i2p.itoopie/router/doc/udp.html

<code>$Id: udp.html,v 1.9 2005/04/05 10:28:54 jrandom Exp $</code>

<h1>Secure Semireliable UDP (SSU)</h1>
<b>DRAFT</b>

<p>
The goal of this protocol is to provide secure, authenticated,
semireliable, and unordered message delivery, exposing only a minimal
amount of data easily discernible to third parties.  It should 
support high degree communication as well as TCP-friendly congestion
control, and may include PMTU detection.   It should be capable of
efficiently moving bulk data at rates sufficient for home users.
In addition, it should support techniques for addressing network 
obstacles, like most NATs or firewalls.</p>

<h2><a name="addressing">Addressing and introduction</a></h2>

<p>To contact an SSU peer, one of two sets of information is necessary:
a direct address, for when the peer is publicly reachable, or an 
indirect address, for using a third party to introduce the peer.  
There is no restriction on the number of addresses a peer may have.</p>

<pre>
    Direct: udp://host:port/introKey
  Indirect: udp://tag@relayhost:port/relayIntroKey/targetIntroKey
</pre>

<p>These introduction keys are delivered through an external channel 
and must be used when establishing a session key.  For the indirect
address, the peer must first contact the relayhost and ask them for
an introduction to the peer known at that relayhost under the given
tag.  If possible, the relayhost sends a message to the addressed
peer telling them to contact the requesting peer, and also gives 
the requesting peer the IP and port on which the addressed peer is
located.  In addition, the peer establishing the connection must 
already know the public keys of the peer they are connecting to (but
not necessary to any intermediary relay peer).</p>

<h2><a name="header">Header</a></h2>

<p>All UDP datagrams begin with a MAC and an IV, followed by a variable
size payload encrypted with the appropriate key.  The MAC used is 
HMAC-SHA256, truncated to 16 bytes, while the key is a full AES256 
key.  The specific construct of the MAC is the first 16 bytes from:</p>
<pre>
  HMAC-SHA256(payload || IV || payloadLength, macKey)
</pre>

<p>The payload itself is AES256/CBC encrypted with the IV and the 
sessionKey, with replay prevention addressed within its body, 
explained below.  The payloadLength in the MAC is a 2 byte unsigned 
integer in 2s complement.</p>

<h2><a name="payload">Payload</a></h2>

<p>Within the AES encrypted payload, there is a minimal common structure
to the various messages - a one byte flag and a four byte sending 
timestamp (*seconds* since the unix epoch).  The flag byte contains 
the following bitfields:</p>
<pre>
  bits 0-3: payload type
     bit 4: rekey?
     bit 5: extended options included
  bits 6-7: reserved
</pre>

<p>If the rekey flag is set, 64 bytes of keying material follow the 
timestamp.  If the extended options flag is set, a one byte option 
size value is appended to, followed by that many extended option 
bytes, which are currently uninterpreted.</p>

<p>When rekeying, the first 32 bytes of the keying material is fed 
into a SHA256 to produce the new MAC key, and the next 32 bytes are
fed into a SHA256 to produce the new session key, though the keys are
not immediately used.  The other side should also reply with the 
rekey flag set and that same keying material.  Once both sides have 
sent and received those values, the new keys should be used and the 
previous keys discarded.  It may be useful to keep the old keys 
around briefly, to address packet loss and reordering.</p>

<pre>
 Header: 37+ bytes
 +----+----+----+----+----+----+----+----+
 |                  MAC                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                   IV                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |flag|        time       | (optionally  |
 +----+----+----+----+----+              |
 | this may have 64 byte keying material |
 | and/or a one+N byte extended options) |
 +---------------------------------------|
</pre>

<h2><a name="messages">Messages</a></h2>

<h3><a name="sessionRequest">SessionRequest (type 0)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Alice to Bob</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>256 byte X, to begin the DH agreement</li>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Bob's IP address</li>
        <li>N bytes, currently uninterpreted (later, for challenges)</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>introKey</td></tr>
</table>

<pre>
 +----+----+----+----+----+----+----+----+
 |         X, as calculated from DH      |
 |                                       |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
 |size| that many byte IP address (4-16) |
 +----+----+----+----+----+----+----+----+
 |           arbitrary amount            |
 |        of uninterpreted data          |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
</pre>

