feat(net): active-open TCP, and fix two bugs that broke TCP on the wire

Adds the client half of TCP -- tcp_connect/tcp_send/tcp_recv/tcp_close --
and wires SOCK_STREAM sockets to it, so the kernel can originate an
outbound stream rather than only answer inbound ones. Inbound segments
are now matched against the connection table before being offered to the
listeners; previously anything not addressed to a listening port was
answered with RST, which would have rejected the SYN-ACK completing our
own handshake.

Transmission is stop-and-wait: one segment outstanding, retransmitted
until acknowledged. Every blocking call is bounded against the timer tick
and halts the CPU between checks, so a silent peer costs a timeout rather
than a hang. Connections linger in TIME_WAIT so a late duplicate cannot
be mistaken for traffic on a new incarnation.

Verifying it against a real peer uncovered two pre-existing bugs that had
made TCP unusable in both directions. Neither was visible from inside the
guest, which is why the servers had looked fine:

1. The TCP and UDP checksums were computed by reading the header a
   uint16_t at a time, immediately after the caller stored zero into that
   same header's checksum field. Nothing stops the compiler reordering
   those word reads around that store, and at -O2 it did: the stale
   checksum field was folded into its own result. Every TCP segment the
   kernel had ever sent carried a wrong checksum and was silently dropped
   by the peer -- no client could complete a handshake with sshd or
   httpd, and it is the likely reason DHCP and DNS replies never arrived.
   The checksum is now summed a byte at a time through unsigned char
   pointers, which may alias any object and so cannot be reordered, and
   which also drops the alignment assumption that reading a packed header
   as uint16_t made -- a fault rather than a slowdown on ARM. The helper
   is shared by IP, TCP and UDP instead of being open-coded four times.

2. net_receive derived the datagram length from the Ethernet frame rather
   than from the IP header's total_length, so the padding on any frame
   shorter than 60 bytes was delivered as protocol payload. A bare ACK or
   FIN appeared to carry six bytes, which pushed the acknowledgement
   number out of step with the peer and turned every close into a FIN
   retransmission storm answered with RST.

Also adds `tcpconnect` and `httpget` applets, and gives netstat the real
per-connection state and direction from the engine's table.

Verified on x86_64 against a real TCP stack, checked packet by packet in
a capture: `httpget` fetched a 349-byte page from a server on the host --
handshake, 81-byte request, response, orderly four-way close, every
sequence number exact, zero bad checksums, no retransmissions. In the
other direction curl on the host now fetches a live page from the guest's
httpd, which had never worked. Builds clean on x86_64, aarch64 and armv8i.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Fh5z6V7nGPbHvqVG5p4CXX

net/ipv4/tcp.c

@@ -1,3 +1,29 @@
+/*
+ * TCP for SUB-OS.
+ *
+ * The engine drives both ends of a connection:
+ *
+ *   Passive open  -- an inbound SYN to a listening service (sshd, httpd)
+ *                    creates a server connection, which is answered straight
+ *                    out of the receive path.
+ *   Active open   -- tcp_connect() performs a real three-way handshake, after
+ *                    which tcp_send/tcp_recv carry a byte stream and
+ *                    tcp_close() shuts it down in order.
+ *
+ * Inbound segments are matched against the connection table first and only
+ * then offered to the listeners, which is what lets a reply to one of our own
+ * outbound connections through -- previously anything that was not addressed
+ * to a listening port was answered with RST, so the SYN-ACK completing our own
+ * handshake would have been rejected.
+ *
+ * Transmission is stop-and-wait rather than a sliding window: one segment is
+ * outstanding at a time and is retransmitted until acknowledged. That is
+ * enough to be correct over a lossy path and keeps the retransmission state to
+ * a single sequence number, at the cost of throughput on a long fat link.
+ *
+ * Every blocking call is bounded against the timer tick and halts the CPU
+ * between checks, so a silent peer costs a timeout rather than a hang.
+ */
 #include <net/tcp.h>
 #include <net/net.h>
 #include <net/ssh.h>
@@ -6,66 +32,68 @@
 #include <lib/string.h>
 #include <lib/printf.h>
 #include <kernel/printk.h>
+#include <crypto/crypto.h>
+#include <arch/arch.h>
 
 #define MAX_TCP_CONNS 32
 
-typedef struct {
-    uint32_t src_ip;
-    uint32_t dst_ip;
-    uint8_t  zero;
-    uint8_t  protocol;
-    uint16_t tcp_len;
-} __attribute__((packed)) tcp_pseudo_header_t;
+// The timer tick is 100 Hz, so one tick is 10 ms.
+#define TCP_RTO_TICKS        50   // 500 ms retransmission timeout
+#define TCP_MAX_RETRIES       5
+#define TCP_CLOSE_TICKS      50   // how long tcp_close waits for the shutdown
+#define TCP_TIME_WAIT_TICKS 200   // 2 s; a deliberately shortened 2*MSL
+
+#define TCP_EPHEMERAL_LO 49152
+#define TCP_EPHEMERAL_HI 65535
 
 static tcp_conn_t tcp_connections[MAX_TCP_CONNS];
 
