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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 for reliable delivery.
</p>


<h2>NTCP Protocol Specification</h2>

<h3>Standard Message Format</h3>
<p>
  After establishment,
  the NTCP transport sends individual I2NP messages, with a simple checksum.
  The unencrypted message is encoded as follows:
<pre>
 *  +-------+-------+--//--+---//----+-------+-------+-------+-------+
 *  | sizeof(data)  | data | padding | Adler checksum of sz+data+pad |
 *  +-------+-------+--//--+---//----+-------+-------+-------+-------+
</pre>
  The data is then AES/256/CBC encrypted. The session key for the encryption
  is negotiated during establishment (using Diffie-Hellman 2048 bit).
  The establishment between two routers is implemented in the EstablishState class
  and detailed below.
  The IV for AES/256/CBC encryption is the last 16 bytes of the previous encrypted message.
</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.
The message is encrypted just as standard messages are.


<h3>Checksums</h3>
The standard and time sync messages use the Adler-32 checksum
as defined in the <a href="http://tools.ietf.org/html/rfc1950">ZLIB Specification</a>.


<h3>Establishment Sequence</h3>
In the establish state, there is a 4-phase message sequence to exchange DH keys and signatures.
In the first two messages there is a 2048-bit Diffie Hellman exchange.
Then, DSA signatures of the critical data are exchanged to confirm the connection.
<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), sk, hX_xor_Bob.identHash[16:31])---&gt;
 *  &lt;----------------------E(S(X+Y+Alice.identHash+tsA+tsB)+padding, sk, prev)

</pre>

<pre>
  Legend:
    X, Y: 256 byte DH public 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 id="DH">DH Key Exchange</h4>
<p>
The initial 2048-bit DH key exchange
uses the same shared prime (p) and generator (g) as that used for I2P's
<a href="how_cryptography.html#elgamal">ElGamal encryption</a>.
</p>

<p>
The DH key exchange consists of a number of steps, displayed below.
The mapping between these steps and the messages sent between I2P routers,
is marked in bold.
  <ol>
    <li>Alice generates a secret 226-bit integer x.
        She then calculates X = g^x mod p.
    </li>
    <li>Alice sends X to Bob <b>(Message 1)</b>.</li>
    <li>Bob generates a secret 226-bit integer y.
        He then calculates Y = g^y mod p.</li>
    <li>Bob sends Y to Alice.<b>(Message 2)</b></li>
    <li>Alice can now compute sessionKey = Y^x mod p.</li>
    <li>Bob can now compute sessionKey = X^y mod p.</li>
    <li>Both Alice and Bob now have a shared key sessionKey = g^(x*y) mod p.</li>
  </ol>
The sessionKey is then used to exchange identities in <b>Message 3</b> and <b>Message 4</b>.
</p>

<h4>Message 1 (Session Request)</h4>
This is the DH request.
Alice already has Bob's
<a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>,
IP address, and port, as contained in his
<a href="common_structures_spec.html#struct_RouterInfo">Router Info</a>,
which was published to the
<a href="how_networkdatabase.html">network database</a>.
Alice sends Bob:
<pre>
 *  X+(H(X) xor Bob.identHash)-----------------------------&gt;

    Size: 288 bytes
</pre>
Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |         X, as calculated from DH      |
 +                                       +
 |                                       |
 ~               .   .   .               ~
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |              HXxorHI                  |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +----+----+----+----+----+----+----+----+

  X: 256 byte X from Diffie Hellman

  HXxorHI:  SHA256 Hash(X) xored with SHA256 Hash(Bob's <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>)
            (32 bytes)

</pre>

<p><b>Notes:</b>
<ul><li>
Bob verifies HXxorHI using his own router hash. If it does not verify,
Alice has contacted the wrong router, and Bob drops the connection.
</li></ul>


<h4>Message 2 (Session Created)</h4>
This is the DH reply. Bob sends Alice:
<pre>
 *  &lt;----------------------------------------Y+E(H(X+Y)+tsB+padding, sk, Y[239:255])

    Size: 304 bytes
</pre>
Unencrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |         Y as calculated from DH       |
 +                                       +
 |                                       |
 ~               .   .   .               ~
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |              HXY                      |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +----+----+----+----+----+----+----+----+
 |        tsB        |     padding       |
 +----+----+----+----+                   +
 |                                       |
 +----+----+----+----+----+----+----+----+

  Y: 256 byte Y from Diffie Hellman

  HXY:  SHA256 Hash(X concatenated with Y)
        (32 bytes)

  tsB: 4 byte timestamp (seconds since the epoch)

  padding: 12 bytes random data

</pre>


Encrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |         Y as calculated from DH       |
 +                                       +
 |                                       |
 ~               .   .   .               ~
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |             encrypted data            |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +----+----+----+----+----+----+----+----+

