Add gnutls_dtls_set_data_mtu()

[gnutls.git] / lib / gnutls_dtls.c

/*
 * Copyright (C) 2009-2012 Free Software Foundation, Inc.
 *
 * Authors: Jonathan Bastien-Filiatrault
 *          Nikos Mavrogiannopoulos
 *
 * This file is part of GNUTLS.
 *
 * The GNUTLS library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * as published by the Free Software Foundation; either version 3 of
 * the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>
 *
 */

/* Functions that relate to DTLS retransmission and reassembly.
 */

#include "gnutls_int.h"
#include "gnutls_errors.h"
#include "debug.h"
#include "gnutls_dtls.h"
#include "gnutls_record.h"
#include <gnutls_mbuffers.h>
#include <gnutls_buffers.h>
#include <gnutls_constate.h>
#include <gnutls_state.h>
#include <gnutls/dtls.h>
#include <timespec.h>

/* returns a-b in ms */
unsigned int
_dtls_timespec_sub_ms (struct timespec *a, struct timespec *b)
{
  return (a->tv_sec * 1000 + a->tv_nsec / (1000 * 1000) -
          (b->tv_sec * 1000 + b->tv_nsec / (1000 * 1000)));
}

void
_dtls_async_timer_delete (gnutls_session_t session)
{
  if (session->internals.dtls.async_term != 0)
    {
      _gnutls_dtls_log ("DTLS[%p]: Deinitializing previous handshake state.\n", session);
      session->internals.dtls.async_term = 0; /* turn off "timer" */

      _dtls_reset_hsk_state(session);
      _gnutls_handshake_io_buffer_clear (session);
      _gnutls_epoch_gc(session);
    }
}

/* This function fragments and transmits a previously buffered
 * outgoing message. It accepts mtu_data which is a buffer to
 * be reused (should be set to NULL initially).
 */
static inline int
transmit_message (gnutls_session_t session,
                  mbuffer_st *bufel, uint8_t **buf)
{
  uint8_t *data, *mtu_data;
  int ret = 0;
  unsigned int offset, frag_len, data_size;
  const unsigned int mtu = gnutls_dtls_get_data_mtu(session) - DTLS_HANDSHAKE_HEADER_SIZE;

  if (bufel->type == GNUTLS_CHANGE_CIPHER_SPEC)
    {
      _gnutls_dtls_log ("DTLS[%p]: Sending Packet[%u] fragment %s(%d)\n",
                        session, bufel->handshake_sequence,
                        _gnutls_handshake2str (bufel->htype),
                        bufel->htype);

      return _gnutls_send_int (session, bufel->type, -1,
        bufel->epoch, 
        _mbuffer_get_uhead_ptr(bufel), 
        _mbuffer_get_uhead_size(bufel), 0);
    }

  if (*buf == NULL) *buf = gnutls_malloc(mtu + DTLS_HANDSHAKE_HEADER_SIZE);
  if (*buf == NULL)
    return gnutls_assert_val(GNUTLS_E_MEMORY_ERROR);

  mtu_data = *buf;

  data = _mbuffer_get_udata_ptr( bufel);
  data_size = _mbuffer_get_udata_size(bufel);

  /* Write fixed headers
   */

  /* Handshake type */
  mtu_data[0] = (uint8_t) bufel->htype;

  /* Total length */
  _gnutls_write_uint24 (data_size, &mtu_data[1]);

  /* Handshake sequence */
  _gnutls_write_uint16 (bufel->handshake_sequence, &mtu_data[4]);

  /* Chop up and send handshake message into mtu-size pieces. */
  for (offset=0; offset <= data_size; offset += mtu)
    {
      /* Calculate fragment length */
      if(offset + mtu > data_size)
        frag_len = data_size - offset;
      else
        frag_len = mtu;

      /* Fragment offset */
      _gnutls_write_uint24 (offset, &mtu_data[6]);

