Last night, I copied a large stack of files from my work machine to my home machine via a single, batch scp. Both machines, Mac Minis, were otherwise idle. My home DSL is ostensibly 6Mb. Each file is a 10-13MB DNG raw file.

So the question is where the following odd performance numbers come from:


The initial batch of files were being transmitted a factor of ten slower than the later files with what looks like a gradual ramp-up in performance. Any ideas?

Well, the batch SCP probably means it used one TCP connection, which was in linear growth mode the whole time. Something probably knocked it down out of the exponential ramp up by dropping a packet early on.

We could speculate as to if this was intentional on your ISPs part or just a random coincidence.

Is the behavior reproducible?

TCP usually implements a variety of congestion control and avoidance algorithms that are transparent to the TCP protocol itself. That is, using them doesn't require any collaboration from the remote end. Many of these implement a ramping feature. While your numbers seem to ramp fairly slowly based on my familiarity with those algorithms, I'd be surprised if they weren't the culprits.

Matthew's hypothesis is probably as good as anything. My only other hypothesis was that maybe TimeMachine backups on the source computer could well have been running and gumming up the works. Still it does seem a bit odd when you know the sending machine has the bandwidth, you know the local pipe is empty, and you know you're getting 10% of peak.