What is the IOTA transaction data structure?

Question

I'm vaguely familiar with Bitcoin's UTXO-based transaction structure and Ethereum's account-based/gas-including transaction structure.

What does an IOTA transaction contain?

Answer 1

Trinary Note

• In the "normal" world we have bytes & bits, where 1 byte is 8 bits giving us 2^8 = 256 possible data values for 1 byte.
• In the trinary world we have trytes & trits, where 1 tryte is 3 trits giving us 3^3 = 27 possible data values for 1 tryte. This is what IOTA uses to stores its transaction data.
  
  

Transaction Composition

One IOTA transaction consists of 2673 trytes. Note, however, that a user will usually broadcast a collection of multiple transactions (also called a bundle). One bundle could consist of

1. Withdrawal transaction (acquire funds to be spent from some address)
2. Payment transactions (pay some other address a sub or total amount of the funds withdrawn)
3. Change transaction (deposit unspent funds back to one of the user's addresses)

Note that bundles are not restricted to a specific number of transactions. For exmaple, one could have a bundle that consists of just one transaction (when a value of 0 is "transferred").

The data segments within one transaction are as follows.

Core Data

1. Signature Message Fragment contains the transaction signature = 2187 trytes
2. Address is either receipt address (if value > 0) or withdrawal address (if value < 0) = 81 trytes
3. Value is the amount of IOTA transferred = 27 trytes
4. Obsolete Tag is some arbitrary user-defined value = 27 trytes
5. Time Stamp of when the transaction was issued = 9 trytes
6. Current Index of this transaction in its bundle = 9 trytes
7. Last Index is the total number of transactions in the bundle = 9 trytes

Hashes

8. Bundle Hash is the hash of the entire bundle = 81 trytes
9. Trunk Hash is the hash of the 1st transaction referenced/approved = 81 trytes
10. Branch Hash is the hash of the 2nd transaction referenced/approved = 81 trytes

Another Tag

11. Tag is another arbitrary user-defined value = 27 trytes

Additional Time Stamps

12. Attachment Time is the time stamp for when PoW is completed = 9 trytes
13. Attachment Time (lower bound) is a slot for future use = 9 trytes
14. Attachment Time (upper bound) is a slot for future use = 9 trytes

Proof-of-Work Data

15. Nonce is the PoW nonce of the transaction = 27 trytes
    
    

Additional Links

https://github.com/iotaledger/iri/.../controllers/TransactionViewModel.java
https://github.com/alon-e/iotaWayback
https://iota.readme.io/docs/the-anatomy-of-a-transaction