[security-dev 00058]: Re: DTLS design

Christian Uebber briefkasten at uebber.de
Tue Jan 29 03:39:08 PST 2008

The network and security layer are not as easily separable in the case  
of DTLS as they are in the case of TLS over TCP. During the exchange  
of control data (handshake, change_cipher_spec, alert) the network  
layer must provide reliability, but not anytime else.

As the SSLEngine must be extended anyway for a complete DTLS  
implementation, I'm going to follow your suggested route and do that  
first. I have had this option on the table anyways. Calling it the  
"nio centered approach" i was referring, in a historical sense, to the  
two traditional branches of network implementations in Java: the  
legacy blocking sockets with attached SSL functionality and the  
branch, which started with nio, where everything is nice, fresh, and  
clean. I may sometimes be prone to sloppy, not very precise English,  
when I'm missing precise vocabulary to express exactly what I want to  
say. But I think we'll work it out.

To make SSLEngine DTLS-able most changes do not affect its interface  
and could be implemented internally; for that alone we wouldn't even  
need a separate DTLSEngine. After all it's just  another record format  
and a slightly modified handshake. I haven't spend much time on the  
issue of how to integrate the reliable transport for the handshake  
phase into the completely network-independent design. I'll work that  
out and get back to you. If any ideas pop into your mind, let me know.


Am 29.01.2008 um 09:52 schrieb Andreas Sterbenz:

> Christian,
> 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?
> Andreas.
> 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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