Consistently suggest ephemeral port for QUIC client

net/quic/port_suggester.cc

+// Copyright 2013 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/port_suggester.h"
+
+#include "base/logging.h"
+#include "net/base/host_port_pair.h"
+
+namespace net {
+
+PortSuggester::PortSuggester(const HostPortPair& server, uint64 seed)
+    : call_count_(0),
+      previous_suggestion_(-1) {
+  unsigned char hash_bytes[base::kSHA1Length];
+  base::SHA1HashBytes(
+      reinterpret_cast<const unsigned char*>(server.host().data()),
+      server.host().length(), hash_bytes);
+  COMPILE_ASSERT(sizeof(seed_) < sizeof(hash_bytes), seed_larger_than_hash);
+  memcpy(&seed_, hash_bytes, sizeof(seed_));
+  seed_ ^= seed ^ server.port();
+}
+
+int PortSuggester::SuggestPort(int min, int max) {
+  // Sometimes our suggestion can't be used, so we ensure that if additional
+  // calls are made, then each call (probably) provides a new suggestion.
+  if (++call_count_ > 1) {
+    // Evolve the seed.
+    unsigned char hash_bytes[base::kSHA1Length];
+    base::SHA1HashBytes(reinterpret_cast<const unsigned char *>(&seed_),
+                        sizeof(seed_), hash_bytes);
+    memcpy(&seed_, hash_bytes, sizeof(seed_));
+  }
+  DCHECK_LE(min, max);
+  DCHECK_GT(min, 0);
+  int range = max - min + 1;
+  // Ports (and hence the extent of the |range|) are generally under 2^16, so
+  // the tiny non-uniformity in the pseudo-random distribution is not
+  // significant.
+  previous_suggestion_ = static_cast<int>(seed_ % range) + min;
+  return previous_suggestion_;
+}
+
+int PortSuggester::previous_suggestion() const {
+  DCHECK_LT(0u, call_count_);
+  return previous_suggestion_;
+}
+
+}  // namespace net