a) When GSO is in use we should not cap the segment to maxPayloadSize in
sender.maybeSendSegment as the GSO logic will cap the segment to the correct
size. Without this the host GSO is not used as we end up breaking up large
segments into small MSS sized segments before writing the packets to the
host.
b) The check to not split a segment due to it not fitting in the receiver window
when there are pending segments is incorrect as segments in writeList can be
really large as we just take the write call's buffer size and create a single
large segment. So a write of say 128KB will just be 1 segment in the
writeList.
The linux code checks if 1 MSS sized segments fits in the receiver's window
and if not then does not split the current segment. gVisor's check was
incorrect that it was checking if the whole segment which could be >>> 1 MSS
would fit in the receiver's window. This was causing us to prematurely stop
sending and falling back to retransmit timer/probe from the other end to send
data.
This was seen when running HTTPD benchmarks where @ HEAD when sending large
files the benchmark was taking forever to run.
The tcp_splitseg_mss_test.go is being deleted as the test as written doesn't
test what is intended correctly. This is because GSO is enabled by default and
the reason the MSS+1 sized segment is sent is because GSO is in use. A proper
test will require disabling GSO on linux and netstack which is going to take a
bit of work in packetimpact to do it correctly.
Separately a new test probably should be written that verifies that a segment >
availableWindow is not split if the availableWindow is < 1 MSS.
Fixes#3107
PiperOrigin-RevId: 319172089
Tests the effect of reordering on retransmission and window size.
Test covers the expected behavior of both Linux and netstack, however, netstack
does not behave as expected. Further, the current expected behavior of netstack
is not ideal and should be adjusted in the future.
PiperOrigin-RevId: 316015184
Bhasker Hariharan
Fabricio Voznika
Ian Gudger
gVisor bot