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Denis Angell
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docs/skills/protocol.md
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271
docs/skills/protocol.md
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# Protocol and Serialization
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The protocol layer defines XRPL's wire format, type system, and validation rules. It owns the canonical binary encoding required for signatures and consensus, the macro-driven registries for features/transactions/ledger entries, and the typed object model (`STBase` hierarchy) that every transaction and ledger object inhabits.
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## Layered Type System
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```
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Asset = std::variant<Issue, MPTIssue> ← unified asset identity (XRP/IOU/MPT)
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Issue = (Currency, AccountID) ← XRP iff currency==zero
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MPTIssue = wraps MPTID (192-bit: seq32 || account160)
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Amount types (lean, runtime polymorphic via Asset):
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XRPAmount = int64 drops ← integral, no asset
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IOUAmount = (mantissa, exponent) ← 15-digit decimal floating point
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MPTAmount = int64 ← integral, no asset
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STAmount = unified wire/serialized form holding Asset + value
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- holds<Issue>(), holds<MPTIssue>(), native(), integral()
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- canonicalize() normalizes (mantissa, exponent) per asset rules
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```
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Conversion utilities in `AmountConversions.h`: `toSTAmount`, `toAmount<T>`, `getAsset<T>`. Lean→STAmount is implicit-friendly; STAmount→lean is explicit (`get<>` throws on type mismatch).
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## Key Invariants
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- **Canonical field ordering:** sort by `(SerializedTypeID << 16) | fieldValue`, NOT by raw Field ID bytes — wrong sort breaks signatures
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- **Field ID encoding:** 1–3 bytes; both type and field codes <16 → single byte `(type<<4)|name`
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- **Hash domain separation:** every signable payload prepends a 4-byte `HashPrefix` (`STX\0`, `SMT\0`, `VAL\0`, `STX\0`, `BCH\0`, `CLM\0`, `LWR\0`, etc.) — never share hashes across domains
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- **STObject access semantics:** `obj[sfFoo]` throws `FieldErr` if absent; `obj[~sfFoo]` returns `std::optional`
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- **Amendment IDs are deterministic:** `featureFoo == sha512Half(Slice("Foo"))` — never change a feature name
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- **Singletons everywhere:** `SField`, `LedgerFormats`, `TxFormats`, `InnerObjectFormats`, `Permission`, `Feature` registry all use Meyer's singletons; registration completes before `main()` via static init
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- **Multi-sign signers MUST be sorted ascending by AccountID** (no duplicates, count in [1,32], cannot include tx account)
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- **`vfFullyCanonicalSig` always set** by signer; verifiers normalize ECDSA S to low form
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- **Amendment-gated arithmetic:** `getSTNumberSwitchover()` is a `LocalValue<bool>` (per-coroutine) selecting legacy vs `Number`-based normalization in `IOUAmount`/`STAmount`
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## Macro-Driven Registries (X-Macros)
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Single source of truth for each registry; `.macro` files included multiple times with redefined macros to generate enum, declarations, and definitions.
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| Macro file | Used for | Add requires |
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|---|---|---|
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| `features.macro` | `XRPL_FEATURE`, `XRPL_FIX`, `XRPL_RETIRE_*` | Bump `numFeatures` in `Feature.h` |
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| `transactions.macro` | `TRANSACTION(tag, value, name, delegable, amendment, privileges, fields)` | nothing — count derived |
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| `ledger_entries.macro` | `LEDGER_ENTRY(tag, value, name, rpcName, fields)` + `LEDGER_ENTRY_DUPLICATE` for name collisions | nothing |
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| `sfields.macro` | `TYPED_SFIELD(name, TYPE, code)`, `UNTYPED_SFIELD` | nothing |
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| `permissions.macro` | `PERMISSION(name, txType, value)` (granular permissions ≥65537) | nothing |
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Pattern uses `#pragma push_macro/pop_macro` to protect macro names. `UNWRAP(...)` strips outer parens around field-list initializers so commas don't confuse the preprocessor. `LEDGER_ENTRY_DUPLICATE` exists because `DepositPreauth` is both a transaction type and ledger entry type — `JSS()` can't emit the same string twice.
