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Updated August 2010 for release 0.8

<h2>NTCP (NIO-based TCP)</h2>

<p>
NTCP
is one of two <a href="transport.html">transports</a> currently implemented in I2P.
The other is <a href="udp.html">SSU</a>.
NTCP
is a Java NIO-based transport
introduced in I2P release 0.6.1.22.
Java NIO (new I/O) does not suffer from the 1 thread per connection issues of the old TCP transport.
</p><p>

By default,
NTCP uses the IP/Port
auto-detected by SSU. When enabled on config.jsp,
SSU will notify/restart NTCP when the external address changes
or when the firewall status changes.
Now you can enable inbound TCP without a static IP or dyndns service.
</p><p>

The NTCP code within I2P is relatively lightweight (1/4 the size of the SSU code)
because it uses the underlying Java TCP transport.
</p>


<h2>NTCP Protocol Specification</h2>


<h3>Standard Message Format</h3>
<p>
  The NTCP transport sends individual I2NP messages AES/256/CBC encrypted with
  a simple checksum.  The unencrypted message is encoded as follows:
<pre>
 *  +-------+-------+--//--+---//----+-------+-------+-------+-------+
 *  | sizeof(data)  | data | padding | Adler checksum of sz+data+pad |
 *  +-------+-------+--//--+---//----+-------+-------+-------+-------+
</pre>
  That message is then encrypted with the DH/2048 negotiated session key
  (station to station authenticated per the EstablishState class) using the
  last 16 bytes of the previous encrypted message as the IV.
</p>

<p>
0-15 bytes of padding are required to bring the total message length
(including the six size and checksum bytes) to a multiple of 16.
The maximum message size is currently 16 KB.
Therefore the maximum data size is currently 16 KB - 6, or 16378 bytes.
The minimum data size is 1.
</p>

<h3>Time Sync Message Format</h3>
<p>
  One special case is a metadata message where the sizeof(data) is 0.  In
  that case, the unencrypted message is encoded as:
<pre>
 *  +-------+-------+-------+-------+-------+-------+-------+-------+
 *  |       0       |      timestamp in seconds     | uninterpreted             
 *  +-------+-------+-------+-------+-------+-------+-------+-------+
 *          uninterpreted           | Adler checksum of bytes 0-11  |
 *  +-------+-------+-------+-------+-------+-------+-------+-------+
</pre>
Total length: 16 bytes. The time sync message is sent at approximately 15 minute intervals.


<h3>Establishment Sequence</h3>
In the establish state, the following communication happens.
There is a 2048-bit Diffie Hellman exchange.
For more information see the <a href="how_cryptography.html#tcp">cryptography page</a>.
<pre>
 * Alice                   contacts                      Bob
 * =========================================================
 *  X+(H(X) xor Bob.identHash)-----------------------------&gt;
 *  &lt;----------------------------------------Y+E(H(X+Y)+tsB+padding, sk, Y[239:255])
 *  E(sz+Alice.identity+tsA+padding+S(X+Y+Bob.identHash+tsA+tsB+padding), sk, hX_xor_Bob.identHash[16:31])---&gt;
 *  &lt;----------------------E(S(X+Y+Alice.identHash+tsA+tsB)+padding, sk, prev)

</pre>

<pre>
Hints for documentors:
X, Y: 256 byte DH keys
H(): 32 byte SHA256 Hash
E(data, session key, IV): AES256 Encrypt
S(): 40 byte DSA Signature
tsA, tsB: timestamps (4 bytes, seconds since epoch)
sk: 32 byte Session key
sz: 2 byte size of Alice identity to follow
</pre>

<h4>Step 1</h4>
Alice sends Bob:
<pre>
 *  X+(H(X) xor Bob.identHash)-----------------------------&gt;

    Size: 288 bytes
</pre>
Todo: Explain this in words.


<h4>Step 2</h4>
Bob sends Alice:
<pre>
 *  &lt;----------------------------------------Y+E(H(X+Y)+tsB+padding, sk, Y[239:255])

    Size: 304 bytes
</pre>
Todo: Explain this in words.


<h4>Step 3</h4>
Alice sends Bob:
<pre>
 *  E(sz+Alice.identity+tsA+padding+S(X+Y+Bob.identHash+tsA+tsB+padding), sk, hX_xor_Bob.identHash[16:31])---&gt;

    Size: 448 bytes (typ. for 387 byte identity)
</pre>
Todo: Explain this in words.


<h4>Step 4</h4>
Bob sends Alice:
<pre>
 *  &lt;----------------------E(S(X+Y+Alice.identHash+tsA+tsB)+padding, sk, prev)

    Size: 48 bytes
</pre>
Todo: Explain this in words.

<h4>Finally</h4>
The connection is established and standard messages may be exchanged.



<h3>Check Connection Message</h3>
Alternately, when Bob receives a connection, it could be a
check connection (perhaps prompted by Bob asking for someone
to verify his listener).
Check Connection is not currently used.
However, for the record, check connections are formatted as follows.
     A check info connection will receive 256 bytes containing:
<ul>
     <li> 32 bytes of uninterpreted, ignored data
     <li> 1 byte size
     <li> that many bytes making up the local router's IP address (as reached by the remote side)
     <li> 2 byte port number that the local router was reached on
     <li> 4 byte i2p network time as known by the remote side (seconds since the epoch)
     <li> uninterpreted padding data, up to byte 223
     <li> xor of the local router's identity hash and the SHA256 of bytes 32 through bytes 223
</ul>
</pre>

<h2>Discussion</h2>
Now on the <a href="ntcp_discussion.html">NTCP Discussion Page</a>.

<h2><a name="future">Future Work</a></h2>
<p>The maximum message size should be increased to approximately 32 KB.
</p>

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