Client Server Architecture A network architecture in which each computer or process on the network is either a client or a server.
Components Clients Servers Communication Networks Server Client
Clients Applications that run on computers Rely on servers for ◦ Files Clients are Applications ◦ Devices ◦ Processing power Example: E-mail client ◦ An application that enables you to send and receive e-mail
Servers Computers or processes that manage network resources ◦ Disk drives (file servers) Servers Manage ◦ Printers (print servers) Resources ◦ Network traffic (network servers) Example: Database Server ◦ A computer system that processes database queries
Communication NetworksNetworks Connect Clients and Servers
Client–Server Computing Process takes place ◦ on the server and ◦ on the client Client-Server Servers Computing Optimizes Computing Resources ◦ Store and protect data ◦ Process requests from clients Clients ◦ Make requests ◦ Format data on the desktop
Application Functions Software application functions are separated into three distinct parts Server: Data Management Client: Presentation & Application Logic
Application Components 3 Data Management 2 Client Types 2 Application Logic Fat Thin Client 1 Presentation Client 3 Logical TiersDatabase Applications: Most common use of client-server architectures
Middleware Software that connects two otherwise separate applications Example: Middleware product linking Database Server: a database system to a Web server Manages Data Middleware Links Applications Web Server: Presents Dynamic Pages Client: Requests Data via Web
Types of Servers From A to Z Application Servers List Servers Audio/Video Servers Mail Servers Chat Servers News Servers Fax Servers Proxy Servers FTP Servers Telnet Servers Groupware Servers Web Servers IRC Servers Z39.50 ServersSource: http://webopedia.lycos.com
ADVANTAGES OF CLIENT-SERVER Advantages often cited include: ◦ Centralization - access, resources, and data security are controlled through the server ◦ Scalability - any element can be upgraded when needed ◦ Flexibility - new technology can be easily integrated into the system ◦ Interoperability - all components (clients, network, servers) work together
DISADVANTAGES OF CLIENT-SERVER Disadvantages often cited include: ◦ Dependability - when the server goes down, operations cease ◦ Higher than anticipated costs ◦ Can cause network congestion
CLIENT-SERVERARCHITECTURES There are basically two types of client-server architectures ◦ Two tier architectures ◦ Three tier architectures The choice between the two should be made based on combination of: ◦ Schedule for project implementation ◦ Expected system changes and enhancements
TWO-TIER ARCHITECTURES Application components are distributed between the Server server and client software In addition to part of the application software, the server also stores the data, Network and all data accesses are through the server. The presentation (to the user)PC PC PC is handled strictly by the client Clients software.
TWO-TIER ARCHITECTURES(cont.) The PC clients assume the bulk of the responsibility for the application logic. The server assumes the bulk of the responsibility for data integrity checks, query capabilities, data extraction and most of the data intensive tasks, including sending the appropriate data to the appropriate clients.
TWO-TIER ARCHITECTURES,ADVANTAGES The commonly cited advantages of two-tier systems include: ◦ Fast application development time ◦ Available tools are robust and lend themselves to fast prototyping to insure user needs a met accurately and completely. ◦ Conducive to environments with homogeneous clients, homogeneous applications, and static business rules.
TWO-TIER ARCHITECTURES,DISADVANTAGES The commonly cited disadvantages of two- tier systems include: ◦ Not suitable for dispersed, heterogeneous environments with rapidly changing business rules. ◦ Because the bulk of the application logic is on the client, there is the problem of client software version control and new version redistribution. ◦ Security can be complicated because a user may require separate passwords for each SQL server accessed.
THREE-TIER ARCHITECTURES 3-tier architectures attempt to overcome some of theServer Server limitations of the 2-tier architecture by separating presentation, processing, and data into 3 separate and Network distinct entities. The software in the client PC PC PC handles the presentation (to the user) using similar tools Clients as in the 2-tier architecture.
THREE-TIER ARCHITECTURES(cont.) When data or processing are required by the presentation client, a call is made to the middle-tier functionality server. This tier performs calculations, does reports, and makes any needed client calls to other servers (e.g.. a data base server).
THREE-TIER ARCHITECTURES(cont.) Middle tier servers are usually coded in a highly portable, non-proprietary language such as C or C++. Middle tier servers may be multithreaded and can be accessed by multiple clients. The calling mechanism from client to server and from server to server is by means of RPC’s.
THREE-TIER ARCHITECTURES,ADVANTAGES (cont.) Commonly cited advantages include: ◦ Having separate functionality servers allows for the parallel development of individual tiers by application specialists. ◦ Provides for more flexible resource allocation. Can reduce network traffic by having the functionality servers strip data to the precise structure needed before sending it to the clients.