-static uint16_t tcp_checksum(uint32_t src_ip, uint32_t dst_ip, const tcp_header_t* tcp, const void* payload, uint16_t payload_len) {
-    uint16_t tcp_total_len = sizeof(tcp_header_t) + payload_len;
+static uint16_t tcp_checksum(uint32_t src_ip, uint32_t dst_ip, const tcp_header_t* tcp,
+                             const void* payload, uint16_t payload_len) {
+    uint16_t tcp_total_len = (uint16_t)(sizeof(tcp_header_t) + payload_len);
 
-    tcp_pseudo_header_t ph;
-    ph.src_ip = src_ip;
-    ph.dst_ip = dst_ip;
-    ph.zero = 0;
-    ph.protocol = IP_PROTO_TCP;
-    ph.tcp_len = htons(tcp_total_len);
+    // The pseudo-header, laid out explicitly rather than as a packed struct:
+    // src and dst already hold network order, so their bytes are summed as-is.
+    uint8_t tail[4] = {
+        0, IP_PROTO_TCP,
+        (uint8_t)(tcp_total_len >> 8), (uint8_t)(tcp_total_len & 0xFF)
+    };
 
     uint32_t sum = 0;
-    const uint16_t* ptr = (const uint16_t*)&ph;
-    for (size_t i = 0; i < sizeof(ph) / 2; i++) {
-        sum += *ptr++;
-    }
+    sum = net_csum_add(sum, &src_ip, sizeof(src_ip));
+    sum = net_csum_add(sum, &dst_ip, sizeof(dst_ip));
+    sum = net_csum_add(sum, tail, sizeof(tail));
+    sum = net_csum_add(sum, tcp, sizeof(tcp_header_t));
+    if (payload && payload_len > 0) sum = net_csum_add(sum, payload, payload_len);
 
-    ptr = (const uint16_t*)tcp;
-    for (size_t i = 0; i < sizeof(tcp_header_t) / 2; i++) {
-        sum += *ptr++;
-    }
-
-    if (payload && payload_len > 0) {
-        ptr = (const uint16_t*)payload;
-        size_t len = payload_len;
-        while (len > 1) {
-            sum += *ptr++;
-            len -= 2;
-        }
-        if (len > 0) {
-            sum += *(const uint8_t*)ptr;
-        }
-    }
-
-    while (sum >> 16) {
-        sum = (sum & 0xFFFF) + (sum >> 16);
-    }
-    return (uint16_t)(~sum);
+    return net_csum_finish(sum);
 }
 
 void tcp_init(void) {
     memset(tcp_connections, 0, sizeof(tcp_connections));
-    printk(KERN_INFO "NET: TCP (Transmission Control Protocol) state machine online\n");
+    printk(KERN_INFO "NET: TCP state machine online (active + passive open)\n");
 }
 
-void tcp_send_packet(uint32_t dst_ip, uint16_t src_port, uint16_t dst_port,
-                     uint32_t seq, uint32_t ack, uint8_t flags,
-                     const void* payload, uint16_t payload_len) {
+// ===========================================================================
+// Sequence arithmetic
+//
+// Sequence numbers wrap at 2^32, so they are compared by the sign of their
+// difference rather than by magnitude.
+// ===========================================================================
+static inline bool seq_ge(uint32_t a, uint32_t b) { return (int32_t)(a - b) >= 0; }
+static inline bool seq_gt(uint32_t a, uint32_t b) { return (int32_t)(a - b) >  0; }
+
+static inline uint64_t ms_to_ticks(uint32_t ms) {
+    uint64_t t = ms / 10;
+    return t ? t : 1;
+}
+
+// ===========================================================================
+// Transmission
+// ===========================================================================
+static void tcp_tx_full(uint32_t dst_ip, uint16_t src_port, uint16_t dst_port,
+                        uint32_t seq, uint32_t ack, uint8_t flags,
+                        const void* payload, uint16_t payload_len, uint16_t window) {
     net_if_t* netif = net_get_primary_if();
     uint32_t src_ip = netif ? netif->ip : 0x0A00020F; // 10.0.2.15
 
@@ -80,7 +108,7 @@ void tcp_send_packet(uint32_t dst_ip, uint16_t src_port, uint16_t dst_port,
     tcp->ack_num  = htonl(ack);
     tcp->data_offset_reserved = (sizeof(tcp_header_t) / 4) << 4;
     tcp->flags    = flags;
-    tcp->window_size = htons(65535);
+    tcp->window_size = htons(window);
     tcp->urgent_pointer = 0;
 
     if (payload && payload_len > 0) {
@@ -94,31 +122,292 @@ void tcp_send_packet(uint32_t dst_ip, uint16_t src_port, uint16_t dst_port,
     kfree(buf);
 }
 
