BEP 15

The message flow in [BEP15] is as follows:

Client                        Tracker
  Connect Req. ------------->
    <-------------- Connect Resp.
  Announce Req. ------------->
    <-------------- Announce Resp.
  Announce Req. ------------->
    <-------------- Announce Resp.

The connect phase is required to prevent IP address spoofing. The tracker returns a connection ID that the client uses in subsequent announces. This connection ID expires in one minute at the client, and in two minutes at the tracker. This is not necessary in I2P because of repliable datagrams.

We propose two mechanisms for I2P, compatibility mode and fast mode.

Compatibility Mode

In compatibility mode, we keep the same message flow as BEP 15, for ease of adoption in existing UDP-capable client code bases:

Client                        Tracker
  Connect Req. ------------->       (Repliable)
    <-------------- Connect Resp.   (Raw)
  Announce Req. ------------->      (Raw)
    <-------------- Announce Resp.  (Raw)
  Announce Req. ------------->      (Raw)
    <-------------- Announce Resp.  (Raw)
           ...

This mode is also useful if the client plans to send multiple announces within one minute to a single tracker, as only the connect message must be repliable.

I2P Fast Mode

In fast mode, we omit the connect phase, as it is not required to prevent address spoofing. This significantly simplifies the client-side implementation.

Client                        Tracker
  Announce Req. ------------->      (Repliable)
    <-------------- Announce Resp.  (Raw)

This mode omits a round-trip, but requires every announce request to be repliable.

Security Analysis

The primary goal of an announce protocol is to impose a cost on address spoofing. In compatibility mode, the client must actually exist and bundle a real leaseset. It must have inbound tunnels to receive the Connect Response. These tunnels could be zero-hop and built instantly, but that would expose the creator.

However, in fast mode, the destination and leaseset could be fake. Multiple fake destinations and leasesets can be rapidly generated without actually building tunnels. The Announce Request messages could then be sent out any tunnel.

Connect Request

Client to tracker. 16 bytes. Must be repliable. Same as in [BEP15].

Offset  Size            Name            Value
0       64-bit integer  protocol_id     0x41727101980 // magic constant
8       32-bit integer  action          0 // connect
12      32-bit integer  transaction_id

Connect Response

Tracker to client. 16 bytes. Must be raw. Same as in [BEP15].

Offset  Size            Name            Value
0       32-bit integer  action          0 // connect
4       32-bit integer  transaction_id
8       64-bit integer  connection_id

The response MUST be sent to the I2CP "to port" that was received as the request "from port".

Announce Request

Client to tracker. 98 bytes. Same as in [BEP15] except as noted below.

If preceded by a connect request/response, must be raw, with the connection_id received in the connect response.

If NOT preceded by a connect request/response, must be repliable, and the connection_id is ignored.

Offset  Size            Name            Value
0       64-bit integer  connection_id
8       32-bit integer  action          1 // announce
12      32-bit integer  transaction_id
16      20-byte string  info_hash
36      20-byte string  peer_id
56      64-bit integer  downloaded
64      64-bit integer  left
72      64-bit integer  uploaded
80      32-bit integer  event           0 // 0: none; 1: completed; 2: started; 3: stopped
84      32-bit integer  IP address      0 // default
88      32-bit integer  key
92      32-bit integer  num_want        -1 // default
96      16-bit integer  port
98      TBD             additional data TBD

Changes from [BEP15]:

• connection_id is ignored if repliable
• IP address is ignored
• key is ignored
• port is probably ignored
• Explicitly indidate that the protocol is extensible, with possible additional data starting at port 98.

The response MUST be sent to the I2CP "to port" that was received as the request "from port". Do not use the port from the announce request.

Announce Response

Tracker to client. 20+ bytes. Must be raw. Same as in [BEP15] except as noted below.

Offset  Size            Name            Value
0           32-bit integer  action          1 // announce
4           32-bit integer  transaction_id
8           32-bit integer  interval
12          32-bit integer  leechers
16          32-bit integer  seeders
20          16-bit integer  count of hashes to follow
22   32 * n 32-byte hash    binary hashes
...
22 + 32 * n TBD             additional data TBD

Changes from [BEP15]:

• Add a hash count before the hashes, so that the response format is extensible with additional data after the hashes.
• Instead of 6-byte IPv4+port or 18-byte IPv6+port, we return a multiple of 32-byte "compact responses" with the SHA-256 binary peer hashes. As with TCP compact responses, we do not include a port.
• Explicitly indidate that the protocol is extensible, with possible additional data starting after the hashes

The response MUST be sent to the I2CP "to port" that was received as the request "from port". Do not use the port from the announce request.

I2P datagrams have a very large maximum size of about 16 KB; however, for reliable delivery, datagrams larger than 4 KB should be avoided. For bandwidth efficiency, trackers should probably limit the maximum peers to about 50.

Scrape

Scrape request/response from [BEP15] is not required by this proposal, but may be implemented if desired, no changes required. The scrape request is always repliable (unless there is a previous connect request/response) and the scrape response is always raw.

Error Response

Error response from [BEP15] is not required by this proposal, but may be implemented if desired, no changes required. The error response is always raw.

Announce URL

As in clearnet, UDP announce URLs are of the form "udp://host:port/path". The path is ignored and may be empty. If the ":port" part is omitted, use an I2CP port of 0.

Clients

For a given tracker hostname, a client should prefer UDP over HTTP URLs, and should not announce to both.

Clients wihout existing BEP 15 support should implement fast mode only, as it is much simpler. Clients with existing BEP 15 support should require only small modifications. Evaluate both fast and compatibility modes and choose whatever is best for the existing code base.

If a client support DHT or other datagram protocols, it should probably select a different port as the request "from port" so that the replies come back to that port and are not mixed up with DHT messages. The client only receives raw datagrams as replies. Trackers will never send a repliable datagram to the client.

Clients with a default list of opentrackers should update the list to add UDP URLs after the known opentrackers are known to support UDP.

Clients may or may not implement retransmission of requests. Retransmissions, if implemented, should use an initial timeout of at least 15 seconds, and double the timeout for each retransmission (exponential backoff).

Trackers

Trackers must implement both compatibility mode and fast mode. Trackers with existing BEP 15 support should require only small modifications. This proposal differs from the 2014 proposal, in that the tracker must support reception of repliable and raw datagrams on the same port.

For an integrated application (router and client in one process, for example the ZzzOT Java plugin), it should be straightforward to implement and route the streaming and datagram traffic separately.

For an external tracker application that currently uses an HTTP server tunnel to receive announce requests, the implementation could be quite difficult. A specialized tunnel could be developed to translate datagrams to local HTTP requests/responses. Or, a specialized tunnel that handles both HTTP requests and datagrams could be designed that would forward the datagrams to the external process. These design decisions will depend heavily on the specific router and tracker implementations, and are outside the scope of this proposal.