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## Field Identity (`SField`)
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- **Field code** = `(SerializedTypeID << 16) | fieldValue` — packs type family and per-type index; canonical sort key
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- `SField` instances are immutable singletons created at static init via `private_access_tag_t` (only definable inside `SField.cpp`)
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- `TypedField<T>` adds compile-time payload type; `OptionaledField<T>` via `operator~(sfField)`
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- Metadata flags (`fieldMeta`): `sMD_ChangeOrig`, `sMD_ChangeNew`, `sMD_DeleteFinal`, `sMD_Create`, `sMD_Always`, `sMD_BaseTen` (decimal display), `sMD_PseudoAccount`, `sMD_NeedsAsset` (drives `STTakesAsset` association)
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- `IsSigning::no` excludes fields from signing hash (`sfTxnSignature`, `sfSigners`, `sfSignature`, etc.)
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- `isBinary()` ⇔ `fieldValue<256` (wire-representable); `isDiscardable()` ⇔ `fieldValue>256` (JSON-only, e.g., `sfHash`, `sfIndex`)
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## Wire Format Reference
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| Item | Encoding |
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|---|---|
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| XRP STAmount | 8 bytes; bit63=0, bit62=sign(1=pos), 62-bit value |
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| MPT STAmount | 8 bytes header (bit63=0, bit61=1) + 192-bit MPTID |
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| IOU STAmount | bit63=1, bit62=sign, 8-bit (offset+97), 54-bit mantissa, +20B currency, +20B issuer |
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| AccountID | 20 bytes, VL-prefixed when standalone (`STAccount` mimics `STBlob` wire format) |
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| STArray | elements between markers; ends with `STI_ARRAY,1` (`0xf1`) |
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| STObject | fields in canonical order; ends with `STI_OBJECT,1` (`0xe1`) |
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| VL prefix | 1 byte (0–192), 2 bytes (193–12480), 3 bytes (12481–918744); else `std::overflow_error` |
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| STIssue | 160-bit currency; if zero → XRP; if next 160 = `noAccount()` → MPT (then 32-bit seq); else IOU issuer |
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| LP token currency | byte0 = `0x03`; bytes 1-19 = `sha512Half(min(asset1,asset2), max(asset1,asset2))` low bits |
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| Order book quality | `(exponent+100) << 56 \| mantissa`; embedded in last 8 bytes of directory key (big-endian) so SHAMap order = price order |
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| NFTokenID (256-bit) | flags(2) + transferFee(2) + issuer(20) + cipheredTaxon(4) + serial(4); low 96 bits = page sort key |
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## Canonical Hashes
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```
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TXN → transactionID SND → txNode (with metadata)
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MLN → leafNode MIN → innerNode
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LWR → ledgerMaster STX → txSign (single-sig)
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SMT → txMultiSign VAL → validation
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PRP → proposal MAN → manifest
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CLM → paymentChannelClaim BCH → batch
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```
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All hashes use `sha512Half` (first 256 bits of SHA-512). `HashPrefix` constants are protocol-immutable.
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## STObject and STVar
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- `STObject` stores `std::vector<detail::STVar>`; iterators expose `STBase const&` via transform iterator
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- `STVar` is type-erased with 72-byte inline buffer (small-object optimization); larger types heap-allocate
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- `copy()`/`move()` virtuals on every ST type delegate to `STBase::emplace()` for placement-new into `STVar`'s buffer
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- **Two modes:**
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- **Free** (`mType==nullptr`): linear field scan, accepts any field
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- **Templated** (`mType` set): O(1) field lookup via `SOTemplate::indices_`, template enforced
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- `applyTemplate()` validates after deserialization; `set(SOTemplate)` initializes empty object with template
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- Deserialization depth capped at 10 to prevent stack exhaustion
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### Proxy Access Pattern
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```cpp
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auto amt = tx[sfAmount]; // ValueProxy: throws FieldErr if absent
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auto dst = tx[~sfDestination]; // OptionalProxy: std::optional
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tx[sfFlags] = 0; // proxy.assign() — soeDEFAULT zero is silently removed
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tx[~sfDestTag] = std::nullopt; // remove field (only valid for soeOPTIONAL)
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```
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Proxies forbid removing `soeREQUIRED` or `soeDEFAULT` fields.