      /* Fragment length */
      _gnutls_write_uint24 (frag_len, &mtu_data[9]);

      memcpy (&mtu_data[DTLS_HANDSHAKE_HEADER_SIZE], data+offset, frag_len);

      _gnutls_dtls_log ("DTLS[%p]: Sending Packet[%u] fragment %s(%d) with "
                        "length: %u, offset: %u, fragment length: %u\n",
                        session, bufel->handshake_sequence,
                        _gnutls_handshake2str (bufel->htype),
                        bufel->htype, data_size, offset, frag_len);

      ret = _gnutls_send_int (session, bufel->type, bufel->htype, 
        bufel->epoch, mtu_data, DTLS_HANDSHAKE_HEADER_SIZE + frag_len, 0);
      if (ret < 0)
        {
          gnutls_assert();
          break;
        }
   }

  return ret;
}

static int drop_usage_count(gnutls_session_t session, mbuffer_head_st *const send_buffer)
{
  int ret;
  mbuffer_st *cur;

  for (cur = send_buffer->head;
       cur != NULL; cur = cur->next)
    {
      ret = _gnutls_epoch_refcount_dec(session, cur->epoch);
      if (ret < 0)
        return gnutls_assert_val(ret);
    }

  return 0;
}

/* This function is to be called from record layer once
 * a handshake replay is detected. It will make sure
 * it transmits only once per few seconds. Otherwise
 * it is the same as _dtls_transmit().
 */
int _dtls_retransmit(gnutls_session_t session)
{
  return _dtls_transmit(session);
}

/* Checks whether the received packet contains a handshake
 * packet with sequence higher that the previously received.
 * It must be called only when an actual packet has been
 * received.
 *
 * Returns: 0 if expected, negative value otherwise.
 */
static int is_next_hpacket_expected(gnutls_session_t session)
{
int ret;

  /* htype is arbitrary */
  ret = _gnutls_recv_in_buffers(session, GNUTLS_HANDSHAKE, GNUTLS_HANDSHAKE_FINISHED);
  if (ret < 0)
    return gnutls_assert_val(ret);
  
  ret = _gnutls_parse_record_buffered_msgs(session);
  if (ret < 0)
    return gnutls_assert_val(ret);

  if (session->internals.handshake_recv_buffer_size > 0)
    return 0;
  else
    return gnutls_assert_val(GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET);
}

void _dtls_reset_hsk_state(gnutls_session_t session)
{
  session->internals.dtls.flight_init = 0;
  drop_usage_count(session, &session->internals.handshake_send_buffer);
  _mbuffer_head_clear(&session->internals.handshake_send_buffer);
}


#define UPDATE_TIMER { \
      session->internals.dtls.actual_retrans_timeout_ms *= 2; \
      session->internals.dtls.actual_retrans_timeout_ms %= MAX_DTLS_TIMEOUT; \
    }

#define RESET_TIMER \
      session->internals.dtls.actual_retrans_timeout_ms = session->internals.dtls.retrans_timeout_ms

#define TIMER_WINDOW session->internals.dtls.actual_retrans_timeout_ms

/* This function transmits the flight that has been previously
 * buffered.
 *
 * This function is called from the handshake layer and calls the
 * record layer.
 */
int
_dtls_transmit (gnutls_session_t session)
{
int ret;
uint8_t* buf = NULL;
unsigned int timeout;

  /* PREPARING -> SENDING state transition */
  mbuffer_head_st *const send_buffer =
    &session->internals.handshake_send_buffer;
  mbuffer_st *cur;
  gnutls_handshake_description_t last_type = 0;
  unsigned int diff;
  struct timespec now;
  
  gettime(&now);

