusnetwork

corporatenetwork

IXP

RegionalNetwork

RegionalNetwork

local ISP

local ISP

IXP

IXP

Backbone Network

Backbone Network

RegionalNetwork

RegionalNetwork


The infrastructure of the Internet consists of a federation of connected networks that are each independently managed (“autonomous system”) Note: Each “autononmous system may consist of

multiple IP networks Hierarchy of network service providers

Tier-1: nation or worldwide network (US: less than 20) Tier-2: regional networks (in US: less than 100) Tier-3: local Internet service provider (in US: several

thousand)


Location where a network (ISP, corporate network, or regional network) gets access to the Internet is called a Point-of-Presence (POP).

Locations (Tier-1 or Tier-2) networks are connected for the purpose of exchanging traffic are called peering points. Public peering: Traffic is swapped in a specific

location, called Internet exchange points (IXPs) Private peering: Two networks establish a direct link

to each other.

Tier-1 ISP: e.g., Sprint

Sprint US backbone network

Who is Who on the Internet ?

Internet Society (ISOC): Founded in 1992, an international nonprofit professional organization that provides administrative support for the Internet. Founded in 1992, ISOC is the organizational home for the standardization bodies of the Internet.

Internet Engineering Task Force (IETF): Forum that coordinates the development of new protocols and standards. Organized into working groups that are each devoted to a specific topic or protocol. Working groups document their work in reports, called Request For Comments (RFCs).

IRTF (Internet Research Task Force): The Internet Research Task Force is a composed of a number of focused, long-term and small Research Groups.

Internet Architecture Board (IAB): a technical advisory group of the Internet Society, provides oversight of the architecture for the protocols and the standardization process

The Internet Engineering Steering Group (IESG): The IESG is responsible for technical management of IETF activities and the Internet standards process. Standards. Composed of the Area Directors of the IETF working groups.

Internet Standardization Process

Working groups present their work i of the Internet are published as RFC (Request for Comments).

RFCs are the basis for Internet standards. Not all RFCs become Internet Standards ! (There are

>3000 RFCs and less than 70 Internet standards A typical (but not only) way of standardization is:

Internet Drafts RFC Proposed Standard Draft Standard (requires 2 working implementation) Internet Standard (declared by IAB)

Assigning Identifiers for the Internet

Who gives University the domain name “netlab.edu” and who assigns it the network prefix “128.143.0.0/16”? Who assigns port 80 as the default port for web servers?

The functions associated with the assignment of numbers is referred to as Internet Assigned Number Authority (IANA).

Early days of the Internet: IANA functions are administered by a single person (Jon Postel).

Today: Internet Corporation for Assigned Names and Numbers (ICANN)

assumes the responsibility for the assignment of technical protocol parameters, allocation of the IP address space, management of the domain name system, and others.

Management of IP address done by Regional Internet Registries (RIRs): APNIC (Asia Pacific Network Information Centre) RIPE NCC (Réseaux IP Européens Network Coordination Centre) ARIN (American Registry for Internet Numbers)

Domain names are administered by a large number of private organizations that are accredited by ICANN.

Summary

Layered Internet architecture Reduce complexity Higher layer views lower layer as service provider Application layer, transport layer, network layer, and

link layer

IP Addressing

Next:

IP addressing

Data link protocols and ARP

Notes about lab

IP Addressing

Addressing defines how addresses are allocated and the structure of addresses

IPv4 Classful IP addresses (obsolete) Classless inter-domain routing (CIDR) (RFC

854, current standard)

IP Version 6 addresses

What is an IP Address?

Why Addresses?

End-to-end argument (principle) Reading:

http://web.mit.edu/Saltzer/www/publications/endtoend/endtoend.pdf

Keep it Simple, Stupid

What is an IP Address?

An IP address is a unique global address for a network interface.

An IP address uniquely identifies a network location. http://www.arin.net/whois http://www.iana.org/ipaddress/ip-addresses.htm

Routers forwards a packet based on the destination address of the packet.

