This page was last updated in 2023-10 and is accurate for router version 0.9.59.


See [STREAMING] for an overview of the Streaming protocol.

Protocol Versions

The streaming protocol does not include a version field. The versions listed below are for Java I2P. Implementations and actual crypto support may vary. There is no way to determine if the far-end supports any particular version or feature. The table below is for general guidance as to the release dates for various features.

The features listed below are for the protocol itself. Various options for configuration are documented in [STREAMING] along with the Java I2P version in which they were implemented.

Router Version Streaming Features
0.9.58 Bob's hash in NACKs field of SYN packet
0.9.36 I2CP protocol number enforced
0.9.20 FROM_INCLUDED no longer required in RESET
0.9.18 PINGs and PONGs may include a payload
0.9.15 EdDSA Ed25519 sig type
0.9.12 ECDSA P-256, P-384, and P-521 sig types
0.9.11 Variable-length signatures
0.7.1 Protocol numbers defined in I2CP

Protocol Specification

Packet Format

The format of a single packet in the streaming protocol is shown below. The minimum header size, without NACKs or option data, is 22 bytes.

There is no length field in the streaming protocol. Framing is provided by the lower layers - I2CP and I2NP.

| send Stream ID    | rcv Stream ID     |
| sequence  Num     | ack Through       |
| nc |  nc*4 bytes of NACKs (optional)
     | rd |  flags  | opt size| opt data
   ...  (optional, see below)           |
|   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, greater than zero. 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, greater than zero.
                   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

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

NACK count :: 1 byte Integer
              The number of 4-byte NACKs in the next field,
              or 8 when used together with SYNCHRONIZE for replay prevention
              as of 0.9.58; see below.

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.
         Also used together with SYNCHRONIZE for replay prevention
         as of 0.9.58; see below.

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

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 5 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. A value of 0 requests an immediate ack.
7 MAX_PACKET_SIZE_INCLUDED 3 2 byte [Integer] The maximum length of the payload. Send with SYNCHRONIZE.
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 OFFLINE_SIGNATURE 4 variable length [OfflineSig] Contains the offline signature section from LS2. See proposal 123 and the common structures specification. FROM_INCLUDED must also be set. Contains an [OfflineSig]: 1) Expires timestamp (4 bytes, seconds since epoch, rolls over in 2106) 2) Transient sig type (2 bytes) 3) Transient [SigningPublicKey] (length as implied by sig type) 4) [Signature] of expires timestamp, transient sig type, and public key, by the destination public key. Length of sig as implied by by the destination public key sig type.
12-15 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.

As of release 0.9.39, the OFFLINE_SIGNATURE option is supported. If this option is present, the transient [SigningPublicKey] is used to verify any signed packets, and the signature length and type are inferred from the transient [SigningPublicKey] in the option.

  • 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.

Replay prevention

To prevent Bob from using a replay attack by storing a valid signed SYNCHRONIZE packet received from Alice and later sending it to a victim Charlie, Alice must include Bob's destination hash in the SYNCHRONIZE packet as follows:

Set NACK count field to 8
Set the NACKs field to Bob's 32-byte destination hash

Upon reception of a SYNCHRONIZE, if the NACK count field is 8, Bob must interpret the NACKs field as a 32-byte destination hash, and must verify that it matches his destination hash. He must also verify the signature of the packet as usual, as that covers the entire packet including the NACK count and NACKs fields. If the NACK count is 8 and the NACKs field does not match, Bob must drop the packet.

This is required for versions 0.9.58 and higher. This is backward-compatible with older versions, because NACKs are not expected in a SYNCHRONIZE packet. Destinations do not and cannot know what version the other end is running.

No change is necessary for the SYNCHRONIZE ACK packet sent from Bob to Alice; do not include NACKs in that packet.