  /* If we have already sent a flight and we are operating in a 
   * non blocking way, check if it is time to retransmit or just
   * return.
   */
  if (session->internals.dtls.flight_init != 0 && session->internals.dtls.blocking == 0)
    {
      /* just in case previous run was interrupted */
      ret = _gnutls_io_write_flush (session);
      if (ret < 0)
        {
          gnutls_assert();
          goto cleanup;
        }

      if (session->internals.dtls.last_flight == 0 || !_dtls_is_async(session))
        {
          /* check for ACK */
          ret = _gnutls_io_check_recv(session, 0);
          if (ret == GNUTLS_E_TIMEDOUT)
            {
              /* if no retransmission is required yet just return 
               */
              if (_dtls_timespec_sub_ms(&now, &session->internals.dtls.last_retransmit) < TIMER_WINDOW)
                {
                  gnutls_assert();
                  goto nb_timeout;
                }
            }
          else /* received something */
            {
              if (ret == 0)
                {
                  ret = is_next_hpacket_expected(session);
                  if (ret == GNUTLS_E_AGAIN || ret == GNUTLS_E_INTERRUPTED)
                    goto nb_timeout;
                  if (ret < 0 && ret != GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET)
                    {
                      gnutls_assert();
                      goto cleanup;
                    }
                  if (ret == 0) goto end_flight;
                  /* if ret == GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET retransmit */
                }
              else
                goto nb_timeout;
            }
        }
    }

  do 
    {
      timeout = TIMER_WINDOW;

      diff = _dtls_timespec_sub_ms(&now, &session->internals.dtls.handshake_start_time);
      if (diff >= session->internals.dtls.total_timeout_ms) 
        {
          _gnutls_dtls_log("Session timeout: %u ms\n", diff);
          ret = gnutls_assert_val(GNUTLS_E_TIMEDOUT);
          goto end_flight;
        }

      diff = _dtls_timespec_sub_ms(&now, &session->internals.dtls.last_retransmit);
      if (session->internals.dtls.flight_init == 0 || diff >= TIMER_WINDOW)
        {
          _gnutls_dtls_log ("DTLS[%p]: %sStart of flight transmission.\n", session,  (session->internals.dtls.flight_init == 0)?"":"re-");
          for (cur = send_buffer->head;
               cur != NULL; cur = cur->next)
            {
              ret = transmit_message (session, cur, &buf);
              if (ret < 0)
                {
                  gnutls_assert();
                  goto end_flight;
                }

              last_type = cur->htype;
            }
          gettime(&session->internals.dtls.last_retransmit);

          if (session->internals.dtls.flight_init == 0)
            {
              session->internals.dtls.flight_init = 1;
              RESET_TIMER;
              timeout = TIMER_WINDOW;

              if (last_type == GNUTLS_HANDSHAKE_FINISHED)
                {
              /* On the last flight we cannot ensure retransmission
               * from here. _dtls_wait_and_retransmit() is being called
               * by handshake.
               */
                  session->internals.dtls.last_flight = 1;
                }
              else
                session->internals.dtls.last_flight = 0;
            }
          else
            {
              UPDATE_TIMER;
            }
        }

      ret = _gnutls_io_write_flush (session);
      if (ret < 0)
        {
          ret = gnutls_assert_val(ret);
          goto cleanup;
        }

      /* last message in handshake -> no ack */
      if (session->internals.dtls.last_flight != 0)
        {
          /* we don't wait here. We just return 0 and
           * if a retransmission occurs because peer didn't receive it
           * we rely on the record or handshake
           * layer calling this function again.
           */
          ret = 0;
          goto cleanup;
        }
      else /* all other messages -> implicit ack (receive of next flight) */
        {
          if (session->internals.dtls.blocking != 0)
            ret = _gnutls_io_check_recv(session, timeout);
          else
            {
              ret = _gnutls_io_check_recv(session, 0);
              if (ret == GNUTLS_E_TIMEDOUT)
                {
                  goto nb_timeout;
                }
            }
          
          if (ret == 0)
            {
              ret = is_next_hpacket_expected(session);
              if (ret == GNUTLS_E_AGAIN || ret == GNUTLS_E_INTERRUPTED)
                goto nb_timeout;