IPv4 Addresses

Application dataTCP HeaderEthernet Header Ethernet Trailer

Ethernet frame

IP Header

version(4 bits)

headerlength

Type of Service/TOS(8 bits)

Total Length (in bytes)(16 bits)

Identification (16 bits)flags

(3 bits)Fragment Offset (13 bits)

Source IP address (32 bits)

Destination IP address (32 bits)

TTL Time-to-Live(8 bits)

Protocol(8 bits)

Header Checksum (16 bits)

32 bits

IP v.4 Addresses


Ethernet frame

IP Header

0x4 0x5 0x00 4410

9d08 0102 00000000000002

128.143.137.144

128.143.71.21

12810 0x06 8bff

32 bits

IP v.4 Addressing

An IP address is often written in dotted decimal notation

Each byte is identified by a decimal number in the range [0..255]:

1000111110000000 10001001 10010000

1st Byte

= 128

2nd Byte

= 143

3rd Byte

= 137

4th Byte

= 144

128.143.137.144

Structure of an IP address

network prefixnetwork prefix host numberhost number

An IP address encodes both a network number (network prefix) and an interface number (host number). network prefix identifies a network the host number identifies a specific host

(actually, interface on the network).

0 31

How long the network prefix is?

Before 1993: The network prefix is implicitly defined (class-based addressing)

After 1993: The network prefix is indicated by a netmask.

Before 1993: Class-based addressing

The Internet address space was divided up into classes: Class A: Network prefix is 8 bits long

Class B: Network prefix is 16 bits long

Class C: Network prefix is 24 bits long Class D is multicast address Class E is reserved

Classful IP Adresses (Until 1993)

Each IP address contained a key which identifies the class: Class A: IP address starts with “0”

Class B: IP address starts with “10”

Class C: IP address starts with “110” Class D: IP address starts with “1110” Class E: IP address starts wit “11110”

The old way: Internet Address Classes

Class C network id host11 0

Network Prefix24 bits

Host Number8 bits

bit # 0 1 23 242 313

Class B 1 network id host

bit # 0 1 15 162

Network Prefix16 bits

Host Number16 bits

031

Class A 0Network Prefix

8 bits

bit # 0 1 7 8

Host Number24 bits

31

Class D multicast group id11 1bit # 0 1 2 313

04

Class E (reserved for future use)11 1bit # 0 1 2 313

14

05



Class Leading bits Start End CIDR equivalent

Class A 0 0.0.0.0 127.255.255.255 /8

Class B 10 128.0.0.0 191.255.255.255 /16

Class C 110 192.0.0.0 223.255.255.255 /24

Class D (multicast) 1110 224.0.0.0 239.255.255.255 NA

Class E (reserved) 1111 240.0.0.0 255.255.255.255 NA

Problems with Classful IP Addresses

Fast growing routing table size Each router must have an entry for every network prefix ~ 221 = 2,097,152 class C networks In 1993, the size of routing tables started to outgrow the

capacity of routers

Other problems with classful addresses

Address depletion for large networks Class A and Class B addresses were gone

How many class A/B network prefixes can there be?

Limited flexibility for network addresses: Class A and B addresses are overkill (>64,000 addresses) Class C address is insufficient (256 addresses)

Classless Inter-domain routing (CIDR) 1993

Full description RFC 1518 & 1519

Network prefix is of variable length

Addresses are allocated hierarchically

Routers aggregate multiple address prefixes into one routing entry to minimize routing table size

CIDR network prefix is variable length

A network mask specifies the number of bits used to identify a network in an IP address.

How?

