3 hubs bridges switches

8/8/2019 3 Hubs Bridges Switches

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Lecture 3 #1

Hubs, Bridges and Switches

Lecture 3


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Lecture 3 #2

Interconnecting LANs

Q: Why not just one big LAN? Limited amount of supportable traffic: on single

LAN, all stations must share bandwidth

limited length: 802.3 (Ethernet) specifiesmaximum cable length

large collision domain (can collide with manystations)

limited number of stations: 802.5 (token ring)have token passing delays at each station


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Lecture 3 #3

Hubs Physical Layer devices: essentially repeaters

operating at bit levels: repeat received bits on oneinterface to all other interfaces

Hubs can be arranged in a hierarchy (or multi-tier

design), with backbone hub at its top


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Lecture 3 #4

Hubs (more)

Each connected LAN referred to as LAN segment

Hubs do not isolate collision domains: node may collidewith any node residing at any segment in LAN

Hub Advantages: simple, inexpensive device

Multi-tier provides graceful degradation: portionsof the LAN continue to operate if one hub

malfunctionsextends maximum distance between node pairs

(100m per Hub)


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Lecture 3 #5

Hub limitations

single collision domain results in no increase in maxthroughput

multi-tier throughput same as single segment

throughput individual LAN restrictions pose limits on number

of nodes in same collision domain and on totalallowed geographical coverage

cannot connect different Ethernet types (e.g.,10BaseT and 100baseT) Why?


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Lecture 3 #6

Bridges

Link Layer devices: operate on Ethernetframes, examining frame header andselectively forwarding frame based on its

destination Bridge isolates collision domains since it

buffers frames

When frame is to be forwarded onsegment, bridge uses CSMA/CD to accesssegment and transmit


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Lecture 3 #7

Bridges (more)

Bridge advantages: Isolates collision domains resulting in higher

total max throughput, and does not limit the

number of nodes nor geographical coverage

Can connect different type Ethernet since it isa store and forward device

Transparent: no need for any change to hostsLAN adapters


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Lecture 3 #8

Backbone Bridge


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Lecture 3 #9

Interconnection Without Backbone

Not recommended for two reasons:

- single point of failure at Computer Science hub- all traffic between EE and SE must path over

CS segment


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Lecture 3 #10

Bridges: frame filtering, forwarding

bridges filter packets same-LAN -segment frames not forwarded onto

other LAN segments

forwarding:how to know on which LAN segment to forward

frame?


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Lecture 3 #11

Bridge Filtering

bridges learn which hosts can be reached throughwhich interfaces: maintain filtering tables

when frame received, bridge learns location of

sender: incoming LAN segment records sender location in filtering table

filtering table entry:

(Node LAN Address, Bridge Interface, Time Stamp)

stale entries in Filtering Table dropped (TTL can be60 minutes)


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Lecture 3 #12

Bridge Operation

bridge procedure(in_MAC, in_port,out_MAC)Set filtering table (in_MAC) to in_port /*learning*/lookup in filtering table (out_MAC) receive out_portif

(out_port not valid) /* no entry found for destination */then flood; /* forward on all but the interface onwhich the frame arrived*/

if (in_port = out_port) /*destination is on LAN on whichframe was received */

then drop the frame

Otherwise (out_port is valid) /*entry found for destination */then forward the frame on interface indicate


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Lecture 3 #13

Bridge Learning: example

Suppose C sends frame to D and D replies back withframe to C

C sends frame, bridge has no info about D, sofloods to both LANs bridge notes that C is on port 1

frame ignored on upper LAN

frame received by D


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Lecture 3 #14

Bridge Learning: example

D generates reply to C, sends

bridge sees frame from D

bridge notes that D is on interface 2

bridge knows C on interface 1, so selectivelyforwards frame out via interface 1

C 1


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Lecture 3 #15

What will happen with loops?

Incorrect learning

A

B

1 1

22

A , 1 A , 122


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Lecture 3 #16


Frame looping

A

C

1 1

22

C,?? C,??