              if (ret == GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKET)
                {
                  ret = GNUTLS_E_TIMEDOUT;
                  goto keep_up;
                }
              if (ret < 0)
                {
                  gnutls_assert();
                  goto cleanup;
                }
              goto end_flight;
            }
        }

keep_up:
      gettime(&now);
    } while(ret == GNUTLS_E_TIMEDOUT);

  if (ret < 0)
    {
      ret = gnutls_assert_val(ret);
      goto end_flight;
    }

  ret = 0;

end_flight:
  _gnutls_dtls_log ("DTLS[%p]: End of flight transmission.\n", session);
  _dtls_reset_hsk_state(session);

cleanup:
  if (buf != NULL)
    gnutls_free(buf);

  /* SENDING -> WAITING state transition */
  return ret;

nb_timeout:
  if (buf != NULL)
    gnutls_free(buf);

  RETURN_DTLS_EAGAIN_OR_TIMEOUT(session, ret);
}

/* Waits for the last flight or retransmits
 * the previous on timeout. Returns 0 on success.
 */
int _dtls_wait_and_retransmit(gnutls_session_t session)
{
int ret;

  if (session->internals.dtls.blocking != 0)
    ret = _gnutls_io_check_recv(session, TIMER_WINDOW);
  else
    ret = _gnutls_io_check_recv(session, 0);

  if (ret == GNUTLS_E_TIMEDOUT)
    {
      ret = _dtls_retransmit(session);
      if (ret == 0)
        {
          RETURN_DTLS_EAGAIN_OR_TIMEOUT(session, 0);
        }
      else
        return gnutls_assert_val(ret);
    }

  RESET_TIMER;
  return 0;
}


#define window_table rp->record_sw
#define window_size rp->record_sw_size

/* FIXME: could we modify that code to avoid using
 * uint64_t?
 */

static void rot_window(struct record_parameters_st * rp, int places)
{
  window_size -= places;
  memmove(window_table, &window_table[places], window_size*sizeof(window_table[0]));
}

#define MOVE_SIZE 20
/* Checks if a sequence number is not replayed. If replayed
 * returns a negative error code, otherwise zero.
 */
int _dtls_record_check(struct record_parameters_st *rp, uint64 * _seq)
{
uint64_t seq = 0, diff;
unsigned int i, offset = 0;

  for (i=2;i<8;i++)
    {
      seq <<= 8;
      seq |= _seq->i[i] & 0xff;
    }

  if (window_size == 0)
    {
      window_size = 1;
      window_table[0] = seq;
      return 0;
    }

  if (seq <= window_table[0])
    {
      return -1;
    }

  if (window_size == DTLS_RECORD_WINDOW_SIZE) 
    {
      rot_window(rp, MOVE_SIZE);
    }

  if (seq < window_table[window_size-1])
    {
      /* is between first and last */
      diff = window_table[window_size-1] - seq;

      if (diff >= window_size) 
        return -1;

      offset = window_size-1-diff;

      if (window_table[offset] == seq)
        return -1;
      else
        window_table[offset] = seq;
    }
  else /* seq >= last */
    {
      if (seq == window_table[window_size-1]) 
        {
          return -1;
        }

      diff = seq - window_table[window_size-1];
      if (diff <= DTLS_RECORD_WINDOW_SIZE - window_size)
        { /* fits in our empty space */
          offset = diff + window_size-1;

          window_table[offset] = seq;
          window_size = offset + 1;
        }
      else
        {
          if (diff > DTLS_RECORD_WINDOW_SIZE/2)
            { /* difference is too big */
              window_table[DTLS_RECORD_WINDOW_SIZE-1] = seq;
              window_size = DTLS_RECORD_WINDOW_SIZE;
            }
          else
            {
              rot_window(rp, diff);
              offset = diff + window_size-1;
              window_table[offset] = seq;
              window_size = offset + 1;            
            }
        }
    }
  return 0;
}


