Introduction to Networking and Distributed Systems

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Introduction to Networking and Distributed Systems

• Operating Systems
• Fall 2002

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Overview

• Intro to Networking
• Review of Basic principles
• layers, protocols, etc
• TCP/IP and the Internet
• Implementation issues
• Intro to Distributed systems
• client/server

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Communication in everyday life
Jaime les lapins
I like rabbits
L Dutch
L Dutch
Ik hou van konijnen
Ik hou van konijnen
Fax
Fax
L Dutch
L Dutch
Ik hou van konijnen
Ik hou van konijnen
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Computer based communication
mail
mail
message
mail
mail
encode
encode
message
rep
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Computer based communication
mail
mail
encode
encode
message
rep
mail
mail
encode
encode
packets
packets
mess
rep
pkt
age
rep
pkt
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Computer based communication
mail
mail
encode
encode
packets
packets
route
route
route
message
rep
pkt
addr
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Computer based communication
mail
mail
encode
encode
packets
packets
route
route
route
error
error
error
message
rep
pkt
addr
ECC
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Layered organization

• Computer networks are structured as layer
  (protocol) stacks
• Each layer provides services to the layer above
• Layers communicate by protocols

service
service
protocol
API
API
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Protocols

• Protocol is a collection of message formats and
  their interpretation needed to implement the
  layer service
• format headerdata
• headers convey the protocol specific information

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Standards and openness

• Protocols must be standardized for
  interoperability
• Layer APIs must be standardized for portability
• Existing standards
• ISO OSI Layered model (eight-layer model)
• TCP/IP is a de-facto model of the Internet

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ISO OSI Layered model
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
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Lower OSI Layers

• Physical layer
• Voltages to represent 0/1, how many bits per
  second can be sent, simplex/duplex, hardware
• Data Link layer
• Grouping bits into frames,
• point-to-point reliability, error
  detection/correction

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Network and Transport

• Network layer
• Getting data from one host to another
• Routing, addressing, fragmentation/reassembly
• Transport layer
• End-to-end application connectivity
• Reliable transport connection
• Messages are not lost and delivered in the order
  they were sent
• Transmission Control Protocol (TCP)

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Session and Presentation

• Session layer
• Enhanced transport layer session control
• Presentation layer
• Message structure e.g., records
• Data representation
• compression and encryption

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Application Layer

• General-purpose high level services and
  abstractions
• File transfer (FTP)
• Hyper-text transfer (HTTP)
• Remote Procedure Call (RPC)
• Network File System (NFS)
• Object Request Brokers
• CORBA, Java RMI
• Etc...

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TCP/IP Protocol Suite
TCP/IP protocol suite
OSI model
Application
Application
Presentation
Session
TCP/UDP
OS
Transport
Network
IP
Network access
Datalink
Hardware
Physical
Physical
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Physical Networks

• Each computer is directly attached to some
  physical network
• Network Interface Cards (NIC)cables
• Usually limited geographical scale (Local Area
  NetworkLAN)
• MAN, WAN
• supports routing, addressing, network access
  within its boundaries
• LAN Ethernet (10Mbit/sec, 100Mbit/sec), Token
  ring, Fibre channel, etc...

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Physical networks characteristics

• Bandwidth Number of bits per second
• Latency The time it takes to send a single bit
• Ethernet
• Shared bus bandwidth 10Mbit/sec
• Switched Ethernet bw 100Mbit/sec

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Ethernet LAN

• Shared bus
• Each NIC has a unique address
• Messages are broadcast
• If collision occurs, wait for a randomly chosen
  time period and retry

Maximum Transfer Unit (MTU) 1536 Bytes
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Internet protocol (IP)

• Creates an illusion of a single physical network
  over a collection of heterogeneous LANs
• Global addressing, routing
• Fragmentation, reassembly to overcome differences
  in the MTU sizes
• Architecture LANs interconnected by routers
  (gateways)

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Internet Protocol (IP)

• Connectionless
• Each packet is routed independently based on the
  IP address in the header
• Connections are supported by the transport layer
  (TCP)

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The Internet
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The Internet
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TCP/IP transport protocols

• Connectivity between application end-points
• Application end-point is uniquely identified by
  (IP address,port number)
• Transmission Control Protocol (TCP)
• Reliable end-to-end byte stream,
• connection oriented
• User Datagram Protocol (UDP)
• Unreliable, connectionless

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Implementation issues

• Reliability
• Flow control
• Routing

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Reliability

• Physical networks are inherently unreliable
• Messages can be both corrupted and lost
• Dealing with corruption
• Error detection, error correction
• Dealing with message loss
• retransmission

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Error detection/correction

• Parity bit is the simplest error detection
  technique
• Parity bit is the binary sum of the message bits
• Detects corruption of a single bit
• More parity bits are needed for error correction

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Error correction

• Forward Error Correction (FEC)
• A bit at position holds parity of bits
  whose positions includes in its binary
  representation
• The positions of wrong parity bits yield the
  position of the erroneous bit
• Corrects a single bit error
• In practice more sophisticated techniques are
  used (e.g., CRC)

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Dealing with message loss

• Timeouts
• Send a message, wait for an acknowledgment (ACK)
• If an acknowledgment does not arrive during a
  predefined timeout period retransmit the message
• If ack is lost a message can be sent more than
  once!
• Negative acknowledgments

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Flow control

• Sender and received have limited buffer space to
  hold outgoing and incoming messages
• Flow control is needed to avoid buffer overflow
• The simplest technique
• The receiver buffer size in known in advance
• Sender never sends a new message until it knows
  that the receiver has enough space to accommodate
  it

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Routing

• Each router holds a table indicating the next
  router (hop) on the way to each destination
• The next hop router must reside on the same
  physical network (LAN)
• Routers exchange messages to update their routing
  tables
• Protocols Distance Vector (Bellman-Ford), Open
  Shortest Path First (OSPF)

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Distributed Systems

• A distributed system (middleware) is an
  implementation of an application level service
• Distributed systems support high level
  abstractions
• Remote Procedure Call (RPC) everything looks
  like a local function call
• Network File System (NFS) everything looks like
  a local file
• Object Based systems Everything looks like a
  local object

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Client/Server Paradigm

• Distributed systems are usually structured
  according to the client/server model
• A server implements a service
• Clients request service from the server using a
  pre-defined protocol
• WWW, FTP, Telnet, NFS are all client/server based

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Internet Well-known services

• Well-known services must be supported by all the
  computers connected to the Internet
• Ftp, telnet, rlogin, nfs
• Domain Name Service (DNS)
• Symbolic name lt-gt IP address
• Uniquely identified by the port number of the
  server
• HTTP 80, FTP 20

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RPC

• A server registers a procedure implementation
• A client calls a function which looks local to
  the client
• E.g., add(a,b)
• The RPC implementation packs (marshals) the
  function name and parameter values into a message
  and sends it to the server

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RPC

• Server accepts the message, unpacks (unmarshals)
  parameters and calls the local function
• Return values is then marshaled into a message
  and sent back to the client
• Marshaling/unmarshaling must take into account
  differences in data representation

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RPC

• Transport both TCP and UDP
• Data types atomic types and non-recursive
  structures
• Pointers are not supported
• Complex memory objects (e.g., linked lists) are
  not supported
• NFS is built on top of RPC

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Next

• Storage Area Networks and
• Modern storage architectures