The first IPv6 related network code was added to the Linux kernel 2.1.8 in November 1996 by Pedro Roque. It was based on the BSD API:

diff -u --recursive --new-file v2.1.7/linux/include/linux/in6.h
¬ linux/include/linux/in6.h 
--- v2.1.7/linux/include/linux/in6.h Thu Jan 1 02:00:00 1970 
+++ linux/include/linux/in6.h Sun Nov 3 11:04:42 1996 
@@ -0,0 +1,99 @@ 
+/* 
+ * Types and definitions for AF_INET6 
+ * Linux INET6 implementation 
+ * + * Authors: 
+ * Pedro Roque <******> 
+ * 
+ * Source: 
+ * IPv6 Program Interfaces for BSD Systems 
+ * <draft-ietf-ipngwg-bsd-api-05.txt>

The shown lines were copied from patch-2.1.8 (e-mail address was blanked on copy&paste).

Because of lack of manpower, the IPv6 implementation in the kernel was unable to follow the discussed drafts or newly released RFCs. In October 2000, a project was started in Japan, called USAGI, whose aim was to implement all missing, or outdated IPv6 support in Linux. It tracks the current IPv6 implementation in FreeBSD made by the KAME project. From time to time they create snapshots against current vanilla Linux kernel sources.

Until kernel development series 2.5.x was started, the USAGI patch was so big, that Linux networking maintainers were unable to include it completly in the production source of the Linux kernel 2.4.x series.

During kernel development series 2.5.x, USAGI tried to insert all of their current extensions into this.

Many of the long-term developed IPv6 related patches by USAGI and others are integrated into vanilla kernel series 2.6.x.

USAGI and others are still working on implementation of newer features like mobililty and others. From time to time, new extension patches are released and also integration into vanilla kernel series is made.