How would a node detect a reorganization/orphan of the blockchain?

[mustyoshi]

Such as it was downloading the wrong chain, and suddenly the chain ends?

Like if 2-3 blocks back was suddenly marked as orphaned, how would the qt client understand this had happened?

[oakpacific]

A full node should be able to see every blockchain, sans the possibility of some MITM attacks.

[mustyoshi]

A full node should be able to see every blockchain, sans the possibility of some MITM attacks.

What if the node is more like a 9/10ths node?

[oakpacific]

A full node should be able to see every blockchain, sans the possibility of some MITM attacks.

What if the node is more like a 9/10ths node?

If you don't download the full copy of the blockchain you're not considered a full node. In that case(you download just the block headers) you're susceptible to sybil attack by some entities controlling more than 50% of the hashpower feeding you the wrong chain.

[mustyoshi]

A full node should be able to see every blockchain, sans the possibility of some MITM attacks.

What if the node is more like a 9/10ths node?

If you don't download the full copy of the blockchain you're not considered a full node. In that case(you download just the block headers) you're susceptible to sybil attack by some entities controlling more than 50% of the hashpower feeding you the wrong chain.

Is there any way to defend against a sybil attack if I don't download every transaction, I do have some transactions, but not all of them?

How does a full node defend against an attacker with >50% of the hashpower?