Introduction to Networking and Distributed Systems Operating Systems

Introduction to Networking and Distributed Systems Operating Systems Fall 2002 OS Fall’ 02

Overview § Intro to Networking Review of Basic principles: w layers, protocols, etc… TCP/IP and the Internet Implementation issues § Intro to Distributed systems client/server OS Fall’ 02

Communication in everyday life I like rabbits J’aime les lapins L: Dutch Ik hou van konijnen Fax# L: Dutch Ik hou van konijnen OS Fall’ 02

Computer based communication mail message mail encode message rep OS Fall’ 02

Computer based communication mail encode message rep mail encode packets mess rep pkt age rep pkt OS Fall’ 02

Computer based communication mail encode packets route message rep pkt addr OS Fall’ 02

Computer based communication mail encode packets route error message rep pkt addr ECC OS Fall’ 02

Layered organization § Computer networks are structured as layer (protocol) stacks § Each layer provides services to the layer above § Layers communicate by protocols service API service protocol OS Fall’ 02 API

Protocols § Protocol is a collection of message formats and their interpretation needed to implement the layer service format: header+data headers convey the protocol specific information OS Fall’ 02

Standards and openness § Protocols must be standardized for interoperability § Layer APIs must be standardized for portability § Existing standards: ISO OSI Layered model (“eight-layer model”) TCP/IP is a de-facto model of the Internet OS Fall’ 02

ISO OSI Layered model Application Presentation Session Transport Network Data Link Physical OS Fall’ 02

Lower OSI Layers § Physical layer: Voltages to represent 0/1, how many bits per second can be sent, simplex/duplex, hardware § Data Link layer: Grouping bits into frames, point-to-point reliability, error detection/correction OS Fall’ 02

Network and Transport § Network layer: Getting data from one host to another w Routing, addressing, fragmentation/reassembly § Transport layer: End-to-end application connectivity Reliable transport connection: w Messages are not lost and delivered in the order they were sent w Transmission Control Protocol (TCP) OS Fall’ 02

Session and Presentation § Session layer: Enhanced transport layer: session control § Presentation layer: Message structure: e. g. , records Data representation compression and encryption OS Fall’ 02

Application Layer § General-purpose high level services and abstractions File transfer (FTP) Hyper-text transfer (HTTP) Remote Procedure Call (RPC) Network File System (NFS) Object Request Brokers w CORBA, Java RMI Etc. . . OS Fall’ 02

TCP/IP Protocol Suite OSI model TCP/IP protocol suite Application Presentation Session Transport TCP/UDP Network IP Datalink Network access Physical OS Fall’ 02 OS Hardware

Physical Networks § Each computer is directly attached to some physical network Network Interface Cards (NIC)+cables Usually limited geographical scale (Local Area Network=LAN) w MAN, WAN supports routing, addressing, network access within its boundaries § LAN: Ethernet (10 Mbit/sec, 100 Mbit/sec), Token ring, Fibre channel, etc. . . OS Fall’ 02

Physical networks characteristics § Bandwidth: Number of bits per second § Latency: The time it takes to send a single bit § Ethernet: Shared bus bandwidth: 10 Mbit/sec Switched Ethernet bw: 100 Mbit/sec OS Fall’ 02

Ethernet LAN § Shared bus § Each NIC has a unique address § Messages are broadcast If collision occurs, wait for a randomly chosen time period and retry Maximum Transfer Unit (MTU) = 1536 Bytes OS Fall’ 02

Internet protocol (IP( § Creates an illusion of a single physical network over a collection of heterogeneous LANs Global addressing, routing Fragmentation, reassembly to overcome differences in the MTU sizes § Architecture: LANs interconnected by routers (gateways) OS Fall’ 02

Internet Protocol (IP( § Connectionless Each packet is routed independently based on the IP address in the header § Connections are supported by the transport layer (TCP) OS Fall’ 02

The Internet OS Fall’ 02

The Internet OS Fall’ 02

TCP/IP transport protocols § Connectivity between application endpoints Application end-point is uniquely identified by (IP address, port number) § Transmission Control Protocol (TCP) Reliable end-to-end byte stream, connection oriented § User Datagram Protocol (UDP) Unreliable, connectionless OS Fall’ 02

Implementation issues § Reliability § Flow control § Routing OS Fall’ 02

Reliability § Physical networks are inherently unreliable Messages can be both: corrupted and lost § Dealing with corruption Error detection, error correction § Dealing with message loss retransmission OS Fall’ 02

Error detection/correction § Parity bit is the simplest error detection technique Parity bit is the binary sum of the message bits Detects corruption of a single bit § More parity bits are needed for error correction OS Fall’ 02

Error correction § Forward Error Correction (FEC) A bit at position holds parity of bits whose positions includes in its binary representation § The positions of wrong parity bits yield the position of the erroneous bit Corrects a single bit error § In practice more sophisticated techniques are used (e. g. , CRC) OS Fall’ 02

Dealing with message loss § Timeouts Send a message, wait for an acknowledgment (ACK) If an acknowledgment does not arrive during a predefined timeout period: retransmit the message w If ack is lost a message can be sent more than once! § Negative acknowledgments OS Fall’ 02

Flow control § Sender and received have limited buffer space to hold outgoing and incoming messages § Flow control is needed to avoid buffer overflow § The simplest technique: The receiver buffer size in known in advance Sender never sends a new message until it knows that the receiver has enough space to accommodate it OS Fall’ 02

Routing § Each router holds a table indicating the next router (hop) on the way to each destination The next hop router must reside on the same physical network (LAN) § Routers exchange messages to update their routing tables § Protocols: Distance Vector (Bellman. Ford), Open Shortest Path First (OSPF) OS Fall’ 02

Distributed Systems § A distributed system (middleware) is an implementation of an application level service § Distributed systems support high level abstractions w Remote Procedure Call (RPC): everything looks like a local function call w Network File System (NFS): everything looks like a local file w Object Based systems: Everything looks like a local object OS Fall’ 02

Client/Server Paradigm § Distributed systems are usually structured according to the client/server model § A server implements a service § Clients request service from the server using a pre-defined protocol § WWW, FTP, Telnet, NFS are all client/server based OS Fall’ 02

Internet Well-known services § Well-known services must be supported by all the computers connected to the Internet Ftp, telnet, rlogin, nfs § Domain Name Service (DNS) Symbolic name <-> IP address § Uniquely identified by the port number of the server HTTP: 80, FTP: 20 OS Fall’ 02

RPC § A server registers a procedure implementation § A client calls a function which looks local to the client E. g. , add(a, b) § The RPC implementation packs (marshals) the function name and parameter values into a message and sends it to the server OS Fall’ 02

RPC § Server accepts the message, unpacks (unmarshals) parameters and calls the local function § Return values is then marshaled into a message and sent back to the client § Marshaling/unmarshaling must take into account differences in data representation OS Fall’ 02

RPC § Transport: both TCP and UDP § Data types: atomic types and non- recursive structures § Pointers are not supported § Complex memory objects (e. g. , linked lists) are not supported § NFS is built on top of RPC OS Fall’ 02

Next: § Storage Area Networks and § Modern storage architectures OS Fall’ 02