diff --git a/apps/CMakeLists.txt b/apps/CMakeLists.txt
index e77e5fb..b372bdb 100644
--- a/apps/CMakeLists.txt
+++ b/apps/CMakeLists.txt
@@ -1 +1,9 @@
+add_library (stream_copy STATIC bidirectional_stream_copy.cc)
+
+add_sponge_exec (udp_tcpdump ${LIBPCAP})
+add_sponge_exec (tcp_native stream_copy)
+add_sponge_exec (tun)
+add_sponge_exec (tcp_udp stream_copy)
+add_sponge_exec (tcp_ipv4 stream_copy)
 add_sponge_exec (webget)
+add_sponge_exec (tcp_benchmark)
diff --git a/apps/bidirectional_stream_copy.cc b/apps/bidirectional_stream_copy.cc
new file mode 100644
index 0000000..ee9794b
--- /dev/null
+++ b/apps/bidirectional_stream_copy.cc
@@ -0,0 +1,91 @@
+#include "bidirectional_stream_copy.hh"
+
+#include "byte_stream.hh"
+#include "eventloop.hh"
+
+#include <algorithm>
+#include <iostream>
+#include <unistd.h>
+
+using namespace std;
+
+void bidirectional_stream_copy(Socket &socket) {
+    constexpr size_t max_copy_length = 65536;
+    constexpr size_t buffer_size = 1048576;
+
+    EventLoop _eventloop{};
+    FileDescriptor _input{STDIN_FILENO};
+    FileDescriptor _output{STDOUT_FILENO};
+    ByteStream _outbound{buffer_size};
+    ByteStream _inbound{buffer_size};
+    bool _outbound_shutdown{false};
+    bool _inbound_shutdown{false};
+
+    socket.set_blocking(false);
+    _input.set_blocking(false);
+    _output.set_blocking(false);
+
+    // rule 1: read from stdin into outbound byte stream
+    _eventloop.add_rule(
+        _input,
+        Direction::In,
+        [&] {
+            _outbound.write(_input.read(_outbound.remaining_capacity()));
+            if (_input.eof()) {
+                _outbound.end_input();
+            }
+        },
+        [&] { return (not _outbound.error()) and (_outbound.remaining_capacity() > 0) and (not _inbound.error()); },
+        [&] { _outbound.end_input(); });
+
+    // rule 2: read from outbound byte stream into socket
+    _eventloop.add_rule(socket,
+                        Direction::Out,
+                        [&] {
+                            const size_t bytes_to_write = min(max_copy_length, _outbound.buffer_size());
+                            const size_t bytes_written = socket.write(_outbound.peek_output(bytes_to_write), false);
+                            _outbound.pop_output(bytes_written);
+                            if (_outbound.eof()) {
+                                socket.shutdown(SHUT_WR);
+                                _outbound_shutdown = true;
+                            }
+                        },
+                        [&] { return (not _outbound.buffer_empty()) or (_outbound.eof() and not _outbound_shutdown); },
+                        [&] { _outbound.end_input(); });
+
+    // rule 3: read from socket into inbound byte stream
+    _eventloop.add_rule(
+        socket,
+        Direction::In,
+        [&] {
+            _inbound.write(socket.read(_inbound.remaining_capacity()));
+            if (socket.eof()) {
+                _inbound.end_input();
+            }
+        },
+        [&] { return (not _inbound.error()) and (_inbound.remaining_capacity() > 0) and (not _outbound.error()); },
+        [&] { _inbound.end_input(); });
+
+    // rule 4: read from inbound byte stream into stdout
+    _eventloop.add_rule(_output,
+                        Direction::Out,
+                        [&] {
+                            const size_t bytes_to_write = min(max_copy_length, _inbound.buffer_size());
+                            const size_t bytes_written = _output.write(_inbound.peek_output(bytes_to_write), false);
+                            _inbound.pop_output(bytes_written);
+
+                            if (_inbound.eof()) {
+                                _output.close();
+                                _inbound_shutdown = true;
+                            }
+                        },
+                        [&] { return (not _inbound.buffer_empty()) or (_inbound.eof() and not _inbound_shutdown); },
+                        [&] { _inbound.end_input(); });
+
+    // loop until completion
+    while (true) {
+        if (EventLoop::Result::Exit == _eventloop.wait_next_event(-1)) {
+            return;
+        }
+    }
+}
diff --git a/apps/bidirectional_stream_copy.hh b/apps/bidirectional_stream_copy.hh
new file mode 100644
index 0000000..13d8892
--- /dev/null
+++ b/apps/bidirectional_stream_copy.hh
@@ -0,0 +1,9 @@
+#ifndef SPONGE_APPS_BIDIRECTIONAL_STREAM_COPY_HH
+#define SPONGE_APPS_BIDIRECTIONAL_STREAM_COPY_HH
+
+#include "socket.hh"
+
+//! Copy socket input/output to stdin/stdout until finished
+void bidirectional_stream_copy(Socket &socket);
+
+#endif  // SPONGE_APPS_BIDIRECTIONAL_STREAM_COPY_HH
diff --git a/apps/tcp_benchmark.cc b/apps/tcp_benchmark.cc
new file mode 100644
index 0000000..b39c723
--- /dev/null
+++ b/apps/tcp_benchmark.cc
@@ -0,0 +1,116 @@
+#include "tcp_connection.hh"
+
+#include <chrono>
+#include <cstdlib>
+#include <iomanip>
+#include <iostream>
+#include <string>
+
+using namespace std;
+using namespace std::chrono;
+
+constexpr size_t len = 100 * 1024 * 1024;
+
+void move_segments(TCPConnection &x, TCPConnection &y, vector<TCPSegment> &segments, const bool reorder) {
+    while (not x.segments_out().empty()) {
+        segments.emplace_back(move(x.segments_out().front()));
+        x.segments_out().pop();
+    }
+    if (reorder) {
+        for (auto it = segments.rbegin(); it != segments.rend(); ++it) {
+            y.segment_received(move(*it));
+        }
+    } else {
+        for (auto it = segments.begin(); it != segments.end(); ++it) {
+            y.segment_received(move(*it));
+        }
+    }
+    segments.clear();
+}
+
+void main_loop(const bool reorder) {
+    TCPConfig config;
+    TCPConnection x{config}, y{config};
+
+    string string_to_send(len, 'x');
+    for (auto &ch : string_to_send) {
+        ch = rand();
+    }
+
+    Buffer bytes_to_send{string(string_to_send)};
+    x.connect();
+    y.end_input_stream();
+
+    bool x_closed = false;
+
+    string string_received;
+    string_received.reserve(len);
+
+    const auto first_time = high_resolution_clock::now();
+
+    auto loop = [&] {
+        // write input into x
+        while (bytes_to_send.size() and x.remaining_outbound_capacity()) {
+            const auto want = min(x.remaining_outbound_capacity(), bytes_to_send.size());
+            const auto written = x.write(string(bytes_to_send.str().substr(0, want)));
+            if (want != written) {
+                throw runtime_error("want = " + to_string(want) + ", written = " + to_string(written));
+            }
+            bytes_to_send.remove_prefix(written);
+        }
+
+        if (bytes_to_send.size() == 0 and not x_closed) {
+            x.end_input_stream();
+            x_closed = true;
+        }
+
+        // exchange segments between x and y but in reverse order
+        vector<TCPSegment> segments;
+        move_segments(x, y, segments, reorder);
+        move_segments(y, x, segments, false);
+
+        // read output from y
+        const auto available_output = y.inbound_stream().buffer_size();
+        if (available_output > 0) {
+            string_received.append(y.inbound_stream().read(available_output));
+        }
+
+        // time passes
+        x.tick(1000);
+        y.tick(1000);
+    };
+
+    while (not y.inbound_stream().eof()) {
+        loop();
+    }
+
+    if (string_received != string_to_send) {
+        throw runtime_error("strings sent vs. received don't match");
+    }
+
+    const auto final_time = high_resolution_clock::now();
+
+    const auto duration = duration_cast<nanoseconds>(final_time - first_time).count();
+
+    const auto gigabits_per_second = len * 8.0 / double(duration);
+
+    cout << fixed << setprecision(2);
+    cout << "CPU-limited throughput" << (reorder ? " with reordering: " : "                : ") << gigabits_per_second
+         << " Gbit/s\n";
+
+    while (x.active() or y.active()) {
+        loop();
+    }
+}
+
+int main() {
+    try {
+        main_loop(false);
+        main_loop(true);
+    } catch (const exception &e) {
+        cerr << e.what() << "\n";
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/apps/tcp_ipv4.cc b/apps/tcp_ipv4.cc
new file mode 100644
index 0000000..fc8e819
--- /dev/null
+++ b/apps/tcp_ipv4.cc
@@ -0,0 +1,162 @@
+#include "bidirectional_stream_copy.hh"
+#include "tcp_config.hh"
+#include "tcp_sponge_socket.hh"
+#include "tun.hh"
+
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <iostream>
+#include <random>
+#include <string>
+#include <tuple>
+
+using namespace std;
+
+constexpr const char *TUN_DFLT = "tun144";
+const string LOCAL_ADDRESS_DFLT = "169.254.144.9";
+
+static void show_usage(const char *argv0, const char *msg) {
+    cout << "Usage: " << argv0 << " [options] <host> <port>\n\n"
+         << "   Option                                                          Default\n"
+         << "   --                                                              --\n\n"
+
+         << "   -l              Server (listen) mode.                           (client mode)\n"
+         << "                   In server mode, <host>:<port> is the address to bind.\n\n"
+
+         << "   -a <addr>       Set source address (client mode only)           " << LOCAL_ADDRESS_DFLT << "\n"
+         << "   -s <port>       Set source port (client mode only)              (random)\n\n"
+
+         << "   -w <winsz>      Use a window of <winsz> bytes                   " << TCPConfig::MAX_PAYLOAD_SIZE
+         << "\n\n"
+
+         << "   -t <tmout>      Set rt_timeout to tmout                         " << TCPConfig::TIMEOUT_DFLT << "\n\n"
+
+         << "   -d <tundev>     Connect to tun <tundev>                         " << TUN_DFLT << "\n\n"
+
+         << "   -Lu <loss>      Set uplink loss to <rate> (float in 0..1)       (no loss)\n"
+         << "   -Ld <loss>      Set downlink loss to <rate> (float in 0..1)     (no loss)\n\n"
+
+         << "   -h              Show this message.\n\n";
+
+    if (msg != nullptr) {
+        cout << msg;
+    }
+    cout << endl;
+}
+
+static void check_argc(int argc, char **argv, int curr, const char *err) {
+    if (curr + 3 >= argc) {
+        show_usage(argv[0], err);
+        exit(1);
+    }
+}
+
+static tuple<TCPConfig, FdAdapterConfig, bool, char *> get_config(int argc, char **argv) {
+    TCPConfig c_fsm{};
+    FdAdapterConfig c_filt{};
+    char *tundev = nullptr;
+
+    int curr = 1;
+    bool listen = false;
+
+    string source_address = LOCAL_ADDRESS_DFLT;
+    string source_port = to_string(uint16_t(random_device()()));
+
+    while (argc - curr > 2) {
+        if (strncmp("-l", argv[curr], 3) == 0) {
+            listen = true;
+            curr += 1;
+
+        } else if (strncmp("-a", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -a requires one argument.");
+            source_address = argv[curr + 1];
+            curr += 2;
+
+        } else if (strncmp("-s", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -s requires one argument.");
+            source_port = argv[curr + 1];
+            curr += 2;
+
+        } else if (strncmp("-w", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -w requires one argument.");
+            c_fsm.recv_capacity = strtol(argv[curr + 1], nullptr, 0);
+            curr += 2;
+
+        } else if (strncmp("-t", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -t requires one argument.");
+            c_fsm.rt_timeout = strtol(argv[curr + 1], nullptr, 0);
+            curr += 2;
+
+        } else if (strncmp("-d", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -t requires one argument.");
+            tundev = argv[curr + 1];
+            curr += 2;
+
+        } else if (strncmp("-Lu", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -Lu requires one argument.");
+            float lossrate = strtof(argv[curr + 1], nullptr);
+            using LossRateUpT = decltype(c_filt.loss_rate_up);
+            c_filt.loss_rate_up =
+                static_cast<LossRateUpT>(static_cast<float>(numeric_limits<LossRateUpT>::max()) * lossrate);
+            curr += 2;
+
+        } else if (strncmp("-Ld", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -Lu requires one argument.");
+            float lossrate = strtof(argv[curr + 1], nullptr);
+            using LossRateDnT = decltype(c_filt.loss_rate_dn);
+            c_filt.loss_rate_dn =
+                static_cast<LossRateDnT>(static_cast<float>(numeric_limits<LossRateDnT>::max()) * lossrate);
+            curr += 2;
+
+        } else if (strncmp("-h", argv[curr], 3) == 0) {
+            show_usage(argv[0], nullptr);
+            exit(0);
+
+        } else {
+            show_usage(argv[0], string("ERROR: unrecognized option " + string(argv[curr])).c_str());
+            exit(1);
+        }
+    }
+
+    // parse positional command-line arguments
+    if (listen) {
+        c_filt.source = {"0", argv[curr + 1]};
+        if (c_filt.source.port() == 0) {
+            show_usage(argv[0], "ERROR: listen port cannot be zero in server mode.");
+            exit(1);
+        }
+    } else {
+        c_filt.destination = {argv[curr], argv[curr + 1]};
+        c_filt.source = {source_address, source_port};
+    }
+
+    return make_tuple(c_fsm, c_filt, listen, tundev);
+}
+
+int main(int argc, char **argv) {
+    try {
+        if (argc < 3) {
+            show_usage(argv[0], "ERROR: required arguments are missing.");
