common method to count traffic volume on IX

Nick Hilliard nick at
Wed Sep 18 22:59:35 UTC 2013

On 18/09/2013 18:23, Leo Bicknell wrote:
> Serious question, at an IXP shouldn't IN = OUT nearly perfectly?

if you host multicast on your unicast peering lan, then this will be
affected by the unicast:multicast ratio and the number of recipient ports.
 Most networks which support multicast will also support multicast pruning,
so in reality this counts for very little.

Most IXPs rely on unicast flooding to determine forwarding paths, which
adds a little to the outbound numbers.  So on these IXPs, outbound
aggregate is usually a tiny amount larger than inbound aggregate.  The
larger the network, the smaller this effect.  And on networks which
precompute forwarding paths, the in and out aggregate figures will be equal
+/- counter entropy.


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