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Hi all,

I hope anybody can help me. I'm trying to measure the RTT for TCP packets over LTE. I have running a server with a file (size 1K) and I download this file 10 times with wget on a client, capturing the packets with tcpdump. To get the RTT I analyze this packets by observing both SYN-SYN/ACK time difference and the FIN/ACK - FIN/ACK time difference. I expected to get values in the same magnigute. But I observe, that the SYN-SYN/ACK round trip time is always 3-5 times bigger than FIN/ACK RTT.

Example:

| No. | Time | Source | Destination | Protocol | Info | RTT | | 1 | 0.000 | 100.xx | 195.xx | TCP | SYN | 0.000 | | 2 | 0.156 | 195.xx | 100.xx | TCP | SYN,ACK | 0.156 | | ... | ... | ... | ... | ... | ... | ... | | 8 | 0.3744 | 100.xx | 195.xx | TCP | FIN,ACK | 0.000 | | 9 | 0.4055 | 195.xx | 100.xx | TCP | FIN,ACK | 0.03118 |

As you can see, the FIN,ACK rtt is 5 times smaller than the initial SYN-ACK RTT? I can observe this behaviour for most of the packets. But why ?

What i also observe, the RTT is the first time very big (0.156ms) and than downloading the file again it gets smaller (0.045ms)? I disable caching (wget --no-cache)

Thanks for your help,

Dieter

asked 20 Dec '16, 09:43

DieterMeier's gravatar image

DieterMeier
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accept rate: 0%

edited 20 Dec '16, 10:13

grahamb's gravatar image

grahamb ♦
19.8k330206

A complete guess, but it could be that the first time you connect the mobile device has to do a RACH to get an uplink grant, whereas for subsequent packets you get an uplink grant more quickly?

(21 Dec '16, 12:42) MartinM
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question asked: 20 Dec '16, 09:43

question was seen: 973 times

last updated: 21 Dec '16, 12:42

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