<h3><a name="sessionCreated">SessionCreated (type 1)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Bob to Alice</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>256 byte Y, to complete the DH agreement</li>
	<li>1 byte IP address size</li>
	<li>that many byte representation of Alice's IP address</li>
	<li>2 byte port number (unsigned, big endian 2s complement)</li>
        <li>4 byte relay tag which Alice can publish (else 0x0)</li>
        <li>4 byte timestamp (seconds from the epoch) for use in the DSA 
            signature</li>
        <li>40 byte DSA signature of the critical exchanged data 
            (Alice's IP + Alice's port + Bob's IP + Bob's port + Alice's
            new relay tag + Bob's signed on time), encrypted with another 
            layer of encryption using the negotiated sessionKey.  The IV 
            is reused here.</li>
        <li>8 bytes padding, encrypted with an additional layer of encryption
            using the negotiated session key as part of the DSA block</li>
        <li>N bytes, currently uninterpreted (later, for challenges)</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>introKey, with an additional layer of encryption over the 40 byte
        signature and the following 8 bytes padding.</td></tr>
</table>

<pre>
 +----+----+----+----+----+----+----+----+
 |         Y, as calculated from DH      |
 |                                       |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
 |size| that many byte IP address (4-16) |
 +----+----+----+----+----+----+----+----+
 | Port (A)| public relay tag  |  signed
 +----+----+----+----+----+----+----+----+
   on time |                             |
 +----+----+                             |
 |              DSA signature            |
 |                                       |
 |                                       |
 |                                       |
 |         +----+----+----+----+----+----+
 |         |     (8 bytes of padding) 
 +----+----+----+----+----+----+----+----+
           |                             |
 +----+----+                             |
 |           arbitrary amount            |
 |        of uninterpreted data          |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
</pre>

<h3><a name="sessionConfirmed">SessionConfirmed (type 2)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Alice to Bob</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>1 byte identity fragment info:<pre>
bits 0-3: current identity fragment #
bits 4-7: total identity fragments</pre></li>
        <li>N byte fragment of Alice's identity, sent over a number
            of messages.</li>
        <li>on the last identity fragment, the signed on time is
            included after the identity fragment, and the last 40 
            bytes contain the DSA signature of the critical exchanged 
            data (Alice's IP + Alice's port + Bob's IP + Bob's port
            + Alice's new relay key + Alice's signed on time)</li>
        </ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>sessionKey</td></tr>
</table>

<pre>
 <b>Fragment 1 through N-1</b>
 +----+----+----+----+----+----+----+----+
 |info| fragment of Alice's full         |
 +----+                                  |
 |            identity keys              |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
 
 <b>Fragment N:</b>
 +----+----+----+----+----+----+----+----+
 |info| fragment of Alice's full         |
 +----+                                  |
 |            identity keys              |
                 .   .   .               
 |                                       |
 +----+----+----+----+----+----+----+----+
 |  signed on time   |                   |
 +----+----+----+----+                   |
 |  arbitrary amount of uninterpreted    |
 |        data, up from the end of the   |
 |  identity key to 40 bytes prior to    |
 |       end of the current packet       |
 +----+----+----+----+----+----+----+----+
 | DSA signature                         |
 |                                       |
 |                                       |
 |                                       |
 |                                       |
 +----+----+----+----+----+----+----+----+
</pre>
 
<h3><a name="relayRequest">RelayRequest (type 3)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Alice to Bob</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>4 byte relay tag</li>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Bob's IP address</li>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Alice's IP address</li>
        <li>2 byte port number (of Alice)</li>
        <li>1 byte challenge size</li>
        <li>that many bytes to be relayed to Charlie in the intro</li>
        <li>N bytes, currently uninterpreted</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>introKey (or sessionKey, if Alice/Bob is established)</td></tr>
</table>
 
<pre>
 +----+----+----+----+----+----+----+----+
 |      relay tag    |size| that many    |
 +----+----+----+----+----+         +----|
 | bytes making up Bob's IP address |size|
 +----+----+----+----+----+----+----+----+
 | that many bytes making up Alice's IP  |
 +----+----+----+----+----+----+----+----+
 | Port (A)|size| that many challenge    |
 +----+----+----+                        |
 | bytes to be delivered to Charlie      |
 +----+----+----+----+----+----+----+----+
 | arbitrary amount of uninterpreted data|
 +----+----+----+----+----+----+----+----+
</pre>

<h3><a name="relayResponse">RelayResponse (type 4)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Bob to Alice</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Charlie's IP address</li>
        <li>2 byte port number</li>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Alice's IP address</li>
        <li>2 byte port number</li>
        <li>N bytes, currently uninterpreted</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>introKey (or sessionKey, if Alice/Bob is established)</td></tr>
</table>

