1 哈工大计算机学院李全龙Network Application Development 1 Network Application Development Autumn, 2005 哈工大计算机学院李全龙Network Application Development 1: Introduction 2 Self-introduction 李全龙 : ?Associate Professor ?Office: Room 504 of New Tech. Building ?Tel.: ?0451-86413750(O.) ?0451- 86922927 (H.) ?13351781125 ?Email: liquanlong@hit.edu.cn lql@public.hr.hl.cn 哈工大计算机学院李全龙Network Application Development 1: Introduction 3 Reference ? Douglas E. Comer, David L. Stevens, Internetworking with TCP/IP， Vol. III， Client-Server Programing and Application, Windows Sockets Version(影印版 )，清华大学出版社， 2002年 1月.(Text Book) ? Douglas E. Comer, David L. Stevens, Internetworking with TCP/IP， Vol.I, II， (影印版 )，清华大学出版社， 2002年 1月。 ? Behrouz A. Forouzan & Sophia Chung Fegan, TCP/IP Protocol Suite （影印版） , 清华大学出版社， 2004年 1月。 ? James F. Kurose and Keith W. Ross,Computer Networking-A Top- Down Approach Featuring the Internet (Third Edition)(影印版) , Higher Education Press (China) & Pearson Education, 2001. ? William Stallings, Data & Computer Communications (Sixth Edition), Higher Education Press (China) & Pearson Education, 2001. ? W. Richard Stevens, TCP/IP Illustrated Volume 1,2,3， (影印版 )，机 械工业出版社， 2003年 2月。 哈工大计算机学院李全龙Network Application Development 1: Introduction 4 Assessment ? Course work- 10% ? Lab- 20% ? Assignment (Mini Project)-20% ? Final examinition-50% 哈工大计算机学院李全龙Network Application Development 1: Introduction 5 Course schedule ? Introduction ? Socket API ? Client software design ? Server software design ? Raw socket programming ? Web-based network application development ? RPC/Middleware application development ? P2P application development 哈工大计算机学院李全龙Network Application Development 1: Introduction 6 Relationship with other Courses(1) Physical Data Link Network Transport Session Presentation Application Concepts&Theory Typical Net&Protocols App. Dev & Practice Comput er Netw o r ks N e t Planning & I m p Net A pp De v 2 哈工大计算机学院李全龙Network Application Development 1: Introduction 7 Relationship with other Courses(2) Physical Data Link Network Transport Session Presentation Application Computer Net Net App Dev Net Planning &Implementation 哈工大计算机学院李全龙Network Application Development 1: Introduction 8 Relationship with other Courses(2) Concept&Theory Typical Net&Prot App Dev & Practice Computer Net Net App Dev Net Planning &Implementation 哈工大计算机学院李全龙Network Application Development 1: Introduction 9 Chapter 1: Introduction Chapter goal: ? Network basics ? TCP/IP protocols stack review ? Client-server application model and software design ? Concurrent processing in client-server model 哈工大计算机学院李全龙Network Application Development 1: Introduction 10 Chapter 1: Introduction Chapter goal: ? Network basics ? TCP/IP protocols stack review ? Client-server application model and software design ? Concurrent processing in client-server model 哈工大计算机学院李全龙Network Application Development 1: Introduction 11 What’s the Internet: “nuts and bolts” view ? millions of connected computing devices: hosts, end-systems ? pc’s workstations, servers ? PDA’s phones, toasters running network apps ? communication links ? fiber, copper, radio, satellite ? routers: forward packets (chunks) of data thru network local ISP company network regional ISP router workstation server mobile 哈工大计算机学院李全龙Network Application Development 1: Introduction 12 What’s the Internet: “nuts and bolts” view ? protocols: control sending, receiving of msgs ? e.g., TCP, IP, HTTP, FTP, PPP ? Internet: “network of networks” ? loosely hierarchical ? Internet standards ? RFC: Request for comments ? IETF: Internet Engineering Task Force local ISP company network regional ISP router workstation server mobile 3 哈工大计算机学院李全龙Network Application Development 1: Introduction 13 What’s a protocol? ? Used for communications between entities in a system ? Must speak the same language ? Entities ? User applications ? e-mail facilities ? terminals ? Systems ? Computer ? Terminal ? Remote sensor protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt 哈工大计算机学院李全龙Network Application Development 1: Introduction 14 Key Elements of a Protocol ? Syntax ? Data formats ? Signal levels ? Semantics ? Control information ? Error handling ? Timing ? Speed matching ? Sequencing 哈工大计算机学院李全龙Network Application Development 1: Introduction 15 Protocol Functions ? Encapsulation ? Segmentation and reassembly ? Connection control ? Ordered delivery ? Flow control ? Error control ? Addressing ? Multiplexing ? Transmission services 哈工大计算机学院李全龙Network Application Development 1: Introduction 16 The OSI Model ? The Open Systems Interconnection (OSI) model is defined by International Standard Organization (ISO) in 1984. ? To open communication between different systems without changing the underlying architecture (software and hardware). ? An international standard that referenced by most network vendors for their products and services ? Best tools for teaching people about sending and receiving data on a network. ? Consist of seven layers: Physical (layer 1), data link (layer 2), Network (layer 3), Transport (layer 4), Session (layer 5), Presentation (layer 6) and Application (layer 7). Each of the layer illustrates a particular network function. This separation of networking functions is called layering. 哈工大计算机学院李全龙Network Application Development 1: Introduction 17 7 layers of OSI model Application Presentation Session Transport Network Data link Physical Layer 7: Layer 6: Layer 5: Layer 4: Layer 3: Layer 2: Layer 1: ? Advantages of multi-layered model ? Reduce complexity - simplified teaching and learning ? Standardize interface ? Allow communication between different network hardware and software ? Prevent the change of one layer from affecting the other layers ? Accelerate Evolution Some Characteristics for the 7-layer model ? Peer-to-peer process - layer N of one computer (device) can communicate with layer N on another computer (device) through protocols of that layer. ? A layer provide service for the layer above it 哈工大计算机学院李全龙Network Application Development 1: Introduction 18 Data flow and Peer communication in OSI layers Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 Network Data link Data link Physical Physical Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 PC 1 PC 2Intermediate node link link For PC1 sends a message to PC2 Physical medium Physical medium Transport-layer protocol Session-layer protocol Presentation-layer protocol Application-layer protocol Data flow Peer communication within layer 4 哈工大计算机学院李全龙Network Application Development 1: Introduction 19 Data Encapsulation Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 PC 1 PC 2 User data AH PH SH AH PH SH TH NH AH PH SH TH NH DHDT 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Application Presentation Session Transport Network Data link Physical 7 6 5 4 3 2 1 User data AH PH SH AH PH SH TH AH PH SH TH NH AH PH SH TH NH DHDT 10101010101011111110000000101010 A-PDU S-PDU P-PDU T-PDU N-PDU Network Wrapping of data with necessary protocol information before network transit (adding header or trailer or both) 哈工大计算机学院李全龙Network Application Development 1: Introduction 20 Why Data Encapsulation ? ?To add control information (in form of header or trailer or both) to the data being encapsulated in order to ensure accurate and secure communication. The data after encapsulated is called protocol data unit (PDU) The control information falls into the three categories: ? Address: The address of the sender and/or receiver may be indicated. ? Error-detecting code: some sort of frame check sequence ? Protocol control: additional information is included to implement the protocol functions such as: priority, quality of service and security ?Note: Headers are added to the data at layers 6,5,4,3, and 2. Trailers are usually added only at layer2. 