Handle TCP segment split cases as per MSS.

- Always split segments larger than MSS.
  Currently, we base the segment split decision as a function of the
  send congestion window and MSS, which could be greater than the MSS
  advertised by remote.
- While splitting segments, ensure the PSH flag is reset when there
  are segments that are queued to be sent.
- With TCP_CORK, hold up segments up until MSS. Fix a bug in computing
  available send space before attempting to coalesce segments.

Fixes #2832

PiperOrigin-RevId: 314802928

1 files changed, 28 insertions, 3 deletions

diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index 3a19c4468..acacb42e4 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -618,6 +618,20 @@ func (s *sender) splitSeg(seg *segment, size int) {
 	nSeg.data.TrimFront(size)
 	nSeg.sequenceNumber.UpdateForward(seqnum.Size(size))
 	s.writeList.InsertAfter(seg, nSeg)
+
+	// The segment being split does not carry PUSH flag because it is
+	// followed by the newly split segment.
+	// RFC1122 section 4.2.2.2: MUST set the PSH bit in the last buffered
+	// segment (i.e., when there is no more queued data to be sent).
+	// Linux removes PSH flag only when the segment is being split over MSS
+	// and retains it when we are splitting the segment over lack of sender
+	// window space.
+	// ref: net/ipv4/tcp_output.c::tcp_write_xmit(), tcp_mss_split_point()
+	// ref: net/ipv4/tcp_output.c::tcp_write_wakeup(), tcp_snd_wnd_test()
+	if seg.data.Size() > s.maxPayloadSize {
+		seg.flags ^= header.TCPFlagPsh
+	}
+
 	seg.data.CapLength(size)
 }
 
@@ -739,7 +753,7 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 	if !s.isAssignedSequenceNumber(seg) {
 		// Merge segments if allowed.
 		if seg.data.Size() != 0 {
-			available := int(seg.sequenceNumber.Size(end))
+			available := int(s.sndNxt.Size(end))
 			if available > limit {
 				available = limit
 			}
@@ -782,8 +796,11 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 					//   sent all at once.
 					return false
 				}
-				if atomic.LoadUint32(&s.ep.cork) != 0 {
-					// Hold back the segment until full.
+				// With TCP_CORK, hold back until minimum of the available
+				// send space and MSS.
+				// TODO(gvisor.dev/issue/2833): Drain the held segments after a
+				// timeout.
+				if seg.data.Size() < s.maxPayloadSize && atomic.LoadUint32(&s.ep.cork) != 0 {
 					return false
 				}
 			}
@@ -843,9 +860,17 @@ func (s *sender) maybeSendSegment(seg *segment, limit int, end seqnum.Value) (se
 		if available == 0 {
 			return false
 		}
+
+		// The segment size limit is computed as a function of sender congestion
+		// window and MSS. When sender congestion window is > 1, this limit can
+		// be larger than MSS. Ensure that the currently available send space
+		// is not greater than minimum of this limit and MSS.
 		if available > limit {
 			available = limit
 		}
+		if available > s.maxPayloadSize {
+			available = s.maxPayloadSize
+		}
 
 		if seg.data.Size() > available {
 			s.splitSeg(seg, available)