TCP-Connection Establishment = How to measure time based on Ping RRT?

I would be greatful for help, understanding how long it takes to establish a TCP connection when I have the Ping RoundTripTip:

According to Wikipedia a TCP Connection will be established in three steps:

1.SYN-SENT (=>CLIENT TO SERVER)
2.SYN/ACK-RECEIVED (=>SERVER TO CLIENT)
3.ACK-SENT (=>CLIENT TO SERVER)

My Questions:

1. Is it correct, that the third transmission (ACK-SENT) will not yet carry any payload (my data) but is only used for the connection establishement.(This leads to the conclusion, that the fourth packt will be the first packt to hold any payload....)

2. Is it correct to assume, that when my Ping RoundTripTime is 开发者_运维技巧20 milliseconds, that in the example given above, the TCP Connection establishment would at least require 30 millisecons, before any data can be transmitted between the Client and Server?

Thank you very much

Tom

Those things are basically correct, though #2 assumes that the round-trip time is symmetric.

To measure this, called the "Time to Syn/ACK" (which is NOT the Time to Establish a connection - the connection is only half-open when in that state, you need the 3rd packet acknowledging the establishment to consider it established), you usually need professional tools that include their own TCP stack, enabling that kind of measurement. The most used one is called the Spirent Avalanche, but you also have Ixia's IxLoad or BreakingPoint Systems box (BPS has now been acquired by Ixia btw).

Note that, yes, 3rd packet won't have any data, and that is also true of the first two. They are only Syn and Syn+Ack flagged (those are TCP flags), and contain no application data. This initial exchange, called the Three-way Handshake therefore causes some overhead, which is why TCP is typically not used in real-time applications (voice, live video, etc..).

Also as stated, you can't assume that Latency = RTT/2. It is in fact very complicated to measure one-way latency above layer 3 (IP) - and you are already at layer 4 (TCP) here. This blog post covers in details the challenge of this: http://synsynack.wordpress.com/2012/04/09/realistic-latency-measurement-in-the-application-layers/