哈工大计算机学院李全龙Network Application Development 1: Introduction 21 Functions of the Physical Layer 10101010111101 DL - PDU From data link layer Physical layer 10101010111101 DL - PDU To data link layer Physical layer ? Physical characteristics of interfaces and media - interface between the devices and the transmission medium (e.g. network card). It also defines the type of transmission medium (e.g. wire) ? Representation of bits - encoding the bit (0s and 1s ) into electrical or optical signals. This layer also defines the type of encoding (e.g. ASK or FSK) ? Data rate - the transmission rate in term of number of bit sent each second (e.g. 56 Kb/s) ? Synchronization of bits - the sender and the receiver clocks must be synchronized 哈工大计算机学院李全龙Network Application Development 1: Introduction 22 Functions of the Physical Layer ? Line configuration - the connection of devices to the medium (point-to-point or multipoint configuration) ? Physical topology - the configuration for the devices to be connected to form a network (mesh topology, star topology, ring topology or bus topology) ? Transmission mode - The physical layer also defines the direction of transmission between two devices: Simplex, half- duplex, or full-duplex Example for the physical layer protocol: ? RS-232 哈工大计算机学院李全龙Network Application Development 1: Introduction 23 Functions of the Data Link Layer 10101010111101 DHDT N - PDU Data link layer Frame To physical layer From network layer 10101010111101 DHDT N - PDU Data link layer Frame From physical layer To network layer The data link layer is responsible for node-to-node delivery, it makes the physical layer appear error free to the upper layer (network layer). It also take care of the following functions: ? Framing - The data link layer divides the stream of bits received from the network layer into manageable data units called frames ? Physical addressing - the data link layer adds a header to the frame to define the physical address of the sender (source address) and/or receiver (destination address) of the frame 哈工大计算机学院李全龙Network Application Development 1: Introduction 24 Functions of the Data Link Layer 20 25 32 48 62 Data 20 62DT Trailer Source address destination address ? Flow control - If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer will impose a flow control mechanism to prevent overwhelming the receiver ? Error control - mechanism that can detect and retransmit damaged or lost frames and also prevent duplication of frames. Error control is normally achieved by the trailer at the end of the frame ? Access control - When one or more devices are connected to the same link, data link layer protocols are necessary to determine which device has control over the link at any given time. 5 哈工大计算机学院李全龙Network Application Development 1: Introduction 25 Functions of the Network Layer N-PDU NH T- PDU Network layer packet To data link layer From transport layer N-PDU NH T- PDU Network layer packet From data link layer To transport layer ? Logical addressing - the network layer adds a header to the incoming packet from the upper layer with a logical address of the sender. If a packet passes the network boundary, this universal (logical) address helps to distinguish whether the packet belong to the source or destination systems The network layer is responsible for the source-to-destination delivery of a packet possibly across multiple networks, if two systems are connected to the same link, no need for a network layer. The functions of a network layer are: 哈工大计算机学院李全龙Network Application Development 1: Introduction 26 Functions of the Network Layer ? Routing - Router (or gateways) connect independent networks together, network layer routes the packet to their final destination by router (gateway). F R K L 15 D 78 79 DT Data S D 02 46 DT Data S D 23 09 DT Data S D 29 79 The above figure shows the case for a computer "S" communicating with a remote computer "D". S and D are the logical addresses of the source and destination computers respectively, where 02 and 79 are the local physical addresses of the source and destination computers respectively. T Ring Router Router U X 46 23 61 09 29 S A 02 37 C 哈工大计算机学院李全龙Network Application Development 1: Introduction 27 Functions of the Transport Layer T-PDU S- PDU Transport layer To Network layer From session layer TH TH TH T-PDU T-PDU T-PDU S- PDU Transport layer From Network layer To session layer TH TH TH T-PDU T-PDU segments segments ? responsible for source-to-destination (end-to-end) delivery of the entire message ? segmentation occurs and this layer also ensures the whole message arrives intact and in order ? a logical path can be set up (fixed pathway) between the source and destination for all packets in a message for additional security. ? This layer has more control over sequencing, flow and error detection and correction 哈工大计算机学院李全龙Network Application Development 1: Introduction 28 Functions of the Transport Layer Data x y Data-1 x y S D DHDT Data-2 x y S D DHDT Data-1 x y S D Data-2 x y S D Transport layer Network layer Data link layer S Data x y Data-1 x y S D DHDT Data-2 x y S D DHDT Data-1 x y S D Data-2 x y S D Transport layer Network layer Data link layer D Internet ? Segmentation and re-assembly - A message is divided into transmittable segments (can be handled by network layer) each contains a sequence number (for correct re-assembling at the destination). ? Service-point addressing - the transport layer is responsible for the delivery of the entire message to the correct process on the computer ( that may be running several programs) by including a service-point address (port address) in its header. 哈工大计算机学院李全龙Network Application Development 1: Introduction 29 Functions of the Transport Layer ? Connection control - The transport layer can be either connectionless or connection-oriented. A connectionless transport layer deliver each segments as an independent packet to the destination. A connection- oriented transport layer firstly makes a connection with the transport layer at the destination, then deliver the packets and after all data are transferred without error, the connection is terminated. ? Flow control - Transport layer perform end to end flow control which differs from the data link layer which is responsible for flow control in single link. ? Error control - The transport layer performs end to end error control which make sure the entire message arrives at the receiving transport layer without error (damage, loss or duplication). Error correction is done by re-transmission. 哈工大计算机学院李全龙Network Application Development 1: Introduction 30 Functions of the Session Layer S-PDU P- PDU To transport layer From Presentation layer SH Session layer syn syn syn S-PDU P- PDU From transport layer To Presentation layer SH Session layer syn syn syn ? Dialog control - The session layer allows two systems to enter into a dialog. The communication between two processes can be controlled in either half-duplex (one way at a time) or full-duplex (two ways at a time). The session layer is the network dialog controlling. It establishes, maintains, and synchronizes the interaction between communication systems. ? Synchronization - The session layer allows a process to add checkpoints (synchronization points) into a stream of data, when data loss during transmission occurred, more efficient re-transmission can be performed. 