You might be interested to know that many times, packets in a TCP session take asymmetric routes across the internet. In my experience, the most common reasons for this are business rules related to cost and human error.
If you think about how IP works, you’ll see that this doesn’t particularly matter but that it can make understanding the routing more difficult.
Boise State University, and the University of Idaho are two schools at opposite ends of the state of Idaho. UIdaho in the north is close to Spokane, and almost all of its connectivity comes from Seattle. Boise is closer to Salt Lake so most of its connectivity comes via Portland or Salt Lake City. The middle of the state between the two schools is mountains and very, very little large scale connectivity at all, except there was a small line way bad in the day because the UofIdaho had remote classrooms in the southern part of the state. Sometime in the late 90's a network engineer from BSU and one from UofI realized that they both had switches and routing kit in the same building so they ran an ethernet cable between them.
The effect was catastrophic. It turns out that both networks happily started announcing BPG to each other, which in turn announced the connection to the internet as a whole. Suddenly there was a very short jump between networks in Seattle and networks in Salt Lake City. That poor little t1 (iirc) was absolutely getting saturated. But, interestingly only in one direction. See Boise announced the route, but Idaho didn't so the traffic was effectively only failing in one direction.
Needless to say the cable as disconnected and years later when I worked at the UofIdaho it was still well known that the two networks shouldn't ever be connected again! (Which was ironic because I was working on a program to setup I2 at both universities)
If you think about how IP works, you’ll see that this doesn’t particularly matter but that it can make understanding the routing more difficult.