-static tcp_conn_t* find_or_alloc_conn(uint32_t r_ip, uint16_t r_port, uint16_t l_port) {
+void tcp_send_packet(uint32_t dst_ip, uint16_t src_port, uint16_t dst_port,
+                     uint32_t seq, uint32_t ack, uint8_t flags,
+                     const void* payload, uint16_t payload_len) {
+    tcp_tx_full(dst_ip, src_port, dst_port, seq, ack, flags, payload, payload_len, 65535);
+}
+
+// Free space in the receive ring, which is what we advertise as our window so
+// a peer stops before overrunning a reader that has fallen behind.
+static uint16_t tcp_window(const tcp_conn_t* c) {
+    if (!c->rx_buf) return 65535;
+    return (uint16_t)(TCP_RX_BUF_SIZE - c->rx_count);
+}
+
+// Send on a connection at an explicit sequence number. Retransmissions reuse
+// the original segment's sequence number, which is why this is separate from
+// the connection's current send position.
+static void tcp_tx_seq(tcp_conn_t* c, uint32_t seq, uint8_t flags,
+                       const void* payload, uint16_t payload_len) {
+    tcp_tx_full(c->remote_ip, c->local_port, c->remote_port,
+                seq, c->ack_num, flags, payload, payload_len, tcp_window(c));
+}
+
+static void tcp_tx(tcp_conn_t* c, uint8_t flags, const void* payload, uint16_t payload_len) {
+    tcp_tx_seq(c, c->seq_num, flags, payload, payload_len);
+}
+
+// ===========================================================================
+// Connection table
+// ===========================================================================
+static void conn_free(tcp_conn_t* c) {
+    if (c->rx_buf) { kfree(c->rx_buf); c->rx_buf = NULL; }
+    memset(c, 0, sizeof(*c));
+}
+
+// A connection in TIME_WAIT still occupies its slot so that a stray duplicate
+// from the old incarnation cannot be mistaken for traffic on a new one.
+static void reap_time_wait(void) {
+    uint64_t now = pit_get_ticks();
     for (size_t i = 0; i < MAX_TCP_CONNS; i++) {
-        if (tcp_connections[i].in_use &&
-            tcp_connections[i].remote_ip == r_ip &&
-            tcp_connections[i].remote_port == r_port &&
-            tcp_connections[i].local_port == l_port) {
-            return &tcp_connections[i];
+        tcp_conn_t* c = &tcp_connections[i];
+        if (c->in_use && c->state == TCP_STATE_TIME_WAIT &&
+            (now - c->closed_at) >= TCP_TIME_WAIT_TICKS) {
+            conn_free(c);
         }
     }
+}
+
+static tcp_conn_t* find_conn(uint32_t r_ip, uint16_t r_port, uint16_t l_port) {
+    for (size_t i = 0; i < MAX_TCP_CONNS; i++) {
+        tcp_conn_t* c = &tcp_connections[i];
+        if (c->in_use && c->remote_ip == r_ip &&
+            c->remote_port == r_port && c->local_port == l_port) {
+            return c;
+        }
+    }
+    return NULL;
+}
+
+static tcp_conn_t* alloc_conn(void) {
+    reap_time_wait();
     for (size_t i = 0; i < MAX_TCP_CONNS; i++) {
         if (!tcp_connections[i].in_use) {
+            memset(&tcp_connections[i], 0, sizeof(tcp_conn_t));
             tcp_connections[i].in_use = true;
-            tcp_connections[i].remote_ip = r_ip;
-            tcp_connections[i].remote_port = r_port;
-            tcp_connections[i].local_port = l_port;
-            tcp_connections[i].local_ip = 0x0A00020F;
-            tcp_connections[i].state = TCP_STATE_CLOSED;
-            tcp_connections[i].seq_num = 1000;
-            tcp_connections[i].ack_num = 0;
             return &tcp_connections[i];
         }
     }
     return NULL;
 }
 
