Why is the TCP throughput test around 80-90Mbps?

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Contents 1 Observations 2 Questions 3 Hypothesis 4 Causes 5 Investigations

Observations

• The TCP throughput transfer from Frascati to Catania is constantly around 80 Mbps whilst I would be expecting several hunderds of Mbps with the MDM devices?

Questions

1. Why are we getting only 77Mbps from Frascati to Catania in general whilst we would expect up to 1Gbps from the MDM service?

Hypothesis

There can be several causes for that behavior. Causes that are related to the equipment themselves (BWCTL interface configured as a Fast Ethernet interface, interface connected to a Fast Ethernet one), network bottlenecks. The stability of the measurements around 80Mbps over several days indicates rather a Fast Ethernet configuration/connection.

Causes

The BWCTL interface in Frascati is a 100Mbps one. This is due to a physical limitation (no 1Gbps interface available in the near future).

Investigations

• Let's first look at the connection at which the BWCTL interface is connected. One can suspect it is connected with a Fast Ethernet connection (100Mbps) either because no Gigabit ethernet is made available or because of a configuration problem.
• Looking at: http://www.win-labor.dfn.de/bwctl/MDM/2008/01/16/hardwaredata.txt, searching from Catania, we can find that:
  1. the IP address for the BWCT test in catania (search fro catania) is: GARR_Catania - rt-ct1-bwctl.ct1.garr.net - 193.206.137.170
  2. Now let's look at the interface corresponding to the IP address: it is eth0

  ifconfig:
  eth0:
  Hardwareaddress: 00:E0:81:74:29:C9 
  inet addr: 193.206.137.170 Mask 255.255.255.252
  [snip]

• 1. Let's check below what the speed of eth0:

  ethtool:
  eth0:
  Advertised auto-negotiation: Yes
  Auto-negotiation: on
  Speed: 1000Mb/s
  Duplex: Full
  [snip]

• 1. Seems OK at first sight: the BWCTL interface is the eth0, configured at 1000Mb/s full duplex.
  2. Let's now look at the Frascati :the IP address for the BWCT test in catania is: GARR_Catania - rt-ct1-bwctl.ct1.garr.net - 193.206.137.170
  3. Now let's look at the interface corresponding to the IP address: it is eth0

  ifconfig:
  eth0:
  Hardwareaddress: 00:E0:81:74:33:66 
  [snip]

• 1. Let's check below what the speed of eth0:

  ethtool:
  eth0:
  Advertised auto-negotiation: Yes
  Auto-negotiation: on
  Speed: 100Mb/s
  Duplex: Full

• 1. So here it is, the BWCTL interface in Frascati is configured to be limited to 100Mbps. This is actually because of a physical limitation (no 1Gbps interface was available).
  2. If you were confronted to such issue, (configuration or FastEthernet limitation), please contact the Service Desk for investigating this particular point.