diff --git a/td/mtproto/SessionConnection.cpp b/td/mtproto/SessionConnection.cpp
index ad044ac05..b9a40ea40 100644
--- a/td/mtproto/SessionConnection.cpp
+++ b/td/mtproto/SessionConnection.cpp
@@ -25,6 +25,7 @@
 #include "td/utils/SliceBuilder.h"
 #include "td/utils/Time.h"
 #include "td/utils/tl_parsers.h"
+#include "td/utils/TlDowncastHelper.h"
 
 #include "td/mtproto/mtproto_api.h"
 #include "td/mtproto/mtproto_api.hpp"
@@ -171,22 +172,6 @@ namespace mtproto {
  *
  */
 
-class OnPacket {
-  const MsgInfo &info_;
-  SessionConnection *connection_;
-  Status *status_;
-
- public:
-  OnPacket(const MsgInfo &info, SessionConnection *connection, Status *status)
-      : info_(info), connection_(connection), status_(status) {
-  }
-
-  template <class T>
-  void operator()(const T &func) const {
-    *status_ = connection_->on_packet(info_, func);
-  }
-};
-
 unique_ptr<RawConnection> SessionConnection::move_as_raw_connection() {
   return std::move(raw_connection_);
 }
@@ -230,7 +215,6 @@ Status SessionConnection::on_packet_container(const MsgInfo &info, Slice packet)
   };
 
   TlParser parser(packet);
-  parser.fetch_int();
   int32 size = parser.fetch_int();
   if (parser.get_error()) {
     return Status::Error(PSLICE() << "Failed to parse mtproto_api::rpc_container: " << parser.get_error());
@@ -243,7 +227,6 @@ Status SessionConnection::on_packet_container(const MsgInfo &info, Slice packet)
 
 Status SessionConnection::on_packet_rpc_result(const MsgInfo &info, Slice packet) {
   TlParser parser(packet);
-  parser.fetch_int();
   uint64 req_msg_id = parser.fetch_long();
   if (parser.get_error()) {
     return Status::Error(PSLICE() << "Failed to parse mtproto_api::rpc_result: " << parser.get_error());
@@ -275,7 +258,7 @@ Status SessionConnection::on_packet_rpc_result(const MsgInfo &info, Slice packet
       return callback_->on_message_result_ok(req_msg_id, std::move(object), info.size);
     }
     default:
-      packet.remove_prefix(4 + sizeof(req_msg_id));
+      packet.remove_prefix(sizeof(req_msg_id));
       return callback_->on_message_result_ok(req_msg_id, as_buffer_slice(packet), info.size);
   }
 }
@@ -285,12 +268,15 @@ Status SessionConnection::on_packet(const MsgInfo &info, const T &packet) {
   LOG(ERROR) << "Unsupported: " << to_string(packet);
   return Status::OK();
 }
+
 Status SessionConnection::on_packet(const MsgInfo &info, const mtproto_api::destroy_auth_key_ok &destroy_auth_key) {
   return on_destroy_auth_key(destroy_auth_key);
 }
+
 Status SessionConnection::on_packet(const MsgInfo &info, const mtproto_api::destroy_auth_key_none &destroy_auth_key) {
   return on_destroy_auth_key(destroy_auth_key);
 }
+
 Status SessionConnection::on_packet(const MsgInfo &info, const mtproto_api::destroy_auth_key_fail &destroy_auth_key) {
   return on_destroy_auth_key(destroy_auth_key);
 }
@@ -479,52 +465,66 @@ Status SessionConnection::on_slice_packet(const MsgInfo &info, Slice packet) {
   if (info.seq_no & 1) {
     send_ack(info.message_id);
   }
-  TlParser parser(packet);
-  tl_object_ptr<mtproto_api::Object> object = mtproto_api::Object::fetch(parser);
-  parser.fetch_end();
-  if (parser.get_error()) {
-    // msg_container is not real tl object
-    if (packet.size() >= 4 && as<int32>(packet.begin()) == mtproto_api::msg_container::ID) {
-      return on_packet_container(info, packet);
-    }
-    if (packet.size() >= 4 && as<int32>(packet.begin()) == mtproto_api::rpc_result::ID) {
-      return on_packet_rpc_result(info, packet);
-    }
-
-    // It is an update... I hope.
-    auto status = auth_data_->check_update(info.message_id);
-    auto recheck_status = auth_data_->recheck_update(info.message_id);
-    if (recheck_status.is_error() && recheck_status.code() == 2) {
-      LOG(WARNING) << "Receive very old update from " << get_name() << " created in " << (Time::now() - created_at_)
-                   << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
-                   << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
-                   << ", seq_no = " << info.seq_no << " and original size " << info.size << ": " << status << ' '
-                   << recheck_status;
-    }
-    if (status.is_error()) {
-      if (status.code() == 2) {
-        LOG(WARNING) << "Receive too old update from " << get_name() << " created in " << (Time::now() - created_at_)
-                     << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
-                     << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
-                     << ", seq_no = " << info.seq_no << " and original size " << info.size << ": " << status;
-        callback_->on_session_failed(Status::Error("Receive too old update"));
-        return status;
-      }
-      VLOG(mtproto) << "Skip update " << info.message_id << " of size " << info.size << " with seq_no " << info.seq_no
-                    << " from " << get_name() << " created in " << (Time::now() - created_at_) << ": " << status;
-      return Status::OK();
-    } else {
-      VLOG(mtproto) << "Got update from " << get_name() << " created in " << (Time::now() - created_at_)
-                    << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
-                    << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
-                    << ", seq_no = " << info.seq_no << " and original size " << info.size;
-      return callback_->on_update(as_buffer_slice(packet));
-    }
+  if (packet.size() < 4) {
+    callback_->on_session_failed(Status::Error("Receive too small packet"));
+    return Status::Error(PSLICE() << "Receive packet of size " << packet.size());
   }
 
