1. CS-3013 & CS-502, Summer 2006 Network Input & Output1 CS-3013 & CS-502, Summer 2006

2. Network Input & Output2 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

3. CS-3013 & CS-502, Summer 2006 Network Input & Output3 Computer A Process 1 Computer B Process i Computer C Process j The Network

4. CS-3013 & CS-502, Summer 2006 Network Input & Output4 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

5. CS-3013 & CS-502, Summer 2006 Network Input & Output5 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

6. CS-3013 & CS-502, Summer 2006 Network Input & Output6 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

7. CS-3013 & CS-502, Summer 2006 Network Input & Output7 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

8. CS-3013 & CS-502, Summer 2006 Network Input & Output8 The OSI 7-layer model (in a nutshell) Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Silbershatz, §§16.6-16.7

9. CS-3013 & CS-502, Summer 2006 Network Input & Output9 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) Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

10. CS-3013 & CS-502, Summer 2006 Network Input & Output10 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 Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

11. CS-3013 & CS-502, Summer 2006 Network Input & Output11 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 Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

12. CS-3013 & CS-502, Summer 2006 Network Input & Output12 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 Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer

13. CS-3013 & CS-502, Summer 2006 Network Input & Output13 Example of OSI Model Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Sending Process Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer Receiving Process Bits DHDataDT NHData THData SHData PHData AHData

14. CS-3013 & CS-502, Summer 2006 Network Input & Output14 TCP/IP – a subset Physical Layer Data Link Layer Network Layer Transport Layer Session Layer Presentation Layer Application Layer HTTP, DNS, Telnet, SMTP, FTP, SSH, etc. TCP-UDP IP Not Defined Device Hardware

15. CS-3013 & CS-502, Summer 2006 Network Input & Output15 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.)

16. CS-3013 & CS-502, Summer 2006 Network Input & Output16 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

17. CS-3013 & CS-502, Summer 2006 Network Input & Output17 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.

18. CS-3013 & CS-502, Summer 2006 Network Input & Output18 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

19. CS-3013 & CS-502, Summer 2006 Network Input & Output19 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

20. CS-3013 & CS-502, Summer 2006 Network Input & Output20 Next Project 3 – a simple HTTP server and web client