more specs

2026-07-21 09:02:05 +00:00 · 2026-03-11 07:32:57 +00:00
parent 0bba2efc45
commit ea2a62ab7e
3 changed files with 609 additions and 7 deletions
@@ -1,7 +1,84 @@
 # Compression

-> Compression support for SimpleX protocols.
+> Zstd compression for SimpleX protocol messages.

-## Zstd
+**Source file**: [`Compression.hs`](../src/Simplex/Messaging/Compression.hs)
+
+## Overview
+
+Optional Zstd compression for SMP message bodies. Short messages bypass compression entirely to avoid overhead. The `Compressed` type carries a tag byte indicating whether the payload is compressed or passthrough, making it self-describing on the wire.
+
+## Types
+
+### `Compressed`
+
+**Source**: `Compression.hs:17-22`
+
+```haskell
+data Compressed
+  = Passthrough ByteString   -- short messages, left intact
+  | Compressed Large          -- Zstd-compressed, 2-byte length prefix
+```
+
+Wire encoding (`Compression.hs:30-38`):
+
+```
+Passthrough → '0' ++ smpEncode ByteString  (1-byte tag + 1-byte length + data)
+Compressed  → '1' ++ smpEncode Large       (1-byte tag + 2-byte length + data)
+```
+
+Tags are `'0'` (0x30) and `'1'` (0x31) — same ASCII convention as `Maybe` encoding.
+
+`Passthrough` uses standard `ByteString` encoding (max 255 bytes, 1-byte length prefix). `Compressed` uses `Large` encoding (max 65535 bytes, 2-byte Word16 length prefix), since compressed output can exceed 255 bytes for larger inputs.
+
+## Constants
+
+| Constant | Value | Purpose | Source |
+|----------|-------|---------|--------|
+| `maxLengthPassthrough` | 180 | Messages at or below this length are not compressed | `Compression.hs:24-25` |
+| `compressionLevel` | 3 | Zstd compression level | `Compression.hs:27-28` |
+
+The 180-byte threshold was "sampled from real client data" — messages above this length show rapidly increasing compression ratio. Below 180 bytes, compression overhead (FFI call, dictionary-less Zstd startup) outweighs savings.

 ## Functions
+
+### `compress1`
+
+**Source**: `Compression.hs:40-43`
+
+```haskell
+compress1 :: ByteString -> Compressed
+```
+
+Compress a message body:
+- If `B.length bs <= 180` → `Passthrough bs`
+- Otherwise → `Compressed (Large (Z1.compress 3 bs))`
+
+No context or dictionary — each message is independently compressed ("1" in `compress1` refers to single-shot compression).
+
+### `decompress1`
+
+**Source**: `Compression.hs:45-53`
+
+```haskell
+decompress1 :: Int -> Compressed -> Either String ByteString
+```
+
+Decompress with size limit:
+- `Passthrough bs` → `Right bs` (no check needed — already bounded by encoding)
+- `Compressed (Large bs)` → check `Z1.decompressedSize bs`:
+  - If size is known and within `limit` → decompress
+  - If size unknown or exceeds `limit` → `Left` error
+
+The size limit check happens **before** decompression, using Zstd's frame header (which includes the decompressed size when the compressor wrote it). This prevents decompression bombs — an attacker cannot cause unbounded memory allocation by sending a small compressed payload that expands to gigabytes.
+
+The `Z1.decompress` result is pattern-matched for three cases:
+- `Z1.Error e` → `Left e`
+- `Z1.Skip` → `Right mempty` (zero-length output)
+- `Z1.Decompress bs'` → `Right bs'`
+
+## Security notes
+
+- **Decompression bomb protection**: `decompress1` requires an explicit size limit and checks `decompressedSize` before allocating. Callers must pass an appropriate limit (typically the SMP block size).
+- **No dictionary/context**: Each message is independently compressed. No shared state between messages that could leak information across compression boundaries.
