[Bloat] Speed tests - attribution of latency to relevant network hops

[Bill Ver Steeg (versteb)]

You do have to be careful using ttl tricks and/or ICMP packets for fine-grained timing measurements. On many middleboxes, there is a "fast path" that is done in a highly optimized manner (often in silicon) and a "slow path". In a non-zero number of cases, you will be in the fast path for normal packets and in the slow path for unusual packets. 

What you describe will work for the vast majority of cases, but there will be situations in which you will get misleading results. You would need to handle the timing differences when the slow path was MUCH slower than the fast path and it distorted the results. You would also have to handle some weird edge conditions..... The worst case that springs to mind would be a bloated buffer somewhere in a device's fast path (due to lots of "normal" packets), but the slow path on the same device is not congested and thus actually has less delay for this single packet.

Bvs
  

-----Original Message-----
From: bloat-bounces at lists.bufferbloat.net [mailto:bloat-bounces at lists.bufferbloat.net] On Behalf Of Jan Ceuleers
Sent: Thursday, July 30, 2015 1:18 AM
To: Mikael Abrahamsson; David Lang
Cc: bloat
Subject: Re: [Bloat] Speed tests - attribution of latency to relevant network hops

On 30/07/15 06:52, Mikael Abrahamsson wrote:
> On Wed, 29 Jul 2015, David Lang wrote:
> 
>> unless you measure it per hop, how are you going to attribute it to 
>> each hop? and unless you have a server at that layer to talk to, how 
>> do you know what the latency or bandwidth is?
> 
> Measuring latency is doable (using the same mechanism that traceroute 
> with for instance max-ttl 5), but I don't know how much of this is 
> available to your web application?
> 
> If you sent 5 packets with TTL 1-5 and measured the time to get back 
> the ttl-expired-in-transit ICMP, you could get an indication where the 
> latency increase was happening.

Yes, that's what I had in mind.
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