  Y: 256 byte Y from Diffie Hellman

  encrypted data: 48 bytes <a href="how_cryptography.html#AES">AES encrypted</a> using the DH session key and
                  the last 16 bytes of Y as the IV

</pre>


<p><b>Notes:</b>
<ul><li>
Alice may drop the connection if the clock skew with Bob is too high as calculated using tsB.
</li></ul>
</p>


<h4>Message 3 (Session Confirm A)</h4>
This contains Alice's router identity, and a DSA signature of the critical data. Alice sends Bob:
<pre>
 *  E(sz+Alice.identity+tsA+padding+S(X+Y+Bob.identHash+tsA+tsB), sk, hX_xor_Bob.identHash[16:31])---&gt;

    Size: 448 bytes (typ. for 387 byte identity)
</pre>
Unencrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |   sz    | Alice's Router Identity     |
 +----+----+                             +
 |                                       |
 ~               .   .   .               ~
 |                                       |
 +                        +----+----+----+
 |                        |     tsA       
 +----+----+----+----+----+----+----+----+
      |             padding              |
 +----+                                  +
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |              signature                |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +----+----+----+----+----+----+----+----+

  sz: 2 byte size of Alice's router identity to follow (should always be 387)

  ident: Alice's 387 byte <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>

  tsA: 4 byte timestamp (seconds since the epoch)

  padding: 15 bytes random data

  signature: the 40 byte <a href="common_structures_spec.html#type_Signature">DSA signature</a> of the following concatenated data:
             X, Y, Bob's <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>, tsA, tsB.
             Alice signs it with the <a href="common_structures_spec.html#type_SigningPrivateKey">private signing key</a> associated with the <a href="common_structures_spec.html#type_SigningPublicKey">public signing key</a> in her <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>

</pre>

Encrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |             encrypted data            |
 ~               .   .   .               ~
 |                                       |
 +----+----+----+----+----+----+----+----+

  encrypted data: 448 bytes <a href="how_cryptography.html#AES">AES encrypted</a> using the DH session key and
                  the last 16 bytes of HXxorHI (i.e., the last 16 bytes of message #1) as the IV

</pre>


<p><b>Notes:</b>
<ul><li>
Bob verifies the signature, and on failure, drops the connection.
</li><li>
Bob may drop the connection if the clock skew with Alice is too high as calculated using tsA.
</li></ul>
</p>



<h4>Message 4 (Session Confirm B)</h4>
This is a DSA signature of the critical data. Bob sends Alice:
<pre>
 *  &lt;----------------------E(S(X+Y+Alice.identHash+tsA+tsB)+padding, sk, prev)

    Size: 48 bytes
</pre>
Unencrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |              signature                |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +                                       +
 |                                       |
 +----+----+----+----+----+----+----+----+
 |               padding                 |
 +----+----+----+----+----+----+----+----+


  signature: the 40 byte <a href="common_structures_spec.html#type_Signature">DSA signature</a> of the following concatenated data:
             X, Y, Alice's <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>, tsA, tsB.
             Bob signs it with the <a href="common_structures_spec.html#type_SigningPrivateKey">private signing key</a> associated with the <a href="common_structures_spec.html#type_SigningPublicKey">public signing key</a> in his <a href="common_structures_spec.html#struct_RouterIdentity">Router Identity</a>

  padding: 8 bytes random data

</pre>


Encrypted Contents:
<pre>
 +----+----+----+----+----+----+----+----+
 |                                       |
 +                                       +
 |             encrypted data            |
 ~               .   .   .               ~
 |                                       |
 +----+----+----+----+----+----+----+----+

  encrypted data: 48 bytes <a href="how_cryptography.html#AES">AES encrypted</a> using the DH session key and
                  the last 16 bytes of the encrypted contents of message #2 as the IV

</pre>

<p><b>Notes:</b>
<ul><li>
Alice verifies the signature, and on failure, drops the connection.
</li></ul>
</p>




<h4>After Establishment</h4>
<p>
The connection is established, and standard or time sync messages may be exchanged.
All subsequent messages are AES encrypted using the negotiated DH session key.
Alice will use the last 16 bytes of the encrypted contents of message #3 as the next IV.
Bob will use the last 16 bytes of the encrypted contents of message #4 as the next IV.
</p>



<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>
<ul><li>
The maximum message size should be increased to approximately 32 KB.
</li><li>
A set of fixed packet sizes may be appropriate to further hide the data 
fragmentation to external adversaries, but the tunnel, garlic, and end to 
end padding should be sufficient for most needs until then.
However, there is currently no provision for padding beyond the next 16-byte boundary,
to create a limited number of message sizes.
</li><li>
Memory utilization (including that of the kernel) for NTCP should be compared to that for SSU.
</li><li>
Can the establishment messages be randomly padded somehow, to frustrate
identification of I2P traffic based on initial packet sizes?
</li><li>
Review and possibly disable 'check connection'
</li></ul>
</p>

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