+            return EXIT_FAILURE;
+        }
+
+        auto [c_fsm, c_filt, listen, tun_dev_name] = get_config(argc, argv);
+        LossyTCPOverIPv4SpongeSocket tcp_socket(LossyTCPOverIPv4OverTunFdAdapter(
+            TCPOverIPv4OverTunFdAdapter(TunFD(tun_dev_name == nullptr ? TUN_DFLT : tun_dev_name))));
+
+        if (listen) {
+            tcp_socket.listen_and_accept(c_fsm, c_filt);
+        } else {
+            tcp_socket.connect(c_fsm, c_filt);
+        }
+
+        bidirectional_stream_copy(tcp_socket);
+        tcp_socket.wait_until_closed();
+    } catch (const exception &e) {
+        cerr << "Exception: " << e.what() << endl;
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/apps/tcp_native.cc b/apps/tcp_native.cc
new file mode 100644
index 0000000..da9024b
--- /dev/null
+++ b/apps/tcp_native.cc
@@ -0,0 +1,43 @@
+#include "bidirectional_stream_copy.hh"
+
+#include <cstdlib>
+#include <cstring>
+#include <iostream>
+
+using namespace std;
+
+void show_usage(const char *argv0) {
+    cerr << "Usage: " << argv0 << " [-l] <host> <port>\n\n"
+         << "  -l specifies listen mode; <host>:<port> is the listening address." << endl;
+}
+
+int main(int argc, char **argv) {
+    try {
+        bool server_mode = false;
+        if (argc < 3 || ((server_mode = (strncmp("-l", argv[1], 3) == 0)) && argc < 4)) {
+            show_usage(argv[0]);
+            return EXIT_FAILURE;
+        }
+
+        // in client mode, connect; in server mode, accept exactly one connection
+        auto socket = [&] {
+            if (server_mode) {
+                TCPSocket listening_socket;                 // create a TCP socket
+                listening_socket.set_reuseaddr();           // reuse the server's address as soon as the program quits
+                listening_socket.bind({argv[2], argv[3]});  // bind to specified address
+                listening_socket.listen();                  // mark the socket as listening for incoming connections
+                return listening_socket.accept();           // accept exactly one connection
+            }
+            TCPSocket connecting_socket;
+            connecting_socket.connect({argv[1], argv[2]});
+            return connecting_socket;
+        }();
+
+        bidirectional_stream_copy(socket);
+    } catch (const exception &e) {
+        cerr << "Exception: " << e.what() << endl;
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/apps/tcp_udp.cc b/apps/tcp_udp.cc
new file mode 100644
index 0000000..54d7351
--- /dev/null
+++ b/apps/tcp_udp.cc
@@ -0,0 +1,136 @@
+#include "bidirectional_stream_copy.hh"
+#include "tcp_config.hh"
+#include "tcp_sponge_socket.hh"
+
+#include <cstdlib>
+#include <cstring>
+#include <iostream>
+#include <limits>
+#include <random>
+#include <string>
+#include <tuple>
+
+using namespace std;
+
+constexpr uint16_t DPORT_DFLT = 1440;
+
+static void show_usage(const char *argv0, const char *msg) {
+    cout << "Usage: " << argv0 << " [options] <host> <port>\n\n"
+
+         << "   Option                                                          Default\n"
+         << "   --                                                              --\n\n"
+
+         << "   -l              Server (listen) mode.                           (client mode)\n"
+         << "                   In server mode, <host>:<port> is the address to bind.\n\n"
+
+         << "   -w <winsz>      Use a window of <winsz> bytes                   " << TCPConfig::MAX_PAYLOAD_SIZE
+         << "\n\n"
+
+         << "   -t <tmout>      Set rt_timeout to tmout                         " << TCPConfig::TIMEOUT_DFLT << "\n\n"
+
+         << "   -Lu <loss>      Set uplink loss to <rate> (float in 0..1)       (no loss)\n"
+         << "   -Ld <loss>      Set downlink loss to <rate> (float in 0..1)     (no loss)\n\n"
+
+         << "   -h              Show this message and quit.\n\n";
+
+    if (msg != nullptr) {
+        cout << msg;
+    }
+    cout << endl;
+}
+
+static void check_argc(int argc, char **argv, int curr, const char *err) {
+    if (curr + 3 >= argc) {
+        show_usage(argv[0], err);
+        exit(1);
+    }
+}
+
+static tuple<TCPConfig, FdAdapterConfig, bool> get_config(int argc, char **argv) {
+    TCPConfig c_fsm{};
+    FdAdapterConfig c_filt{};
+
+    int curr = 1;
+    bool listen = false;
+
+    while (argc - curr > 2) {
+        if (strncmp("-l", argv[curr], 3) == 0) {
+            listen = true;
+            curr += 1;
+
+        } else if (strncmp("-w", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -w requires one argument.");
+            c_fsm.recv_capacity = strtol(argv[curr + 1], nullptr, 0);
+            curr += 2;
+
+        } else if (strncmp("-t", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -t requires one argument.");
+            c_fsm.rt_timeout = strtol(argv[curr + 1], nullptr, 0);
+            curr += 2;
+
+        } else if (strncmp("-Lu", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -Lu requires one argument.");
+            float lossrate = strtof(argv[curr + 1], nullptr);
+            using LossRateUpT = decltype(c_filt.loss_rate_up);
+            c_filt.loss_rate_up =
+                static_cast<LossRateUpT>(static_cast<float>(numeric_limits<LossRateUpT>::max()) * lossrate);
+            curr += 2;
+
+        } else if (strncmp("-Ld", argv[curr], 3) == 0) {
+            check_argc(argc, argv, curr, "ERROR: -Lu requires one argument.");
+            float lossrate = strtof(argv[curr + 1], nullptr);
+            using LossRateDnT = decltype(c_filt.loss_rate_dn);
+            c_filt.loss_rate_dn =
+                static_cast<LossRateDnT>(static_cast<float>(numeric_limits<LossRateDnT>::max()) * lossrate);
+            curr += 2;
+
+        } else if (strncmp("-h", argv[curr], 3) == 0) {
+            show_usage(argv[0], nullptr);
+            exit(0);
+
+        } else {
+            show_usage(argv[0], std::string("ERROR: unrecognized option " + std::string(argv[curr])).c_str());
+            exit(1);
+        }
+    }
+
+    if (listen) {
+        c_filt.source = {"0", argv[argc - 1]};
+    } else {
+        c_filt.destination = {argv[argc - 2], argv[argc - 1]};
+    }
+
+    return make_tuple(c_fsm, c_filt, listen);
+}
+
+int main(int argc, char **argv) {
+    try {
+        if (argc < 3) {
+            show_usage(argv[0], "ERROR: required arguments are missing.");
+            exit(1);
+        }
+
+        // handle configuration and UDP setup from cmdline arguments
+        auto [c_fsm, c_filt, listen] = get_config(argc, argv);
+
+        // build a TCP FSM on top of the UDP socket
+        UDPSocket udp_sock;
+        if (listen) {
+            udp_sock.bind(c_filt.source);
+        }
+        LossyTCPOverUDPSpongeSocket tcp_socket(LossyTCPOverUDPSocketAdapter(TCPOverUDPSocketAdapter(move(udp_sock))));
+        if (listen) {
+            tcp_socket.listen_and_accept(c_fsm, c_filt);
+        } else {
+            tcp_socket.connect(c_fsm, c_filt);
+        }
+
+        bidirectional_stream_copy(tcp_socket);
+        tcp_socket.wait_until_closed();
+    } catch (const exception &e) {
+        cerr << "Exception: " << e.what() << endl;
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/apps/tun.cc b/apps/tun.cc
new file mode 100644
index 0000000..e12e7b4
--- /dev/null
+++ b/apps/tun.cc
@@ -0,0 +1,57 @@
+#include "tun.hh"
+
+#include "ipv4_datagram.hh"
+#include "parser.hh"
+#include "tcp_segment.hh"
+#include "util.hh"
+
+#include <cstdlib>
+#include <exception>
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+int main() {
+    try {
+        TunFD tun("tun144");
+        while (true) {
+            auto buffer = tun.read();
+            cout << "\n\n***\n*** Got packet:\n***\n";
+            hexdump(buffer.data(), buffer.size());
+
+            IPv4Datagram ip_dgram;
+
+            cout << "attempting to parse as ipv4 datagram... ";
+            if (ip_dgram.parse(move(buffer)) != ParseResult::NoError) {
+                cout << "failed.\n";
+                continue;
+            }
+
+            cout << "success! totlen=" << ip_dgram.header().len << ", IPv4 header contents:\n";
+            cout << ip_dgram.header().to_string();
+
+            if (ip_dgram.header().proto != IPv4Header::PROTO_TCP) {
+                cout << "\nNot TCP, skipping.\n";
+                continue;
+            }
+
+            cout << "\nAttempting to parse as a TCP segment... ";
+
+            TCPSegment tcp_seg;
+
+            if (tcp_seg.parse(ip_dgram.payload(), ip_dgram.header().pseudo_cksum()) != ParseResult::NoError) {
+                cout << "failed.\n";
+                continue;
+            }
+
+            cout << "success! payload len=" << tcp_seg.payload().size() << ", TCP header contents:\n";
+            cout << tcp_seg.header().to_string() << endl;
+        }
+    } catch (const exception &e) {
+        cout << "Exception: " << e.what() << endl;
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/apps/udp_tcpdump.cc b/apps/udp_tcpdump.cc
new file mode 100644
index 0000000..a3bdfbb
--- /dev/null
+++ b/apps/udp_tcpdump.cc
@@ -0,0 +1,293 @@
+#include "parser.hh"
+#include "tcp_header.hh"
+#include "tcp_segment.hh"
+#include "util.hh"
+
+#include <arpa/inet.h>
+#include <cstdint>
+#include <cstdlib>
+#include <cstring>
+#include <iomanip>
+#include <iostream>
+#include <pcap/pcap.h>
+#include <sstream>
+#include <string>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <vector>
+
+using namespace std;
+
+static void show_usage(const char *arg0, const char *errmsg) {
+    cout << "Usage: " << arg0 << " [-i <intf>] [-F <file>] [-h|--help] <expression>\n\n"
+         << "  -i <intf>    only capture packets from <intf> (default: all)\n\n"
+
+         << "  -F <file>    reads in a filter expression from <file>\n"
+         << "               <expression> is ignored if -F is supplied.\n\n"
+
+         << "  -h, --help   show this message\n\n"
+         << "  <expression> a filter expression in pcap-filter(7) syntax\n";
+
+    if (errmsg != nullptr) {
+        cout << '\n' << errmsg;
+    }
+    cout << endl;
+}
+
+static void check_arg(char *arg0, int argc, int curr, const char *errmsg) {
+    if (curr + 1 >= argc) {
+        show_usage(arg0, errmsg);
+        exit(1);
+    }
+}
+
+static int parse_arguments(int argc, char **argv, char **dev_ptr) {
+    int curr = 1;
+    while (curr < argc) {
+        if (strncmp("-i", argv[curr], 3) == 0) {
+            check_arg(argv[0], argc, curr, "ERROR: -i requires an argument");
+            *dev_ptr = argv[curr + 1];
+            curr += 2;
+
+        } else if ((strncmp("-h", argv[curr], 3) == 0) || (strncmp("--help", argv[curr], 7) == 0)) {
+            show_usage(argv[0], nullptr);
+            exit(0);
+
+        } else {
+            break;
+        }
+    }
+
+    return curr;
+}
+
+static string inet4_addr(const uint8_t *data) {
+    char addrbuf[128];
+    auto *addr = reinterpret_cast<const in_addr *>(data);
+    if (inet_ntop(AF_INET, addr, static_cast<char *>(addrbuf), 128) == nullptr) {
+        return "unknown";
+    }
+    return string(static_cast<char *>(addrbuf));
+}
+
+static string inet6_addr(const uint8_t *data) {
+    char addrbuf[128];
+    auto *addr = reinterpret_cast<const in6_addr *>(data);
+    if (inet_ntop(AF_INET6, addr, static_cast<char *>(addrbuf), 128) == nullptr) {
+        return "unknown";
+    }
+    return string(static_cast<char *>(addrbuf));
+}
+
+static int process_ipv4_ipv6(int len, const uint8_t *data, string &src_addr, string &dst_addr) {
+    // this is either an IPv4 or IPv6 packet, we hope
+    if (len < 1) {
+        return -1;
+    }
+    int data_offset = 0;
+    const uint8_t pt = data[0] & 0xf0;
+    if (pt == 0x40) {
+        // check packet length and proto
+        data_offset = (data[0] & 0x0f) * 4;
+        if (len < data_offset) {
+            return -1;
+        }
+        if (data[9] != 0x11) {
+            cerr << "Not UDP; ";
+            return -1;
+        }
+        src_addr = inet4_addr(data + 12);
+        dst_addr = inet4_addr(data + 16);
+    } else if (pt == 0x60) {
+        // check packet length
+        if (len < 42) {
+            return -1;
+        }
+        data_offset = 40;
+        uint8_t nxt = data[6];
+        while (nxt != 0x11) {
+            if (nxt != 0 && nxt != 43 && nxt != 60) {
+                cerr << "Not UDP or fragmented; ";
+                return -1;
+            }
+            nxt = data[data_offset];
+            data_offset += 8 * (1 + data[data_offset + 1]);
+            if (len < data_offset + 2) {
+                return -1;
+            }
+        }
+        src_addr = inet6_addr(data + 8);
+        dst_addr = inet6_addr(data + 24);
+    } else {
+        return -1;
+    }
+
+    return data_offset + 8;  // skip UDP header
+}
+
+int main(int argc, char **argv) {
+    char *dev = nullptr;
+    const int exp_start = parse_arguments(argc, argv, &dev);
+
+    // create pcap handle
+    if (dev != nullptr) {
+        cout << "Capturing on interface " << dev;
+    } else {
+        cout << "Capturing on all interfaces";
+    }
+    pcap_t *p_hdl = nullptr;
+    const int dl_type = [&] {
+        char errbuf[PCAP_ERRBUF_SIZE] = {
+            0,
+        };
+        p_hdl = pcap_open_live(dev, 65535, 0, 100, static_cast<char *>(errbuf));
+        if (p_hdl == nullptr) {
+            cout << "\nError initiating capture: " << static_cast<char *>(errbuf) << endl;
