QUIC transport parameters — RFC 9000 §18

Transport parameters are exchanged inside the TLS 1.3 quic_transport_parameters extension (ID 57). Each parameter is:

Transport Parameter {
    Parameter ID (i),
    Parameter Length (i),     ← byte length of value
    Parameter Value (..),
}

Scalar parameters encode their value as a QUIC varint inside the value region; flag parameters have a zero-length value; complex parameters (preferred_address) carry a sub-structure.

Parameter catalogue

ID	Name	Type	Default	Carrier
0x00	original_destination_connection_id	CID (≤ 20 bytes)	absent	server only
0x01	max_idle_timeout	varint ms	0 (disabled)	both
0x02	stateless_reset_token	16 bytes	absent	server only
0x03	max_udp_payload_size	varint	65527	both (≥ 1200)
0x04	initial_max_data	varint	0	both
0x05	initial_max_stream_data_bidi_local	varint	0	both
0x06	initial_max_stream_data_bidi_remote	varint	0	both
0x07	initial_max_stream_data_uni	varint	0	both
0x08	initial_max_streams_bidi	varint	0	both (≤ 2⁶⁰)
0x09	initial_max_streams_uni	varint	0	both (≤ 2⁶⁰)
0x0A	ack_delay_exponent	varint u8	3	both (≤ 20)
0x0B	max_ack_delay	varint ms	25	both
0x0C	disable_active_migration	flag	absent	both
0x0D	preferred_address	struct	absent	server only
0x0E	active_connection_id_limit	varint	2	both (≥ 2)
0x0F	initial_source_connection_id	CID	required	both
0x10	retry_source_connection_id	CID	absent	server only after Retry
0x20	max_datagram_frame_size	varint	absent	both (RFC 9221)
0x2AB2	grease_quic_bit	flag	absent	both (RFC 9287)

The type is core.net.quic.transport_params.TransportParams:

public type TransportParams is {
    original_destination_connection_id: Maybe<ConnectionId>,
    max_idle_timeout_ms:                 UInt64,
    stateless_reset_token:               Maybe<[Byte; 16]>,
    max_udp_payload_size:                UInt64,
    initial_max_data:                    UInt64,
    initial_max_stream_data_bidi_local:  UInt64,
    initial_max_stream_data_bidi_remote: UInt64,
    initial_max_stream_data_uni:         UInt64,
    initial_max_streams_bidi:            UInt64,
    initial_max_streams_uni:             UInt64,
    ack_delay_exponent:                  UInt8,
    max_ack_delay_ms:                    UInt64,
    disable_active_migration:            Bool,
    preferred_address:                   Maybe<PreferredAddress>,
    active_connection_id_limit:          UInt64,
    initial_source_connection_id:        Maybe<ConnectionId>,
    retry_source_connection_id:          Maybe<ConnectionId>,
    max_datagram_frame_size:             Maybe<UInt64>,
    grease_quic_bit:                     Bool,
};

TransportParams.defaults() returns RFC 9000 §18.2 defaults.

Bounds theorem (V9)

core.net.quic.transport_params.TransportParams.bounds_ok() enforces the §18.2 constraints that MUST hold for every valid parameter set:

initial_max_streams_bidi     ≤ 2^60    (MAX_STREAMS_LIMIT)
initial_max_streams_uni      ≤ 2^60
ack_delay_exponent           ≤ 20      (MAX_ACK_DELAY_EXPONENT)
active_connection_id_limit   ≥ 2       (MIN_ACTIVE_CID_LIMIT)
max_udp_payload_size         ≥ 1200    (MIN_MAX_UDP_PAYLOAD_SIZE)

The call site treats a false return as fatal (TransportError.ProtocolViolation). The SMT layer discharges V9 at compile time (v9_transport_params_theorem).

Wire encoding

Each scalar parameter is three nested varints: id, length, value. Byte-exact example for a minimum server parameter block with max_idle_timeout=0 ms, max_udp_payload_size=1200 B, and initial_max_data=5:

0x01 0x01 0x00                ← max_idle_timeout = 0
0x03 0x02 0x44 0xB0           ← max_udp_payload_size = 1200
0x04 0x01 0x05                ← initial_max_data = 5

The 1200-byte max_udp_payload_size spills into a 2-byte varint: top 2 bits 01 (14-bit prefix) + 14-bit value 0x4B0 → wire bytes 0x44 0xB0 (0x4000 | 0x04B0).

Empty-body parameters (disable_active_migration, grease_quic_bit) emit [id_varint, 0x00]:

0x0C 0x00                     ← disable_active_migration present
0x6A 0xB2 0x00                ← grease_quic_bit (id 0x2AB2 → 2-byte varint)

The greased grease_quic_bit ID (0x2AB2) encodes as a 2-byte QUIC varint: top 2 bits 01 (14-bit prefix) plus value 0x2AB2 → wire bytes 0x6A 0xB2. See transport_params_encode_kat and rfc9000_transport_params_roundtrip.

Preferred address

Server-only parameter 0x0D carrying a migration hint:

PreferredAddress {
    ipv4_addr: [Byte; 4],
    ipv4_port: UInt16,
    ipv6_addr: [Byte; 16],
    ipv6_port: UInt16,
    connection_id: ConnectionId,            ← length byte + CID bytes
    stateless_reset_token: [Byte; 16],
}

Either IPv4 or IPv6 may be all-zero meaning "not offered". The CID length is a single byte followed by the CID bytes. See preferred_address_roundtrip.

API

mount core.net.quic.transport_params.{TransportParams, encode, decode};

let mut tp = TransportParams.defaults();
tp.max_udp_payload_size = 1200_u64;
tp.initial_max_data = 1_048_576_u64;
tp.initial_max_streams_bidi = 100_u64;

// Encode into scratch buffer.
let mut wire: List<Byte> = [];
encode(&tp, &mut wire);

// Decode + validate bounds (callers treat `!bounds_ok()` as fatal).
let parsed = decode(wire.as_slice()).unwrap();
assert(parsed.bounds_ok());

References

• vcs/specs/L2-standard/net/quic/transport_params_defaults.vr
• vcs/specs/L2-standard/net/quic/transport_params_bounds.vr
• vcs/specs/L2-standard/net/quic/transport_params_encode_kat.vr
• vcs/specs/L2-standard/net/quic/transport_params_optionals_roundtrip.vr
• vcs/specs/L2-standard/net/quic/rfc9000_transport_params_roundtrip.vr
• vcs/specs/L2-standard/net/quic/preferred_address_roundtrip.vr
• vcs/specs/L2-standard/net/quic/v9_transport_params_theorem.vr