6 哈工大计算机学院李全龙Network Application Development 1: Introduction 31 Functions of the Presentation Layer P-PDU Encoded, encrypted, and compressed data PH A- PDU Presentation layer To session layer From application layer P-PDU Decoded, decrypted, and decompressed data PH A- PDU Presentation layer From session layer To application layer The presentation layer is concerned with the syntax and semantics of the information exchange between two systems. ? Translation - The presentation layer changes (encodes) the sender- dependent format of the message into a common format that can be readable by any receiving system ? Encryption - The presentation layer can transform the original information into an encrypted form to assure privacy. ? Compression - Data compression can reduce the number of bits to be transmitted, which is important for multimedia transmission. 哈工大计算机学院李全龙Network Application Development 1: Introduction 32 Functions of the Application Layer This layers enable the user (human or software) to access the network by user interfaces. Sometimes, No header or trailers are added at this layer. Typical services provided by this layer are: ? Network virtual terminal - remote log-in other host in the internet ? File transfer, access, and management - store, retrieve or manage the files from a remote computer. e.g. File Transfer Protocol (FTP) ? Mail services - e.g. Simple Mail Transfer Protocol (SMTP) ? Directory services - access for global information and services e.g. Hyper Text Transfer Protocol (HTTP) Application layer User A (U.S.) HTTP FTP SMTP L7 data Application layer User B (Africa) HTTP FTP SMTP L7 data 哈工大计算机学院李全龙Network Application Development 1: Introduction 33 Application Presentation Session Transport Network Data link Physical Summary for the functions of OSI-layers allow access to network Translate, encrypt, and compress data Establish, manage, and terminate session Provide reliable end-to- end message delivery and error recoveryMove packets from source to destination; to provide internetworking Organize bits into frames; to provide node-to-node deliveryTransmit bits over a medium; to provide mechanical and electrical specification 哈工大计算机学院李全龙Network Application Development 1: Introduction 34 Chapter 1: Introduction Chapter goal: ? Network basics ? TCP/IP protocols stack review ? Client-server application model and software design ? Concurrent processing in client-server model 哈工大计算机学院李全龙Network Application Development 1: Introduction 35 TCP/IP Protocol Suite ? Dominant commercial protocol architecture ? Specified and extensively used before OSI ? Developed by research funded US Department of Defense ? Used by the Internet 哈工大计算机学院李全龙Network Application Development 1: Introduction 36 TCP/IP Protocol Architecture(1) ? Application Layer ? Communication between processes or applications ? End to end or transport layer (TCP/UDP/…) ? End to end transfer of data ? May include reliability mechanism (TCP) ? Hides detail of underlying network ? Network Layer ? Routing of data 7 哈工大计算机学院李全龙Network Application Development 1: Introduction 37 TCP/IP Protocol Architecture(2) ? Data link Layer ? data transfer between neighboring network elements ? Physical Layer ? Transmission medium ? Signal rate and encoding 哈工大计算机学院李全龙Network Application Development 1: Introduction 38 PDUs in TCP/IP 哈工大计算机学院李全龙Network Application Development 1: Introduction 39 Some Protocols in TCP/IP Suite 哈工大计算机学院李全龙Network Application Development 1: Introduction 40 IPX/SPX ? Developed by Xerox originally,be applied in Novell Netware OS later ? Hosts that running Windows NT/XP/2000/98/95 can connect to Novell Netware by IPX/SPX ? IPX/SPX is called NWLlink in Windows NT ? IPX/SPX can be routed like TCP/IP 哈工大计算机学院李全龙Network Application Development 1: Introduction 41 IPX/SPX vs OSI Physical Data LinkMedia+Protocols Such as Ethernet NetworkIPX TransportSPX SessionNetBios Presentation ApplicationNCP SAP RIP IPX/SPX OSI Model 哈工大计算机学院李全龙Network Application Development 1: Introduction IPX Protocols ? IPX — Inter-networks Packet eXchange like IP ? SPX — Serial Packet eXchange like TCP ? RIP—routing updates ? SAP—service advertisements ? NCP — NetWare Core Protocol ? NetBIOS emulation ? Design options – Protocol filters – Hierarchical design – EIGRP and NLSP as routing protocols 8 哈工大计算机学院李全龙Network Application Development 1: Introduction Novell Servers Support Several Client Stacks NetBIOS SPX IPX Open Data-Link Interface (ODI) 802.3 802.5 FDDI PPP IPX IP DDP NCP NFS AFP SAA Gateway 哈工大计算机学院李全龙Network Application Development 1: Introduction 44 AppleTalk ? Designed to connect Macintosh computers originally ? Now can be integrated to NetWare or Microsoft networks ? Be suitable to small net,not large scale 哈工大计算机学院李全龙Network Application Development 1: Introduction 45 AppleTalk vs OSI Application Presentation Session Transport Network Data Link Physical 哈工大计算机学院李全龙Network Application Development 1: Introduction AppleTalk Protocols ? AppleShare —file sharing ? AFP —AppleTalk interFace Protocol ? ASP —AppleTalk Session protocol ? ATP —AppleTalk Transaction protocol ? RTMP—routing updates ? ZIP—zone information ? NBP—name-to-number translation ? DDP —Datagram Deliver Protocol 哈工大计算机学院李全龙Network Application Development 1: Introduction AppleTalk Protocol Suite Physical and data link layers MacTCP supports IP stack AppleTalk network applications IP network applications AppleTalk higher layers RTMP AARPDDP ATP ADSP NBP ChooserRouting ZIP 哈工大计算机学院李全龙Network Application Development 1: Introduction Windows Networking ? Based on NetBIOS – Session-layer protocol – Name Query—search for a server by name ? Transport protocols – NetBEUI – NWLink—NetBIOS over IPX – NBT—NetBIOS over TCP/IP ? Design options – Use NWLink or NBT – Protocol filters – Hierarchical designs 9 哈工大计算机学院李全龙Network Application Development 1: Introduction M.S Windows NetBIOS Protocol Stack Application Transport Session Network Data Link Physical Presentation Redirector Server Message Block (SMB) NetBIOS Network interface card 1 7 6 2 3 4 5 OSI ReferenceNetBIOS 哈工大计算机学院李全龙Network Application Development 1: Introduction NetBIOS 7 ? NetBIOS is a session—layer protocol used for ? File sharing ? Printer sharing ? Messaging ? Authentication ? Name resolution Name Query - Broadcast Where is File_Server? Name Query - Broadcast Where is Print_Server? Print_ Server File_ Server 哈工大计算机学院李全龙Network Application Development 1: Introduction Windows Network Transport Mechanisms ? NetBIOS can be transported by ? NetBEUI ? NetBIOS over IPX—NWLink ? NetBIOS over TCP/IP—NBT NetBIOS NetBEUI IPX TCP/IP 哈工大计算机学院李全龙Network Application Development 1: Introduction NetBEUI Protocol Stack ? Network-layer addressing not included ? Application traffic must be bridged Application Transport Session Network Data Link Physical Presentation Redirector Server Message Block (SMB) NetBIOS Network interface card 1 7 6 2 3 4 5 OSI ReferenceNetBEUI NetBEUI 哈工大计算机学院李全龙Network Application Development 1: Introduction NetBIOS over IPX—NWLink ? Technical requirements ? Recommended for small- to medium-size networks ? Needs IPX to be operational already ? Carried as IPX type-20 packets Application Data Link NetBIOS Physical IPX 哈工大计算机学院李全龙Network Application Development 1: Introduction NetBIOS over TCP/IP—NBT ? Technical requirements ? Recommended for medium-size to large-scale networks ? Needed for networks that include a WAN link ? Depends on an IP address assignment policy Application Data Link NetBIOS Physical TCP IP 10 哈工大计算机学院李全龙Network Application Development 1: Introduction 55 Internet Protocol (IP) ? Part of TCP/IP ? Used by the Internet ? Specifies