+static tcp_conn_t* alloc_conn_server(uint32_t r_ip, uint16_t r_port, uint16_t l_port) {
+    tcp_conn_t* c = alloc_conn();
+    if (!c) return NULL;
+
+    net_if_t* netif = net_get_primary_if();
+    c->local_ip    = netif ? netif->ip : 0x0A00020F;
+    c->remote_ip   = r_ip;
+    c->remote_port = r_port;
+    c->local_port  = l_port;
+    c->role        = TCP_ROLE_SERVER;
+    c->state       = TCP_STATE_CLOSED;
+    c->seq_num     = 1000;
+    c->ack_num     = 0;
+    return c;
+}
+
+static uint16_t alloc_ephemeral_port(void) {
+    for (int attempt = 0; attempt < 512; attempt++) {
+        uint16_t p = (uint16_t)(TCP_EPHEMERAL_LO +
+                     (prng_rand32() % (TCP_EPHEMERAL_HI - TCP_EPHEMERAL_LO + 1)));
+        bool taken = false;
+        for (size_t i = 0; i < MAX_TCP_CONNS; i++) {
+            if (tcp_connections[i].in_use && tcp_connections[i].local_port == p) {
+                taken = true;
+                break;
+            }
+        }
+        if (!taken) return p;
+    }
+    return 0;
+}
+
+// ===========================================================================
+// Receive ring
+// ===========================================================================
+
+// Store what fits and report how much was taken, so the acknowledgement only
+// ever covers bytes actually kept.
+static uint16_t rx_append(tcp_conn_t* c, const uint8_t* data, uint16_t len) {
+    if (!c->rx_buf || !data) return 0;
+
+    uint16_t stored = 0;
+    while (stored < len && c->rx_count < TCP_RX_BUF_SIZE) {
+        uint16_t tail = (uint16_t)((c->rx_head + c->rx_count) % TCP_RX_BUF_SIZE);
+        c->rx_buf[tail] = data[stored++];
+        c->rx_count++;
+    }
+    return stored;
+}
+
+// ===========================================================================
+// Client state machine
+// ===========================================================================
+static void tcp_client_input(tcp_conn_t* c, uint32_t seq, uint32_t ack, uint8_t flags,
+                             const uint8_t* payload, uint16_t payload_len) {
+    if (flags & TCP_FLAG_RST) {
+        c->reset = true;
+        c->state = TCP_STATE_CLOSED;
+        return;
+    }
+
+    if (c->state == TCP_STATE_SYN_SENT) {
+        if ((flags & (TCP_FLAG_SYN | TCP_FLAG_ACK)) == (TCP_FLAG_SYN | TCP_FLAG_ACK)) {
+            // The SYN we sent occupies one sequence number, so a SYN-ACK that
+            // completes our handshake acknowledges exactly iss + 1.
+            if (ack != c->iss + 1) return;
+            c->snd_una = ack;
+            c->ack_num = seq + 1;
+            c->state   = TCP_STATE_ESTABLISHED;
+            tcp_tx(c, TCP_FLAG_ACK, NULL, 0);
+        } else if (flags & TCP_FLAG_SYN) {
+            // Simultaneous open: both ends sent a SYN at once.
+            c->ack_num = seq + 1;
+            c->state   = TCP_STATE_SYN_RCVD;
+            tcp_tx_seq(c, c->iss, TCP_FLAG_SYN | TCP_FLAG_ACK, NULL, 0);
+        }
+        return;
+    }
+
+    if (c->state == TCP_STATE_SYN_RCVD) {
+        if ((flags & TCP_FLAG_ACK) && seq_ge(ack, c->iss + 1)) {
+            c->snd_una = ack;
+            c->state   = TCP_STATE_ESTABLISHED;
+        }
+        return;
+    }
+
+    if (c->state == TCP_STATE_CLOSED) return;
+
+    // A lingering connection only has to keep acknowledging a retransmitted FIN.
+    if (c->state == TCP_STATE_TIME_WAIT) {
+        if (flags & TCP_FLAG_FIN) tcp_tx(c, TCP_FLAG_ACK, NULL, 0);
+        return;
+    }
+
+    // Advance the send window, ignoring anything that acknowledges more than
+    // has actually been sent.
+    if ((flags & TCP_FLAG_ACK) && seq_gt(ack, c->snd_una) && seq_ge(c->seq_num, ack)) {
+        c->snd_una = ack;
+    }
+
+    // Accept in-order data. Anything else earns a duplicate acknowledgement
+    // telling the peer where we actually are.
+    if (payload_len > 0 && !c->peer_fin) {
+        if (seq == c->ack_num) {
+            c->ack_num += rx_append(c, payload, payload_len);
+        }
+        tcp_tx(c, TCP_FLAG_ACK, NULL, 0);
+    }
+
+    // A FIN sits after the segment's data, so it is only in order once every
+    // preceding byte has been taken.
