[180210] in North American Network Operators' Group
Re: Predicting TCP throughput
daemon@ATHENA.MIT.EDU (Valdis.Kletnieks@vt.edu)
Thu May 28 03:18:26 2015
X-Original-To: nanog@nanog.org
To: Glen Kent <glen.kent@gmail.com>
In-Reply-To: Your message of "Thu, 28 May 2015 06:45:44 +0530."
<CAPLq3UOR1j67BsdrBN7h13UnrUrSR-cGFSi4RFsCNOZM5mJQ6Q@mail.gmail.com>
From: Valdis.Kletnieks@vt.edu
Date: Thu, 28 May 2015 03:18:10 -0400
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
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On Thu, 28 May 2015 06:45:44 +0530, Glen Kent said:
> If i see an RTT of 150ms and packet loss of 0.01% between points A and B
> and the maximum throughput then between these as, say 250Mbps. Then can i
> say that i will *always* get the same (or in a close ballpark) throughput
> not matter what time of the day i run these tests.
Only if you control the network load across the entire path.
As a simplified example, assume you did your test at 2AM and there's no
other activity, there's a bottleneck 1Gbps link in the path, and you get 250Mbps.
(yes, that result indicates probable misconfig, but bear with me.. :)
You test again at 11AM, and now there's 7 other streams trying to pump 250Mbps
across that link. All 8 should probably drop back to 125Mbps.
For extra credit, factor in bufferbloat pushing your RTT through the roof
under congestion, and similar misbehaviors....
Oh, and that 0.01% packet loss is going to play heck with tcp slow-start
and opening the window - to quote RFC3649:
This document proposes HighSpeed TCP, a modification to TCP's
congestion control mechanism for use with TCP connections with large
congestion windows. In a steady-state environment, with a packet
loss rate p, the current Standard TCP's average congestion window is
roughly 1.2/sqrt(p) segments. This places a serious constraint on
the congestion windows that can be achieved by TCP in realistic
environments. For example, for a Standard TCP connection with 1500-
byte packets and a 100 ms round-trip time, achieving a steady-state
throughput of 10 Gbps would require an average congestion window of
83,333 segments, and a packet drop rate of at most one congestion
event every 5,000,000,000 packets (or equivalently, at most one
congestion event every 1 2/3 hours). The average packet drop rate of
at most 2*10^(-10) needed for full link utilization in this
environment corresponds to a bit error rate of at most 2*10^(-14),
and this is an unrealistic requirement for current networks.
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