bitcoin-elixir v0.0.1 Bitcoin.Protocol.Messages.GetHeaders
Return a headers packet containing the headers of blocks starting right after the last known hash in the block locator object, up to hash_stop or 2000 blocks, whichever comes first. To receive the next block headers, one needs to issue getheaders again with a new block locator object. The getheaders command is used by thin clients to quickly download the block chain where the contents of the transactions would be irrelevant (because they are not ours). Keep in mind that some clients may provide headers of blocks which are invalid if the block locator object contains a hash on the invalid branch.
For the block locator object in this packet, the same rules apply as for the getblocks packet.
https://en.bitcoin.it/wiki/Protocol_specification#getheaders