Topic 2: Interposes Communication Dr. Ayman Srour Faculty of Applied Engineering and Urban Planning University of Palestine

2.1 Overview of Network Principles The networks in the distributed systems are built from a variety of transmission media, hardware devices and software components transmission media: including wire, cable, fibre and wireless channel. hardware devices: including routers, switches, bridges, hubs, etc. software components: including protocol stacks, communication handlers and drivers. The characteristics of such networks impact the performance and behaviour of a distributed system.

2.1 Overview of Network Principles communication subsystem: is a collection of hardware and software components that provide the communication facilities for a distributed system. Hosts: the computers and other devices that use the network for communicate purposes. Node: any computer or switching device attached to a network. E.g., the Internet is a single communication subsystem providing communication between all of the hosts that are connected to it. We need to have some understanding of such principles in order to evaluate the effect of network characteristics on the distributed system’s performance, scalability, reliability, security and mobility.

2.1 Overview of Network Principles Performance: concern with the factors that affecting the speed messages transition between two interconnected computers, including latency and Data transfer rate. Scalability: The potential future size of the Internet is commensurate with the population of the planet. It is realistic to expect it to include several billion nodes and hundreds of millions of active hosts. Reliability: the ability to detect and recover communication errors. Usually occurs in the application-level software. Security: it is necessary enable distributed applications to move beyond the restrictions imposed by firewalls there is a need to produce a secure network environment in which a wide range of distributed applications can be deployed with end-to-end authentication, privacy and security. Mobility: Mobile devices such as laptop computers and Internet-capable mobile phones are moved frequently between locations and reconnected at convenient network connection points or even used while on the move.

2.1 Overview of Network Principles  Types of Networks: o Local Area Networks (LANs): LANs support the exchange of messages at relatively high speeds between computers connected by a single communication medium. A segment is a section of a cable that can connect many computers in,e.g., a building. No routing is required in a segment. Larger LANs can be constructed by connecting more that one segment using a hub. LANs can be implemented using the Ethernet, Token Rings.

2.1 Overview of Network Principles  Types of Networks: o Metropolitan Area Networks (MANs): Based on the high bandwidth copper and fibre optic cables which are installed across towns and cities. Can be implemented in many technologies ranging from the Ethernet to ATM. o Wireless Networks: Supports mobile terminals. Examples include: WLANs, WPANs, GSM.

2.1 Layered Protocols  Protocols: Communications in distributed systems is based on message exchange. To resolve the numerous issues & levels involved in communication, the Open Systems Interconnection (OSI) model is developed. The OSI is designed to allow open systems to interconnect & communicate. An open system is one that is prepared to communicate with any other open systems by using standard rules that govern the format, contents, and meanings of messages sent or received. These rules are called protocols.

2.1 Layered Protocols  Protocols: protocol is a set of rules to communicate between processes to perform a task. A protocol contains a specification of a set of messages and their contents. Network protocols are arranged into layers, where each layer acts as an interface to the layer above it with the latter extending the network properties Generally there are 2 types of protocols Connection-oriented: in which a “virtual connection” is set up between a sending and receiving process and is used for the transmission of a stream of data. Sequencing and acknowledgement are included. Connectionless : which individual messages, known as datagrams, are transmitted to specified destinations. No sequencing or acknowledgement The OSI model divides communication into 7 layers.

2.1 Layered Protocols

 Protocols: Application layer: Protocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service. (HTTP, FTP, Telnet, SMTP, CORBA IIOP). Presentation layer: Protocols at this level transmit data in a network representation that is independent of the representations used in individual computers. Encryption is performed in this layer if needed. (XDR, CORBA Data Representation, encryption). Session layer: At this level communication between processes is established and error recovery is performed. It is not required for connectionless communication. Transport layer: This is the lowest level at which messages (rather than packets) are handled. Messages are addressed to communication ports. This layer also constructs an end-to-end reliable connection- oriented channel between two parties. Its tasks include flow control, error detection and correction and sequencing (TCP).

2.1 Layered Protocols  Protocols: Network layer: Transfers data packets between computers in a specific network. Also concerned with the use of networking to create channels needed for communication. Routing is delt with in this layer (IP). Data link layer: responsible for error-free transmission of packets between nodes that are directly linked by a physical connection, i.e. between pairs of routers. Physical layer: The circuits and hardware that drives the network (Bit transmission using proper communication medium.