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## SOEStyle (Field Presence)
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| Style | Meaning |
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|---|---|
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| `soeREQUIRED` | must be present |
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| `soeOPTIONAL` | may be absent; if present, may carry default value |
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| `soeDEFAULT` | may be absent; if present, must NOT equal default — auto-removed when assigned default |
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`SOETxMPTIssue` flag on amount/issue fields: `soeMPTSupported`, `soeMPTNotSupported`, `soeMPTNone`. Omitting `soeMPTSupported` silently rejects MPT amounts in that field.
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## Common Bug Patterns
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- Adding to `transactions.macro` without adding to `sfields.macro` → silent serialization failures
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- Forgetting to bump `numFeatures` after `XRPL_FEATURE` → out-of-bounds access in `FeatureBitset`
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- Hand-built binary blobs in non-canonical field order → signature verification failures
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- Omitting `soeMPTSupported` on amount field → MPT payments silently rejected
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- Mutating `sfTransactionType` inside `STTx` assembler callback → `LogicError` (caught at startup)
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- Storing `STBase` subclasses directly in `std::vector` → field names lost on copy-assignment slide; use `STArray`/`STObject` instead
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- Storing `Currency` as `"XRP"` ISO code (`badCurrency()`) instead of zero → silently rejected; `to_currency()` legacy returns `badCurrency()` rather than failing
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- Forgetting to call `associateAsset(sle, asset)` near end of `doApply()` for vault/loan transactors → unrounded `STNumber` values
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- Returning `tec*` from `preflight()` → `NotTEC` type prevents this at compile time (would allow fee theft on unsigned tx)
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## Key Patterns
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### Amendment Registration
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```cpp
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// In features.macro:
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XRPL_FEATURE(MyFeature, Supported::yes, VoteBehavior::DefaultNo)
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XRPL_FIX (MyBugFix, Supported::yes, VoteBehavior::DefaultNo)
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XRPL_RETIRE_FEATURE(OldFeature) // code removed; remains registered for ledger compat
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```
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Lifecycle: `Supported::no/DefaultNo` → `Supported::yes/DefaultNo` → (rare) `DefaultYes` for critical fixes. **Never** revert `Supported::yes` to `no` (would amendment-block existing nodes).
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### NotTEC vs TER
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```cpp
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NotTEC preflight(...); // can only return tel/tem/tef/ter/tes (no tec)
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TER doApply(...); // can return any code including tec*
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```
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`TERSubset<Trait>` enforces this at compile time via `enable_if`. `TERtoInt(v)` is the authorized free-function conversion (member `explicit operator` would be too permissive in initializer contexts).
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### Signing/Verifying
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```cpp
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sign(st, HashPrefix::txSign, KeyType::secp256k1, sk); // writes sfSignature
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verify(st, HashPrefix::txSign, pubKey); // returns bool
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// Multi-sign optimization (shared body, per-signer suffix):
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auto s = startMultiSigningData(obj);
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finishMultiSigningData(signerAccountID, s); // append signer ID to shared payload
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```
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`addWithoutSigningFields()` excludes signature fields from the signed payload — this is what breaks the circularity.
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### STNumber + STTakesAsset
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```cpp
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// Vault/Loan/LoanBroker fields use STNumber (no asset embedded).