+- **Passthrough for short messages**: Messages ≤ 180 bytes are never compressed, avoiding timing side channels from compression ratio differences on short, potentially-predictable messages.
@@ -2,10 +2,345 @@

 > Binary and string encoding used across all SimpleX protocols.

-## Binary Encoding
+**Source files**: [`Encoding.hs`](../src/Simplex/Messaging/Encoding.hs), [`Encoding/String.hs`](../src/Simplex/Messaging/Encoding/String.hs), [`Parsers.hs`](../src/Simplex/Messaging/Parsers.hs)

-## String Encoding
+## Overview
+
+Two encoding layers serve different purposes:
+
+- **`Encoding`** — Binary wire format for SMP protocol transmissions. Compact, no delimiters between fields. Used in all on-the-wire protocol messages.
+- **`StrEncoding`** — Human-readable string format for configuration, URIs, logs, and JSON serialization. Uses base64url for binary data, decimal for numbers, comma-separated lists, space-separated tuples.
+
+Both are typeclasses with `MINIMAL` pragmas requiring `encode` + (`decode` | `parser`), with the missing one derived from the other.
+
+## Binary Encoding (`Encoding` class)
+
+**Source**: `Encoding.hs:38-52`
+
+```haskell
+class Encoding a where
+  smpEncode :: a -> ByteString
+  smpDecode :: ByteString -> Either String a  -- default: parseAll smpP
+  smpP :: Parser a                            -- default: smpDecode <$?> smpP
+```
+
+### Length-prefix conventions
+
+| Type | Prefix | Max size | Source |
+|------|--------|----------|--------|
+| `ByteString` | 1-byte length (Word8 as Char) | 255 bytes | `Encoding.hs:102-106` |
+| `Large` (newtype) | 2-byte length (Word16 big-endian) | 65535 bytes | `Encoding.hs:135-143` |
+| `Tail` (newtype) | None — consumes rest of input | Unlimited | `Encoding.hs:126-132` |
+| Lists (`smpEncodeList`) | 1-byte count prefix, then concatenated items | 255 items | `Encoding.hs:155-159` |
+| `NonEmpty` | Same as list (fails on count=0) | 255 items | `Encoding.hs:173-178` |
+
+### Scalar types
+
+| Type | Encoding | Bytes | Source |
+|------|----------|-------|--------|
+| `Char` | Raw byte | 1 | `Encoding.hs:54-58` |
+| `Bool` | `'T'` / `'F'` (0x54 / 0x46) | 1 | `Encoding.hs:60-70` |
+| `Word16` | Big-endian | 2 | `Encoding.hs:72-76` |
+| `Word32` | Big-endian | 4 | `Encoding.hs:78-82` |
+| `Int64` | Two big-endian Word32s (high then low) | 8 | `Encoding.hs:84-99` |
+| `SystemTime` | `systemSeconds` as Int64 (nanoseconds dropped) | 8 | `Encoding.hs:145-149` |
+| `Text` | UTF-8 then ByteString encoding (1-byte length prefix) | 1 + len | `Encoding.hs:161-165` |
+| `String` | `B.pack` then ByteString encoding | 1 + len | `Encoding.hs:167-171` |
+
+### `Maybe a`
+
+**Source**: `Encoding.hs:116-124`
+
+```
+Nothing → '0' (0x30)
+Just x  → '1' (0x31) ++ smpEncode x
+```
+
+Tags are ASCII characters `'0'`/`'1'`, not binary 0x00/0x01.
+
+### Tuples
+
+**Source**: `Encoding.hs:180-220`
+
+Tuples (2 through 8) encode as simple concatenation — no length prefix, no separator. Fields are parsed sequentially using each component's `smpP`. This works because each component's parser knows how many bytes to consume (via its own length prefix or fixed size).
