Transcript CSCI 6450: Principles of Distributed Systems - royal hackeres 2010-14

Network Technologies
&
Principles
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Network Technologies &
Principles
 Communication Subsystem.
 Types of Network.
 Principles of Network.
 Distributed Protocols
Communication Subsystem
 The hardware and software within a distributed system
which provides the communication facilities is known as the
communication subsystem.
 Consists of:
– Transmission media: providing the physical
connectivity, e.g. wire, cable, fiber and wireless channels;
– Hardware devices: providing the linkage, e.g. routers,
bridges, hubs, repeaters, network interfaces and
gateways;
– Software components: managing the communication,
e.g. protocol stacks, communication handlers and
drivers.
Network Types
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 Local Area Networks (LANs)
– High-speed communication on proprietary grounds (oncampus).
– Based on twisted copper wire, coaxial cable or optical
fibre.
– Total system bandwidth is high and latency is low.
– Most typical solution: Ethernet with 100 Mbps
 Metropolitan Area Networks (MANs)
– High-speed communication for nodes distributed over
medium-range distances, usually belonging to one
organization.
– Based on high bandwidth copper and optical fibre.
– Providing "back-bone" to interconnect LAN's.
– Technology often based on ATM, FDDI or DSL.
 Wide Area Networks
– Communication over long distances (cities,
countries, or continents).
– Covers computers of different organizations.
– High degree of heterogeneity of underlying
computing infrastructure.
– Involves routers to manage network and route
messages to their destinations.
– Speeds up to a few Mbps possible, but around 50100 Kbps more typical.
– Most prominent example: the Internet.
 Wireless Networks
– End user equipment accesses network through short
or mid range radio or infrared signal transmission
– Wireless WANs:
• GSM (up to about 20 Kbps), UMTS (up to Mbps),
PCS.
– Wireless LANs/MANs:
• WaveLAN (2-11 Mbps, radio up to 150 meters).
– Wireless Personal Area Networks:
• Bluetooth (up to 2 Mbps on low power radio
signal, < 10 m distance).
Network Principles
Packet Transmission
 A packet is a sequence of binary data with addressing
information to identify the source and destination
computers.
 A network message with arbitrary length is divided
before transmission into packets of restricted length.
 Restricted length packets are used:
– To allow each computer in the network to allocate
sufficient buffer storage to hold largest possible
incoming packet.
– To avoid long waiting for communication
channels to be free if long messages ware
transmitted without subdivision.
Network Principles
Switching Schemes
 A switching system is required to transmit information
between two arbitrary nodes in the network using shared
communications link.
 Four types of switching are used in computer network:
– Broadcast:
• Requires no switches.
• All messages are sent to all connected computers.
• Each computer is responsible extracting messages
addressed to itself.
• Used approach in Ethernet and wireless networks.
– Circuit switching:
• Approach taken in the telephone system.
• A physical link is established between the sender
and the receiver.
– Packet switching:
• Otherwise known as store-and-forward (postal
system).
• At each switching node (connection point) a
computer manages the packets by reading each
one into memory, examining its destination, and
choosing an outgoing circuit appropriately.
– Frame relay:
• Reading in and storing the whole of each
packet introduces a performance overhead
which can become significant.
• In ATM networks a frame of fixed size is used
in place of a packet and only its header needs
to be examined.
• The remainder of the frame is simply relayed
as a stream of bits.
Networking Performance
Parameters
 Latency - time to transfer “empty” message
 Bandwidth or data transfer rate - how many
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bits/sec can be transferred (how thick the “pipe” is)
message_transfer_time = latency +
msg_length / data_transfer_rate
 Consider: a modem connection vs. a van of
magnetic tapes traveling an interstate highway
 QoS: Quality of Service (bandwidth/latency
guarantees for particular connections)
OSI Protocol Stack
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 OSI - Open Systems Interconnect
 Application - application interfaces (httpd, ftp)
 Presentation - network representation for
data
 Session - connections, encryption
 Transport - message  packets
 Network - network-specific packets, routing
 Data Link - transmission of packets between
“directly” connected machines + error issues
 Physical - hardware (“I can touch it”)
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Communication Through Layers
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
TCP/IP Protocol Stack
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 ISO stack is good as a model for understanding networks
 Layers in “real” network stacks aren’t so differentiated
 TCP/IP stack has won primarily because of the free
implementation shipped in early versions of BSD Unix
 Addresses above IP are (port, address) combinations
Application
Transport
Network
Application
UDP
TCP
IP
Physical
Transport Protocols
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 UDP (User Datagram Protocol)
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Connectionless
Fast setup
Easy one-to-many communication
Datagram-oriented (fixed size chunks of data)
Packet reordering
Packet loss (no flow control, bad packets dropped)
Packet duplication
(Absolute) maximum datagram length: 64K
Usable maximum is more complicated
8K is generally safe for modern systems
Transport Protocols, Cont.
 TCP (Transmission Control Protocol)
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Connection-oriented
Byte stream-oriented
Slower setup
Consumes file handles: one per connection
Flow control, automatic retransmission
• No packet reordering (delivery is FIFO)
• No packet loss
• No duplication
– Theoretically “no” limit on size of objects that
can be dumped into a TCP stream
– In practice, limits exist
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