Hubs, Bridges and Switches

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Hubs, Bridges and Switches

• Lecture 3

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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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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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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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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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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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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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Backbone Bridge
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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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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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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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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_port
• if (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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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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Bridge Learning example
C 1

• 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

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What will happen with loops?Incorrect learning
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What will happen with loops?Frame looping
C
2
2
C,??
C,??
1
1
A
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What will happen with loops?Frame looping
B
2
2
B,2
B,1
1
1
A
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Loop-free tree
C
B
A message from Awill mark As location
A
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Loop-free tree
C
B
A ?
A message from Awill mark As location
A
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Loop-free tree
C
A ?
B
A ?
A message from Awill mark As location
A
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Loop-free tree
A ?
A ?
C
A ?
B
A ?
A ?
A message from Awill mark As location
A
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Loop-free tree
A ?
A ?
C
A ?
B
A ?
A ?
A message from Awill mark As location
A
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Loop-free tree
A ?
A ?
C
A ?
B
A ?
A ?
So a message toA will go by marks
A message from Awill mark As location
A
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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

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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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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

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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 bridge
• So we solve in order
• How to find a root bridge?
• How to compute a ST of bridges?
• How to compute a ST LAN segments?

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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?!

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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

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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

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Why does this work?

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

?
E
?
?
D
?
C
A
0
?
G
?
B
?
F
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Why does this work?

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

?
E
1
1
D
?
C
A
1
0
G
?
1
B
F
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Why does this work?

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

2
E
1
1
D
2
C
A
1
0
G
?
1
B
F
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Why does this work?

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

2
E
1
1
D
2
C
A
1
0
G
3
1
B
F
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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...)

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3. ST of LAN segments

• Assumption given a ST of the bridges
• Idea 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 ports LAN

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Spanning Tree ConceptsPath 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

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Spanning Tree ConceptsRoot 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

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Example Spanning Tree

• Protocol operation
• Pick a root
• Each bridge picks a root port

B8
B3
B5
B7
B2
B1
B6
B4
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Example Spanning Tree
B8
Spanning Tree
B3
B5
B1
root port
B7
B2
B7
B4
B5
B6
B2
B1
Root
B8
B3
B6
B4
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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!

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Example Spanning Tree
B8
Forwarding Tree
B3
B5
B1
root port
B7
B2
B2
B4
B5
B7
B1
Root
B8
Designated Bridge
B6
B4
Note B3, B6 forward nothing
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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
  bridges
• Bridge id port number

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Spanning Tree AlgorithmImplementation

• Keep 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 by ID
• Thats enough!

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Spanning Tree AlgorithmSelect Designated Bridges

• Bridges send BPDU frames to its attached LANs
• sender port ID
• bridge and port ID of the bridge the sending
  bridge considers root
• root path cost for the sending bridge
• 3. Best bridge wins, and it knows it (and winning
  port)
• (lowest ID/cost/priority)

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Forwarding/Blocking State

• 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.
• All ports send BPDU messages
• To adjust to changes

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Spanning Tree Protocol Execution
B8
B3
B5
B7
B2
B1
(B1,rootB1, dist0)
(B1,rootB1,dist0)
B6
B4
(B4, rootB1, dist1)
(B6, RootB1dist1)
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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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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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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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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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Ethernet Switches

• cut-through switching frame forwarded from input
  to output port without awaiting for assembly of
  entire frame
• slight reduction in latency
• combinations of shared/dedicated, 10/100/1000
  Mbps interfaces

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Ethernet Switches (more)
Dedicated
Shared
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Optional Wireless LAN and PPP
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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
• BSSs combined to form distribution system (DS)

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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 devices
• battlefield
• IETF MANET (Mobile Ad hoc Networks) working
  group

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IEEE 802.11 MAC Protocol CSMA/CA

• 802.11 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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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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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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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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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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Point to Point Data Link Control

• one sender, one receiver, one link easier than
  broadcast link
• no Media Access Control
• no need for explicit MAC addressing
• e.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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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 others network address

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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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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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PPP Data Frame

• info upper layer data being carried
• check cyclic redundancy check (CRC) for error
  detection

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Byte Stuffing

• data transparency requirement data field must
  be allowed to include flag pattern lt01111110gt
• Q is received lt01111110gt data or flag?
• Sender adds (stuffs) extra lt 01111101gt byte
  before each lt 01111110gt or lt01111101gt data byte
• Receiver
• Receive 01111101
• discard the byte,
• Next byte is data
• Receive 01111110 flag byte

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Byte Stuffing
flag byte pattern in data to send
flag byte pattern plus stuffed byte in
transmitted data
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PPP Data Control Protocol

• Before 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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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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Configuration Messages BPDU