+
+### Combinators
+
+| Function | Signature | Purpose | Source |
+|----------|-----------|---------|--------|
+| `_smpP` | `Parser a` | Space-prefixed parser (`A.space *> smpP`) | `Encoding.hs:151-152` |
+| `smpEncodeList` | `[a] -> ByteString` | 1-byte count + concatenated items | `Encoding.hs:155-156` |
+| `smpListP` | `Parser [a]` | Parse count then that many items | `Encoding.hs:158-159` |
+| `lenEncode` | `Int -> Char` | Int to single-byte length char | `Encoding.hs:108-110` |
+
+## String Encoding (`StrEncoding` class)
+
+**Source**: `Encoding/String.hs:56-67`
+
+```haskell
+class StrEncoding a where
+  strEncode :: a -> ByteString
+  strDecode :: ByteString -> Either String a  -- default: parseAll strP
+  strP :: Parser a                            -- default: strDecode <$?> base64urlP
+```
+
+Key difference from `Encoding`: the default `strP` parses base64url input first, then applies `strDecode`. This means types that only implement `strDecode` will automatically accept base64url-encoded input.
+
+### Instance conventions
+
+| Type | Encoding | Source |
+|------|----------|--------|
+| `ByteString` | base64url (non-empty required) | `String.hs:70-76` |
+| `Word16`, `Word32` | Decimal string | `String.hs:114-124` |
+| `Int`, `Int64` | Signed decimal | `String.hs:138-148` |
+| `Char`, `Bool` | Delegates to `Encoding` (`smpEncode`/`smpP`) | `String.hs:126-136` |
+| `Maybe a` | Empty string = `Nothing`, otherwise `strEncode a` | `String.hs:108-112` |
+| `Text` | UTF-8 bytes, parsed until space/newline | `String.hs:97-99` |
+| `SystemTime` | `systemSeconds` as Int64 (decimal) | `String.hs:150-152` |
+| `UTCTime` | ISO 8601 string | `String.hs:154-156` |
+| `CertificateChain` | Comma-separated base64url blobs | `String.hs:158-162` |
+| `Fingerprint` | base64url of fingerprint bytes | `String.hs:164-168` |
+
+### Collection encoding
+
+| Type | Separator | Source |
+|------|-----------|--------|
+| Lists (`strEncodeList`) | Comma `,` | `String.hs:171-175` |
+| `NonEmpty` | Comma (fails on empty) | `String.hs:178-180` |
+| `Set a` | Comma | `String.hs:182-184` |
+| `IntSet` | Comma | `String.hs:186-188` |
+| Tuples (2-6) | Space (` `) | `String.hs:193-221` |
+
+### `Str` newtype
+
+**Source**: `String.hs:84-89`
+
+Raw string (not base64url-encoded). Parses until space, consumes trailing space. Used for string-valued protocol fields that should not be base64-encoded.
+
+### `TextEncoding` class
+
+**Source**: `String.hs:51-53`
+
+```haskell
+class TextEncoding a where
+  textEncode :: a -> Text
+  textDecode :: Text -> Maybe a
+```
+
+Separate from `StrEncoding` — operates on `Text` rather than `ByteString`. Used for types that need Text representation (e.g., enum display names).
