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Distributed Computing - API for Internet Protocols
- 1. API for Internet Protocols Unit - II
- 2. API for Internet Protocols • Characteristics of Interprocess Communication • Sockets • UDP Datagram Communication • TCP Stream Communication
- 3. Characteristics of Interprocess Communication • Message passing between a pair of processes can be supported by two message communication operations, send and receive, defined in terms of destinations and messages. • To communicate, one process sends a message (a sequence of bytes) to a destination and another process at the destination receives the message. • It involves synchronization of two processes • Characteristics • Synchronous and Asynchronous communication • Message Destination • Reliability • Ordering
- 4. Characteristics of Interprocess Communication • Synchronous and Asynchronous Communication • A queue is associated with each message destination. • Sending processes cause messages to be added to remote queues and receiving processes remove messages from local queues. • Communication between the sending and receiving processes may be either synchronous or asynchronous. • In the synchronous form of communication • the sending and receiving processes synchronize at every message. • Both send and receive process are blocking operations. • 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.
- 5. Characteristics of Interprocess Communication • In Asynchronous form of communication • the send operation is nonblocking • 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. • In the non-blocking variant, the receiving process proceeds with its program after issuing a receive operation, which provides a buffer to be filled in the background, but it must separately receive notification that its buffer has been filled, by polling or interrupt.
- 6. Characteristics of Interprocess Communication • Message Destinations • In Internet protocols, messages are sent to (Internet address, local port) pairs. • 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. • Processes may use multiple ports to receive messages. • Reliability • Reliable communication in terms of validity and integrity • Validity – point to point message service • Integrity – messages must arrive uncorrupted and without duplication • Ordering • Applications require that messages delivered in sender order
- 7. Sockets • Both TCP and UDP Communication uses Socket Abstraction • Provide communication endpoint between processes • For a process to receive messages, its socket must be bound to a local port and one of the Internet addresses of the computer on which it runs. • Messages sent to a particular Internet address and port number can be received only by a process whose socket is associated with that Internet address and port number.
- 8. Sockets • Java API for Internet Address • Java provides a class, InetAddress, that represents Internet addresses. • Users of this class refer to computers by Domain Name System (DNS) hostnames. InetAddress aComputer = InetAddress.getByName("bruno.dcs.qmul.ac.uk"); • instances of InetAddress that contain Internet addresses can be created by calling a static method of InetAddress, giving a DNS hostname as the argument. • The method uses the DNS to get the corresponding Internet address. • the class encapsulates the details of the representation of Internet addresses. • the interface for this class is not dependent on the number of bytes needed to represent Internet addresses – 4 bytes in IPv4 and 16 bytes in IPv6.
- 9. 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 which 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, in addition to the message, allowing the recipient to send a reply.
- 10. UDP Datagram Communication • Issues related to Datagram Communication • Message Size • If the message is too big on its array, it will be truncated on arrival • Blocking • Sockets provide non-blocking sends and blocking receives datagram communication • The send operation returns when it has handed the message to the underlying UDP and IP protocols, which are responsible for transmitting it to its destination. • On arrival, the message is placed in a queue for the socket that is bound to the destination port. • The message can be collected from the queue by an outstanding or future invocation of receive on that socket. • Messages are discarded at the destination if no process already has a socket bound to the destination port. • Timeouts • Receive form any
- 11. UDP Datagram Communication • Failure Model for UDP Datagram • Omission Failures • Messages may be dropped occasionally, either because of a checksum error or because no buffer space is available at the source or destination. • Ordering • Messages can sometimes be delivered out of sender order. • Use of UDP Datagram • Voice over IP (VOIP) runs over UDP • Sources of overhead • the need to store state information at the source and destination • the transmission of extra messages • latency for the sender.
- 12. UDP Datagram Communication • Java API for UDP Datagram • Communication by means of two classes - DatagramPacket and DatagramSocket • An instance of DatagramPacket may be transmitted between processes when one process sends it and another receives it. • This class provides another constructor for use when receiving a message. • A received message is put in the DatagramPacket together with its length and the Internet address and port of the sending socket. • The message can be retrieved from the DatagramPacket by means of the method getData. • The methods getPort and getAddress access the port and Internet address.