THREE-TIER ARCHITECTURES,DISADVANTAGES Often cited disadvantages of 3-tier architectures include: ◦ Creates an increased need for network traffic management, server load balancing, and fault tolerance. ◦ Current tools are relatively immature and are more complex. ◦ Maintenance tools are currently inadequate for maintaining server libraries. This is a potential obstacle for simplifying maintenance and promoting code reuse throughout the organization.
What is Peer-to-Peer? A model of communication where every node in the network acts alike. As opposed to the Client-Server model, where one node provides services and other nodes use the services.
Advantages of P2P Computing No central point of failure ◦ E.g., the Internet and the Web do not have a central point of failure. ◦ Most internet and web services use the client-server model (e.g. HTTP), so a specific service does have a central point of failure. Scalability ◦ Since every peer is alike, it is possible to add more peers to the system and scale to larger networks.
Disadvantages of P2P Computing Decentralized coordination ◦ How to keep global state consistent? ◦ Need for distributed coherency protocols. All nodes are not created equal. ◦ Computing power, bandwidth have an impact on overall performance. Programmability ◦ As a corollary of decentralized coordination.
P2P Computing Applications File sharing Process sharing Collaborative environments
P2P File Sharing Applications Improves data availability Replication to compensate for failures. E.g., Napster, Gnutella, Freenet, KaZaA (FastTrack), your DFS project.
P2P Process Sharing Applications For large-scale computations Data analysis, data mining, scientific computing E.g., SETI@Home, Folding@Home, distributed.net, World-Wide Computer
P2P Collaborative Applications For remote real-time human collaboration. Instant messaging, virtual meetings, shared whiteboards, teleconferencing, tele-presence. E.g., talk, IRC, ICQ, AOL Messenger, Yahoo! Messenger, Jabber, MS Netmeeting, NCSA Habanero, Games
Distributed systems Data Networking &Client-Server Communication
Distributed systemsIndependent machines work cooperativelywithout shared memory They have to talk somehow Interconnect is the network
Modes of connectionCircuit-switched ◦ dedicated path ◦ guaranteed (fixed) bandwidth ◦ [almost] constant latencyPacket-switched ◦ shared connection ◦ data is broken into chunks called packets ◦ each packet contains destination address ◦ available bandwidth ≤ channel capacity ◦ variable latency
What’s in the data? For effective communication ◦ same language, same conventions For computers: ◦ electrical encoding of data ◦ where is the start of the packet? ◦ which bits contain the length? ◦ is there a checksum? where is it? how is it computed? ◦ what is the format of an address? ◦ byte ordering
ProtocolsThese instructions and conventionsare known as protocols
ProtocolsExist at different levels understand format of humans vs. whales address and how to different wavelengths compute checksum versus request web page French vs. Hungarian
Layering To ease software development and maximizeflexibility: ◦ Network protocols are generally organized in layers ◦ Replace one layer without replacing surrounding layers ◦ Higher-level software does not have to know how to format an Ethernet packet … or even know that Ethernet is being used
Layering Most popular model of guiding(not specifying) protocol layers is OSI reference model Adopted and created by ISO 7 layers of protocols
OSI Reference Model: Layer 1 Transmits and receives raw data to communication medium. Does not care about contents. voltage levels, speed, connectors1 Physical Physical Examples: RS-232, 10BaseT
OSI Reference Model: Layer 2 Detects and corrects errors. Organizes data into packets before passing it down. Sequences packets (if necessary). Accepts acknowledgements from receiver.2 Data Link Data Link1 Physical Physical Examples: Ethernet MAC, PPP
OSI Reference Model: Layer 3 Relay and route information to destination. Manage journey of packets and figure out intermediate hops (if3 Network Network needed).2 Data Link Data Link1 Physical Physical Examples: IP, X.25
OSI Reference Model: Layer 4 Provides a consistent interface for end-to-end (application-to- application) communication. Manages4 Transport Transport flow control.3 Network Network Network interface is similar to a mailbox.2 Data Link Data Link1 Physical Physical Examples: TCP, UDP
OSI Reference Model: Layer 5 Services to coordinate dialogue and manage data exchange.5 Session Session Software implemented Transport switch.4 Transport Manage multiple logical3 Network Network connections.2 Data Link Data Link Keep track of who is talking: establish & end1 Physical Physical Examples: HTTP 1.1, SSL, communications. NetBIOS
OSI Reference Model: Layer 6 Data representation6 Presentation Presentation Concerned with the5 Session Session meaning of data bits4 Transport Transport Convert between3 Network Network machine representations2 Data Link Data Link1 Physical Physical Examples: XDR, ASN.1, MIME, MIDI
