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I googled a lot about this question, but I didn't find anything useful.

If, let's say, the router just drop the packets when the transfer rate exceed the limitation, then there should be TCP retransmissoin packets when downloading from Internet, but I didn't see any. Or, if it is happened in the data link layer, like setting the interpacket gap, then why downloading from LAN can reach the max speed of the router?

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When the router's forwarding capacity is exceeded, ingress packets are dropped - the router physically receives the packets but due to lack of buffering they can't be stored.

When the downlink's capacity is exceeded, packets are dropped by the upstream router - your router never sees them.

Both events cause TCP retransmissions. However, TCP's congestion control kicks in and the sender adapts the stream speed that's exceeding link and buffer capacities, so after a short while retransmissions become rare.

When you're downloading from wherever and monitoring the receiver, you won't be able to see retransmissions because the initial transmissions haven't reached you. Only the sender sees retransmissions.

The data link layer has nothing to do with this. The Ethernet IPG (physical layer) is completely unrelated, it's fixed and can't be changed.

  • But the cwnd in congestion control keeps growing after retransmission. Doesn't that implie that transmission speed will growing again till another packet loss? So I guess there should be a retransmission periodically. – ryuu Feb 10 '18 at 11:16
  • Yes, that's true. TCP will carefully try to ramp up the sending speed until there are retransmissions again. – Zac67 Feb 10 '18 at 11:22
  • Well, that's weird, I just did my experimentation again(on a different network, it's faster), and yes, there are some retransmissions, periodically. Maybe I should have observed a little bit longer. – ryuu Feb 10 '18 at 11:44
  • Glad that it's answered then. ;-) – Zac67 Feb 10 '18 at 13:56
  • The edited answer really sobered me up, I should monitor on a sender. It's just Wireshark can detect retransmissions, even on a receiver. Thanks a lot. – ryuu Feb 10 '18 at 15:55

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