Re: Forward Erasure Correction (FEC) and network performance

["Patrick W. Gilmore" <patrick () ianai net>]

On Apr 10, 2009, at 11:03 AM, Marshall Eubanks wrote:

> I work with FEC in various ways, mostly to protect video streams  
> against packet loss, including as co-chair
> of the IETF FECFRAME WG and in the Video Services Forum. Most FEC is  
> driven by congestion in the edge, RF issues on wireless LANs, etc.,  
> but there is always
> the chance of loss in transit over the wider network. In many  
> important cases, in fact, (e.g., transfer of video from a content  
> creator to
> an IPTV service provider or Enterprise to Enterprise video  
> conferencing) the loss at the edges can be controlled, leaving only  
> network transit to
> worry about.
>
> This question has thus come up from time to time, and I was hoping  
> that the assembled NANOG might be able
> to either answer it or provide pointers to the literature :
>
> What level of packet loss would trigger response from network  
> operators ? How bad does a sustained packet loss need
> to be before it is viewed as a problem to be fixed ? Conversely,  
> what is a typical packet loss fraction during periods
> of good network performance ?
>
> If there is some consensus around this, it would effectively set an  
> upper bound for the need for FEC in network transit.

There will be two consensuses (consensai?).

People who _use_ the network will tell you that a network provider  
will fix a network when they complain, and never before.  You have 50%  
packet loss?  Trying to shove 40 Gbps down a GigE?  Provider doesn't  
care, or notice.

People who _run_ the network will tell you: "No packet loss is  
acceptable!"  They'll explain how they constantly monitor their  
network, have SLAs, give you a tour of their show-NOC, etc.  But when  
you read the SLA, you realize they are measuring packet loss between  
their core routers in city pairs.  And frequently they don't even  
notice when those hit 2 or 3% packet loss.

If you try to send a packet anywhere other than those cities and those  
router, say down your own transit link, or to a peer, or another  
customer, well, that's not monitored.  And packet loss on those links  
are not covered by the SLA in many cases.  Even if it is covered, it  
will only be covered from the time you open a ticket.  (See point  
about provider not caring until you complain.)

There are a few networks who try harder than others, but no network is  
perfect.  And although you did not say it, I gather than you are not  
planning to use one of the better networks, you need to use them all.


In Summary: How much packet loss is typical?  Truthfully, 0% most  
(i.e. > 50%) of the time.  The rest of the time, it varies between a  
fraction of a percent and double-digit percentages.  Good luck on  
figuring out a global average.

--
TTFN,
patrick