Re: [dnssd] Fwd: New Version Notification for draft-huitema-dnssd-tls-privacy-00.txt

[Christian Huitema <[e-mail]>]

On 3/11/2019 12:03 AM, Bob Bradley wrote:
>> As designed, the answer is yes, the client would send 20 packets. I
>> understand very well that there is an alternative design in which the
>> server sends a packet announcing its arrival, and then every interested
>> client discovers the server and contacts it. I believe that the scaling
>> is actually equivalent:
>>
>> 1) In my design's worst case, the client sends N packets, and P servers
>> who are present perform O(N) trial decryptions. Total O(P.N2).
>>
>> 2) In the server announce design, P arriving servers send P packets upon
>> arrival on the network, and O(N) clients perform N trial decryptions.
>> Total O(P.N2) as well.
> In (1), there are N multicast packets per client and P unicast responses from paired servers. In (2), there is 1 multicast request per client and P unicast from paired servers. Many devices act as both client and server. Multicast vs unicast can make a big difference in the number of packets processed by each device.
>
> As an example, my device has 40 paired devices and the network has about 300 devices browsing for and offering services (by looking at mDNS). If we assume other devices have a similar number of paired devices then:
>
> Approach 1: 12000 multicast requests (and trial decryptions) and 40 unicast responses.
> Approach 2: 300 multicast requests (and trial decryptions) and 40 unicast responses.

Using your numbers, there would be 12000 trial decryptions in approach 2
as well. Each client has to try 40 different server keys to see which
one would work.

But I am not convinced at all that this 40/300 split is something we
will see in privacy oriented applications. If we are looking at
application pairing rather than device pairing, then the server and
client role are very flexible, the ratio of client and server will be
close to parity, and the number of pairing per application could be very
small.

-- Christian Huitema