When this occurs it usually means that your connection has minor packet loss or TCP delays in transfer, very hard to identify. You will likely find that UDP will transfer full speed. TCP is doing what it is supposed to do, which is slow down, to attempt to correct congestion, but in reality may be caused by transmission delays.
Here are two things that can cause random sporatic delays in transmission. 1) Switches with Flow control on. (should be turned off). I recently had transmission increased from 3-5mbps to 24mbps on a path, simply by disabling Flow control. (Flow control really only meant to be used on switch when connected directly to an end user PC, not a backbone connection). This can be common, because many switches default with flow control on, and its something you can't check yourself, if its at your upstream first hop. 2) Bandwidth management on hardware with inadequate timers, and high traffic load. (If Linux, should be using kernels 2.6.21 or higher, for high_res_timers, if high speed links). 3) Basic hardware failure. A flaky NIC port. A Bad cable. Wrong NIC Driver. Actually Resulting in very small amounts of packet loss or sporatic transfer. 4) TX Buffers to low. Or other similar NIC driver optimization. I think Linux defaults to 100, but for Gig_e should be like 1000. Should check optimization suggestions for your specific brand NIC. Those are the kind of things that might show failures when testing under load, but where there is free capacity on link still. 5) Window size incorrectly adjusting. Manually test with hard set window sizes, to compare results. You can sniff at layer2, to see if there are excessive acks or retransmissions, which sometimes can be an indicator that something is wrong. Should also be noted whether problem occurs both with significant other traffic on connection, or if no other traffic passing when testing. Timing delays are accumulative, and small individually, so they often can't be identified at a device easilly, by testing a single hop or link individually. Tom DeReggi RapidDSL & Wireless, Inc IntAirNet- Fixed Wireless Broadband ----- Original Message ----- From: "Scott Carullo" <[email protected]> To: <[email protected]> Sent: Sunday, May 10, 2009 11:26 PM Subject: [WISPA] One connection bandwidth=x / 2 commections bandwidth=2x why? > > On our main upstream connection 100mb fiber a speedtest to BHN yeilds > about > 7MB max when 15 is there... > > Open two connections tcp and now the transfer rate doubles (from same > server to same client). > > What would cause this? > > Scott Carullo > Brevard Wireless > 321-205-1100 x102 > > > > -------------------------------------------------------------------------------- > WISPA Wants You! Join today! > http://signup.wispa.org/ > -------------------------------------------------------------------------------- > > WISPA Wireless List: [email protected] > > Subscribe/Unsubscribe: > http://lists.wispa.org/mailman/listinfo/wireless > > Archives: http://lists.wispa.org/pipermail/wireless/ -------------------------------------------------------------------------------- WISPA Wants You! Join today! http://signup.wispa.org/ -------------------------------------------------------------------------------- WISPA Wireless List: [email protected] Subscribe/Unsubscribe: http://lists.wispa.org/mailman/listinfo/wireless Archives: http://lists.wispa.org/pipermail/wireless/
