DTLS: Use bio callback to get fragment packet. v5.0.156, v6.0.47 (#3565)

1. The MTU is effective, with the certificate being split into two DTLS records to comply with the limit.
1. The issue occurs when using BIO_get_mem_data, which retrieves all DTLS packets in a single call, even though each is smaller than the MTU.
1. An alternative callback is available for using BIO_new with BIO_s_mem.
1. Improvements to the MTU setting were made, including adding the DTLS_set_link_mtu function and removing the SSL_set_max_send_fragment function.
1. The handshake process was refined, calling SSL_do_handshake only after ICE completion, and using SSL_read to handle handshake messages.
1. The session close code was improved to enable immediate closure upon receiving an SSL CloseNotify or fatal message.

------

Co-authored-by: chundonglinlin <chundonglinlin@163.com>

trunk/doc/CHANGELOG.md

@@ -8,6 +8,7 @@ The changelog for SRS.
 
 ## SRS 6.0 Changelog
 
+* v6.0, 2023-06-05, Merge [#3565](https://github.com/ossrs/srs/pull/3565): DTLS: Use bio callback to get fragment packet. v6.0.47 (#3565)
 * v6.0, 2023-05-29, Merge [#3513](https://github.com/ossrs/srs/pull/3513): SSL: Fix SSL_get_error get the error of other coroutine. v6.0.46 (#3513)
 * v6.0, 2023-05-14, Merge [#3534](https://github.com/ossrs/srs/pull/3534): Replace sprintf with snprintf to eliminate compile warnings. v6.0.45 (#3534)
 * v6.0, 2023-05-13, Merge [#3541](https://github.com/ossrs/srs/pull/3541): asan: Fix memory leak in asan by releasing global IPs when run_directly_or_daemon fails. v6.0.44 (#3541)
@@ -60,6 +61,7 @@ The changelog for SRS.
 
 ## SRS 5.0 Changelog
 
+* v5.0, 2023-06-05, Merge [#3565](https://github.com/ossrs/srs/pull/3565): DTLS: Use bio callback to get fragment packet. v5.0.156 (#3565)
 * v5.0, 2023-05-29, Merge [#3513](https://github.com/ossrs/srs/pull/3513): SSL: Fix SSL_get_error get the error of other coroutine. v5.0.155 (#3513)
 * v5.0, 2023-05-13, Merge [#3541](https://github.com/ossrs/srs/pull/3541): asan: Fix memory leak in asan by releasing global IPs when run_directly_or_daemon fails. v5.0.154 (#3541)
 * v5.0, 2023-05-12, Merge [#3539](https://github.com/ossrs/srs/pull/3539): WHIP: Improve HTTP DELETE for notifying server unpublish event. v5.0.153 (#3539)

trunk/src/app/srs_app_rtc_conn.cpp

@@ -2240,11 +2240,12 @@ srs_error_t SrsRtcConnection::on_dtls_alert(std::string type, std::string desc)
     srs_error_t err = srs_success;
 
     // CN(Close Notify) is sent when client close the PeerConnection.
-    if (type == "warning" && desc == "CN") {
+    // fatal, IP(Illegal Parameter) is sent when DTLS failed.
+    if (type == "fatal" || (type == "warning" && desc == "CN")) {
         SrsContextRestore(_srs_context->get_id());
         switch_to_context();
 
-        srs_trace("RTC: session destroy by DTLS alert, username=%s", username_.c_str());
+        srs_trace("RTC: session destroy by DTLS alert(%s %s), username=%s", type.c_str(), desc.c_str(), username_.c_str());
         _srs_rtc_manager->remove(this);
     }