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// In doApply(), after all mutations:
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associateAsset(*sle, vaultAsset); // rounds all sMD_NeedsAsset fields, removes zero-defaults
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```
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## Critical Files
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### Foundations
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- `include/xrpl/protocol/SField.h`, `src/libxrpl/protocol/SField.cpp` — field registry, X-macro expansion
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- `include/xrpl/protocol/Feature.h`, `src/libxrpl/protocol/Feature.cpp` — `numFeatures`, `FeatureBitset`, registration
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- `include/xrpl/protocol/Rules.h` — per-coroutine active amendment set; `isFeatureEnabled()` queries thread-local
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- `include/xrpl/protocol/HashPrefix.h` — protocol-immutable domain separators
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### Macro Tables (single sources of truth)
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- `include/xrpl/protocol/detail/features.macro`
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- `include/xrpl/protocol/detail/transactions.macro`
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- `include/xrpl/protocol/detail/ledger_entries.macro`
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- `include/xrpl/protocol/detail/sfields.macro`
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- `include/xrpl/protocol/detail/permissions.macro`
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### Type System Roots
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- `STBase.h/cpp` — polymorphic root, `emplace()` SOO helper, `JsonOptions`
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- `STObject.h/cpp` — heterogeneous container, proxy system, template enforcement
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- `STVar` (`detail/STVar.h`) — 72-byte inline variant, depth guard at 10
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- `SOTemplate.h/cpp` — schema with O(1) field index; move-only
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### Format Registries
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- `TxFormats.h/cpp`, `LedgerFormats.h/cpp`, `InnerObjectFormats.h/cpp` — all inherit `KnownFormats<Key, Derived>` with `forward_list<Item>` (pointer-stable) + dual flat_maps
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### Amount/Asset Stack
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- `Asset.h/cpp` — variant of Issue/MPTIssue; `visit()`, `equalTokens()`, `BadAsset` sentinel
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- `Issue.h/cpp`, `MPTIssue.h/cpp` — XRP/IOU and MPT identity
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- `STAmount.h/cpp` — unified serialized amount; `canMul`/`canAdd`/`canSubtract` safety checks; `mulRound`/`mulRoundStrict` (legacy vs precise rounding)
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- `IOUAmount.h/cpp`, `XRPAmount.h`, `MPTAmount.h/cpp` — lean representations
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- `Number` (in `xrpl/basics/`) — high-precision arithmetic; `MantissaRange::large` enabled by SingleAssetVault/LendingProtocol amendments
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### Cryptography
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- `PublicKey.h/cpp` — 33-byte unified format (0xED prefix for Ed25519); `ECDSACanonicality` enum (canonical vs fullyCanonical), libsecp256k1 normalization
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- `SecretKey.h/cpp` — `secure_erase` in dtor; deleted `==`/`<<`; XRPL-specific secp256k1 derivation via `Generator`
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- `Seed.h/cpp` — 128-bit; `parseGenericSeed()` cascades hex→base58→RFC1751→passphrase, rejecting other key types first
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- `digest.h` — `sha512Half`, `sha512_half_hasher_s` (secure erase variant)
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- `tokens.h/cpp` — Base58Check; fast path uses base 58^10 intermediate (10–15× speedup, gated on non-MSVC for `__int128`)
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### Wire I/O
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- `Serializer.h/cpp` — accumulator; `addVL`, `addFieldID`, big-endian integers, `getSHA512Half()`
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- `SerialIter` — non-owning forward cursor over a byte buffer; throws on underrun
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- `Sign.h/cpp` — `sign`/`verify` with HashPrefix prepended to `addWithoutSigningFields()` output
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### Higher-Level Objects
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- `STTx.h/cpp` — caches `tid_` and `tx_type_`; `passesLocalChecks` (memos, pseudo-tx, MPT support, batch nesting); `sterilize()` round-trip
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- `STLedgerEntry.h/cpp` (alias `SLE`) — typed ledger object; `thread()` updates `sfPreviousTxnID`; `isThreadedType()` gated by `fixPreviousTxnID`
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- `STValidation.h/cpp` — lazy `valid_` cache; `mTrusted` separate from validity; `lookupNodeID` callback decouples manifest system
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### Indexes and Keys
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- `Indexes.h/cpp` — `keylet::*` factories with `LedgerNameSpace` tagged hashing; `keylet::quality()` embeds 64-bit quality in last 8 bytes (big-endian)
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- `Keylet.h/cpp` — type-tagged `(uint256, LedgerEntryType)`; `ltANY` wildcard, `ltCHILD` rejects directories
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- NFT pages: composite keys (high 160 = owner AccountID, low 96 = token range); `nft::pageMask` is the boundary
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### Validation Helpers (return NotTEC, preflight-time)
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- `AMMCore.h/cpp` — `invalidAMMAsset`, `invalidAMMAssetPair`, `invalidAMMAmount`; `ammLPTCurrency()` uses canonical `std::minmax`
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- `Permissions.h/cpp` — singleton; `isDelegable()` checks granular vs transaction-level (`<UINT16_MAX` boundary), amendment, delegable flag
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### RPC/JSON Boundary
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- `STParsedJSON.h/cpp` — depth cap 64; field-path-qualified errors via `make_name`; recognizes `"Payment"`, `"tesSUCCESS"`, etc.