1000111110000000 10001001 10010000

1111111111111111 1111111 00000000

128 59 16 144

255 255 255 0

Addr

Mask

CIDR notation

CIDR notation of an IP address: 128.143.137.144/24 /24 is the prefix length. It states that the first 24 bits are the

network prefix of the address (and the remaining 8 bits are available for specific host addresses)

CIDR notation can nicely express blocks of addresses An address block

[128.195.0.0, 128.195.255.255] can be represented by an address prefix 128.195.0.0/16

How many addresses are there in a /x address block? 2 (32-x)

CIDR hierarchical address allocation

IP addresses are hierarchically allocated. An ISP obtains an address block from a Regional Internet Registry An ISP allocates a subdivision of the address block to an organization An organization recursively allocates subdivision of its address block to

its networks A host in a network obtains an address within the address block assigned

to the network

ISP128.0.0.0/8

128.1.0.0/16

Foo.com

128.2.0.0/16

Library CS

128.59.0.0/16

128.59.44.0/24 128.59.16.0/24

University

Bar.com

128.59.16.150

Hierarchical address allocation

ISP obtains an address block 128.0.0.0/8 [128.0.0.0, 128.255.255.255] ISP allocates 128.59.0.0/16 ([128.59.0.0, 128.59.255.255]) to the

university. University allocates 128.59.16.0/24 ([128.59.16.0, 128.59.16.255]) to the

CS department’s network A host on the CS department’s network gets one IP address

128.59.16.150

128.0.0.0 - 128.255.255.255

128.59.0.0 – 128.59.255.255

128.59.16.[0 – 255]128.59.16.150

CIDR allows route aggregation

ISP1 announces one address prefix 128.0.0.0./8 to ISP2 ISP2 can use one routing entry to reach all networks

connected to ISP1

ISP1128.0.0.0/8

128.1.0.0/16

Foo.com

128.2.0.0/16

Library CS

128.59.0.0/16

UniversityBar.com

IISP3

You can reach 128.0.0.0/8 via ISP1

128.0.0.0/8 ISP1

CIDR summary

A network prefix is of variable length: a.b.c.d/x

Addresses are hierarchical allocated

Routers aggregate multiple address prefixes into one routing entry to minimize routing table size.

Security is still an issue Secure Routing & Path validation

What problems CIDR does not solve (I)

An multi-homing site still adds one entry into global routing tables

Mutil-home.com

128.0.0.0/8204.0.0.0/8

204.1.0.0/16

ISP2 ISP1

You can reach 128.0.0.0/8And 204.1.0.0/16 via ISP1

ISP3

204.1.0.0/16 ISP1204.1.0.0/16128.0.0.0/8 ISP1

What problems CIDR does not solve (II)

A site switches provider without renumbering still adds one entry into global routing tables

Switched.com

128.0.0.0/8204.0.0.0/8

204.1.0.0/16

204.1.0.0/16

ISP2 ISP1

You can reach 128.0.0.0/8And 204.1.0.0/16 via ISP1

ISP3

128.0.0.0/8 ISP1

204.1.0.0/16 ISP1

Global routing tables continue to grow

Source: http://bgp.potaroo.net/as4637/

Special IPv4 Addresses

Reserved or (by convention) special addresses: Loopback interfaces

all addresses 127.0.0.1-127.255.255.255 are reserved for loopback interfaces

Most systems use 127.0.0.1 as loopback address loopback interface is associated with name “localhost”

Broadcast address Host number is all ones, e.g., 128.143.255.255 Broadcast goes to all hosts on the network Often ignored due to security concerns

Test / Experimental addresses 10.0.0.0 - 10.255.255.255 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255

Convention (but not a reserved address) Default gateway has host number set to ‘1’, e.g., 128.195.4.1

Special IPv4 Addresses (RFC 3330)

AddressesCIDR Equivalent

Purpose RFC Class# of addresses

0.0.0.0 - 0.255.255.255 0.0.0.0/8 Zero Addresses RFC 1700 A 16,777,216

10.0.0.0 - 10.255.255.255 10.0.0.0/8 Private IP addresses RFC 1918 A 16,777,216

127.0.0.0 - 127.255.255.255 127.0.0.0/8

Localhost Loopback Address

RFC 1700 A 16,777,216

169.254.0.0 - 169.254.255.255 169.254.0.0/16 Zeroconf RFC 3330 B 65,536

172.16.0.0 - 172.31.255.255 172.16.0.0/12 Private IP addresses RFC 1918 B 1,048,576