/**
 * gnutls_dtls_set_timeouts:
 * @session: is a #gnutls_session_t structure.
 * @retrans_timeout: The time at which a retransmission will occur in milliseconds
 * @total_timeout: The time at which the connection will be aborted, in milliseconds.
 *
 * This function will set the timeouts required for the DTLS handshake
 * protocol. The retransmission timeout is the time after which a
 * message from the peer is not received, the previous messages will
 * be retransmitted. The total timeout is the time after which the
 * handshake will be aborted with %GNUTLS_E_TIMEDOUT.
 *
 * The DTLS protocol recommends the values of 1 sec and 60 seconds
 * respectively.
 *
 * If the retransmission timeout is zero then the handshake will operate
 * in a non-blocking way, i.e., return %GNUTLS_E_AGAIN.
 *
 * Since: 3.0
 **/
void gnutls_dtls_set_timeouts (gnutls_session_t session, unsigned int retrans_timeout,
  unsigned int total_timeout)
{
  session->internals.dtls.retrans_timeout_ms = retrans_timeout;
  session->internals.dtls.total_timeout_ms  = total_timeout;
}

/**
 * gnutls_dtls_set_mtu:
 * @session: is a #gnutls_session_t structure.
 * @mtu: The maximum transfer unit of the interface
 *
 * This function will set the maximum transfer unit of the interface
 * that DTLS packets are expected to leave from.
 *
 * Since: 3.0
 **/
void gnutls_dtls_set_mtu (gnutls_session_t session, unsigned int mtu)
{
  session->internals.dtls.mtu  = mtu;
}

/* returns overhead imposed by the record layer (encryption/compression)
 * etc. It does not include the record layer headers, since the caller
 * needs to cope with rounding to multiples of blocksize, and the header
 * is outside that.
 *
 * blocksize: will contain the block size when padding may be required or 1
 *
 * It may return a negative error code on error.
 */
static int _gnutls_record_overhead_rt(gnutls_session_t session, unsigned int *blocksize)
{
record_parameters_st *params;
int total = 0, ret, iv_size;

  if (session->internals.initial_negotiation_completed == 0)
    return GNUTLS_E_INVALID_REQUEST;

  ret = _gnutls_epoch_get (session, EPOCH_WRITE_CURRENT, &params);
  if (ret < 0)
    return gnutls_assert_val(ret);

  /* requires padding */
  iv_size = _gnutls_cipher_get_iv_size(params->cipher_algorithm);

  if (_gnutls_cipher_is_block (params->cipher_algorithm) == CIPHER_BLOCK)
    {
      *blocksize = iv_size;

      if (!IS_DTLS(session))
        total += MAX_PAD_SIZE;
      else
        total += iv_size; /* iv_size == block_size */
    }
  else
    {
      *blocksize = 1;
    }
  
  if (params->mac_algorithm == GNUTLS_MAC_AEAD)
    total += _gnutls_cipher_get_tag_size(params->cipher_algorithm);
  else
    {
      ret = _gnutls_hmac_get_algo_len(params->mac_algorithm);
      if (ret < 0)
        return gnutls_assert_val(ret);
      total+=ret;
    }

  if (params->compression_algorithm != GNUTLS_COMP_NULL)
    total += EXTRA_COMP_SIZE;
  
  /* We always pad with at least one byte; never 0. */
  total++;

  return total;
}

/**
 * gnutls_dtls_get_data_mtu:
 * @session: is a #gnutls_session_t structure.
 *
 * This function will return the actual maximum transfer unit for
 * application data. I.e. DTLS headers are subtracted from the
 * actual MTU.
 *
 * Returns: the maximum allowed transfer unit.
 *
 * Since: 3.0
 **/
unsigned int gnutls_dtls_get_data_mtu (gnutls_session_t session)
{
int mtu = session->internals.dtls.mtu;
int blocksize = 0;
int overhead;
 
  mtu -= RECORD_HEADER_SIZE(session);

  overhead = _gnutls_record_overhead_rt(session, &blocksize);
  if (overhead < 0)
    return mtu;

  if (blocksize)
    mtu -= mtu % blocksize;

