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Network Input & Output

This course covers various topics related to network input and output in computer networks, including the protocol stack, network connections, socket interface, distributed computing, I/O devices, memory management, and real-time operating systems.

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Network Input & Output

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  1. Network Input & Output CS-3013 & CS-502, Summer 2006 Network Input & Output

  2. 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 Network Input & Output

  3. Computer A Computer B Computer C The Network Process 1 Process i Process j Network Input & Output

  4. 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 Network Input & Output

  5. Principal Abstraction – Socket • Originally created in BSD Unix • Subsequently, part of most OS’s • Allows opening a connection between two processes across network • Connection: • a serial conversation between two processes, threads, etc., on different machines • organized as a sequence of messages or datagrams • distinct from all other connections Network Input & Output

  6. 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 communicate with each other • Via network-aware devices • 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 Network Input & Output

  7. 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 Network Input & Output

  8. Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer The OSI 7-layer model(in a nutshell) Silbershatz, §§16.6-16.7 Network Input & Output

  9. 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 an abstract link 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 Network Input & Output

  10. 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 Network Input & Output

  11. 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 Network Input & Output

  12. 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 Network Input & Output

  13. 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 Network Input & Output

  14. 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 Network Input & Output

  15. Some Terms • Packet: • A unit of communication at Data Link layer • aka datagram • 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 < 1024, used by agreement for standard services (telnet, ftp, smtp, pop, etc.) Network Input & Output

  16. 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 Network Input & Output

  17. 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 n:p • p is a well-known port • Process b on machine n is listening on p • Receives request from m:q • Process b forks a process or spawns a thread c to talk with m:q, then resumes listening on p • Thread/process c • Creates a new socket r for this connection • Replies to m:q with return address n:r • a and c continue to communicate over this pair of sockets until they are finished. Network Input & Output

  18. 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 Network Input & Output

  19. Connection-less communication • UDP – User Datagram Protocol • Used when a certain number of errors can be tolerated and also • Where recovery from those errors is easy Network Input & Output

  20. Next Project 3 – a simple HTTP server and web client Network Input & Output

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