+
+### JSON bridge functions
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `strToJSON` | `StrEncoding a => a -> J.Value` via `decodeLatin1 . strEncode` | `String.hs:229-231` |
+| `strToJEncoding` | Same, for Aeson encoding | `String.hs:233-235` |
+| `strParseJSON` | `StrEncoding a => String -> J.Value -> JT.Parser a` — parse JSON string via `strP` | `String.hs:237-238` |
+| `textToJSON` | `TextEncoding a => a -> J.Value` | `String.hs:240-242` |
+| `textToEncoding` | Same, for Aeson encoding | `String.hs:244-246` |
+| `textParseJSON` | `TextEncoding a => String -> J.Value -> JT.Parser a` | `String.hs:248-249` |

 ## Parsers

-## Functions
+**Source**: [`Parsers.hs`](../src/Simplex/Messaging/Parsers.hs)
+
+### Core parsing functions
+
+| Function | Signature | Purpose | Source |
+|----------|-----------|---------|--------|
+| `parseAll` | `Parser a -> ByteString -> Either String a` | Parse consuming all input (fails if bytes remain) | `Parsers.hs:64-65` |
+| `parse` | `Parser a -> e -> ByteString -> Either e a` | `parseAll` with custom error type (discards error string) | `Parsers.hs:61-62` |
+| `parseE` | `(String -> e) -> Parser a -> ByteString -> ExceptT e IO a` | `parseAll` lifted into `ExceptT` | `Parsers.hs:67-68` |
+| `parseE'` | `(String -> e) -> Parser a -> ByteString -> ExceptT e IO a` | Like `parseE` but allows trailing input | `Parsers.hs:70-71` |
+| `parseRead1` | `Read a => Parser a` | Parse a word then `readMaybe` it | `Parsers.hs:76-77` |
+| `parseString` | `(ByteString -> Either String a) -> String -> a` | Parse from `String` (errors with `error`) | `Parsers.hs:89-90` |
+
+### `base64P`
+
+**Source**: `Parsers.hs:44-53`
+
+Standard base64 parser (not base64url — uses `+`/`/` alphabet). Takes alphanumeric + `+`/`/` characters, optional `=` padding, then decodes. Contrast with `base64urlP` in `Encoding/String.hs` which uses `-`/`_` alphabet.
+
+### JSON options helpers
+
+Platform-conditional JSON encoding for cross-platform compatibility (Haskell ↔ Swift).
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `enumJSON` | All-nullary constructors as strings, with tag modifier | `Parsers.hs:101-106` |
+| `sumTypeJSON` | Platform-conditional: `taggedObjectJSON` on non-Darwin, `singleFieldJSON` on Darwin | `Parsers.hs:109-114` |
+| `taggedObjectJSON` | `{"type": "Tag", "data": {...}}` format | `Parsers.hs:119-128` |
+| `singleFieldJSON` | `{"Tag": value}` format | `Parsers.hs:137-149` |
+| `defaultJSON` | Default options with `omitNothingFields = True` | `Parsers.hs:151-152` |
+
+Pattern synonyms for JSON field names:
+- `TaggedObjectJSONTag = "type"` (`Parsers.hs:131`)
+- `TaggedObjectJSONData = "data"` (`Parsers.hs:134`)
+- `SingleFieldJSONTag = "_owsf"` (`Parsers.hs:117`)
+
+### String helpers
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `fstToLower` | Lowercase first character | `Parsers.hs:92-94` |
+| `dropPrefix` | Remove prefix string, lowercase remainder | `Parsers.hs:96-99` |
+| `textP` | Parse rest of input as UTF-8 `String` | `Parsers.hs:154-155` |
+
+## Auxiliary Types and Utilities
+
+### TMap
+
+**Source**: [`TMap.hs`](../src/Simplex/Messaging/TMap.hs)
+
+```haskell
+type TMap k a = TVar (Map k a)
+```
+
+STM-based concurrent map. Wraps `Data.Map.Strict` in a `TVar`. All mutations use `modifyTVar'` (strict) to prevent thunk accumulation.
+
+| Function | Notes | Source |
+|----------|-------|--------|
+| `emptyIO` | IO allocation (`newTVarIO`) | `TMap.hs:32-34` |
+| `singleton` | STM allocation | `TMap.hs:36-38` |
+| `clear` | Reset to empty | `TMap.hs:40-42` |
+| `lookup` / `lookupIO` | STM / non-transactional IO read | `TMap.hs:48-54` |
+| `member` / `memberIO` | STM / non-transactional IO membership | `TMap.hs:56-62` |
+| `insert` / `insertM` | Insert value / insert from STM action | `TMap.hs:64-70` |
+| `delete` | Remove key | `TMap.hs:72-74` |
+| `lookupInsert` | Atomic lookup-then-insert (returns old value) | `TMap.hs:76-78` |
+| `lookupDelete` | Atomic lookup-then-delete | `TMap.hs:80-82` |
+| `adjust` / `update` / `alter` / `alterF` | Standard Map operations lifted to STM | `TMap.hs:84-100` |
+| `union` | Merge `Map` into `TMap` | `TMap.hs:102-104` |
+
+`lookupIO`/`memberIO` use `readTVarIO` — single-read outside STM transaction, useful when you need a snapshot without composing with other STM operations.
