[security-dev 00057]: Re: DTLS design

Andreas Sterbenz Andreas.Sterbenz at Sun.COM
Tue Jan 29 00:52:38 PST 2008


as you point out, in a datagram world some things are more complicated and 
there are more choices to make than in a socket world. How to handle 
handshaking, buffer data, etc. This is in many ways similar to the issues 
that came up for TLS with non-blocking I/O. There we made the decision to 
decouple the TLS aspects (SSLEngine) from the I/O aspects. That way the 
choices of how to do I/O, how many threads to use and for which tasks, 
etc. are left to the application developer.

A similar approach may be useful here. In fact, you may be able to use the 
existing SSLEngine API and write an implementation that does DTLS instead 
of SSL/TLS. Note that SSLEngine is not tied to java.nio at all. The only 
link to NIO in the API is the use of ByteBuffers as a generic abstraction 
for a memory region.

Of course, having a drop-in replacement for DatagramSocket is still 
useful. That could be implemented on top of the DTLS engine as as simple 
way for applications to use DTLS without the extra flexibility of the engine.

What do you think?


Christian Uebber wrote:
> Dear Andreas,
> due to some other current business my DTLS efforts were set back for a 
> couple of weeks. But I'm happy to be back on the task now. First I'd 
> like to discuss some architectural issues. I have put much time into 
> working out the best way of integrating DTLS into the current APIs as 
> transparently as possible. All solutions have specific trade-offs.
> The main class most users would use is going to 
> be TLSDatagramSocket extending the well known DatagramSocket. The first 
> design choice would be - DTLS differentiates between client and server - 
> if we should keep an unified DatagramSocket class without a dedicated 
> DatagramServerSocket. I would say yes and I think that's an easy one. 
> Users shouldn't be forced to understand different Java networking 
> concepts if all what is added is an additional security layer. There are 
> also sessions to be managed, but this is also possible in a transparent 
> way without a dedicated server class. For security reasons a method as 
> acceptHandshakeRequests(boolean b) could be added to enable or disable 
> server like behavior.
> In any case threads accessing the main I/O methods send(DatagramPacket 
> p) and receive() should never be blocked by security layer events, as 
> they are needed for bulk transfers from and to anywhere at any time. Of 
> course there must be mechanisms (e.g. callbacks) for users wanting 
> extended control.
> TLS over TCP could attach session information to created sockets in a 
> 1:1 relationship. As we have just one (TLS)DatagramSocket socket for any 
> potential endpoint (1:n) this cannot be done easily in the case of UDP. 
> Since Java programmers never had to care about anything else but single 
> DatagramPackets in their application space, this shouldn't change for 
> anybody who doesn't need to deal with cryptographic details. So my 
> proposal would be letting sessions be managed by the socket, but 
> exposing control to those who need it.
> Trying to accomplish full transparency on the receiving side is quite 
> easy. All packets for which there is no established session object and 
> which are not handshake requests are silently dropped. Handshake 
> requests instead get forwared to a Handshaker class, packets belonging 
> to an established session get unwrapped and are then fired through the 
> send-method of the attached TLSDatagramSocket.
> Transparency on the sending side implies some trade-off decisions in 
> cases where the send-method gets packets for destinations without an 
> already established session.
>    1. Full Transparency:
>          1. Session initiation is started, packets for an unestablished
>             target are buffered until the session is established and
>             then sent (or dropped if establishment failed).
>          2. Session initiation is started, packets get dropped (nobody
>             promised that UDP would be reliable).
>    2. Manual Session Initiation: A method as initiateSession(InetAddress
>       a, int port, CallbackHandler c) must be called before any packet
>       can be send to a specific endpoint.
> Buffers could fill up quite quickly under heavy load in the case of 1.1. 
> In the case of 1.2 no promises are broken, but it's not really good 
> style. An average handshake can take a second and a lot of packets can 
> already be lost during this time. The case No. 2 wouldn't be usable as a 
> drop-in replacement anymore.
> Maybe some aspects could be combined for the best solution. What do you 
> think?
> But that should be it for today... I'm looking forward to your remarks.
> Christian
> PS I've emailed the signed SCA today. It should get registered soon.
> Am 30.11.2007 um 02:42 schrieb Andreas Sterbenz:
>> Christian Uebber wrote:
>>> Is anybody already working on this for Java DatagramSockets? I'd be 
>>> interested in doing the work. Integration into and reuse of the 
>>> existing JSSE code would also be my preferred way to go.
>> That sounds like a great idea. We at Sun don't have any current plans 
>> to implement DTLS due to a lack of resources, but we could assist by 
>> answering questions about JSSE or commenting on your code. There are 
>> also some architectural issues about fitting a secure datagram 
>> transport into the current Networking APIs that we may want to discuss.
>> BTW, you may want to look into signing the contributor agreement: 
>> http://openjdk.java.net/contribute/
>> Andreas.

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