  return mtu - overhead;
}

/**
 * gnutls_dtls_set_data_mtu:
 * @session: is a #gnutls_session_t structure.
 * @mtu: The maximum unencrypted transfer unit of the session
 *
 * This function will set the maximum size of the *unencrypted* records
 * which will be sent over a DTLS session. It is equivalent to calculating
 * the DTLS packet overhead with the current encryption parameters, and
 * calling gnutls_dtls_set_mtu() with that value. In particular, this means
 * that you may need to call this function again after any negotiation or
 * renegotiation, in order to ensure that the MTU is still sufficient to
 * account for the new protocol overhead.
 *
 * Returns: %GNUTLS_E_SUCCESS (0) on success, or a negative error code.
 *
 * Since: 3.1
 **/
int gnutls_dtls_set_data_mtu (gnutls_session_t session, unsigned int mtu)
{
  int blocksize;
  int overhead = _gnutls_record_overhead_rt(session, &blocksize);

  /* You can't call this until the session is actually running */
  if (overhead < 0)
          return GNUTLS_E_INVALID_SESSION;

  /* Add the overhead inside the encrypted part */
  mtu += overhead;

  /* Round it up to the next multiple of blocksize */
  mtu += blocksize - 1;
  mtu -= mtu % blocksize;

  /* Add the *unencrypted header size */
  mtu += RECORD_HEADER_SIZE(session);

  gnutls_dtls_set_mtu(session, mtu);
  return GNUTLS_E_SUCCESS;
}

/**
 * gnutls_dtls_get_mtu:
 * @session: is a #gnutls_session_t structure.
 *
 * This function will return the MTU size as set with
 * gnutls_dtls_set_mtu(). This is not the actual MTU
 * of data you can transmit. Use gnutls_dtls_get_data_mtu()
 * for that reason.
 *
 * Returns: the set maximum transfer unit.
 *
 * Since: 3.0
 **/
unsigned int gnutls_dtls_get_mtu (gnutls_session_t session)
{
  return session->internals.dtls.mtu;
}

/**
 * gnutls_dtls_get_timeout:
 * @session: is a #gnutls_session_t structure.
 *
 * This function will return the milliseconds remaining
 * for a retransmission of the previously sent handshake
 * message. This function is useful when DTLS is used in
 * non-blocking mode, to estimate when to call gnutls_handshake()
 * if no packets have been received.
 *
 * Returns: the remaining time in milliseconds.
 *
 * Since: 3.0
 **/
unsigned int gnutls_dtls_get_timeout (gnutls_session_t session)
{
struct timespec now;
unsigned int diff;

  gettime(&now);
  
  diff = _dtls_timespec_sub_ms(&now, &session->internals.dtls.last_retransmit);
  if (diff >= TIMER_WINDOW)
    return 0;
  else
    return TIMER_WINDOW - diff;
}

#define COOKIE_SIZE 16
#define COOKIE_MAC_SIZE 16

/*   MAC
 * 16 bytes
 *
 * total 19 bytes
 */

#define C_HASH GNUTLS_MAC_SHA1
#define C_HASH_SIZE 20

/**
 * gnutls_dtls_cookie_send:
 * @key: is a random key to be used at cookie generation
 * @client_data: contains data identifying the client (i.e. address)
 * @client_data_size: The size of client's data
 * @prestate: The previous cookie returned by gnutls_dtls_cookie_verify()
 * @ptr: A transport pointer to be used by @push_func
 * @push_func: A function that will be used to reply
 *
 * This function can be used to prevent denial of service
 * attacks to a DTLS server by requiring the client to
 * reply using a cookie sent by this function. That way
 * it can be ensured that a client we allocated resources
 * for (i.e. #gnutls_session_t) is the one that the 
 * original incoming packet was originated from.
 *
 * Returns: the number of bytes sent, or a negative error code.  
 *
 * Since: 3.0
 **/
int gnutls_dtls_cookie_send(gnutls_datum_t* key, void* client_data, size_t client_data_size, 
  gnutls_dtls_prestate_st* prestate,
  gnutls_transport_ptr_t ptr, gnutls_push_func push_func)
{
uint8_t hvr[20+DTLS_HANDSHAKE_HEADER_SIZE+COOKIE_SIZE];
int hvr_size = 0, ret;
uint8_t digest[C_HASH_SIZE];

  if (key == NULL || key->data == NULL || key->size == 0)
    return gnutls_assert_val(GNUTLS_E_INVALID_REQUEST);