2.1 Layered Protocols  Protocols: The Internet protocol suite (I.e., a complete set of protocol layers) provides two types of transport-layer service: TCP is a connection-oriented service that transmits streams of bytes across a pre-established connection UDP is a connectionless service that transmits messages of up to 64 kilobytes to a specified destination (a communication identifier which is IP address + port number).

2.1 Layered Protocols  Routing: Routing in a network is required when there is no direct link between nodes. Routers at connection points of such network cooperate to deliver packets from source to destination. Packets are transmitted through a number of hops between routers. A routing algorithm is used in each of the routers and implemented as a program in the network layer. A routing algorithm make decisions regarding the path that each packet has to go through in the network.

2.1 Layered Protocols  Internet Protocols: IP is a network protocol providing basic transmission mechanism for the Internet. is usually based on another network protocol specific to subnet (e.g. the Ethernet). Transmits datagrams between hosts via routers if necessary. The IP protocol provides an unreliable (best-offer) service as there is no guarantee of delivery. The only error-detection mechanism used in IP is a header checksum which detect whether the header was corrupted during transmission. The IP protocol implements part of the Network layer. IP datagrams are then put in packets suitable for transmission in the underlying network (e.g. Ethernet). Routing is the responsibility of the IP protocol from source to destination.

2.1 Layered Protocols  TCP/IP Protocol Layers

2.1 Layered Protocols  TCP Protocol: A transport layer protocol that support process-to-process communication. Ports are used to address messages to specific processes in communicating hosts. TCP provides a connection-oriented reliable service where a guarantee is provided to deliver messages in the same order. Communicating processes, at the start of communication agree on establishing an end-to-end reliable connection. The network, e.g. the routers have no knowledge of such connection agreement. IP packets carrying the TCP part don’t necessarily follow the same route in the network.

2.1 Layered Protocols  TCP Protocol: Several reliability mechanisms are supported in TCP including: o Sequencing: divide the data to be communicated into a sequence of data segments and transmit them as IP packets. The receiver uses the sequence number to order the received segments to re-construct the data stream which is then presented to the receiving process. o Flow Control: Aims to control the rate at which segments (packets) are communicated. A segment acknowledgment mechanism is used. From time to time, the receiver informs the transmitter of the highest sequence number received together with a window size. o Retransmission: If any segment is not acknowledged within a specified timeout, the sender retransmit that segment. o Buffering: The incoming buffer at the receiver is used to balance the difference in flow between the sender and receiver. o Checksum: Each segment includes a checksum covering the header and the data. If a received segment does not match its checksum, retransmission is performed. UDP is another transport layer protocol that does not provide the reliability mechanisms reviewed above (except checksums).

2.2 Interprocess Communication  Introduction: TCP (and UDP) protocols provide a mean for processes at different hosts to communicate through the exchange of We have discussed the TCP protocol (and UDP) without describing how a distributed application can use such protocols. The Java programming language provides an API to implement these protocols. Before considering the programming aspects, we need to describe the characteristics of interprocess communication.

2.2 Interprocess Communication  Introduction:

2.2 Interprocess Communication  Interprocess Communication Characteristics: Message passing between two processes can be supported through two operations send and receive: o One process makes a send operation to send a message. o The destination process must support a receive operation to receive the message. Characteristics include: o Synchronous VS. asynchronous. o Addressing/Message destinations. o Reliability. o Ordering.

2.2 Interprocess Communication  Interprocess Communication Characteristics: Synchronous and asynchronous communication: o A queue is associated with each message destination. o Sending processes cause messages to be added to remote queues and receiving processes remove messages from local queues. o synchronous communication: the sending and receiving processes synchronize at every message. Whenever a send is issued the sending process (or thread) is blocked until the corresponding receive is issued. Whenever a receive is issued by a process (or thread), it blocks until a message arrives. o asynchronous communication: the use of the send operation is non-blocking in that the sending process is allowed to proceed as soon as the message has been copied to a local buffer, and the transmission of the message proceeds in parallel with the sending process. The receive operation can have blocking and non-blocking variants.

2.2 Interprocess Communication  Interprocess Communication Characteristics: Addressing/Message destinations: o Messages are sent to (Internet address, local port) pairs. o A local port is a message destination within a computer, specified as an integer. A port has exactly one receiver but can have many senders. o Any process that knows the number of a port can send a message to it. o Servers generally publicize their port numbers for use by clients.

