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I run a Tor relay. I have often seen that there is a significant discrepancy between the amount of data sent and that received. For instance, this morning log states:

Tor's uptime is 7 days 18:00 hours, with 551 circuits open. I've sent 167.80 GB and received 140.35 GB. 

That's a nearly 20% difference. I had a view of a relay as little more than a passing station on the road, except for the occasional authentication to the network.

Does anyone know the origin and meaning of this discrepancy? By the way, I am not hosting any hidden services.

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It is normal that one receives less data then was send because of packet loss. I think 20% is a bit too much to just blame packet loss but it could be. – Mixxiphoid Oct 11 '13 at 10:36
Have you seen this question on tor.stackexchange or is this question eventually from you? – nixda Oct 11 '13 at 12:19
No, I had not seen this. Thanks for pointing this out to me. – MariusMatutiae Oct 11 '13 at 15:09
@nixda If you post your comment as an answer, I will accept it. Sorry for taking so long. – MariusMatutiae Mar 6 at 11:39

In addition to the link about being a directory service.

ToR adds an additional layer of encryption each hop a request goes through, that would mean that outbound traffic would be larger than the original request.

TOR instead uses, at a minimum, three servers to pass your traffic on, each encapsulated with its own layer of encryption.

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