Linux IPv6: Which One to Deploy?

If you are hesitant as to which IPv6 implementation to adopt for your Linux server, this article will help you decide.
The Linux Kernel and IPv6

The Linux kernel has its own IPv6 implementation. However, as mentioned previously, based on the TAHI Project results, this implementation proved to be not as good as other implementations. However, this is not a big surprise given that no major development activity has happened for a while. Another factor is that the original writer of the Linux kernel IPv6 code, Pedro Roque, has left the community, and since then, Alexey Kuznetsov and others have made quite a few enhancements over the years; however, this is not a major development. The USAGI Project has submitted some small fixes, but the question of a complete integration with the kernel stack is still an open issue.

Conformance Tests

At this point, we already have two IPv6 test networks in our lab. One network had Linux nodes with the USAGI IPv6 stack and the other had Linux nodes with the kernel IPv6 stack. Much work has been put to perform the setup and solve routing and tunneling issues. However, the question was still which implementation to adopt.

To be able to answer this question objectively, Ericsson Research (Budapest) has performed conformance tests for the latest version of the official Linux kernel (at that time kernel 2.4.5) and the USAGI IPv6 implementation (based on 2.4.0). The tests were based on the University of New Hampshire InterOperability Lab IPv6 Test Description document (see Resources).

The result of each test case can be:

  • Pass: the implementation passes the test.

  • Fail: the implementation fails the test.

  • Inc: the verdict is inconclusive if we cannot decide whether the implementation is capable of passing the test. For example, when the test consists of three request/reply sequences and we do not get an answer for the tester's second request, then the verdict is inconclusive.

    The Conformance Lab conducted four types of testing: basic specification, address autoconfiguration, redirect and neighbor discovery. Below, we explain these tests and present the results.

  • Basic specification: this series of tests covers the base specification for IPv6. The base specification specifies the basic IPv6 header and the initially defined IPv6 extension headers and options. It also discusses packet-size issues, the semantics of flow labels and traffic classes and the effects of IPv6 on upper-layer protocols (see Figure 1).

  • Figure 1. Specification Test Results

  • Address autoconfiguration: these tests cover address autoconfiguration for IPv6. They are designed to verify conformance with the IPv6 stateless address autoconfiguration specification (see Figure 2).

  • Figure 2. Address Autoconfiguration Test Results

  • Redirect: the redirect tests cover the redirect function of the neighbor discovery specification for IPv6. Redirect messages are sent by routers to redirect a host to a better first-hop router for a specific destination or to inform hosts that a destination is in fact a neighbor, i.e., on-link (see Figure 3).

  • Figure 3. Redirect Test Results

  • Neighbor discovery: these tests cover the neighbor discovery specification for IPv6. The neighbor discovery protocol is used by nodes (hosts and routers) to determine the link layer address for neighbors known to reside on attached links as well as to purge cached values that become invalid quickly. Hosts also use neighbor discovery to find neighboring routers that are willing to forward packets on their behalf. Finally, nodes use the protocol actively to keep track of neighbors that are reachable and those that are not. When a router or the path to a router fails, a host actively searches for functioning alternates (see Figures 4 and 5).

  • Figure 4. Neighbor Discovery Test Results

    Figure 5. Test Result Summary

Based on these results, we can see that the USAGI implementation had better results than the Linux kernel implementation; it passed more tests, failed fewer tests and had less inconclusive cases than the Linux kernel implementation.