- It is not a goal to support transport-specific interfaces (for example, DTLS for
- It is not a goal to support PMTU discovery.
The implementation, in both client and server modes, must interoperate successfully with at least two other DTLS implementations.
It is important to support DTLS to satisfy secure-transport requirements for the increasing number of datagram-compatible applications. RFC 4347 lists a number of reasons why TLS is not sufficient for these types of applications:
- “TLS is the most widely deployed protocol for securing network traffic. … However, TLS must run over a reliable transport channel—typically TCP. It therefore cannot be used to secure unreliable datagram traffic.”
- “… an increasing number of application layer protocols have been designed that use UDP transport. In particular, protocols such as the Session Initiation Protocol (SIP) and electronic gaming protocols are increasingly popular.”
- “In many cases, the most desirable way to secure client/server applications would be to use TLS; However, the requirement for datagram semantics automatically prohibits use of TLS. Thus, a datagram-compatible variant of TLS is very desirable.”
Protocols that support DTLS include, but are not limited, to:
- RFC 5238, Datagram Transport Layer Security (DTLS) over the Datagram Congestion Control Protocol (DCCP)
- RFC 6083, Datagram Transport Layer Security (DTLS) for Stream Control Transmission Protocol (SCTP)
- RFC 5764, Datagram Transport Layer Security (DTLS) Extension to Establish Keys for the Secure Real-time Transport Protocol (SRTP)
- RFC 7252, Constrained Application Protocol (CoAP)
Google Chrome and Firefox now support DTLS-SRTP for Web Real-Time Communication (WebRTC). DTLS version 1.0 and 1.2 are supported by the major TLS vendors and implementations including OpenSSL, GnuTLS, and Microsoft SChannel.
We expect the DTLS API and implementation to be fairly small. The new API should be transport-independent and similar to
javax.net.ssl.SSLEngine. Further details on the API will be added here as the work progresses. Some initial design considerations are as follows:
- The DTLS API and implementation will not manage read timeouts. It will be the responsibility of the application to determine an appropriate timeout value and when and how to trigger the timeout event.
- A new API will likely be added to set the maximum application datagram size (PMTU minus the DTLS per-record overhead). If the size is not specified explicitly, however, then the DTLS implementation should adjust the size automatically. If a fragment is lost two or three times, the implementation may reduce the size of the maximum application datagram size until it is small enough.
- The DTLS implementation should consume or produce at most one TLS record for each unwrap or wrap operation, so that the record can be delivered in the datagram layer individually or can be reassembled more easily if the delivery is out of order.
- It is the application’s responsibility to assemble the out-of-order application data accordingly if necessary. The DTLS API should provide access to the application data in each DTLS message.