Finding peers

If a peer is in a network behind a NAT (Network Address Translation) table, the process for finding a peer is more complicated.

These diagrams are intended as a rough-guide.

Non-NAT simple case

The bootnode can gossip to Charlie who can then directly contact Alice.

sequenceDiagram
    Alice IP1 PORT1-->>Bootnode: Hello (ENR with no IP)
    Bootnode-->>Alice IP1 PORT1: Hi, I notice your address is <IP1>:<PORT1>
    Alice IP1 PORT1-->>Alice IP1 PORT1: Updates ENR (<IP1>:<PORT1>)
    Bootnode-->>Charlie: Meet Alice (ENR with <IP1>:<PORT1>)
    Charlie->>Alice IP1 PORT1: Hello Alice at <IP1>:<PORT1>
    Alice IP1 PORT1->>Charlie: Hello (ENR <IP1>:<PORT1>)

NAT problem

The bootnode can gossip to Charlie, but Charlie is a stranger from the NAT's perspective. It doesn't know who on the internal network is the recipient.

The NAT remembers who it has spoken to.
Messages from the bootnode are expected.
Messages from Charlie are not expected, and its not clear who they are for. Perhaps the smart fridge?

sequenceDiagram
    Alice IP1 PORT1-->>NAT IP2 PORT2: Hello bootnode (ENR with no IP)
    Note right of NAT IP2 PORT2: Stores Bootnode
    NAT IP2 PORT2-->>NAT IP2 PORT2: Maps from internal IP
    NAT IP2 PORT2-->>Bootnode: Hello bootnode (ENR with no IP)
    Bootnode-->>NAT IP2 PORT2: Hi, I notice your address is <IP2>:<PORT2>
    NAT IP2 PORT2-->>NAT IP2 PORT2: Maps to internal IP
    NAT IP2 PORT2-->>Alice IP1 PORT1: Hi, I notice your address is <IP2>:<PORT2>
    Alice IP1 PORT1-->>Alice IP1 PORT1: Updates ENR (<IP2>:<PORT2>)
    Alice IP1 PORT1-->>NAT IP2 PORT2: Thanks bootnode (ENR with <IP2>:<PORT2>)
    NAT IP2 PORT2-->>Bootnode: Thanks boodnode (ENR with <IP2>:<PORT2>)
    Bootnode-->>Charlie: Meet Alice (ENR with <IP2>:<PORT2>)
    Charlie->>NAT IP2 PORT2: Hello Alice at <IP2>:<PORT2>
    Note right of NAT IP2 PORT2: No map on record. Who is this for?
    Note right of Charlie: Hmm Alice didn't respond.

The NAT solution

If Alice knows she is behind a NAT, she can pass a message which goes:

"I'm behind a NAT. Send your requests via peers and I'll reach out to you."

The bootnode gossips to Charlie
Charlie sees "NAT" in Alices ENR
Charlie asks the bootnode to introduce him to Alice
Alice reaches out to Charlie
The NAT now has a mapping for Charlie-Alice messages.

Part 1: NAT detection

Alice can suspect that she is behind a NAT probabilistically. If 2 minutes after connecting with a bootnode, no strangers (like Charlie) have reached out, a NAT is likely.

sequenceDiagram
    Alice IP1 PORT1-->>NAT IP2 PORT2: Hello bootnode (ENR with no IP)
    Note right of NAT IP2 PORT2: Stores Bootnode
    NAT IP2 PORT2-->>NAT IP2 PORT2: Maps from internal IP
    NAT IP2 PORT2-->>Bootnode: Hello bootnode (ENR with no IP)
    Bootnode-->>NAT IP2 PORT2: Hi, I notice your address is <IP2>:<PORT2>
    NAT IP2 PORT2-->>NAT IP2 PORT2: Maps to internal IP
    NAT IP2 PORT2-->>Alice IP1 PORT1: Hi, I notice your address is <IP2>:<PORT2>
    Alice IP1 PORT1-->>Alice IP1 PORT1: Updates ENR (<IP2>:<PORT2>)
    Alice IP1 PORT1-->>NAT IP2 PORT2: Thanks bootnode (ENR with <IP2>:<PORT2>)
    NAT IP2 PORT2-->>Bootnode: Thanks boodnode (ENR with <IP2>:<PORT2>)
    Note right of Alice IP1 PORT1: ... Hmm no strangers. Must be a NAT.

Part 2: NAT communication

Alice can put "NAT" in her ENR. Now when Charlie tries to get in touch, he knows to go via a peer.

Continued from above, skipping Charlie's failed attempt to contact Alice directly.