+    if ((flags & TCP_FLAG_FIN) && !c->peer_fin && c->ack_num == seq + payload_len) {
+        c->ack_num++;
+        c->peer_fin = true;
+        tcp_tx(c, TCP_FLAG_ACK, NULL, 0);
+
+        if (c->state == TCP_STATE_ESTABLISHED)    c->state = TCP_STATE_CLOSE_WAIT;
+        else if (c->state == TCP_STATE_FIN_WAIT1) c->state = TCP_STATE_CLOSING;
+        else if (c->state == TCP_STATE_FIN_WAIT2) {
+            c->state = TCP_STATE_TIME_WAIT;
+            c->closed_at = pit_get_ticks();
+        }
+    }
+
+    // Our own FIN also consumes a sequence number: once snd_una reaches
+    // seq_num there is nothing of ours left outstanding.
+    if (c->snd_una == c->seq_num) {
+        if (c->state == TCP_STATE_FIN_WAIT1) {
+            c->state = TCP_STATE_FIN_WAIT2;
+        } else if (c->state == TCP_STATE_CLOSING) {
+            c->state = TCP_STATE_TIME_WAIT;
+            c->closed_at = pit_get_ticks();
+        } else if (c->state == TCP_STATE_LAST_ACK) {
+            c->state = TCP_STATE_CLOSED;
+        }
+    }
+}
+
+// ===========================================================================
+// Server state machine
+//
+// Behaviour is unchanged from the original listener: SSH and HTTP are served
+// straight out of the receive path, one request per segment.
+// ===========================================================================
+static void tcp_server_input(tcp_conn_t* c, uint32_t src_ip, uint16_t src_port,
+                             uint16_t dst_port, uint32_t seq_num, uint8_t flags,
+                             const uint8_t* payload, uint16_t payload_len) {
+    bool is_ssh_port  = (dst_port == sshd_get_port() && sshd_is_running());
+    bool is_http_port = (dst_port == 80 && httpd_is_running());
+
+    if (flags & TCP_FLAG_SYN) {
+        c->state   = TCP_STATE_SYN_RCVD;
+        c->ack_num = seq_num + 1;
+        c->seq_num = 10000;
+
+        tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                        TCP_FLAG_SYN | TCP_FLAG_ACK, NULL, 0);
+        c->seq_num++;
+        return;
+    }
+
+    if (flags & TCP_FLAG_FIN) {
+        c->ack_num = seq_num + 1;
+        tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                        TCP_FLAG_ACK | TCP_FLAG_FIN, NULL, 0);
+        conn_free(c);
+        return;
+    }
+
+    if (flags & TCP_FLAG_RST) {
+        conn_free(c);
+        return;
+    }
+
+    if (c->state == TCP_STATE_SYN_RCVD) {
+        c->state = TCP_STATE_ESTABLISHED;
+        if (is_ssh_port && payload_len == 0) {
+            const char* banner = SSH_BANNER;
+            uint16_t blen = (uint16_t)strlen(banner);
+            tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                            TCP_FLAG_PSH | TCP_FLAG_ACK, banner, blen);
+            c->seq_num += blen;
+            return;
+        }
+    }
+    c->state = TCP_STATE_ESTABLISHED;
+
+    if (payload_len == 0) return;
+    c->ack_num = seq_num + payload_len;
+
+    if (is_ssh_port) {
+        char resp[1024];
+        int rlen = sshd_process_packet((const char*)payload, payload_len, resp, sizeof(resp));
+        if (rlen > 0) {
+            tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                            TCP_FLAG_PSH | TCP_FLAG_ACK, resp, (uint16_t)rlen);
+            c->seq_num += rlen;
+        } else {
+            tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                            TCP_FLAG_ACK, NULL, 0);
+        }
+    } else if (is_http_port) {
+        char resp[2048];
+        int rlen = httpd_handle_request((const char*)payload, resp, sizeof(resp));
+        if (rlen > 0) {
+            tcp_send_packet(src_ip, dst_port, src_port, c->seq_num, c->ack_num,
+                            TCP_FLAG_PSH | TCP_FLAG_ACK, resp, (uint16_t)rlen);
+            c->seq_num += rlen;
+        }
+    }
+}
+
+// ===========================================================================
+// Receive dispatch
+// ===========================================================================
 void tcp_receive(const uint8_t* packet, uint16_t length, uint32_t src_ip) {
     if (!packet || length < sizeof(tcp_header_t)) return;
 
