Network - Middleware PDF

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Network - Middleware Middleware alters the OSI model by replacing session and presentation layers with middleware protocols. In terms of the TCP/IP model, middleware protocols are placed between the application and transport layers. It is a good place to augment TCP/IP with dependability features, e.g., security and fault tolerance. Middleware protocols abstract away from the difficulties of using raw UDP and TCP, and allow high-level IPC to occur. TCP is connection-based, so a connection needs to be established and closed down before messages can be sent. Messages are sent using the send and receive primitives, and can be used synchronously or asynchronously. Destinations of messages are specified as a tuple of IP address and port as dictated by TCP and UDP. Sockets A common implementation of this kind of middleware is using the "socket" abstraction. On UNIX-based systems, two type of sockets exist, ones in the UNIX domain, for local sockets tied to the VFS and ones in the INET domain, for processes on different machines. Different types of sockets also exist: SOCK_STREAM creates a TCP-based socket, SOCK_DGRAM for a UDP based socket and SOCK_RAW to handle other socket types (bypasses the TCP/UDP stage and uses the IP level directly). Common socket operations include:  socket(domain, type, procedure) - create a socket  bind(s, port) - associate the socket s with a specified port  listen(port, backlog) - server listens for connection requests on a port  connect(s, serverport) - client connects to a server using socket s  accept(s, clientport) - server accepts connection  sendto(s, data, clientport) - client/server sends data to a port  receive(ip, port) - client/server receives data from a port The sockets abstraction is extensively used and realises many application level services, such as FTP, Telnet, HTTP, etc...

Most client-server communication is synchronous (the client blocks until the server replies), and acknowledgements are redundant, however asynchronous communication allows clients to continue without waiting (e.g., when no reply is expected from the server, or the client accepts the reply when it arrives). Typically, request-reply type communication is mapped to TCP, as UDP may get omissions, lost replies, duplicates, unordered messages, etc... Using UDP/TCP in this way means distribution is apparent to the application, but one of the goals of distributed systems is transparency. Middleware provides a transparent layer between the application and transport protocols....