Is there any data on packet duplication?

Saku Ytti saku at ytti.fi
Tue Jun 23 04:41:41 UTC 2020


Hey Hal,

> How often do packets magically get duplicated within the network so that the
> target receives 2 copies?  That seems like something somebody at NANOG might
> have studied and given a talk on.

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.
This requires an unfortunate (but typical) combination of ARP timeout
and MAC timeout, so that sender still has ARP cache, while switch
doesn't have MAC cache.

In the opposite direction this same topology can cause loops, when PE
routers still have a customer MAC in the ARP table, but L2 switch
doesn't have the MAC.

I wouldn't personally add code in applications to handle this case
more gracefully.
-- 
  ++ytti


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