@@ -127,96 +416,204 @@ void tcp_receive(const uint8_t* packet, uint16_t length, uint32_t src_ip) {
     uint16_t dst_port = ntohs(tcp->dst_port);
     uint32_t seq_num  = ntohl(tcp->seq_num);
     uint32_t ack_num  = ntohl(tcp->ack_num);
-    uint8_t flags     = tcp->flags;
-    (void)ack_num;
+    uint8_t  flags    = tcp->flags;
 
     uint8_t header_len = (tcp->data_offset_reserved >> 4) * 4;
+    if (header_len < sizeof(tcp_header_t) || header_len > length) return;
+
     const uint8_t* payload = packet + header_len;
-    uint16_t payload_len = length > header_len ? length - header_len : 0;
+    uint16_t payload_len = length - header_len;
 
-    // Check if target port is SSH (22) or HTTP (80)
-    bool is_ssh_port = (dst_port == sshd_get_port() && sshd_is_running());
+    // 1. An established connection -- ours or a peer's -- owns this segment.
+    tcp_conn_t* conn = find_conn(src_ip, src_port, dst_port);
+    if (conn) {
+        if (conn->role == TCP_ROLE_CLIENT) {
+            tcp_client_input(conn, seq_num, ack_num, flags, payload, payload_len);
+        } else {
+            tcp_server_input(conn, src_ip, src_port, dst_port, seq_num, flags,
+                             payload, payload_len);
+        }
+        return;
+    }
+
+    // 2. Otherwise a listening service may want to accept it.
+    bool is_ssh_port  = (dst_port == sshd_get_port() && sshd_is_running());
     bool is_http_port = (dst_port == 80 && httpd_is_running());
-
-    if (!is_ssh_port && !is_http_port) {
-        // Port not open: send TCP RST
-        if (!(flags & TCP_FLAG_RST)) {
-            tcp_send_packet(src_ip, dst_port, src_port, 0, seq_num + 1, TCP_FLAG_RST | TCP_FLAG_ACK, NULL, 0);
+    if (is_ssh_port || is_http_port) {
+        conn = alloc_conn_server(src_ip, src_port, dst_port);
+        if (conn) {
+            tcp_server_input(conn, src_ip, src_port, dst_port, seq_num, flags,
+                             payload, payload_len);
         }
         return;
     }
 
-    tcp_conn_t* conn = find_or_alloc_conn(src_ip, src_port, dst_port);
-    if (!conn) return;
-
-    // Handle TCP SYN (Connection Initiation from Client)
-    if (flags & TCP_FLAG_SYN) {
-        conn->state = TCP_STATE_SYN_RCVD;
-        conn->ack_num = seq_num + 1;
-        conn->seq_num = 10000;
-
-        // Send SYN-ACK
-        tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                        TCP_FLAG_SYN | TCP_FLAG_ACK, NULL, 0);
-        conn->seq_num++;
-        return;
+    // 3. Nothing is listening and nothing matches: refuse the segment.
+    if (!(flags & TCP_FLAG_RST)) {
+        tcp_send_packet(src_ip, dst_port, src_port, 0, seq_num + 1,
+                        TCP_FLAG_RST | TCP_FLAG_ACK, NULL, 0);
     }
+}
 
-    // Handle TCP FIN (Connection Termination)
-    if (flags & TCP_FLAG_FIN) {
-        conn->ack_num = seq_num + 1;
-        tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                        TCP_FLAG_ACK | TCP_FLAG_FIN, NULL, 0);
-        conn->in_use = false;
-        conn->state = TCP_STATE_CLOSED;
-        return;
-    }
+// ===========================================================================
+// Active open API
+// ===========================================================================
+tcp_conn_t* tcp_connect(uint32_t dst_ip, uint16_t dst_port, uint32_t timeout_ms) {
+    if (dst_ip == 0 || dst_port == 0) return NULL;
 
-    // Handle TCP RST
-    if (flags & TCP_FLAG_RST) {
-        conn->in_use = false;
-        conn->state = TCP_STATE_CLOSED;
-        return;
-    }
+    tcp_conn_t* c = alloc_conn();
+    if (!c) return NULL;
 
-    // Handle Data / ACK
-    if (conn->state == TCP_STATE_SYN_RCVD) {
-        conn->state = TCP_STATE_ESTABLISHED;
-        if (is_ssh_port && payload_len == 0) {
-            const char* banner = SSH_BANNER;
-            uint16_t blen = (uint16_t)strlen(banner);
-            tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                            TCP_FLAG_PSH | TCP_FLAG_ACK, banner, blen);
-            conn->seq_num += blen;
-            return;
+    c->rx_buf = (uint8_t*)kzalloc(TCP_RX_BUF_SIZE);
+    if (!c->rx_buf) { conn_free(c); return NULL; }
+
+    c->local_port = alloc_ephemeral_port();
+    if (c->local_port == 0) { conn_free(c); return NULL; }
+
+    net_if_t* netif = net_get_primary_if();
+    c->local_ip    = netif ? netif->ip : 0x0A00020F;
+    c->remote_ip   = dst_ip;
+    c->remote_port = dst_port;
+    c->role        = TCP_ROLE_CLIENT;
+    c->iss         = prng_rand32();
+    c->seq_num     = c->iss;
+    c->snd_una     = c->iss;
+    c->ack_num     = 0;
+    c->state       = TCP_STATE_SYN_SENT;
+
+    tcp_tx_seq(c, c->iss, TCP_FLAG_SYN, NULL, 0);
+    c->seq_num = c->iss + 1;   // the SYN consumes one sequence number
+
+    uint64_t t0   = pit_get_ticks();
+    uint64_t wait = ms_to_ticks(timeout_ms);
+    uint64_t last = t0;
+
+    while ((pit_get_ticks() - t0) < wait) {
+        if (c->state == TCP_STATE_ESTABLISHED) return c;
+        if (c->reset || c->state == TCP_STATE_CLOSED) break;
+
+        if ((pit_get_ticks() - last) >= TCP_RTO_TICKS) {
+            tcp_tx_seq(c, c->iss, TCP_FLAG_SYN, NULL, 0);
+            last = pit_get_ticks();
         }
+        arch_halt();
     }
-    conn->state = TCP_STATE_ESTABLISHED;
 
-    if (payload_len > 0) {
-        conn->ack_num = seq_num + payload_len;
+    conn_free(c);
+    return NULL;
+}
 