/* send
 *  struct {
 *    ContentType type - 1 byte GNUTLS_HANDSHAKE;
 *    ProtocolVersion version; - 2 bytes (254,255)
 *    uint16 epoch; - 2 bytes (0, 0)
 *    uint48 sequence_number; - 4 bytes (0,0,0,0)
 *    uint16 length; - 2 bytes (COOKIE_SIZE+1+2)+DTLS_HANDSHAKE_HEADER_SIZE
 *    uint8_t fragment[DTLSPlaintext.length];
 *  } DTLSPlaintext;
 *
 *
 * struct {
 *    HandshakeType msg_type; 1 byte - GNUTLS_HANDSHAKE_HELLO_VERIFY_REQUEST
 *    uint24 length; - COOKIE_SIZE+3
 *    uint16 message_seq; - 2 bytes (0,0)
 *    uint24 fragment_offset; - 3 bytes (0,0,0)
 *    uint24 fragment_length; - same as length
 * }
 *
 * struct {
 *   ProtocolVersion server_version;
 *   uint8_t cookie<0..32>;
 * } HelloVerifyRequest;
 */ 

  hvr[hvr_size++] = GNUTLS_HANDSHAKE;
  /* version */
  hvr[hvr_size++] = 254;
  hvr[hvr_size++] = 255;
  
  /* epoch + seq */
  memset(&hvr[hvr_size], 0, 8);
  hvr_size += 7;
  hvr[hvr_size++] = prestate->record_seq;

  /* length */
  _gnutls_write_uint16(DTLS_HANDSHAKE_HEADER_SIZE+COOKIE_SIZE+3, &hvr[hvr_size]);
  hvr_size += 2;

  /* now handshake headers */
  hvr[hvr_size++] = GNUTLS_HANDSHAKE_HELLO_VERIFY_REQUEST;
  _gnutls_write_uint24(COOKIE_SIZE+3, &hvr[hvr_size]);
  hvr_size += 3;
  
  /* handshake seq */
  hvr[hvr_size++] = 0;
  hvr[hvr_size++] = prestate->hsk_write_seq;

  _gnutls_write_uint24(0, &hvr[hvr_size]);
  hvr_size += 3;

  _gnutls_write_uint24(COOKIE_SIZE+3, &hvr[hvr_size]);
  hvr_size += 3;

  /* version */
  hvr[hvr_size++] = 254;
  hvr[hvr_size++] = 255;
  hvr[hvr_size++] = COOKIE_SIZE;

  ret = _gnutls_hmac_fast(C_HASH, key->data, key->size, client_data, client_data_size, digest);
  if (ret < 0)
    return gnutls_assert_val(ret);

  memcpy(&hvr[hvr_size], digest, COOKIE_MAC_SIZE);
  hvr_size+= COOKIE_MAC_SIZE;

  ret = push_func(ptr, hvr, hvr_size);
  if (ret < 0)
    ret = GNUTLS_E_PUSH_ERROR;

