2.13. Network Implementation
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## 2.13. Network Implementation

The first protocol suite implemented in 4.2BSD was DARPA's Transmission Control Protocol/Internet Protocol (TCP/IP). The CSRG chose TCP/IP as the first network to incorporate into the socket IPC framework, because a 4.1BSD-based implementation was publicly available from a DARPA-sponsored project at Bolt, Beranek, and Newman (BBN). That was an influential choice: The 4.2BSD implementation is the main reason for the extremely widespread use of this protocol suite. Later performance and capability improvements to the TCP/IP implementation have also been widely adopted. The TCP/IP implementation is described in detail in Chapter 13.

The release of 4.3BSD added the Xerox Network Systems (XNS) protocol suite, partly building on work done at the University of Maryland and at Cornell University. This suite was needed to connect isolated machines that could not communicate using TCP/IP.

The release of 4.4BSD added the ISO protocol suite because of the latter's increasing visibility both within and outside the United States. Because of the somewhat different semantics defined for the ISO protocols, some minor changes were required in the socket interface to accommodate these semantics. The changes were made such that they were invisible to clients of other existing protocols. The ISO protocols also required extensive addition to the two-level routing tables provided by the kernel in 4.3BSD. The greatly expanded routing capabilities of 4.4BSD include arbitrary levels of routing with variable-length addresses and network masks.

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