MTU to CDN's

William Herrin bill at
Fri Jan 19 01:44:20 UTC 2018

On Thu, Jan 18, 2018 at 7:41 PM, Jared Mauch <jared at> wrote:
>> On Jan 18, 2018, at 7:32 PM, William Herrin <bill at> wrote:
>> On Thu, Jan 18, 2018 at 7:14 PM, Jared Mauch <jared at> wrote:
>>> lets say i can
>>> send you a 9K packet.  If you receive that frame, and realize you need
>>> to fragment, then it’s your routers job to slice 9000 into 5 x 1500.
>> In practice, no, because the packet you sent had the "don't fragment"
>> bit set.
> Which packet?  Is there a specific CDN that does this?  I’d be curious to see
> data vs speculation.


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.


William Herrin ................ herrin at  bill at
Dirtside Systems ......... Web: <>

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