h2: HTTP/2 Frames and HPACK🔗ℹ

1 Frames🔗ℹ

Every frame is a frame? — length, type, flags, stream id, payload. The nine-byte header is decoded with bytes->frame; building helpers exist for the payloads that matter early: SETTINGS, PING, GOAWAY, WINDOW_UPDATE, RST_STREAM, and HEADERS with padding.

struct (struct frame (length type flags stream-id payload))

length : exact-nonnegative-integer?

type : exact-nonnegative-integer?

flags : exact-nonnegative-integer?

stream-id : exact-nonnegative-integer?

payload : bytes?

procedure (frame->bytes f) → bytes?

f : frame?

procedure (bytes->frame bstr pos)   →   frame?   exact-nonnegative-integer?

bstr : bytes?

pos : exact-nonnegative-integer?

Decode one frame at pos; the caller buffers at least length+9 bytes. Stream ids keep 31 bits (the reserved bit reads 0).

procedure (frames->bytes fs) → bytes?

fs : (listof frame?)

value connection-preface : bytes?

The fixed client preface PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n (§3.5).

Frame type constants:

Error codes:

procedure (make-settings-frame pairs) → frame?

pairs : (listof (cons/c exact-nonnegative-integer? exact-nonnegative-integer?))

procedure (settings-payload->list payload)

→ (listof (cons/c exact-nonnegative-integer? exact-nonnegative-integer?))

payload : bytes?

procedure (make-ping-frame ack? opaque) → frame?

ack? : boolean?

opaque : exact-nonnegative-integer?

procedure (make-goaway-frame   last-stream-id             error-code             debug-bytes)   →   frame?

last-stream-id : exact-nonnegative-integer?

error-code : exact-nonnegative-integer?

debug-bytes : bytes?

GOAWAY frames are always stream 0; last-stream-id rides in the payload.

procedure (make-window-update-frame   stream-id             increment)   →   frame?

stream-id : exact-nonnegative-integer?

increment : exact-nonnegative-integer?

procedure (make-rst-stream-frame   stream-id             error-code)   →   frame?

stream-id : exact-nonnegative-integer?

error-code : exact-nonnegative-integer?

procedure (make-headers-frame   stream-id             block            [ #:end-stream end-stream             #:pad-length pad-length])   →   frame?

stream-id : exact-nonnegative-integer?

block : bytes?

end-stream : boolean? = #t

pad-length : (or/c exact-nonnegative-integer? #f) = #f

procedure (headers-block->bytes payload) → bytes?

payload : bytes?

Strips HPACK-unrelated padding from a PADDED HEADERS/DATA payload — call it only when the PADDED flag is set.

2 HPACK🔗ℹ

procedure (make-hpack-ctx max-size) → hpack-ctx?

max-size : exact-nonnegative-integer?

procedure (hpack-ctx? v) → boolean?

v : any/c

procedure (hpack-ctx-size ctx) → exact-nonnegative-integer?

ctx : hpack-ctx?

The dynamic-table context (in octets, per §4.1’s 32-byte entry overhead). Decoder contexts track inserts, evictions, and size updates; encoder contexts may stay fresh — the built-in encoder uses only static-table indexing and literals without indexing, which is always legal.

procedure (hpack-decode-headers ctx bstr)

→ (listof (cons/c string? string?))

ctx : hpack-ctx?

bstr : bytes?

Full §6: indexed, literal with/without/never indexing, dynamic table size updates. Invalid data (including an indexed field with index 0) raises.

procedure (hpack-encode-headers ctx headers) → bytes?

ctx : hpack-ctx?

headers : (listof (cons/c string? string?))

Exact static-table matches become one indexed byte (e.g. (:method . GET) → 130, same shape as the RFC’s own examples); everything else goes out as literal without indexing, Huffman-coded when the table actually makes it shorter (ASCII only).

procedure (hpack-encode-integer value prefix-bits) → bytes?

value : exact-nonnegative-integer?

prefix-bits : exact-integer?

procedure (hpack-decode-integer bstr pos prefix-bits)

→   exact-nonnegative-integer? exact-nonnegative-integer?

bstr : bytes?

pos : exact-nonnegative-integer?

prefix-bits : exact-integer?

§5.1; the prefix’s high bits are the caller’s opcode bits.

§5.2 string literals with the Huffman flag.

3 Huffman🔗ℹ

procedure (huffman-encode s) → bytes?

s : string?

RFC 7541 Appendix B coding, padded with 1 bits (never a whole byte).

procedure (huffman-decode bstr) → string?

bstr : bytes?

Rejects padding that is not a prefix of EOS, mid-stream EOS, and invalid codes.

procedure (huffman-encoded-length-bits s) → exact-nonnegative-integer?

s : string?

For size decisions before encoding.

4 Status and limits🔗ℹ

Frames + HPACK are complete and golden-vector tested against RFC 7541’s appendix C. The connection layer — preface exchange, stream state machine, flow control, and a working h2c client — is the next milestone; until it lands, this package is the protocol core other code can build on, tested the same way the RFC tests itself.