Add support for TCP_USER_TIMEOUT option.

The implementation follows the linux behavior where specifying
a TCP_USER_TIMEOUT will cause the resend timer to honor the
user specified timeout rather than the default rto based timeout.

Further it alters when connections are timedout due to keepalive
failures. It does not alter the behavior of when keepalives are
sent. This is as per the linux behavior.

PiperOrigin-RevId: 285099795

1 files changed, 17 insertions, 4 deletions

diff --git a/pkg/tcpip/transport/tcp/protocol.go b/pkg/tcpip/transport/tcp/protocol.go
index 89b965c23..bc718064c 100644
--- a/pkg/tcpip/transport/tcp/protocol.go
+++ b/pkg/tcpip/transport/tcp/protocol.go
@@ -162,13 +162,26 @@ func (*protocol) HandleUnknownDestinationPacket(r *stack.Route, id stack.Transpo
 func replyWithReset(s *segment) {
 	// Get the seqnum from the packet if the ack flag is set.
 	seq := seqnum.Value(0)
+	ack := seqnum.Value(0)
+	flags := byte(header.TCPFlagRst)
+	// As per RFC 793 page 35 (Reset Generation)
+	//   1.  If the connection does not exist (CLOSED) then a reset is sent
+	//   in response to any incoming segment except another reset.  In
+	//   particular, SYNs addressed to a non-existent connection are rejected
+	//   by this means.
+
+	//   If the incoming segment has an ACK field, the reset takes its
+	//   sequence number from the ACK field of the segment, otherwise the
+	//   reset has sequence number zero and the ACK field is set to the sum
+	//   of the sequence number and segment length of the incoming segment.
+	//   The connection remains in the CLOSED state.
 	if s.flagIsSet(header.TCPFlagAck) {
 		seq = s.ackNumber
+	} else {
+		flags |= header.TCPFlagAck
+		ack = s.sequenceNumber.Add(s.logicalLen())
 	}
-
-	ack := s.sequenceNumber.Add(s.logicalLen())
-
-	sendTCP(&s.route, s.id, buffer.VectorisedView{}, s.route.DefaultTTL(), stack.DefaultTOS, header.TCPFlagRst|header.TCPFlagAck, seq, ack, 0 /* rcvWnd */, nil /* options */, nil /* gso */)
+	sendTCP(&s.route, s.id, buffer.VectorisedView{}, s.route.DefaultTTL(), stack.DefaultTOS, flags, seq, ack, 0 /* rcvWnd */, nil /* options */, nil /* gso */)
 }
 
 // SetOption implements TransportProtocol.SetOption.