interface with higher layer ? e.g. TCP ? Only protocol at Layer 3 ? Fundamental in suite ? Defines ? Internet addressing ? Internet packet format ? Internet routing 哈工大计算机学院李全龙Network Application Development 1: Introduction 56 IP Addressing: introduction ? IP address: 32-bit identifier for host, router interface ? interface: connection between host, router and physical link ? router’s typically have multiple interfaces ? host may have multiple interfaces ? IP addresses associated with interface, not host, router 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2223.1.3.1 223.1.3.27 223.1.1.1 = 11011111 00000001 00000001 00000001 223 111 哈工大计算机学院李全龙Network Application Development 1: Introduction 57 IP Addressing 哈工大计算机学院李全龙Network Application Development 1: Introduction 58 IP Addressing ? IP address: ? network part (high order bits) ? host part (low order bits) ? What’s a network ? (from IP address perspective) ? device interfaces with same network part of IP address ? can physically reach each other without intervening router 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 network consisting of 3 IP networks (for IP addresses starting with 223, first 24 bits are network address) LAN 哈工大计算机学院李全龙Network Application Development 1: Introduction 59 IP Addressing How to find the networks? ? Detach each interface from router, host ? create “islands of isolated networks 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.2223.1.2.1 223.1.2.6 223.1.3.2223.1.3.1 223.1.3.27 223.1.1.2 223.1.7.0 223.1.7.1 223.1.8.0223.1.8.1 223.1.9.1 223.1.9.2 Interconnected system consisting of six networks 哈工大计算机学院李全龙Network Application Development 1: Introduction 60 IP Addresses 0network host 10 network host 110 network host 1110 multicast address A B C D class 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 32 bits given notion of “network”, let’s re-examine IP addresses: “class-full” addressing: E 1111 240.0.0.0 to 255.255.255.255 11 哈工大计算机学院李全龙Network Application Development 1: Introduction 61哈工大计算机学院李全龙Network Application Development 1: Introduction 62 哈工大计算机学院李全龙Network Application Development 1: Introduction 63 Special Addresses DestinationAny127Loopback address DestinationSpecificAll 0sSpecific host on this network SourceAll 0sAll 0sThis host on this network DestinationAll 1sAll 1sLimited broadcast Address DestinationAll 1sSpecificDirect broadcast Address NoneAll 0sSpecificNetwork Address Source or Destination HostIDNetIDSpecial Address 哈工大计算机学院李全龙Network Application Development 1: Introduction 64 Private Addresses 256192.168.0 to 192.168.255 C 16172.16 to 172.31B 110.0.0A BlocksNetIDsClass 哈工大计算机学院李全龙Network Application Development 1: Introduction 65 Subnets and Subnet Masks ? Allow arbitrary complexity of internetworked LANs within organization ? Insulate overall internet from growth of network numbers and routing complexity ? Site looks to rest of internet like single network ? Each LAN assigned subnet number ? Host portion of address partitioned into subnet number and host number ? Local routers route within subnetted network ? Subnet mask indicates which bits are subnet number and which are host number 哈工大计算机学院李全龙Network Application Development 1: Introduction 66 Subnets and Subnet Mask 12 哈工大计算机学院李全龙Network Application Development 1: Introduction 67 Rules 哈工大计算机学院李全龙Network Application Development 1: Introduction 68 哈工大计算机学院李全龙Network Application Development 1: Introduction 69哈工大计算机学院李全龙Network Application Development 1: Introduction 70 哈工大计算机学院李全龙Network Application Development 1: Introduction 71哈工大计算机学院李全龙Network Application Development 1: Introduction 72 .224 thru .25500000 thru 11111111Eighth Subnetwork .192 thru .22300000 thru 11111110Seventh Subnetwork .160 thru .19100000 thru 11111101Sixth Subnetwork .128 thru .15900000 thru 11111100Fifth Subnetwork .96 thru .12700000 thru 11111011Fourth Subnetwork .64 thru .9500000 thru 11111010Third Subnetwork .32 thru .6300000 thru 11111001Second Subnetwork .0 thru .3100000 thru 11111000First Subnetwork Fourth Octet of a Class "C" Network 13 哈工大计算机学院李全龙Network Application Development 1: Introduction 73 .224 thru .25500000 thru 11111111Eighth Subnetwork .192 thru .22300000 thru 11111110Seventh Subnetwork .160 thru .19100000 thru 11111101Sixth Subnetwork .128 thru .15900000 thru 11111100Fifth Subnetwork .96 thru .12700000 thru 11111011Fourth Subnetwork .64 thru .9500000 thru 11111010Third Subnetwork .32 thru .6300000 thru 11111001Second Subnetwork .0 thru .3100000 thru 11111000First Subnetwork Which numbers in a subnetwork are reserved for broadcasts/subnetwork ? 哈工大计算机学院李全龙Network Application Development 1: Introduction 74 How does the router handle IP addresses and subnet masks? 哈工大计算机学院李全龙Network Application Development 1: Introduction 75 How does the router handle IP addresses and subnet masks? 哈工大计算机学院李全龙Network Application Development 1: Introduction 76 How does the router handle IP addresses and subnet masks? 哈工大计算机学院李全龙Network Application Development 1: Introduction 77 How does the router handle IP addresses and subnet masks? 哈工大计算机学院李全龙Network Application Development 1: Introduction 78 Supernetting ? Classes A and B are almost deleted ? Class C addresses are available ? One organazition can combine several class C blocks ? But how? ? Randomly ? S set of rules ? Ramdomly assigning: increase the size of the routing table ? So supernetting 14 哈工大计算机学院李全龙Network Application Development 1: Introduction 79 IP addressing: CIDR ? classful addressing: ? inefficient use of address space, address space exhaustion ? e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network ? CIDR: Classless InterDomain Routing ? network portion of address of arbitrary length ? address format: a.b.c.d/x, where x is # bits in network portion of address 11001000 00010111 00010000 00000000 network part host part 200.23.16.0/23 哈工大计算机学院李全龙Network Application Development 1: Introduction 80 Hierarchical addressing: route aggregation “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16” 200.23.20.0/23 Organization 2 . . . . . . Hierarchical addressing allows efficient advertisement of routing information: 哈工大计算机学院李全龙Network Application Development 1: Introduction 81 Hierarchical addressing: more specific routes ISPs-R-Us has a more specific route to Organization 1 “Send me anything with addresses beginning 200.23.16.0/20” 200.23.16.0/23 200.23.18.0/23 200.23.30.0/23 Fly-By-Night-ISP Organization 0 Organization 7 Internet Organization 1 ISPs-R-Us “Send me anything with addresses beginning 199.31.0.0/16 or 200.23.18.0/23” 200.23.20.0/23 Organization 2 . . . . . . 哈工大计算机学院李全龙Network Application Development 1: Introduction 82 IP datagram format Ver/4 Length/16 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum/16 time to live/8 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len/4 type of service/8 “type” of data flgs/3 fragment offset/13 upper layer/8 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. 哈工大计算机学院李全龙Network Application Development 1: Introduction 83 IP v6 - Version Number ? IP v 1-3 defined and replaced ? IP v4 - current version ? IP v5 - streams protocol ? IP v6 - replacement for IP v4 ? During development it was called IPng ? Next Generation 哈工大计算机学院李全龙Network Application Development 1: Introduction 84 Why IPv6?