STREAMS's design is a modular architecture for implementing full-duplex I/O between kernel or user space processes and between device drivers. Its most frequent uses have been in developing terminal I/O (line discipline) and networking subsystems. In System V Release 4, the entire terminal interface was reimplemented using STREAMS. An important concept in STREAMS is the ability to push drivers — custom code modules which can modify the functionality of a network interface or other device — together to form a stack. Several of these drivers can be chained together in order.
STREAMS was based on the Streams I/O subsystem introduced in the Eighth Edition Research Unix (V8) by Dennis Ritchie, where it was used for the terminal I/O subsystem and the Internet protocol suite. This version, not yet called STREAMS in capitals, fit the new functionality under the existing device I/O system calls (open, close, read, write, and ioctl), and its application was limited to terminal I/O and protocols providing pipe-like I/O semantics.
This I/O system was ported to System V Release 3 by Robert Israel, Gil McGrath, Dave Olander, Her-Daw Che, and Maury Bach as part of a wider framework intended to support a variety of transport protocols, including TCP, ISO Class 4 transport, SNA LU 6.2, and the AT&T NPACK protocol (used in RFS). It was first released with the Network Support Utilities (NSU) package of UNIX System V Release 3. This port added the putmsg, getmsg, and poll system calls, which are nearly equivalent in purpose to the send, recv, and select calls from Berkeley sockets. The putmsg and getmsg system calls were originally called send and recv, but were renamed to avoid namespace conflict. In System V Release 4, STREAMS was extended and used for the terminal I/O framework and pipes, providing useful new functionality like bidirectional pipes and file descriptor passing. A port for Unicos was also produced. Eric S. Raymond quotes Ritchie as saying about the complexity of System V STREAMS when compared to his V8 Streams that "Streams means something different when shouted".
Concurrent with the System V Release 3 port, AT&T developed protocol-independent STREAMS message passing guidelines for the link, network, and transport layers of the OSI model (layers 2-4). Due to the typically close implementation coupling of the network and transport protocols in a given protocol stack, and the typical practice of implementing layers 5-7 outside of the kernel, only the link and transport layer STREAMS service interfaces were later standardized by X/Open. In conjunction with the transport message passing model, the Transport Layer Interface (later adopted as the X/Open Transport Interface) was defined to provide a transport protocol-independent API for application development. Also, a library supporting the session, presentation and application layers was defined and later standardized by The Open Group.
STREAMS was required for conformance with the Single UNIX Specification versions 1 (UNIX 95) and 2 (UNIX 98), but as a result of the refusal of the BSD and Linux developers to provide STREAMS, was marked as optional for POSIX compliance by the Austin Group in version 3 (UNIX 03).
STREAMS has mostly been used in the System V Unix world; however, other implementations exist:
- Plan 9 originally used a multi-processor variant of Research Unix's Streams. During the transition to the third edition of Plan 9, Streams were further simplified to simple I/O queues.
- An implementation written at Mentat was used in Novell NetWare for its TCP/IP stack, and licensed by Apple for use in Mac OS starting in version 7.5.2, as part of the Open Transport networking system. (In OS X, the Classic Environment used the STREAMS architecture, but the native networking architecture uses the Berkeley sockets API and is derived from the BSD networking code.)
- FreeBSD has basic support for STREAMS-related system calls, as required by SVR4 binary compatibility layer.
- The Windows NT kernel offered a full port of STREAMS as the streams.sys binary. NT DDK even had a chapter on STREAMS, going as late as NT4 though in NT4 DDK it was declared obsolete. The original TCP/IP stack for Windows NT 3.1 was implemented atop STREAMS by Spider Systems, and used the streams.sys binary. From NT 3.5 up, TCP/IP was remade completely, by adopting the one from MS LAN Manager for OS/2 1.x.
Linux does not include STREAMS functionality without third-party add-ons. Caldera had "pushed" for STREAMS to be included in Linux ca. 1998, to support its Netware for Linux, but it was rejected outright by the Linux kernel developers on technical grounds (mainly performance). The compatibility layers in Linux for other operating systems convert STREAMS operations into sockets as early as possible. The implementation used by Caldera was "LiS", by a company called GCOM; it later figured in the legal battles by Caldera's successor, the SCO Group, against Linux, with SCO claiming that Linux with STREAMS infringed what it believed to be its copyrights to System V.
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