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Let's say node is on a chain of length N+4, and receives a block from the chain of the same length N+4, but diverged by 4 blocks.

  1. Is the information of the length and accumulated difficulty broadcasted along with mined block?

  2. How will the recipient node handle this situation?

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This phenomenon is known as a chain-split (i.e. the bitcoin network can no longer agree which chain is the canonical chain).

Nodes will be network-partitioned until one chain becomes dominant.

Therefore:

Is the information of the length and accumulated difficulty broadcasted along with mined block?

No.

How will the recipient node handle this situation?

Nodes will handle equal branches on a First Come, First Serve basis, so there will be a network partition where some nodes agree chain A with length N+4 is the canonical chain and the other set of nodes will assume chain B with length N+4 is the canonical chain. In the event of such a chain-split, the race is on for miners to produce a new block-tip and broadcast it across the bitcoin network ASAP to achieve convergence. When the contention is resolved, the node can switch to longer branch and obsolete the old branch.

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  • But then, if I recieve a block that is not for the tip of my blockchain, I will reject it immediately if along I don't recieve the information that it is for the tip of some other heavier chain. I will then never switch to another chain.
    – croraf
    Commented Oct 22, 2017 at 13:15
  • Sure, but say you have adopted chain A with length N+4, and suddenly you receive a block-tip of length N+5 that mines on top of chain B, your node will immediately recognise that the block-tip you just received may belong to a longer & heavier chain, so your node will probe the peer that has sent your that block tip for all the relevant blocks.
    – rny
    Commented Oct 22, 2017 at 13:17
  • How will my node immediately recognise it was a block for the heavier chain, and not some erroneous block for example?
    – croraf
    Commented Oct 22, 2017 at 13:18
  • But I dont know if proof of work is correct as it was made with different inputs (unknown to me).
    – croraf
    Commented Oct 22, 2017 at 13:22
  • So I looked the bitcoin core implementation and it looks like all blocks are stored so as long as they obey consensus rules. And there's a function in src/validation.cpp that activates the best chain.
    – rny
    Commented Oct 22, 2017 at 13:29

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