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Networks & Communication (continued) CS-502 Operating Systems (Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Modern Operating Systems, 2nd ed., by Tanenbaum, and Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen) CS-502 Fall 2007 Networks & Communication 1 Principal Abstraction – Socket • Originally created in BSD Unix • Now, part of most operating systems • Allows opening a connection between two processes across network • Connection: – a serial conversation between two end points • e.g., processes, threads, tasks on different machines – organized as a sequence of messages or datagrams – distinct from all other connections • One of the four principal abstractions provided by most modern operating systems CS-502 Fall 2007 Networks & Communication 2 Definition — Protocol • Formal set of rules that govern the formats, contents, and meanings of messages from computer to computer, process to process, etc. • Must be agreed to by all parties to a communication • May be defined in terms of other protocols CS-502 Fall 2007 Networks & Communication 3 The OSI 7-layer model (in a nutshell) Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Silbershatz, §§16.6-16.7 Physical Layer CS-502 Fall 2007 Networks & Communication 4 Annotated OSI 7-Layer Stack Silbershatz, page 630 CS-502 Fall 2007 Networks & Communication 5 Summary — OSI 7-layer model Sending Process Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer CS-502 Fall 2007 Receiving Process Data AH PH Data SH TH NH DH Data Data Data Data Data DT Application Layer Presentation Layer Session Layer Transport Layer Network Layer Data Link Layer Physical Layer Bits Networks & Communication 6 Circular Definition • Socket: – End point of a connection – 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 – i.e., a serial conversation between endpoints • Usually two-way CS-502 Fall 2007 Networks & Communication 7 Connection • The backbone of most message-oriented communication protocols • Each party retains knowledge of the other • Each party retains information about state of the other (vis a vis the protocol itself) • Each party “knows” if connection is broken • … • Note: some popular protocols are “connectionless” – one side retains no state information about other side CS-502 Fall 2007 Networks & Communication 8 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. CS-502 Fall 2007 Networks & Communication 9 Typical Client-Server Connection • Create socket • On server side • Bind • I.e., connect socket to port # (usually well-known port) • Listen • Sit and wait for a communication to come in • Accept • Create new socket for purpose of responding to this caller CS-502 Fall 2007 Networks & Communication 10 Notes • Responder to request for connection does not have to be the original server machine • Delegate workload to other server systems • Systems often include a connection ID as part of request to open connection • Unique or randomly chosen • Reduces spoofing of server responses • Unix/Linux will not re-use a socket # within 30 seconds • To avoid confusion between old connection and new CS-502 Fall 2007 Networks & Communication 11 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 sequence # 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 • Window defines how many packet buffers to maintain for efficient transmission • Allows many packets in “flight” CS-502 Fall 2007 Networks & Communication 12 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 … Packet i+k rec’d i time CS-502 Fall 2007 Networks & Communication 13 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 … Packet i+k rec’d i rec’d i time CS-502 Fall 2007 Networks & Communication 14 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 … Packet i+k rec’d i rec’d i rec’d i+2 time CS-502 Fall 2007 Networks & Communication 15 Reliable Connections (continued) Packet i Packet i+1 Packet i+2 Packet i+3 … Packet i+k rec’d i lost rec’d i rec’d i+2 … rec’d i+2 time CS-502 Fall 2007 Networks & Communication 16 Reliable Connections (continued) • If acknowledgement received for packet i • Delete from buffer all packets i • If no acknowledgement received within a reasonable time for packet k • Retransmit from buffer all packets k • Result • • • • CS-502 Fall 2007 Recovers from loss of packets Recovers from loss of acknowledgements Works well for reasonably reliable internet Doesn’t work so well for noisy, unreliable networks Networks & Communication 17 Reminder • How do we know if a packet is received correctly? • Cyclic Redundancy Check (CRC) – Polynomial computed from packet header and body – Usually 16 or 32 bits, computed by hardware – Appended to message – Recomputed on reception, compared with transmitted CRC – Equal packet received correctly CS-502 Fall 2007 Networks & Communication 18 Examples of Connection-based Protocols • Telnet (virtual terminal) – 2-way communication by character stream – Line-by-line organization • SMTP (Simple Mail Transport Protocol) – For sending mail – Layered on top of telnet protocol • POP (Post Office Protocol) – For receiving your mail – Layered on top of telnet protocol • FTP (File Transfer Protocol) – For transmitting ASCII or binary files – Binary data transmission not layered on telnet protocol • … CS-502 Fall 2007 Networks & Communication 19 Connection-less communication • Some communication protocols don’t need the overhead of reliable connections – When some number of errors can be tolerated – Where recovery from those errors is easy • UDP – User Datagram Protocol – The internet connection-less protocol (layer 4) – Breaks messages into packets • Reassembles at destination – Messages delivered completely or not at all – Does not send acknowledgement of correct receipt CS-502 Fall 2007 Networks & Communication 20 Examples • HTTP (HyperText Transport Protocol) – Web server responds directly to requests – If client does not get response, retries request • NFS (Network File System) – For access to files on servers as if they are local – If client does not get response, retries request • RPC (Remote Procedure Call) – Next topic • … CS-502 Fall 2007 Networks & Communication 21 Summary • • • • Socket, connection Network stack, 7-layer model Establishing a connection Reliable transmission • Reading assignment – Silbershatz Chapter 16 CS-502 Fall 2007 Networks & Communication 22