+
+### SessionVar
+
+**Source**: [`Session.hs`](../src/Simplex/Messaging/Session.hs)
+
+Race-safe session management using TMVar + monotonic ID.
+
+```haskell
+data SessionVar a = SessionVar
+  { sessionVar   :: TMVar a    -- result slot
+  , sessionVarId :: Int        -- monotonic ID from TVar counter
+  , sessionVarTs :: UTCTime    -- creation timestamp
+  }
+```
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `getSessVar` | Lookup or create session. Returns `Left new` or `Right existing` | `Session.hs:24-33` |
+| `removeSessVar` | Delete session only if ID matches (prevents removing a replacement) | `Session.hs:35-39` |
+| `tryReadSessVar` | Non-blocking read of session result | `Session.hs:41-42` |
+
+The ID-match check in `removeSessVar` (`sessionVarId v == sessionVarId v'`) prevents a race where:
+1. Thread A creates session #5, starts work
+2. Thread B creates session #6 (replacing #5 in TMap)
+3. Thread A finishes, tries to remove — ID mismatch, removal blocked
+
+### ServiceScheme
+
+**Source**: [`ServiceScheme.hs`](../src/Simplex/Messaging/ServiceScheme.hs)
+
+```haskell
+data ServiceScheme = SSSimplex | SSAppServer SrvLoc
+data SrvLoc = SrvLoc HostName ServiceName
+```
+
+URI scheme for SimpleX service addresses. `SSSimplex` encodes as `"simplex:"`, `SSAppServer` as `"https://host:port"`.
+
+`simplexChat :: ServiceScheme` is the constant `SSAppServer (SrvLoc "simplex.chat" "")` (`ServiceScheme.hs:38-39`).
+
+### SystemTime
+
+**Source**: [`SystemTime.hs`](../src/Simplex/Messaging/SystemTime.hs)
+
+```haskell
+newtype RoundedSystemTime (t :: Nat) = RoundedSystemTime { roundedSeconds :: Int64 }
+type SystemDate = RoundedSystemTime 86400    -- day precision
+type SystemSeconds = RoundedSystemTime 1     -- second precision
+```
+
+Phantom-typed time rounding. The `Nat` type parameter specifies rounding granularity in seconds.
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `getRoundedSystemTime` | Get current time rounded to `t` seconds | `SystemTime.hs:40-43` |
+| `getSystemDate` | Alias for day-rounded time | `SystemTime.hs:45-47` |
+| `getSystemSeconds` | Second-precision (no rounding needed, just drops nanoseconds) | `SystemTime.hs:49-51` |
+| `roundedToUTCTime` | Convert back to `UTCTime` | `SystemTime.hs:53-55` |
+
+`RoundedSystemTime` derives `FromField`/`ToField` for SQLite storage and `FromJSON`/`ToJSON` for API serialization.
