Why is `branch` needed to parse operations?

Question

I am trying to manually parse an operation. By asking the (testnet) tezos node to parse an operation, one can get data to compare:

tezos-client -P 18732 rpc post chains/main/blocks/head/helpers/parse/operations \
  with '{ "operations": [{"branch":"BMUQYkq83VVyhgRLjsWqJZhmEVKyzj22ogLP3QTNC4ziEwqaLHU",
                          "data":"6c00b07ee75b490cfd65c4fe5cef27888754c76751418c0bc2b724e852ac02c48aa401000011dbc55dddc843df043d6f36a162d2f4a8c2176d004bd63386731a940609ded1d37bef4ea373304c05389000eb1d16fd44480a9b6231c8859fd26b78d98ab410cfd399f329495caa64a1856b53a13c19f141168306" }]}'

which gives:

[ { "branch": "BMUQYkq83VVyhgRLjsWqJZhmEVKyzj22ogLP3QTNC4ziEwqaLHU",
    "contents":
      [ { "kind": "transaction",
          "source": "tz1bjFeJnrEU3zjhDW8uG1Xtd7MRyHkp5VQd", "fee": "1420",
          "counter": "596930", "gas_limit": "10600", "storage_limit": "300",
          "amount": "2688324",
          "destination": "tz1MGTTLMhikC7qkfduaS9XKNMvHEkSxMq4X" } ],
    "signature":
      "sigXukL1KWLCddQtZYPxp3NdXpqSVufPQBU1QwdYQCM8dsFvow6UwdBoFgJynfdDUNuwDPguTK6Jq9YyoFgi5PtrQ8cs4c8e" } ]

Converting the source, tz1bjFeJnrEU3zjhDW8uG1Xtd7MRyHkp5VQd from Base58 to hex gives 06a19fb07ee75b490cfd65c4fe5cef27888754c76751419acb5dba. When I remove the first three and the last four bytes I get a match: b07ee75b490cfd65c4fe5cef27888754c7675141.

Similarly with the destination.

The tezos node however requires branch as an input and I do not understand why. Now, I have not decoded the amount yet, but so far I cannot see what I need branch for. What is it I do not understand? What piece of the puzzle do I not get by omitting that in my effort to parse the operation?

Answer 1

Every transaction in current and past Tezos protocols must reference a recent block (the "branch"). This acts as

1. an informal checkpointing mechanism
2. a replay protection mechanism
3. an effective time-to-live limit on transactions

edit: As to why it is required for parsing, it is because the node uses the branch to determine which protocol it should use to parse the operation.