+            exit(1);
+        }
+        int dlt = pcap_datalink(p_hdl);
+        // need to handle: DLT_RAW, DLT_NULL, DLT_EN10MB, DLT_LINUX_SLL
+        if (dlt != DLT_RAW && dlt != DLT_NULL && dlt != DLT_EN10MB && dlt != DLT_LINUX_SLL
+#ifdef DLT_LINUX_SLL2
+            && dlt != DLT_LINUX_SLL2
+#endif
+        ) {
+            cout << "\nError: unsupported datalink type " << pcap_datalink_val_to_description(dlt) << endl;
+            exit(1);
+        }
+        cout << " (type: " << pcap_datalink_val_to_description(dlt) << ")\n";
+        return dlt;
+    }();
+
+    // compile and set filter
+    {
+        struct bpf_program p_flt {};
+        stringstream f_stream;
+        for (int i = exp_start; i < argc; ++i) {
+            f_stream << argv[i] << ' ';
+        }
+        string filter_expression = f_stream.str();
+        cout << "Using filter expression: " << filter_expression << "\n";
+        if (pcap_compile(p_hdl, &p_flt, filter_expression.c_str(), 1, PCAP_NETMASK_UNKNOWN) != 0) {
+            cout << "Error compiling filter expression: " << pcap_geterr(p_hdl) << endl;
+            return EXIT_FAILURE;
+        }
+        if (pcap_setfilter(p_hdl, &p_flt) != 0) {
+            cout << "Error configuring packet filter: " << pcap_geterr(p_hdl) << endl;
+            return EXIT_FAILURE;
+        }
+        pcap_freecode(&p_flt);
+    }
+
+    int next_ret = 0;
+    struct pcap_pkthdr *pkt_hdr = nullptr;
+    const uint8_t *pkt_data = nullptr;
+    cout << setfill('0');
+    while ((next_ret = pcap_next_ex(p_hdl, &pkt_hdr, &pkt_data)) >= 0) {
+        if (next_ret == 0) {
+            // timeout; just listen again
+            continue;
+        }
+
+        size_t hdr_off = 0;
+        int start_off = 0;
+        // figure out where in the datagram to look based on link type
+        if (dl_type == DLT_NULL) {
+            hdr_off = 4;
+            if (pkt_hdr->caplen < hdr_off) {
+                cerr << "[INFO] Skipping malformed packet.\n";
+                continue;
+            }
+            const uint8_t pt = pkt_data[3];
+            if (pt != 2 && pt != 24 && pt != 28 && pt != 30) {
+                cerr << "[INFO] Skipping non-IP packet.\n";
+                continue;
+            }
+        } else if (dl_type == DLT_EN10MB) {
+            hdr_off = 14;
+            if (pkt_hdr->caplen < hdr_off) {
+                cerr << "[INFO] Skipping malformed packet.\n";
+                continue;
+            }
+            const uint16_t pt = (pkt_data[12] << 8) | pkt_data[13];
+            if (pt != 0x0800 && pt != 0x86dd) {
+                cerr << "[INFO] Skipping non-IP packet.\n";
+                continue;
+            }
+        } else if (dl_type == DLT_LINUX_SLL) {
+            hdr_off = 16;
+            if (pkt_hdr->caplen < hdr_off) {
+                cerr << "[INFO] Skipping malformed packet.\n";
+                continue;
+            }
+            const uint16_t pt = (pkt_data[14] << 8) | pkt_data[15];
+            if (pt != 0x0800 && pt != 0x86dd) {
+                cerr << "[INFO] Skipping non-IP packet.\n";
+                continue;
+            }
+#ifdef DLT_LINUX_SLL2
+        } else if (dl_type == DLT_LINUX_SLL2) {
+            if (pkt_hdr->caplen < 20) {
+                cerr << "[INFO] Skipping malformed packet.\n";
+                continue;
+            }
+            const uint16_t pt = (pkt_data[0] << 8) | pkt_data[1];
+            hdr_off = 20;
+            if (pt != 0x0800 && pt != 0x86dd) {
+                cerr << "[INFO] Skipping non-IP packet.\n";
+                continue;
+            }
+#endif
+        } else if (dl_type != DLT_RAW) {
+            cerr << "Mysterious datalink type. Giving up.";
+            return EXIT_FAILURE;
+        }
+
+        // now actually parse the packet
+        string src{}, dst{};
+        if ((start_off = process_ipv4_ipv6(pkt_hdr->caplen - hdr_off, pkt_data + hdr_off, src, dst)) < 0) {
+            cerr << "Error parsing IPv4/IPv6 packet. Skipping.\n";
+            continue;
+        }
+
+        // hdr_off + start_off is now the start of the UDP payload
+        const size_t payload_off = hdr_off + start_off;
+        const size_t payload_len = pkt_hdr->caplen - payload_off;
+
+        string_view payload{reinterpret_cast<const char *>(pkt_data) + payload_off, payload_len};
+
+        // try to parse UDP payload as TCP packet
+        auto seg = TCPSegment{};
+        if (const auto res = seg.parse(string(payload), 0); res > ParseResult::BadChecksum) {
+            cout << "(did not recognize TCP header) src: " << src << " dst: " << dst << '\n';
+        } else {
+            const TCPHeader &tcp_hdr = seg.header();
+            uint32_t seqlen = seg.length_in_sequence_space();
+
+            cout << src << ':' << tcp_hdr.sport << " > " << dst << ':' << tcp_hdr.dport << "\n    Flags ["
+
+                 << (tcp_hdr.urg ? "U" : "") << (tcp_hdr.psh ? "P" : "") << (tcp_hdr.rst ? "R" : "")
+                 << (tcp_hdr.syn ? "S" : "") << (tcp_hdr.fin ? "F" : "") << (tcp_hdr.ack ? "." : "")
+
+                 << "] cksum 0x" << hex << setw(4) << tcp_hdr.cksum << dec
+                 << (res == ParseResult::NoError ? " (correct)" : " (incorrect!)")
+
+                 << " seq " << tcp_hdr.seqno;
+
+            if (seqlen > 0) {
+                cout << ':' << (tcp_hdr.seqno + seqlen);
+            }
+
+            cout << " ack " << tcp_hdr.ackno << " win " << tcp_hdr.win << " length " << payload_len << endl;
+        }
+        hexdump(payload.data(), payload.size(), 8);
+    }
+
+    pcap_close(p_hdl);
+    if (next_ret == -1) {
+        cout << "Error listening for packet: " << pcap_geterr(p_hdl) << endl;
+        return EXIT_FAILURE;
+    }
+
+    return EXIT_SUCCESS;
+}
diff --git a/libsponge/tcp_connection.cc b/libsponge/tcp_connection.cc
new file mode 100644
index 0000000..aab4055
--- /dev/null
+++ b/libsponge/tcp_connection.cc
@@ -0,0 +1,49 @@
+#include "tcp_connection.hh"
+
+#include <iostream>
+
+// Dummy implementation of a TCP connection
+
+// For Lab 4, please replace with a real implementation that passes the
+// automated checks run by `make check`.
+
+template <typename... Targs>
+void DUMMY_CODE(Targs &&... /* unused */) {}
+
+using namespace std;
+
+size_t TCPConnection::remaining_outbound_capacity() const { return {}; }
+
+size_t TCPConnection::bytes_in_flight() const { return {}; }
+
+size_t TCPConnection::unassembled_bytes() const { return {}; }
+
+size_t TCPConnection::time_since_last_segment_received() const { return {}; }
+
+void TCPConnection::segment_received(const TCPSegment &seg) { DUMMY_CODE(seg); }
+
+bool TCPConnection::active() const { return {}; }
+
+size_t TCPConnection::write(const string &data) {
+    DUMMY_CODE(data);
+    return {};
+}
+
+//! \param[in] ms_since_last_tick number of milliseconds since the last call to this method
+void TCPConnection::tick(const size_t ms_since_last_tick) { DUMMY_CODE(ms_since_last_tick); }
+
+void TCPConnection::end_input_stream() {}
+
+void TCPConnection::connect() {}
+
+TCPConnection::~TCPConnection() {
+    try {
+        if (active()) {
+            cerr << "Warning: Unclean shutdown of TCPConnection\n";
+
+            // Your code here: need to send a RST segment to the peer
+        }
+    } catch (const exception &e) {
+        std::cerr << "Exception destructing TCP FSM: " << e.what() << std::endl;
+    }
+}
diff --git a/libsponge/tcp_connection.hh b/libsponge/tcp_connection.hh
new file mode 100644
index 0000000..b8a907d
--- /dev/null
+++ b/libsponge/tcp_connection.hh
@@ -0,0 +1,99 @@
+#ifndef SPONGE_LIBSPONGE_TCP_FACTORED_HH
+#define SPONGE_LIBSPONGE_TCP_FACTORED_HH
+
+#include "tcp_config.hh"
+#include "tcp_receiver.hh"
+#include "tcp_sender.hh"
+#include "tcp_state.hh"
+
+//! \brief A complete endpoint of a TCP connection
+class TCPConnection {
+  private:
+    TCPConfig _cfg;
+    TCPReceiver _receiver{_cfg.recv_capacity};
+    TCPSender _sender{_cfg.send_capacity, _cfg.rt_timeout, _cfg.fixed_isn};
+
+    //! outbound queue of segments that the TCPConnection wants sent
+    std::queue<TCPSegment> _segments_out{};
+
+    //! Should the TCPConnection stay active (and keep ACKing)
+    //! for 10 * _cfg.rt_timeout milliseconds after both streams have ended,
+    //! in case the remote TCPConnection doesn't know we've received its whole stream?
+    bool _linger_after_streams_finish{true};
+
+  public:
+    //! \name "Input" interface for the writer
+    //!@{
+
+    //! \brief Initiate a connection by sending a SYN segment
+    void connect();
+
+    //! \brief Write data to the outbound byte stream, and send it over TCP if possible
+    //! \returns the number of bytes from `data` that were actually written.
+    size_t write(const std::string &data);
+
+    //! \returns the number of `bytes` that can be written right now.
+    size_t remaining_outbound_capacity() const;
+
+    //! \brief Shut down the outbound byte stream (still allows reading incoming data)
+    void end_input_stream();
+    //!@}
+
+    //! \name "Output" interface for the reader
+    //!@{
+
+    //! \brief The inbound byte stream received from the peer
+    ByteStream &inbound_stream() { return _receiver.stream_out(); }
+    //!@}
+
+    //! \name Accessors used for testing
+
+    //!@{
+    //! \brief number of bytes sent and not yet acknowledged, counting SYN/FIN each as one byte
+    size_t bytes_in_flight() const;
+    //! \brief number of bytes not yet reassembled
+    size_t unassembled_bytes() const;
+    //! \brief Number of milliseconds since the last segment was received
+    size_t time_since_last_segment_received() const;
+    //!< \brief summarize the state of the sender, receiver, and the connection
+    TCPState state() const { return {_sender, _receiver, active(), _linger_after_streams_finish}; };
+    //!@}
+
+    //! \name Methods for the owner or operating system to call
+    //!@{
+
+    //! Called when a new segment has been received from the network
+    void segment_received(const TCPSegment &seg);
+
+    //! Called periodically when time elapses
+    void tick(const size_t ms_since_last_tick);
+
+    //! \brief TCPSegments that the TCPConnection has enqueued for transmission.
+    //! \note The owner or operating system will dequeue these and
+    //! put each one into the payload of a lower-layer datagram (usually Internet datagrams (IP),
+    //! but could also be user datagrams (UDP) or any other kind).
+    std::queue<TCPSegment> &segments_out() { return _segments_out; }
+
+    //! \brief Is the connection still alive in any way?
+    //! \returns `true` if either stream is still running or if the TCPConnection is lingering
+    //! after both streams have finished (e.g. to ACK retransmissions from the peer)
+    bool active() const;
+    //!@}
+
+    //! Construct a new connection from a configuration
+    explicit TCPConnection(const TCPConfig &cfg) : _cfg{cfg} {}
+
+    //! \name construction and destruction
+    //! moving is allowed; copying is disallowed; default construction not possible
+
+    //!@{
+    ~TCPConnection();  //!< destructor sends a RST if the connection is still open
+    TCPConnection() = delete;
+    TCPConnection(TCPConnection &&other) = default;
+    TCPConnection &operator=(TCPConnection &&other) = default;
+    TCPConnection(const TCPConnection &other) = delete;
+    TCPConnection &operator=(const TCPConnection &other) = delete;
+    //!@}
+};
+
+#endif  // SPONGE_LIBSPONGE_TCP_FACTORED_HH
diff --git a/libsponge/tcp_helpers/fd_adapter.cc b/libsponge/tcp_helpers/fd_adapter.cc
new file mode 100644
index 0000000..16c8d4a
--- /dev/null
+++ b/libsponge/tcp_helpers/fd_adapter.cc
@@ -0,0 +1,57 @@
+#include "fd_adapter.hh"
+
+#include <iostream>
+#include <stdexcept>
+#include <utility>
+
+using namespace std;
+
+//! \details This function first attempts to parse a TCP segment from the next UDP
+//! payload recv()d from the socket.
+//!
+//! If this succeeds, it then checks that the received segment is related to the
+//! current connection. When a TCP connection has been established, this means
+//! checking that the source and destination ports in the TCP header are correct.
+//!
+//! If the TCP FSM is listening (i.e., TCPOverUDPSocketAdapter::_listen is `true`)
+//! and the TCP segment read from the wire includes a SYN, this function clears the
+//! `_listen` flag and calls calls connect() on the underlying UDP socket, with
+//! the result that future outgoing segments go to the sender of the SYN segment.
+//! \returns a std::optional<TCPSegment> that is empty if the segment was invalid or unrelated
+optional<TCPSegment> TCPOverUDPSocketAdapter::read() {
+    auto datagram = _sock.recv();
+
+    // is it for us?
+    if (not listening() and (datagram.source_address != config().destination)) {
+        return {};
+    }
+
+    // is the payload a valid TCP segment?
+    TCPSegment seg;
+    if (ParseResult::NoError != seg.parse(move(datagram.payload), 0)) {
+        return {};
+    }
+
+    // should we target this source in all future replies?
+    if (listening()) {
+        if (seg.header().syn and not seg.header().rst) {
+            config_mutable().destination = datagram.source_address;
+            set_listening(false);
+        } else {
+            return {};
+        }
+    }
+
+    return seg;
+}
+
+//! Serialize a TCP segment and send it as the payload of a UDP datagram.
