Performance improvement: lookup leases by address first when address is available

Edit: TLDR: This issue is about looking up leases by address hint in renews and v6 requests. Currently, they are first searched by DUID/client ID. Because these are not primary keys, but indexes, this lookup is slower. Searching by address which is a primary key should be significantly faster. I imagine this as a few lines of code changed, although it is affecting the allocation engine. There are concerns that this might affect host reservations, RADIUS or other aspects. I expect that to come up in tests.

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When designing database scehmas, it is desirable to look at access patterns in order to define keys or indexes or maybe entire table definitions. When looking at the DHCP protocol, we can easily figure out that the key that we most want to use in a lookup is the client's identifier which is usually the DUID or client ID. This is not the case for current Kea since address is clearly chosen as sole primary key across all backends for the lease databases. This is probably because it is chosen based on it's unique properties. A v6 client can have multiple addresses per DUID and maybe it's the same for v4 in same strange use cases. This could have been solved by storing addresses as a list in DUID/clientID-keyed tables at the cost of complexity. But that is not my proposal.

We can leverage the current table structure by looking up by address when possible. This is effectively the case when an address hint is provided. A well known case when this happens is during the renew process, but I think I remember reading from a RFC that it can be provided in other DHCP messages as well. But a well-behaved client should (RFC SHOULD?) always send the address in their RENEW (I think it's still called REQUEST for DHCPv4?) and RENEWs are 99% of the DHCP messages sent in networks with high uptime. So it is an optimization for RENEW.

Concerns:

• Security? Honoring DHCP clients requesting address directly might lead to address starvation?

  • No, after lookup by address finds nothing, the usual DISCOVER & SOLICIT messages go through with looking up by DUID/clientID. And then it will find the lease that belonged to the malicious client. Even if it did, add that to the list of security issues. Clients spoofing their DUID/clientID doesn't do the same?

• Are we sure we aren't providing the address to the wrong client?

  • Yes. We can check in-memory/in-server if the DUIDs/clientIDs match. Even though what harm can the right lease given to the wrong client do?

• Does this affect the ability to reserve addresses/prefixes, RADIUS, hooks or other use-cases?

  • To be investigated. I don't know. Because firstly I don't understand why host reservations are looked up before leases. This optimization is less impactful if we still search by DUID/clientID in hosts first. I would move the address lookup in the lease database at the beginning of the packet processing. But then why not move the DUID/clientID lookup at the beginning as well? If the client has an active lease, doesn't it stop there?