[e2e] Internet packet dynamics

[David G. Andersen]

On Sun, Mar 14, 2004 at 01:16:57PM -0800, Sam Liang scribed:
>   Thanks for the info. 
>   You said 38% of the time, the loss rate is less than 0.2%, which means
> that 62% of the time, it's higher than 0.2%. I think this implies two
> things. First, such loss rate seems good enough for voice over IP, even
> without error recovery. Second, for video streaming, 0.2% is still pretty
> bad.  I am trying to evaluate the severity of packet loss effect on today's
> Internet on real-time multimedia communication.

  Oftentimes it is.  It depends wildly on the connection,
though - voip over DSL would probably be unpleasant without
a DSL router that supported QoS.

  Some of the high loss periods were short outages, and some of them
were congestion.  I didn't break them down in the paper since it's
hard to distinguish with low-rate probing, but it would be
interesting data to see.
 
>   One quick question. In your paper, you seem to suggest that sending
> the same packet back-to-back along the same path get about the same
> benefit as sending packets along different paths. If the packet loss is
> caused by congestion, aren't you aggravating the congestion condition by
> doubling your transmission rate?  And isn't it going to increase the
> packet loss rate?

  Depends on the cause of the losses.  My conclusion from the
paper was that using different paths was more effective against
outages (long-lasting or transient), but that a 20ms spacing between
packets was nearly as effective as using alternate paths when trying
to combat congestion.  This makes some sense if you believe that most
congestion probably occurs at the access links.

 Aren't I aggravating it by sending twice?  Absolutely. :)
I don't think that the basic mesh scheme is appropriate for bulk
transfers.  I think it's great for low-rate control traffic, SNMP
data, and streams where you were planning on sending them at a fixed
bitrate anyway (like the air traffic control example from the  
original Mesh paper at SOSP 2001).  Some of our newer work
looks at the benefits of applying duplication _very_ selectively - 
to, say, TCP SYN packets and DNS packets - and it turns out that that's
a huge win without imposing awful amounts of overhead or impairing
friendliness.

  -Dave
> 
>   Thanks,
> 
> Sam
>   

-- 
work: dga at lcs.mit.edu                          me:  dga at pobox.com
      MIT Laboratory for Computer Science           http://www.angio.net/