OSI Reference Model: Layer 77 Application Application Collection of application- Presentation specific protocols6 Presentation5 Session Session4 Transport Transport3 Network Network2 Data Link Data Link Examples: email (SMTP, POP, IMAP)1 Physical Physical file transfer (FTP) directory services (LDAP)
Local Area Network (LAN)Communications network ◦ small area (building, set of buildings) ◦ same, sometimes shared, transmission medium ◦ high data rate (often): 1 Mbps – 1 Gbps ◦ Low latency ◦ devices are peers any device can initiate a data transfer with any other deviceMost elements on a LAN are workstations ◦ endpoints on a LAN are called nodes
Connecting nodes to LANs network computerAdapter ◦ expansion slot (PCI, PC Card, USB dongle) ◦ usually integrated onto main boardNetwork adapters are referred to as Network Interface Cards (NICs) or adapters
MediaWires (or RF, IR) connecting together the devicesthat make up a LANTwisted pair ◦ Most common: STP: shielded twisted pair UTP: unshielded twisted pair (e.g. Telephone cable, Ethernet 10BaseT)Coaxial cable ◦ Thin (similar to TV cable) ◦ Thick (e.g., 10Base5, ThickNet)FiberWireless
Hubs, routers, bridgesHub ◦ Device that acts as a central point for LAN cables ◦ Take incoming data from one port & send to all other portsSwitch ◦ Moves data from input to output port. ◦ Analyzes packet to determine destination port and makes a virtual connection between the ports.Concentrator or repeater ◦ Regenerates data passing through itBridge ◦ Connects two LANs or two segments of a LAN ◦ Connection at data link layer (layer 2)Router ◦ Determines the next network point to which a packet should be forwarded ◦ Connects different types of local and wide area networks at network layer (layer 3)
Clients and Servers Send messages to applications ◦ not just machines Client must get data to the desired process ◦ server process must get data back to client process To offer a service, a server must get a transport address for a particular service ◦ well-defined location
Connection-oriented Protocols analogous to phone call1. establish connection dial phone number2. [negotiate protocol] [decide on a language]3. exchange data speak4. terminate connectionhang upvirtual circuit service ◦ provides illusion of having a dedicated circuit ◦ messages guaranteed to arrive in-order ◦ application does not have to address each messagevs. circuit-switched service
Connectionless Protocols- no call setup- send/receive data (each packet addressed)- no termination
Connectionless Protocols analogous to mailbox- no call setup- send/receive data drop letter in mailbox (each packet addressed) (each letter addressed)- no termination datagram service ◦ client is not positive whether message arrived at destination ◦ no state has to be maintained at client or server ◦ cheaper but less reliable than virtual circuit service
Ethernet Layers 1 & 2 of OSI model ◦ Physical (1) Cables: 10Base-T, 100Base-T, 1000Base-T, etc. ◦ Data Link (2) Ethernet bridging Data frame parsing Data frame transmission Error detection Unreliable, connectionless communication
IP – Internet ProtocolBorn in 1969 as a research network of 4 machinesFunded by DoD’s ARPAGoal:build an efficient fault-tolerant networkthat could connect heterogeneousmachines and link separately connectednetworks.
Internet ProtocolConnectionless protocol designed to handle theinterconnection of a large number of local andwide-area networks that comprise the internetIP can route from one physical network toanother
IP AddressingEach machine on an IP network is assigned aunique 32-bit number for each networkinterface: ◦ IP address, not machine addressA machine connected to several physicalnetworks will have several IP addresses ◦ One for each network
IP Address space32-bit addresses → >4 billion addresses! Routers would need a table of 4 billion entries Design routing tables so one entry can match multiple addresses ◦ hierarchy: addresses physically close will share a common prefix
IP Addressing: networks & hosts cs.rutgers.edu remus.rutgers.edu 18.104.22.168 22.214.171.124 80 06 04 02 80 06 0D 03network # host # first 16 bits identify Rutgers external routers need only one entry ◦ route 128.6.*.* to Rutgers
IP Addressing: networks & hosts IP address ◦ network #: identifies network machine belongs to ◦ host #: identifies host on the network use network number to route packet to correct network use host number to identify specific machine
IP Addressing Expectation: ◦ a few big networks and many small ones ◦ create different classes of networks ◦ class use leading bits to identifynet # leading bits bits for network bits for host A 0 7 (128) 24 (16M) B 10 14 (16K) 16 (64K) C 110 21 (2M) 8 (256)To allow additional networks within an organization: use high bits of host number for a “network within a network” – subnet
Running out of addresses Huge growth Wasteful allocation of networks ◦ Lots of unused addresses Every machine connected to the internet needed a worldwide-unique IP address Solutions: CIDR, NAT, IPv6
