As we have seen the Lightning Network uses a sourced base Onion Routing Protocol to deliver a payment from a sender to the recipient.
For this the sending node has to be able to construct a path of payment channels that connects it with the recipient.
Thus the sender has to be aware of the Channel Graph.
The channel graph is the interconnected set of publicly advertised channels (more on that later), and the nodes that these channels interlink.
As channels are backed by a funding transaction that is happening on chain one might falsely believe that Lightning Nodes could just extract the existing channels from the Bitcoin Blockchain.
However this only possible to a certain extend.
The Funding transactions are Pay to Whitness Script addresses and the nature of the script will only be revealed once the funding transaction output is spent.
Even if the nature of the script was known we emphasize that not all 2 - of - 2 multisignature scripts correspond to payment channels.
Since not even every Pay to Whitness Script address that we see in the Bitcoin Blockchain corresponds to a payment channel this approach is not helpful.
There are actually more reasons why looking at the Bitcoin Blockchain might not be helpful.
For example on the Lightnign Network the Bitcoin keys that are used for signing are rotated by the nodes for every channel and update.
Thus even if we could reliably detect funding transactions on the Bitcoin Blockchain we would not know which two nodes on the Lightning Network own that particular channel.
Thus we could node create the Channel Graph that would be used to create candidate paths during the payment delivery via onion routing.
The Lightning Network solves the afore mentioned problem by implementing a gossip protocol.
Gossip protocols are typical for peer 2 peer networks and are being used so that nodes can share information with other nodes without talking to those other nodes directly.
For that Lightning Nodes open encrypted peer 2 peer connections to each other and share novel information that they have received from other peers.
As soon as a node wants to share some information - for example about a newly created channel - it sends a message to all its peers.
Uppon receiving a message a node decides if the received message was novel and if so it will forward the information to its peers.
In this way if the peer 2 peer network is connected all new information that is necessary for the operation of the network will eventually be propagated to all other peers.
Obviously if a new peer initially wants to join the network it needs to know some other peers on the Network initially.
Only then the new peer could connect to them in order to learn about the existance of other peers.
In this chapter, we'll explore exactly _how_ Lightning nodes discover each
other, and also the channel graph itself. The channel graph is the
interconnected set of publicly advertised channels (more on that later), and
the nodes that these channels interlink. When most refer to the _network_ part
other, and also the channel graph itself.
When most refer to the _network_ part
of the Lightning Network, they're referring to the Channel Graph which itself
is a unique authenticated data structured _anchored_ in the base Bitcoin
is a unique authenticated data structure _anchored_ in the base Bitcoin
blockchain.
However the Lightning network is also a peer to peer network of nodes that are have just opened an encrypted peer connection.
Usually for 2 peers to maintain a payment channel they need to talk to each other directly which means that there will be a peer connection between them.
This suggests that the Channel Graph is a subnetwork of the peer to peer network.
However this is not true.
As payment channels do not need to be closed if one or both peers are going offline they can continue to exist even though the peer connection is terminated.
It might be usefull to get familiar with the different terminology that we have used througout the book for similar concepts / actions depending on weather they happen on the Channel Graph or on the peer 2 peer network.
Rene Pickhardt
4 years ago