<pre>
 +----+----+----+----+----+----+----+----+
 |size| that many bytes making up        |
 +----+                        +----+----+
 | Charlie's IP address        | Port (C)|
 +----+----+----+----+----+----+----+----+
 |size| that many bytes making up        |
 +----+                        +----+----+
 | Alice's IP address          | Port (A)|
 +----+----+----+----+----+----+----+----+
 | arbitrary amount of uninterpreted data|
 +----+----+----+----+----+----+----+----+
</pre>

<h3><a name="relayIntro">RelayIntro (type 5)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Bob to Charlie</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>1 byte IP address size</li>
        <li>that many byte representation of Alice's IP address</li>
        <li>2 byte port number (of Alice)</li>
        <li>1 byte challenge size</li>
        <li>that many bytes relayed from Alice</li>
        <li>N bytes, currently uninterpreted</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>sessionKey</td></tr>
</table>

<pre>
 +----+----+----+----+----+----+----+----+
 |size| that many bytes making up        |
 +----+                        +----+----+
 | Charlie's IP address        | Port (C)|
 +----+----+----+----+----+----+----+----+
 |size| that many bytes of challenge     |
 +----+                                  |
 | data relayed from Alice               |
 +----+----+----+----+----+----+----+----+
 | arbitrary amount of uninterpreted data|
 +----+----+----+----+----+----+----+----+
</pre>

<h3><a name="data">Data (type 6)</a></h3>
<table border="1">
<tr><td align="right" valign="top"><b>Peer:</b></td>
    <td>Any</td></tr>
<tr><td align="right" valign="top"><b>Data:</b></td>
    <td><ul>
        <li>1 byte flags:<pre>
   bit 0: explicit ACKs included
   bit 1: explicit NACKs included
   bit 2: numACKs included
   bit 3: explicit congestion notification
   bit 4: request previous ACKs
   bit 5: want reply
   bit 6: extended data included
   bit 7: reserved</pre></li>
        <li>if explicit ACKs are included:<ul>
	  <li>a 1 byte number of ACKs</li>
          <li>that many 4 byte MessageIds being fully ACKed</li>
	  </ul></li>
        <li>if explicit NACKs are included:<ul>
          <li>a 1 byte number of NACKs</li>
          <li>that many 4 byte MessageIds + 1 byte fragmentNum NACKs</li>
	  </ul></li>
        <li>if numACKs included:<ul>
          <li>a 2 byte number for how many messages were fully 
              received in the last minute.</li></ul></li>
        <li>If extended data included:<ul>
          <li>1 byte data size</li>
          <li>that many bytes of extended data (currently uninterpreted)</li</ul></li>
        <li>1 byte number of fragments</li>
        <li>that many message fragments:<ul>
          <li>4 byte messageId</li>
          <li>1 byte fragment info:<pre>
bits 0-4: fragment #
   bit 5: isLast (1 = true)
bits 6-7: unused</pre></li>
          <li>2 byte fragment size</li>
          <li>that many bytes</li>
          <li>1 byte fragment size</li></ul>
        <li>N bytes padding, uninterpreted</li>
	</ul></td></tr>
<tr><td align="right" valign="top"><b>Key used:</b></td>
    <td>sessionKey</td></tr>
</table>

<pre>
 +----+----+----+----+----+----+----+----+
 |flag| (additional headers, determined  |
 +----+                                  |
 | by the flags, such as ACKs, NACKs, or |
 | simple rate of full ACKs)             |
 +----+----+----+----+----+----+----+----+
 |#frg|     messageId     |info|fragSize |
 +----+----+----+----+----+----+----+----+
 | that many bytes of fragment data      |
                  .  .  .                                       
 |                                       |
 +----+----+----+----+----+----+----+----+
 |     messageId     |info|fragSize |    |
 +----+----+----+----+----+----+----+    |
 | that many bytes of fragment data      |
                  .  .  .                                       
 |                                       |
 +----+----+----+----+----+----+----+----+
 |     messageId     |info|fragSize |    |
 +----+----+----+----+----+----+----+    |
 | that many bytes of fragment data      |
                  .  .  .                                       
 |                                       |
 +----+----+----+----+----+----+----+----+
 | arbitrary amount of uninterpreted data|
 +----+----+----+----+----+----+----+----+
</pre>

<h2><a name="congestioncontrol">Congestion control</a></h2>

<p>SSU's need for only semireliable delivery, TCP-friendly operation,
and the capacity for high throughput allows a great deal of latitude in
congestion control.  The congestion control algorithm outlined below is
meant to be both efficient in bandwidth as well as simple to implement.</p>