  return ret;
}

/**
 * gnutls_dtls_cookie_verify:
 * @key: is a random key to be used at cookie generation
 * @client_data: contains data identifying the client (i.e. address)
 * @client_data_size: The size of client's data
 * @_msg: An incoming message that initiates a connection.
 * @msg_size: The size of the message.
 * @prestate: The cookie of this client.
 *
 * This function will verify an incoming message for
 * a valid cookie. If a valid cookie is returned then
 * it should be associated with the session using
 * gnutls_dtls_prestate_set();
 *
 * Returns: %GNUTLS_E_SUCCESS (0) on success, or a negative error code.  
 *
 * Since: 3.0
 **/
int gnutls_dtls_cookie_verify(gnutls_datum_t* key, 
  void* client_data, size_t client_data_size, 
  void* _msg, size_t msg_size, gnutls_dtls_prestate_st* prestate)
{
gnutls_datum_t cookie;
int ret;
unsigned int pos, sid_size;
uint8_t * msg = _msg;
uint8_t digest[C_HASH_SIZE];

  if (key == NULL || key->data == NULL || key->size == 0)
    return gnutls_assert_val(GNUTLS_E_INVALID_REQUEST);

  /* format:
   * version - 2 bytes
   * random - 32 bytes
   * session_id - 1 byte length + content
   * cookie - 1 byte length + content
   */

  pos = 34+DTLS_RECORD_HEADER_SIZE+DTLS_HANDSHAKE_HEADER_SIZE;

  if (msg_size < pos+1)
    return gnutls_assert_val(GNUTLS_E_UNEXPECTED_PACKET_LENGTH);

  sid_size = msg[pos++];

  if (sid_size > 32 || msg_size < pos+sid_size+1)
    return gnutls_assert_val(GNUTLS_E_UNEXPECTED_PACKET_LENGTH);

  pos += sid_size;
  cookie.size = msg[pos++];

  if (msg_size < pos+cookie.size+1)
    return gnutls_assert_val(GNUTLS_E_UNEXPECTED_PACKET_LENGTH);
  
  cookie.data = &msg[pos];
  if (cookie.size != COOKIE_SIZE)
    {
      if (cookie.size > 0) _gnutls_audit_log(NULL, "Received cookie with illegal size %d. Expected %d\n", (int)cookie.size, COOKIE_SIZE);
      return gnutls_assert_val(GNUTLS_E_BAD_COOKIE);
    }

  ret = _gnutls_hmac_fast(C_HASH, key->data, key->size, client_data, client_data_size, digest);
  if (ret < 0)
    return gnutls_assert_val(ret);

  if (memcmp(digest, cookie.data, COOKIE_MAC_SIZE) != 0)
    return gnutls_assert_val(GNUTLS_E_BAD_COOKIE);
  
  prestate->record_seq = msg[10]; /* client's record seq */
  prestate->hsk_read_seq =  msg[DTLS_RECORD_HEADER_SIZE+5]; /* client's hsk seq */
  prestate->hsk_write_seq = 0;/* we always send zero for this msg */
  
  return 0;
}

/**
 * gnutls_dtls_prestate_set:
 * @session: a new session
 * @prestate: contains the client's prestate
 *
 * This function will associate the prestate acquired by
 * the cookie authentication with the client, with the newly 
 * established session.
 *
 * Since: 3.0
 **/
void gnutls_dtls_prestate_set(gnutls_session_t session, gnutls_dtls_prestate_st* prestate)
{
  record_parameters_st *params;
  int ret;

  if (prestate == NULL)
    return;

  /* we do not care about read_params, since we accept anything
   * the peer sends.
   */
  ret = _gnutls_epoch_get (session, EPOCH_WRITE_CURRENT, &params);
  if (ret < 0)
    return;

  params->write.sequence_number.i[7] = prestate->record_seq;

  session->internals.dtls.hsk_read_seq = prestate->hsk_read_seq;
  session->internals.dtls.hsk_write_seq = prestate->hsk_write_seq + 1;
}

/**
 * gnutls_record_get_discarded:
 * @session: is a #gnutls_session_t structure.
 *
 * Returns the number of discarded packets in a
 * DTLS connection.
 *
 * Returns: The number of discarded packets.
 *
 * Since: 3.0
 **/
unsigned int gnutls_record_get_discarded (gnutls_session_t session)
{
  return session->internals.dtls.packets_dropped;
}