[TLS] Record layer corner cases

Hi all,

In San Diego I gave a short presentation on how various TLS
implementations handle some corner cases in the TLS record protocol:

http://www3.ietf.org/proceedings/06nov/slides/tls-3.ppt

The conclusion was that receiving these corner cases is not handled
well, probably because real implementations never send them.
Thus, I'd suggest explicitly forbidding some of these corner
cases in TLS 1.2. In particular:

- Implementations MUST NOT fragment Handshake, Alert, or Change
  cipher spec messages to multiple TLS records unless the message 
  exceeds the maximum fragment length (either 2^14 or negotiated 
  with max_fragment_length extension).

- Implementations MUST NOT send zero-length fragments of Handshake, 
  Alert, or Change Cipher Spec content types (zero-length fragments
  of Application data would still be allowed; presumably they might
  be useful against traffic analysis).

- Implementations MUST NOT send content types not defined in this
  document except when explicitly negotiated using e.g. a TLS
  extension (current text says unknown content types SHOULD
  be just ignored).

- The maximum fragment size is a strict limit (i.e. change
  "the length should not exceed 2^14" to MUST NOT).

Comments?

Best regards,
Pasi

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