2.2 Interprocess Communication  Interprocess Communication Characteristics: Reliability: o Defines reliable communication in terms of validity and integrity o a point-to-point message service can be described as reliable if messages are guaranteed to be delivered despite a ‘reasonable’ number of packets being dropped or lost. o In contrast, a point-to-point message service can be described as unreliable if messages are not guaranteed to be delivered in the face of even a single packet dropped or lost. Ordering : o Some applications require that messages be delivered in sender order -that is, the order in which they were transmitted by the sender o The delivery of messages out of sender order is regarded as a failure by such applications.

2.2 Interprocess Communication  sockets: Sockets represent the endpoints of communication in TCP or UDP. For an intercommunication to occur between two processes, a message is transmitted between the socket of one process to the socket of the other process (as illustrated in the figure on the next slide). A computer has a large number of possible port numbers which can be used by local processes to receive messages. The socket of the receiving process must be bound to a local port and an IP address of the computer on which the process is running. A process can use multiple ports to receive messages but different processes cannot share one port. A process can use the same socket for sending and receiving. Each socket can only provide support for one protocol (TCP or UDP).

2.2 Interprocess Communication

 sockets: Java API for Internet addresses: Java provides a class, InetAddress, that represents Internet addresses. By using this class user can refer to computers by DNS hostname. For example, to get an object representing the Internet address of the host whose DNS name is bruno.dcs.qmul.ac.uk, use: This method can throw an UnknownHostException.

2.2 Interprocess Communication  UDP datagram communication: A datagram sent by UDP is transmitted from a sending process to a receiving process without acknowledgement or retries. To send or receive messages a process must first create a socket bound to an Internet address of the local host and a local port. A server will bind its socket to a server port – one that it makes known to clients so that they can send messages to it. A client binds its socket to any free local port. The receive method returns the Internet address and port of the sender, allowing the recipient to send a reply.

2.2 Interprocess Communication  UDP datagram Communication: Message size: The receiving process needs to specify an array of bytes of a particular size in which to receive a message. Blocking: Sockets normally provide non-blocking sends and blocking receives for datagram communication (a non-blocking receive is an option in some implementations). Timeouts: in case of receive blocks, it is not appropriate that a process that has invoked a receive operation should wait indefinitely in situations where the sending process may have crashed or the expected message may have been lost. Receive from any: the receive method does not specify an origin for messages. Instead, an invocation of receive gets a message addressed to its socket from any origin.

2.2 Interprocess Communication  UDP Interprocess Communication Implementation : The Java API for UDP datagram communication is provided in the classes DatagramPacket and DatagramSocket. included in the java.net package. DatagramPacket class: o This class provides a constructor that makes an instance out of an array of bytes comprising a message, the length of the message and the Internet address and local port number of the destination socket. o This class provides another constructor for use when receiving a message. o A received message is put in the DatagramPacket together with its length and the Internet address and port of the sending socket. o Include methods like: getData, getPort and getAddress;.

2.2 Interprocess Communication  UDP Interprocess Communication Implementation : DatagramSocket class: o This class supports sockets for sending and receiving UDP datagrams. It provides a constructor that takes a port number as its argument, for use by processes that need to use a particular port. o It also provides a no-argument constructor that allows the system to choose a free local port. o send and receive methods: These methods are for transmitting datagrams between a pairof sockets. The argument of send is an instance of DatagramPacket containing a message and its destination. The argument of receive is an empty DatagramPacket in which to put the message, its length and its origin. o setSoTimeout method : This method allows a timeout to be set. With a timeout set, the receive method will block for the time specified and then throw an InterruptedIOException. o Connect method: This method is used for connecting to a particular remote port and Internet address, in which case the socket is only able to send messages to and receive messages from that address.

2.2 Interprocess Communication

 TCP Interprocess Communication Implementation : The Java API for TCP stream implementation is provided in the classes ServerSockets and Socket. included in the java.net package. ServerSocket class: o Used by the server to create a socket and associate it with a port. o The socket listens for connect requests from clients. o The class’s accept method get a request from the queue, or blocks if the queue is empty. o The result of executing accept is an instance of Socket which provides access to streams for communication with client Socket class: o Its constructor is used by the client to create a socket specifying DNS hostname and port number of the server. The newly created socket is connected with the remote computer. o UnknownHostException is thrown by the constructor if the hostname is not recognised. o IOException is raised if an IO error occurs.

2.2 Interprocess Communication