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I use tcpdump to get packets of sender by downloading file from sender. The sender use Tcp reno. But it seems that cwnd doesn't increase immediately after get enough acks. See 2 pictures. details in wireshark , tcptrace in wireshark.

The cwnd is actually increasing, but not immediately. I don't know why.

Leavrth Le
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  • the MSS is 1460. – Leavrth Le Nov 25 '20 at 02:52
  • Could you elaborate on what you expected to see? It seems from your trace there was a loss detected right at the top, so one may expect TCP to be at the linear growth phase, no? – haggai_e Nov 26 '20 at 06:11
  • This is the linear growth phase, but I don't know why the cwnds per RTT are <2, 4, 4, 4...,6,6...,6, > instead <2, 3, 4, 5, 6, 7,...> – Leavrth Le Nov 26 '20 at 13:00
  • What is the RTT? – haggai_e Nov 26 '20 at 13:57
  • Some tutorials say that in the congestion avoidance phase, there will be cwnd+=1/cwnd every time a packet is received. RTT is Round Trip Time, which means the time from when packets are sent to when they come back. I think there may be cwnd packets sent to receiver each RTT. For example, time 19.444198 and time 19.412402, I think sender has no more cwnd so it wait for a ack at time 19.444167. That how I infer cwnd. I want to know if this is wrong. Or I want to know how to infer the cwnd for each period. Thanks in advance. – Leavrth Le Nov 27 '20 at 02:04
  • Thanks, I knew what RTT is, I meant to ask what is the RTT in your system :) If I understand correctly, cwnd should grow by 1 every RTT in the congestion avoidance phase. If cwnd>1 that would mean not every ack would cause cwnd to grow. – haggai_e Nov 28 '20 at 10:55
  • The sender is on Centos7. I think RTT is determinded by network situation, maybe you're talking about RTO. Today I ask my teacher, he advice me to have a local test to avoid complex network. Anyway, Thank you for your attention! – Leavrth Le Nov 30 '20 at 06:12

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