+//! \param[in] seg is the TCP segment to write
+void TCPOverUDPSocketAdapter::write(TCPSegment &seg) {
+    seg.header().sport = config().source.port();
+    seg.header().dport = config().destination.port();
+    _sock.sendto(config().destination, seg.serialize(0));
+}
+
+//! Specialize LossyFdAdapter to TCPOverUDPSocketAdapter
+template class LossyFdAdapter<TCPOverUDPSocketAdapter>;
diff --git a/libsponge/tcp_helpers/fd_adapter.hh b/libsponge/tcp_helpers/fd_adapter.hh
new file mode 100644
index 0000000..6ee779b
--- /dev/null
+++ b/libsponge/tcp_helpers/fd_adapter.hh
@@ -0,0 +1,70 @@
+#ifndef SPONGE_LIBSPONGE_FD_ADAPTER_HH
+#define SPONGE_LIBSPONGE_FD_ADAPTER_HH
+
+#include "file_descriptor.hh"
+#include "lossy_fd_adapter.hh"
+#include "socket.hh"
+#include "tcp_config.hh"
+#include "tcp_header.hh"
+#include "tcp_segment.hh"
+
+#include <optional>
+#include <utility>
+
+//! \brief Basic functionality for file descriptor adaptors
+//! \details See TCPOverUDPSocketAdapter and TCPOverIPv4OverTunFdAdapter for more information.
+class FdAdapterBase {
+  private:
+    FdAdapterConfig _cfg{};  //!< Configuration values
+    bool _listen = false;    //!< Is the connected TCP FSM in listen state?
+
+  protected:
+    FdAdapterConfig &config_mutable() { return _cfg; }
+
+  public:
+    //! \brief Set the listening flag
+    //! \param[in] l is the new value for the flag
+    void set_listening(const bool l) { _listen = l; }
+
+    //! \brief Get the listening flag
+    //! \returns whether the FdAdapter is listening for a new connection
+    bool listening() const { return _listen; }
+
+    //! \brief Get the current configuration
+    //! \returns a const reference
+    const FdAdapterConfig &config() const { return _cfg; }
+
+    //! \brief Get the current configuration (mutable)
+    //! \returns a mutable reference
+    FdAdapterConfig &config_mut() { return _cfg; }
+
+    //! Called periodically when time elapses
+    void tick(const size_t) {}
+};
+
+//! \brief A FD adaptor that reads and writes TCP segments in UDP payloads
+class TCPOverUDPSocketAdapter : public FdAdapterBase {
+  private:
+    UDPSocket _sock;
+
+  public:
+    //! Construct from a UDPSocket sliced into a FileDescriptor
+    explicit TCPOverUDPSocketAdapter(UDPSocket &&sock) : _sock(std::move(sock)) {}
+
+    //! Attempts to read and return a TCP segment related to the current connection from a UDP payload
+    std::optional<TCPSegment> read();
+
+    //! Writes a TCP segment into a UDP payload
+    void write(TCPSegment &seg);
+
+    //! Access the underlying UDP socket
+    operator UDPSocket &() { return _sock; }
+
+    //! Access the underlying UDP socket
+    operator const UDPSocket &() const { return _sock; }
+};
+
+//! Typedef for TCPOverUDPSocketAdapter
+using LossyTCPOverUDPSocketAdapter = LossyFdAdapter<TCPOverUDPSocketAdapter>;
+
+#endif  // SPONGE_LIBSPONGE_FD_ADAPTER_HH
diff --git a/libsponge/tcp_helpers/ipv4_datagram.cc b/libsponge/tcp_helpers/ipv4_datagram.cc
new file mode 100644
index 0000000..9839485
--- /dev/null
+++ b/libsponge/tcp_helpers/ipv4_datagram.cc
@@ -0,0 +1,41 @@
+#include "ipv4_datagram.hh"
+
+#include "parser.hh"
+#include "util.hh"
+
+#include <stdexcept>
+#include <string>
+
+using namespace std;
+
+ParseResult IPv4Datagram::parse(const Buffer buffer) {
+    NetParser p{buffer};
+    _header.parse(p);
+    _payload = p.buffer();
+
+    if (_payload.size() != _header.payload_length()) {
+        return ParseResult::PacketTooShort;
+    }
+
+    return p.get_error();
+}
+
+BufferList IPv4Datagram::serialize() const {
+    if (_payload.size() != _header.payload_length()) {
+        throw runtime_error("IPv4Datagram::serialize: payload is wrong size");
+    }
+
+    IPv4Header header_out = _header;
+    header_out.cksum = 0;
+    const string header_zero_checksum = header_out.serialize();
+
+    // calculate checksum -- taken over header only
+    InternetChecksum check;
+    check.add(header_zero_checksum);
+    header_out.cksum = check.value();
+
+    BufferList ret;
+    ret.append(header_out.serialize());
+    ret.append(_payload);
+    return ret;
+}
diff --git a/libsponge/tcp_helpers/ipv4_datagram.hh b/libsponge/tcp_helpers/ipv4_datagram.hh
new file mode 100644
index 0000000..e86232f
--- /dev/null
+++ b/libsponge/tcp_helpers/ipv4_datagram.hh
@@ -0,0 +1,32 @@
+#ifndef SPONGE_LIBSPONGE_IPV4_DATAGRAM_HH
+#define SPONGE_LIBSPONGE_IPV4_DATAGRAM_HH
+
+#include "buffer.hh"
+#include "ipv4_header.hh"
+
+//! \brief [IPv4](\ref rfc::rfc791) Internet datagram
+class IPv4Datagram {
+  private:
+    IPv4Header _header{};
+    BufferList _payload{};
+
+  public:
+    //! \brief Parse the segment from a string
+    ParseResult parse(const Buffer buffer);
+
+    //! \brief Serialize the segment to a string
+    BufferList serialize() const;
+
+    //! \name Accessors
+    //!@{
+    const IPv4Header &header() const { return _header; }
+    IPv4Header &header() { return _header; }
+
+    const BufferList &payload() const { return _payload; }
+    BufferList &payload() { return _payload; }
+    //!@}
+};
+
+using InternetDatagram = IPv4Datagram;
+
+#endif  // SPONGE_LIBSPONGE_IPV4_DATAGRAM_HH
diff --git a/libsponge/tcp_helpers/ipv4_header.cc b/libsponge/tcp_helpers/ipv4_header.cc
new file mode 100644
index 0000000..899b203
--- /dev/null
+++ b/libsponge/tcp_helpers/ipv4_header.cc
@@ -0,0 +1,159 @@
+#include "ipv4_header.hh"
+
+#include "util.hh"
+
+#include <arpa/inet.h>
+#include <iomanip>
+#include <sstream>
+
+using namespace std;
+
+//! \param[in,out] p is a NetParser from which the IP fields will be extracted
+//! \returns a ParseResult indicating success or the reason for failure
+//! \details It is important to check for (at least) the following potential errors
+//!          (but note that NetParser inherently checks for certain errors;
+//!          use that fact to your advantage!):
+//!
+//! - data stream is too short to contain a header
+//! - wrong IP version number
+//! - the header's `hlen` field is shorter than the minimum allowed
+//! - there is less data in the header than the `doff` field claims
+//! - there is less data in the full datagram than the `len` field claims
+//! - the checksum is bad
+ParseResult IPv4Header::parse(NetParser &p) {
+    Buffer original_serialized_version = p.buffer();
+
+    const size_t data_size = p.buffer().size();
+    if (data_size < IPv4Header::LENGTH) {
+        return ParseResult::PacketTooShort;
+    }
+
+    const uint8_t first_byte = p.u8();
+    ver = first_byte >> 4;     // version
+    hlen = first_byte & 0x0f;  // header length
+    tos = p.u8();              // type of service
+    len = p.u16();             // length
+    id = p.u16();              // id
+
+    const uint16_t fo_val = p.u16();
+    df = static_cast<bool>(fo_val & 0x4000);  // don't fragment
+    mf = static_cast<bool>(fo_val & 0x2000);  // more fragments
+    offset = fo_val & 0x1fff;                 // offset
+
+    ttl = p.u8();     // ttl
+    proto = p.u8();   // proto
+    cksum = p.u16();  // checksum
+    src = p.u32();    // source address
+    dst = p.u32();    // destination address
+
+    if (data_size < 4 * hlen) {
+        return ParseResult::PacketTooShort;
+    }
+    if (ver != 4) {
+        return ParseResult::WrongIPVersion;
+    }
+    if (hlen < 5) {
+        return ParseResult::HeaderTooShort;
+    }
+    if (data_size != len) {
+        return ParseResult::TruncatedPacket;
+    }
+
+    p.remove_prefix(hlen * 4 - IPv4Header::LENGTH);
+
+    if (p.error()) {
+        return p.get_error();
+    }
+
+    InternetChecksum check;
+    check.add({original_serialized_version.str().data(), size_t(4 * hlen)});
+    if (check.value()) {
+        return ParseResult::BadChecksum;
+    }
+
+    return ParseResult::NoError;
+}
+
+//! Serialize the IPv4Header to a string (does not recompute the checksum)
+string IPv4Header::serialize() const {
+    // sanity checks
+    if (ver != 4) {
+        throw runtime_error("wrong IP version");
+    }
+    if (4 * hlen < IPv4Header::LENGTH) {
+        throw runtime_error("IP header too short");
+    }
+
+    string ret;
+    ret.reserve(4 * hlen);
+
+    const uint8_t first_byte = (ver << 4) | (hlen & 0xf);
+    NetUnparser::u8(ret, first_byte);  // version and header length
+    NetUnparser::u8(ret, tos);         // type of service
+    NetUnparser::u16(ret, len);        // length
+    NetUnparser::u16(ret, id);         // id
+
+    const uint16_t fo_val = (df ? 0x4000 : 0) | (mf ? 0x2000 : 0) | (offset & 0x1fff);
+    NetUnparser::u16(ret, fo_val);  // flags and offset
+
+    NetUnparser::u8(ret, ttl);    // time to live
+    NetUnparser::u8(ret, proto);  // protocol number
+
+    NetUnparser::u16(ret, cksum);  // checksum
+
+    NetUnparser::u32(ret, src);  // src address
+    NetUnparser::u32(ret, dst);  // dst address
+
+    ret.resize(4 * hlen);  // expand header to advertised size
+
+    return ret;
+}
+
+uint16_t IPv4Header::payload_length() const { return len - 4 * hlen; }
+
+//! \details This value is needed when computing the checksum of an encapsulated TCP segment.
+//! ~~~{.txt}
+//!   0      7 8     15 16    23 24    31
+//!  +--------+--------+--------+--------+
+//!  |          source address           |
+//!  +--------+--------+--------+--------+
+//!  |        destination address        |
+//!  +--------+--------+--------+--------+
+//!  |  zero  |protocol|  payload length |
+//!  +--------+--------+--------+--------+
+//! ~~~
+uint32_t IPv4Header::pseudo_cksum() const {
+    uint32_t pcksum = (src >> 16) + (src & 0xffff);  // source addr
+    pcksum += (dst >> 16) + (dst & 0xffff);          // dest addr
+    pcksum += proto;                                 // protocol
+    pcksum += payload_length();                      // payload length
+    return pcksum;
+}
+
+//! \returns A string with the header's contents
+std::string IPv4Header::to_string() const {
+    stringstream ss{};
+    ss << hex << boolalpha << "IP version: " << +ver << '\n'
+       << "IP hdr len: " << +hlen << '\n'
+       << "IP tos: " << +tos << '\n'
+       << "IP dgram len: " << +len << '\n'
+       << "IP id: " << +id << '\n'
+       << "Flags: df: " << df << " mf: " << mf << '\n'
+       << "Offset: " << +offset << '\n'
+       << "TTL: " << +ttl << '\n'
+       << "Protocol: " << +proto << '\n'
+       << "Checksum: " << +cksum << '\n'
+       << "Src addr: " << +src << '\n'
+       << "Dst addr: " << +dst << '\n';
+    return ss.str();
+}
+
+std::string IPv4Header::summary() const {
+    stringstream ss{};
+    ss << hex << boolalpha << "IPv" << +ver << ", "
+       << "len=" << +len << ", "
+       << "protocol=" << +proto << ", " << (ttl >= 10 ? "" : "ttl=" + ::to_string(ttl) + ", ")
+       << "src=" << inet_ntoa({htobe32(src)}) << ", "
+       << "dst=" << inet_ntoa({htobe32(dst)});
+    return ss.str();
+}
diff --git a/libsponge/tcp_helpers/ipv4_header.hh b/libsponge/tcp_helpers/ipv4_header.hh
new file mode 100644
index 0000000..b9eaf83
--- /dev/null
+++ b/libsponge/tcp_helpers/ipv4_header.hh
@@ -0,0 +1,71 @@
+#ifndef SPONGE_LIBSPONGE_IPV4_HEADER_HH
+#define SPONGE_LIBSPONGE_IPV4_HEADER_HH
+
+#include "parser.hh"
+
+//! \brief [IPv4](\ref rfc::rfc791) Internet datagram header
+//! \note IP options are not supported
+struct IPv4Header {
+    static constexpr size_t LENGTH = 20;         //!< [IPv4](\ref rfc::rfc791) header length, not including options
+    static constexpr uint8_t DEFAULT_TTL = 128;  //!< A reasonable default TTL value
+    static constexpr uint8_t PROTO_TCP = 6;      //!< Protocol number for [tcp](\ref rfc::rfc793)
+
+    //! \struct IPv4Header
+    //! ~~~{.txt}
+    //!   0                   1                   2                   3
+    //!   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |Version|  IHL  |Type of Service|          Total Length         |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |         Identification        |Flags|      Fragment Offset    |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |  Time to Live |    Protocol   |         Header Checksum       |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |                       Source Address                          |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |                    Destination Address                        |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //!  |                    Options                    |    Padding    |
+    //!  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+    //! ~~~
+
+    //! \name IPv4 Header fields
+    //!@{
+    uint8_t ver = 4;            //!< IP version
+    uint8_t hlen = LENGTH / 4;  //!< header length (multiples of 32 bits)
+    uint8_t tos = 0;            //!< type of service
+    uint16_t len = 0;           //!< total length of packet
+    uint16_t id = 0;            //!< identification number
+    bool df = true;             //!< don't fragment flag
+    bool mf = false;            //!< more fragments flag
+    uint16_t offset = 0;        //!< fragment offset field
+    uint8_t ttl = DEFAULT_TTL;  //!< time to live field
+    uint8_t proto = PROTO_TCP;  //!< protocol field
+    uint16_t cksum = 0;         //!< checksum field
+    uint32_t src = 0;           //!< src address
+    uint32_t dst = 0;           //!< dst address
+    //!@}
+
+    //! Parse the IP fields from the provided NetParser
+    ParseResult parse(NetParser &p);
+
+    //! Serialize the IP fields
+    std::string serialize() const;
+
+    //! Length of the payload
+    uint16_t payload_length() const;
+
+    //! [pseudo-header's](\ref rfc::rfc793) contribution to the TCP checksum
+    uint32_t pseudo_cksum() const;
+
+    //! Return a string containing a header in human-readable format
+    std::string to_string() const;
+
+    //! Return a string containing a human-readable summary of the header
+    std::string summary() const;
+};
+
+//! \struct IPv4Header
+//! This struct can be used to parse an existing IP header or to create a new one.
