Re: [TLS] RFC 5878 - why?

[[e-mail] (Martin Rex)]

Martin Rex wrote:
> Marsh Ray wrote:
> > > Trevor Perrin
> > > Sent: Monday, September 16, 2013 9:16 PM
> > > Subject: [TLS] RFC 5878 - why?
> > > 
> > > Does anyone know of a reason to use RFC 5878?  Extension data can already
> > > be sent in TLS handshakes:
> > 
> > But probably the bigger reason is that some percentage of TLS servers
> > in the wild will crash or (worse) hang when they receive a Client Hello
> > with too much data. Nobody is really sure how much data is safe to send
> > in the Client Hello, so getting preapproved with a small extension is
> > safest.
> 
> I hope that, rather than crashing, the most widespread behaviour among
> servers will be to simply abort the handshake with a fatal Handshake
> failure alert when the ClientHello handshake message exceeds some
> reasonable size (maybe 16 KByte).  Ours does.


Actually, another reason that comes to mind when looking at the
handshake message flowchart here:

  http://tools.ietf.org/html/rfc5878#page-4

taking into account the last sentence before that chart:

   Successful session resumption uses the same authorization information
   as the original session.

and compare it with the handshake message flowchart for the
abbreviated TLS handshake (aka session resume):

  http://tools.ietf.org/html/rfc2246#page-32


For a potentially large TLS extension that carries fairly static data,
you might not want to have to send it on every proposed TLS session resume.

-Martin