Extreme congestion (was Re: inter-domain link recovery)

Thu Aug 16 18:49:02 UTC 2007

Mikael Abrahamsson wrote:
> 
> On Thu, 16 Aug 2007, michael.dillon at bt.com wrote:
> 
>> How many people have noticed that when you replace a circuit with a 
>> higher capacity one, the traffic on the new circuit is suddenly 
>> greater than 100% of the old one. Obviously this doesn't happen all 
>> the time, such as when you have a 40% threshold for initiating a 
>> circuit upgrade, but if you do your upgrades when they are 80% or 90% 
>> full, this does happen.
> 
> I'd say this might happen on links connected to devices with small 
> buffers such as with a 7600 with lan cards, foundry device or alike. If 
> you look at the same behaviour of a deep packet buffer device such as 
> juniper or cisco GSR/CRS-1 the behaviour you're describing doesn't exist 
> (at least not that I have noticed).

Depends on your traffic type and I think this really depends on the 
granularity of your study set (when you are calculating 80-90% usage). 
If you upgrade early, or your (shallow) packet buffers convince to 
upgrade late, the effects might be different.

If you do upgrades assuming the same amount of latency and packet loss 
on any circuit, you should see the same effect irrespective of buffer 
depth. (for any production equipment by a main vendor).

Deeper buffers allow you to run closer to 100% (longer) with fewer 
packet drops at the cost of higher latency. The assumption being that 
more congested devices with smaller buffers are dropping some packets 
here and there and causing those sessions to back off in a way the 
deeper buffer systems don't.

Its a business case whether its better to upgrade early or buy gear that 
lets you upgrade later.

DJ