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It's a common practice to send gratuitous ARPs after moving an IP address to another device to speed up healing of a service. To fulfill service availability constraints, the timing of network peers updating their caches to point to a new location of an IP address can be particularly important. Sometimes neigh_update calls won't touch neither lladdr nor state, for example if an update arrives in locktime interval. The neigh->updated value is tested by the protocol specific neigh code, which in turn will influence whether NEIGH_UPDATE_F_OVERRIDE gets set in the call to neigh_update() or not. As a result, we may effectively ignore the update request, bailing out of touching the neigh entry, except that we still bump its timestamps inside neigh_update. This may be a problem for updates arriving in quick succession. For example, consider the following scenario: A service is moved to another device with its IP address. The new device sends three gratuitous ARP requests into the network with ~1 seconds interval between them. Just before the first request arrives to one of network peer nodes, its neigh entry for the IP address transitions from STALE to DELAY. This transition, among other things, updates neigh->updated. Once the kernel receives the first gratuitous ARP, it ignores it because its arrival time is inside the locktime interval. The kernel still bumps neigh->updated. Then the second gratuitous ARP request arrives, and it's also ignored because it's still in the (new) locktime interval. Same happens for the third request. The node eventually heals itself (after delay_first_probe_time seconds since the initial transition to DELAY state), but it just wasted some time and require a new ARP request/reply round trip. This unfortunate behaviour both puts more load on the network, as well as reduces service availability. This patch changes neigh_update so that it bumps neigh->updated (as well as neigh->confirmed) only once we are sure that either lladdr or entry state will change). In the scenario described above, it means that the second gratuitous ARP request will actually update the entry lladdr. Ideally, we would update the neigh entry on the very first gratuitous ARP request. The locktime mechanism is designed to ignore ARP updates in a short timeframe after a previous ARP update was honoured by the kernel layer. This would require tracking timestamps for state transitions separately from timestamps when actual updates are received. This would probably involve changes in neighbour struct. Therefore, the patch doesn't tackle the issue of the first gratuitous APR ignored, leaving it for a follow-up. Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.