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- `ErrorCodes.h/cpp` — append-only enum; `sortedErrorInfos` validated at compile time
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- `MultiApiJson.h` — per-API-version `Json::Value` array; composes with `forAllApiVersions` from `ApiVersion.h`
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- `jss.h` — every JSON key as `Json::StaticString` via `JSS(name)` macro; PascalCase = protocol fields, snake_case = RPC
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### Specialized Types
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- `STIssue`, `STAccount` (160-bit, VL-encoded), `STBitString<Bits>`, `STInteger<T>`, `STBlob`, `STArray`, `STVector256`, `STCurrency`, `STPathSet`, `STXChainBridge`, `STNumber` (asset-contextual)
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- `Quality.h/cpp` — inverted encoding (lower uint64 = higher quality); `ceil_in`/`ceil_out` proportional scaling; `_strict` variants honor Number rounding mode
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- `QualityFunction.h/cpp` — linear `q(out)=m*out+b`; AMMTag (slope from pool) vs CLOBLikeTag (m=0); `combine()` for multi-step strands
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- `XChainAttestations.h/cpp` — Attestations:: namespace (full, with signature) vs xrpl:: (stored); `match()` returns three-state `AttestationMatch`
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### Pseudo-Account Fields (sMD_PseudoAccount)
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- `sfAMMID`, `sfVaultID`, `sfLoanBrokerID` — 256-bit hash representing a synthesized account address
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## Numeric Encoding Reference
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```
|
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IOU canonical: mantissa ∈ [10^15, 10^16), exponent ∈ [-96, +80]
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||||
zero = (mantissa=0, exponent=-100) — sorts below smallest positive
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||||
XRP max: cMaxNativeN = 10^17 drops (100 billion XRP)
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MPT max: maxMPTokenAmount = INT64_MAX = 0x7FFFFFFFFFFFFFFF
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||||
Transfer rate: Rate{value} where value/1_000_000_000 = 1.0 (parityRate = 1:1)
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||||
NFT transfer fee: uint16 basis points (0–50000), convert via nft::transferFeeAsRate (×10000)
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```
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## Protocol-Stable Constants (NEVER CHANGE)
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- `LedgerEntryType` numeric values (in ledger objects)
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- `TxType` numeric values (in signed transactions)
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- `SerializedTypeID` and `SField` codes (in serialized fields)
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- `LedgerNameSpace` discriminator characters (in keylet derivation)
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- `HashPrefix` enum values (in signature/hash domain separation)
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- `error_code_i` numeric values (clients depend on them; append-only)
|
||||
- `FLAG_LEDGER_INTERVAL = 256` (drives consensus timing)
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- `INITIAL_XRP = 100B × 10^6 drops` (validated by `static_assert` against `Number::maxRep`)
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- NFT taxon LCG constants (`384160001 * seq + 2459`)
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- All flag bit values (`tf*`, `lsf*`, `asf*`)
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||||
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||||
Changing any requires an amendment with explicit detection logic for old/new behavior.
