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add notes about how to handle peer discovery and gossip topics prior to genesis

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Danny Ryan 2020-04-06 09:57:23 -06:00
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specs/phase0/p2p-interface.md
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@ -105,6 +105,7 @@ It consists of four main sections:
- [Discovery](#discovery)
- [Why are we using discv5 and not libp2p Kademlia DHT?](#why-are-we-using-discv5-and-not-libp2p-kademlia-dht)
- [What is the difference between an ENR and a multiaddr, and why are we using ENRs?](#what-is-the-difference-between-an-enr-and-a-multiaddr-and-why-are-we-using-enrs)
- [Why do we not form ENRs and find peers until genesis block/state is known?](#why-do-we-not-form-enrs-and-find-peers-until-genesis-blockstate-is-known)
- [Compression/Encoding](#compressionencoding)
- [Why are we using SSZ for encoding?](#why-are-we-using-ssz-for-encoding)
- [Why are we compressing, and at which layers?](#why-are-we-compressing-and-at-which-layers)
@ -247,6 +248,8 @@ Topics are plain UTF-8 strings and are encoded on the wire as determined by prot
- `Name` - see table below
- `Encoding` - the encoding strategy describes a specific representation of bytes that will be transmitted over the wire. See the [Encodings](#Encoding-strategies) section for further details.
*Note*: `ForkDigestValue` is composed of values that are not known until the genesis block/state are available. Due to this, clients SHOULD NOT subscribe to gossipsub topics until these genesis values are known.
Each gossipsub [message](https://github.com/libp2p/go-libp2p-pubsub/blob/master/pb/rpc.proto#L17-L24) has a maximum size of `GOSSIP_MAX_SIZE`. Clients MUST reject (fail validation) messages that are over this size limit. Likewise, clients MUST NOT emit or propagate messages larger than this limit.
The `message-id` of a gossipsub message MUST be:
@ -752,6 +755,8 @@ where the fields of `ENRForkID` are defined as
* `next_fork_version` is the fork version corresponding to the next planned hard fork at a future epoch. If no future fork is planned, set `next_fork_version = current_fork_version` to signal this fact
* `next_fork_epoch` is the epoch at which the next fork is planned and the `current_fork_version` will be updated. If no future fork is planned, set `next_fork_epoch = FAR_FUTURE_EPOCH` to signal this fact
*Note*: `fork_digest` is composed of values that are not not known until the genesis block/state are available. Due to this, clients SHOULD NOT form ENRs and begin peer discovery until genesis values are known.
Clients SHOULD connect to peers with `fork_digest`, `next_fork_version`, and `next_fork_epoch` that match local values.
Clients MAY connect to peers with the same `fork_digest` but a different `next_fork_version`/`next_fork_epoch`. Unless `ENRForkID` is manually updated to matching prior to the earlier `next_fork_epoch` of the two clients, these connecting clients will be unable to successfully interact starting at the earlier `next_fork_epoch`.
@ -1092,6 +1097,12 @@ discv5 uses ENRs and we will presumably need to:
1. Add `multiaddr` to the dictionary, so that nodes can advertise their multiaddr under a reserved namespace in ENRs. – and/or –
2. Define a bi-directional conversion function between multiaddrs and the corresponding denormalized fields in an ENR (ip, ip6, tcp, tcp6, etc.), for compatibility with nodes that do not support multiaddr natively (e.g. Eth 1.0 nodes).
### Why do we not form ENRs and find peers until genesis block/state is known?
Although client software might very well be running locally prior to the solidification of the eth2 genesis state and block, clients cannot form valid ENRs prior to this point. ENRs contain `fork_digest` which utilizes the `genesis_validators_root` for a cleaner separation between chains so prior to knowing genesis, we cannot use `fork_digest` to cleanly find peers on our intended chain. Once genesis data is known, we can then form ENRs and safely find peers.
When using an eth1 deposit contract for deposits, `fork_digest` will be known at least `MIN_GENESIS_DELAY` (24 hours in mainnet configuration) before `genesis_time`, providing ample time to find peers and form initial connections and gossip subnets prior to genesis.
## Compression/Encoding
### Why are we using SSZ for encoding?