+
+#endif  // SPONGE_LIBSPONGE_IPV4_HEADER_HH
diff --git a/libsponge/tcp_helpers/lossy_fd_adapter.hh b/libsponge/tcp_helpers/lossy_fd_adapter.hh
new file mode 100644
index 0000000..e76f933
--- /dev/null
+++ b/libsponge/tcp_helpers/lossy_fd_adapter.hh
@@ -0,0 +1,72 @@
+#ifndef SPONGE_LIBSPONGE_LOSSY_FD_ADAPTER_HH
+#define SPONGE_LIBSPONGE_LOSSY_FD_ADAPTER_HH
+
+#include "file_descriptor.hh"
+#include "tcp_config.hh"
+#include "tcp_segment.hh"
+#include "util.hh"
+
+#include <optional>
+#include <random>
+#include <utility>
+
+//! An adapter class that adds random dropping behavior to an FD adapter
+template <typename AdapterT>
+class LossyFdAdapter {
+  private:
+    //! Fast RNG used by _should_drop()
+    std::mt19937 _rand{get_random_generator()};
+
+    //! The underlying FD adapter
+    AdapterT _adapter;
+
+    //! \brief Determine whether or not to drop a given read or write
+    //! \param[in] uplink is `true` to use the uplink loss probability, else use the downlink loss probability
+    //! \returns `true` if the segment should be dropped
+    bool _should_drop(bool uplink) {
+        const auto &cfg = _adapter.config();
+        const uint16_t loss = uplink ? cfg.loss_rate_up : cfg.loss_rate_dn;
+        return loss != 0 && uint16_t(_rand()) < loss;
+    }
+
+  public:
+    //! Conversion to a FileDescriptor by returning the underlying AdapterT
+    operator const FileDescriptor &() const { return _adapter; }
+
+    //! Construct from a FileDescriptor appropriate to the AdapterT constructor
+    explicit LossyFdAdapter(AdapterT &&adapter) : _adapter(std::move(adapter)) {}
+
+    //! \brief Read from the underlying AdapterT instance, potentially dropping the read datagram
+    //! \returns std::optional<TCPSegment> that is empty if the segment was dropped or if
+    //!          the underlying AdapterT returned an empty value
+    std::optional<TCPSegment> read() {
+        auto ret = _adapter.read();
+        if (_should_drop(false)) {
+            return {};
+        }
+        return ret;
+    }
+
+    //! \brief Write to the underlying AdapterT instance, potentially dropping the datagram to be written
+    //! \param[in] seg is the packet to either write or drop
+    void write(TCPSegment &seg) {
+        if (_should_drop(true)) {
+            return;
+        }
+        return _adapter.write(seg);
+    }
+
+    //! \name
+    //! Passthrough functions to the underlying AdapterT instance
+
+    //!@{
+    void set_listening(const bool l) { _adapter.set_listening(l); }      //!< FdAdapterBase::set_listening passthrough
+    const FdAdapterConfig &config() const { return _adapter.config(); }  //!< FdAdapterBase::config passthrough
+    FdAdapterConfig &config_mut() { return _adapter.config_mut(); }      //!< FdAdapterBase::config_mut passthrough
+    void tick(const size_t ms_since_last_tick) {
+        _adapter.tick(ms_since_last_tick);
+    }  //!< FdAdapterBase::tick passthrough
+    //!@}
+};
+
+#endif  // SPONGE_LIBSPONGE_LOSSY_FD_ADAPTER_HH
diff --git a/libsponge/tcp_helpers/tcp_config.hh b/libsponge/tcp_helpers/tcp_config.hh
index 65542c7..02662d5 100644
--- a/libsponge/tcp_helpers/tcp_config.hh
+++ b/libsponge/tcp_helpers/tcp_config.hh
@@ -12,7 +12,7 @@
 class TCPConfig {
   public:
     static constexpr size_t DEFAULT_CAPACITY = 64000;  //!< Default capacity
-    static constexpr size_t MAX_PAYLOAD_SIZE = 1452;   //!< Max TCP payload that fits in either IPv4 or UDP datagram
+    static constexpr size_t MAX_PAYLOAD_SIZE = 1000;   //!< Conservative max payload size for real Internet
     static constexpr uint16_t TIMEOUT_DFLT = 1000;     //!< Default re-transmit timeout is 1 second
     static constexpr unsigned MAX_RETX_ATTEMPTS = 8;   //!< Maximum re-transmit attempts before giving up
 
diff --git a/libsponge/tcp_helpers/tcp_over_ip.cc b/libsponge/tcp_helpers/tcp_over_ip.cc
new file mode 100644
index 0000000..ca028c6
--- /dev/null
+++ b/libsponge/tcp_helpers/tcp_over_ip.cc
@@ -0,0 +1,90 @@
+#include "tcp_over_ip.hh"
+
+#include "ipv4_datagram.hh"
+#include "ipv4_header.hh"
+#include "parser.hh"
+
+#include <arpa/inet.h>
+#include <stdexcept>
+#include <unistd.h>
+#include <utility>
+
+using namespace std;
+
+//! \details This function attempts to parse a TCP segment from
+//! the IP datagram's payload.
+//!
+//! If this succeeds, it then checks that the received segment is related to the
+//! current connection. When a TCP connection has been established, this means
+//! checking that the source and destination ports in the TCP header are correct.
+//!
+//! If the TCP connection is listening (i.e., TCPOverIPv4OverTunFdAdapter::_listen is `true`)
+//! and the TCP segment read from the wire includes a SYN, this function clears the
+//! `_listen` flag and records the source and destination addresses and port numbers
+//! from the TCP header; it uses this information to filter future reads.
+//! \returns a std::optional<TCPSegment> that is empty if the segment was invalid or unrelated
+optional<TCPSegment> TCPOverIPv4Adapter::unwrap_tcp_in_ip(const InternetDatagram &ip_dgram) {
+    // is the IPv4 datagram for us?
+    // Note: it's valid to bind to address "0" (INADDR_ANY) and reply from actual address contacted
+    if (not listening() and (ip_dgram.header().dst != config().source.ipv4_numeric())) {
+        return {};
+    }
+
+    // is the IPv4 datagram from our peer?
+    if (not listening() and (ip_dgram.header().src != config().destination.ipv4_numeric())) {
+        return {};
+    }
+
+    // does the IPv4 datagram claim that its payload is a TCP segment?
+    if (ip_dgram.header().proto != IPv4Header::PROTO_TCP) {
+        return {};
+    }
+
+    // is the payload a valid TCP segment?
+    TCPSegment tcp_seg;
+    if (ParseResult::NoError != tcp_seg.parse(ip_dgram.payload(), ip_dgram.header().pseudo_cksum())) {
+        return {};
+    }
+
+    // is the TCP segment for us?
+    if (tcp_seg.header().dport != config().source.port()) {
+        return {};
+    }
+
+    // should we target this source addr/port (and use its destination addr as our source) in reply?
+    if (listening()) {
+        if (tcp_seg.header().syn and not tcp_seg.header().rst) {
+            config_mutable().source = {inet_ntoa({htobe32(ip_dgram.header().dst)}), config().source.port()};
+            config_mutable().destination = {inet_ntoa({htobe32(ip_dgram.header().src)}), tcp_seg.header().sport};
+            set_listening(false);
+        } else {
+            return {};
+        }
+    }
+
+    // is the TCP segment from our peer?
+    if (tcp_seg.header().sport != config().destination.port()) {
+        return {};
+    }
+
+    return tcp_seg;
+}
+
+//! Takes a TCP segment, sets port numbers as necessary, and wraps it in an IPv4 datagram
+//! \param[in] seg is the TCP segment to convert
+InternetDatagram TCPOverIPv4Adapter::wrap_tcp_in_ip(TCPSegment &seg) {
+    // set the port numbers in the TCP segment
+    seg.header().sport = config().source.port();
+    seg.header().dport = config().destination.port();
+
+    // create an Internet Datagram and set its addresses and length
+    InternetDatagram ip_dgram;
+    ip_dgram.header().src = config().source.ipv4_numeric();
+    ip_dgram.header().dst = config().destination.ipv4_numeric();
+    ip_dgram.header().len = ip_dgram.header().hlen * 4 + seg.header().doff * 4 + seg.payload().size();
+
+    // set payload, calculating TCP checksum using information from IP header
+    ip_dgram.payload() = seg.serialize(ip_dgram.header().pseudo_cksum());
+
+    return ip_dgram;
+}
diff --git a/libsponge/tcp_helpers/tcp_over_ip.hh b/libsponge/tcp_helpers/tcp_over_ip.hh
new file mode 100644
index 0000000..2d9ab4e
--- /dev/null
+++ b/libsponge/tcp_helpers/tcp_over_ip.hh
@@ -0,0 +1,19 @@
+#ifndef SPONGE_LIBSPONGE_TCP_OVER_IP_HH
+#define SPONGE_LIBSPONGE_TCP_OVER_IP_HH
+
+#include "buffer.hh"
+#include "fd_adapter.hh"
+#include "ipv4_datagram.hh"
+#include "tcp_segment.hh"
+
+#include <optional>
+
+//! \brief A converter from TCP segments to serialized IPv4 datagrams
+class TCPOverIPv4Adapter : public FdAdapterBase {
+  public:
+    std::optional<TCPSegment> unwrap_tcp_in_ip(const InternetDatagram &ip_dgram);
+
+    InternetDatagram wrap_tcp_in_ip(TCPSegment &seg);
+};
+
+#endif  // SPONGE_LIBSPONGE_TCP_OVER_IP_HH
diff --git a/libsponge/tcp_helpers/tcp_sponge_socket.cc b/libsponge/tcp_helpers/tcp_sponge_socket.cc
new file mode 100644
index 0000000..0e3e64d
--- /dev/null
+++ b/libsponge/tcp_helpers/tcp_sponge_socket.cc
@@ -0,0 +1,295 @@
+#include "tcp_sponge_socket.hh"
+
+#include "parser.hh"
+#include "tun.hh"
+#include "util.hh"
+
+#include <cstddef>
+#include <exception>
+#include <iostream>
+#include <stdexcept>
+#include <string>
+#include <sys/socket.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <utility>
+
+using namespace std;
+
+static constexpr size_t TCP_TICK_MS = 10;
+
+//! \param[in] condition is a function returning true if loop should continue
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::_tcp_loop(const function<bool()> &condition) {
+    auto base_time = timestamp_ms();
+    while (condition()) {
+        auto ret = _eventloop.wait_next_event(TCP_TICK_MS);
+        if (ret == EventLoop::Result::Exit or _abort) {
+            break;
+        }
+
+        if (_tcp.value().active()) {
+            const auto next_time = timestamp_ms();
+            _tcp.value().tick(next_time - base_time);
+            _datagram_adapter.tick(next_time - base_time);
+            base_time = next_time;
+        }
+    }
+}
+
+//! \param[in] data_socket_pair is a pair of connected AF_UNIX SOCK_STREAM sockets
+//! \param[in] datagram_interface is the interface for reading and writing datagrams
+template <typename AdaptT>
+TCPSpongeSocket<AdaptT>::TCPSpongeSocket(pair<FileDescriptor, FileDescriptor> data_socket_pair,
+                                         AdaptT &&datagram_interface)
+    : LocalStreamSocket(move(data_socket_pair.first))
+    , _thread_data(move(data_socket_pair.second))
+    , _datagram_adapter(move(datagram_interface)) {
+    _thread_data.set_blocking(false);
+}
+
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::_initialize_TCP(const TCPConfig &config) {
+    _tcp.emplace(config);
+
+    // Set up the event loop
+
+    // There are four possible events to handle:
+    //
+    // 1) Incoming datagram received (needs to be given to
+    //    TCPConnection::segment_received method)
+    //
+    // 2) Outbound bytes received from local application via a write()
+    //    call (needs to be read from the local stream socket and
+    //    given to TCPConnection::data_written method)
+    //
+    // 3) Incoming bytes reassembled by the TCPConnection
+    //    (needs to be read from the inbound_stream and written
+    //    to the local stream socket back to the application)
+    //
+    // 4) Outbound segment generated by TCP (needs to be
+    //    given to underlying datagram socket)
+
+    // rule 1: read from filtered packet stream and dump into TCPConnection
+    _eventloop.add_rule(_datagram_adapter,
+                        Direction::In,
+                        [&] {
+                            auto seg = _datagram_adapter.read();
+                            if (seg) {
+                                _tcp->segment_received(move(seg.value()));
+                            }
+
+                            // debugging output:
+                            if (_thread_data.eof() and _tcp.value().bytes_in_flight() == 0 and not _fully_acked) {
+                                cerr << "DEBUG: Outbound stream to "
+                                     << _datagram_adapter.config().destination.to_string()
+                                     << " has been fully acknowledged.\n";
+                                _fully_acked = true;
+                            }
+                        },
+                        [&] { return _tcp->active(); });
+
+    // rule 2: read from pipe into outbound buffer
+    _eventloop.add_rule(
+        _thread_data,
+        Direction::In,
+        [&] {
+            const auto data = _thread_data.read(_tcp->remaining_outbound_capacity());
+            const auto len = data.size();
+            const auto amount_written = _tcp->write(move(data));
+            if (amount_written != len) {
+                throw runtime_error("TCPConnection::write() accepted less than advertised length");
+            }
+
+            if (_thread_data.eof()) {
+                _tcp->end_input_stream();
+                _outbound_shutdown = true;
+
+                // debugging output:
+                cerr << "DEBUG: Outbound stream to " << _datagram_adapter.config().destination.to_string()
+                     << " finished (" << _tcp.value().bytes_in_flight() << " byte"
+                     << (_tcp.value().bytes_in_flight() == 1 ? "" : "s") << " still in flight).\n";
+            }
+        },
+        [&] { return (_tcp->active()) and (not _outbound_shutdown) and (_tcp->remaining_outbound_capacity() > 0); },
+        [&] {
+            _tcp->end_input_stream();
+            _outbound_shutdown = true;
+        });
+
+    // rule 3: read from inbound buffer into pipe
+    _eventloop.add_rule(
+        _thread_data,
+        Direction::Out,
+        [&] {
+            ByteStream &inbound = _tcp->inbound_stream();
+            // Write from the inbound_stream into
+            // the pipe, handling the possibility of a partial
+            // write (i.e., only pop what was actually written).
