Re: [Anima-signaling] GRASP and unicast UDP

On 17/08/2016 18:23, Toerless Eckert wrote:
> Thanks a lot for the insight. How about trying CoAP ?

Two comments on that.

1) I woke up before the alarm went off this morning, and had exactly that
thought. But I'd have to study CoAP in detail to understand whether it
helps. Maybe Carsten can advise on this.

(In GRASP, you may get multiple simultaneous requests or multiple simultaneous
discovery responses. The problem in a nutshell is that with TCP you have
accept(), which fires up a new socket for each incoming transaction, but with
UDP you have to manage sharing the socket yourself.)

2) I also realised that we can't leave it open in GRASP whether discovery
uses TCP or UDP for the discovery response. Otherwise discovery might not work
between two random implementations.

   Brian

> 
> On Wed, Aug 17, 2016 at 04:42:40PM +1200, Brian E Carpenter wrote:
>> To entertain myself while waiting for comments on other matters, I spent
>> quite some time (and swear words) on hacking the prototype GRASP to use
>> UDP instead of TCP for unicast operations (i.e. discovery responses,
>> synchronization responses, and negotiation).
>>
>> My conclusion is that it's a fairly silly thing to do, except possibly
>> for discovery, which only involved a normal amount of debugging. I'm sure
>> this is something that DNS server developers are very familiar with:
>> writing a robust server handling things over UDP is a great deal harder
>> than doing it with TCP. In fact I gave up when I realised that for
>> the negotiation case (where there are an unpredictable number of messages
>> in both directions) I was basically going to have to rewrite half of TCP,
>> and still not have a robust solution. Without a lot more work, the UDP
>> solution is riddled with timing-dependent issues. (At one point, adding
>> a print statement for debugging fixed the bug, so that I simply couldn't
>> debug it...)
>>
>> Of course, all this could be solved with more work, but I'm really not
>> sure we should recommend UDP at all, except for discovery responses.
>> I can't really imagine a real autonomic node that doesn't have TCP code.
>>
>> Comments?
>>
>>     Brian
>>
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> 