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Quoted from the wiki of BitComet:

http://wiki.bitcomet.com/add_port_mapping_in_nat_router

To get the best speeds, BitComet needs an open listening port, just as most other P2P applications do.

http://wiki.bitcomet.com/bitcomet_status_bar

An yellow light means BitComet successfully requested the probe, but did not detect it at your listen port. This indicates that the port is blocked by one or more firewalls. Your client can therefore reach other clients but other clients can not initiate contact with your client.

If your listen port is blocked you can still transfer files with BitComet, but your transfer speed probably won't be very high.

http://wiki.bitcomet.com/bitcomet_options

No listening Mode: will greatly slow down your upload/download rate. Not recommended. (Exceptions are those who are situated behind routers or firewalls which they cannot access and configure. For them, this setting will provide the means to still be able to download even though at much lower speeds).

E.g. I can't not configure the outermost firewall, so the listening port used by a P2P client is blocked, thus I can only be a client not a host. But how would it greatly slow down the download speed? I can still connect to all the hosts as a client.

Is it merely because I would lose those client peer connections if I can be a host?

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E.g. I can't not configure the outermost firewall, so the listening port used by a P2P client is blocked, thus I can only be a client not a host. But how would it greatly slow down the download speed? I can still connect to all the hosts as a client.

No; you cannot connect to all the hosts, because many of them are in the same situation as you – many of them are ordinary PCs, and many of them are behind a NAT router or firewall that they often didn't correctly configure.

You can still connect only to hosts behind a correctly configured firewall, and of course to hosts that have their own dedicated IP address (servers, seedboxes, etc.) And depending on situation, those systems might indeed provide the full download speed that you're expecting; even just a single seedbox peer would often be enough.

However, the help article was not written with dedicated seedboxes in mind – remember that the core design of BitTorrent was to distribute the load across all ordinary users providing chunks of data to each other, so ideally you would have had ~50 peers collectively providing you a high download speed despite each peer individually having slow upload.

So the article's assumption is likely that if you could only use a small portion of available peers (because of the 1st paragraph), let's say ~10 peers, and those peers were ordinary home systems (and not extraordinarily fast servers), then the sum of their upload speeds available to you would be much lower than otherwise.


Terminology note: The opposite of 'client' is 'server', not 'host'. A host means any end node that participates in the network, whether it's acting a client, or a server, or both simultaneously.

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    OK I see. So my guess is right. If I am blocked, then I lost those potential peer connections (those which also can't be a server but only a client) if I could be a server. – Rick May 16 at 5:22
  • Yes, that's right. – grawity May 16 at 5:25
  • Got it. Thank you for your time :-). Many thanks~ – Rick May 16 at 5:26
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The BitTorrent protocol does not consistently use the same TCP port number. Apparently the range it typically uses is 6881-6889 but it can and does randomize the port numbers it uses (which is why it's harder to block than most other protocols, like FTP for example): https://wiki.theory.org/index.php/BitTorrentSpecification

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