+
+### Util
+
+**Source**: [`Util.hs`](../src/Simplex/Messaging/Util.hs)
+
+Selected utilities used across the codebase:
+
+**Monadic combinators**:
+
+| Function | Signature | Purpose | Source |
+|----------|-----------|---------|--------|
+| `<$?>` | `MonadFail m => (a -> Either String b) -> m a -> m b` | Lift fallible function into parser | `Util.hs:119-121` |
+| `$>>=` | `(Monad m, Monad f, Traversable f) => m (f a) -> (a -> m (f b)) -> m (f b)` | Monadic bind through nested monad | `Util.hs:165-167` |
+| `ifM` / `whenM` / `unlessM` | Monadic conditionals | `Util.hs:147-157` |
+| `anyM` | Short-circuit `any` for monadic predicates (strict) | `Util.hs:159-161` |
+
+**Error handling**:
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `tryAllErrors` | Catch all exceptions (including async) into `ExceptT` | `Util.hs:273-275` |
+| `catchAllErrors` | Same with handler | `Util.hs:281-283` |
+| `tryAllOwnErrors` | Catch only "own" exceptions (re-throws async cancellation) | `Util.hs:322-324` |
+| `catchAllOwnErrors` | Same with handler | `Util.hs:330-332` |
+| `isOwnException` | `StackOverflow`, `HeapOverflow`, `AllocationLimitExceeded` | `Util.hs:297-304` |
+| `isAsyncCancellation` | Any `SomeAsyncException` except own exceptions | `Util.hs:306-310` |
+| `catchThrow` | Catch exceptions, wrap in Left | `Util.hs:289-291` |
+| `allFinally` | `tryAllErrors` + `final` + `except` (like `finally` for ExceptT) | `Util.hs:293-295` |
+
+The own-vs-async distinction is critical: `catchOwn`/`tryAllOwnErrors` never swallow async cancellation (`ThreadKilled`, `UserInterrupt`, etc.), only synchronous exceptions and resource exhaustion (`StackOverflow`, `HeapOverflow`, `AllocationLimitExceeded`).
+
+**STM**:
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `tryWriteTBQueue` | Non-blocking bounded queue write, returns success | `Util.hs:256-261` |
+
+**Database result helpers**:
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `firstRow` | Extract first row with transform, or Left error | `Util.hs:346-347` |
+| `maybeFirstRow` | Extract first row as Maybe | `Util.hs:349-350` |
+| `firstRow'` | Like `firstRow` but transform can also fail | `Util.hs:355-356` |
+
+**Collection utilities**:
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `groupOn` | `groupBy` using equality on projected key | `Util.hs:358-359` |
+| `groupAllOn` | `groupOn` after `sortOn` (groups non-adjacent elements) | `Util.hs:372-373` |
+| `toChunks` | Split list into `NonEmpty` chunks of size n | `Util.hs:376-380` |
+| `packZipWith` | Optimized ByteString zipWith (direct memory access) | `Util.hs:236-254` |
+
+**Miscellaneous**:
+
+| Function | Purpose | Source |
+|----------|---------|--------|
+| `safeDecodeUtf8` | Decode UTF-8 replacing errors with `'?'` | `Util.hs:382-386` |
+| `bshow` / `tshow` | `show` to `ByteString` / `Text` | `Util.hs:123-129` |
+| `threadDelay'` | `Int64` delay (handles overflow by looping) | `Util.hs:391-399` |
+| `diffToMicroseconds` / `diffToMilliseconds` | `NominalDiffTime` conversion | `Util.hs:401-407` |
+| `labelMyThread` | Label current thread for debugging | `Util.hs:409-410` |
+| `encodeJSON` / `decodeJSON` | `ToJSON a => a -> Text` / `FromJSON a => Text -> Maybe a` | `Util.hs:415-421` |
+| `traverseWithKey_` | `Map` traversal discarding results | `Util.hs:423-425` |
+
+## Security notes
+
+- **Length prefix overflow**: `ByteString` encoding uses 1-byte length — silently truncates strings > 255 bytes. Callers must ensure size bounds before encoding. `Large` extends to 65535 bytes via Word16 prefix.
+- **`Tail` unbounded**: `Tail` consumes all remaining input with no size check. Only safe when total message size is already bounded (e.g., within a padded SMP block).
+- **base64 vs base64url**: `Parsers.base64P` uses standard alphabet (`+`/`/`), while `String.base64urlP` uses URL-safe alphabet (`-`/`_`). Mixing them causes silent decode failures.