192.0.2.0 - 192.0.2.255 192.0.2.0/24Documentation and Examples

RFC 3330 C 256

192.88.99.0 - 192.88.99.255 192.88.99.0/24

IPv6 to IPv4 relay Anycast

RFC 3068 C 256

192.168.0.0 - 192.168.255.255 192.168.0.0/16 Private IP addresses RFC 1918 C 65,536

198.18.0.0 - 198.19.255.255 198.18.0.0/15

Network Device Benchmark

RFC 2544 C 131,072

224.0.0.0 - 239.255.255.255 224.0.0.0/4 Multicast RFC 3171 D 268,435,456

240.0.0.0 - 255.255.255.255 240.0.0.0/4 Reserved RFC 1700 E 268,435,456

IP Addressing (Summary)

Addressing defines how addresses are allocated and the structure of addresses

IPv4 Classful IP addresses (obsolete) Classless inter-domain routing (CIDR) (current

standard)

IP Version 6 addresses

IPv6 - IP Version 6

IP Version 6 Designed to be the successor to the currently used IPv4 Specification completed in 1994 Makes improvements to IPv4 (no revolutionary changes)

One (not the only !) feature of IPv6 is a significant increase in of the IP address to 128 bits (16 bytes)

IPv6 will solve – for the foreseeable future – the problems with IP addressing

1024 addresses per square inch on the surface of the Earth.

IPv6 Header


Ethernet frame

IPv6 Header

version(4 bits)

Traffic Class(8 bits)

Flow Label(24 bits)

Payload Length (16 bits)Next Header

(8 bits)Hop Limits (8 bits)

Source IP address (128 bits)

32 bits

Destination IP address (128 bits)

Notation of IPv6 addresses

Convention: The 128-bit IPv6 address is written as eight 16-bit integers (using hexadecimal digits for each integer)

CEDF:BP76:3245:4464:FACE:2E50:3025:DF12

Short notation:Abbreviations of leading zeroes:CEDF:BP76:0000:0000:009E:0000:3025:DF12 CEDF:BP76:0:0:9E :0:3025:DF12 “:0000:0000:0000” can be written as “::”CEDF:BP76:0:0:FACE:0:3025:DF12 CEDF:BP76::FACE:0:3025:DF12

IPv4 address in IPv6

IPv6 addresses derived from IPv4 addresses have 96 leading zero bits.

Convention allows to use IPv4 notation for the last 32 bits.::80:8F:89:90 ::128.143.137.144

IPv6 vs. IPv4: Address Comparison

IPv4 has a maximum of 232 4 billion addresses

IPv6 has a maximum of 2128 = (232)4

4 billion x 4 billion x 4 billion x 4 billion addresses

Is IPv6 widely deployed?

Data Link Layer

The main tasks of the data link layer are: Transfer data from the network layer of one machine to the

network layer of another machine Convert the raw bit stream of the physical layer into groups

of bits (“frames”)

NetworkLayer

Data LinkLayer

PhysicalLayer

NetworkLayer

Data LinkLayer

PhysicalLayer

TCP/IP Protocol Stack

ApplicationLayer

TransportLayer

NetworkLayer

(Data) LinkLayer

The TCP/IP protocol stack runs on top of multiple data link layers.

Two data link layer technologies

•Broadcast

•Point-to-Point

Logical LinkControl (LLC)

Media AccessControl (MAC)

Sublayer inLocal AreaNetworks

Two types of networks at the data link layer

Broadcast Networks: All stations share a single communication channel

Point-to-Point Networks: Pairs of hosts (or routers) are directly connected

Typically, local area networks (LANs) are broadcast and wide area networks (WANs) are point-to-point

Broadcast Network Point-to-Point Network

Local Area Networks

Local area networks (LANs) connect computers within a building or a enterprise network

Almost all LANs are broadcast networks Typical topologies of LANs are bus or ring or star We will work with Ethernet LANs. Ethernet has a bus or

star topology.

