High latency, TCP window size, single or multi session

5 pts.
Tags:
Filezilla
Networking
TCP
Telecom
Dear Gurus: The situation is between 2 place with high latency e.g. 60ms from China to Taiwan. The IPLC link is 20mbps, the test PC hase been tcp optimisation increase window size. If one FTP test session still can not reach for example 10%(vs 100Mbps LAN), 10Mbps, the provider say have to multiple session can reach above 15%(vs LAN) 15Mbps. Is this reasonable? Thanks!


Software/Hardware used:
filezilla , WinXP NB
1

Answer Wiki

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Yes – I have seen situations where a single client and single session are not enough to drive a link to full capacity. Multiple sessions can drive the link to full utilization. And 60ms is not really high latency – my global WAN has links with 200ms or higher latency between locations. See this website for a nifty calculator (Network Protocol Specialists WAN delay calculator) that can calculate application delay based on file size & latency.

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  • WestTech
    Have seen same issue with connections just across town. In some cases, it may be that the telco is rate limiting by session, which is our suspicion in our case where cable ISP connection is not fully utilized unless multiple sessions are active. FTP should be able to load a pipe quite full given correct packet size and adequate window size and scaling factor, but as you see it isn't necessarily the case. Do you get similar results from speed test sites? Or do the tests manage to load the pipe to expected max utilization? Some such on-line tests can launch multiple sessions, etc. to test and ensure you are getting the bandwidth you are paying for.
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