diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 20624d2..bb5537f 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -85,6 +85,7 @@
 #define DFL_SERVER_PORT         "4433"
 #define DFL_LISTEN_ADDR         "localhost"
 #define DFL_LISTEN_PORT         "5556"
+#define DFL_PACK                0
 
 #define USAGE                                                               \
     "\n usage: udp_proxy param=<>...\n"                                     \
@@ -108,6 +109,8 @@
     "    protect_len=%%d     default: (don't protect packets of this size)\n" \
     "\n"                                                                    \
     "    seed=%%d             default: (use current time)\n"                \
+    "    pack=%%d             default: 0     (don't merge)\n"               \
+    "                         options: t > 0 (merge for t milliseconds)\n"  \
     "\n"
 
 /*
@@ -128,6 +131,8 @@
     int bad_ad;                 /* inject corrupted ApplicationData record  */
     int protect_hvr;            /* never drop or delay HelloVerifyRequest   */
     int protect_len;            /* never drop/delay packet of the given size*/
+    int merge;                  /* merge packets into single datagram for
+                                 * at most \c merge milliseconds if > 0     */
 
     unsigned int seed;          /* seed for "random" events                 */
 } opt;
@@ -152,6 +157,7 @@
     opt.server_port    = DFL_SERVER_PORT;
     opt.listen_addr    = DFL_LISTEN_ADDR;
     opt.listen_port    = DFL_LISTEN_PORT;
+    opt.merge          = DFL_PACK;
     /* Other members default to 0 */
 
     for( i = 1; i < argc; i++ )
@@ -193,6 +199,10 @@
             if( opt.drop < 0 || opt.drop > 20 || opt.drop == 1 )
                 exit_usage( p, q );
         }
+        else if( strcmp( p, "pack" ) == 0 )
+        {
+            opt.merge = atoi( q );
+        }
         else if( strcmp( p, "mtu" ) == 0 )
         {
             opt.mtu = atoi( q );
@@ -290,6 +300,94 @@
 
 typedef struct
 {
+    mbedtls_net_context *ctx;
+
+    const char *description;
+
+    unsigned long packet_lifetime;
+    size_t num_datagrams;
+
+    unsigned char data[MAX_MSG_SIZE];
+    unsigned len;
+
+} ctx_buffer;
+
+static ctx_buffer outbuf[2];
+
+static int ctx_buffer_flush( ctx_buffer *buf )
+{
+    int ret;
+
+    mbedtls_printf( "  %05lu flush    %s: %u bytes, %lu datagrams, "
+                    "last %ld ms\n", ellapsed_time(),
+                    buf->description, buf->len, buf->num_datagrams,
+                    ellapsed_time() - buf->packet_lifetime );
+
+    ret = mbedtls_net_send( buf->ctx, buf->data, buf->len );
+
+    buf->len           = 0;
+    buf->num_datagrams = 0;
+
+    return( ret );
+}
+
+static inline int ctx_buffer_check( ctx_buffer *buf )
+{
+    if( buf->len > 0 &&
+        ellapsed_time() - buf->packet_lifetime >= (size_t) opt.merge )
+    {
+        return( ctx_buffer_flush( buf ) );
+    }
+
+    return( 0 );
+}
+
+static int ctx_buffer_append( ctx_buffer *buf,
+                              const unsigned char * data,
+                              size_t len )
+{
+    int ret;
+
+    if( len > sizeof( buf->data ) )
+    {
+        mbedtls_printf( "  ! buffer size %lu too large (max %lu)\n",
+                        len, sizeof( buf->data ) );
+        return( -1 );
+    }
+
+    if( sizeof( buf->data ) - buf->len < len )
+    {
+        if( ( ret = ctx_buffer_flush( buf ) ) <= 0 )
+            return( ret );
+    }
+
+    memcpy( buf->data + buf->len, data, len );
+
+    buf->len += len;
+    if( ++buf->num_datagrams == 1 )
+        buf->packet_lifetime = ellapsed_time();
+
+    return( len );
+}
+
+static int dispatch_data( mbedtls_net_context *ctx,
+                          const unsigned char * data,
+                          size_t len )
+{
+    ctx_buffer *buf = NULL;
+    if( outbuf[0].ctx == ctx )
+        buf = &outbuf[0];
+    else if( outbuf[1].ctx == ctx )
+        buf = &outbuf[1];
+
+    if( buf == NULL )
+        return( mbedtls_net_send( ctx, data, len ) );
+
+    return( ctx_buffer_append( buf, data, len ) );
+}
+
+typedef struct
+{
     mbedtls_net_context *dst;
     const char *way;
     const char *type;
@@ -301,10 +399,10 @@
 void print_packet( const packet *p, const char *why )
 {
     if( why == NULL )
-        mbedtls_printf( "  %05lu %s %s (%u bytes)\n",
+        mbedtls_printf( "  %05lu dispatch %s %s (%u bytes)\n",
                 ellapsed_time(), p->way, p->type, p->len );
     else
-        mbedtls_printf( "        %s %s (%u bytes): %s\n",
+        mbedtls_printf( "        dispatch %s %s (%u bytes): %s\n",
                 p->way, p->type, p->len, why );
     fflush( stdout );
 }
@@ -323,17 +421,17 @@
         ++buf[p->len - 1];
 
