Kernel: Add support for TCP window size scaling

This should allow us to eventually properly saturate high-bandwidth
network links when using TCP, once other nonoptimal parts of our
network stack are improved.

Kernel/Net/IPv4Socket.cpp

@@ -38,7 +38,7 @@ MutexProtected<IPv4Socket::List>& IPv4Socket::all_sockets()
 
 ErrorOr<NonnullOwnPtr<DoubleBuffer>> IPv4Socket::try_create_receive_buffer()
 {
-    return DoubleBuffer::try_create("IPv4Socket: Receive buffer"sv, 256 * KiB);
+    return DoubleBuffer::try_create("IPv4Socket: Receive buffer"sv, receive_buffer_size);
 }
 
 ErrorOr<NonnullRefPtr<Socket>> IPv4Socket::create(int type, int protocol)

Kernel/Net/IPv4Socket.h

@@ -68,6 +68,8 @@ public:
     BufferMode buffer_mode() const { return m_buffer_mode; }
 
 protected:
+    static constexpr size_t receive_buffer_size = 256 * KiB;
+
     IPv4Socket(int type, int protocol, NonnullOwnPtr<DoubleBuffer> receive_buffer, OwnPtr<KBuffer> optional_scratch_buffer);
     virtual StringView class_name() const override { return "IPv4Socket"sv; }
 

Kernel/Net/NetworkTask.cpp

@@ -430,6 +430,19 @@ void handle_tcp(IPv4Packet const& ipv4_packet, UnixDateTime const& packet_timest
     dbgln_if(TCP_DEBUG, "handle_tcp: got socket {}; state={}", socket->tuple().to_string(), TCPSocket::to_string(socket->state()));
 
     socket->receive_tcp_packet(tcp_packet, ipv4_packet.payload_size());
+    Optional<u8> send_window_scale;
+    if (tcp_packet.has_syn()) {
+        tcp_packet.for_each_option([&send_window_scale](auto const& option) {
+            if (option.kind() != TCPOptionKind::WindowScale)
+                return;
+            if (option.length() != sizeof(TCPOptionWindowScale))
+                return;
+            auto scale = static_cast<TCPOptionWindowScale const&>(option).value();
+            if (scale > 14)
+                return; // Maximum allowed as per RFC7323
+            send_window_scale = scale;
+        });
+    }
 
     switch (socket->state()) {
     case TCPSocket::State::Closed:
@@ -459,6 +472,8 @@ void handle_tcp(IPv4Packet const& ipv4_packet, UnixDateTime const& packet_timest
             client->set_ack_number(tcp_packet.sequence_number() + payload_size + 1);
             [[maybe_unused]] auto rc2 = client->send_tcp_packet(TCPFlags::SYN | TCPFlags::ACK);
             client->set_state(TCPSocket::State::SynReceived);
+            if (send_window_scale.has_value())
+                client->set_send_window_scale(*send_window_scale);
             return;
         }
         default:
@@ -472,6 +487,8 @@ void handle_tcp(IPv4Packet const& ipv4_packet, UnixDateTime const& packet_timest
             socket->set_ack_number(tcp_packet.sequence_number() + payload_size + 1);
             (void)socket->send_tcp_packet(TCPFlags::SYN | TCPFlags::ACK);
             socket->set_state(TCPSocket::State::SynReceived);
+            if (send_window_scale.has_value())
+                socket->set_send_window_scale(*send_window_scale);
             return;
         case TCPFlags::ACK | TCPFlags::SYN:
             socket->set_ack_number(tcp_packet.sequence_number() + payload_size + 1);
@@ -479,6 +496,8 @@ void handle_tcp(IPv4Packet const& ipv4_packet, UnixDateTime const& packet_timest
             socket->set_state(TCPSocket::State::Established);
             socket->set_setup_state(Socket::SetupState::Completed);
             socket->set_connected(true);
+            if (send_window_scale.has_value())
+                socket->set_send_window_scale(*send_window_scale);
             return;
         case TCPFlags::ACK | TCPFlags::FIN:
             socket->set_ack_number(tcp_packet.sequence_number() + payload_size + 1);

Kernel/Net/TCP.h

@@ -21,21 +21,59 @@ struct TCPFlags {
     };
 };
 