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@@ -1,64 +0,0 @@
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# Protocol and Serialization
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Macro-driven system for defining features, transactions, ledger entries, and serialized fields. Canonical binary format is required for signatures and consensus.
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## Key Invariants
|
||||
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||||
- Fields are sorted by (type code, field code) for canonical serialization; sorting by Field ID bytes produces WRONG results
|
||||
- Field ID encoding: 1-3 bytes depending on type/field code values (both < 16 = 1 byte)
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||||
- Signing hash prefix: `0x53545800` for single-signing, `0x534D5400` for multi-signing
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||||
- `STObject[sfFoo]` returns value or default; `STObject[~sfFoo]` returns optional (nothing if absent)
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||||
- All ST types inherit from `STBase`; `STVar` provides type-erased storage with stack/heap allocation
|
||||
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||||
## Macro System
|
||||
|
||||
- `XRPL_FEATURE(name, supported, vote)` / `XRPL_FIX` / `XRPL_RETIRE` in `features.macro`
|
||||
- `TRANSACTION(tag, value, class, delegation, fields)` in `transactions.macro`
|
||||
- `LEDGER_ENTRY(type, code, class, name, fields)` in `ledger_entries.macro`
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||||
- `TYPED_SFIELD(name, TYPE, code)` in `sfields.macro`
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||||
- Adding any new definition requires updating the count in the corresponding header
|
||||
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||||
## Serialization Format
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||||
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||||
- XRP Amount: 8 bytes, MSB=0, next bit=1 for positive, remaining 62 bits = value
|
||||
- Token Amount: 8 bytes mantissa/exponent + 20 bytes currency + 20 bytes issuer
|
||||
- AccountID: 20 bytes, length-prefixed when top-level
|
||||
- STArray: elements between start (`0xf0`) and end (`0xf1`) markers
|
||||
- STObject: fields in canonical order between start (`0xe0`) and end (`0xe1`) markers
|
||||
- Length prefixing: 1 byte (0-192), 2 bytes (193-12480), 3 bytes (12481-918744)
|
||||
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||||
## Common Bug Patterns
|
||||
|
||||
- Adding a field to `transactions.macro` without adding it to `sfields.macro` causes silent serialization failures
|
||||
- Forgetting to increment `numFeatures` after adding to `features.macro` causes out-of-bounds access
|
||||
- Non-canonical field ordering in hand-built binary blobs causes signature verification failures
|
||||
- `soeMPTSupported` flag on amount fields enables MPT token support; omitting it silently rejects MPT payments
|
||||
|
||||
## Key Patterns
|
||||
|
||||
### Amendment Registration
|
||||
```cpp
|
||||
// In features.macro — REQUIRED format:
|
||||
XRPL_FEATURE(MyNewFeature, Supported::yes, VoteBehavior::DefaultNo)
|
||||
XRPL_FIX (MyBugFix, Supported::yes, VoteBehavior::DefaultNo)
|
||||
// MUST also increment numFeatures in Feature.h — omitting causes OOB access
|
||||
```
|
||||
|
||||
### STObject Field Access
|
||||
```cpp
|
||||
// Safe: optional access — returns std::optional, never throws
|
||||
if (auto const val = tx[~sfAmount])
|
||||
use(*val);
|
||||
|
||||
// Throws if field is absent — only safe when preflight guarantees presence
|
||||
auto const amount = tx[sfAmount];
|
||||
```
|
||||
|
||||
## Key Files
|
||||
|
||||
- `include/xrpl/protocol/detail/features.macro` - amendment definitions
|
||||
- `include/xrpl/protocol/detail/transactions.macro` - transaction types
|
||||
- `include/xrpl/protocol/detail/ledger_entries.macro` - ledger objects
|
||||
- `include/xrpl/protocol/detail/sfields.macro` - field definitions
|
||||
- `include/xrpl/protocol/Feature.h` - `numFeatures` constant
|
||||
- `src/libxrpl/protocol/STObject.cpp` - core serialized object
|
||||
Reference in New Issue
Block a user