(Theoretical Reasons) only compelling reason: more IP addresses! ? for billions of new users (Japan, China, India,…) ? for billions of new devices (mobile phones, cars, appliances,…) ? for always-on access (cable, xDSL, ethernet-to-the- home,…) ? for applications that are difficult, expensive, or impossible to operate through NATs (IP telephony, peer-to-peer gaming, home servers,…) ? to phase out NATs to improve the robustness, security, performance, and manageability of the Internet 15 哈工大计算机学院李全龙Network Application Development 1: Introduction 85 Other Benefits of IPv6 ? server-less plug-and-play possible ? end-to-end, IP-layer authentication & encryption possible ? elimination of “triangle routing” for mobile IP ? other minor improvements NON-benefits: ? quality of service (same QoS capabilities as IPv4) ? flow label field in IPv6 header may enable more efficient flow classification by routers, but does not add any new capability ? routing (same routing protocols as IPv4) ? except larger address allows more levels of hierarchy ? except customer multihoming is defeating hierarchy 哈工大计算机学院李全龙Network Application Development 1: Introduction 86 IPv6 ? Background ? Technology Overview ? Deployment Strategies ? Current Status 哈工大计算机学院李全龙Network Application Development 1: Introduction 87 IPv6 Header compared to IPv4 Header Ver. Time to Live Source Address Total Length Type of Service Hdr Len Identification Fragment Offset Flg Protocol Header Checksum Destination Address Options... Ver. Traffic Class Source Address Payload Length Next Header Hop Limit Destination Address shaded fields have no equivalent in the other version IPv6 header is twice as long (40 bytes) as IPv4 header without options (20 bytes) Flow Label 哈工大计算机学院李全龙Network Application Development 1: Introduction 88 Summary of Header Changes ? Revised ? Addresses increased 32 bits -> 128 bits ? Time to Live -> Hop Limit ? Protocol -> Next Header ? Type of Service -> Traffic Class ? Streamlined ? Fragmentation fields moved out of base header ? IP options moved out of base header ? Header Checksum eliminated ? Header Length field eliminated ? Length field excludes IPv6 header ? Alignment changed from 32 to 64 bits ? Extended ? Flow Label field added 哈工大计算机学院李全龙Network Application Development 1: Introduction 89 Text Representation of Addresses “preferred” form: 1080:0:FF:0:8:800:200C:417A compressed form: FF01:0:0:0:0:0:0:43 becomes FF01::43 IPv4-embedded: 0:0:0:0:0:FFFF:13.1.68.3 or ::FFFF:13.1.68.3 哈工大计算机学院李全龙Network Application Development 1: Introduction 90 Text Representation of Addresses (cont.) address prefix: 2002:43c:476b::/48 (note: no masks in IPv6!) zone qualifiers: FE80::800:200C:417A%3 in URLs: http://[3FFE::1:800:200C:417A]:8000 (square-bracket convention also used anywhere else there’s a conflict with address syntax) 16 哈工大计算机学院李全龙Network Application Development 1: Introduction 91 Basic Address Types unicast: for one-to-one communication multicast: for one-to-many communication anycast: for one-to-nearest communication M M M A A A U 哈工大计算机学院李全龙Network Application Development 1: Introduction 92 ICMP: Internet Control Message Protocol ? used by hosts, routers, gateways to communication network-level information ? error reporting: unreachable host, network, port, protocol ? echo request/reply (used by ping) ? network-layer “above” IP: ? ICMP msgs carried in IP datagrams ? ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header 哈工大计算机学院李全龙Network Application Development 1: Introduction 93 ICMP Message Formats 哈工大计算机学院李全龙Network Application Development 1: Introduction 94 ARP: Address Resolution Protocol ? Each IP node (Host, Router) on LAN has ARP module, table ? ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> < ………………………….. > ? TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B given B’s IP address? 哈工大计算机学院李全龙Network Application Development 1: Introduction 95 ARP Table 哈工大计算机学院李全龙Network Application Development 1: Introduction 96 How do devices use ARP tables to send data?(1) 17 哈工大计算机学院李全龙Network Application Development 1: Introduction 97 How do devices use ARP tables to send data?(2) 哈工大计算机学院李全龙Network Application Development 1: Introduction 98 How do devices use ARP tables to send data?(3) 哈工大计算机学院李全龙Network Application Development 1: Introduction 99 How do devices use ARP tables to send data?(4) 哈工大计算机学院李全龙Network Application Development 1: Introduction 100 How do devices use ARP tables to send data?(5) 哈工大计算机学院李全龙Network Application Development 1: Introduction 101 How do devices use ARP tables to send data?(6) 哈工大计算机学院李全龙Network Application Development 1: Introduction 102 How do devices use ARP tables to send data?(7) 18 哈工大计算机学院李全龙Network Application Development 1: Introduction 103 How do devices use ARP tables to send data?(8) 哈工大计算机学院李全龙Network Application Development 1: Introduction 104 What happens if the device is unable to locate the destination MAC address in its ARP table? 哈工大计算机学院李全龙Network Application Development 1: Introduction 105 ARP message format (IP to Ethernet) 哈工大计算机学院李全龙Network Application Development 1: Introduction 106 ARP message format (IP to Ethernet) ‘1’-Ethernet 哈工大计算机学院李全龙Network Application Development 1: Introduction 107 ARP message format (IP to Ethernet) Upper layer protocol address type, for IP,it’s value is 0806H 哈工大计算机学院李全龙Network Application Development 1: Introduction 108 ARP message format (IP to Ethernet) Hardware Address Length 19 哈工大计算机学院李全龙Network Application Development 1: Introduction 109 ARP message format (IP to Ethernet) Protocol Address Length 哈工大计算机学院李全龙Network Application Development 1: Introduction 110 ARP message format (IP to Ethernet) ‘1’-ARP request ‘2’-ARP response ‘3’-RARP request ‘4’-RARP response 哈工大计算机学院李全龙Network Application Development 1: Introduction 111 ARP message format (IP to Ethernet) Sender Hardware Address 哈工大计算机学院李全龙Network Application Development 1: Introduction 112 ARP message format (IP to Ethernet) Sender IP Address 哈工大计算机学院李全龙Network Application Development 1: Introduction 113 ARP message format (IP to Ethernet) Target Hardware Address 哈工大计算机学院李全龙Network Application Development 1: Introduction 114 ARP message format (IP to Ethernet) Target IP Address 20 哈工大计算机学院李全龙Network Application Development 1: Introduction 115 ARP protocol ? A knows B's IP address, wants to learn physical address of B ? A broadcasts ARP query pkt, containing B's IP address ? all machines on LAN receive ARP query ? B receives ARP packet, replies to A with its (B's) physical layer address ? A caches (saves) IP-to-physical address pairs until information becomes old (times out) ? soft state: information that times out (goes away) unless refreshed 哈工大计算机学院李全龙Network Application Development 1: Introduction 116 Routing in the Internet ? The Global Internet consists of Autonomous Systems (AS) interconnected with each other: ? Stub AS: small corporation ? Multihomed AS: large corporation (no transit) ? Transit AS: provider ? Two-level routing: ? Intra-AS: administrator is responsible for choice ? Inter-AS: unique standard 哈工大计算机学院李全龙Network Application Development 1: Introduction 117 Internet AS Hierarchy Inter-AS border (exterior gateway) routers Intra-AS interior (gateway) routers 哈工大计算机学院李全龙Network Application Development 1: Introduction 118 Intra-AS Routing ? Also known as Interior Gateway Protocols (IGP) ? Most common IGPs: ? RIP: Routing Information Protocol ? OSPF: Open Shortest Path First ? IGRP: Interior Gateway Routing Protocol (Cisco propr.) 