-        if (is_ssh_port) {
-            char resp[1024];
-            int rlen = sshd_process_packet((const char*)payload, payload_len, resp, sizeof(resp));
-            if (rlen > 0) {
-                tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                                TCP_FLAG_PSH | TCP_FLAG_ACK, resp, (uint16_t)rlen);
-                conn->seq_num += rlen;
-            } else {
-                tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                                TCP_FLAG_ACK, NULL, 0);
-            }
-        } else if (is_http_port) {
-            char resp[2048];
-            int rlen = httpd_handle_request((const char*)payload, resp, sizeof(resp));
-            if (rlen > 0) {
-                tcp_send_packet(src_ip, dst_port, src_port, conn->seq_num, conn->ack_num,
-                                TCP_FLAG_PSH | TCP_FLAG_ACK, resp, (uint16_t)rlen);
-                conn->seq_num += rlen;
+int tcp_send(tcp_conn_t* c, const void* data, uint16_t len) {
+    if (!c || !c->in_use || !data) return -1;
+    if (c->state != TCP_STATE_ESTABLISHED && c->state != TCP_STATE_CLOSE_WAIT) return -1;
+    if (len == 0) return 0;
+
+    const uint8_t* p = (const uint8_t*)data;
+    uint16_t sent = 0;
+
+    while (sent < len) {
+        uint16_t chunk = (uint16_t)(len - sent);
+        if (chunk > TCP_MSS) chunk = TCP_MSS;
+
+        uint32_t seg_seq = c->seq_num;
+        uint32_t seg_end = seg_seq + chunk;
+
+        // Publish the segment as sent before waiting: the acknowledgement is
+        // only accepted if it falls within what seq_num says we have sent.
+        c->seq_num = seg_end;
+
+        int tries = 0;
+        for (;;) {
+            tcp_tx_seq(c, seg_seq, TCP_FLAG_PSH | TCP_FLAG_ACK, p + sent, chunk);
+
+            uint64_t t0 = pit_get_ticks();
+            while ((pit_get_ticks() - t0) < TCP_RTO_TICKS) {
+                if (seq_ge(c->snd_una, seg_end) || c->reset) break;
+                arch_halt();
             }
+
+            if (seq_ge(c->snd_una, seg_end)) break;
+            if (c->reset) return sent ? (int)sent : -1;
+            if (++tries >= TCP_MAX_RETRIES) return sent ? (int)sent : -1;
+        }
+
+        sent += chunk;
+    }
+
+    return (int)sent;
+}
+
+int tcp_recv(tcp_conn_t* c, void* buf, uint16_t len, uint32_t timeout_ms) {
+    if (!c || !c->in_use || !buf || len == 0) return -1;
+
+    uint64_t t0   = pit_get_ticks();
+    uint64_t wait = ms_to_ticks(timeout_ms);
+
+    while (c->rx_count == 0) {
+        if (c->reset) return -1;
+        if (c->peer_fin) return 0;                       // stream ended
+        if ((pit_get_ticks() - t0) >= wait) return 0;     // nothing arrived
+        arch_halt();
+    }
+
+    uint16_t before = c->rx_count;
+    uint16_t n = (c->rx_count < len) ? c->rx_count : len;
+
+    uint8_t* out = (uint8_t*)buf;
+    for (uint16_t i = 0; i < n; i++) {
+        out[i] = c->rx_buf[(c->rx_head + i) % TCP_RX_BUF_SIZE];
+    }
+
+    c->rx_head = (uint16_t)((c->rx_head + n) % TCP_RX_BUF_SIZE);
+    c->rx_count = (uint16_t)(c->rx_count - n);
+
+    // If the window had closed down far enough that the peer may have stalled,
+    // tell it immediately that there is room again.
+    if (before >= TCP_RX_BUF_SIZE / 2 && c->state == TCP_STATE_ESTABLISHED) {
+        tcp_tx(c, TCP_FLAG_ACK, NULL, 0);
+    }
+
+    return (int)n;
+}
+
+void tcp_close(tcp_conn_t* c) {
+    if (!c || !c->in_use) return;
+
+    if (c->state == TCP_STATE_ESTABLISHED || c->state == TCP_STATE_CLOSE_WAIT) {
+        // Closing after the peer already sent its FIN is the passive case and
+        // finishes in LAST_ACK; closing first is the active case.
+        bool passive = (c->state == TCP_STATE_CLOSE_WAIT);
+
+        tcp_tx(c, TCP_FLAG_FIN | TCP_FLAG_ACK, NULL, 0);
+        c->seq_num++;   // our FIN consumes a sequence number
+        c->state = passive ? TCP_STATE_LAST_ACK : TCP_STATE_FIN_WAIT1;
+
+        uint64_t t0 = pit_get_ticks();
+        while ((pit_get_ticks() - t0) < TCP_CLOSE_TICKS) {
+            if (c->reset) break;
+            if (c->state == TCP_STATE_CLOSED || c->state == TCP_STATE_TIME_WAIT) break;
+            arch_halt();
         }
     }
+
+    if (c->state == TCP_STATE_TIME_WAIT) {
+        // Hold the slot so a late duplicate cannot land on a new connection.
+        // The receive buffer is not needed for that, so give it back now.
+        if (c->rx_buf) { kfree(c->rx_buf); c->rx_buf = NULL; }
+        c->rx_head = c->rx_count = 0;
+        c->closed_at = pit_get_ticks();
+        return;
+    }
+
+    conn_free(c);
+}
+
+bool tcp_is_established(const tcp_conn_t* c) {
+    return c && c->in_use && c->state == TCP_STATE_ESTABLISHED;
+}
+
+bool tcp_peer_closed(const tcp_conn_t* c) {
+    return c && c->in_use && c->peer_fin;
 }
 
