Lecture 3 #1

Hubs, Bridges and Switches

Lecture 3

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) specifies

maximum cable length (need small a) large “collision domain” (can collide with many

stations) limited number of stations: 802.5 (token ring)

have token passing delays at each station

Lecture 3 #3

Hubs Physical Layer devices: essentially repeaters

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

Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

Lecture 3 #4

Hubs (more)

Each connected LAN referred to as LAN segment Hubs do not isolate collision domains: node may

collide with any node residing at any segment in LAN

Hub Advantages: simple, inexpensive device Multi-tier provides graceful degradation: portions

of the LAN continue to operate if one hub malfunctions

extends maximum distance between node pairs (100m per Hub)

Lecture 3 #5

Hub limitations

single collision domain results in no increase in max throughput multi-tier throughput same as single

segment throughput individual LAN restrictions pose limits on

number of nodes in same collision domain and on total allowed geographical coverage

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

What happens to a?

Lecture 3 #6

Bridges

Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

Bridge isolates collision domains since it buffers frames

When frame is to be forwarded on segment, bridge uses CSMA/CD to access segment and transmit

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 is a store and forward device

Transparent: no need for any change to hosts LAN adapters

Lecture 3 #8

Backbone Bridge

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 pass over CS segment

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?

Lecture 3 #11

Bridge Filtering

bridges learn which hosts can be reached through which 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

be 60 minutes)

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 on which the frame arrived*/

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

then drop the frame

Otherwise (out_port is valid) /*entry found for destination */

then forward the frame on interface indicate

Lecture 3 #13

Bridge Learning: example

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

C sends frame, bridge has no info about D, so floods to both LANs bridge notes that C is on port 1 frame ignored on upper LAN frame received by D

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 selectively

forwards frame out via interface 1

C 1

Lecture 3 #15

What will happen with loops?Incorrect learning

A

B

1 1

22

A , 1 A , 122

Lecture 3 #16

What will happen with loops?Frame looping

A

C

1 1

22

C,?? C,??

Lecture 3 #17

What will happen with loops?Frame looping

A

B

1 1

22

B,2 B,1

Lecture 3 #18

Loop-free: tree

A

B

C

A message from Awill mark A’s location

Lecture 3 #19

Loop-free: tree

A

B

C

A message from Awill mark A’s location

A:

Lecture 3 #20

Loop-free: tree

A

B

CA:

A:

A message from Awill mark A’s location

Lecture 3 #21

Loop-free: tree

A

B

CA: A:

A:

A:

A:

A message from Awill mark A’s location

Lecture 3 #22

Loop-free: tree

A

B

CA: A:

A:

A:

A:

A message from Awill mark A’s location

Lecture 3 #23

Loop-free: tree

A

B

C

A:

A: A:

A:

A:

So a message toA will go by marks…

A message from Awill mark A’s location

Lecture 3 #24

Bridges Spanning Tree for increased reliability, desirable to have redundant,

alternative paths from source to dest with multiple paths, cycles result - bridges may

multiply and forward frame forever solution: organize bridges in a spanning tree by

disabling subset of interfaces

Disabled

Lecture 3 #25

Introducing Spanning Tree

Allow a path between every LAN without causing loops (loop-free environment)

Bridges communicate with special configuration messages (BPDUs)

Standardized by IEEE 802.1D

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

Lecture 3 #26

How to construct a spanning tree? Bridges run a distributed spanning tree

algorithm Select what ports (and bridges) should

actively forward frames Standardized in IEEE 802.1 specification

Lecture 3 #27

Overview of STP

We make a series of simplifications: Build a ST of bridges (in fact, need to

span LAN segments!) Assume that we are given a root

bridgeSo 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?

Lecture 3 #28

1. Choosing a root bridge

Assume each bridge has a unique identifier

Each bridge remembers smallest ID seen so far (my_root_ID)

Periodically, send my_root_ID to all neighbors (“flooding”)

When receiving ID, update if necessary Is that enough?!

Lecture 3 #29

2. Compute ST Given a root

Idea: each node finds its shortest paths to the root shortest paths tree

Output: At each node, parent pointer (and distance)

How: Bellman-Ford algorithm

Lecture 3 #30

Distributed Bellman-Ford

Assumption: There is a unique root node s

Idea: Each node, periodically, tells all its neighbors what is its distance from s

But how can they tell? s: easy. dists = 0 always! Another node v:

Mark neighbor with least distance as “parent”

Lecture 3 #31

Why does this work?

Suppose all nodes start with distance , and suppose that updates are sent every time unit.

CD

B

E

F

G

A 0

Lecture 3 #32

Why does this work?

Suppose all nodes start with distance , and suppose that updates are sent every time unit.

CD

B

E

F

G

A 0

1 1

1

1

Lecture 3 #33

Why does this work?

Suppose all nodes start with distance , and suppose that updates are sent every time unit.

CD

B

E

F

G

A 0

1 1

2

1

1

2

Lecture 3 #34

Why does this work?

Suppose all nodes start with distance , and suppose that updates are sent every time unit.

CD

B

E

F

G

A 0

1 1

2

1

1

3

2

Lecture 3 #35

Bellman-Ford: properties

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

Works when the system operates asynchronously.

