[TLS] ServerHello layout in TLS 1.3

[Hannes Mehnert <[e-mail]>]

Hi,

while implementing the TLS 1.3 draft, I came across the modified
ServerHello message - it does neither contain a SessionID nor a
CompressionMethod field.  This makes code which supports both TLS 1.3
and earlier versions more complicated*:  a client sending out a
ClientHello does not know which protocol version the server is willing
to talk.

Thus, depending on the protocol version inside of the server hello, the
remainder of the packet needs to be parsed.

I'm not sure whether it is worth to safe two bytes (empty SessionID and
NULL CompressionMethod) to introduce such an intrinsic dependency.

Is there a reason behind this change, apart from cleanup?


Thanks,

Hannes

*: It is not hard to implement a conditional here, but it makes the
control flow of the code unnecessary complex imho, especially since up
to TLS-1.2 client and server hello can be parsed by the same function
(modulo the list vs single ciphersuite and compression method, see
https://github.com/mirleft/ocaml-tls/blob/99384104e09b21a736872854e0a69615a4545780/lib/reader.ml#L295-L327).