David G' Messerschmitt

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Chapter 18

• by
• David G. Messerschmitt

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Objectives

• Major functions of the network
• Impact of network on application performance
• Collective issues in networking

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Impact of network on applications

• Communication service provided the application
• What does application have to do for itself?
• Impact on application performance
• Message (packet) latency
• Message (packet) loss
• Message (packet) corruption

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Internet protocol architecture
Application
Remote method invocation (RMI)
Middleware
Datagram (UDP)
Bytestream session (TCP)
OS
Internetworking (IP)
Network
Subnetworks
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Network topology
Switches
Hosts
Backbone links
Access links
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Properties of a communication link
Bits in transit
Bits waiting to be sent
Bits that have arrived
00110110
010101110101010
00001011011011
Bitrate number transmitted per second (Bitrate
is sometimes called bandwidth) Propagation
delay input-output delay experienced by each bit
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Sending packets on a link
0101101011010011011010110101010101011010
Packet 1
Packet 2
Packet 3
Bitrate gt packets per sec x average size of a
packet
There must be some way (bit patterns) for the
receiver to distinguish packets
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Statistical multiplexing
Messages on three incoming links
Single outgoing link
Queues
What are some other examples of statistical
sharing?
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Congestion

• Cause irregular packet arrivals, and irregular
  packet length
• Direct effect sometimes more packets have to
  wait in queues for space on output link
• In severe cases, packets discarded
• Indirect effect waiting time in queues
  contributes to packet (and hence message) latency

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Packet structure
Header
Payload

• Header
• Indicate beginning of packet
• Destination address for forwarding
• Other information specified by network protocols
• Payload
• Unstructured data to be delivered to application

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Packet forwarding
Packet switch
Packet
?
?
Output link
?
Routing table
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Two basic functions in packet switching

• Packet forwarding
• Transmitting each packet on the appropriate
  output link
• Based on routing table
• Routing
• Updating the routing table
• Objective each packet gets closer to destination
  via less congested links

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Routing

• Full route not written down
• Each packet switch has gt1 output link
• Routing table
• Reducing table size
• use wildcards 141.211. next hop is X

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Routing function

• Switches (routers) talk to each other
• Im now accepting traffic for 141.211.
• Called advertising a route
• Is anyone accepting traffic for 141.211.?
• Update entries in own routing table
• Sophistication is in deciding whether to accept
  traffic

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Routing Dangers

• Long routes
• Circular routes
• HopLimit limits damage
• Decrement HopLimit at each router
• Discard packet if HopLimit0
• Hijacking routes
• Advertise a route, but dont deliver
• Route flapping
• Frequent updates to routing table

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Packet delivery not guaranteed

• Intermediate switch not responding
• Temporary malfunction
• Queues full (congestion)
• Bad routing
• Too long or circular
• Corruption of packet, especially header
• Network does not inform sender
• Network may not know!

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Why packets?

• Fairness short message doesnt have to wait for
  long message
• Allows messages to be sent concurrently
• Reduces statistical waiting time
• Store and forward delay reduced
• Data garbling may necessitate resending only a
  packet, not a whole message

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IP Addresses

• Every host gets a distinct address
• Can be dynamically assigned
• IPv4 (current standard)
• Each address 32 bits
• Divide into 8 bit segments
• Example 141.211.203.32
• 4 billion addresses
• IPv6 (future standard)
• Each address 128 bits
• 1500 addresses per square foot

• Recall three ways of locating something
• Address
• Name
• Reference

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Domain Name Service

• IP addresses are inconvenient for people
• 32 bits hard to remember
• 128 bits very hard to remember
• Domain names
• e.g. www.sims.berkeley.edu
• Domain Name Service (DNS)
• get an IP address from a domain name

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Hierarchy in location

• Addresses hierarchical in topology
• Maximize wild cards and distribute address
  administration
• Names hierarchical in administration
• Single administered organizations often
  distributed topologically (e.g. ibm.com)
• DNS decouples these two issues