<p>Data is transmitted in volleys of up to 1 second, sending N bytes within
P packets.  The volley a packet is a part of is defined by the second field
in the encrypted <a href="#payload">payload</a>.  The receiver of a volley
should send back a full set of ACKs and NACKs for message IDs received in
the previous volley - these ACKs and NACKs should be included in all messages
sent until either the volley changes again or the the receiver gets a message
in the current volley stating that the previous ACKs are no longer required.
Subsequent responses from the receiver in the current volley should not 
contain the ACKs.</p>

<p>After receiving a volley with at least one data message fragment, the 
receiver should send back at least one message with the ACKs.  Each time
subsequent messages arrive on the current volley with the "request previous
ACKs" flag set, if no messages in the current volley have arrived without
that being set the receiver should send back a data message with the ACKs,
if the receiver has the bandwidth to do so.</p>

<p>The number of bytes sent in each volley (N) should be initialized as
8192 bytes (an arbitrarily high value).  At the beginning of a volley, if
the ACKs/NACKs received for the volley two periods back contain any NACKs,
N should be set to the minimum of N and the number of bytes fully ACKed,
though no less than 1/2 of N.  If there were no NACKs and all of the 
messages sent were ACKed, N is increased by the average packet size.  If 
a message is received in a volley with the explicit congestion 
notification bit set, at the beginning of the next volley N is set to 
1/2 N.</p>

<p>Messages which are partially sent or NACKed have the unsent fragments
transmitted in the next volley, unless the message expiration occurs, in
which case it is dropped entirely.</p>

<p>The simplest possible implementation does not need to pad the packets to
any particular size, but instead just places a single message fragment into
a packet and sends it off (careful not to exceed the MTU).  A more efficient
strategy would be to bundle multiple message fragments into the same packet,
so long as it doesn't exceed the MTU, but this is not necessary.  Eventually,
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.</p>

<h3><a name="congestionscenarios">Congestion scenarios</a></h3>

<b>Unidirectional transfer</b><br />
<pre>
        Alice                         Bob
    Data 1, volley 1, no ACKs---------&gt;
    Data 2, volley 1, no ACKs---------&gt;
    Data 3, volley 1, no ACKs---------&gt;
    Data 4, volley 1, no ACKs---------&gt;
    Data 5, volley 2, want ACKs-------&gt;
    Data 6, volley 2, want ACKs-------&gt;         // want ACK since ACKs not received
          &lt;------------------ACK 1, 2, 3, 4     // automatically sent
          &lt;------------------ACK 1, 2, 3, 4     // sent due to Data 6
    Data 7, volley 2, no ACKs---------&gt;         // no further ACKs required
    Data 8, volley 2, no ACKs---------&gt;
    Data 9, volley 3, want ACKs-------&gt;         // new volley, we want ACKs!
          &lt;------------------ACK 5, 6, 7, 8     // automatically sent
    Data 10, volley 3, no ACKs---------&gt;
    Data 11, volley 3, no ACKs---------&gt;
    Data 12, volley 3, no ACKs---------&gt;
          &lt;------------------ACK 9, 10, 11, 12  // automatically sent
</pre>

<b>Bidirectional transfer</b><br />
<pre>
        Alice                                        Bob
    Data 1, volley 1, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 1, volley 1, no ACKs
    Data 2, volley 1, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 2, volley 1, no ACKs
    Data 3, volley 1, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 3, volley 1, no ACKs
    Data 4, volley 1, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 4, volley 1, no ACKs
    Data 5, volley 2, want ACKs, ACK 1, 2, 3, 4-------&gt;              // new volley, send ACKs
          &lt;---------------Data 5, volley 2, no ACKs, ACK 1, 2, 3, 4  // received ACKs, no need to ask for them
    Data 6, volley 2, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 6, volley 2, no ACKs
    Data 7, volley 2, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 8, volley 2, no ACKs
    Data 8, volley 2, no ACKs-------------------------&gt;
          &lt;-----------------------------Data 9, volley 2, no ACKs
    ACK 5, 6, 7, 8, 9---------------------------------&gt;
          &lt;-----------------------------------ACK 5, 6, 7, 8
</pre>

<h2><a name="keys">Keys</a></h2>

<p>All encryption used is AES256/CBC with 32 byte keys and 16 byte IVs.
The MAC and session keys are negotiated as part of the DH exchange, used
for the HMAC and encryption, respectively.  Prior to the DH exchange, 
the publicly knowable introKey is used for the MAC and encryption.</p>

