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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) specifies
maximum cable length large “collision domain” (can collide with many
stations) 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 one interface 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
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)
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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?
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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
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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 is a store and forward device
Transparent: no need for any change to hosts LAN 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 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)
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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 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
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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
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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 selectively
forwards 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
What will happen with loops?Frame looping
A
C
1 1
22
C,?? C,??
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Lecture 3 #17
What will happen with loops?Frame looping
A
B
1 1
22
B,2 B,1
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Lecture 3 #18
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)
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Lecture 3 #19
Spanning Tree Requirements Each bridge is assigned a unique
identifier A broadcast address for bridges on a
LAN A unique port identifier for all ports on
all bridges MAC address Bridge id + port number
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Lecture 3 #20
Spanning Tree Concepts:Root Bridge The bridge with the lowest bridge ID
value is elected the root bridge One root bridge chosen among all
bridges Every other bridge calculates a path to
the root bridge
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Lecture 3 #21
Spanning Tree Concepts:Path Cost A cost associated with each port on
each bridge 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
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Lecture 3 #22
Spanning Tree Concepts:Root Port The port on each bridge that is on the
path towards the root bridge 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
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Lecture 3 #23
Spanning Tree Concepts:Root Path Cost The minimum cost path to the root
bridge The cost starts at the root bridge Each bridge computes root path cost
independently based on their view of the network
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Lecture 3 #24
Spanning Tree Concepts: Designated Bridge Only one bridge on a LAN at one time is
chosen the designated bridge This bridge provides the minimum cost
path to the root bridge for the LAN Only the designated bridge passes
frames towards the root bridge
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Lecture 3 #25
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Protocol operation:1. Picks a root2. For each LAN,
picks a designated bridgethat is closest to the root.
3. All bridges on a LANsend packets towards the root via the designated bridge.
B8
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Lecture 3 #26
Example Spanning Tree
B3
B5
B7B2
B1
B6 B4
Root
B8
B2 B4 B5 B7
B8
B1
Spanning Tree:
Designated Bridge
root port
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Lecture 3 #27
Spanning Tree Algorithm:An Overview 1. Determine the root bridge among all bridges 2. Each bridge determines its root port
The port in the direction of the root bridge 3. Determine the designated bridge on each
LAN The bridge which accepts frames to forward towards
the root bridge The frames are sent on the root port of the
designated bridge
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Lecture 3 #28
Spanning Tree Algorithm:Selecting Root Bridge Initially, each bridge considers itself to
be the root bridge Bridges send BDPU frames to its
attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
Best one wins (lowest root ID/cost/priority)
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Lecture 3 #29
Spanning Tree Algorithm:Selecting Root Ports Each bridge selects one of its ports
which has the minimal cost to the root bridge
In case of a tie, the lowest uplink (transmitter) bridge ID is used
In case of another tie, the lowest port ID is used
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Lecture 3 #30
Spanning Tree Algorithm:Select Designated Bridges
Initially, each bridge considers itself to be the designated bridge
Bridges send BDPU frames to its attached LANs The bridge and port ID of the sending bridge The bridge and port ID of the bridge the sending bridge
considers root The root path cost for the sending bridge
3. Best one wins (lowest ID/cost/priority)
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Lecture 3 #31
Forwarding/Blocking State Root and designated bridges will
forward frames to and from their attached LANs
All other ports are in the blocking state
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Lecture 3 #32
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 #33
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
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Lecture 3 #34
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)
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Lecture 3 #35
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)
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Lecture 3 #36
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!
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Lecture 3 #37
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
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Lecture 3 #38
Ethernet Switches (more)Dedicated
Shared
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Lecture 3 #39
Optional: Wireless LAN and PPP
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Lecture 3 #40
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)
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Lecture 3 #41
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
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Lecture 3 #42
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?
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Lecture 3 #43
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
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Lecture 3 #44
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 #45
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
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Lecture 3 #46
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
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Lecture 3 #47
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!)
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Lecture 3 #48
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) connection livenes: detect, signal link failure to
network layer network layer address negotiation: endpoint can
learn/configure each other’s network address
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Lecture 3 #49
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!!!
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Lecture 3 #50
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 #51
PPP Data Frame
info: upper layer data being carried check: cyclic redundancy check (CRC)
for error detection
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Lecture 3 #52
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
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Lecture 3 #53
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
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Lecture 3 #54
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
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Lecture 3 #55
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
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Lecture 3 #56
Configuration Messages: BPDU