On Wed, Feb 24, 2021 at 5:03 AM Gil Pedersen [off-list ref] 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?
thanks,
neal