18. Transport Parameter Encoding

The extension_data field of the quic_transport_parameters extension defined in [QUIC-TLS] contains the QUIC transport parameters. They are encoded as a sequence of transport parameters, as shown in Figure 20:

Transport Parameters {
  Transport Parameter (..),
  ...
}

Figure 20: Sequence of Transport Parameters

Each transport parameter is encoded as an (identifier, length, value) tuple, as shown in Figure 21:

Transport Parameter {
  Transport Parameter ID (i),
  Transport Parameter Length (i),
  Transport Parameter Value (..),
}

Figure 21: Transport Parameter Encoding

The Transport Parameter Length field contains the length of the Transport Parameter Value field in bytes.

QUIC encodes transport parameters into a sequence of bytes, which is then included in the cryptographic handshake.

18.1 Reserved Transport Parameters

Transport parameters with an identifier of the form 31 * N + 27 for integer values of N are reserved to exercise the requirement that unknown transport parameters be ignored. These transport parameters have no semantics and can carry arbitrary values.

18.2 Transport Parameter Definitions

This section details the transport parameters defined in this document.

original_destination_connection_id (0x00): This parameter is the value of the Destination Connection ID field from the first Initial packet sent by the client.

max_idle_timeout (0x01): The maximum idle timeout is a value in milliseconds that is encoded as an integer. If this value is absent or zero, the idle timeout is disabled.

stateless_reset_token (0x02): A stateless reset token is used in verifying a stateless reset; see Section 10.3.

max_udp_payload_size (0x03): The maximum UDP payload size parameter is an integer value that limits the size of UDP payloads that the endpoint is willing to receive.

initial_max_data (0x04): The initial maximum data parameter is an integer value that contains the initial value for the maximum amount of data that can be sent on the connection.

initial_max_stream_data_bidi_local (0x05): This parameter is an integer value specifying the initial flow control limit for locally-initiated bidirectional streams.

initial_max_stream_data_bidi_remote (0x06): This parameter is an integer value specifying the initial flow control limit for peer-initiated bidirectional streams.

initial_max_stream_data_uni (0x07): This parameter is an integer value specifying the initial flow control limit for unidirectional streams.

initial_max_streams_bidi (0x08): The initial maximum bidirectional streams parameter is an integer value that contains the initial maximum number of bidirectional streams the endpoint that receives this transport parameter is permitted to initiate.

initial_max_streams_uni (0x09): The initial maximum unidirectional streams parameter is an integer value that contains the initial maximum number of unidirectional streams the endpoint that receives this transport parameter is permitted to initiate.

ack_delay_exponent (0x0a): The acknowledgment delay exponent is an integer value indicating an exponent used to decode the ACK Delay field in the ACK frame (Section 19.3). If this value is absent, a default value of 3 is assumed.

max_ack_delay (0x0b): The maximum acknowledgment delay is an integer value indicating the maximum amount of time in milliseconds by which the endpoint will delay sending acknowledgments. If this value is absent, a default of 25 milliseconds is assumed.

disable_active_migration (0x0c): The disable active migration transport parameter is included if the endpoint does not support active connection migration (Section 9) on the address being used during the handshake.

preferred_address (0x0d): The server's preferred address is used to effect a change in server address at the end of the handshake, as described in Section 9.6.

active_connection_id_limit (0x0e): The active connection ID limit is an integer value specifying the maximum number of connection IDs from the peer that an endpoint is willing to store.

initial_source_connection_id (0x0f): This is the value that the endpoint included in the Source Connection ID field of the first Initial packet it sends for the connection.

retry_source_connection_id (0x10): This is the value that the server included in the Source Connection ID field of a Retry packet.