The sequence field is a 4-byte unsigned integer that appears in each transaction input.
Originally intended for "high-frequency trades". it now serves different purposes including Replace-By-Fee (RBF) and relative timelocks.

Let's examine our transaction to understand this better:

In our example transaction, we can see the sequence value is 0xffffffff. This is the default sequence value (4294967295 in decimal) which:

• Disables relative timelocks (BIP 68)
• Indicates RBF (Replace-By-Fee) is not enabled
• Represents the traditional "final" sequence number

1- Sequence Values

We will cover Replace-By-Fee (RBF) and relative timelocks in detail in separate topics. Now just understand what a sequence number is and its range.

2- Implementation Example

Here's how you might parse a transaction input's sequence number:

def parse_sequence(raw_tx: bytes, offset: int = 0) -> tuple[int, int]:
    """
    Parse a sequence number from raw transaction bytes
    
    Args:
        raw_tx: Raw transaction bytes
        offset: Starting position in bytes
        
    Returns:
        (sequence, new_offset)
    """
    # Read 4 bytes for sequence
    sequence_bytes = raw_tx[offset:offset + 4]
    
    # Convert to integer (little-endian)
    sequence = int.from_bytes(sequence_bytes, 'little')
    
    return sequence, offset + 4

3- Historical Context

4- Modern Uses

Replace-By-Fee (RBF)

When a transaction's input has a sequence number below 0xf0000000, it signals that the transaction can be replaced by a version with higher fees. This is useful when network fees increase and you want to speed up confirmation of your transaction.

Relative Timelocks

BIP 68 introduced relative timelocks using sequence numbers below 0x80000000. This allows an input to specify a minimum age (in blocks or time) relative to its previous output before it can be spent.

def is_rbf_enabled(sequence: int) -> bool:
    """Check if RBF is enabled for this sequence number"""
    return sequence < 0xf0000000

def is_relative_timelock(sequence: int) -> bool:
    """Check if relative timelock is enabled"""
    return sequence < 0x80000000