+- **`safeDecodeUtf8`**: Replaces invalid UTF-8 with `'?'` rather than failing. Suitable for logging/display, not for security-critical string comparison.
@@ -2,8 +2,198 @@

 > Version ranges and compatibility checking for protocol evolution.

-## Version Ranges
+**Source files**: [`Version.hs`](../src/Simplex/Messaging/Version.hs), [`Version/Internal.hs`](../src/Simplex/Messaging/Version/Internal.hs)

-## Compatibility
+## Overview
+
+All SimpleX protocols use version negotiation during handshake. Each party advertises a `VersionRange` (min..max supported), and negotiation produces a `Compatible` proof value if the ranges overlap — choosing the highest mutually-supported version.
+
+The `Compatible` newtype can only be constructed internally (constructor is not exported), so the type system enforces that compatibility was actually checked.
+
+## Types
+
+### `Version v`
+
+**Source**: `Version/Internal.hs:11-12`
+
+```haskell
+newtype Version v = Version Word16
+```
+
+Phantom-typed version number. The phantom `v` distinguishes version spaces (e.g., SMP versions vs Agent versions vs XFTP versions) at the type level, preventing accidental comparison across protocols.
+
+- `Encoding`: 2 bytes big-endian (via Word16 instance)
+- `StrEncoding`: decimal string
+- JSON: numeric value
+- Derives: `Eq`, `Ord`, `Show`
+
+The constructor is exported from `Version.Internal` but not from `Version`, so application code cannot fabricate versions — they must come from protocol constants or parsing.
+
+### `VersionRange v`
+
+**Source**: `Version.hs:46-50`
+
+```haskell
+data VersionRange v = VRange
+  { minVersion :: Version v
+  , maxVersion :: Version v
+  }
+```
+
+Invariant: `minVersion <= maxVersion` (enforced by smart constructors).
+
+The `VRange` constructor is not exported — only the pattern synonym `VersionRange` (read-only, `Version.hs:41-44`) is public.
+
+- `Encoding`: two Word16s concatenated (4 bytes total, `Version.hs:80-84`)
+- `StrEncoding`: `"min-max"` or `"v"` if min == max (`Version.hs:86-93`)
+- JSON: `{"minVersion": n, "maxVersion": n}`
+
+### `VersionScope v`
+
+**Source**: `Version.hs:64`
+
+```haskell
+class VersionScope v
+```
+
+Empty typeclass used as a constraint on version operations. Each protocol declares its version scope:
+
+```haskell
+instance VersionScope SMP
+instance VersionScope Agent
+```
+
+This prevents accidentally mixing version ranges from different protocols in negotiation functions.
+
+### `Compatible a`
+
+**Source**: `Version.hs:117-122`
+
+```haskell
+newtype Compatible a = Compatible_ a
+
+pattern Compatible :: a -> Compatible a
+pattern Compatible a <- Compatible_ a
+```
+
+Proof that compatibility was checked. The `Compatible_` constructor is not exported — `Compatible` is a read-only pattern synonym. The only way to obtain a `Compatible` value is through `compatibleVersion`, `compatibleVRange`, `proveCompatible`, or the internal `mkCompatibleIf`.
+
+### `VersionI` / `VersionRangeI` type classes
+
+**Source**: `Version.hs:95-115`
+
+Multi-param typeclasses with functional dependencies for generic version/range operations. Allow extension types that wrap `Version` or `VersionRange` to participate in negotiation:
+
+```haskell
+class VersionScope v => VersionI v a | a -> v where
+  type VersionRangeT v a          -- associated type: range form
+  version :: a -> Version v
+  toVersionRangeT :: a -> VersionRange v -> VersionRangeT v a
+
+class VersionScope v => VersionRangeI v a | a -> v where
+  type VersionT v a               -- associated type: version form
+  versionRange :: a -> VersionRange v
+  toVersionRange :: a -> VersionRange v -> a
+  toVersionT :: a -> Version v -> VersionT v a
+```
+
+Identity instances exist for `Version v` and `VersionRange v` themselves.

