MTU to CDN's

Jared Mauch jared at
Fri Jan 19 13:58:19 UTC 2018

> On Jan 18, 2018, at 8:44 PM, William Herrin <bill at> wrote:
>> Which packet?  Is there a specific CDN that does this?  I’d be curious to see
>> data vs speculation.
> Howdy,
> Path MTU discovery (which sets the DF bit on TCP packets) is enabled
> by default on -every- operating system that's shipped for decades now.
> If you don't want it, you have to explicitly disable it. Disabling it
> for any significant quantity of traffic is considered antisocial since
> routers generally can't fragment in the hardware fast path.

I’m not seeing this in a PCAP capture to at least one CDN, either from my
host or from the CDN endpoint.

I suspect you’re mistaken.

- Jared


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