int tls1_heartbeat(SSL *s)
{
    unsigned char *buf, *p;
    int ret;
    unsigned int payload = 18;  /* Sequence number + random bytes */
    unsigned int padding = 16;  /* Use minimum padding */

    /* Only send if peer supports and accepts HB requests... */
    if (!(s->tlsext_heartbeat & SSL_TLSEXT_HB_ENABLED) ||
        s->tlsext_heartbeat & SSL_TLSEXT_HB_DONT_SEND_REQUESTS) {
        SSLerr(SSL_F_TLS1_HEARTBEAT, SSL_R_TLS_HEARTBEAT_PEER_DOESNT_ACCEPT);
        return -1;
    }

    /* ...and there is none in flight yet... */
    if (s->tlsext_hb_pending) {
        SSLerr(SSL_F_TLS1_HEARTBEAT, SSL_R_TLS_HEARTBEAT_PENDING);
        return -1;
    }

    /* ...and no handshake in progress. */
    if (SSL_in_init(s) || s->in_handshake) {
        SSLerr(SSL_F_TLS1_HEARTBEAT, SSL_R_UNEXPECTED_MESSAGE);
        return -1;
    }

    /*
     * Check if padding is too long, payload and padding must not exceed 2^14
     * - 3 = 16381 bytes in total.
     */
    OPENSSL_assert(payload + padding <= 16381);

    /*-
     * Create HeartBeat message, we just use a sequence number
     * as payload to distuingish different messages and add
     * some random stuff.
     *  - Message Type, 1 byte
     *  - Payload Length, 2 bytes (unsigned int)
     *  - Payload, the sequence number (2 bytes uint)
     *  - Payload, random bytes (16 bytes uint)
     *  - Padding
     */
    buf = OPENSSL_malloc(1 + 2 + payload + padding);
    p = buf;
    /* Message Type */
    *p++ = TLS1_HB_REQUEST;
    /* Payload length (18 bytes here) */
    s2n(payload, p);
    /* Sequence number */
    s2n(s->tlsext_hb_seq, p);
    /* 16 random bytes */
    RAND_pseudo_bytes(p, 16);
    p += 16;
    /* Random padding */
    RAND_pseudo_bytes(p, padding);

    ret = ssl3_write_bytes(s, TLS1_RT_HEARTBEAT, buf, 3 + payload + padding);
    if (ret >= 0) {
        if (s->msg_callback)
            s->msg_callback(1, s->version, TLS1_RT_HEARTBEAT,
                            buf, 3 + payload + padding,
                            s, s->msg_callback_arg);

        s->tlsext_hb_pending = 1;
    }

    OPENSSL_free(buf);

    return ret;
}

Unknown