Internet Checksum Carry Fold Trace
Trace exact Internet-checksum end-around carry folding of an unsigned integer sum without complementing it or reading packet fields.
Description
Trace exact Internet-checksum end-around carry folding of an unsigned integer sum without complementing it or reading packet fields.
End-around carry reduces an accumulated sum to 16 bits by adding overflow bits back into the low word. A trace makes repeated folds visible.
When to use this tool
Trace exact Internet-checksum end-around carry folding of an unsigned integer sum without complementing it or reading packet fields.
- Prepare the supplied sum fields for this operation.
- Compare the complete example with the representation used by your own input before carrying the result into another step.
How to use the tool and how it works
- Enter sum. Use the exact text syntax, units and index conventions explained below.
- Run the tool. If input validation fails, correct the supplied fields before interpreting any result.
- Read result.folded, result.trace. Interpret false, null and empty lists using the distinctions below; they can be valid outputs.
Parse sum as a nonnegative exact integer. While above 65,535, replace it by (sum AND 65,535) plus (sum shifted right 16). Return the final number and decimal-text trace including the input.1
| Input field | Meaning and representation | Accepted input |
|---|---|---|
| sum | Exact decimal integer text; keep it quoted in JSON rather than converting through a floating-point number. | Optional leading + or - and 1 to 256 decimal digits; no whitespace |
| Output field | Interpretation |
|---|---|
| result | Object containing the named outputs below. |
| result.folded | The folded value under the stated formula or text convention. |
| result.trace | The trace list in the ordering and representation described above. |
{
"input": {
"sum": "131071"
},
"output": {
"result": {
"folded": 1,
"trace": [
"131071",
"65536",
"1"
]
}
}
}Limitations and assumptions
- The tool does not complement the folded result or assemble protocol words and pseudoheaders.
- All declared input fields are required and unexpected top-level fields are rejected. Numbers must be finite; lists are limited to 128 entries and ordinary text fields to 8,192 UTF-16 code units, with tighter limits where stated.
- JSON text inputs must parse within bounded nesting and size limits. Numeric outputs outside the safe finite range are rejected; large exact integer results use decimal strings. Indexes and half-open spans are zero-based unless a rank is explicitly described as one-based.
Alternative or Complementary methods
Use the complete specified checksum algorithm and independent test vectors when verifying a payload. Field packing, parameter shape and arithmetic agreement are separate checks; none supplies cryptographic authenticity.
References
Similar or alternative tools
- Internet Checksum Complement Tool
Take the bounded ones-complement of one explicit 16-bit Internet-checksum word; this is not a packet checksum calculation.
- Internet Checksum Residue Check
Verify an explicit Internet-checksum word list including its checksum against all-one folded residue; no packet-field or protocol coverage assumptions.