[Kea-users] yet another question about multiple subnets %)

[Simon]

3 <babut at yandex.ru> wrote:

> "shared network" is not about how to allocate multiple addresses(it doesn't matter if we have one pool or several), but about how to combine several pools into one.

Almost - it’s about how to have multiple (IPv4) subnets/(IPv6) prefixes on one wire.

> A client connected to a shared network may be assigned a lease (address or prefix) from any of the pools defined within the subnets belonging to the shared network. Internally, the server selects one of the subnets belonging to a shared network and tries to allocate a lease from this subnet. If the server is unable to allocate a lease from the selected subnet (e.g., due to pool exhaustion), it uses another subnet from the same shared network and tries to allocate a lease from this subnet. The server typically allocates all leases available in a given subnet before it starts allocating leases from other subnets belonging to the same shared network. 


> but i want several addresses AT THE SAME TIME. this is stated in rfc8415. and here is what is said about rfc8415 in the kea documentation:
> --------
> The server will allocate, renew, or rebind a maximum of one lease for a particular IA option (IA_NA or IA_PD) sent by a client. RFC 8415 allows for multiple addresses or prefixes to be allocated for a single IA.

As has already been said, your client will need to ask for multiple addresses/prefixes. The client knows (for IPv6) the prefixes configured for any link from the RAs it receives.

However, as this thread has highlighted, one of the complications of multiple prefixes by default (having only one is simply multiple prefixes where “n=1”) is that there is then the gap in how to inform clients how they are expected to use them. Having been following a couple of IETF mailing lists, this is not yet a solved problem - and I suspect may never be due to the complexity involved to cover all situations.
One particular variation involves having two (or more) upstream internet providers - each providing a different prefix. There is currently no portable way to inform clients how to use the different prefixes - hence calls by some for NPT (network prefix translation) so that the network layer (i.e. routers) can route packets according to admin defined rules (e.g. email via a cheap but high latency link, web browsing via an expensive but low latency link), or switch routing on the fly (e.g. switching to a backup link if the primary fails).

Having said that, if there is a GUA and a ULA advertised, then it would be logical for a client to request addresses from both and use them accordingly - ULA for communications with other ULA addresses, GUA for everything else.

Simon