 size_t tcp_get_connections_count(void) {
@@ -231,3 +628,12 @@ const tcp_conn_t* tcp_get_connection(size_t index) {
     if (index >= MAX_TCP_CONNS || !tcp_connections[index].in_use) return NULL;
     return &tcp_connections[index];
 }
+
+const char* tcp_state_name(uint8_t state) {
+    static const char* const names[] = {
+        "CLOSED", "LISTEN", "SYN_SENT", "SYN_RCVD", "ESTABLISHED",
+        "FIN_WAIT1", "FIN_WAIT2", "CLOSE_WAIT", "CLOSING", "LAST_ACK", "TIME_WAIT"
+    };
+    if (state >= sizeof(names) / sizeof(names[0])) return "UNKNOWN";
+    return names[state];
+}

net/ipv4/udp.c

@@ -41,29 +41,22 @@ static volatile uint64_t stat_drop = 0;
 static uint16_t udp_checksum(uint32_t src_ip, uint32_t dst_ip,
                              const udp_header_t* udp, const void* payload,
                              uint16_t payload_len) {
-    udp_pseudo_header_t ph;
-    ph.src_ip = src_ip;
-    ph.dst_ip = dst_ip;
-    ph.zero = 0;
-    ph.protocol = IP_PROTO_UDP;
-    ph.udp_len = htons((uint16_t)(sizeof(udp_header_t) + payload_len));
+    uint16_t udp_len = (uint16_t)(sizeof(udp_header_t) + payload_len);
+
+    // The pseudo-header, laid out explicitly rather than as a packed struct:
+    // src and dst already hold network order, so their bytes are summed as-is.
+    uint8_t tail[4] = {
+        0, IP_PROTO_UDP, (uint8_t)(udp_len >> 8), (uint8_t)(udp_len & 0xFF)
+    };
 
     uint32_t sum = 0;
-    const uint16_t* p = (const uint16_t*)&ph;
-    for (size_t i = 0; i < sizeof(ph) / 2; i++) sum += *p++;
+    sum = net_csum_add(sum, &src_ip, sizeof(src_ip));
+    sum = net_csum_add(sum, &dst_ip, sizeof(dst_ip));
+    sum = net_csum_add(sum, tail, sizeof(tail));
+    sum = net_csum_add(sum, udp, sizeof(udp_header_t));
+    if (payload && payload_len > 0) sum = net_csum_add(sum, payload, payload_len);
 
-    p = (const uint16_t*)udp;
-    for (size_t i = 0; i < sizeof(udp_header_t) / 2; i++) sum += *p++;
-
-    if (payload && payload_len > 0) {
-        p = (const uint16_t*)payload;
-        size_t len = payload_len;
-        while (len > 1) { sum += *p++; len -= 2; }
-        if (len > 0) sum += *(const uint8_t*)p;
-    }
-
-    while (sum >> 16) sum = (sum & 0xFFFF) + (sum >> 16);
-    uint16_t cksum = (uint16_t)(~sum);
+    uint16_t cksum = net_csum_finish(sum);
     // A zero checksum means "not computed" on the wire; send 0xFFFF instead.
     return cksum ? cksum : 0xFFFF;
 }
@@ -189,14 +182,8 @@ void udp_send_broadcast(uint16_t src_port, uint16_t dst_port,
     ip->src_ip = 0x00000000;
     ip->dst_ip = 0xFFFFFFFF;
 
-    // IPv4 header checksum.
-    {
-        uint32_t sum = 0;
-        const uint16_t* p = (const uint16_t*)ip;
-        for (size_t i = 0; i < sizeof(ip_header_t) / 2; i++) sum += *p++;
-        while (sum >> 16) sum = (sum & 0xFFFF) + (sum >> 16);
-        ip->checksum = (uint16_t)(~sum);
-    }
+    // IPv4 header checksum, over the header just written (with checksum zero).
+    ip->checksum = net_checksum(ip, sizeof(ip_header_t));
 
     udp_header_t* udp = (udp_header_t*)(buf + sizeof(ip_header_t));
     udp->src_port = htons(src_port);