+            const size_t amount_to_write = min(size_t(65536), inbound.buffer_size());
+            const std::string buffer = inbound.peek_output(amount_to_write);
+            const auto bytes_written = _thread_data.write(move(buffer), false);
+            inbound.pop_output(bytes_written);
+
+            if (inbound.eof() or inbound.error()) {
+                _thread_data.shutdown(SHUT_WR);
+                _inbound_shutdown = true;
+
+                // debugging output:
+                cerr << "DEBUG: Inbound stream from " << _datagram_adapter.config().destination.to_string()
+                     << " finished " << (inbound.error() ? "with an error/reset.\n" : "cleanly.\n");
+                if (_tcp.value().state() == TCPState::State::TIME_WAIT) {
+                    cerr << "DEBUG: Waiting for lingering segments (e.g. retransmissions of FIN) from peer...\n";
+                }
+            }
+        },
+        [&] {
+            return (not _tcp->inbound_stream().buffer_empty()) or
+                   ((_tcp->inbound_stream().eof() or _tcp->inbound_stream().error()) and not _inbound_shutdown);
+        });
+
+    // rule 4: read outbound segments from TCPConnection and send as datagrams
+    _eventloop.add_rule(_datagram_adapter,
+                        Direction::Out,
+                        [&] {
+                            while (not _tcp->segments_out().empty()) {
+                                _datagram_adapter.write(_tcp->segments_out().front());
+                                _tcp->segments_out().pop();
+                            }
+                        },
+                        [&] { return not _tcp->segments_out().empty(); });
+}
+
+//! \brief Call [socketpair](\ref man2::socketpair) and return connected Unix-domain sockets of specified type
+//! \param[in] type is the type of AF_UNIX sockets to create (e.g., SOCK_SEQPACKET)
+//! \returns a std::pair of connected sockets
+static inline pair<FileDescriptor, FileDescriptor> socket_pair_helper(const int type) {
+    int fds[2];
+    SystemCall("socketpair", ::socketpair(AF_UNIX, type, 0, static_cast<int *>(fds)));
+    return {FileDescriptor(fds[0]), FileDescriptor(fds[1])};
+}
+
+//! \param[in] datagram_interface is the underlying interface (e.g. to UDP, IP, or Ethernet)
+template <typename AdaptT>
+TCPSpongeSocket<AdaptT>::TCPSpongeSocket(AdaptT &&datagram_interface)
+    : TCPSpongeSocket(socket_pair_helper(SOCK_STREAM), move(datagram_interface)) {}
+
+template <typename AdaptT>
+TCPSpongeSocket<AdaptT>::~TCPSpongeSocket() {
+    try {
+        if (_tcp_thread.joinable()) {
+            cerr << "Warning: unclean shutdown of TCPSpongeSocket\n";
+            // force the other side to exit
+            _abort.store(true);
+            _tcp_thread.join();
+        }
+    } catch (const exception &e) {
+        cerr << "Exception destructing TCPSpongeSocket: " << e.what() << endl;
+    }
+}
+
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::wait_until_closed() {
+    shutdown(SHUT_RDWR);
+    if (_tcp_thread.joinable()) {
+        cerr << "DEBUG: Waiting for clean shutdown... ";
+        _tcp_thread.join();
+        cerr << "done.\n";
+    }
+}
+
+//! \param[in] c_tcp is the TCPConfig for the TCPConnection
+//! \param[in] c_ad is the FdAdapterConfig for the FdAdapter
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::connect(const TCPConfig &c_tcp, const FdAdapterConfig &c_ad) {
+    if (_tcp) {
+        throw runtime_error("connect() with TCPConnection already initialized");
+    }
+
+    _initialize_TCP(c_tcp);
+
+    _datagram_adapter.config_mut() = c_ad;
+
+    cerr << "DEBUG: Connecting to " << c_ad.destination.to_string() << "... ";
+    _tcp->connect();
+
+    const TCPState expected_state = TCPState::State::SYN_SENT;
+
+    if (_tcp->state() != expected_state) {
+        throw runtime_error("After TCPConnection::connect(), state was " + _tcp->state().name() + " but expected " +
+                            expected_state.name());
+    }
+
+    _tcp_loop([&] { return _tcp->state() == TCPState::State::SYN_SENT; });
+    cerr << "done.\n";
+
+    _tcp_thread = thread(&TCPSpongeSocket::_tcp_main, this);
+}
+
+//! \param[in] c_tcp is the TCPConfig for the TCPConnection
+//! \param[in] c_ad is the FdAdapterConfig for the FdAdapter
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::listen_and_accept(const TCPConfig &c_tcp, const FdAdapterConfig &c_ad) {
+    if (_tcp) {
+        throw runtime_error("listen_and_accept() with TCPConnection already initialized");
+    }
+
+    _initialize_TCP(c_tcp);
+
+    _datagram_adapter.config_mut() = c_ad;
+    _datagram_adapter.set_listening(true);
+
+    cerr << "DEBUG: Listening for incoming connection... ";
+    _tcp_loop([&] {
+        const auto s = _tcp->state();
+        return (s == TCPState::State::LISTEN or s == TCPState::State::SYN_RCVD or s == TCPState::State::SYN_SENT);
+    });
+    cerr << "new connection from " << _datagram_adapter.config().destination.to_string() << ".\n";
+
+    _tcp_thread = thread(&TCPSpongeSocket::_tcp_main, this);
+}
+
+template <typename AdaptT>
+void TCPSpongeSocket<AdaptT>::_tcp_main() {
+    try {
+        if (not _tcp.has_value()) {
+            throw runtime_error("no TCP");
+        }
+        _tcp_loop([] { return true; });
+        shutdown(SHUT_RDWR);
+        if (not _tcp.value().active()) {
+            cerr << "DEBUG: TCP connection finished "
+                 << (_tcp.value().state() == TCPState::State::RESET ? "uncleanly" : "cleanly.\n");
+        }
+        _tcp.reset();
+    } catch (const exception &e) {
+        cerr << "Exception in TCPConnection runner thread: " << e.what() << "\n";
+        throw e;
+    }
+}
+
+//! Specialization of TCPSpongeSocket for TCPOverUDPSocketAdapter
+template class TCPSpongeSocket<TCPOverUDPSocketAdapter>;
+
+//! Specialization of TCPSpongeSocket for TCPOverIPv4OverTunFdAdapter
+template class TCPSpongeSocket<TCPOverIPv4OverTunFdAdapter>;
+
+//! Specialization of TCPSpongeSocket for LossyTCPOverUDPSocketAdapter
+template class TCPSpongeSocket<LossyTCPOverUDPSocketAdapter>;
+
+//! Specialization of TCPSpongeSocket for LossyTCPOverIPv4OverTunFdAdapter
+template class TCPSpongeSocket<LossyTCPOverIPv4OverTunFdAdapter>;
+
+CS144TCPSocket::CS144TCPSocket() : TCPOverIPv4SpongeSocket(TCPOverIPv4OverTunFdAdapter(TunFD("tun144"))) {}
+
+void CS144TCPSocket::connect(const Address &address) {
+    TCPConfig tcp_config;
+    tcp_config.rt_timeout = 100;
+
+    FdAdapterConfig multiplexer_config;
+    multiplexer_config.source = {"169.254.144.9", to_string(uint16_t(random_device()()))};
+    multiplexer_config.destination = address;
+
+    TCPOverIPv4SpongeSocket::connect(tcp_config, multiplexer_config);
+}
diff --git a/libsponge/tcp_helpers/tcp_sponge_socket.hh b/libsponge/tcp_helpers/tcp_sponge_socket.hh
new file mode 100644
index 0000000..606d04d
--- /dev/null
+++ b/libsponge/tcp_helpers/tcp_sponge_socket.hh
@@ -0,0 +1,129 @@
+#ifndef SPONGE_LIBSPONGE_TCP_SPONGE_SOCKET_HH
+#define SPONGE_LIBSPONGE_TCP_SPONGE_SOCKET_HH
+
+#include "byte_stream.hh"
+#include "eventloop.hh"
+#include "fd_adapter.hh"
+#include "file_descriptor.hh"
+#include "tcp_config.hh"
+#include "tcp_connection.hh"
+#include "tcp_over_ip.hh"
+#include "tuntap_adapter.hh"
+
+#include <atomic>
+#include <cstdint>
+#include <optional>
+#include <thread>
+#include <vector>
+
+//! Multithreaded wrapper around TCPConnection that approximates the Unix sockets API
+template <typename AdaptT>
+class TCPSpongeSocket : public LocalStreamSocket {
+  private:
+    //! Stream socket for reads and writes between owner and TCP thread
+    LocalStreamSocket _thread_data;
+
+    //! Adapter to underlying datagram socket (e.g., UDP or IP)
+    AdaptT _datagram_adapter;
+
+    //! Set up the TCPConnection and the event loop
+    void _initialize_TCP(const TCPConfig &config);
+
+    //! TCP state machine
+    std::optional<TCPConnection> _tcp{};
+
+    //! eventloop that handles all the events (new inbound datagram, new outbound bytes, new inbound bytes)
+    EventLoop _eventloop{};
+
+    //! Process events while specified condition is true
+    void _tcp_loop(const std::function<bool()> &condition);
+
+    //! Main loop of TCPConnection thread
+    void _tcp_main();
+
+    //! Handle to the TCPConnection thread; owner thread calls join() in the destructor
+    std::thread _tcp_thread{};
+
+    //! Construct LocalStreamSocket fds from socket pair, initialize eventloop
+    TCPSpongeSocket(std::pair<FileDescriptor, FileDescriptor> data_socket_pair, AdaptT &&datagram_interface);
+
+    std::atomic_bool _abort{false};  //!< Flag used by the owner to force the TCPConnection thread to shut down
+
+    bool _inbound_shutdown{false};  //!< Has TCPSpongeSocket shut down the incoming data to the owner?
+
+    bool _outbound_shutdown{false};  //!< Has the owner shut down the outbound data to the TCP connection?
+
+    bool _fully_acked{false};  //!< Has the outbound data been fully acknowledged by the peer?
+
+  public:
+    //! Construct from the interface that the TCPConnection thread will use to read and write datagrams
+    explicit TCPSpongeSocket(AdaptT &&datagram_interface);
+
+    //! Close socket, and wait for TCPConnection to finish
+    //! \note Calling this function is only advisable if the socket has reached EOF,
+    //! or else may wait foreever for remote peer to close the TCP connection.
+    void wait_until_closed();
+
+    //! Connect using the specified configurations; blocks until connect succeeds or fails
+    void connect(const TCPConfig &c_tcp, const FdAdapterConfig &c_ad);
+
+    //! Listen and accept using the specified configurations; blocks until accept succeeds or fails
+    void listen_and_accept(const TCPConfig &c_tcp, const FdAdapterConfig &c_ad);
+
+    //! When a connected socket is destructed, it will send a RST
+    ~TCPSpongeSocket();
+
+    //! \name
+    //! This object cannot be safely moved or copied, since it is in use by two threads simultaneously
+
+    //!@{
+    TCPSpongeSocket(const TCPSpongeSocket &) = delete;
+    TCPSpongeSocket(TCPSpongeSocket &&) = delete;
+    TCPSpongeSocket &operator=(const TCPSpongeSocket &) = delete;
+    TCPSpongeSocket &operator=(TCPSpongeSocket &&) = delete;
+    //!@}
+
+    //! \name
+    //! Some methods of the parent Socket wouldn't work as expected on the TCP socket, so delete them
+
+    //!@{
+    void bind(const Address &address) = delete;
+    Address local_address() const = delete;
+    Address peer_address() const = delete;
+    void set_reuseaddr() = delete;
+    //!@}
+};
+
+using TCPOverUDPSpongeSocket = TCPSpongeSocket<TCPOverUDPSocketAdapter>;
+using TCPOverIPv4SpongeSocket = TCPSpongeSocket<TCPOverIPv4OverTunFdAdapter>;
+using LossyTCPOverUDPSpongeSocket = TCPSpongeSocket<LossyTCPOverUDPSocketAdapter>;
+using LossyTCPOverIPv4SpongeSocket = TCPSpongeSocket<LossyTCPOverIPv4OverTunFdAdapter>;
+
+//! \class TCPSpongeSocket
+//! This class involves the simultaneous operation of two threads.
+//!
+//! One, the "owner" or foreground thread, interacts with this class in much the
+//! same way as one would interact with a TCPSocket: it connects or listens, writes to
+//! and reads from a reliable data stream, etc. Only the owner thread calls public
+//! methods of this class.
+//!
+//! The other, the "TCPConnection" thread, takes care of the back-end tasks that the kernel would
+//! perform for a TCPSocket: reading and parsing datagrams from the wire, filtering out
+//! segments unrelated to the connection, etc.
+//!
+//! There are a few notable differences between the TCPSpongeSocket and TCPSocket interfaces:
+//!
