Hi Martin, I have implemented the record size extension into mbed TLS. It can be found at https://github.com/ARMmbed/mbedtls/pull/1088
There is only one problem that remains to be addressed IMHO. This extension was developed to address the problem of devices with small RAM. Application developers have to configure their embedded TLS stack in such a way that the parameters configured with this TLS extensions match the available hardware. The record_size_limit helps a lot already but does not quite to the final goal since it uses an artificial metric for deciding when to fragment records. Currently, a developer has to understand various security concepts to get this right, namely * Ciphersuite negotiated (and the overhead associated with it, such as tag length), * DTLS vs. TLS record layer header differences, * potential compression being applied. Additionally, there is, of course, other header information that needs to be considered in the overall buffer size calculation, such as TCP or UDP, IP and potentially any lower layer headers. I just think that this is too much to ask for from an ordinary developer. Hence, I would suggest to use a different metric so that the developer can be certain that at least from a DTLS/TLS layer there are not records being sent that exceed the indicated threshold. If you think that this idea is worthwhile to entertain then I will make a proposal. Ciao Hannes _______________________________________________ TLS mailing list TLS@ietf.org https://www.ietf.org/mailman/listinfo/tls
