[openssl-dev] Calculating DTLS payload MTU

[Matt Caswell]

On 05/10/16 20:59, David Woodhouse wrote:
> 
>>
>>
>> On 05/10/16 15:37, David Woodhouse wrote:
>>> On Wed, 2016-10-05 at 14:40 +0100, David Woodhouse wrote:
>>>> How's this for a start...
>>>
>>> Now I think I have it right for CCM too, although having to use
>>> strstr() for that makes me *very* sad. Next up, Chacha20-Poly1305...
>>> and then maybe I can stop worrying about new modes and ciphersuites
>>> because those won't be added in OpenSSL 1.1 and we can get OpenSSL do
>>> to this for itself before 1.2? :)
>>
>> Or even 1.1.1! Why don't you pull this together into a github PR?
> 
> Sure. Any thoughts on what you want it to look like? Not just
> SSL_CIPHER_get_overhead() because it'seems not constant -- you have to
> account for blocksize rounding and padding in CBC ciphers. So just
> SSL_CIPHER_get_data_mtu(int link_mtu)?

Hmmmm....well its not constant even by cipher. It depends on the
transport (IPv6 has a bigger overhead than IPv4).

We already have:

DTLS_set_link_mtu(SSL *s, int mtu)

So why not:

DTLS_get_data_mtu(SSL *s)

> 
> Or are there other use cases we want it for, and we *should* try returning
> a full set of three "add this, round up to this, add this" integers?
> 
> As for the actual numbers... am I on the right track so far?
> 


I've not thought about it in great detail, but it looks ok at first glance.

Matt