•Bus LAN •Ring LAN

MAC and LLC

In any broadcast network, the stations must ensure that only one station transmits at a time on the shared communication channel

The protocol that determines who can transmit on a broadcast channel are called Medium Access Control (MAC) protocol

The MAC protocol are implemented in the MAC sublayer which is the lower sublayer of the data link layer

The higher portion of the data link layer is often called Logical Link Control (LLC) Logical Link

Control

Medium AccessControlD

ata

Link

Laye

r

to Physical Layer

to Network Layer

IEEE 802 Standards

IEEE 802 is a family of standards for LANs, which defines an LLC and several MAC sublayers

80

2.3

80

2.4

80

2.5

80

2.1

1

802.2

802.1

IEEE 802 standard

MediumAccessControl

PhysicalLayer

Logical LinkControl

IEEEReference

Model

PhysicalLayer

Data LinkLayer

HigherLayer

Higher layer issues

LLC

CS

MA

/CS

Token

bus

Token

ring

Wireless

lan

Ethernet

Speed: 10Mbps -10 Gbps Standard: 802.3, Ethernet II (DIX)

Most popular physical layers for Ethernet:

10Base5 Thick Ethernet: 10 Mbps coax cable 10Base2 Thin Ethernet: 10 Mbps coax cable 10Base-T 10 Mbps Twisted Pair 100Base-TX 100 Mbps over Category 5 twisted pair 100Base-FX 100 Mbps over Fiber Optics 1000Base-FX 1Gbps over Fiber Optics 10000Base-FX 1Gbps over Fiber Optics (for wide area

links)

Bus Topology

Ethernet

10Base5 and 10xBase2 Ethernets has a bus topology

Starting with 10Base-T, stations are connected to a hub in a star configuration

Star Topology

Hub

Ethernet Hubs vs. Ethernet Switches An Ethernet switch is a packet switch for Ethernet frames

Buffering of frames prevents collisions. Each port is isolated and builds its own collision domain

An Ethernet Hub does not perform buffering: Collisions occur if two frames arrive at the same time.

HighS

peedB

ackplane

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

OutputBuffers

InputBuffers

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

CSMA/CD

Hub Switch

Ethernet and IEEE 802.3: Any Difference?

There are two types of Ethernet frames in use, with subtle differences:

“Ethernet” (Ethernet II, DIX (Digital-Intel-Xerox) An industry standards from 1982 that is based on the first

implementation of CSMA/CD by Xerox. Predominant version of CSMA/CD in the US.

802.3: IEEE’s version of CSMA/CD from 1985. Interoperates with 802.2 (LLC) as higher layer.

Difference for our purposes: Ethernet and 802.3 use different methods to encapsulate an IP datagram.

Ethernet II, DIX Encapsulation (RFC 894)

802.3 MAC

destinationaddress

6

sourceaddress

6

type

2

data

46-1500

CRC

4

0800

2

IP datagram

38-1492

0806

2

ARP request/reply

28

PAD

10

0835

2

RARP request/reply

28

PAD

10

IEEE 802.2/802.3 Encapsulation (RFC 1042)

802.3 MAC

destinationaddress

6

sourceaddress

6

length

2

DSAPAA

1

SSAPAA

1

cntl03

1

org code0

3

type

2

data

38-1492

CRC

4

802.2 LLC 802.2 SNAP

- destination address, source address:MAC addresses are 48 bit

- lengt h : frame length in number of bytes- DSAP, SSAP : always set to 0xaa- Ctrl: set t o 3- org code: set to 0- type field identifies the content of the

data field- CRC: cylic redundancy check

0800

2

IP datagram

38-1492

0806

2

ARP request/reply

28

PAD

10

0835

2

RARP request/reply

28

PAD

10

Dial-Up Access

AccessRouter

Modems

Point-to-Point (serial) links Many data link connections are

point-to-point serial links: Dial-in or DSL access connects hosts to

access routers Routers are connected by

high-speed point-to-point links

Here, IP hosts and routers are connected by a serial cable

Data link layer protocols for point-to-point links are simple: Main role is encapsulation of IP datagrams No media access control needed

