Extreme congestion (was Re: inter-domain link recovery)
dzlboi at gmail.com
Wed Aug 15 17:06:14 UTC 2007
Congestion and applications...
A tier 1 provider does not care what traffic it carries. That is all a
function of the application not the network.
A tier 2 provider may do traffic shaping, etc.
A tier 3 provider may decide to block traffic paterns.
More or less... The network was intended to move data from one machine to
another... The less manipulation in the middle the better... No
manipulation of the payload is the name of the game.
That being said. It's entirely a function of the application to timeout and
drop out of order packets, etc.
ONS is designed around this principle.
In streaming data... often it is better to get bad or missing data than to
try and put out of order or bad data in the buffer...
A good example is digital over-the-air tv... If you didn't build in enough
error correction... then you'll have digital breakup, etc. It is
impossible to recover any of that data.
If reliable transport of data is required... That is a function of the
ONS is an Optical Networking Standard in the development stage.
On 8/15/07, Stephen Wilcox <steve.wilcox at packetrade.com> wrote:
> Hey Sean,
> On Wed, Aug 15, 2007 at 11:35:43AM -0400, Sean Donelan wrote:
> > On Wed, 15 Aug 2007, Stephen Wilcox wrote:
> > >(Check slide 4) - the simple fact was that with something like 7 of 9
> > >cables down the redundancy is useless .. even if operators maintained
> > >N+1 redundancy which is unlikely for many operators that would imply
> > >50% of capacity was actually used with 50% spare.. however we see
> > >around 78% of capacity is lost. There was simply to much traffic and
> > >not enough capacity.. IP backbones fail pretty badly when faced with
> > >extreme congestion.
> > Remember the end-to-end principle. IP backbones don't fail with extreme
> > congestion, IP applications fail with extreme congestion.
> Hmm I'm not sure about that... a 100% full link dropping packets causes
> many problems:
> L7: Applications stop working, humans get angry
> L4: TCP/UDP drops cause retransmits, connection drops, retries etc
> L3: BGP sessions drop, OSPF hellos are lost.. routing fails
> L2: STP packets dropped.. switching fails
> I believe any or all of the above could occur on a backbone which has just
> failed massively and now has 20% capacity available such as occurred in SE
> > Should IP applications respond to extreme congestion conditions better?
> alert('Connection dropped')
> "Ping timed out"
> kinda icky but its not the applications job to manage the network
> > Or should IP backbones have methods to predictably control which IP
> > applications receive the remaining IP bandwidth? Similar to the
> > network special information tone -- All Circuits are Busy. Maybe we've
> > found a new use for ICMP Source Quench.
> yes and no.. for a private network perhaps, but for the Internet backbone
> where all traffic is important (right?), differentiation is difficult unless
> applied at the edge and you have major failure and congestion i dont see
> what you can do that will have any reasonable effect. perhaps you are a
> government contractor and you reserve some capacity for them and drop
> everything else but what is really out there as a solution?
> FYI I have seen telephone networks fail badly under extreme congestion.
> CO's have small CPUs that dont do a whole lot - setup calls, send busy
> signals .. once a call is in place it doesnt occupy CPU time as the path is
> locked in place elsewhere. however, if something occurs to cause a serious
> amount of busy ccts then CPU usage goes thro the roof and you can cause
> cascade failures of whole COs
> telcos look to solutions such as call gapping to intervene when they
> anticipate major congestion, and not rely on the network to handle it
> > Even if the IP protocols recover "as designed," does human impatience
> > there is a maximum recovery timeout period before humans start making
> > problem worse?
> i'm not sure they were designed to do this.. the arpanet wasnt intended to
> be massively congested.. the redundant links were in place to cope with loss
> of a node and usage was manageable.
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