+  int32 constructor_id = as<int32>(packet.begin());
+  if (constructor_id == mtproto_api::msg_container::ID) {
+    return on_packet_container(info, packet.substr(4));
+  }
+  if (constructor_id == mtproto_api::rpc_result::ID) {
+    return on_packet_rpc_result(info, packet.substr(4));
+  }
+
+  TlDowncastHelper<mtproto_api::Object> helper(constructor_id);
   Status status;
-  downcast_call(*object, OnPacket(info, this, &status));
-  return status;
+  bool is_mtproto_api = downcast_call(static_cast<mtproto_api::Object &>(helper), [&](auto &dummy) {
+    // a constructor from mtproto_api
+    using Type = std::decay_t<decltype(dummy)>;
+    TlParser parser(packet.substr(4));
+    auto object = Type::fetch(parser);
+    parser.fetch_end();
+    if (parser.get_error()) {
+      status = parser.get_status();
+    } else {
+      status = this->on_packet(info, static_cast<const Type &>(*object));
+    }
+  });
+  if (is_mtproto_api) {
+    return status;
+  }
+
+  // It is an update... I hope.
+  status = auth_data_->check_update(info.message_id);
+  auto recheck_status = auth_data_->recheck_update(info.message_id);
+  if (recheck_status.is_error() && recheck_status.code() == 2) {
+    LOG(WARNING) << "Receive very old update from " << get_name() << " created in " << (Time::now() - created_at_)
+                 << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
+                 << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
+                 << ", seq_no = " << info.seq_no << " and original size " << info.size << ": " << status << ' '
+                 << recheck_status;
+  }
+  if (status.is_error()) {
+    if (status.code() == 2) {
+      LOG(WARNING) << "Receive too old update from " << get_name() << " created in " << (Time::now() - created_at_)
+                   << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
+                   << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
+                   << ", seq_no = " << info.seq_no << " and original size " << info.size << ": " << status;
+      callback_->on_session_failed(Status::Error("Receive too old update"));
+      return status;
+    }
+    VLOG(mtproto) << "Skip update " << info.message_id << " of size " << info.size << " with seq_no " << info.seq_no
+                  << " from " << get_name() << " created in " << (Time::now() - created_at_) << ": " << status;
+    return Status::OK();
+  } else {
+    VLOG(mtproto) << "Got update from " << get_name() << " created in " << (Time::now() - created_at_)
+                  << " in container " << container_id_ << " from session " << auth_data_->get_session_id()
+                  << " with message_id " << info.message_id << ", main_message_id = " << main_message_id_
+                  << ", seq_no = " << info.seq_no << " and original size " << info.size;
+    return callback_->on_update(as_buffer_slice(packet));
+  }
 }
 
 Status SessionConnection::parse_packet(TlParser &parser) {
@@ -579,6 +579,7 @@ void SessionConnection::on_message_failed(uint64 id, Status status) {
     on_message_failed_inner(id);
   }
 }
+
 void SessionConnection::on_message_failed_inner(uint64 id) {
   auto it = service_queries_.find(id);
   if (it == service_queries_.end()) {
diff --git a/td/mtproto/SessionConnection.h b/td/mtproto/SessionConnection.h
index 9ef287ca9..3da23a4fc 100644
--- a/td/mtproto/SessionConnection.h
+++ b/td/mtproto/SessionConnection.h
@@ -203,8 +203,6 @@ class SessionConnection final
   SessionConnection::Callback *callback_ = nullptr;
   BufferSlice *current_buffer_slice_;
 
-  friend class OnPacket;
-
   BufferSlice as_buffer_slice(Slice packet);
   auto set_buffer_slice(BufferSlice *buffer_slice) TD_WARN_UNUSED_RESULT {
     auto old_buffer_slice = current_buffer_slice_;