net/tcp: Support initial sequence number described in RFC 6528

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

net/tcp/Kconfig

@@ -158,6 +158,17 @@ config NET_TCP_CC_NEWRENO
 			The TCP Congestion Control defines four congestion control algorithms,
 			slow start, congestion avoidance, fast retransmit, and fast recovery.
 
+config NET_TCP_ISN_RFC6528
+	bool "Use Initial Sequence Number Algorithm from RFC 6528"
+	default n
+	depends on CRYPTO
+	---help---
+		Initial Sequence Number Generation Algorithm from RFC 6528:
+			ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
+
+			M is the 4 microsecond timer, and F() is a pseudorandom
+			function (PRF) which is MD5 (suggested by RFC 6528).
+
 config NET_TCP_WINDOW_SCALE
 	bool "Enable TCP/IP Window Scale Option"
 	default n

net/tcp/tcp.h

@@ -896,7 +896,7 @@ uint32_t tcp_addsequence(FAR uint8_t *seqno, uint16_t len);
  *
  ****************************************************************************/
 
-void tcp_initsequence(FAR uint8_t *seqno);
+void tcp_initsequence(FAR struct tcp_conn_s *conn);
 
 /****************************************************************************
  * Name: tcp_nextsequence

net/tcp/tcp_conn.c

@@ -1200,7 +1200,7 @@ FAR struct tcp_conn_s *tcp_alloc_accept(FAR struct net_driver_s *dev,
       conn->rport            = tcp->srcport;
       conn->tcpstateflags    = TCP_SYN_RCVD;
 
-      tcp_initsequence(conn->sndseq);
+      tcp_initsequence(conn);
 #if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
       conn->rexmit_seq       = tcp_getsequence(conn->sndseq);
 #endif
@@ -1506,13 +1506,6 @@ int tcp_connect(FAR struct tcp_conn_s *conn, FAR const struct sockaddr *addr)
    */
 
   conn->tcpstateflags = TCP_SYN_SENT;
-  tcp_initsequence(conn->sndseq);
-
-  /* Save initial sndseq to rexmit_seq, otherwise it will be zero */
-
-#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
-  conn->rexmit_seq = tcp_getsequence(conn->sndseq);
-#endif
 
   conn->tx_unacked = 1;    /* TCP length of the SYN is one. */
   conn->nrtx       = 0;
@@ -1528,6 +1521,16 @@ int tcp_connect(FAR struct tcp_conn_s *conn, FAR const struct sockaddr *addr)
   conn->sndseq_max = 0;
 #endif
 
+  /* Set initial sndseq when we have both local/remote addr and port */
+
+  tcp_initsequence(conn);
+
+  /* Save initial sndseq to rexmit_seq, otherwise it will be zero */
+
+#if !defined(CONFIG_NET_TCP_WRITE_BUFFERS)
+  conn->rexmit_seq = tcp_getsequence(conn->sndseq);
+#endif
+
 #ifdef CONFIG_NET_TCP_CC_NEWRENO
   /* Initialize the variables of congestion control. */
 

net/tcp/tcp_seqno.c

@@ -43,23 +43,84 @@
 #include <nuttx/config.h>
 #if defined(CONFIG_NET) && defined(CONFIG_NET_TCP)
 
-#include <stdint.h>
+#include <crypto/md5.h>
 #include <debug.h>
+#include <stdint.h>
 
 #include <nuttx/clock.h>
 #include <nuttx/net/netconfig.h>
 #include <nuttx/net/netdev.h>
 
 #include "devif/devif.h"
+#include "tcp/tcp.h"
 #include "utils/utils.h"
 
 /****************************************************************************
  * Private Data
  ****************************************************************************/
 
-/* g_tcpsequence is used to generate initial TCP sequence numbers */
+/* These fields are used to generate initial TCP sequence numbers */
 
+#ifdef CONFIG_NET_TCP_ISN_RFC6528
+/* RFC 6528, Section 3: Key lengths of 128 bits should be adequate. */
+
+static uint32_t g_tcp_isnkey[4];
+#else
 static uint32_t g_tcpsequence;
+#endif
+
+/****************************************************************************
+ * Private Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: tcp_isn_rfc6528
+ *
+ * Description:
+ *   Calculate the initial sequence number described in RFC 6528.
+ *   ISN = M + F(localip, localport, remoteip, remoteport, secretkey)
+ *
+ ****************************************************************************/
+
+#ifdef CONFIG_NET_TCP_ISN_RFC6528
+static uint32_t tcp_isn_rfc6528(FAR struct tcp_conn_s *conn)
+{
+  const size_t addrlen = net_ip_domain_select(conn->domain,
+                                  sizeof(in_addr_t), sizeof(net_ipv6addr_t));
+  MD5_CTX ctx;
+  uint32_t digest[MD5_DIGEST_LENGTH / 4];
+  uint32_t m;
+
+  /* Make sure we have a secret key */
+
+  if (g_tcp_isnkey[0] == 0)
+    {
+      net_getrandom(g_tcp_isnkey, sizeof(g_tcp_isnkey));
+    }
+
+  /* M is the 4 microsecond timer */
+
+  m = TICK2USEC(clock_systime_ticks()) / 4;
+
+  /* F() is suggested to be MD5 */
+
+  md5init(&ctx);
+
+  /* Calculate F(localip, localport, remoteip, remoteport, secretkey) */
+
+  md5update(&ctx, net_ip_binding_laddr(&conn->u, conn->domain), addrlen);
+  md5update(&ctx, &conn->lport, sizeof(conn->lport));
+  md5update(&ctx, net_ip_binding_raddr(&conn->u, conn->domain), addrlen);
+  md5update(&ctx, &conn->rport, sizeof(conn->rport));
+  md5update(&ctx, g_tcp_isnkey, sizeof(g_tcp_isnkey));
+
+  md5final((FAR uint8_t *)digest, &ctx);
+
+  /* ISN = M + F(localip, localport, remoteip, remoteport, secretkey) */
+
+  return m + digest[0];
+}
+#endif
 
 /****************************************************************************
  * Public Functions
@@ -142,8 +203,11 @@ uint32_t tcp_addsequence(FAR uint8_t *seqno, uint16_t len)
  *
  ****************************************************************************/
 
-void tcp_initsequence(FAR uint8_t *seqno)
+void tcp_initsequence(FAR struct tcp_conn_s *conn)
 {
+#ifdef CONFIG_NET_TCP_ISN_RFC6528
+  tcp_setsequence(conn->sndseq, tcp_isn_rfc6528(conn));
+#else
   /* If g_tcpsequence is already initialized, just copy it */
 
   if (g_tcpsequence == 0)
@@ -164,7 +228,8 @@ void tcp_initsequence(FAR uint8_t *seqno)
         }
     }
 
-  tcp_setsequence(seqno, g_tcpsequence);
+  tcp_setsequence(conn->sndseq, g_tcpsequence);
+#endif
 }
 
 /****************************************************************************
@@ -180,7 +245,9 @@ void tcp_initsequence(FAR uint8_t *seqno)
 
 void tcp_nextsequence(void)
 {
+#ifndef CONFIG_NET_TCP_ISN_RFC6528
   g_tcpsequence++;
+#endif
 }
 
 #endif /* CONFIG_NET && CONFIG_NET_TCP */