Re: [TLS] ServerHello layout in TLS 1.3

On Sat, Dec 12, 2015 at 4:08 AM, Hannes Mehnert <hannes@mehnert.org> wrote:

> 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?
>

I'm not aware of ay reason besides cleanup. I don't feel strongly
about the current state.

-Ekr

> 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
> ).
>
>
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