Transcript Chapter8

Computer Networks
Chapter 8 – Generic Connectionless
Service
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Connectionless Network Issues
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Data Transfer
Addresses
Hop count
Service Classes
Network feedback
Fragmentation and Reassembly
Maximum Packet Size discover
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Data Transfer
• Still need three levels in interface
– PHY
– Link
– Network
• Data transfer by send/receive message
• Network packet header contains:
– Source & destination NSAPs
– Hop count to purge looping packets
– Maybe other stuff, too
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Addresses
• Network Service Access Point (NSAP)
• Fixed size
– Larger for scalability, configurability
– AppleTalk – 24 bits!
– IPv4 – 32
– IPX – 60
– IPv6 – 128
• Hierarchical for ease of routing
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Hop Count
• Need for Hop Count
– Routing changes
– Topology changes
– Limit extent of travel (bcast/mcast)
• Hop Count Implementation
– Called Time to Live (TTL) in IP, CLNP
– Too hard to measure real time, so count hops
– Increments in IPX, decrements in others
– IPv6 calls it “Hop Limit”
• Use by Traceroute
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Service Class Information
• Need SC info in every packet
• Priority
• Bandwidth reservation/service guarantees
– Requires path setup, state!
• Special route computation … really?
– Delay
– Bandwidth
– Residual error rate
– Cost ($$$)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Network Feedback
• Advisory information from network
• Error reports
– Delivery failure, …
• Congestion information
– Reduce usage, …
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Fragmentation and Reassembly
• X.25 uses one bit … connection-oriented!
• Datagram Network has:
– Lost packets
– Duplicate packets
– Out of order arrivals
• Repeated fragmentation: use offset
• Destination must know when complete
– Total length (CLNP)/More fragments flag (IP)
– Track “holes”
• Reassembly timeout
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Maximum Packet Size Discovery
• Maximum Transmission Unit (MTU)
– Largest DG that can be sent w/o fragmenting
– Want to avoid fragmenting – costly….
• Flag in header for legal to fragment
– If must fragment, then delivery error sent back
• Approaches
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“Too big” error message – binary chop
Provide MTU size in error message – no help
Truncate and deliver – one packet does it
Fragment to MTUs, not evenly
• All fail if different routes used….
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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