Is there any data on packet duplication?

William Herrin bill at herrin.us
Tue Jun 23 05:11:59 UTC 2020


On Mon, Jun 22, 2020 at 9:43 PM Saku Ytti <saku at ytti.fi> wrote:
> I can't tell you how common it is, because that type of visibility is
> not easy to acquire, But I can explain at least one scenario when it
> occasionally happens.
>
> 1) Imagine a ring of L2 metro ethernet
> 2) Ring is connected to two PE routers, for redundancy
> 3) Customers are connected to ring ports and backhauled over VLAN to PE
>
> If there is very little traffic from Network=>Customer, the L2 metro
> forgets the MAC of customer subinterfaces (or VRRP) on the PE routers.
> Then when the client sends a packet to the Internet, the L2 floods it
> to all eligible ports, and it'll arrive to both PE routers, which will
> continue to forward it to the Internet.

Hi Saku,

That's what spanning tree and its compatriots are for. Otherwise,
ordinary broadcast traffic (like those arp packets) would travel in a
loop, flooding the network and it would just about instantly collapse
when you first turned it on.

A slightly more likely scenario is a wifi link. 802.11 employs layer-2
retries across the wireless segment. When the packet is successfully
transmitted but the ack is garbled, the packet may be sent a second
time.

Even then I wouldn't expect duplicated packets to be more than a very
small fraction of a percent. Hal, if you're seeing a non-trivial
amount of identical packets, my best guess is that the client is
sending identical packets for some reason. NTP you say? How does
iburst work during initial sync up?

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/


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