哈工大计算机学院李全龙Network Application Development 1: Introduction 119 RIP ( Routing Information Protocol) ? Distance vector algorithm ? Included in BSD-UNIX Distribution in 1982 ? Distance metric: # of hops (max = 15 hops) ? Can you guess why? ? Distance vectors: exchanged every 30 sec via Response Message (also called advertisement) ? Each advertisement: route to up to 25 destination nets 哈工大计算机学院李全龙Network Application Development 1: Introduction 120 RIP (Routing Information Protocol) Destination Network Next Router Num. of hops to dest. w A2 y B2 z B7 x -- 1 …. …. .... w xy z A C D B Routing table in D 21 哈工大计算机学院李全龙Network Application Development 1: Introduction 121 RIP: Link Failure and Recovery If no advertisement heard after 180 sec --> neighbor/link declared dead ? routes via neighbor invalidated ? new advertisements sent to neighbors ? neighbors in turn send out new advertisements (if tables changed) ? link failure info quickly propagates to entire net ? poison reverse used to prevent ping-pong loops (infinite distance = 16 hops) 哈工大计算机学院李全龙Network Application Development 1: Introduction 122 RIP Table processing ? RIP routing tables managed by application- level process called route-d (daemon) ? advertisements sent in UDP packets, periodically repeated 哈工大计算机学院李全龙Network Application Development 1: Introduction 123 RIP Table example (continued) Router: giroflee.eurocom.fr ? Three attached class C networks (LANs) ? Router only knows routes to attached LANs ? Default router used to “go up” ? Route multicast address: 224.0.0.0 ? Loopback interface (for debugging) Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454 哈工大计算机学院李全龙Network Application Development 1: Introduction 124 OSPF (Open Shortest Path First) ? “open”: publicly available ? Uses Link State algorithm ? LS packet dissemination ? Topology map at each node ? Route computation using Dijkstra’s algorithm ? OSPF advertisement carries one entry per neighbor router ? Advertisements disseminated to entire AS (via flooding) 哈工大计算机学院李全龙Network Application Development 1: Introduction 125 OSPF “advanced” features (not in RIP) ? Security: all OSPF messages authenticated (to prevent malicious intrusion) ? Multiple same-cost paths allowed (only one path in RIP) ? For each link, multiple cost metrics for different TOS (eg, satellite link cost set “low” for best effort; high for real time) ? Integrated uni- and multicast support: ? Multicast OSPF (MOSPF) uses same topology data base as OSPF ? Hierarchical OSPF in large domains. 哈工大计算机学院李全龙Network Application Development 1: Introduction 126 Hierarchical OSPF 22 哈工大计算机学院李全龙Network Application Development 1: Introduction 127 Hierarchical OSPF ? Two-level hierarchy: local area, backbone. ? Link-state advertisements only in area ? each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. ? Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers. ? Backbone routers: run OSPF routing limited to backbone. ? Boundary routers: connect to other ASs. 哈工大计算机学院李全龙Network Application Development 1: Introduction 128 IGRP (Interior Gateway Routing Protocol) ? CISCO proprietary; successor of RIP (mid 80s) ? Distance Vector, like RIP ? several cost metrics (delay, bandwidth, reliability, load etc) ? uses TCP to exchange routing updates ? Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation 哈工大计算机学院李全龙Network Application Development 1: Introduction 129 Inter-AS routing 哈工大计算机学院李全龙Network Application Development 1: Introduction 130 Internet inter-AS routing: BGP ? BGP (Border Gateway Protocol): the de facto standard ? Path Vector protocol: ? similar to Distance Vector protocol ? each Border Gateway broadcast to neighbors (peers) entire path (I.e, sequence of ASs) to destination ? E.g., Gateway X may send its path to dest. Z: Path (X,Z) = X,Y1,Y2,Y3,…,Z 哈工大计算机学院李全龙Network Application Development 1: Introduction 131 Internet inter-AS routing: BGP Suppose: gateway X send its path to peer gateway W ? W may or may not select path offered by X ? cost, policy (don’t route via competitors AS), loop prevention reasons. ? If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) ? Note: X can control incoming traffic by controlling it route advertisements to peers: ? e.g., don’t want to route traffic to Z -> don’t advertise any routes to Z 哈工大计算机学院李全龙Network Application Development 1: Introduction 132 Internet inter-AS routing: BGP ? BGP messages exchanged using TCP. ? BGP messages: ? OPEN: opens TCP connection to peer and authenticates sender ? UPDATE: advertises new path (or withdraws old) ? KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request ? NOTIFICATION: reports errors in previous msg; also used to close connection 23 哈工大计算机学院李全龙Network Application Development 1: Introduction 133 IGMP ? Internet Group Management Protocol ? RFC 1112 ? Host and router exchange of multicast group info ? Use broadcast LAN to transfer info among multiple hosts and routers 哈工大计算机学院李全龙Network Application Development 1: Introduction 134 IGMP Format 哈工大计算机学院李全龙Network Application Development 1: Introduction 135 IGMP Fields ? Version ? 1 ? Type ? 1 - query sent by router ? O - report sent by host ? Checksum ? Group address ? Zero in request message ? Valid group address in report message 哈工大计算机学院李全龙Network Application Development 1: Introduction 136 IGMP Operation ? To join a group, hosts sends report message ? Group address of group to join ? In IP datagram to same multicast destination address ? All hosts in group receive message ? Routers listen to all multicast addresses to hear all reports ? Routers periodically issue request message ? Sent to all-hosts multicast address ? Host that want to stay in groups must read all- hosts messages and respond with report for each group it is in 哈工大计算机学院李全龙Network Application Development 1: Introduction 137 Group Membership in IPv6 ? Function of IGMP included in ICMP v6 ? New group membership termination message to allow host to leave group 哈工大计算机学院李全龙Network Application Development 1: Introduction 138 UDP: User Datagram Protocol [RFC 768] ? “no frills,” “bare bones” Internet transport protocol ? “best effort” service, UDP segments may be: ? lost ? delivered out of order to app ? connectionless: ? no handshaking between UDP sender, receiver ? each UDP segment handled independently of others Why is there a UDP? ? no connection establishment (which can add delay) ? simple: no connection state at sender, receiver ? small segment header ? no congestion control: UDP can blast away as fast as desired 24 哈工大计算机学院李全龙Network Application Development 1: Introduction 139 UDP: more ? often used for streaming multimedia apps ? loss tolerant ? rate sensitive ? other UDP uses (why?): ? DNS ? SNMP ? reliable transfer over UDP: add reliability at application layer ? application-specific error recover! source port # dest port # 32 bits Application data (message) UDP segment format length checksum Length, in bytes of UDP segment, including header 哈工大计算机学院李全龙Network Application Development 1: Introduction 140 TCP: Overview RFCs: 793, 1122, 1323, 2018, 2581 ? full duplex data: ? bi-directional data flow in same connection ? MSS: maximum segment size ? connection-oriented: ? handshaking (exchange of control msgs) init’s sender, receiver state before data exchange ? flow controlled: ? sender will not overwhelm receiver ? Congestion controlled ? point-to-point: ? one sender, one receiver ? reliable, in-order byte stream: ? no “message boundaries” ? pipelined: ? TCP congestion and flow control set window size ? send & receive buffers socket door TCP send buffer TCP receive buffer socket door segment application writes data application reads data 哈工大计算机学院李全龙Network Application Development 1: Introduction 141 TCP segment structure source port # dest port # 32 bits application data (variable length) sequence number acknowledgement number rcvr window size ptr urgent datachecksum FSRPAU head len not used Options (variable length) URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) # bytes rcvr willing to accept counting by bytes of data (not segments!) Internet checksum (as