+//! - a TCPSpongeSocket can only accept a single connection
+//! - listen_and_accept() is a blocking function call that acts as both [listen(2)](\ref man2::listen)
+//!   and [accept(2)](\ref man2::accept)
+//! - if TCPSpongeSocket is destructed while a TCP connection is open, the connection is
+//!   immediately terminated with a RST (call `wait_until_closed` to avoid this)
+
+//! Helper class that makes a TCPOverIPv4SpongeSocket behave more like a (kernel) TCPSocket
+class CS144TCPSocket : public TCPOverIPv4SpongeSocket {
+  public:
+    CS144TCPSocket();
+    void connect(const Address &address);
+};
+
+#endif  // SPONGE_LIBSPONGE_TCP_SPONGE_SOCKET_HH
diff --git a/libsponge/tcp_helpers/tcp_state.cc b/libsponge/tcp_helpers/tcp_state.cc
index 4840695..1488fe2 100644
--- a/libsponge/tcp_helpers/tcp_state.cc
+++ b/libsponge/tcp_helpers/tcp_state.cc
@@ -2,6 +2,83 @@
 
 using namespace std;
 
+bool TCPState::operator==(const TCPState &other) const {
+    return _active == other._active and _linger_after_streams_finish == other._linger_after_streams_finish and
+           _sender == other._sender and _receiver == other._receiver;
+}
+
+bool TCPState::operator!=(const TCPState &other) const { return not operator==(other); }
+
+string TCPState::name() const {
+    return "sender=`" + _sender + "`, receiver=`" + _receiver + "`, active=" + to_string(_active) +
+           ", linger_after_streams_finish=" + to_string(_linger_after_streams_finish);
+}
+
+TCPState::TCPState(const TCPState::State state) {
+    switch (state) {
+        case TCPState::State::LISTEN:
+            _receiver = TCPReceiverStateSummary::LISTEN;
+            _sender = TCPSenderStateSummary::CLOSED;
+            break;
+        case TCPState::State::SYN_RCVD:
+            _receiver = TCPReceiverStateSummary::SYN_RECV;
+            _sender = TCPSenderStateSummary::SYN_SENT;
+            break;
+        case TCPState::State::SYN_SENT:
+            _receiver = TCPReceiverStateSummary::LISTEN;
+            _sender = TCPSenderStateSummary::SYN_SENT;
+            break;
+        case TCPState::State::ESTABLISHED:
+            _receiver = TCPReceiverStateSummary::SYN_RECV;
+            _sender = TCPSenderStateSummary::SYN_ACKED;
+            break;
+        case TCPState::State::CLOSE_WAIT:
+            _receiver = TCPReceiverStateSummary::FIN_RECV;
+            _sender = TCPSenderStateSummary::SYN_ACKED;
+            _linger_after_streams_finish = false;
+            break;
+        case TCPState::State::LAST_ACK:
+            _receiver = TCPReceiverStateSummary::FIN_RECV;
+            _sender = TCPSenderStateSummary::FIN_SENT;
+            _linger_after_streams_finish = false;
+            break;
+        case TCPState::State::CLOSING:
+            _receiver = TCPReceiverStateSummary::FIN_RECV;
+            _sender = TCPSenderStateSummary::FIN_SENT;
+            break;
+        case TCPState::State::FIN_WAIT_1:
+            _receiver = TCPReceiverStateSummary::SYN_RECV;
+            _sender = TCPSenderStateSummary::FIN_SENT;
+            break;
+        case TCPState::State::FIN_WAIT_2:
+            _receiver = TCPReceiverStateSummary::SYN_RECV;
+            _sender = TCPSenderStateSummary::FIN_ACKED;
+            break;
+        case TCPState::State::TIME_WAIT:
+            _receiver = TCPReceiverStateSummary::FIN_RECV;
+            _sender = TCPSenderStateSummary::FIN_ACKED;
+            break;
+        case TCPState::State::RESET:
+            _receiver = TCPReceiverStateSummary::ERROR;
+            _sender = TCPSenderStateSummary::ERROR;
+            _linger_after_streams_finish = false;
+            _active = false;
+            break;
+        case TCPState::State::CLOSED:
+            _receiver = TCPReceiverStateSummary::FIN_RECV;
+            _sender = TCPSenderStateSummary::FIN_ACKED;
+            _linger_after_streams_finish = false;
+            _active = false;
+            break;
+    }
+}
+
+TCPState::TCPState(const TCPSender &sender, const TCPReceiver &receiver, const bool active, const bool linger)
+    : _sender(state_summary(sender))
+    , _receiver(state_summary(receiver))
+    , _active(active)
+    , _linger_after_streams_finish(active ? linger : false) {}
+
 string TCPState::state_summary(const TCPReceiver &receiver) {
     if (receiver.stream_out().error()) {
         return TCPReceiverStateSummary::ERROR;
diff --git a/libsponge/tcp_helpers/tcp_state.hh b/libsponge/tcp_helpers/tcp_state.hh
index 4489e23..b7d37c0 100644
--- a/libsponge/tcp_helpers/tcp_state.hh
+++ b/libsponge/tcp_helpers/tcp_state.hh
@@ -21,7 +21,41 @@
 //! sender/receiver states and two variables that belong to the
 //! overarching TCPConnection object.
 class TCPState {
+  private:
+    std::string _sender{};
+    std::string _receiver{};
+    bool _active{true};
+    bool _linger_after_streams_finish{true};
+
   public:
+    bool operator==(const TCPState &other) const;
+    bool operator!=(const TCPState &other) const;
+
+    //! \brief Official state names from the [TCP](\ref rfc::rfc793) specification
+    enum class State {
+        LISTEN = 0,   //!< Listening for a peer to connect
+        SYN_RCVD,     //!< Got the peer's SYN
+        SYN_SENT,     //!< Sent a SYN to initiate a connection
+        ESTABLISHED,  //!< Three-way handshake complete
+        CLOSE_WAIT,   //!< Remote side has sent a FIN, connection is half-open
+        LAST_ACK,     //!< Local side sent a FIN from CLOSE_WAIT, waiting for ACK
+        FIN_WAIT_1,   //!< Sent a FIN to the remote side, not yet ACK'd
+        FIN_WAIT_2,   //!< Received an ACK for previously-sent FIN
+        CLOSING,      //!< Received a FIN just after we sent one
+        TIME_WAIT,    //!< Both sides have sent FIN and ACK'd, waiting for 2 MSL
+        CLOSED,       //!< A connection that has terminated normally
+        RESET,        //!< A connection that terminated abnormally
+    };
+
+    //! \brief Summarize the TCPState in a string
+    std::string name() const;
+
+    //! \brief Construct a TCPState given a sender, a receiver, and the TCPConnection's active and linger bits
+    TCPState(const TCPSender &sender, const TCPReceiver &receiver, const bool active, const bool linger);
+
+    //! \brief Construct a TCPState that corresponds to one of the "official" TCP state names
+    TCPState(const TCPState::State state);
+
     //! \brief Summarize the state of a TCPReceiver in a string
     static std::string state_summary(const TCPReceiver &receiver);
 
diff --git a/libsponge/tcp_helpers/tuntap_adapter.cc b/libsponge/tcp_helpers/tuntap_adapter.cc
new file mode 100644
index 0000000..e171b13
--- /dev/null
+++ b/libsponge/tcp_helpers/tuntap_adapter.cc
@@ -0,0 +1,6 @@
+#include "tuntap_adapter.hh"
+
+using namespace std;
+
+//! Specialize LossyFdAdapter to TCPOverIPv4OverTunFdAdapter
+template class LossyFdAdapter<TCPOverIPv4OverTunFdAdapter>;
diff --git a/libsponge/tcp_helpers/tuntap_adapter.hh b/libsponge/tcp_helpers/tuntap_adapter.hh
new file mode 100644
index 0000000..b6506a5
--- /dev/null
+++ b/libsponge/tcp_helpers/tuntap_adapter.hh
@@ -0,0 +1,42 @@
+#ifndef SPONGE_LIBSPONGE_TUNFD_ADAPTER_HH
+#define SPONGE_LIBSPONGE_TUNFD_ADAPTER_HH
+
+#include "tcp_over_ip.hh"
+#include "tun.hh"
+
+#include <optional>
+#include <unordered_map>
+#include <utility>
+
+//! \brief A FD adapter for IPv4 datagrams read from and written to a TUN device
+class TCPOverIPv4OverTunFdAdapter : public TCPOverIPv4Adapter {
+  private:
+    TunFD _tun;
+
+  public:
+    //! Construct from a TunFD
+    explicit TCPOverIPv4OverTunFdAdapter(TunFD &&tun) : _tun(std::move(tun)) {}
+
+    //! Attempts to read and parse an IPv4 datagram containing a TCP segment related to the current connection
+    std::optional<TCPSegment> read() {
+        InternetDatagram ip_dgram;
+        if (ip_dgram.parse(_tun.read()) != ParseResult::NoError) {
+            return {};
+        }
+        return unwrap_tcp_in_ip(ip_dgram);
+    }
+
+    //! Creates an IPv4 datagram from a TCP segment and writes it to the TUN device
+    void write(TCPSegment &seg) { _tun.write(wrap_tcp_in_ip(seg).serialize()); }
+
+    //! Access the underlying TUN device
+    operator TunFD &() { return _tun; }
+
+    //! Access the underlying TUN device
+    operator const TunFD &() const { return _tun; }
+};
+
+//! Typedef for TCPOverIPv4OverTunFdAdapter
+using LossyTCPOverIPv4OverTunFdAdapter = LossyFdAdapter<TCPOverIPv4OverTunFdAdapter>;
+
+#endif  // SPONGE_LIBSPONGE_TUNFD_ADAPTER_HH
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 0fff79e..e6e33ba 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -1,4 +1,4 @@
-add_library (spongechecks STATIC byte_stream_test_harness.cc)
+add_library (spongechecks STATIC send_equivalence_checker.cc tcp_fsm_test_harness.cc byte_stream_test_harness.cc)
 
 macro (add_test_exec exec_name)
     add_executable ("${exec_name}" "${exec_name}.cc")
@@ -7,6 +7,12 @@ macro (add_test_exec exec_name)
 endmacro (add_test_exec)
 
 add_test_exec (tcp_parser ${LIBPCAP})
+add_test_exec (ipv4_parser ${LIBPCAP})
+add_test_exec (fsm_active_close)
+add_test_exec (fsm_passive_close)
+add_test_exec (fsm_ack_rst_relaxed)
+add_test_exec (fsm_ack_rst_win_relaxed)
+add_test_exec (fsm_stream_reassembler_cap)
 add_test_exec (fsm_stream_reassembler_single)
 add_test_exec (fsm_stream_reassembler_seq)
 add_test_exec (fsm_stream_reassembler_dup)
@@ -14,16 +20,23 @@ add_test_exec (fsm_stream_reassembler_holes)
 add_test_exec (fsm_stream_reassembler_many)
 add_test_exec (fsm_stream_reassembler_overlapping)
 add_test_exec (fsm_stream_reassembler_win)
-add_test_exec (fsm_stream_reassembler_cap)
+add_test_exec (fsm_connect_relaxed)
+add_test_exec (fsm_listen_relaxed)
+add_test_exec (fsm_reorder)
+add_test_exec (fsm_loopback)
+add_test_exec (fsm_loopback_win)
+add_test_exec (fsm_retx_relaxed)
+add_test_exec (fsm_retx_win)
+add_test_exec (fsm_winsize)
+add_test_exec (wrapping_integers_cmp)
+add_test_exec (wrapping_integers_unwrap)
+add_test_exec (wrapping_integers_wrap)
+add_test_exec (wrapping_integers_roundtrip)
 add_test_exec (byte_stream_construction)
 add_test_exec (byte_stream_one_write)
 add_test_exec (byte_stream_two_writes)
 add_test_exec (byte_stream_capacity)
 add_test_exec (byte_stream_many_writes)
-add_test_exec (wrapping_integers_cmp)
-add_test_exec (wrapping_integers_unwrap)
-add_test_exec (wrapping_integers_wrap)
-add_test_exec (wrapping_integers_roundtrip)
 add_test_exec (recv_connect)
 add_test_exec (recv_transmit)
 add_test_exec (recv_window)
diff --git a/tests/ipv4_parser.data b/tests/ipv4_parser.data
new file mode 100644
index 0000000..20ac240
Binary files /dev/null and b/tests/ipv4_parser.data differ
diff --git a/tun.sh b/tun.sh
new file mode 100755
index 0000000..0c46eb5
--- /dev/null
+++ b/tun.sh
@@ -0,0 +1,111 @@
+#!/bin/bash
+
+show_usage () {
+    echo "Usage: $0 <start | stop | restart | check> [tunnum ...]"
+    exit 1
+}
+
+start_tun () {
+    local TUNNUM="$1" TUNDEV="tun$1"
+    ip tuntap add mode tun user "${SUDO_USER}" name "${TUNDEV}"
+    ip addr add "${TUN_IP_PREFIX}.${TUNNUM}.1/24" dev "${TUNDEV}"
+    ip link set dev "${TUNDEV}" up
+    ip route change "${TUN_IP_PREFIX}.${TUNNUM}.0/24" dev "${TUNDEV}" rto_min 10ms
+
+    # Apply NAT (masquerading) only to traffic from CS144's network devices
+    iptables -t nat -A PREROUTING -s ${TUN_IP_PREFIX}.${TUNNUM}.0/24 -j CONNMARK --set-mark ${TUNNUM}
+    iptables -t nat -A POSTROUTING -j MASQUERADE -m connmark --mark ${TUNNUM}
+    echo 1 > /proc/sys/net/ipv4/ip_forward
+}
+
+stop_tun () {
+    local TUNDEV="tun$1"
+    iptables -t nat -D PREROUTING -s ${TUN_IP_PREFIX}.${1}.0/24 -j CONNMARK --set-mark ${1}
+    iptables -t nat -D POSTROUTING -j MASQUERADE -m connmark --mark ${1}
+    ip tuntap del mode tun name "$TUNDEV"
+}
+
+start_all () {
+    while [ ! -z "$1" ]; do
+        local INTF="$1"; shift
+        start_tun "$INTF"
+    done
+}
+
+stop_all () {
+    while [ ! -z "$1" ]; do
+        local INTF="$1"; shift
+        stop_tun "$INTF"
+    done
+}
+
+restart_all() {
+    stop_all "$@"
+    start_all "$@"
+}
+
+check_tun () {
+    [ "$#" != 1 ] && { echo "bad params in check_tun"; exit 1; }
+    local TUNDEV="tun${1}"
+    # make sure tun is healthy: device is up, ip_forward is set, and iptables is configured
+    ip link show ${TUNDEV} &>/dev/null || return 1
+    [ "$(cat /proc/sys/net/ipv4/ip_forward)" = "1" ] || return 2
+}
+
+check_sudo () {
+    if [ "$SUDO_USER" = "root" ]; then
+        echo "please execute this script as a regular user, not as root"
+        exit 1
+    fi
+    if [ -z "$SUDO_USER" ]; then
+        # if the user didn't call us with sudo, re-execute
+        exec sudo $0 "$MODE" "$@"
+    fi
+}
+
+# check arguments
+if [ -z "$1" ] || ([ "$1" != "start" ] && [ "$1" != "stop" ] && [ "$1" != "restart" ] && [ "$1" != "check" ]); then
+    show_usage
+fi
+MODE=$1; shift
+
+# set default argument
+if [ "$#" = "0" ]; then
+    set -- 144 145
+fi
+
+# execute 'check' before trying to sudo
+# - like start, but exit successfully if everything is OK
+if [ "$MODE" = "check" ]; then
+    declare -a INTFS
+    MODE="start"
+    while [ ! -z "$1" ]; do
+        INTF="$1"; shift
+        check_tun ${INTF}
+        RET=$?
