[timf]

The docs say they use the time it takes to do a TCP open between their agents because they "believe that TCP Open times are more representative of Internet users' actual experience".

That is, they think pings could be treated in a different way but with a TCP connection establishment you are measuring what most internet users want to do over the link.

As for different readings from different directions you can get that with both pings and TCP open, it's just maybe more noticable and exacerbated with the longer and multiple-message nature of a TCP open.

[rrival]

and icmp can get deprioritized on busy routers, yaddayaddayadda. pathchar (old old) was doing interesting things in measuring link health.