tcp: (fixes #2653) to not save smaller ts

RELEASE_NOTES

@@ -32,6 +32,7 @@ New user-visible features
 Bugs fixed
 ----------
 - Bug 2505 - network:  Avoid asserts in Header/Trailer deserialization
+- Bug 2653 - tcp: Avoid saving smaller TS in case of packet reordering
 - Bug 2764 - wifi: WifiSpectrumModelId doesn't distinguish 11ax from legacy
 - Bug 2766 - core:  Modify logging for int64x64 to avoid stack overflow
 - Bug 2824 - ICMP opcode fr fragment timeout drop is wrong

src/internet/model/tcp-socket-base.cc

@@ -3845,6 +3845,14 @@ TcpSocketBase::ProcessOptionTimestamp (const Ptr<const TcpOption> option,
 
   Ptr<const TcpOptionTS> ts = DynamicCast<const TcpOptionTS> (option);
 
+  // This is valid only when no overflow occours. It happens
+  // when a connection last longer than 50 days.
+  if (m_tcb->m_rcvTimestampValue > ts->GetTimestamp ())
+    {
+      // Do not save a smaller timestamp (probably there is reordering)
+      return;
+    }
+
   m_tcb->m_rcvTimestampValue = ts->GetTimestamp ();
   m_tcb->m_rcvTimestampEchoReply = ts->GetEcho ();