 ## Functions
+
+### Construction
+
+| Function | Signature | Purpose | Source |
+|----------|-----------|---------|--------|
+| `mkVersionRange` | `Version v -> Version v -> VersionRange v` | Construct range, `error` if min > max | `Version.hs:67-70` |
+| `safeVersionRange` | `Version v -> Version v -> Maybe (VersionRange v)` | Safe construction, `Nothing` if invalid | `Version.hs:72-75` |
+| `versionToRange` | `Version v -> VersionRange v` | Singleton range (min == max) | `Version.hs:77-78` |
+
+### Compatibility checking
+
+#### `isCompatible`
+
+**Source**: `Version.hs:124-125`
+
+```haskell
+isCompatible :: VersionI v a => a -> VersionRange v -> Bool
+```
+
+Check if a single version falls within a range.
+
+#### `isCompatibleRange`
+
+**Source**: `Version.hs:127-130`
+
+```haskell
+isCompatibleRange :: VersionRangeI v a => a -> VersionRange v -> Bool
+```
+
+Check if two version ranges overlap: `min1 <= max2 && min2 <= max1`.
+
+#### `proveCompatible`
+
+**Source**: `Version.hs:132-133`
+
+```haskell
+proveCompatible :: VersionI v a => a -> VersionRange v -> Maybe (Compatible a)
+```
+
+If version is compatible, wrap in `Compatible` proof. Returns `Nothing` if out of range.
+
+### Negotiation
+
+#### `compatibleVersion`
+
+**Source**: `Version.hs:135-140`
+
+```haskell
+compatibleVersion :: VersionRangeI v a => a -> VersionRange v -> Maybe (Compatible (VersionT v a))
+```
+
+Negotiate a single version from two ranges. Returns `min(max1, max2)` — the highest mutually-supported version. Returns `Nothing` if ranges don't overlap.
+
+#### `compatibleVRange`
+
+**Source**: `Version.hs:143-148`
+
+```haskell
+compatibleVRange :: VersionRangeI v a => a -> VersionRange v -> Maybe (Compatible a)
+```
+
+Compute the intersection of two version ranges: `(max(min1,min2), min(max1,max2))`. Returns `Nothing` if the intersection is empty (i.e., ranges don't overlap).
+
+#### `compatibleVRange'`
+
+**Source**: `Version.hs:151-156`
+
+```haskell
+compatibleVRange' :: VersionRangeI v a => a -> Version v -> Maybe (Compatible a)
+```
+
+Cap a version range's maximum at a given version. Returns `Nothing` if the cap is below the range's minimum.
+
+## Protocol version constants
+
+Version constants for each protocol are defined in their respective Transport modules. For SMP, key gates include:
+
+- `currentSMPAgentVersion`, `supportedSMPAgentVRange` — current negotiation range
+- `serviceCertsSMPVersion = 16` — service certificate handshake
+- `rcvServiceSMPVersion = 19` — service subscription commands
+
+See [`transport.md`](transport.md) and [`rcv-services.md`](rcv-services.md) for protocol-specific version constants.
+
+## Negotiation protocol
+
+During handshake:
+1. Client sends its `VersionRange` to server
+2. Server computes `compatibleVRange clientRange serverRange`
+3. If `Nothing` → reject connection (incompatible)
+4. If `Just (Compatible agreedRange)` → use `maxVersion agreedRange` as the effective protocol version
+
+The `Compatible` proof flows through the connection setup, ensuring all subsequent version-gated code paths have evidence that negotiation occurred.
+
+## Security notes
+
+- **No downgrade attack protection in negotiation itself** — an active MITM could modify the version range to force a lower version. Protection comes from the TLS layer (authentication prevents MITM) and from servers setting minimum version floors.
+- **`mkVersionRange` uses `error`** — only safe for compile-time constants. Runtime construction must use `safeVersionRange`.