[ENet-discuss] peer->roundTripTime

[Lee Salzman]

Well, it appears to be simple round-off error. Due to the way I average 
things there is margin of error of +-8ms in the RTTs.

Lee

intripoon wrote:
> Hi!
> 
> I know about that. Actually, my program doesn't do much more than servicing
> enet, sending a packet once per second and print out the rtt every 5
> seconds.
> 
> What I do is get the system time in microseconds by using the
> PerformanceCounter, put that value in a packet and send it to the other
> peer, which again gets the system time in microseconds using the
> PerformanceCounterr. The difference is about 300 microseconds, averaged over
> a few interations. So it actually takes about 300 microseconds to send a
> packet from one peer to another on the same pc, in two different processes.
> At the same time, the rtt returned by enet is 7 milliseconds, telling me the
> averaged time for a packet transmission is 3500 microseconds. Factor 10
> off?!?
> 
> I didn't know of http://enet.bespin.org until Bjorn's post. Googleing for
> enet results in the cubrik page for me. However, I downloaded the 1.0 from
> the bespin page and it is probably newer because some files are larger than
> in my previous version. The issue described above is the same in both
> versions though.
> 
> The reason why I'm curious about it is because I planed to use the rtt value
> to correct time differences between the peers. But a 6.5 ms error on
> localhost is too much for my application. I could send packets containing
> timestamps between the peers and calculate the rtt and the time correction
> myself. However, that'ld require to send additional packets. The possibility
> to use all packets sent to calculate the rtt has to be done in enet's layer
> - or I would have to introduce another layer on top of enet adding the
> required information to each packet. But all this is already done in enet,
> but for some reason it seems to be quite unprecise. Any idea why?
> 
> Best regards
> 	Marc
> 
> 
> -----Ursprüngliche Nachricht-----
> Von: enet-discuss-bounces at cubik.org [mailto:enet-discuss-bounces at cubik.org]
> Im Auftrag von Lee Salzman
> Gesendet: Mittwoch, 27. Dezember 2006 17:37
> An: Discussion of the ENet library
> Betreff: Re: [ENet-discuss] peer->roundTripTime
> 
> Remember, ENet does pretty much all work in enet_host_service(). So if 
> you aren't constantly servicing the host, then the frequency at which 
> you call it is going to have a small effect on RTTs. It's never been a 
> big deal in so far as what ENet's designed for.
> 
> Lee
> 
> intripoon wrote:
>> Hi !
>>
>> I wonder about the peer->roundTripTime value. Between every pc in my local
>> network, all peers have a roundTripTime of 7 to each other, no matter what
> I
>> do. Shouldn't that be value be smaller? Even on one pc having two
> processes
>> connecting to each other by enet peers, that value is 7. I also tried
>> sending a high performance counter value from one process to the other to
>> check if it really takes 3.5 ms to send a packet - but it doesn't.
>>
>> Somewhere I read, the rtt value is computed by comparing the time of a
>> reliably sent packet and it's returning ack packet. Do maybe ack packets
> get
>> delayed for some reason? Maybe waiting for more possible ack packets to be
>> sent? Maybe something like that explains the 7 ms?
>>
>> Is there a way to configure enet to return a more correct value in this
>> case?
>>
>> I'm not quite sure which version of enet I'm using. Is there a version
>> string somewhere? It's not too old though.
>>
>> Best regards
>> 	Marc
>>