- 13. UDP Datagram Communication • Java API for UDP Datagram - DatagramPacket UDP client sends a message to the server and gets a reply
- 14. UDP Datagram Communication • Java API for UDP Datagram – DatagramSocket • 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. • It also provides a no-argument constructor that allows the system to choose a free local port. • Methods • send and receive • These methods are for transmitting datagrams between a pair of 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. • setSoTimeout • This method allows a timeout to be set. • connect • 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.
- 15. UDP Datagram Communication • Java API for UDP Datagram - DatagramSocket UDP server repeatedly receives a request and sends it back to the client
- 16. TCP Stream Communication • abstraction of a stream of bytes to which data may be written and from which data may be read. • characteristics of the network are hidden by the stream abstraction: • Message sizes • The application can choose how much data it writes to a stream or reads from it. • It may deal in very small or very large sets of data. • Lost messages • The TCP protocol uses an acknowledgement scheme. • Flow control • The TCP protocol attempts to match the speeds of the processes that read from and write to a stream. • Message duplication and ordering • Message identifiers are associated with each IP packet, which enables the recipient to detect and reject duplicates, or to reorder messages that do not arrive in sender order. • Message destinations • A pair of communicating processes establish a connection before they can communicate over a stream. • Once a connection is established, the processes simply read from and write to the stream without needing to use Internet addresses and ports.
- 17. TCP Stream Communication • The client role involves creating a stream socket bound to any port and then making a connect request asking for a connection to a server at its server port. • The server role involves creating a listening socket bound to a server port and waiting for clients to request connections. • The listening socket maintains a queue of incoming connection requests. • In the socket model, when the server accepts a connection, a new stream socket is created for the server to communicate with a client, meanwhile retaining its socket at the server port for listening for connect requests from other clients. • The pair of sockets in the client and server are connected by a pair of streams, one in each direction. • Thus each socket has an input stream and an output stream.
- 18. TCP Stream Communication • Issues related to Stream Communication • Matching of Data items • Two communicating processes need to agree as to the contents of the data transmitted over a stream • Blocking • The data written to a stream is kept in a queue at the destination socket. • When a process attempts to read data from an input channel, it will get data from the queue or it will block until data becomes available. • The process that writes data to a stream may be blocked by the TCP flow-control mechanism. • Threads • When a server accepts a connection, it generally creates a new thread in which to communicate with the new client.
- 19. TCP Stream Communication • Failure Model for TCP Stream Communication • To satisfy the integrity property, • use checksums to detect and reject corrupt packets • sequence numbers to detect and reject duplicate packets • For the sake of the validity property, • use timeouts and retransmissions to deal with lost packets • When a connection is broken, the side effects are • The processes using the connection cannot distinguish between network failure and failure of the process at the other end of the connection. • The communicating processes cannot tell whether the messages they have sent recently have been received or not. • Use of TCP Stream • HTTP - communication between web browsers and web servers • FTP - allows directories on a remote computer to be browsed • Telnet - access by means of a terminal session to a remote computer. • SMTP - to send mail between computers.
- 20. TCP Stream Communication • Java API for TCP Communication • Communication by means of two classes – ServerSocket and Socket • ServerSocket • This class is intended for use by a server to create a socket at a server port for listening for connect requests from clients. • Its accept method gets a connect request from the queue or, if the queue is empty, blocks until one arrives. • The result of executing accept is an instance of Socket – a socket to use for communicating with the client.
- 21. TCP Stream Communication TCP client makes connection to server, sends request and receives reply
- 22. TCP Stream Communication • Java API for TCP Communication • Socket • This class is for use by a pair of processes with a connection. • The client uses a constructor to create a socket, specifying the DNS hostname and port of a server. • This constructor not only creates a socket associated with a local port but also connects it to the specified remote computer and port number. • It can throw an UnknownHostException if the hostname is wrong or an IOException if an IO error occurs. • The Socket class provides the methods getInputStream and getOutputStream for accessing the two streams associated with a socket. • The return types of these methods are InputStream and OutputStream, respectively – abstract classes that define methods for reading and writing bytes.
- 23. TCP Stream Communication TCP server makes a connection for each client and then echoes the client’s request