UDP clamped on service provider links

[morrowc.lists at gmail.com (Christopher Morrow)]

On Fri, Jul 31, 2015 at 8:07 AM, John Kristoff <jtk at cymru.com> wrote:
> On Thu, 30 Jul 2015 21:18:10 -0500
> Jason Baugher <jason at thebaughers.com> wrote:
>
>> In one case, when we were having an issue with a SIP trunk, we
>> re-numbered our end to another IP in the same subnet. Same path from
>> A to Z, but the packet loss mysteriously disappeared using the new
>> IP. It sure seems like they are throttling somewhere.
>
> Not knowing how you evaluated the two paths, but if MPLS was not
> considered, it may have perhaps been due in part to ECMP behavior.
> While not ruling out UDP limits, it is plausible that the changed
> source IP address resulted in a less congested path to be chosen.

ding! this sounds like the most plausible answer... I wouldn't expect
L3 to limit udp/5060/6061/SIP traffic, as a common carrier that also
runs a SIP trunking service they:
  1) probably know what SIP traffic is
  2) don't want to get bitten being seen as preferring their own
network offerings over other external ones (or perhaps accidentally
impacting actual customers).