         print_packet( p, "corrupted" );
-        if( ( ret = mbedtls_net_send( dst, buf, p->len ) ) <= 0 )
+        if( ( ret = dispatch_data( dst, buf, p->len ) ) <= 0 )
         {
-            mbedtls_printf( "  ! mbedtls_net_send returned %d\n", ret );
+            mbedtls_printf( "  ! dispatch returned %d\n", ret );
             return( ret );
         }
     }
 
     print_packet( p, why );
-    if( ( ret = mbedtls_net_send( dst, p->buf, p->len ) ) <= 0 )
+    if( ( ret = dispatch_data( dst, p->buf, p->len ) ) <= 0 )
     {
-        mbedtls_printf( "  ! mbedtls_net_send returned %d\n", ret );
+        mbedtls_printf( "  ! dispatch returned %d\n", ret );
         return( ret );
     }
 
@@ -344,9 +442,9 @@
     {
         print_packet( p, "duplicated" );
 
-        if( ( ret = mbedtls_net_send( dst, p->buf, p->len ) ) <= 0 )
+        if( ( ret = dispatch_data( dst, p->buf, p->len ) ) <= 0 )
         {
-            mbedtls_printf( "  ! mbedtls_net_send returned %d\n", ret );
+            mbedtls_printf( "  ! dispatch returned %d\n", ret );
             return( ret );
         }
     }
@@ -471,10 +569,14 @@
     int ret;
 
     mbedtls_net_context listen_fd, client_fd, server_fd;
+    struct timeval tm;
 
     int nb_fds;
     fd_set read_fds;
 
+    tm.tv_sec  = 0;
+    tm.tv_usec = 0;
+
     mbedtls_net_init( &listen_fd );
     mbedtls_net_init( &client_fd );
     mbedtls_net_init( &server_fd );
@@ -560,6 +662,19 @@
         nb_fds = listen_fd.fd;
     ++nb_fds;
 
+    if( opt.merge > 0 )
+    {
+        outbuf[0].ctx = &server_fd;
+        outbuf[0].description = "S <- C";
+        outbuf[0].num_datagrams = 0;
+        outbuf[0].len = 0;
+
+        outbuf[1].ctx = &client_fd;
+        outbuf[1].description = "S -> C";
+        outbuf[1].num_datagrams = 0;
+        outbuf[1].len = 0;
+    }
+
     while( 1 )
     {
         FD_ZERO( &read_fds );
@@ -567,7 +682,10 @@
         FD_SET( client_fd.fd, &read_fds );
         FD_SET( listen_fd.fd, &read_fds );
 
-        if( ( ret = select( nb_fds, &read_fds, NULL, NULL, NULL ) ) <= 0 )
+        ctx_buffer_check( &outbuf[0] );
+        ctx_buffer_check( &outbuf[1] );
+
+        if( ( ret = select( nb_fds, &read_fds, NULL, NULL, &tm ) ) < 0 )
         {
             perror( "select" );
             goto exit;
@@ -589,6 +707,7 @@
                                         &client_fd, &server_fd ) ) != 0 )
                 goto accept;
         }
+
     }
 
 exit: