[Bloat] Goodput fraction w/ AQM vs bufferbloat

[Fred Baker]

On May 8, 2011, at 5:34 AM, Richard Scheffenegger wrote:
> Goodput can really only be measured at the sender; by definition, any retransmitted packet will reduce goodput vs throughput; In your example, where each segment is retransmitted once, goodput would be - at most - 0.5, not 1.0... IMHO defining the data volume after the bottleneck by itself as goodput is also a bit short-sighted, because a good fraction of that data may still be discarded by TCP for numerous reasons, ultimately (ie, legacy go-back-n RTO recovery by the sender)...

Actually, I didn't say that every packet was retransmitted once. I said that every dropped packet was retransmitted once. And Goodput will never exceed the bit rate of the bottleneck in the path, apart from compression (which in effect applies a multiplier to the bottleneck bandwidth).

> But back to my original question: When looking at modern TCP stacks, with TSO, if the bufferbloat allows the senders cwnd to grow beyond thresholds which allow the aggressive use of TSO (64kB or even 256kB of data allowed in the senders cwnd), the effective sending rate of such a burst will be wirespeed (no interleaving segments of other sessions). As pointed out in other mails to this thread, if the bottleneck has then 1/10th the capacity of the senders wire (and is potentially shared among multiple senders), at least 90% of all the sent data of such a TSO segment train will be dropped in a single burst of loss... With proper AQM, and some (single segment) loss earlier, cwnd may never grow to trigger TSO in that way, and the goodput (1 segment out of 64kB data, vs. 58kB out of 64kB data) is obviously shifted extremely to the scenario with AQM...

Again, possibly, but not necessarily. If we have a constrained queue and are using tail drop, it is possible for a single burst sent to a full queue to be entirely lost. The question is, in the course of a file transfer, how many packets are lost. Before you make sweeping statements, I would strongly suggest that you mock up the situation and take a tcpdump.