12 ethernet-wifi

Week 12Ethernet

WiFi

Agenda

• Ethernet

• Spanning Tree

• WiFi

• IP version 4

Ethernet Frames

• DIX Format

• proposed by Digital, Intel and Xerox

Preamble[8 bytes]

Destination address

Type[2 bytes]

CRC [32 bits]

Source address

Data[46-1500 bytes

Used to mark the beginning of the frameAllows the receiver to synchronise its

clock to the sender’s clock

Indication of the type of packet containedinside the frame

Upper layer protocol must ensure thatthe payload of the Ethernet frame is

at least 46 bytes and at most 1500 bytes

The Ethernet zoo

10BASE5 Thick coaxial cable, 500m

10BASE2 Thin coaxial cable, 185m

10BASE-T Two pairs of category 3+ UTP

10BASE-F 10 Mb/s over optical fiber

100BASE-TX Category 5 UTP or STP, 100 m maximum

100BASE-FX Two multimode optical fiber, 2 km maximum

1000BASE-CX Two pairs shielded twisted pair, 25m maximum

1000BASE-SX Two multimode or single mode optical fibers with lasers

10 Gbps optical fiber but also cat 6 twisted pair

40-100 Gbps being developed, standard expected in 2010, 40Gbps one

meter long for switch backplanes, 10 meters for copper cable and 100 meters for fiber optics

Ethernet switch

• A switch is a relay that operates in the datalink layer

Host A Host BSwitch

Physical Phys. Phys.

Datalink

Network Network

Datalink

Physical

How to favour high-

speed links ?

Switch 1

Switch 7

Switch 9

Switch 22

Switch 44

Switch 2

Link costs

BandwidthRecommended

link cost range

Recommended

link cost value

10 Mbps 50-600 100

100 Mbps 10-60 19

1000 Mbps 3-10 4

Selection of root

• Root priority vectors

• Port 1: 8,7+100,9

• Port 2 : 8,9+1,22

• Port 3 : 8,4+10,17

• Port 4: 8,4+10,18

S911

234

R=8,C=7,T=9

R=8,C=9,T=22

R=8,C=4,T=17R=8,C=4,T=18

• Switch S91's BPDU

• R=8, C=10,T=91

Switches and hubs

• How should the spanning tree work with

hubs ?

S11

4

Hub1S3

1

2

Hub2

2

The states of the

ports• Root port

• Port having the best root priority vector

• Only one root port per switch !

• Designated port

• Ports where the switch's BPDU is better than best BDPU received

• Blocked ports

• Ports where the switch's BPDU is worse than best BDPU received

The root switch

• What is the state of the ports of the root

switch ?

• How to influence the selection of the

root switch ?

S11

234

Corner cases

• Parallel links

• Backup links to same LAN

S2S31

2 3

4

S1

2 4

S1

Spanning tree

1

234 1

1

1

1

2

22

2

3

3

3

4S222S111

S333

S444

S555

1 Gbps, cost =10

10 Gbps, cost =1

BPDU format• Simplified BPDU format

BPDU Header

Root Id

Switch identifier

Root path cost

Protocol IdentifierProtocol version

Configuration BPDU or topology changeFlags

Identifier of the switch sending the BPDU

Port identifier : used when a switch has severalports attached to the same LAN

Current root identifier

Port identifier

Message age

Max age

Hello time

Forward delay

Port states and

activityReceive

BPDUs

Transmit

BPDUs

Blocked yes no

Root yes no

Designated yes yes

Learn

Addresses

Forward Data

Frames

Inactive no no

Active yes yes

Failures• Failure (power-off) of the root switch

• A new root needs to be elected

• Failure of a designated switch

• Another switch should replace

• Failure of a link

• a disabled link should be enabled

• If the network is split we have two

separated networks

Dealing with failures• Regular transmission of BPDUs

• Default Hello timer is two seconds

• BPDUs stored in the switches age and are

removed when they timeout

• Failure notification mechanism

• When switch detects important failure, it

sends a topology change BPDU to Root

• Upon reception of TC BPDU all switches

stop forwarding data frames and recompute

ST

Full duplex Ethernet

ObservationsIn many networks, Ethernet is a often a point-to-point technology

host-to-switchswitch to switch

Twisted-pairs and fiber-based physical layers allow to send and receive at the same time