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

Lecture 3 #36

3. ST of LAN segments

Assumption: given a ST of the bridgesIdea: Each segment has at least one bridge

attached. 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 distance announcement on each port. Mark port as “designated port” iff best on that port’s LAN

Lecture 3 #37

Spanning Tree Concepts:Path Cost A cost associated with each port on

each bridge (“weight” of the segment) default is 1

The cost associated with transmission onto the LAN connected to the port Can be manually or automatically assigned Can be used to alter the path to the root

bridge

Lecture 3 #38

Spanning Tree Concepts:Root Port Each non-root bridge has a Root port:

The port on the path towards the root bridge parent pointer

The root port is part of the lowest cost path towards the root bridge

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

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

Lecture 3 #40

Example Spanning Tree

B3

B5

B7B2

B1

B4B6

Root

B4 B5 B6

B8

B1

Spanning Tree:

root port

B3

B7B2

B8

Lecture 3 #41

Spanning Tree Concepts: Designated Port Each LAN has a single designated port This is the port reporting minimum cost

path to the root bridge for the LAN Only designated and root ports remain

active!

Lecture 3 #42

Example Spanning Tree

B3

B5

B7B2

B1

B6 B4

Root

B8

B2 B4 B5 B7

B8

B1

Forwarding Tree:

Designated Bridge

root port

Note: B3, B6 forward nothing

Lecture 3 #43

Spanning Tree Requirements

Each bridge has a unique identifierA broadcast address for bridges on

a LANA unique port identifier for all ports

on all bridgesBridge id + port number

Lecture 3 #44

Spanning Tree Algorithm:ImplementationKeep pumping a single message:

(my root ID, my cost to root, my ID)BPDU: Bridge Protocol Data UnitUpdate var’s when receiving: My_root_ID: smallest seen so far My_cost_to_root: smallest received to

my_root_ID + link cost Break ties by ID

That’s enough!

Lecture 3 #45

Spanning Tree Algorithm:Select Designated Bridges

Bridges send BPDU frames to its attached LANssender port IDbridge 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 (and winning port) (lowest ID/cost/priority)

Lecture 3 #46

Forwarding/Blocking State

1. Only root and designated ports are active for data forwarding

Other ports are in the blocking state: no forwarding!

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

2. All ports send BPDU messages

To adjust to changes

B3B5

B7B2

B1

B6 B4

Root

B8

Designated Bridge

root port

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)

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 routing algorithms

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

Lecture 3 #49

Routers vs. Bridges

Bridges + and - + Bridge operation is simpler requiring less

processing- Topologies are restricted with bridges: a

spanning tree must be built to avoid cycles - Bridges do not offer protection from broadcast

storms (endless broadcasting by a host will be forwarded by a bridge)

Lecture 3 #50

Routers vs. Bridges

Routers + and -+ arbitrary topologies can be supported, cycling is

limited 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) while routers used in large networks (thousands of hosts)

Lecture 3 #51

Ethernet Switches

layer 2 (frame) forwarding, filtering using LAN addresses

Switching: A-to-B and A’-to-B’ simultaneously, no collisions

large number of interfaces often: individual hosts,

star-connected into switch Ethernet, but no

collisions!

Lecture 3 #52

Ethernet Switches

cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frameslight reduction in latency

combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Lecture 3 #53

Ethernet Switches (more)Dedicated

Shared

Lecture 3 #54

Optional: Wireless LAN and PPP

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):

base station BSS’s combined to

form distribution system (DS)

Lecture 3 #56

Ad Hoc Networks Ad hoc network: IEEE 802.11 stations can

dynamically form network without AP Applications:

“laptop” meeting in conference room, car

interconnection of “personal” devicesbattlefield

IETF MANET (Mobile Ad hoc Networks) working group

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 busy

then binary backoff

802.11 CSMA receiver:if received OK return ACK after SIFS

Why?

Lecture 3 #58

IEEE 802.11 MAC Protocol

802.11 CSMA Protocol: others

NAV: Network Allocation Vector

802.11 frame has transmission time field

others (hearing data) defer access for NAV time units

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

Lecture 3 #60

Collision Avoidance: RTS-CTS exchange CSMA/CA: explicit

channel reservation sender: send short

RTS: Request To Send receiver: reply with

short CTS: Clear To Send

CTS reserves channel for sender, notifying (possibly hidden) stations

avoid hidden station collisions

Lecture 3 #61

Collision Avoidance: RTS-CTS exchange

RTS and CTS short: collisions less likely, of

shorter duration end result similar to

collision detection IEEE 802.11 allows:

CSMA CSMA/CA: reservations polling from AP

Lecture 3 #62

Point to Point Data Link Control one sender, one receiver, one link:

easier than broadcast link:no Media Access Controlno 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” in protocol stack!)

Lecture 3 #63

PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer

datagram in data link frame carry network layer data of any network layer

protocol (not just IP) at same time ability to demultiplex upwards

bit transparency: must carry any bit pattern in the data field

error detection (no correction) No error management (ack) required

connection liveness: detect, signal link failure to network layer

network layer address negotiation: endpoint can learn/configure each other’s network address

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-ordering all relegated to higher layers!!!

Note the difference with 802.11

Reason:

different

error rates

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)

Lecture 3 #66

PPP Data Frame

info: upper layer data being carried check: cyclic redundancy check (CRC)

for error detection

Lecture 3 #67

Byte Stuffing “data transparency” requirement: data field

must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> or <01111101> data byte

Receiver: Receive 01111101

• discard the byte, • Next byte is data

Receive 01111110: flag byte

Lecture 3 #68

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

Lecture 3 #69

PPP Data Control ProtocolBefore exchanging network-

layer data, data link peers must

configure PPP link (max. frame length, authentication)

learn/configure network layer information

for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

Lecture 3 #70

Data Link: Summary

principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP

various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP

Chapter 5 Kurose and Ross

Lecture 3 #71

Configuration Messages: BPDU