(taken from Wikipedia)
4.3BSD was released in June 1986. Its main changes were to improve the performance of many of the new contributions of 4.2BSD that had not been as heavily tuned as the 4.1BSD code. Prior to the release, BSD's implementation of TCP/IP had diverged considerably from BBN's official implementation. After several months of testing, DARPA determined that the 4.2BSD version was superior and would remain in 4.3BSD.
The files here come from the CSRG Archives, and were donated by Kirk McKusick.
For more information about 4.3BSD, see Twenty Years of Berkeley Unix by Kirk McKusick.