S1 S2

HUBHUB

Ethernet full duplex

No collision is possible on a full duplex Ethernet/FastEthernet/GigabitEthernet link

Disable CSMA/CD on such links

AdvantagesImproves bandwidth

Both endpoints can transmit frames at the same time

CSMA/CD is disabled

No constraint on propagation delay anymoreEthernet network can be as large as we want !

No constraint on minimum frame size anymoreWe do not need the frame extension hack for Gigabit Ethernet!

Full duplex Ethernet (3)

DrawbackIf CSMA/CD is disabled, access control is disabled and congestion can occur

How to solve this problem inside Ethernet ?Add buffers to switches

but infinite buffers are impossible and useless anywayCause collisions (e.g. jamming) to force collisions on the inter-switch link and uplink is server is too fast

Drawback : interswitch link could be entirely blockedDevelop a new flow control mechanism inside MAC layer

Pause frame to slowdown transmission

S1 S2Server

Client

FastEthernet (100 Mbps) Ethernet (10 Mbps)

Ethernet flow control

PAUSE frame indicates how much time the upstream should wait before transmitting next frame

S1

serverClient

FastEthernet(100 Mbps)

Ethernet(10 Mbps)

PAUSE [2msec]

Frame1 [10000 bits]

Frame3 [10000 bits]

Frame2 [10000 bits]

100 nsec

Frame1 [10000 bits]

1 microsec

Frame2 [10000 bits]

Sender blocked

Virtual LANs

Allows to build several logical networks on top of a single physical network

S

A B

C

D

F

E

Each port on each switch is associated to a particular VLANAll the hosts that reside on the same VLAN can exchange Ethernet framesA host on VLAN1 cannot send an Ethernet frame towards another host that belongs to VLAN2Broadcast and multicast frames are only sent to the members of the VLAN

VLAN1 : A,E,FVLAN2 : B,C,D

VLANs in campus networks

How to support VLANs in a campus network

S1

A B

C

D

F

E

VLAN1 : A,E,FVLAN2 : B,C,D

S2

Possible solutionsPlace on each switch a tablethat maps each MAC addresson a VLAN iddifficult to manage this table

Change frame format used on inter-switch links to include a VLAN identifiernew header added by first switchnew header removed by last switch

VLAN frame format

DestinationAddress

Address

Identifies the frame as containing VLANtag

Tag control information contains two types of information :- VLAN identifier (12 bits) : up to 4094 different VLANs can be defined- Priority (3 bits) : indicates the importance of the frame and can be used by switches to provide a better service for some frames (e.g. Voice)

Type

CRC [32 bits]

Payload

VLAN Protocol Id

0x8100

Tag Control Info

Agenda

• Ethernet

• Spanning Tree

• WiFi

• IP version 4

The WiFi zoo

Standard Frequency Typical

throughput

Raw

bandwidth

Range in/out

(m)

802 .11 2.4 GHz 0.9 Mbps 2 Mbps 20 / 100

802 .11a 5 GHz 23 Mbps 54 Mbps 35 / 120

802 .11b 2.4 GHz 4.3 Mbps 11 Mbps 38 / 140

802 .11g 2.4 GHz 19 Mbps 54 Mbps 38 / 140

802 .11n 2.4 / 5 GHz 74 Mbps up to 600

Mbps

70 / 250

Source http://en.wikipedia.org/wiki/IEEE_802.11n

http://en.wikipedia.org/wiki/IEEE_802.11n

Practical issues with WLAN deployments

Home environment

A WLAN can interfere with the neighbour’s WLAN

Practical issues with WLAN deployments

Enterprise networks

One access point can interfere with other access pointsreduces significantly overall available bandwidth

The WiFi channel frequencies

WiFi standards operate on several frequencies called channels

Usually about a dozen channels

Why multiple channels ?Some channels my be affected by interference and have a lower performanceSome frequencies are reserved for specific usage in some countriesAllows frequency reuse when there are multiple WiFinetworks in the same area

Unfortunately, many home access points operate by default on the same factory set channel which causes interference and reduced bandwidth

WLAN in enterprise environments

What could be done to improve the performance of WLANs ?

