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

Sean Donelan sean at donelan.com
Wed Aug 15 15:59:54 UTC 2007

On Wed, 15 Aug 2007, Fred Baker wrote:
> On Aug 15, 2007, at 8:35 AM, Sean Donelan wrote:
>> Or should IP backbones have methods to predictably control which IP 
>> applications receive the remaining IP bandwidth?  Similar to the telephone 
>> network special information tone -- All Circuits are Busy.  Maybe we've 
>> found a new use for ICMP Source Quench.
> Source Quench wouldn't be my favored solution here. What I might suggest is 
> taking TCP SYN and SCTP INIT (or new sessions if they are encrypted or UDP) 
> and put them into a lower priority/rate queue. Delaying the start of new work 
> would have a pretty strong effect on the congestive collapse of the existing 
> work, I should think.

I was joking about Source Quench (missing :-), its got a lot of problems.

But I think the fundamental issue is who is responsible for controlling 
the back-off process?  The edge or the middle?

Using different queues implies the middle (i.e. routers).  At best it 
might be the "near-edge," and creating some type of shared knowledge
between past, current and new sessions in the host stacks (and maybe
middle-boxes like NAT gateways).

How fast do you need to signal large-scale back-off over what time period?
Since major events in the real-world also result in a lot of "new" 
traffic, how do you signal new sessions before they reach the affected
region of the network?  Can you use BGP to signal the far-reaches of
the Internet that I'm having problems, and other ASNs should start slowing
things down before they reach my region (security can-o-worms being 

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