CODEWITHSUNDEEP
language-agnostictcpnetwork-programmingnetwork-protocols
Robert S. Barnes
Robert S. Barnes

Reputation: 40578

TCP Slow Start, Congestion Avoidance & Determining Bandwidth

Is there a formula someplace which can be used to determine the minimum number of segments / bytes which need to be transfered across a TCP connection to determine it's bandwidth and which takes into account Slow Start and Congestion Avoidance? I'm aware of the pathrate tool, but I want if possible something a bit simpler that I can incorporate in an app to get a descent ballpark figure. One example of usage would be downloading some data from a webserver in order to determine the optimum number of threads for downloading a bunch of small files automatically. This is related to a previous question I posted: TCP, HTTP and the Multi-Threading Sweet Spot

Upvotes: 1

Views: 666

Answers (2)

Arms
Arms

Reputation: 11

You can fire up scholar.google.com and search for "TCP chirp". However, that requires hires timers, and if you don't write a kernel tcp congestion control algorithm, you'd have to reimplement TCP in userspace. And that by itself will probably not give good results (general purpose OS are not very good at realtime hires timer related stuff, runnning in userspace).

In theory, using TCP chirp you need as few as 4-5 segments (typically, you'd get better resolution with a longer train of segments) to determine the "optimal" bandwidth.

In any case, since you can not know which path is used (ie. satellite link or tv broadcast in the forward direction), you may need a considerable amount of data (10+ MB, perhaps even 1GB) to get a decent measurement over arbitrary paths. (Satellites can have many dozend MB/s bandwidth, but also latencies in the 1000-3000 ms range; and TCP takes a couple round-trip times to open up cwnd (I'd say around 10 RTTs before a measurement should be started)...

Upvotes: 1

Vadym Stetsiak
Vadym Stetsiak

Reputation: 1970

I do not think that there is a fixed number of bytes required to be sent to determine the bandwidth. This number can depend on network type and speed.

Bandwidth is a measure of some resource transferred over a time interval. To get real data you need to measure it. Here are some hints how to do that

Upvotes: 0

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