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Lecture 3 #17


Frame looping

A

B

1 1

22

B,2 B,1


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Lecture 3 #18

Loop-free: tree

A

B

C

A message from Awill mark As location


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Lecture 3 #19

Loop-free: tree

A

B

C


A: q


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Lecture 3 #20

Loop-free: tree

A

B

CA: q

A: q



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Lecture 3 #21

Loop-free: tree

A

B

CA: qA:p

A:n

A:n

A: q



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Lecture 3 #22

Loop-free: tree

A

B

CA: qA:p

A:n

A:n

A: q



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Lecture 3 #23

Loop-free: tree

A

B

C

A: q

A: qA:p

A:n

A:n

So a message toA will go by marks



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Lecture 3 #24

Bridges Spanning Tree

for increased reliability, desirable to haveredundant, alternative paths from source to dest

with multiple paths, cycles result - bridges maymultiply and forward frame forever

solution: organize bridges in a spanning tree bydisabling subset ofinterfaces

Disabled


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Lecture 3 #25

Introducing Spanning Tree

Allow a path between every LAN withoutcausing loops (loop-free environment)

Bridges communicate with specialconfiguration messages (BPDUs)

Standardized by IEEE 802.1D

Note: redundant paths are good, active redundant paths are bad(they cause loops)


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Lecture 3 #26

How to construct a spanning tree?

Bridges run a distributed spanning treealgorithmSelect what ports (and bridges) should actively

forward frames Standardized in IEEE 802.1 specification


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Lecture 3 #27

Overview of STP

We make a series of simplifications:

Build a ST of bridges (in fact, need tospan LAN segments!)

Assume that we are given a root bridge

So we solve in order:

1. How to find a root bridge?

2. How to compute a ST of bridges?3. How to compute a ST LAN segments?


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Lecture 3 #28

1. Choosing a root bridge

Assume each bridge has a unique identifier

Each bridge remembers best ID seen sofar (my_root_ID)

Periodically, send my_root_ID to allneighbors (flooding)

When receiving ID, update if necessary

Is that enough?!


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Lecture 3 #29

2. Compute ST Given a root

Idea: each node finds its shortest paths tothe root shortest paths tree

Output: At each node, parent pointer (and

distance)How: Bellman-Ford algorithm


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Lecture 3 #30

Distributed Bellman-Ford

Assumption: There is a unique root node sIdea: Each node, periodically, tells all its

neighbors what is its distance from s

But how can they tell? s: easy. dist

s= 0 always!

Another node v:

Mark neighbor with least distance asparent


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Lecture 3 #31

Why does this work?

Suppose all nodes start with distance g,and suppose that updates are sent everytime unit.

CD

B

E

F

G

A0

gg

g

g

g

g

g


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Lecture 3 #32

Why does this work?


CD

B

E

F

G

A0

1 1

g

1

1

g

g


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Lecture 3 #33

Why does this work?


CD

B

E

F

G

A0

1 12

1

1

g

2


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Lecture 3 #34

Why does this work?


CD

B

E

F

G

A0

1 12

1

1

3

2


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Lecture 3 #35

Bellman-Ford: properties

Works for any non-negative link weightsw(u,v):

Works when the system operatesasynchronously.

Works regardless of the initial distances!(later...)


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Lecture 3 #36

3. ST of LAN segments

Assumption: given a ST of the bridges

Idea: Each segment has at least one bridgeattached. Only one of them should forward

packets! Choose bridge closest to root. Break ties by bridge ID(and then by port ID...)

Implementation: Bridges listen to all distanceannouncement on each port. Mark port as

designated port iff best on that ports LAN


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Lecture 3 #37

Spanning Tree Concepts:P

ath Cost A cost associated with each port on eachbridge (weight of the segment)default is 1

The cost associated with transmission ontothe LAN connected to the portCan be manually or automatically assigned

Can be used to alter the path to the root bridge


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Lecture 3 #38

Spanning Tree Concepts:

RootP

ort Each non-root bridge has a Root port: Theport on the path towards the root bridgeparent pointer

The root port is part of the lowest costpath towards the root bridge

If port costs are equal on a bridge, theport with the lowest ID becomes root port


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Lecture 3 #39

Example Spanning Tree

B3

B5

B7B2

B1

B6 B4

Protocol operation:1. Pick a root2. Each bridge picks a

root port

B8


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Lecture 3 #40


B3

B5

B7B2

B1

B4B6

Root

B4 B5 B6

B8

B1

Spanning Tree:

rootport

B3

B7B2

B8


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Lecture 3 #41

Spanning Tree Concepts:Designated Port Each LAN has a single designated port

This is the port reporting minimum costpath to the root bridge for the LAN

Only designated and root ports remainactive!