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Forms of broadcast
Simulcast
Multicast
Multicast requires more sophisticated addressing
and routing within the network
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Flow control
Producer
Consumer
Queue
Stream of messages
Flow control
Normally the producer determines what information
is sent, but consumer has to have a way to slow
down producer
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Network congestion

• Fluctuations of traffic can result in overloads
  in given network links
• Failure of statistical multiplexing
• Analogous to processing congestion of a server,
  except resent packets can make the problem worse
• Congestion must be limited in some fashion

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Carried traffic
Congestion instability
Network capacity
Increasing portion of network traffic is
resent packets
Offered traffic
Social optimum
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Questions to address

• What social objectives should a congestion
  control method achieve?
• What technical approaches are available to
  achieve those objectives?
• What is the cost of those approaches?
• Ultimately, what will the customer pay for
  congestion control, and how does that relate to
  the value received?

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Congestion control methods

• Over-provisioning of facilities (mitigation, not
  control)
• Network initiated
• Network ? source flow control, or
• Source notification and policies, policing, or
  pricing incentives, or
• Admission control for sessions
• Sources initiated
• Source detects congestion (necessary resent
  packets is one method), and
• Voluntary or mandatory policies

Examples of each?
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Quality of service (QoS)

• by
• David G. Messerschmitt

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QoS attributes of a packet

• Packet latency
• Time until packet delivered at destination
• Transmission time, propagation time, queuing
  delay, processing time
• Packet loss
• Packet corruption
• Payload only
• Normally network will not deliver corrupt packet

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Transport services

• Raw packet service is not what is needed by
  most applications
• Transport services condition packet service by
  adding layers
• Reliable delivery
• Message service
• Session
• Time stamps
• etc

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Packet latency affects transport service QoS
Stream of packets
Audio coder
Audio decoder
Packet latency
End-to-end delay
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Reliable delivery

• Add acknowledgement for each packet
• Lost packet can be detected by missing ACK
• Lost packet can be retransmitted
• Tradeoff
• Reliable delivery for greater latency
• Latency-sensitive applications must abandon
  reliable delivery (e.g. remote conferencing)

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QoS Guarantees

• Source and network enter session contract
• Source promises not to exceed specified traffic
  parameters for that session
• Rate and burstiness
• Network promises to limit impairments such as
  latency, loss, and corruption

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Achieving QoS

• TCP/IP offers only best-effort
• Every connection gets best-effort service
• Achieving maximum latency guarantees
• Reserve resources
• Or attach priorities to packets
• Contract may allow network to delay or discard
  low-priority packets when necessary
• Application may guarantee traffic shape
• e.g., steady flow rather than bursts

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Pricing and Accounting

• Whats the incentive for using low quality
  service?
• Why should user accept greater latency if less is
  an option?
• Why should application try to minimize bandwidth,
  or shape its traffic?
• Answer is good citizenship, or pricing incentives

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Pricing Today

• Commercial services usually flat rate plus
  connect time (but no per-bit charges)
• e.g., CompuServe, Prodigy, America On-Line
• Internet flat rate, unlimited usage
• Resellers can charge for connect time
• Many people have unlimited use through a
  university or company

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Pricing options
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Congestion Pricing Rationale

• The fixed cost of building the network is high
• The marginal cost of accepting new session is
  nearly zero (assuming not congested)
• Economic efficiency use whenever marginal
  benefit gt 0
• But capacity is fixed (in the short run)
• Person As use may reduce Bs quality of service

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Congestion Pricing

• If network is underutilized, charge nothing
• If network is congested
• Charge person A the amount of Bs lost value
• A will drop out unless his value of use is
  greater than Bs lost value
• Theorem this raises enough money to expand the
  network by the socially optimal amount
• Monitoring and billing overhead

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Big advantage of pricing

• Congestion pricing uses incentives rather than
  forced control or policies to affect
  user/application behavior
• Market mechanism like other goods and services
• User/application can determine freely and
  independently whether use of network during
  periods of congestion is warranted
• Major objection is cost of monitoring and billing