Reduce interference as much as possibleTune channel frequencies Reduce transmission powerSimilar to techniques used in GSM networks

Recent deployments rely on centralized controllers and thin access points

802.11 frame format

Frame control[2 bytes]

Duration/Id[2 bytes]

Address 2[6 bytes]

Address 1[6 bytes]

Standard header- Protocol version [2 bits] : current version 0- Type [2 bits] : control / data / management frame- Subtype [2 bits] : specific subtype of frame- to DS [1 bit] : frame is sent to distribution system- from DS [1 bit] : frame is from distribution system- more fragment [1 bit] : used when packets are fragmented- Retry [1 bit] : retransmission - Power Management [1 bit] : used for power management fct- More data [1 bit] : indicates that there are other frames

for this station at the access point- WEP [1 bit] : 1 if frame has been encrypted with WEP- order [1 bit] : for strictly ordered class

Address 3[6 bytes]

Sequence control [2 bytes]

Frame body[0-2312 bytes]

Frame Check Sequence

Sequence number- 12 bits frame sequence number- 4 bits fragment number

Some 802.11 control frames


Duration[2 bytes]

Receiver address[6 bytes]



Duration[2 bytes]



Transmitter address[6 bytes]


Duration[2 bytes]



ACK frameRTS frame

CTS frame

IP over 802.11

Frame control

Duration/Id[2 bytes]

Address 2[6 bytes]

Address 1[6 bytes]

Address 3[6 bytes]

Sequence control [2 bytes]

IP packet


LLC/SNAP

0x800

LLC/SNAP- 4 bytes header

EtherType- 0x800 for IP, 0x86DD for IPv6

Agenda

• Ethernet

• Spanning Tree

• WiFi

• IP version 4

IP version 4

Ver IHL DS Total length

Payload

32 bits

ChecksumTTL Protocol

Flags FragmentOffset

20 bytesSource IP address

Identification

Destination IP address

Differentiated Services Byte used tospecify Quality of Service expected

for this packet

IP version used to encode header- current version is 4- IP version 6

Header length (default 20 bytes)

Maximum : 64 bytes for entire header including options

Binary flagsMore

Don't Fragment : Packet cannot be fragmented by

intermediate routersAllows to identify the “user” above

the IP layer (e.g. UDP, TPC, ...)Plays similar role to TCP port

numbers

Packet identificationused for fragmentation and

reassembly

Options

Optional header extension

Time to Live

IPv4 addresses

• 32 bits long, one address per interface

• Example

Notation 138.48.26.1/23 or

138.48.26.1 255.255.254.0

• All hosts that belong to the same subnetwork

can directly exchange frames through

datalink layer

ARP : Address

Resolution Protocol

IP: 10.0.1.9Eth : B

IP: 10.0.1.8Eth : C

IP: 10.0.1.11Eth : E

IP: 10.0.1.22Eth : A

10.0.1.22 needs to send an IP packet to 10.0.1.8

1

IP: 10.0.1.9Eth : B

IP: 10.0.1.8Eth : C

IP: 10.0.1.11Eth : E

IP: 10.0.1.22Eth : A

ARP : broadcast frame Addr Eth 10.0.1.8 ?

2

IP: 10.0.1.9Eth : B

IP: 10.0.1.8Eth : C

IP: 10.0.1.11Eth : E

IP: 10.0.1.22Eth : A

10.0.1.8 replies in an Ethernet frame and A knows the MAC address to sendits IP packet

3

ARP : frame formatPreamble[7 bytes]

Delimiter[1byte]

DestinationAddress

Type: 0x806

CRC [32 bits]

Source Address

MAC address of the sender

Broadcast : 111...111

Header

Sender MAC

Sender IP

Target MAC

Common header for all ARP frames- Hardware type Ethernet is 1- Protocol type , IP is 0x0800.- Hardware length : length of MAC address - Protocol length : length of network layer address -Operation : 1 for request, 2 for reply, 3 for RARP request, and 4 for RARP reply.