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Lecture 3 #42


B3

B5

B7B2

B1

B6 B4

Root

B8

B2 B4 B5 B7

B8

B1

Forwarding Tree:

DesignatedBridge

rootport

Note: B3, B6 forward nothing


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Lecture 3 #43

Spanning Tree Requirements

Each bridge has a unique identifier

A broadcast address for bridges on a

LAN A unique port identifier for all ports

on all bridgesBridge id + port number


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Lecture 3 #44

Spanning Tree Algorithm:ImplementationKeep pumping a single message:

(my root ID, my cost to root, my ID)

BPDU: Bridge Protocol Data Unit

Update vars when receiving: My_root_ID: smallest seen so far

My_cost_to_root: smallest received to my_root +link cost

Break ties byID

Thats enough!


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Lecture 3 #45

Spanning Tree Algorithm:Select Designated Bridges

Bridges send BPDU frames to its attachedLANs

sender portIDbridge and port ID of the bridge the sending

bridge considers rootroot path cost for the sending bridge

3. Best bridge wins, and it knows it (andwinning port) (lowest ID/cost/priority)


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Lecture 3 #46

Forwarding/Blocking State

1. Only root and designated ports areactive for data forwarding Other ports are in the blocking state:

no forwarding!

If bridge has no designated port, noforwarding at all block root port too.

2. All ports send BPDU messages To adjust to changes


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Lecture 3 #47

Spanning Tree Protocol: Execution

B3

B5

B7B2

B1

B6 B4

B8

(B1,root=B1, dist=0)(B1,root=B1,dist=0)

(B4, root=B1, dist=1)(B6, Root=B1dist=1)


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Lecture 3 #48

Bridges vs. Routers

both store-and-forward devices routers: network layer devices (examine network layer

headers)

bridges are Link Layer devices

routers maintain routing tables, implement routingalgorithms

bridges maintain filtering tables, implementfiltering, learning and spanning tree algorithms


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Lecture 3 #49

Routers vs. Bridges

Bridges + and -

+ Bridge operation is simpler requiring lessprocessing

- Topologies are restricted with bridges: a spanningtree must be built to avoid cycles

- Bridges do not offer protection from broadcaststorms (endless broadcasting by a host will beforwarded by a bridge)


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Lecture 3 #50

Routers vs. Bridges

Routers + and -

+ arbitrary topologies can be supported, cycling islimited by TTL counters (and good routing protocols)

+ provide firewall protection against broadcast storms- require IP address configuration (not plug and play)

- require higher processing

bridges do well in small (few hundred hosts) whilerouters used in large networks (thousands of hosts)


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Lecture 3 #51

Ethernet Switches

layer 2 (frame) forwarding,filtering using LANaddresses

Switching: A-to-B and A-to-B simultaneously, nocollisions

large number of interfaces

often: individual hosts,star-connected into switchEthernet, but no

collisions!


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Lecture 3 #52

Ethernet Switches

cut-through switching: frame forwardedfrom input to output port without awaitingfor assembly of entire frame

slight reduction in latency combinations of shared/dedicated,

10/100/1000 Mbps interfaces


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Lecture 3 #53

Ethernet Switches (more)

Dedicated

Shared


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Lecture 3 #54

Optional: Wireless LAN and PPP


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Lecture 3 #55

IEEE 802.11 Wireless LAN

wireless LANs: untethered (often mobile) networking IEEE 802.11 standard:

MAC protocol

unlicensed frequency spectrum: 900Mhz, 2.4Ghz

Basic Service Set (BSS)(a.k.a. cell) contains:

wireless hosts

access point (AP): basestation

BSSs combined to formdistribution system (DS)


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Lecture 3 #56

Ad Hoc Networks

Ad hoc network: IEEE 802.11 stations candynamically form network without AP

Applications:

laptop meeting in conference room, carinterconnection of personal devices

battlefield

IETF MANET(Mobile Ad hoc Networks)working group


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Lecture 3 #57

IEEE 802.11 MAC Protocol:CSMA/CA802.11 CSMA: sender- if sense channel idle for

DISF sec.then transmit entire frame(no collision detection)

-if sense channel busythen binary backoff

802.11 CSMA receiver:

if received OKreturn ACK after SIFSWhy?


