Network Stack

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Architecture Overview

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Network Task

• Processes incoming packets from all network adapters
• Dispatches packets to protocol handlers
• Manages delayed TCP ACKs
• Handles TCP retransmission
• Runs the main packet processing loop

1. Network adapter receives packet → triggers interrupt
2. on_receive callback increments pending packets counter
3. NetworkTask wakes up and reads packet from adapter
4. Ethernet frame header parsed to determine EtherType
5. Dispatch to appropriate handler (ARP, IPv4, IPv6)

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Socket Abstraction

enum class SetupState {
    Unstarted,  // Not yet configured
    InProgress, // Connection in progress (TCP SYN sent)
    Completed,  // Setup complete
};

enum class Role {
    None,
    Listener,   // Listening socket
    Accepted,   // Accepted from listener
    Connected,  // Active connection
    Connecting  // Connection in progress
};

• bind() - Bind to local address
• connect() - Establish connection
• listen() / accept() - Server operations
• sendto() / recvfrom() - Data transfer
• setsockopt() / getsockopt() - Socket options
• shutdown() - Close one or both directions

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Protocol Implementations

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TCP (Transmission Control Protocol)

enum class State {
    Closed,
    Listen,
    SynSent,
    SynReceived,
    Established,
    CloseWait,
    LastAck,
    FinWait1,
    FinWait2,
    Closing,
    TimeWait,
};

• Three-way handshake (SYN, SYN-ACK, ACK)
• Sequence number tracking
• Window-based flow control
• Delayed ACK optimization
• Retransmission on timeout
• Congestion control
• Graceful connection termination (FIN)

static void handle_tcp(IPv4Packet const&, 
                       UnixDateTime const& packet_timestamp, 
                       RefPtr<NetworkAdapter> adapter);

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UDP (User Datagram Protocol)

• No connection establishment required
• Best-effort delivery
• Checksum validation
• Multicast support
• Broadcast support (when enabled)

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IPv4 (Internet Protocol version 4)

• Header validation and checksum verification
• Fragmentation and reassembly
• Time-to-live (TTL) processing
• Routing decision making
• Protocol demultiplexing (TCP, UDP, ICMP)

struct IPv4Packet {
    // Header fields
    // Payload access
    // Protocol identification
};

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IPv6 (Internet Protocol version 6)

• 128-bit addressing
• Simplified header format
• Extension headers
• ICMPv6 integration
• Neighbor Discovery Protocol (NDP)

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ICMP (Internet Control Message Protocol)

• Echo request/reply (ping)
• Destination unreachable
• Time exceeded
• Redirect messages

• Echo request/reply
• Neighbor Solicitation/Advertisement
• Router Solicitation/Advertisement
• Packet Too Big messages

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Local Sockets (Unix Domain)

• Stream sockets (SOCK_STREAM)
• Datagram sockets (SOCK_DGRAM)
• File system path binding
• SCM_RIGHTS (file descriptor passing)
• Credential passing

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Network Adapters

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Supported Drivers

• Virtual adapter for local communication
• Automatically configured with 127.0.0.1/8
• No hardware dependencies

• E1000 family support
• PCI-based adapters
• Hardware offload capabilities

• RTL8139 and compatible
• Common in virtual environments

• Virtio-net para-virtualized driver
• Optimized for virtual machines
• High performance with minimal overhead

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Adapter Configuration

• MAC address management
• IPv4 address/netmask configuration
• IPv6 address configuration
• MTU (Maximum Transmission Unit) settings
• Packet receive callback registration
• Link state monitoring

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Routing and ARP

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Routing Table

struct Route {
    IPv4Address destination;
    IPv4Address gateway;
    IPv4Address netmask;
    u16 flags;
    NonnullRefPtr<NetworkAdapter> adapter;
};

• route_to() - Find route for destination
• update_routing_table() - Add/remove routes
• Gateway routing support
• Broadcast routing

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ARP (Address Resolution Protocol)

• ARP request/reply handling
• ARP cache (table) management
• Automatic cache updates
• ARP cache aging and expiration

void update_arp_table(IPv4Address const&, MACAddress const&, UpdateTable);
HashMap<IPv4Address, MACAddress>& arp_table();

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Network Management

• Network adapter registration and enumeration
• Adapter lifecycle management
• Global network subsystem initialization
• Adapter iteration for packet processing

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Socket Options

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SOL_SOCKET Level

• SO_RCVTIMEO / SO_SNDTIMEO - Receive/send timeouts
• SO_ERROR - Get and clear error status
• SO_REUSEADDR - Allow address reuse
• SO_KEEPALIVE - TCP keepalive
• SO_TIMESTAMP - Packet timestamps
• SO_BINDTODEVICE - Bind to specific interface

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IPPROTO_IP Level

• IP_TTL - Time-to-live
• IP_MULTICAST_LOOP - Multicast loopback
• IP_ADD_MEMBERSHIP / IP_DROP_MEMBERSHIP - Multicast groups

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IPPROTO_TCP Level

• TCP_NODELAY - Disable Nagle algorithm

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Packet Buffer Management

size_t buffer_size = 64 * KiB;
auto region = MM.allocate_kernel_region(buffer_size, 
                                        "Kernel Packet Buffer",
                                        Memory::Region::Access::ReadWrite);

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Performance Optimizations

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Delayed TCP ACK

• ACKs batched when possible
• Separate hash table tracks sockets needing ACKs
• Periodic flush (every 500ms) ensures timely acknowledgment

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TCP Retransmission

• Timer-based retransmit queue
• Exponential backoff
• Configurable retransmit limits

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Implementation Notes

• Low latency for packet handling
• Efficient interrupt-driven processing
• Centralized protocol implementation
• Simplified synchronization