IP Special Addresses All bits 0 ◦ Valid only as source address ◦ “all addresses for this machine” ◦ Not valid over network All host bits 1 ◦ Valid only as destination ◦ Broadcast to network All bits 1 ◦ Broadcast to all directly connected networks Leading bits 1110 ◦ Class D network 127.0.0.0: reserved for local traffic ◦ 127.0.0.1 usually assigned to loopback device
IPv6 vs. IPv4IPv4 ◦ 4 byte (32 bit) addressesIPv6: ◦ 16-byte (128 bit) addresses 3.6 x 1038 possible addresses 8 x 1028 times more addresses than IPv4 ◦ 4-bit priority field ◦ Flow label (24-bits)
Getting to the machine IP is a logical network on top of multiplephysical networks OS support for IP: IP driver receive data send data IP driver IP driver receive packet send packet network driver network driver from wire to wire
IP driver responsibilities Get operating parameters from device driver ◦ Maximum packet size (MTU) ◦ Functions to initialize HW headers ◦ Length of HW header Routing packets ◦ From one physical network to another Fragmenting packets Send operations from higher-layers Receiving data from device driver Dropping bad/expired data
Device driver responsibilities Controls network interface card ◦ Comparable to character driver top half bottom half Processes interrupts from network interface ◦ Receive packets ◦ Send them to IP driver Get packets from IP driver ◦ Send them to hardware ◦ Ensure packet goes out without collision
Network device Network card examines packets on wire ◦ Compares destination addresses Before packet is sent, it must be enveloped for the physical network device IP header IP data header payload
Device addressingIP address → ethernet addressAddress Resolution Protocol (ARP) 1. Check local ARP cache 2. Send broadcast message requesting ethernet address of machine with certain IP address 3. Wait for response (with timeout)
Transport-layer protocols over IP IP sends packets to machine ◦ No mechanism for identifying sending or receiving application Transport layer uses a port number to identify the application TCP – Transmission Control Protocol UDP – User Datagram Protocol
TCP – Transmission ControlProtocol Virtual circuit service (connection-oriented) Send acknowledgement for each received packet Checksum to validate data Data may be transmitted simultaneously in both directions
UDP – User Datagram Protocol Datagram service (connectionless) Data may be lost Data may arrive out of sequence Checksum for data but no retransmission ◦ Bad packets dropped
IP header device TCP/UDP IP header IP data header header payloadvers hlen svc type (TOS) total length fragment identification flags fragment offset 20 bytes TTL protocol header checksum source IP address destination IP address options and pad
Headers: TCP & UDP device TCP/UDP IP header IP data header header payload TCP header UDP header src port dest port src port dest port seq number seg length checksum 20 bytes ack numberhdr 8 bytes - flags windowlenchecksum urgent ptr options and pad
Device header (Ethernet II) device TCP/UDP IP header IP data header header payload framedest addr src addr data CRC type6 bytes 6 bytes 2 46-1500 bytes 4 18 bytes + data
Quality of Service Problems in IP Too much traffic ◦ Congestion Inefficient packet transmission ◦ 59 bytes to send 1 byte in TCP/IP! ◦ 20 bytes TCP + 20 bytes IP + 18 bytes ethernet Unreliable delivery ◦ Software to the rescue – TCP/IP Unpredictable packet delivery
Sockets IP lets us send data between machines TCP & UDP are transport layer protocols ◦ Contain port number to identify transport endpoint (application) One popular abstraction for transport layer connectivity: sockets ◦ Developed at Berkeley
SocketsAttempt at generalized IPC modelGoals: ◦ communication between processes should not depend on whether they are on the same machine ◦ efficiency ◦ compatibility ◦ support different protocols and naming conventions
SocketAbstract object from which messages are sentand received ◦ Looks like a file descriptor ◦ Application can select particular style of communication Virtual circuit, datagram, message-based, in-order delivery ◦ Unrelated processes should be able to locate communication endpoints Sockets should be named Name meaningful in the communications domain
Step 5Close connection shutdown(s, how) how: 0: can send but not receive 1: cannot send more data 2: cannot send or receive (=0+1)
Sockets in Javajava.net packageTwo major classes: ◦ Socket: client-side ◦ ServerSocket: server-side
Step 1a (server) Create socket and name itServerSocket svc = new ServerSocket(port)
Step 1b (server) Wait for connection from clientServer req = svc.accept() new socket for client session
Step 1 (client) Create socket and name itSocket s = new Socket(address, port); obtained from: getLocalHost, getByName, or getAllByNameSocket s = new Socket(“cs.rutgers.edu”, 2211);
Step 2Exchange data obtain InputStream/OutputStream from Socket objectBufferedReader in = new BufferedReader( new InputStreamReader( s.getInputStream()));PrintStream out = new PrintStream(s.getOutputStream());
Step 3Terminate connection close streams, close socket in.close(); out.close(); s.close();