-class [[gnu::packed]] TCPOptionMSS {
+enum class TCPOptionKind : u8 {
+    End = 0,
+    Nop = 1,
+    MSS = 2,
+    WindowScale = 3,
+    SACKPermitted = 4,
+    SACK = 5,
+    Timestamp = 6,
+};
+
+class [[gnu::packed]] TCPOption {
+public:
+    TCPOptionKind kind() const { return m_kind; }
+    u8 length() const { return m_length; }
+
+protected:
+    TCPOption(TCPOptionKind kind, u8 length)
+        : m_kind(kind)
+        , m_length(length) {};
+
+private:
+    TCPOptionKind m_kind { TCPOptionKind::End };
+    u8 m_length { sizeof(TCPOption) };
+};
+
+class [[gnu::packed]] TCPOptionMSS : public TCPOption {
 public:
     TCPOptionMSS(u16 value)
-        : m_value(value)
+        : TCPOption(TCPOptionKind::MSS, sizeof(TCPOptionMSS))
+        , m_value(value)
     {
     }
 
     u16 value() const { return m_value; }
 
 private:
-    u8 m_option_kind { 0x02 };
-    u8 m_option_length { sizeof(TCPOptionMSS) };
     NetworkOrdered<u16> m_value;
 };
 
+class [[gnu::packed]] TCPOptionWindowScale : public TCPOption {
+public:
+    TCPOptionWindowScale(u8 value)
+        : TCPOption(TCPOptionKind::WindowScale, sizeof(TCPOptionWindowScale))
+        , m_value(value)
+    {
+    }
+
+    u8 value() const { return m_value; }
+
+private:
+    NetworkOrdered<u8> m_value;
+};
+
 static_assert(AssertSize<TCPOptionMSS, 4>());
 
 class [[gnu::packed]] TCPPacket {
@@ -80,6 +118,28 @@ public:
     void const* payload() const { return ((u8 const*)this) + header_size(); }
     void* payload() { return ((u8*)this) + header_size(); }
 
+    template<typename Callback>
+    void for_each_option(Callback callback) const
+    {
+        auto const* next_option = (u8 const*)this + sizeof(TCPPacket);
+        auto const* options_end = payload();
+        while (next_option < options_end) {
+            if ((size_t)options_end - (size_t)next_option < sizeof(TCPOption))
+                return; // Not enough space left for another option
+            auto const* option = (TCPOption const*)next_option;
+            if (option->kind() == TCPOptionKind::End)
+                return;
+            if (option->kind() == TCPOptionKind::Nop) {
+                next_option += 1;
+                continue;
+            }
+            if (option->length() < sizeof(TCPOption))
+                return; // minimal option length
+            callback(*option);
+            next_option += option->length();
+        }
+    }
+
 private:
     NetworkOrdered<u16> m_source_port;
     NetworkOrdered<u16> m_destination_port;

Kernel/Net/TCPSocket.cpp

@@ -245,10 +245,11 @@ ErrorOr<void> TCPSocket::send_tcp_packet(u16 flags, UserOrKernelBuffer const* pa
 
     auto ipv4_payload_offset = routing_decision.adapter->ipv4_payload_offset();
 
-    bool const has_mss_option = flags == TCPFlags::SYN;
-    const size_t options_size = has_mss_option ? sizeof(TCPOptionMSS) : 0;
-    const size_t tcp_header_size = sizeof(TCPPacket) + options_size;
-    const size_t buffer_size = ipv4_payload_offset + tcp_header_size + payload_size;
+    bool const has_mss_option = flags & TCPFlags::SYN;
+    bool const has_window_scale_option = flags & TCPFlags::SYN;
+    size_t const options_size = (has_mss_option ? sizeof(TCPOptionMSS) : 0) + (has_window_scale_option ? sizeof(TCPOptionWindowScale) : 0);
+    size_t const tcp_header_size = sizeof(TCPPacket) + align_up_to(options_size, 4);
+    size_t const buffer_size = ipv4_payload_offset + tcp_header_size + payload_size;
     auto packet = routing_decision.adapter->acquire_packet_buffer(buffer_size);
     if (!packet)
         return set_so_error(ENOMEM);
@@ -260,7 +261,10 @@ ErrorOr<void> TCPSocket::send_tcp_packet(u16 flags, UserOrKernelBuffer const* pa
     VERIFY(local_port());
     tcp_packet.set_source_port(local_port());
     tcp_packet.set_destination_port(peer_port());
-    tcp_packet.set_window_size(min(available_space_in_receive_buffer(), NumericLimits<u16>::max()));
+    auto window_size = available_space_in_receive_buffer();
+    if ((flags & TCPFlags::SYN) == 0 && m_window_scaling_supported)
+        window_size >>= receive_window_scale();
+    tcp_packet.set_window_size(min(window_size, NumericLimits<u16>::max()));
     tcp_packet.set_sequence_number(m_sequence_number);
     tcp_packet.set_data_offset(tcp_header_size / sizeof(u32));
     tcp_packet.set_flags(flags);
@@ -284,12 +288,20 @@ ErrorOr<void> TCPSocket::send_tcp_packet(u16 flags, UserOrKernelBuffer const* pa
         m_sequence_number += payload_size;
     }
 