+        if [ "$RET" = "0" ]; then
+            continue
+        fi
+
+        if [ "$((RET > 1))" = "1" ]; then
+            MODE="restart"
+        fi
+        INTFS+=($INTF)
+    done
+
+    # address only the interfaces that need it
+    set -- "${INTFS[@]}"
+    if [ "$#" = "0" ]; then
+        exit 0
+    fi
+    echo -e "[$0] Bringing up tunnels ${INTFS[@]}:"
+fi
+
+# sudo if necessary
+check_sudo "$@"
+
+# get configuration
+. "$(dirname "$0")"/etc/tunconfig
+
+# start, stop, or restart all intfs
+eval "${MODE}_all" "$@"
diff --git a/txrx.sh b/txrx.sh
new file mode 100755
index 0000000..13e1b93
--- /dev/null
+++ b/txrx.sh
@@ -0,0 +1,238 @@
+#!/bin/bash
+
+show_usage() {
+    echo "Usage: $0 <-i|-u> <-c|-s> <-R|-S|-D> [-n|-o]"
+    echo "       [-t <rtto>] [-d <size>] [-w <size>] [-l <rate>] [-L <rate>]"
+    echo
+    echo "  Option                                                      Default"
+    echo "  --                                                          --"
+    echo "  -i or -u    Select IP or UDP mode                           (must specify)"
+    echo "  -c or -s    Select client or server mode                    (must specify)"
+    echo "  -R, -S, -D  Receive test, Send test, or Duplex test         (must specify)"
+    echo
+    echo "  -t <rtto>   Set rtto to <rtto> ms                           12"
+    echo "  -d <size>   Set total transfer size to <size>               32"
+    echo "  -w <size>   Set window size to <size>                       1452"
+    echo
+    echo "  -l <rate>   Set downlink loss to <rate> (float in 0..1)     0"
+    echo "  -L <rate>   Set uplink loss to <rate> (float in 0..1)       0"
+    echo
+    echo "  -n          In IP mode, use tcp_native rather tcp_ipv4_ref  False"
+    echo "  -o          In IP mode, use socat rather than tcp_ipv4_ref  False"
+    [ ! -z "$1" ] && { echo; echo ERROR: "$1"; }
+    exit 1
+}
+
+get_cmdline_options () {
+    # prepare to use getopts
+    local OPT= OPTIND=1 OPTARG=
+    CSMODE= RSDMODE= DATASIZE=32 WINSIZE= IUMODE= USE_IPV4= RTTO="-t 12" LOSS_UP= LOSS_DN=
+    while getopts "t:oniucsRSDd:w:p:l:L:" OPT; do
+        case "$OPT" in
+            i|u)
+                [ ! -z "$IUMODE" ] && show_usage "Only one of -i and -u is allowed."
+                IUMODE=$OPT
+                ;;
+            c|s)
+                [ ! -z "$CSMODE" ] && show_usage "Only one of -c and -s is allowed."
+                CSMODE=$OPT
+                ;;
+            R|S|D)
+                [ ! -z "$RSDMODE" ] && show_usage "Only one of -R, -S, and -D is allowed."
+                RSDMODE=$OPT
+                ;;
+            d)
+                DATASIZE="$OPTARG"
+                ;;
+            w)
+                expand_num "$OPTARG" || show_usage "Bad numeric arg \"$OPTARG\" to -w."
+                WINSIZE="-w ${NUM_EXPANDED}"
+                ;;
+            l)
+                LOSS_DN="$OPTARG"
+                ;;
+            L)
+                LOSS_UP="$OPTARG"
+                ;;
+            n|o)
+                [ ! -z "$USE_IPV4" ] && show_usage "Only one of -n and -o is allowed."
+                USE_IPV4=$OPT
+                ;;
+            t)
+                expand_num "$OPTARG" || show_usage "Bad numeric arg \"$OPTARG\" to -t."
+                RTTO="-t ${NUM_EXPANDED}"
+                ;;
+            *)
+                show_usage "Unknown option $OPT"
+                ;;
+        esac
+    done
+    if [ "$OPTIND" != $(($# + 1)) ]; then
+        show_usage "Extraneous arguments detected."
+    fi
+    if [ -z "$CSMODE" ] || [ -z "$RSDMODE" ] || [ -z "$IUMODE" ]; then
+        show_usage "You must specify either -i or -u, either -c or -s, and one of -R, -S, and -D."
+    fi
+    if [ ! -z "$USE_IPV4" ] && [ "$IUMODE" != "i" ]; then
+        show_usage "-n and -o may only be specified in IP mode (-i)."
+    fi
+    # loss param args depend on whether we're applying to ref or test program (test uplink == loss downlink)
+    { [ ! -z "$USE_IPV4" ] && local LD_SWITCH="-Ld" LU_SWITCH="-Lu"; } || local LD_SWITCH="-Lu" LU_SWITCH="-Ld"
+    [ ! -z "$LOSS_DN" ] && LOSS_DN="${LD_SWITCH} ${LOSS_DN}"
+    [ ! -z "$LOSS_UP" ] && LOSS_UP="${LU_SWITCH} ${LOSS_UP}"
+}
+
+expand_num () {
+    [ "$#" != "1" ] && { echo "bad args"; exit 1; }
+    NUM_EXPANDED=$(numfmt --from=iec "$1" 2>/dev/null)
+    return $?
+}
+
+_socat_listen () {
+    coproc socat tcp4-listen:${SERVER_PORT},reuseaddr,reuseport,linger=2 stdio >"$1" <"$2" && sleep 0.1
+    set +u
+    [ -z "$COPROC_PID" ] && { echo "Error in _socat_listen"; exit 1; }
+    set -u
+}
+
+_socat_connect () {
+    socat tcp4-connect:${TEST_HOST}:${SERVER_PORT},reuseaddr,reuseport,linger=2 stdio >"$1" <"$2" ||
+        { echo "Error in _socat_connect"; exit 1; }
+}
+
+_rt_listen () {
+    coproc $3 -l $4 ${SERVER_PORT} >"$1" <"$2" && sleep 0.1
+    set +u
+    [ -z "$COPROC_PID" ] && { echo "Error in _rt_listen"; exit 1; }
+    set -u
+}
+
+_rt_connect () {
+    $3 $4 ${SERVER_PORT} >"$1" <"$2" || { echo "Error in _rt_connect"; exit 1; }
+}
+
+test_listen () {
+    [ "$#" != 2 ] && { echo "bad args"; exit 1; }
+    _rt_listen "$1" "$2" "${TEST_PROG}" "${TEST_HOST}"
+}
+
+test_connect () {
+    [ "$#" != 2 ] && { echo "bad args"; exit 1; }
+    _rt_connect "$1" "$2" "${TEST_PROG}" "${REF_HOST}"
+}
+
+ref_listen () {
+    [ "$#" != 2 ] && { echo "bad args"; exit 1; }
+    if [ "$IUMODE" = "u" ] || [ -z "$USE_IPV4" ] || [ "$USE_IPV4" = "n" ]; then
+        _rt_listen "$1" "$2" "${REF_PROG}" "${REF_HOST}"
+    else
+        _socat_listen "$1" "$2"
+    fi
+}
+
+ref_connect () {
+    [ "$#" != 2 ] && { echo "bad args"; exit 1; }
+    if [ "$IUMODE" = "u" ] || [ -z "$USE_IPV4" ] || [ "$USE_IPV4" = "n" ]; then
+        _rt_connect "$1" "$2" "${REF_PROG}" "${TEST_HOST}"
+    else
+        _socat_connect "$1" "$2"
+    fi
+}
+
+hash_file () {
+    [ "$#" != "1" ] && { echo "bad args"; exit 1; }
+    sha256sum "$1" | cut -d \  -f 1
+}
+
+make_test_file () {
+    if expand_num "$2"; then
+        dd status=none if=/dev/urandom of="$1" bs="${NUM_EXPANDED}" count=1 || { echo "Failed to make test file."; exit 1; }
+    else
+        # can't interpret as a number, so interpret as literal data to send
+        echo -en "$2" >"$1"
+    fi
+}
+
+exit_cleanup () {
+    set +u
+    rm -f "${TEST_IN_FILE}" "${TEST_OUT_FILE}" "${TEST_OUT2_FILE}"
+    [ ! -z "$COPROC_PID" ] && kill ${COPROC_PID}
+}
+
+# make sure tun device is running
+ip link show tun144 &>/dev/null || { echo "please enable tun144 and re-run"; exit 1; }
+ip link show tun145 &>/dev/null || { echo "please enable tun145 and re-run"; exit 1; }
+
+set -u
+trap exit_cleanup EXIT
+
+get_cmdline_options "$@"
+
+. "$(dirname "$0")"/etc/tunconfig
+REF_HOST=${TUN_IP_PREFIX}.144.1
+TEST_HOST=${TUN_IP_PREFIX}.144.1
+SERVER_PORT=$(($((RANDOM % 50000)) + 1025))
+if [ "$IUMODE" = "i" ]; then
+    # IPv4 mode
+    TEST_HOST=${TUN_IP_PREFIX}.144.9
+    if [ -z "$USE_IPV4" ]; then
+        REF_HOST=${TUN_IP_PREFIX}.145.9
+        REF_PROG="./apps/tcp_ipv4 ${RTTO} ${WINSIZE} ${LOSS_UP} ${LOSS_DN} -d tun145 -a ${REF_HOST}"
+        TEST_PROG="./apps/tcp_ipv4 ${RTTO} ${WINSIZE} -d tun144 -a ${TEST_HOST}"
+    else
+        REF_PROG="./apps/tcp_native"
+        TEST_PROG="./apps/tcp_ipv4 ${RTTO} ${WINSIZE} ${LOSS_UP} ${LOSS_DN} -d tun144 -a ${TEST_HOST}"
+    fi
+else
+    # UDP mode
+    REF_PROG="./apps/tcp_udp ${RTTO} ${WINSIZE} ${LOSS_UP} ${LOSS_DN}"
+    TEST_PROG="./apps/tcp_udp ${RTTO} ${WINSIZE}"
+fi
+
+TEST_OUT_FILE=$(mktemp)
+TEST_IN_FILE=$(mktemp)
+make_test_file "${TEST_IN_FILE}" "${DATASIZE}"
+HASH_IN=$(sha256sum ${TEST_IN_FILE} | cut -d \  -f 1)
+HASH_OUT2=
+case "$RSDMODE" in
+    S)  # test sending
+        if [ "$CSMODE" = "c" ]; then
+            ref_listen "${TEST_OUT_FILE}" /dev/null
+            test_connect /dev/null "${TEST_IN_FILE}"
+        else
+            test_listen /dev/null "${TEST_IN_FILE}"
+            ref_connect "${TEST_OUT_FILE}" /dev/null
+        fi
+        ;;
+    R)  # test receiving
+        if [ "$CSMODE" = "c" ]; then
+            ref_listen /dev/null "${TEST_IN_FILE}"
+            test_connect "${TEST_OUT_FILE}" /dev/null
+        else
+            test_listen "${TEST_OUT_FILE}" /dev/null
+            ref_connect /dev/null "${TEST_IN_FILE}"
+        fi
+        ;;
+    D)  # test full-duplex
+        TEST_OUT2_FILE=$(mktemp)
+        if [ "$CSMODE" = "c" ]; then
+            ref_listen "${TEST_OUT_FILE}" "${TEST_IN_FILE}"
+            test_connect "${TEST_OUT2_FILE}" "${TEST_IN_FILE}"
+        else
+            test_listen "${TEST_OUT_FILE}" "${TEST_IN_FILE}"
+            ref_connect "${TEST_OUT2_FILE}" "${TEST_IN_FILE}"
+        fi
+        HASH_OUT2=$(hash_file "${TEST_OUT2_FILE}")
+        ;;
+esac
+if ! wait; then
+    echo ERROR: subprocess failed
+    exit 1
+fi
+
+HASH_OUT=$(hash_file ${TEST_OUT_FILE})
+if [ ! -z "${HASH_OUT2}" ] && [ "${HASH_OUT}" != "${HASH_OUT2}" ] || [ "${HASH_IN}" != "${HASH_OUT}" ]; then
+    echo ERROR: "$HASH_IN" neq "$HASH_OUT" or "$HASH_OUT2"
+    exit 1
+fi
+exit 0
diff --git a/writeups/lab4.md b/writeups/lab4.md
new file mode 100644
index 0000000..2d3d940
--- /dev/null
+++ b/writeups/lab4.md
@@ -0,0 +1,29 @@
+Lab 4 Writeup
+=============
+
+My name: [your name here]
+
+My SUNet ID: [your sunetid here]
+
+I collaborated with: [list sunetids here]
+
+I would like to thank/reward these classmates for their help: [list sunetids here]
+
+This lab took me about [n] hours to do. I [did/did not] attend the lab session.
+
+Program Structure and Design of the TCPConnection:
+[]
+
+Implementation Challenges:
+[]
+
+Remaining Bugs:
+[]
+
+- Optional: I had unexpected difficulty with: [describe]
+
+- Optional: I think you could make this lab better by: [describe]
+
+- Optional: I was surprised by: [describe]
+
+- Optional: I'm not sure about: [describe]