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diff --git a/third_party/libwebrtc/net/dcsctp/rx/traditional_reassembly_streams.h b/third_party/libwebrtc/net/dcsctp/rx/traditional_reassembly_streams.h
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+/*
+ *  Copyright (c) 2021 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#ifndef NET_DCSCTP_RX_TRADITIONAL_REASSEMBLY_STREAMS_H_
+#define NET_DCSCTP_RX_TRADITIONAL_REASSEMBLY_STREAMS_H_
+#include <stddef.h>
+#include <stdint.h>
+
+#include <map>
+#include <string>
+
+#include "absl/strings/string_view.h"
+#include "api/array_view.h"
+#include "net/dcsctp/common/sequence_numbers.h"
+#include "net/dcsctp/packet/chunk/forward_tsn_common.h"
+#include "net/dcsctp/packet/data.h"
+#include "net/dcsctp/rx/reassembly_streams.h"
+
+namespace dcsctp {
+
+// Handles reassembly of incoming data when interleaved message sending
+// is not enabled on the association, i.e. when RFC8260 is not in use and
+// RFC4960 is to be followed.
+class TraditionalReassemblyStreams : public ReassemblyStreams {
+ public:
+  TraditionalReassemblyStreams(absl::string_view log_prefix,
+                               OnAssembledMessage on_assembled_message);
+
+  int Add(UnwrappedTSN tsn, Data data) override;
+
+  size_t HandleForwardTsn(
+      UnwrappedTSN new_cumulative_ack_tsn,
+      rtc::ArrayView<const AnyForwardTsnChunk::SkippedStream> skipped_streams)
+      override;
+
+  void ResetStreams(rtc::ArrayView<const StreamID> stream_ids) override;
+
+  HandoverReadinessStatus GetHandoverReadiness() const override;
+  void AddHandoverState(DcSctpSocketHandoverState& state) override;
+  void RestoreFromState(const DcSctpSocketHandoverState& state) override;
+
+ private:
+  using ChunkMap = std::map<UnwrappedTSN, Data>;
+
+  // Base class for `UnorderedStream` and `OrderedStream`.
+  class StreamBase {
+   protected:
+    explicit StreamBase(TraditionalReassemblyStreams* parent)
+        : parent_(*parent) {}
+
+    size_t AssembleMessage(ChunkMap::iterator start, ChunkMap::iterator end);
+    TraditionalReassemblyStreams& parent_;
+  };
+
+  // Manages all received data for a specific unordered stream, and assembles
+  // messages when possible.
+  class UnorderedStream : StreamBase {
+   public:
+    explicit UnorderedStream(TraditionalReassemblyStreams* parent)
+        : StreamBase(parent) {}
+    int Add(UnwrappedTSN tsn, Data data);
+    // Returns the number of bytes removed from the queue.
+    size_t EraseTo(UnwrappedTSN tsn);
+    bool has_unassembled_chunks() const { return !chunks_.empty(); }
+
+   private:
+    // Given an iterator to any chunk within the map, try to assemble a message
+    // into `reassembled_messages` containing it and - if successful - erase
+    // those chunks from the stream chunks map.
+    //
+    // Returns the number of bytes that were assembled.
+    size_t TryToAssembleMessage(ChunkMap::iterator iter);
+
+    ChunkMap chunks_;
+  };
+
+  // Manages all received data for a specific ordered stream, and assembles
+  // messages when possible.
+  class OrderedStream : StreamBase {
+   public:
+    explicit OrderedStream(TraditionalReassemblyStreams* parent,
+                           SSN next_ssn = SSN(0))
+        : StreamBase(parent), next_ssn_(ssn_unwrapper_.Unwrap(next_ssn)) {}
+    int Add(UnwrappedTSN tsn, Data data);
+    size_t EraseTo(SSN ssn);
+    void Reset() {
+      ssn_unwrapper_.Reset();
+      next_ssn_ = ssn_unwrapper_.Unwrap(SSN(0));
+    }
+    SSN next_ssn() const { return next_ssn_.Wrap(); }
+    bool has_unassembled_chunks() const { return !chunks_by_ssn_.empty(); }
+
+   private:
+    // Try to assemble one or several messages in order from the stream.
+    // Returns the number of bytes assembled if a message was assembled.
+    size_t TryToAssembleMessage();
+    size_t TryToAssembleMessages();
+    // This must be an ordered container to be able to iterate in SSN order.
+    std::map<UnwrappedSSN, ChunkMap> chunks_by_ssn_;
+    UnwrappedSSN::Unwrapper ssn_unwrapper_;
+    UnwrappedSSN next_ssn_;
+  };
+
+  const std::string log_prefix_;
+
+  // Callback for when a message has been assembled.
+  const OnAssembledMessage on_assembled_message_;
+
+  // All unordered and ordered streams, managing not-yet-assembled data.
+  std::map<StreamID, UnorderedStream> unordered_streams_;
+  std::map<StreamID, OrderedStream> ordered_streams_;
+};
+
+}  // namespace dcsctp
+
+#endif  // NET_DCSCTP_RX_TRADITIONAL_REASSEMBLY_STREAMS_H_