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Network Input & Output CS-502 Operating Systems Spring 2006 CS502 Spring 2006 Networks 1 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 CS502 Spring 2006 Networks 2 Computer C Process j Computer A The Network Process 1 Computer B Process i CS502 Spring 2006 Networks 3 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 CS502 Spring 2006 Networks 4 Principal Abstraction – Socket • Originally created in BSD Unix • Subsequently in most OS’s • Allows opening a connection between two processes across network CS502 Spring 2006 Networks 5 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 CS502 Spring 2006 Networks 6 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 CS502 Spring 2006 Networks 7 The OSI 7-layer model (in a nutshell) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS502 Spring 2006 Networks 8 The OSI 7-layer model (continued) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS502 Spring 2006 • 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) Networks 9 The OSI 7-layer model (continued) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS502 Spring 2006 • 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 Networks 10 The OSI 7-layer model (continued) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer • Layer 4 – Transport Layer Physical Layer CS502 Spring 2006 – 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 Networks 11 The OSI 7-layer model (continued) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS502 Spring 2006 • 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 Networks 12 Example of OSI Model Sending Process Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS502 Spring 2006 Receiving Process Data AH PH Data SH TH NH DH Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Data Data Data Data Data DT Physical Layer Bits Networks 13 TCP/IP – a subset Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer HTTP, DNS, Telnet, SMTP, FTP, SSH, etc. Not Defined Not Defined TCP-UDP IP Device Physical Layer CS502 Spring 2006 Hardware Networks 14 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 < 1024, used by agreement for standard services (telnet, ftp, smtp, pop, etc.) CS502 Spring 2006 Networks 15 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 CS502 Spring 2006 Networks 16 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/spawns a process/thread c to talk with m:q, then resumes listening • 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. CS502 Spring 2006 Networks 17 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 CS502 Spring 2006 Networks 18 Connection-less communication • UDP – User Datagram Protocol – Used when a certain number of errors can be tolerated CS502 Spring 2006 Networks 19 Next time • Will assign Project 4 – a simple HTTP server and web client – Using sockets CS502 Spring 2006 Networks 20