> On 24 Feb 2021, at 15.55, Neal Cardwell <ncardw...@google.com> wrote: > > On Wed, Feb 24, 2021 at 5:03 AM Gil Pedersen <kanon...@gmail.com> wrote: >> Sure, I attached a trace from the server that should illustrate the issue. >> >> The trace is cut from a longer flow with the server at 188.120.85.11 and a >> client window scaling factor of 256. >> >> Packet 78 is a TLP, followed by a delayed DUPACK with a SACK from the client. >> The SACK triggers a single segment fast re-transmit with an ignored?? D-SACK >> in packet 81. >> The first RTO happens at packet 82. > > Thanks for the trace! That is very helpful. I have attached a plot and > my notes on the trace, for discussion. > > AFAICT the client appears to be badly misbehaving, and misrepresenting > what has happened. At each point where the client sends a DSACK, > there is an apparent contradiction. Either the client has received > that data before, or it hasn't. If the client *has* already received > that data, then it should have already cumulatively ACKed it. If the > client has *not* already received that data, then it shouldn't send a > DSACK for it. > > Given that, from the server's perspective, the client is > misbehaving/lying, it's not clear what inferences the server can > safely make. Though I agree it's probably possible to do much better > than the current server behavior. > > A few questions. > > (a) is there a middlebox (firewall, NAT, etc) in the path? > > (b) is it possible to capture a client-side trace, to help > disambiguate whether there is a client-side Linux bug or a middlebox > bug?
Yes, this sounds like a sound analysis, and matches my observation. The client is confused about whether it has the data or not. Unfortunately I only have that (un-rooted) device available, so I can't do traces on it. The connection path is Client -> Wi-Fi -> NAT -> NAT -> Internet -> Server (which has a basic UFW firewall). I will try to do a trace on the first NAT router. My first priority is to make the server behave better in this case, but I understand that you would like to investigate the client / connection issue as well? From the server POV, this is clearly an edge case, but a fast re-transmit does seem more appropriate. Btw. the "client SACKs TLP retransmit" note is not correct. This is an old ACK, which can be seen from the ecr value. /Gil