Target IP

http://en.wikipedia.org/wiki/Ethernet

ICMP version 4

Ver IHL DS

IP headerChecksumTTL Protocol

Source IP address

Identification


Data

Type Code

Ver IHL DS Total length



Source IP address

Identification


First 64 bits of payload


Protocol=1 for ICMP

covers entire ICMP message

Additional information abouterror, type of error

Total length

Checksum

32 bits

ICMP headerType and Code indicate the type of

error detectedl Destination unreachable

lnetwork unreachablelhost unreachable

lprotocol unreachablelport unreachable

lfragmentation neededlsource route failed

lRedirectlParameter problem

lTime exceededlTTL exceeded

lreassembly time exceededlEcho requEast et Echo reply

IP over Ethernet Detailed example

ExamplesIP packet from 10.0.1.22 to 10.0.3.11IP packet from 10.0.2.9 to 10.0.1.22IP packet from 10.0.3.11 to 10.0.1.22

IP: 10.0.2.9/24Eth : B10.0.1.0/24 via 10.0.2.110.0.3.0/24 via 10.0.2.2ARP tableEmpty

IP: 10.0.1.8/24R default: 10.0.1.1Eth : CARP tableEmpty

IP: 10.0.1.22/24R default: 10.0.1.1Eth : AARP tableEmpty

R1

IP: 10.0.1.1/24IP: 10.0.2.1/24Eth : R1-WestEth : R1-EastARP tableEmpty

H1 S2 R2

Hub

Router Switch Router

IP: 10.0.3.11/24Eth : FR default:10.0.3.2ARP tableEmpty

IP: 10.0.2.2/24IP: 10.0.3.2/24Eth : R2-WestEth : R2-EastARP tableEmpty

The Internet architecturethat students learn

Physical

Datalink

Network

Transport

Application

O. Bonaventure, Computer networking : Principles, Protocols and Practice, open ebook, http://inl.info.ucl.ac.be/cnp3

Physical

Physical

Datalink

Physical

Datalink

Network

A typical "academic" network

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

Physical

Datalink

The end-to-end principle

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

Physical

Datalink

TCP

In reality

– almost as many middleboxes as routers

– various types of middleboxes are deployed

Sherry, Justine, et al. "Making middleboxes someone else's problem: Network processing as a cloud service." Proceedings of the ACM SIGCOMM 2012 conference. ACM, 2012.

A middlebox zoo

http://www.cisco.com/web/about/ac50/ac47/2.html

Web Security Appliance

NAC Appliance

ACE XMLGateway

Streamer

VPN Concentrator

SSLTerminator

Cisco IOS Firewall

IP Telephony Router

PIX FirewallRight and Left

Voice GatewayVVVV

Content Engine

NAT

How to model those middleboxes ?

• In the official architecture, they do not exist

• In reality...

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

Transport

Application

Physical

Datalink

Network

TCP

Physical

Datalink

Network

Transport

Application

TCP segments processed by a router

Source port Destination port

Checksum Urgent pointer

THL Reserved Flags

Acknowledgment number

Sequence number

Window

Ver IHL ToS Total length


Flags Frag. Offset

Source IP address

Identification


Payload

Options



THL Reserved Flags


Sequence number

Window



Flags Frag. Offset

Source IP address

Identification


Payload

Options

IP

TCP

Network Address Translators

• Preserves IP addresses by using private addresses in LAN

– Packets's addresses are rewritten by NAT

Private addresses Public addresses

TCP segments processed by a NAT



THL Reserved Flags


Sequence number

Window



Flags Frag. Offset

Source IP address

Identification


Payload

Options



THL Reserved Flags


Sequence number

Window



Flags Frag. Offset

Source IP address

Identification


Payload

Options

12 ethernet-wifi

Engineering

Transcript of 12 ethernet-wifi