Point-to-Point Links

Router

Router

Router Router

Data Link Protocols for Point-to-Point links

SLIP (Serial Line IP) First protocol for sending IP datagrams over dial-up links (from

1988) Encapsulation, not much else

PPP (Point-to-Point Protocol):• Successor to SLIP (1992), with added functionality• Used for dial-in and for high-speed routers

HDLC (High-level Data Link Control) :• Widely used and influential standard (1979)• Default protocol for serial links on Cisco routers• Actually, PPP is based on a variant of HDLC

PPP - IP encapsulation

The frame format of PPP is similar to HDLC and the 802.2 LLC frame format:

PPP assumes a duplex circuit Note: PPP does not use addresses Usual maximum frame size is 1500

7E

flag

1

FF

addr

1

03

ctrl

1 2

protocol

<= 1500

data

2

CRC

7E

flag

1

0021 IP datagram

C021 link control data

8021 network control data

Additional PPP functionality

In addition to encapsulation, PPP supports: multiple network layer protocols (protocol multiplexing) Link configuration Link quality testing Error detection Option negotiation Address notification Authentication

The above functions are supported by helper protocols: LCP PAP, CHAP NCP

PPP Support protocols

Link management: The link control protocol (LCP) is responsible for establishing, configuring, and negotiating a data-link connection. LCP also monitors the link quality and is used to terminate the link.

Authentication: Authentication is optional. PPP supports two authentication protocols: Password Authentication Protocol (PAP) and Challenge Handshake Authentication Protocol (CHAP).

Network protocol configuration: PPP has network control protocols (NCPs) for numerous network layer protocols. The IP control protocol (IPCP) negotiates IP address assignments and other parameters when IP is used as network layer.

Address Resolution Protocol(ARP)

NetworkLayer

Link Layer

IP

ARP NetworkAccess RARP

Media

ICMP IGMP

TransportLayer

TCP UDP

Overview

ARP and RARP

Note: The Internet is based on IP addresses Data link protocols (Ethernet, FDDI, ATM) may have different

(MAC) addresses

The ARP and RARP protocols perform the translation between IP addresses and MAC layer addresses

We will discuss ARP for broadcast LANs, particularly Ethernet LANs

RARP

Ethernet MACaddress(48 bit)

ARPIP address(32 bit)

Processing of IP packets by network device drivers

loopbackDriver

IP Input

Put on IPinput queue

ARPdemultiplex

Ethernet Frame

Ethernet

IP destination of packet= local IP address ?

IP destination = multicastor broadcast ?

IP Output

Put on IPinput queue

No: get MACaddress withARP

ARPPacket

IP datagram

No

Yes

YesEthernet

Driver

Address Translation with ARP

ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of 128.143.137.1?”

Argon 128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Request: What is the MAC address of 128.143.71.1?

Address Translation with ARP

ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address

Argon128.143.137.144

00:a0:24:71:e4:44

Router137128.143.137.1

00:e0:f9:23:a8:20

ARP Reply:The MAC address of 128.143.71.1is 00:e0:f9:23:a8:20

ARP Packet Format

Destinationaddress

6

ARP Request or ARP Reply

28

Sourceaddress

6 2

CRC

4

Type0x8060

Padding

10

Ethernet II header

Hardware type (2 bytes)

Hardware addresslength (1 byte)

Protocol addresslength (1 byte)

Operation code (2 bytes)

Target hardware address*

Protocol type (2 bytes)

Source hardware address*

Source protocol address*

Target protocol address*

* Note: The length of the address fields is determined by the corresponding address length fields

Example

ARP Request from Argon:

Source hardware address: 00:a0:24:71:e4:44Source protocol address: 128.143.137.144Target hardware address: 00:00:00:00:00:00Target protocol address: 128.143.137.1

ARP Reply from Router137:

Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: 128.143.137.1 Target hardware address: 00:a0:24:71:e4:44Target protocol address: 128.143.137.144

ARP Cache

Since sending an ARP request/reply for each IP datagram is inefficient, hosts maintain a cache (ARP Cache) of current entries. The entries expire after a time interval.

