nanog mailing list archives
Re: Latency, TCP ACKs and upload needs
From: Leo Bicknell <bicknell () ufp org>
Date: Wed, 20 Apr 2016 07:27:53 -0700
Others have already answered with the technical details. Let me take a stab at some more, uh, variable items. In a message written on Tue, Apr 19, 2016 at 09:29:12PM -0400, Jean-Francois Mezei wrote:
Also, when you establish a TCP connection, do most stacks have a default window size that gives the sender enough "patience" to wait long enough for the ACK ?
Your question is phrased backwards. All will wait for the ACK, the timeouts are long (30-120 seconds). The issue is that you only get one window of data per RTT, so if the window is too small, it will choke the connection. 90%+ of the stacks deployed will be too small. Modern Unix generally has "autotuning" TCP stacks, but I don't think Windows or OS X has those features yet (but I'd be very happy to be wrong on that point). Regardless of satellite uplink/downlink speeds, boxes generally need to be tuned to get maximum performance on satellite.
What i am trying to get at here is whether 25/1 on satellite, in real life with a few apps exchanging data, would actually be able to make use of the 25 download speed or whether the limited 1mbps upload would choke the downloads ?
With a properly tuned stack what you're describing is not a problem. 1460 byte payloads down, maybe 64 byte acks on the return, and with SACK which is widely deployed an ACK every 2-4 packets. You would see about 2,140 packets/sec downstream (25Mbps/1460), and perhaps send 1070 ACKs back upstream, at 64 bytes each, or about 68Kbps. Well under the 1Mbps upstream bandwidth. -- Leo Bicknell - bicknell () ufp org PGP keys at http://www.ufp.org/~bicknell/
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Current thread:
- Latency, TCP ACKs and upload needs Jean-Francois Mezei (Apr 19)
- Re: Latency, TCP ACKs and upload needs Jared Mauch (Apr 19)
- Re: Latency, TCP ACKs and upload needs Eric Kuhnke (Apr 19)
- Re: Latency, TCP ACKs and upload needs joel jaeggli (Apr 19)
- Re: Latency, TCP ACKs and upload needs Eric Kuhnke (Apr 19)
- Re: Latency, TCP ACKs and upload needs Leo Bicknell (Apr 20)
- Re: Latency, TCP ACKs and upload needs Jean-Francois Mezei (Apr 20)
- Re: Latency, TCP ACKs and upload needs Lee (Apr 20)
- Re: Latency, TCP ACKs and upload needs Tony Finch (Apr 20)
- Re: Latency, TCP ACKs and upload needs Eygene Ryabinkin (Apr 21)
- Re: Latency, TCP ACKs and upload needs Chris Welti (Apr 24)
- Re: Latency, TCP ACKs and upload needs Lee (Apr 20)
- Re: Latency, TCP ACKs and upload needs Mikael Abrahamsson (Apr 20)