+    u8* next_option = packet->buffer->data() + ipv4_payload_offset + sizeof(TCPPacket);
     if (has_mss_option) {
         u16 mss = routing_decision.adapter->mtu() - sizeof(IPv4Packet) - sizeof(TCPPacket);
         TCPOptionMSS mss_option { mss };
-        VERIFY(packet->buffer->size() >= ipv4_payload_offset + sizeof(TCPPacket) + sizeof(mss_option));
-        memcpy(packet->buffer->data() + ipv4_payload_offset + sizeof(TCPPacket), &mss_option, sizeof(mss_option));
+        memcpy(next_option, &mss_option, sizeof(mss_option));
+        next_option += sizeof(mss_option);
     }
+    if (has_window_scale_option) {
+        TCPOptionWindowScale window_scale_option { receive_window_scale() };
+        memcpy(next_option, &window_scale_option, sizeof(window_scale_option));
+        next_option += sizeof(window_scale_option);
+    }
+    if ((options_size % 4) != 0)
+        *next_option = to_underlying(TCPOptionKind::End);
 
     tcp_packet.set_checksum(compute_tcp_checksum(local_address(), peer_address(), tcp_packet, payload_size));
 
@@ -339,7 +351,7 @@ void TCPSocket::receive_tcp_packet(TCPPacket const& packet, u16 size)
                         old_adapter->release_packet_buffer(*packet.buffer);
                     TCPPacket& tcp_packet = *(TCPPacket*)(packet.buffer->buffer->data() + packet.ipv4_payload_offset);
                     if (m_send_window_size != tcp_packet.window_size()) {
-                        m_send_window_size = tcp_packet.window_size();
+                        m_send_window_size = tcp_packet.window_size() << m_send_window_scale;
                     }
                     auto payload_size = packet.buffer->buffer->data() + packet.buffer->buffer->size() - (u8*)tcp_packet.payload();
                     unacked_packets.size -= payload_size;
@@ -367,7 +379,7 @@ void TCPSocket::receive_tcp_packet(TCPPacket const& packet, u16 size)
 bool TCPSocket::should_delay_next_ack() const
 {
     // FIXME: We don't know the MSS here so make a reasonable guess.
-    const size_t mss = 1500;
+    size_t const mss = 1500;
 
     // RFC 1122 says we should send an ACK for every two full-sized segments.
     if (m_ack_number >= m_last_ack_number_sent + 2 * mss)

Kernel/Net/TCPSocket.h

@@ -9,6 +9,7 @@
 #include <AK/Error.h>
 #include <AK/Function.h>
 #include <AK/HashMap.h>
+#include <AK/IntegralMath.h>
 #include <AK/SinglyLinkedList.h>
 #include <AK/Time.h>
 #include <Kernel/Library/LockWeakPtr.h>
@@ -135,6 +136,12 @@ public:
     u32 packets_out() const { return m_packets_out; }
     u32 bytes_out() const { return m_bytes_out; }
 
+    void set_send_window_scale(size_t scale)
+    {
+        m_window_scaling_supported = true;
+        m_send_window_scale = scale;
+    }
+
     // FIXME: Make this configurable?
     static constexpr u32 maximum_duplicate_acks = 5;
     void set_duplicate_acks(u32 acks) { m_duplicate_acks = acks; }
@@ -188,6 +195,14 @@ private:
     void enqueue_for_retransmit();
     void dequeue_for_retransmit();
 
+    static constexpr size_t receive_window_scale()
+    {
+        auto buffer_size_bit_length = AK::log2(receive_buffer_size) + 1;
+        if (buffer_size_bit_length < 16)
+            return 0;
+        return buffer_size_bit_length - 16;
+    }
+
     LockWeakPtr<TCPSocket> m_originator;
     HashMap<IPv4SocketTuple, NonnullRefPtr<TCPSocket>> m_pending_release_for_accept;
     Direction m_direction { Direction::Unspecified };
@@ -229,6 +244,8 @@ private:
     // Default to maximum window size. receive_tcp_packet() will update from the
     // peer's advertised window size.
     u32 m_send_window_size { 64 * KiB };
+    bool m_window_scaling_supported { false };
+    size_t m_send_window_scale { 0 };
 
     bool m_no_delay { false };