Transcript 例外(exception）とは何か

Lecture 7
Network Property and ISO Reference Model
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Network Ownership: Private and Public
Connectionless Network and Connect-Oriented Network
Network Performance Measures: Delay and Throughput
Relationship between Delay and Throughput
Network Protocols
ISO Network Reference Model – 7 Layer Network Model
Flow Control
- Stop-And-Go Flow Control
- Sliding Window Flow Control
• Traffic Congestion and Congestion Control
Lecture 7
Network Ownership: Private and Public
Private network: owned by individual, single organization or company
- Restricted users in an organization or company
- Complete, control, installation, management and mantainenance
- Often LAN or Multiple LANs in different buildings, different sites linked by lease lines,
and with external connects, called intranet.
- Can be WAN like bank network
Ichigaya
To Internet
T2
need lease lines from phone company
T1
T1
T1
Koganei
Tama
Public network: owned by common carrier (e.g., phone company like NTT or KDD)
- Operated by common carrier, May be telephone company or other organization
- Analogy telephone network, T-/OC- series, Frame Rely, ATM, CATV, etc.
- One connection per subscriber
* Typical for small corporation on individual
* Communicate with another subscriber
- Multiple connections per subscriber
* Typical for large, multi-site connection
* Communicate among multiple sites
Lecture 7
Connection-oriented Data Transmission
Lecture 7
Connect-Oriented Network & Connectless Network
Connection-Oriented (CO) network
- Sender and receiver establish and maintain a connection when they have data to exchange
- Sender requests “connection” to receiver, waits for network to form connection,
keeps the connection till no longer use, and terminates connection.
- Network receives connection request, form path specified destination and inform sender,
transfers data across connection, and removes connection when send requests.
- The “connection” may not be physically a circuit path
* The connection is a virtual path (VP) or virtual channel (VC) for packet switch network
Connectionless (CL) network
- Sender puts data to network without need to establish a connection similar to postal system
- Sender forms packet, places address of intended recipient in packet, put packet to network
- Network uses destination address to forward and deliver packet
- Characteristics
Header
Payload
Trailer
* Packet contains identification of destination
* Each packet handled independently
* No setup requirement before transmitting data
* No cleanup required after sending data
Dest Addr Sour Addr
Lecture 7
Comparison and Examples of CO and CL
CO
- Connection setup overhead
- More intelligence in network control
- Can reserve bandwidth, reliability, etc.
CL
- Less overhead
- Permits asynchronous use
- Allows broadcast and multicast
Lecture 7
Network Performance Measures: Delay and Throughput
Delay/Latency
Network Performance
- Measure of time for data (bit/packet) to transit network from source to destination
- Propagation delay: time to travel across medium
- Switching delay: time for network component (hub, bridge, packet switch) to forward data
- Access delay:
time to get control of medium (CSMA/CD, token)
- Queuing delay:
time enqueued in packet switches
- The former two are nearly constant, the last two are variable
Throughput
- Measure of rate at which data can be transmitted in network
- Capacity, speed and bandwidth of the underlying hardware
e.g., 34Kbps modem, 1.5Mbps T1, 6.4Mbps ADSL, 1Gbps gigabit Ethernet
- Effective throughput: the rate at which computer can send data
less than network capacity because of frame header in each packet and delay
Utilization
- Ratio of sending data rate to network capacity  U
Relationship between delay and throughput
D = D0 /(1 - U) where D0 is delay when the network is idle and D is total effective delay
- Throughput and delay are not independent: T×D=C, data volume on the network
- As traffic increase, delays increase. When U1, D∞ and T0, congestion
- Network should operates at U<75%, must not operate at U>90%
Network Throughput & Utilization
Lecture 7
Traffic Congestion and Congestion Control
Network congestion
- When the input to some link/node reaches/exceeds maximum bandwidth
- Packets will queue up at the link/node and some packets will be discarded
- Ultimately, network will experience congestion collapse
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30M
50M
20M
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Bandwidth=100Mbps
Congestion
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Congestion control techniques
- Rate control
* Limit rate of data transmission
* Performed by sending computer
* Performed by network
- Network Rate control
* Monitor incoming traffic
* Drop or reject packets over rate
* Called traffic shaping
Network Congestion Wikipedia, http://en.wikipedia.org/wiki/Network_congestion
Lecture 7
Network Protocols
Network Protocol
- Agreement about communication
* formats of messages (syntax)
* meanings of messages (semantics)
* rules for data exchange and connection establishment/termination
* procedures for handling problems
Set of Protocols
- A network includes a set of protocols to work together, called protocol suite/family
- Each protocol in a protocol suite solves part of communication problem
Need for Protocols
- Hardware is low level and can’t be directly interacted with computer
- Need mechanisms to distinguish among
* multiple computers on a network
* multiple applications on a computer
* multiple copies of a single application
- Many problems can occur
* bit error, packet lost, packet duplicated, packets in disorder, congestion, etc.
