Re: Question about global uniqueness of dtls-id

Cullen,

Imagine that call is established between end points A and B through a third
party call control element 3PCC.
3PCC then decides to connect end points A and C. 3PCC sends a request for
an offer to C (INVITE with no SDP in case of SIP).
When 3PCC gets offer from C, 3PCC send this offer to A.
As far as A is concerned this is just a re-offer on an existing connection,
so if tls-id must be different if new DTLS association is needed. If tls-id
just happens to be the same due to collision, this will either be treated
as invalid offer, if fingerprints has changed, or DTLS association will not
be created when it should have.

Essentially, because of 3PCC, connections can migrate between end points,
connecting, disconnecting, and re-connecting multiple end points. For each
end point this looks like a single connection to a 3PCC controller, but
tls-id generated by multiple unrelated end points and should not collide
between any of them.

I agree that 120 bits of randomness is excessive just to avoid collisions.
Amount of randomness was increased in order to securely identify TLS
connection and DTLS associations.

Regards,



_____________
Roman Shpount

On Fri, Sep 1, 2017 at 11:46 AM, Roman Shpount <[email protected]> wrote:

> Cullen,
>
> Imagine that call is established between end points A and B through a
> third party call control element 3PCC.
> 3PCC then decides to connect end points A and C. 3PCC sends a request for
> an offer to C (INVITE with no SDP in case of SIP).
> When 3PCC gets offer from C, 3PCC send this offer to A.
> As far as A is concerned this is just a re-offer on an existing
> connection, so if tls-id must be different if new DTLS association is
> needed. If tls-id just happens to be the same due to collision, this will
> either be treated as invalid offer, if fingerprints has changed, or DTLS
> association will not be created when it should have.
>
> Essentially, because of 3PCC, connections can migrate between end points,
> connecting, disconnecting, and re-connecting multiple end points. For each
> end point this looks like a single connection to a 3PCC controller, but
> tls-id generated by multiple unrelated end points and should not collide
> between any of them.
>
> I agree that 120 bits of randomness is excessive just to avoid collisions.
> Amount of randomness was increased in order to securely identify TLS
> connection and DTLS associations.
>
> Regards,
>
> _____________
> Roman Shpount
>
> On Fri, Sep 1, 2017 at 11:22 AM, Cullen Jennings <[email protected]> wrote:
>
>> There is a bug where we are discussing if the the dtls-id needs to be
>> globally unique or not. Can someone walk me though the logic of why it
>> needs to be globally unique? I'm not finding it in the previous list
>> discussion.
>>
>>
>>
>>
>>
>
>
>

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