Add TTL to Provider Record

[guillaumemichel]

As a result of nodes fetching some content, advertising to the IPFS DHT and quickly churning, some CIDs have a lot of unreachable providers which may be a problem. In order to address this issue, nodes could give a TTL to the provider records they are advertising, given the knowledge of their average uptime. The DHT Servers would store the provider records only for TTL, which should reduce the number of unreachable providers in the IPFS DHT.

In order to implement this change, let's make use of the ttl field in the Provider Record protobuf https://github.com/plprobelab/go-kademlia/blob/dc867cbd3316a89cabaa5be19900cdbf5d2f0805/network/message/ipfsv1/message.proto#L30

Note that this field isn't supported in go-libp2p-kad-dht (so nodes will discard the ttl field), but it can already be parsed by rust-libp2p.

We also need a mechanism on the client/provider that will determine the TTL value, but this value can be provided by the caller (kubo/boxo).

On the Server side, we need to make sure that the TTL field is parsed, and that the Provider Store (once implemented) will only keep the Provider Record for its given TTL (or at most a fixed number e.g 48 hours).

References

• Unreachable providers for popular CIDs network-measurements#49
• Unreachable Providers (client): Decreased Re-Provide Delay/Record TTL ipfs/kubo#9984
• Unreachable Providers (server): Provider-Record Prioritization ipfs/kubo#9982