<p>When using the introKey, both the initial message and any subsequent
reply use the introKey of the responder (Bob) - the responder does 
not need to know the introKey of the requestor (Alice).  The DSA 
signing key used by Bob should already be known to Alice when she 
contacts him, though Alice's DSA key may not already be known by 
Bob.</p>

<p>Upon receiving a message, the receiver checks the from IP address 
with any established sessions - if there is one or more matches,
those session's MAC keys are tested sequentially in the HMAC.  If none
of those verify or if there are no matching IP addresses, the 
receiver tries their introKey in the MAC.  If that does not verify,
the packet is dropped.  If it does verify, it is interpreted 
according to the message type, though if the receiver is overloaded,
it may be dropped anyway.</p>

<p>If Alice and Bob have an established session, but Alice loses the 
keys for some reason and she wants to contact Bob, she may at any 
time simply establish a new session through the SessionRequest and
related messages.  If Bob has lost the key but Alice does not know
that, she will first attempt to prod him to reply, by sending a 
DataMessage with the wantReply flag set, and if Bob continually 
fails to reply, she will assume the key is lost and reestablish a 
new one.</p>

<p>For the DH key agreement,
<a href="http://www.faqs.org/rfcs/rfc3526.html">RFC3526</a> 2048bit
MODP group (#14) is used:</p>
<pre>
  p = 2^2048 - 2^1984 - 1 + 2^64 * { [2^1918 pi] + 124476 }
  g = 2
</pre>

<p>The DSA p, q, and g are shared according to the scope of the 
identity which created them.</p>

<h2><a name="replay">Replay prevention</a></h2>

<p>Replay prevention at the SSU layer occurs by rejecting packets 
with exceedingly old timestamps or those which reuse an IV.  To
detect duplicate IVs, a sequence of Bloom filters are employed to
"decay" periodically so that only recently added IVs are detected.</p>

<p>The messageIds used in DataMessages are defined at layers above
the SSU transport and are passed through transparently.  These IDs
are not in any particular order - in fact, they are likely to be
entirely random.  The SSU layer makes no attempt at messageId 
replay prevention - higher layers should take that into account.</p>

<h2><a name="messageSequences">Message sequences</a></h2>

<h3><a name="establishDirect">Connection establishment (direct)</a></h3>

<pre>
        Alice                         Bob
    SessionRequest---------------------&gt;
          &lt;---------------------SessionCreated
    SessionConfirmed-------------------&gt;
    SessionConfirmed-------------------&gt;
    SessionConfirmed-------------------&gt;
    SessionConfirmed-------------------&gt;
          &lt;--------------------------Data
</pre>

<h3><a name="establishIndirect">Connection establishment (indirect)</a></h3>

<pre>
        Alice                         Bob                  Charlie
    RelayRequest ----------------------&gt;
         &lt;--------------RelayResponse    RelayIntro-----------&gt;
         &lt;--------------------------------------------Data (ignored)
    SessionRequest--------------------------------------------&gt;
         &lt;--------------------------------------------SessionCreated
    SessionConfirmed------------------------------------------&gt;
    SessionConfirmed------------------------------------------&gt;
    SessionConfirmed------------------------------------------&gt;
    SessionConfirmed------------------------------------------&gt;
         &lt;---------------------------------------------------Data
</pre>

<h2><a name="sampleDatagrams">Sample datagrams</a></h2>

<b>Minimal data message (no fragments, no ACKs, no NACKs, etc)</b><br />
<i>(Size: 39 bytes)</i>

<pre>
 +----+----+----+----+----+----+----+----+
 |                  MAC                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                   IV                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |flag|        time       |flag|#frg|    |
 +----+----+----+----+----+----+----+    |
 |  padding to fit a full AES256 block   |
 +----+----+----+----+----+----+----+----+
</pre>

<b>Minimal data message with payload</b><br />
<i>(Size: 46+fragmentSize bytes)</i>

<pre>
 +----+----+----+----+----+----+----+----+
 |                  MAC                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |                   IV                  |
 |                                       |
 +----+----+----+----+----+----+----+----+
 |flag|        time       |flag|#frg| 
 +----+----+----+----+----+----+----+----+
   messageId    |info| fragSize|         |
 +----+----+----+----+----+----+         |
 | that many bytes of fragment data      |
                  .  .  .                                       
 |                                       |
 +----+----+----+----+----+----+----+----+
</pre>