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Lecture 3 #58

IEEE 802.11 MAC Protocol

802.11 CSMA Protocol:others

NAV: NetworkAllocationVector

802.11 frame hastransmission time field

others (hearing data)defer access for NAVtime units


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Lecture 3 #59

Hidden Terminal effect

hidden terminals: A, C cannot hear each other

obstacles, signal attenuation

collisions at B

goal: avoid collisions at B CSMA/CA: CSMA with Collision Avoidance


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Lecture 3 #60

Collision Avoidance: RTS-CTSexchange CSMA/CA: explicit

channel reservation

sender: send shortRTS: request to send

receiver: reply withshort CTS: clear tosend

CTS reserves channel for

sender, notifying(possibly hidden) stations

avoid hidden stationcollisions


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Lecture 3 #61

Collision Avoidance: RTS-CTSexchange RTS and CTS short:

collisions less likely, ofshorter duration

end result similar tocollision detection

IEEE 802.11 allows:

CSMA

CSMA/CA: reservations

polling from AP


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Lecture 3 #62

Point to Point Data Link Control

one sender, one receiver, one link: easierthan broadcast link:

no Media Access Control

no need for explicit MAC addressinge.g., dialup link, ISDN line

popular point-to-point DLC protocols:

PPP

(point-to-point protocol)HDLC: High level data link control (Data

link used to be considered high layer inprotocol stack!)

PPP D F


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Lecture 3 #63

PPP Design Requirements [RFC1557] packet framing: encapsulation of network-layer

datagram in data link frame

carry network layer data of any network layerprotocol (not just IP) at same time

ability to demultiplex upwards

bit transparency: must carry any bit pattern in thedata field

error detection (no correction)

connection livenes: detect, signal link failure tonetwork layer

network layer address negotiation: endpoint canlearn/configure each others network address


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Lecture 3 #64

PPP non-requirements

no error correction/recovery

no flow control

out of order delivery OK

no need to support multipoint links (e.g.,polling)

Error recovery, flow control, data re-orderingall relegated to higher layers!!!


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Lecture 3 #65

PPP Data Frame

Flag: delimiter (framing)

Address: does nothing (only one option)

Control: does nothing; in the future possible

multiple control fields Protocol: upper layer protocol to which frame

delivered (eg, PPP-LCP, IP, IPCP, etc)


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Lecture 3 #66

PPP Data Frame

info: upper layer data being carried

check: cyclic redundancy check (CRC) forerror detection


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Lecture 3 #67

Byte Stuffing

data transparency requirement: data field mustbe allowed to include flag pattern Q: is received data or flag?

Sender: adds (stuffs) extra < 01111101> bytebefore each < 01111110> or data byte

Receiver:Receive 01111101

discard the byte, Next byte is data

Receive 01111110: flag byte


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Lecture 3 #68

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte intransmitted data


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Lecture 3 #69

PPP Data Control Protocol

Before exchanging network-layer data, data link peersmust

configure PPP link (max.frame length,authentication)

learn/configure network

layer information

for IP: carry IP ControlProtocol (IPCP) msgs(protocol field: 8021) toconfigure/learn IPaddress


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Lecture 3 #70

Data Link: Summary

principles behind data link layerservices:error detection, correction

sharing a broadcast channel: multiple access link layer addressing, ARP

various link layer technologiesEthernet

hubs, bridges, switches

IEEE 802.11 LANs

PPP

Chapter 5 Kurose and Ross


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Configuration Messages: BPDU

3 hubs bridges switches

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