Upload speeds dont recover after fast retransmission

asked 2021-06-21 20:54:40 +0000

cabsandy gravatar image

updated 2021-06-21 21:06:50 +0000

Hi there Been using Wireshark for a few years but I'm now in deep, and hopefully someone can help. I am using iperf3 (TCP) to test uploads on a symmetrical 1Gb/sec connection-the client is an Odroid C2, with the server being a hosted Ubuntu box on Digital Ocean. Using 4 threads and an RTT of 14ms, downloads are coming through fine, at 940Mb/sec-but uploads are poor-by poor, I mean it starts well in the first few seconds-and then drops off, ending at about 250Mb/sec (10 second test). Looking at the pcap trace on one of the streams (all 4 look the same), TCP slow start is in play-after about 2 seconds in, with a bytes in flight of just over 1MB, a fast retransmission is seen-as a result, the calculated window size of the receiver (which is the Ubuntu server) drops from 2.08MB to 1.19MB-fair enough,TCP doing what it should and it (window size) recovers back to >2MB within a few ms. But here's the bit I dont get-the sender (Odroid) never recovers back to 1MB bytes in flight-as a result, the speeds remain at a stubbornly low level. I have played about with window sizes and memory buffers on both machines but to no avail-the bit I dont get is after that Fast Rx, why doesn't the upload recover, now that the window size being advertised by the receiver has recovered? This receiver works ok with other tests-and if there was an inherent problem, I would expect the downloads to be poor as well-but they are always great.

I have the pcap at hand-see below




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