Computer Networks

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Computer Networks
Data link layer
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Overview

• Design issues
• Point-to-point links
• Local area Networks
• Data Link layer Switching

• Multiple LANs?
• From 802.x to 802.y
• Transparent bridges
• Spanning Tree bridges
• Remote bridges
• Interconnections ltgt layers
• Virtual LANs

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Multiple LANs?

• Why different LANs in a single organisation?
• Autonomy of owners
• Distance too great for a single LAN
• Load too high for a single LAN
• Reliability a single defective node will cripple
  onle a single LAN
• Security most LANs offer a promiscuous mode
  (receive all packets)
• Solution at data link layer

Bridge switch
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From 802.x to 802.y

• How do bridges work?

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Bridges from 802.x to 802.y

• Common problems different
• data rates
• maximum frame length
• security
• quality of service
• frame formats

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Overview

• Design issues
• Point-to-point links
• Local area Networks
• Data Link layer Switching

• Multiple LANs?
• From 802.x to 802.y
• Transparent bridges
• Spanning Tree bridges
• Remote bridges
• Interconnections ltgt layers
• Virtual LANs

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Transparent bridges

• Goal interconnect LANs with a bridge and
  everything should work perfectly, instantly
• No software changes
• No hardware changes
• No setting of address switches
• No downloading of routing tables
• How? Promiscuous mode receive packet
• Source LAN destination LAN ? discard frame
• Destination LAN unknown ? flooding (on LAN
  level)
• Destination LAN known ? forward frame
• Learn about LANs? Backward learning construct
  routing table
• Initially empty
• Receive frame from node A via LAN L ? A
  reachable via L
• Dynamic topologies attach timestamp to entries

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Transparent bridges
A
L1
L2
Bridge
frame
B
L3
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Transparent bridges
A
L1
L2
Bridge
B
L3
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Transparent bridges
A
L1
L2
Bridge
B
L3
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Spanning Tree bridges

• Bridges in parallel
• To increase reliability ? loops in topology
• To increase forwarding capacity
• Problem
• Frame forwarding/flooding forever
• SolutionSpanning tree Bridges

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Transparent bridges

• Spanning tree bridges
• Solution transparent to hosts
• Bridges construct spanning treeone path from
  every LAN to every other LAN
• All forwarding follows spanning tree
• Spanning tree construction
• Select root of tree bridge with lowest serial
  number (unique by construction broadcast is used
  to inform all bridges)
• Distributed algorithm to add (shortest) paths

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Transparent bridges

• Spanning tree bridges

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Remote bridges

• Interconnect distant LAN
• Bridge on each LAN
• Point-to-point line between bridges
• Routing?
• No difference
• Point-to-point line LAN without hosts

• Protocol
• Select point-to-point protocol
• Inside frame
• Complete MAC frame
• Only LLC frame (without MAC header, trailer)

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Overview

• Design issues
• Point-to-point links
• Local area Networks
• Data Link layer Switching

• Multiple LANs?
• From 802.x to 802.y
• Transparent bridges
• Spanning Tree bridges
• Remote bridges
• Interconnections ltgt layers
• Virtual LANs

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Interconnections ltgt layers

• Which device in which layer?
• Frames, packets and headers

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Interconnections ltgt layers

• Repeater
• Analog devices to interconnect cable segments
• Signal on one segment is amplifies and put on
  other segment
• Not aware of frames, addresses
• Hub
• Frames arriving on one line are sent out on all
  the other lines
• A single collision domain
• Not aware of frames, addresses

Physical layer
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Interconnections ltgt layers

• Bridge
• Interconnects LANs
• Each line has its own collision domain

• Switch
• Interconnects hosts
• cut-through switches
• Forwarding starts when header is received

Data link layer
Difference?
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Overview

• Design issues
• Point-to-point links
• Local area Networks
• Data Link layer Switching

• Multiple LANs?
• From 802.x to 802.y
• Transparent bridges
• Spanning Tree bridges
• Remote bridges
• Interconnections ltgt layers
• Virtual LANs

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Virtual LANs

• Example of a wiring infrastructure
• Logical or physical configuration of LANs?

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Virtual LANs

• LANs should to reflect organizational structure
• Security
• Load
• Broadcasting
• Solutions?
• LAN hub rewiring
• Rewiring in software ? VLAN
• Implemented by specially-designed VLAN aware
  switches/bridges

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Virtual LANs

• Example VLANs White Gray
• How to forward a (broadcast) packet sent out by A?

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Virtual LANs

• How to forward a (broadcast) packet sent out by
  A?
• Forwarded by bridge/switch on all Gray ports
• What is the color of an incoming frame?

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Virtual LANs

• Coloring incoming frames? VLAN color assigned
  to
• Port
• Does not work for bridges
• MAC address
• Use layer 3
• Could be useful
• Violates independence of layers
• Color frames ? IEEE 802.1Q
• Changes in Ethernet header to support VLANs

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IEEE 802.1Q

• Bridges/switches fill in the VLAN color
• Used on lines interconnecting bridges/switches

• Future ethernet cards will be 802.1Q compliant?
• Max length increased from 1518 to 1522 bytes

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IEEE 802.1Q

• VLAN fields
• VLAN protocol ID (gtmax length)
• Pri hard real-time ltgt soft real-time ltgt
  time-insensitive
• CFI Canonical Format indicator 802.5 frame

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IEEE 802.1Q

• Configuration of bridges/switches
• Autoconfiguration
• If only 802.1Q frames arrive
• Learning
• Manual
• For mixed installations

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Computer Networks
Data link layer