Carriage Burst Speeds [was Re: Proving Gig Speed]

Reuben Farrelly reuben-nanog at
Tue Jul 17 09:20:13 UTC 2018

On 17/07/2018 5:49 pm, James Bensley wrote:
> Also there are SO many variables when testing TCP you MUST test using
> UDP if you want to just test the network path. Every OS will behave
> differently with TCP, also with UDP but the variance is a lot lower.

One of the issues I have repeatedly run into is an incorrect burst size 
set on shaped carriage circuits.   In the specific case I have in mind 
now, I don't recall what the exact figures were - but the carrier side 
configuration was set by the carrier to be something like a 64k burst on 
a 20M L3 MPLS circuit.  End to end speed testing results both with 
browsers and with iperf depended enormously on if the test was done with 

Consequently the end customer was unable to get more than 2-3MBit/sec of 
single stream TCP traffic through the link.  The carrier insisted that 
because we could still get 19+ MBit/sec of UDP then there was no issue. 
This was the same for all operating systems.  The end customer certainly 
didn't feel that they were getting the 20Mbit circuit they were sold.

The carrier view was that as we were able to get 20 TCP streams running 
concurrently and max out the link that they were providing the service 
as ordered.  After many months of testing and negotiating we were able 
to get the shaper burst increased temporarily, and the issue completely 
went away.  The customer was able to get over 18MBit/sec of continuous 
TCP throughput on a single stream.  I was told that despite this finding 
and admission that the burst was indeed way too small, the carrier was 
going to continue to provision circuits with almost no burst, because 
this was their "standard configuration".

The common belief seemed to be that a burst was a free upgrade for the 
customer.  I was of the alternate view that this parameter was required 
to be set correctly for TCP to function properly to get their quoted CIR.

I'd be very interested in other's thoughts with regards to testing of 
this.  It seems to me that measuring performance with UDP only means 
that this very critical real-world aspect of a circuit (burst size on a 
shaper) is not tested, and this seems to be a very common 
misconfiguration.  In my case...seen across multiple carriers over many 
years and many dozens of hours spent on "faults" related to it.

[NB: I've always used the rule of thumb that the L3 burst size should be 
about 1/8th the Contracted Line Rate, but there seems to be no consensus 
whatsoever about that...certainly no agreement whatsoever within the 
carrier world]


More information about the NANOG mailing list