[TLS] draft-ietf-tls-record-limit-01

[Hannes Tschofenig <[e-mail]>]

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