Transcript Internetworking

Internetworking
• Different networks
– Different bit rates
– Frame lengths
– Protocols
Internetworking
• Interconnect at
– Physical Layer - Repeaters
• e.g. connecting 802.3 segments
– Layer 2 - Bridges
• LANs with same upper layer 2 LLC
• Different or same MAC Layer
– Layer 3 - Routers
• Typically LAN-WAN, WAN-WAN
– Layer 4 or above - Gateways
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• Emphasis is on Bridges
– Can be local or remote
• Remote – bridge split in half
– Network in between
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• Reason for Interconnecting
– To communicate across legacy
networks
– To interconnect several dispersed
LANs
– To improve performance
– Reliability
– Security
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• Problems
– Different frame formats
• processing delays
– Different Speeds
• queueing delays
– Different frame lengths
• segmentation not possible
– Protocol Differences
• automatic ACKs
– Priorities
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• Types of Bridges
– Transparent Bridges
– Source Routeing (802.5)
• Principle is simple
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• Transparent Bridges
– Main Decision for Bridge
• When should frame be forwarded?
– Forward if destination address is
NOT on the LAN on which it
arrived
– (Otherwise it is already on the
correct LAN for delivery)
Internetworking
• The decision on whether to
Forward raises the following
questions
– How does the bridge know which
port to forward on?
• Maintains an address table which
associates destination addresses with
ports
• (Much the same as a routeing table
in a WAN)
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• What if there is no entry for the
destination address in the table?
• Flood the frame on every port
(Clearly not the port it arrived on)
– How is address table compiled
and maintained?
• By learning (transparently) where
everything is
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• Learning
– Frames arriving at bridge
• Note source address and port
number
• New table entries may be created if
entry doesn’t already exist
• Existing entries are confirmed
– Bridge is continually learning and
maintaining network topology
information
Frame received OK
at some port, X
Forwarding Phase
Destination
Known?
Forward frame on
all LANs except X
Yes
Yes
Destination
on LAN X?
No
Discard frame
No
Forward frame to
next (known) LAN
Source Add.
known?
No
Learning Phase
Yes
Update direction
and time
End
Add Source Add. to
database. Note
direction and time
Internetworking
• Looping
– Because of topology, captured in
routeing information, frames may
circle (loop) around the network
many times.
– This problem also exists in WANs
(Time-to-live field in IP)
– Topology will almost certainly
have included loops for reliability
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• Result - Spanning Tree
– Logical tree which overlays the
actual network
– Achieved by
• Allowing some ports to forward
(those in tree)
Blocking remainder
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• Spanning tree construction
– Identify root bridge
• Bridge with lowest address
• Initially all assume they are root
• Actual root identified after
information exchange
– Identify root port
• Root bridge doesn’t do this
• Bridge port with ‘cheapest’ route to
root bridge
– Identify root bridge for each LAN
• Bridge with ‘cheapest’ route to root
bridge
– Choose designated port for each
LAN
Internetworking
• Source Routeing Bridges
– Typically used with 802.5 LANs
(IBM)
– Source determines the complete
route
• (Other approach is link-by-link)
– Complete route is carried in
header
• Series of Bridge number / LAN
(port) number
• Carried after address information
• e.g. B4/L3, B2/L6 etc
Internetworking
• Route Discovery
– By source
– Broadcast indication frame sent to
destinationalong single route
• (Where are you?)
– Destination responds by sending
broadcast frame along multiple
routes
– Using selective flooding, multiple
copies arrive back at source
– Contain details of route they
followed
– Source selects most appropriate
route
– This is carried in every frame.
Internetworking
• Spanning tree is a solution to
looping problem
– Produces tree which links all
LANs
– Exactly one (enabled) route
between any two interconnected
LANs
– 802.1 Spanning Tree Algorithm is
distributed
– Similar to tree constructing
algorithms used in routeing /
topology design
– Bridges exchange information to
construct tree
Internetworking
• Transparent versus Source
– Transparent - topology
independent
– Source - needs to know about
topology
– No information frame overhead
with transparent
– Route discovery frames are extra
traffic
– Source can give current optimal
route
– Transparent gives current
spanning tree route