Network Input

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Network Input Output

• CS-502 Operating Systems
• Spring 2006

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Computer Networks

• Much more than can be covered in this course
• CS 513, CS 577
• Outline
• Protocol Stack
• Kinds of network connections
• Socket interface
• Textbook Networking is spread out over multiple
  chapters.
• Distributed Computing
• I/O Devices
• Memory Management
• Buffer allocation
• Real Time OS

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Network Goal

• Allow activities on multiple computer systems to
  communicate with each other
• Shared memory (or data)
• Message passing
• Remote Procedure Call
• Create abstractions that make these (relatively)
  transparent

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Principal Abstraction Socket

• Originally created in BSD Unix
• Subsequently in most OSs
• Allows opening a connection between two processes
  across network

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Network Stack

• 1983 Open System Interconnection (OSI) 7 layer
  Reference Model
• Working group of the International Standards
  Organization (ISO)
• Defines seven layers
• Describe how applications
• Running upon network-aware devices
• Communicate with each other
• Most day-to-day protocols
• work on a slightly modified layer system
• E.g. TCP/ IP uses a 6-rather than a 7-layer model

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Network Stack (continued)

• Arrived at 7 layer model
• Software architecture
• Created where a different layer of abstraction is
  needed
• Well defined function
• Layer chosen
• with international standards being defined
• Boundaries chosen
• minimize information flow across interfaces
• Number of layers
• Large enough
• Distinct functions need not be thrown together
• In the same layer out of necessity
• Small enough
• Architecture does not become unwieldy

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The OSI 7-layer model (in a nutshell)
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The OSI 7-layer model (continued)

• Layer 1 Physical Layer
• Defines the physical and electrical
  characteristics of the network.
• Transmitting of raw bits over the communication
  channel
• Layer 2 Data Link Layer
• Take the raw transmission facility and transform
  it into a line that appears free of errors to
  layer 3.
• Error correcting coding (e.g. FEC)
• Rate Control (Slow device not overrun by high
  speed device)

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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The OSI 7-layer model (continued)

• Layer 3 Network Layer
• Controlling the operation of the subnet
• How packets are routed
• Congestion Control
• Accounting function (billing)
• Network Statistics
• Example - IP layer (IPv4, IPv6)
• Differences between v4, v6 source/destination
  addressing
• V4 32 bit addressing
• V6 128 bit addressing

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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The OSI 7-layer model (continued)

• Layer 4 Transport Layer
• Accept data from layer 5
• Split it up into smaller units if need be
• Passes these to the network layer
• Ensures that the packets all arrive correctly at
  the destination
• Isolates layer 5 from changes in the underlying
  hardware
• Type of service to provide
• Reliable or unreliable delivery
• True end-to-end layer
• Example - TCP or UDP

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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The OSI 7-layer model (continued)

• Layer 5 - Session Layer
• Allows users on different machines to establish
  sessions between them
• Example SSL, RTP
• Layer 6 Presentation Layer
• Performs certain functions that are requested
  sufficiently often to warrant finding a general
  solution for them rather than letting each user
  solve the problem
• Example encoding data
• Layer 7 Application Layer
• User layer protocol, multiple protocols required
• Example http, ftp, smtp

Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
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Example of OSI Model
Sending Process
Receiving Process
Data
Application Layer
Application Layer
AH
Data
Presentation Layer
Presentation Layer
PH
Data
Session Layer
Session Layer
SH
Data
Transport Layer
Transport Layer
TH
Data
Network Layer
Network Layer
NH
Data
Data Link Layer
Data Link Layer
DH
Data
DT
Physical Layer
Physical Layer
Bits
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TCP/IP a subset
Application Layer
HTTP, DNS, Telnet,SMTP, FTP, SSH, etc.
Presentation Layer
Not Defined
Session Layer
Not Defined
Transport Layer
TCP-UDP
Network Layer
IP
Data Link Layer
Device
Physical Layer
Hardware
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Some Terms

• Packet
• A unit of communication at Data Link layer
• IP Address
• A four-part number used by Network Layer to
  route a packet from one computer to another
• Port
• A 16-bit number used within one computer to
  identify who/where to send packet to
• Well-known port
• A port with number lt 1024, used by agreement for
  standard services (telnet, ftp, smtp, pop, etc.)

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More Terms

• Socket
• End point of a communication
• Usually used in pairs, one for each direction
• Comprises IP Address Port
• Connection
• A logical linkage between pairs of sockets at two
  endpoints for purposes of a particular
  communication between those endpoints

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Establishing a Connection

• Process a on machine m creates a socket
• OS assigns a new port number q to that socket
• Process a attempts to open a connection to
  machine np
• p is a well-known port
• Process b on machine n is listening on p
• Receives request from mq
• Process b forks/spawns a process/thread c to talk
  with mq, then resumes listening
• Thread/process c
• Creates a new socket r for this connection
• Replies to mq with return address nr
• a and c continue to communicate over this pair of
  sockets until they are finished.

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Reliable Connections

• Transport layer partitions messages into packets
• TCP Transmission Control Protocol
• Sequence number of current packet
• Sequence number of last packet received correctly
• Receiver keeps track of seq. of packets
• Reassembles in right order
• Notify sender of missing, broken packets
• Sender keeps copy of each packet until receipt
  acknowledged
• Retransmits packets if no acknowledgement

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Connection-less communication

• UDP User Datagram Protocol
• Used when a certain number of errors can be
  tolerated

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Next time

• Will assign Project 4 a simple HTTP server and
  web client
• Using sockets