Middleware Technologies [MCA II Yr, Anna University] by Roy Antony Arnold

• Most early client/server applications were implemented using low-level, conversational, peer-to-peer protocols – such as sockets, TLI, CPIC/APPC, NetBIOS and Named Pipes.
• These low-level protocols are hard to code and maintain.
• Instead now, the programmers are using RPCs, MOMs, and ORBs, which provide high level abstraction.
• The term, “peer-to-peer” indicates that the two sides of a communication link use the same protocol interface to conduct a networked conversation.
• The protocol is symmetrical, and it is sometimes called “program-to-program”.
• The peer-to-peer interface not fully mask the underlying network from the programmer.
• Programmer have to handle the transmission timeouts, race conditions, and other network errors.
• The peer-to-peer protocols started as stack-specific APIs.

a) Sockets

• Sockets were introduced in 1981 as the UNIX BSD 4.2 generic interface that would provide Unix-to-Unix communications over network.
• In 1985, SUN OS introduced NFS and RPC over sockets.
• Sockets are supported on virtually every OS.
• The windows socket API, known as WinSock, is a multivendor specification that standardizes the use of TCP/IP under windows.
• In BSD Unix System, sockets are part of the Kernel and provide both a standalone and networked IPC service.

Socket = Net_ID . Host_ID . Port_ID = IP Address + Port Address.

• The three most popular socket types are
  • Stream
  • Datagram
  • Raw
• Stream and datagram sockets interface to the TCP and UDP protocols, and raw sockets interface to the IP protocol.
• A port is an entry point to an application that resides on the host.
• It is represented by a 16-bit integer.
• Ports are commonly used to define the entry points for services provided by the server applications.

b) TLI

• In 1986, AT&T introduced the Transport Layer Interface that provides functionality similar to sockets but in a more network-independent fashion.
• Sockets and TLI are very similar from a programmer’s perspective.
• TLI is just cleaner version of the sockets.
• It should run on IPX/SPX (or) TCP/IP with very few modifications.
• The TLI API consists of 25 API calls.
• Later standardized as XTI, X/Open Transport Interface.

c) NetBIOS:

• It is the premier protocol for LAN-based, program-to-program communications.
• Introduced by IBM and Sytek in 1984 for the IBM PC network.
• It is used as an interface to a variety of stacks – including NetBEUI, TCP/IP, XNS, Vines, OSI and IPX/SPX.
• The NetBIOS services are provided through a set of commands, specified in a structure called the Network Control Block (NCB)
• It does not support the routing of messages to other networks.

d) Named Pipes:

• Provide highly reliable, two-way communications between clients and a server.
• They provide a file-like programming API that abstracts a session-based two-way exchange of data.
• Using named pipes, processes can exchange data as if they were writing to, or reading from, a sequential file.
• These are suitable for implementing server programs that require many-to-one pipelines.
• Important benefit of named pipes are part of the base interprocess communications service.
• Named pipes interface is identical, whether the processes are running on an individual machine or distributed across the network.
• Named pipes run on NetBIOS, IPX/SPX, and TCP/IP stacks.
• Named pipes are built-in networking features in Windows NT, Windows for Workgroups, Windows 95 and Warp Server.
• Unix support for Named Pipes is provided by LAN Manager/X.

e) CPI-C/APPC:

• Common Programming Interface for Communications (CPI-C) build on top of APPC and marks its complexities and irregularities.
• Writing to the CPI-C API allows you to port your programs to all SNA platforms.
• The CPI-C API consists of about 40 calls; APPC consists of over 60 calls.
• Most of these calls deals with configuration and services.
• Advanced program-to-program communication is a protocol which computer programs can use to communicate over a network.
• APPC was developed as a component of IBM’s Systems Network Architecture (SNA).
• APPC is linked with the term LU6.2.
• LU6.2. (Logic Unit Version 6.2) is a device independent SNA Protocol.
• It was developed to allow computers in IBM environments to setup their own communications sessions, rather than rely on a hos computer to do so.
• Contrary to TCP/IP, in which both communication partners always possess a clear role, the communication partners in APPC are equal, i.e., everyone can be both servers and clients equally.
• With the wide success of TCP/IP, APPC has declined.