Contents of the ARP Cache:(128.143.71.37) at 00:10:4B:C5:D1:15 [ether] on eth0

(128.143.71.36) at 00:B0:D0:E1:17:D5 [ether] on eth0

(128.143.71.35) at 00:B0:D0:DE:70:E6 [ether] on eth0

(128.143.136.90) at 00:05:3C:06:27:35 [ether] on eth1

(128.143.71.34) at 00:B0:D0:E1:17:DB [ether] on eth0

(128.143.71.33) at 00:B0:D0:E1:17:DF [ether] on eth0

Proxy ARP

Proxy ARP: Host or router responds to ARP Request that arrives from one of its connected networks for a host that is on another of its connected networks.

128.143.137.1/1600:e0:f9:23:a8:20

128.143.71.1/24

128.143.0.0/16Subnet

128.143.71.0/24Subnet

Router137

ARP Request:What is the MAC addressof 128.143.71.21?

128.143.137.144/16128.143.171.21/2400:20:af:03:98:28

Argon Neon

ARP Reply:The MAC address of128.143.71.21 is00:e0:f9:23:a8:20

Things to know about ARP

What happens if an ARP Request is made for a non-existing host?Several ARP requests are made with increasing time intervals between requests. Entually, ARP gives up (timeout).

On some systems (including Linux) a host periodically sends ARP Requests for all addresses listed in the ARP cache. This refreshes the ARP cache content, but also introduces traffic.

Gratuitous ARP Requests: A host sends an ARP request for its own IP address: Useful for detecting if an IP address has already been assigned.

Vulnerabilities of ARP1. Since ARP does not authenticate requests or replies,

ARP Requests and Replies can be forged

2. ARP is stateless: ARP Replies can be sent without a corresponding ARP Request

3. According to the ARP protocol specification, a node receiving an ARP packet (Request or Reply) must update its local ARP cache with the information in the source fields, if the receiving node already has an entry for the IP address of the source in its ARP cache. (This applies for ARP Request packets and for ARP Reply packets)

Vulnerabilities of ARP

Typical exploitation of these vulnerabilities:

A forged ARP Request or Reply can be used to update the ARP cache of a remote system with a forged entry (ARP Poisoning)

This can be used to redirect IP traffic to other hosts

Some notes on Lab 2

What is a single-segment network?

A single-segment network consists of interfaces connected by a single physical link, either a point-to-point link or a broadcast link.

Interfaces on the same single-segment network have the same network prefix.

128.59.1.100

128.59.1.200

128.59.1.300

128.59.1.1

128.59.2.100

128.59.2.200

128.59.3.100 128.59.3.200

128.59.2.1

128.59.3.1

128.59.1.0/24128.59.2.0/24

128.59.3.0/24

How to identify a single segment IP network

Detach interfaces from routers or hosts Each isolated island is a single segment IP network Each interface on the same single segment IP network

must have the same network address prefix

128.59.1.100

128.59.1.200

128.59.1.300

128.59.1.1 128.59.2.1

128.59.3.1

128.59.2.100

128.59.2.200

128.59.3.100 128.59.3.200

Protocol specification vs implementation

According to the ARP protocol specification, a node receiving an ARP packet (Request or Reply) must update its local ARP cache with the information in the source fields, if the receiving node already has an entry for the IP address of the source in its ARP cache. (This applies for ARP Request packets and for ARP Reply packets)

Implementation may differ from the specification

What you observe in the lab may not be universally true.

Documents

W4140 Network Laboratory Lecture 3 Sept 18 - Fall 2006 Shlomo Hershkop Columbia University