See [STREAMING] for an overview of the Streaming Library.
The format of a single packet in the streaming protocol is:
+----+----+----+----+----+----+----+----+ | send Stream ID | rcv Stream ID | +----+----+----+----+----+----+----+----+ | sequence Num | ack Through | +----+----+----+----+----+----+----+----+ | nc | NACKs ... +----+----+----+----+----+----+----+----+ | rd | flags | opt size| opt data +----+----+----+----+----+----+----+----+ ... | +----+----+----+----+----+----+----+----+ | payload ... +----+----+----+-// sendStreamId :: 4 byte Integer Random number selected by the packet recipient before sending the first SYN reply packet and constant for the life of the connection. 0 in the SYN message sent by the connection originator, and in subsequent messages, until a SYN reply is received, containing the peer's stream ID. receiveStreamId :: 4 byte Integer Random number selected by the packet originator before sending the first SYN packet and constant for the life of the connection. May be 0 if unknown, for example in a RESET packet. sequenceNum :: 4 byte Integer The sequence for this message, starting at 0 in the SYN message, and incremented by 1 in each message except for plain ACKs and retransmissions. If the sequenceNum is 0 and the SYN flag is not set, this is a plain ACK packet that should not be ACKed. ackThrough :: 4 byte Integer The highest packet sequence number that was received on the $receiveStreamId. This field is ignored on the initial connection packet (where $receiveStreamId is the unknown id) or if the NO_ACK flag set. All packets up to and including this sequence number are ACKed, EXCEPT for those listed in NACKs below. NACK count :: 1 byte Integer The number of 4-byte NACKs in the next field NACKs :: $nc * 4 byte Integers Sequence numbers less than ackThrough that are not yet received. Two NACKs of a packet is a request for a 'fast retransmit' of that packet. resendDelay :: 1 byte Integer How long is the creator of this packet going to wait before resending this packet (if it hasn't yet been ACKed). The value is seconds since the packet was created. Currently ignored on receive. flags :: 2 byte value See below. option size :: 2 byte Integer The number of bytes in the next field option data :: 0 or more bytes As specified by the flags. See below. payload :: remaining packet size
Flags and Option Data Fields
The flags field above specifies some metadata about the packet, and in turn may require certain additional data to be included. The flags are as follows. Any data structures specified must be added to the options area in the given order.
Bit order: 15....0 (15 is MSB)
|Bit||Flag||Option Order||Option Data||Function|
|0||SYNCHRONIZE||--||--||Similar to TCP SYN. Set in the initial packet and in the first response. FROM_INCLUDED and SIGNATURE_INCLUDED must also be set.|
|1||CLOSE||--||--||Similar to TCP FIN. If the response to a SYNCHRONIZE fits in a single message, the response will contain both SYNCHRONIZE and CLOSE. SIGNATURE_INCLUDED must also be set.|
|2||RESET||--||--||Abnormal close. SIGNATURE_INCLUDED must also be set. Prior to release 0.9.20, due to a bug, FROM_INCLUDED must also be set.|
|3||SIGNATURE_INCLUDED||4||variable length [Signature]||
Currently sent only with SYNCHRONIZE, CLOSE, and RESET, where it is required, and with ECHO, where it is required for a ping. The signature uses the Destination's [SigningPrivateKey] to sign the entire header and payload with the space in the option data field for the signature being set to all zeroes.
Prior to release 0.9.11, the signature was always 40 bytes. As of release 0.9.11, the signature may be variable-length, see below for details.
|4||SIGNATURE_REQUESTED||--||--||Unused. Requests every packet in the other direction to have SIGNATURE_INCLUDED|
|5||FROM_INCLUDED||2||387+ byte [Destination]||Currently sent only with SYNCHRONIZE, where it is required, and with ECHO, where it is required for a ping. Prior to release 0.9.20, due to a bug, must also be sent with RESET.|
|6||DELAY_REQUESTED||1||2 byte [Integer]||Optional delay. How many milliseconds the sender of this packet wants the recipient to wait before sending any more data. A value greater than 60000 indicates choking.|
|7||MAX_PACKET_SIZE_INCLUDED||3||2 byte [Integer]||Currently sent with SYNCHRONIZE only. Was also sent in retransmitted packets until release 0.9.1.|
|8||PROFILE_INTERACTIVE||--||--||Unused or ignored; the interactive profile is unimplemented.|
|9||ECHO||--||--||Unused except by ping programs. If set, most other options are ignored. See the streaming docs [STREAMING].|
|10||NO_ACK||--||--||This flag simply tells the recipient to ignore the ackThrough field in the header. Currently set in the inital SYN packet, otherwise the ackThrough field is always valid. Note that this does not save any space, the ackThrough field is before the flags and is always present.|
|11-15||unused||Set to zero for compatibility with future uses.|
Variable Length Signature Notes
Prior to release 0.9.11, the signature in the option field was always 40 bytes. As of release 0.9.11, the signature is variable length. The Signature type and length are inferred from the type of key used in the FROM_INCLUDED option and the [Signature] documentation.
- When a packet contains both FROM_INCLUDED and SIGNATURE_INCLUDED (as in SYNCHRONIZE), the inference may be made directly.
- When a packet does not contain FROM_INCLUDED, the inference must be made from a previous SYNCHRONIZE packet.
- When a packet does not contain FROM_INCLUDED, and there was no previous SYNCHRONIZE packet (for example a stray CLOSE or RESET packet), the inference can be made from the length of the remaining options (since SIGNATURE_INCLUDED is the last option), but the packet will probably be discarded anyway, since there is no FROM available to validate the signature. If more option fields are defined in the future, they must be accounted for.
|[Integer]||(1, 2) http://i2pproject.net/spec/common-structures#type-integer|
|[Signature]||(1, 2) http://i2pproject.net/spec/common-structures#type-signature|
|[STREAMING]||(1, 2) http://i2pproject.net/en/docs/api/streaming|