Subject: Re: [linux-audio-dev] It's time to vote (n. 1)
From: Vincent Touquet (vincent_AT_ulyssis.org)
Date: Fri May 25 2001 - 19:03:53 EEST
Joe Pfeiffer wrote:
> Hm, but the packets get sent in order, don't they ?
> So when there is a collision, the ethernet card waits to resend the packet for
> which a collision happened, for some random time (exponentially increasing in
> case of another collision). But that cannot mean, that a packet that is after
> this packet in the queue, gets sent before this very packet is succesfully sent,
> or is it ?
>
> Please explain more if I'm wrong.
>
> That corresponds to my understanding, though I could easily be wrong.
> I think the scenario I suggested -- packets could get out of order due
> to a dropped packet -- is possible, though.
Of course, you're right. Dropped packets are a problem. In TCP they get resent, so
they can be out of order. In UDP, I think everything that arrives (*if* it ever
arrives), arrives in order. Of course, that's part of the reason why TCP causes more
overhead than UDP (the computer the data gets sent to, has to acknowledge having
received the packets, and after some time, if there has been no acknowledgement,
packets get resent etc.). But I think that it is not too bold a statement to say that
in single segment LANs, you won't have dropped packets.
I think that collision resolving is part of the ethernet (physical access layer), so
UDP packets always get sent without collision _once_. When they get dropped by a router
who is flooded eg., then the packet dissapears of course... but packets don't dissapear
because of collisions ...
Also note that even when some packets get dropped, you could introduce some redundancy
into your stream. That has a cost though (less speed) and I don't know if it would be
feasible.
I think the best way is to use a LAN, which doesn't drop packets ;). I even think that
I would even buy seperate dedicated NICs :) (then of course, you are sure you have your
bandwith four your audio and you don't have to fool with QoS stuff).
> Even if it does turn out that packets always arrive in order in a
> single network segment, it would be a bad mistake to assume this in a
> protocol.
Hm, I might agree.
Of course, you could build the 'single network segment restriction' into the protocol ?
I mean, if you can't get it working in another way of course.
The more general the better.
Regards
Vincent
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