Lecture 7
ISO 7-Layer Network Reference Model
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Network protocols are very complex
Protocols are divided into layers
Each layer devoted to one sub-problem
Ease protocols design and implementation
ISO 7-layers reference model
Open System Interaction (OSI) Model
“Layer N” means the Nth layer in ISO model
Not completely same in practical networks
Layer
Layer
Layer
Layer
Layer
Layer
Layer
7:
6:
5:
4:
3:
2:
1:
Application layer - Application-specific protocols such as FTP and SMTP (email)
Presentation layer - Common formats for representation of data
Session layer - Management of sessions such as login to a remote computer
Transport layer - Reliable delivery of data between computers
Network layer - Address assignment and data delivery across a physical network
Link layer - Format of data in frame and delivery of frame through network interface
Physical layer - Basic network hardware - such as RS-232 or Ethernet encoding
OSI model – Wikipedia, http://en.wikipedia.org/wiki/OSI_model
Lecture 7
Layers, Protocol Software and Protocol Stack
- One software module per layer; Modules cooperate
- Incoming or outgoing data passes from one module to another
- Entire modules called protocol stack
Lecture 7
Protocol Layering Principle
Two constraints:
- The software for each layer depends only
on the services of the software provided
by lower layers
- The software at layer n at the destination
receives exactly the same protocol
message sent by layer n at the sender
These constraints mean that protocols can
be tested independently and can be
replaced within a protocol stack
Lecture 7
Layers and Packet Headers
Each layer
- Adds its header to outgoing packet and
- Removes header from incoming packet
Headers are nested at the front as the message traverses the network
Header, here, may include trailer
Lecture 7
Layers, Headers and Data Encapsulation
Anim
Lecture 7
Protocol Techniques
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For
For
For
For
For
For
bit corruption: parity, LRC, checksum, CRC
out-of-order delivery: adding sequence numbers to packets
duplicated packets: using the sequence numbers
lost packets: sending acknowledge (ACK) information and retransmission
reply delay (excessive delay): unique message ID
data overrun: flow control
* Stop-and-go flow control
* Sliding window flow control
P5P4P3P2P1
Sender
Rate Ds in sending
P5P5P2P3P1
network
Data overrun if Ds > Dr
Network throughput Dn (> Ds)
out-of-order: P2P3
duplicated: P5P5
lost packet: P4
Receiver
Rate Dr receiver can process
Lecture 7
Connection-oriented Data Transmission
Lecture 7
Stop-And-Go Flow Control
- Allow sender sends one packet each time and waits
-Sending side
* Transmits one packet
* Waits for acknowledgement from receiver
- Receiving side
* Receives and consumes packet
* Transmits acknowledgement to sender
-Inefficient
* No data sending in waiting time
Anim
Flow Control -Wikipedia,
http://en.wikipedia.org/wiki/Flow_control_(data)
time
time
Lecture 7
Sliding Window Flow Control
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Allows sender to transmit multiple packets before receiving an acknowledgment
Number of packets that can be sent is defined by the protocol and called the window
Window size is determined by the empty buffer in receiver
Receiver tells how many packets can be sent
Sender transmits a number of packets, specified by available window (buffer) size
Receiver sends acknowledgements as packets arrive
Anim
Lecture 7
Comparison of Flow Control
- Sliding widow can send data
much fast than stop-and-go
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L
- For stop-and-go, each packet takes
2L time to deliver where L is the
latency, or network delivery time.
- Sliding window can improve by
number of packets in window:
Tw = Tg * W
Tw is sliding window throughput
Tg is stop-and-go throughput
- Transmission time also limited by
network transmission rate:
Tw = min(B, Tg * W)
B is maximum network bandwidth
Routing table of switch 2
Exercise 7
1. Explain what are the connection-oriented communication and the connectionless
communication. Give some examples for each of the above two types of communications.
2. Describe concepts of delay, throughput, bit corruption (error), disordered packet,
duplicate packet, lost packet, flow control and network congestion, respectively.
3. Professionals sometimes refer to a “knee” in the delay curve. To understand what
they mean, plot the effective delay for values of utilization between 0 and 0.95.
Explain why utilization of a network should not approach to 1.
4. Match the following to one of the ISO 7 layers:
a. Route determination.
b. Flow control.
c. Provides reliable end-to-end transmission of entire message.
d. Reassembly of data packets into a message.
e. Provides format and code conversion services.
f. Provides user services such as electronic mail and file transfer.
g. Provides verification of login and logout.
h. Mechanical, electrical, and functional interface.
5. Why does a stop-and-go protocol have especially low throughput over a satellite channel
that operates at two megabits per second. How to improve throughput using different
flow control method?