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Chapter 18. by David G. Messerschmitt. Objectives. Major functions of the network Impact of network on application performance Collective issues in networking. Impact of network on applications. Communication service provided the application What does application have to do for itself?

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chapter 18

Chapter 18


David G. Messerschmitt

  • Major functions of the network
  • Impact of network on application performance
  • Collective issues in networking
impact of network on applications
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
internet protocol architecture
Internet protocol architecture


Remote method invocation (RMI)


Datagram (UDP)

Bytestream session (TCP)


Internetworking (IP)



network topology
Network topology





Access links

properties of a communication link
Properties of a communication link

Bits in transit

Bits waiting to be sent

Bits that have arrived




Bitrate = number transmitted per second

(Bitrate is sometimes called “bandwidth”)

Propagation delay = input-output delay experienced by each bit

sending packets on a link
Sending packets on a link


Packet 1

Packet 2

Packet 3

Bitrate > # packets per sec x average size of a packet

There must be some way (bit patterns) for the receiver to distinguish packets

statistical multiplexing
Statistical multiplexing

Messages on three

incoming links





What are some other examples of statistical sharing?

  • 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
packet structure
Packet structure



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



Packet forwarding

Packet switch


Output link

Routing table

two basic functions in packet switching
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
Full route not written down

Each packet switch has >1 output link

Routing table :

Reducing table size

use wildcards: 141.211.* next hop is X

routing function
Switches (routers) talk to each other

“I’m 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

Routing function
routing dangers
Long routes

Circular routes

HopLimit limits damage

Decrement HopLimit at each router

Discard packet if HopLimit=0

Hijacking routes

Advertise a route, but don’t deliver

Route flapping

Frequent updates to routing table

Routing Dangers
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!

Packet delivery not guaranteed
why packets
Why packets?
  • Fairness: short message doesn’t 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
ip addresses
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:
    • 4 billion addresses
  • IPv6 (future standard)
    • Each address 128 bits
    • ~1500 addresses per square foot
  • Recall three ways of locating something:
  • Address
  • Name
  • Reference
domain name service
IP addresses are inconvenient for people

32 bits hard to remember

128 bits very hard to remember

Domain names


Domain Name Service (DNS)

get an IP address from a domain name

Domain Name Service
hierarchy in location
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.
  • DNS decouples these two issues
forms of broadcast
Forms of broadcast



Multicast requires more sophisticated addressing and routing within the network

flow control
Flow control





of messages

Flow control

Normally the producer determines what information is sent, but consumer has to have a way to slow down producer

network congestion
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



Congestion instability

Network “capacity”

Increasing portion of

network traffic is resent






questions to address
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?
congestion control methods
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?

quality of service qos

Quality of service (QoS)


David G. Messerschmitt

qos attributes of a packet
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
transport services
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
packet latency affects transport service qos
Packet latency affects transport service QoS

Stream of










reliable delivery
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)
qos guarantees
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
achieving qos
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
pricing and accounting
Pricing and Accounting
  • What’s 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
pricing today
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
congestion pricing rationale
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 > 0
    • But capacity is fixed (in the short run)
    • Person A’s use may reduce B’s quality of service
congestion pricing
Congestion Pricing
  • If network is underutilized, charge nothing
  • If network is congested
    • Charge person A the amount of B’s lost value
    • A will drop out unless his value of use is greater than B’s lost value
  • Theorem: this raises enough money to expand the network by the socially optimal amount
  • Monitoring and billing overhead
big advantage of pricing
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