However, the loss of those 4 small packets causes the sender to reduce the transmit window to just 850 KB (620 packets x 1400 bytes) per round trip. In your case, the sender's transmit window initially ramps up to 2MB (1482 packets x 1400 MSS) per RT. The first set of SACKs start at #3458 and the first set of retransmitted packets are #3569, #3570, #3571 & #3572. The retransmitted packets are the correct size in this capture (ie, not the very large ones due to being captured before the IP layer 'packetised' them). You will then see the retransmissions of the data that the SACKs reported as being lost. There can be multiple left and right edges, indicating multiple 'gaps' in the flow. If you delve into the packet detail of the Dup-ACKs, you'll see the 'left edge' and 'right edge' values that identify them as SACKs.

In this case you have to find the Selective ACKs - which are shown as 'Dup-Acks' in Wireshark.

This is clearly visible, several times, in blue on the Wireshark chart (Statistics - TCP Stream Graphs - Window Scaling).Īn excellent presentation about Congestion Avoidance algorithms was made by Vladimir Gerasimov at SharkFest 2019 did a lot of valuable research and preparation. When there are multiple packet loss events, a chart of the 'Bytes-in-Flight' value forms a sawtooth pattern with fast decreases and slow increases. The subsequent 'gentle' ramp up (usually by one packet per RT) means that overall throughput suffers severely. When a sender detects packet loss it is supposed to 'halve' its transmit window and then ramp up very slowly.Ī halving of bytes-per-round-trip or packets-per-RT results in a halving of throughput. It is some relatively small packet losses at various times which trigger the TCP 'Congestion Avoidance' mechanism/algorithm. However, the reason for the slow transfer is actually quite basic and common. That is, missing from the trace file but were there in real life. Your original 'AIX-to-Cloud' capture file is made more difficult to analyse due to the fact that there are a lot of packets that weren't captured.

Filter tcp retransmission wireshark