Introduce UMA metrics for path MTU discovery.

net/quic/quic_client_session.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "net/quic/quic_client_session.h"
 
 #include "base/callback_helpers.h"
 #include "base/location.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/sparse_histogram.h"
@@ skipping 289 matching lines @@
           wait_for_data_end_time - wait_for_data_start_time;
       const base::HistogramBase::Sample kMaxWaitToRtt = 1000;
       base::HistogramBase::Sample wait_to_rtt =
           static_cast<base::HistogramBase::Sample>(
               100 * wait_time.InMicroseconds() / stats.min_rtt_us);
       UMA_HISTOGRAM_CUSTOM_COUNTS("Net.QuicServerInfo.WaitForDataReadyToRtt",
                                   wait_to_rtt, 0, kMaxWaitToRtt, 50);
     }
   }
 
+  // The MTU used by QUIC is limited to a fairly small set of predefined values
+  // (initial values and MTU discovery values), but does not fare well when
+  // bucketed.  Because of that, a sparse histogram is used here.
+  UMA_HISTOGRAM_SPARSE_SLOWLY("Net.QuicSession.ClientSideMtu",
+                              connection()->max_packet_length());
+  UMA_HISTOGRAM_SPARSE_SLOWLY("Net.QuicSession.ServerSideMtu",
+                              stats.max_received_packet_size);
+
+  UMA_HISTOGRAM_COUNTS("Net.QuicSession.MtuProbesSent",
+                       connection()->mtu_probe_count());
+
   if (stats.max_sequence_reordering == 0)
     return;
   const base::HistogramBase::Sample kMaxReordering = 100;
   base::HistogramBase::Sample reordering = kMaxReordering;
   if (stats.min_rtt_us > 0) {
     reordering = static_cast<base::HistogramBase::Sample>(
         100 * stats.max_time_reordering_us / stats.min_rtt_us);
   }
   UMA_HISTOGRAM_CUSTOM_COUNTS("Net.QuicSession.MaxReorderingTime",
                               reordering, 0, kMaxReordering, 50);
@@ skipping 614 matching lines @@
     return;
 
   // TODO(rch): re-enable this code once beta is cut.
   //  if (stream_factory_)
   //    stream_factory_->OnSessionConnectTimeout(this);
   //  CloseAllStreams(ERR_QUIC_HANDSHAKE_FAILED);
   //  DCHECK_EQ(0u, GetNumOpenStreams());
 }
 
 }  // namespace net