in UDP) 哈工大计算机学院李全龙Network Application Development 1: Introduction 142 Application protocols ? http ? ftp ? tftp ? smtp ? pop ? Dns ? Telnet ? RTSP ? RTP ? RTCP ? RSVP ? …… 哈工大计算机学院李全龙Network Application Development 1: Introduction 143 Chapter 1: Introduction Chapter goal: ? Network basics ? TCP/IP protocols stack review ? Client-server application model and software design ? Concurrent processing in client-server model 哈工大计算机学院李全龙Network Application Development 1: Introduction 144 Why need new applications ? Well-known applications can not satisfy specific requirement ? In fact, over 1/5 traffic is not well-known applications ? New applications (application protocols) are developed every day ? Emphasis shifts from building networks to using them ? More programmers need to know the functional principles and techniques used to design and implement distributed/network applications 25 哈工大计算机学院李全龙Network Application Development 1: Introduction 145 Functionality ?Transport layer and layers below ?Basic communication ?Reliability ?Application layer ?Abstractions ? Files ? Services ? Databases ?Names 哈工大计算机学院李全龙Network Application Development 1: Introduction 146 Dichotomy of Duties ? Network ? Transfers bits ? Operates at application’s request ? Applications determine ? What to send ? When to send ? Where to send ? Meaning of bits 哈工大计算机学院李全龙Network Application Development 1: Introduction 147 How Two Application Programs Make Contact-Rendezvous ? One application ? Begins execution first ? Waits passively at prearranged location ? Another application ? Begins execution later ? Actively contacts first program ? Called client-server interaction ? Why? ? TCP/IP does not provide any mechanisms that automatically create running programs when a message arrives 哈工大计算机学院李全龙Network Application Development 1: Introduction 148 Client-Server Paradigm ? Used by almost all network applications ? Passive program called a server ? Active program called a client 哈工大计算机学院李全龙Network Application Development 1: Introduction 149 Internet Communication Almost all network applications use a form of communication known as the client-server paradigm. A server application waits passively for contact, while a client application initiates communication actively. 哈工大计算机学院李全龙Network Application Development 1: Introduction 150 Characteristics of a Client ? Arbitrary application program ? Becomes client temporarily ? Can also perform other computations ? Invoked directly by user ? Runs locally on user’s computer ? Actively initiates contact with a server ? Contacts one server at a time 26 哈工大计算机学院李全龙Network Application Development 1: Introduction 151 Characteristics of a Server ? Special-purpose, privileged program ? Dedicated to providing one service ? Can handle multiple remote clients simultaneously ? Invoked automatically when system boots ? Executes forever ? Needs powerful computer and operating system ? Waits passively for client contact ? Accepts requests from arbitrary clients ? Servers can handle: ? Authentication ? Authorization ? Data security ? Privacy ? protection哈工大计算机学院李全龙Network Application Development 1: Introduction 152 Direction of Data Flow ?Data can flow ?From client to server only ?From server to client only ?In both directions ?Application protocol determines flow ?Typical scenario ?Client sends request(s) ?Sever sends response(s) 哈工大计算机学院李全龙Network Application Development 1: Introduction 153 Key Idea Although the client initiates contact, information can flow in either or both directions between a client and server. Many services arrange for the client to send one or more requests and the server to return one response for each request. 哈工大计算机学院李全龙Network Application Development 1: Introduction 154 Clients, Servers, and Other Protocols ? Clients and servers are application programs 哈工大计算机学院李全龙Network Application Development 1: Introduction 155 Identifying a Service ? Protocol port number used ? Each service given unique port number, P ? Server ?Informs OS it is using port P ?Waits for requests to arrive ? Client ?Forms request ?Send request to port P on server computer 哈工大计算机学院李全龙Network Application Development 1: Introduction 156 The Point About Ports Transport protocols assign each service a unique port identifier. A server must specify the identifier when it begins execution. A client must specify the identifier when it requests transport protocol software to contact a server. Protocol software on the server computer uses the identifier to direct an incoming request to the correct server. 27 哈工大计算机学院李全龙Network Application Development 1: Introduction 157 In Theory ? Port numbers are merely integers ? Any server could use any port number 哈工大计算机学院李全龙Network Application Development 1: Introduction 158 In Practice ? Protocol port numbers used as service identifiers ? Need uniform numbering ?To allow arbitrary client to contact server on arbitrary machine ?To avoid inventing “directory assistance” mechanism ? Port numbers ?Uniform throughout Internet ?Set by standards bodies 哈工大计算机学院李全龙Network Application Development 1: Introduction 159 Stateless VS. Stateful Servers ? State information: Information that a server maintains about the status of ongoing interactions with clients. ? Stateless servers: Do not keep any state information. ? Stateful servers: keep state information. ? Keeping a small information in a server can reduce the size of messages that the client and server exchange. ? Stateful design leads to complex application protocols. 哈工大计算机学院李全龙Network Application Development 1: Introduction 160 Chapter 1: Introduction Chapter goal: ? Network basics ? TCP/IP protocols stack review ? Client-server application model and software design ? Concurrent processing in client-server model 哈工大计算机学院李全龙Network Application Development 1: Introduction 161 Concurrency in networks ? Concurrency refers to real or apparent simultaneous computing ? Among machines on a single network, many pairs of applications can communicate concurrently ? Concurrency can also occur within a given computer system ? The application programmer designs and constructs each client program without regard to concurrent execution ? Operating system allows users to execute client programs concurrently 哈工大计算机学院李全龙Network Application Development 1: Introduction 162 Concurrency in networks 28 哈工大计算机学院李全龙Network Application Development 1: Introduction 163 Concurrency in servers 哈工大计算机学院李全龙Network Application Development 1: Introduction 164 Terminology and concepts ? Process： abstraction defines the fundamental unit of computation. ? Some system use the terms of task or job instead of process ? Instruction pointer: (the most essential information associated with process) specifies the address at which the process is executing. ? Other information associated with process： ? Identity of the user that owns it ? The compiled program that it is executing ? The memory locations of process’ program text and data 哈工大计算机学院李全龙Network Application Development 1: Introduction 165 Terminology and concepts ? A process is different from a program ? The process concept includes only the active execution of a program, not the code ? After the code has been loaded into a computer, the OS allows one or more processes to execute it. ? Each process has a separate instruction pointer that specifies which instruction it will execute next and its own copy of variables. ? On a uniprocessor architecture, concurrent execution means “apparently simultaneous execution”. 