How Distance can Affect Throughput

slothOffsite backups are important but Round Trip Time should be taken into consideration for particular transfer protocols. TCP connections can be severely hampered under certain conditions from what is known as “Bandwidth – Delay Product” and Wikipedia has the following descriptive article:

In data communications, bandwidth-delay product refers to the product of a data link’s capacity (in bits per second) and its round-trip delay time (in seconds). The result, an amount of data measured in bits (or bytes), is equivalent to the maximum amount of data on the network circuit at any given time, i.e., data that has been transmitted but not yet acknowledged.

To use an extreme example, a tiny 64kB TCP buffer size would constrain a 100Mibit/s interface to 2.6Mibit/s if there is a consistent 200ms latency whereas a 50ms latency would allow 10.5 Mibit/s.

Fortunately modern systems are no longer defaulted with a small TCP buffer size but tweaks were required prior to kernel 2.6. The following system parameters can be explored:

  • (Maximum send window size)
  • (Maximum receive window size)
  • (Reserved memory for TCP send buffer)
  • (Reserved memory for TCP receive buffer)

Always be overly cautious when increasing buffers on production servers because it can potentially cause instability. Improvements can be seen immediately for FTP (more importantly FTPS) over long inflight time but there will still be circumstances where changes won’t seem to show any effect even with the best of configurations in place, such as when SO_SNDBUF and SO_RCVBUF are static for setsockopt().

Sender side autotuning has been present for quite some time but receiver limits are now usually included and this can be verified by reading net.ipv4.tcp_window_scaling and net.ipv4.tcp_moderate_rcvbuf:

sysctl net.ipv4.tcp_window_scaling net.ipv4.tcp_moderate_rcvbuf
net.ipv4.tcp_window_scaling = 1
net.ipv4.tcp_moderate_rcvbuf = 1

Interestingly SSH2 multiplexes sessions over a TCP connection, which means patches to OpenSSH such as High Performance SSH/SCP are necessary to attain greater transfer rates for the likes of SCP.

I would like to take this opportunity to wish everyone a wonderful festive season and upcoming New Year!


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