哈工大计算机学院李全龙Network Application Development 1: Introduction 166 Important concept ? Application programmers build programs for a concurrent environment without knowing whether the underlying hardware consists of a uniprocessor or a multiprocessor. 哈工大计算机学院李全龙Network Application Development 1: Introduction 167 Terminology and concepts ? Threads ? Some OS, such as Win98/2000/XP, provide a second form of concurrent execution known as threads of execution. ? A thread has its own instruction pointer and copy of local variables, and it executes independently of other threads. 哈工大计算机学院李全龙Network Application Development 1: Introduction 168 Terminology and concepts ? A thread mechanism differs from a process mechanism ? Each thread must be associated with a single process. ? Although each thread in process has its own copy of local variables, all threads in a process share access to a single copy of global variables. ? All threads in a process share resources that the OS allocates to the process, including the descriptors used for network communication. ? A concurrent program can be written either to create separate processes or to create multiple threads in a single process. ? One of the potential disadvantages of a multithreaded design is interference. 29 哈工大计算机学院李全龙Network Application Development 1: Introduction 169 Terminology and concepts ? Programs vs. Threads ? A conventional application program is merely a special case ? It consists of a piece of code that is executed by exactly one thread at a time. ? When multiple threads execute a piece of code concurrently ? Each thread has its own, independent copy of the local variables associated with the code. 哈工大计算机学院李全龙Network Application Development 1: Introduction 170 Terminology and concepts ? Procedure calls ? If multiple threads execute code concurrently, they can each be at a different point in the sequence of procedure calls. ? The run-time system for procedure-oriented programming languages uses a stack mechanism to handle procedure calls. ? The run-time system pushes a procedure activation record on the stack whenever it makes a procedure call. ? When multiple threads execute a piece of code concurrently, each has its own run-time stack of procedure activation records. 哈工大计算机学院李全龙Network Application Development 1: Introduction 171 An example of concurrent thread creation ? A sequential C example #include <stdlib.h> #include <stdio.h> #include <process.h> int addem(int); int main(int argc, char *argv[]) { addem(5); return 0;} int addem(int count) {ini, sum; sum = 0; for (i=0; i<=count; ++i) { printf("The value of i is %d\n", i); fflush(stdout); sum += i; } printf("The sum is %d\n", sum); fflush(stdout); return 0; 哈工大计算机学院李全龙Network Application Development 1: Introduction 172 An example of concurrent thread creation ? Output of sequential C example The value of i is 0 The value of i is 1 The value of i is 3 The value of i is 4 The value of i is 5 The sum is 15 哈工大计算机学院李全龙Network Application Development 1: Introduction 173 An example of concurrent thread creation ? A concurrent version ? In Windows, a new thread can be created by calling _beginthread #include <stdlib.h> #include <stdio.h> #include <process.h> int addem(int); int main(int argc, char *argv[]) { _beginthread((void (*)(void *))addem, 0, (void *)5); addem(5); return 0;} int addem(int count) {ini, sum; sum = 0; for (i=0; i<=count; ++i) { printf("The value of i is %d\n", i); fflush(stdout); sum += i;} printf("The sum is %d\n", sum); fflush(stdout); return 0;} 哈工大计算机学院李全龙Network Application Development 1: Introduction 174 An example of concurrent thread creation ? Output of concurrent version The value of i is 0 The value of i is 1 The value of i is 2 The value of i is 0 The value of i is 1 The value of i is 3 The value of i is 4 The value of i is 2 The value of i is 3 The value of i is 4 The value of i is 5 The sum is 15 The value of i is 5 The sum is 15 30 哈工大计算机学院李全龙Network Application Development 1: Introduction 175 Timeslicing ? A timeslicing mechanism attempts to allocate the available processing equally among all available threads. ? Think about next example, and imagine what kind of output it will create. 哈工大计算机学院李全龙Network Application Development 1: Introduction 176 Timeslicing #include <stdlib.h> #include <stdio.h> #include <process.h> int addem(int); int main(int argc, char *argv[]) { _beginthread((void (*)(void *))addem, 0, (void *)10000); addem(10000); return 0;} int addem(int count) {i i, sum; sum = 0; for (i=0; i<=count; ++i) { printf("The value of i is %d\n", i); fflush(stdout); sum += i; } printf("The sum is %d\n", sum); fflush(stdout); return 0; 哈工大计算机学院李全龙Network Application Development 1: Introduction 177 Diverging threads ? In practice, multiple threads in a concurrent program seldom execute a single procedure. ? Usually, each thread executes its own procedure. ? How can thread diverge? ? The first argument to _beginthread specifies a procedure for newly created thread to execute. ? The parent continues execution at the point following the call to _beginthread. ? For example: 哈工大计算机学院李全龙Network Application Development 1: Introduction 178 Diverging threads _beginthread((void (*)(void *))addem, 0, (void *)5); proc2( ); ? The parent thread first calls _beginthread and then calls proc2. ? The child thread begins executing addem with an argument of 5. ? The child thread is created to call addem(5); ? If the call to addem returns, the thread ceases execution. 哈工大计算机学院李全龙Network Application Development 1: Introduction 179 Context switching and protocol software design ? When the OS temporarily stops executing one thread and switches to another, a context switch has occurred. ? Switching context requires use of CPU ? We view context switching as the overhead needed to support concurrent processing. ? To avoid unnecessary overhead, protocol software should be designed to minimize context switching. 哈工大计算机学院李全龙Network Application Development 1: Introduction 180 Concurrency and asynchronous I/O ? In addition to providing support for concurrent use of CPU, some OS allow a single application program to initiate and control concurrent input and output operations. ? In Windows, the select socket function provides a fundamental operation that programmers use to manage concurrent I/O. ? Select function allows a program to ask the OS which source of I/O is ready for use. 31 哈工大计算机学院李全龙Network Application Development 1: Introduction 181 Concurrency under UNIX ? UNIX offers a process abstraction. ? To become concurrent, a UNIX application calls the system function fork. ? fork does not take any arguments. ? fork create another process executing at exactly the same location as the original. ? The value returned by the UNIX fork function differs in the original and newly created processes. ? Concurrent programs use the difference to allow the new process to execute different code than the original process. 哈工大计算机学院李全龙Network Application Development 1: Introduction 182 Executing a separately compiled program ? Both Windows and UNIX provide a mechanism that allows any process to stop executing one application and begin executing an independent program that has been compiled separately and stored on disk. ? Using a family of OS functions named exec. ? Under UNIX, both fork and exec are used to create a new process to run another program. ? Under windows, a single function, CreateProcess, handles both tasks. ? Servers that handle many services can use exec to simplify the server program. 哈工大计算机学院李全龙Network Application Development 1: Introduction 183 Summary Learned: ? Network basics ? TCP/IP protocols review ? Client-server application model and software design ? Concurrent processing in client-server model