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### Description

<!-- Explain what you did, what you expected to happen, and what actually happened. -->

We're trying to connect the same subscriber with Kamailio for load testing. Sometimes it registers successfully but sometimes it throws the following error.

### Troubleshooting

From my understanding by checking the source code, TLS_WR_MBUF_SZ is set as 65536 by default. This is the reason for "write buffer too small". Can someone help me with how to change that?

#### Reproduction

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I don't exactly know how to reproduce as some the clients pass and some fail.

#### Debugging Data

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gdb /path/to/kamailio /path/to/corefile bt full info locals list

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``` (paste your debugging data here) ```

#### Log Messages

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BUG: tls [tls_server.c:849]: tls_encode_f(): write buffer too small (65536/65536 bytes) ERROR: websocket [ws_frame.c:300]: encode_and_send_ws_frame(): sending WebSocket frame ERROR: websocket [ws_frame.c:800]: ws_frame_transmit(): sending message NOTICE: <script>: WebSocket connection from a.b.c.d:1234 has closed ERROR: registrar [reply.c:745]: reg_send_reply(): failed to send 200 OK ERROR: <core> [core/tcp_main.c:3502]: handle_ser_child(): received CON_ERROR for 0x7f9472eec990 (id 100), refcnt 4, flags 0x4018 NOTICE: <script>: Error in saving location data WARNING: sl [../../core/forward.h:173]: msg_send_buffer(): TCP/TLS connection for WebSocket could not be found ERROR: sl [sl_funcs.c:362]: sl_reply_error(): stateless error reply used: I'm terribly sorry, server error occurred (1/SL) WARNING: <core> [core/tcp_read.c:1716]: handle_io(): F_TCPCONN connection marked as bad: 0x7f9472eec990 id 100 refcnt 1

#### SIP Traffic

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### Possible Solutions

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Need to increase the default tls write buffer size from 65536. I think it is set as "#define TLS_WR_MBUF_SZ 65536"

### Additional Information

* **Kamailio Version** - output of `kamailio -v`

version: kamailio 5.0.8 (x86_64/linux) c35e69 flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144, MAX_LISTEN 16, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: c35e69 compiled on 05:04:45 Nov 25 2019 with gcc 4.8.5

* **Operating System**:

<!-- Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...; Kernel details (output of `uname -a`) -->

Linux 3.10.0-957.21.3.el7.x86_64 #1 SMP Tue Jun 18 16:35:19 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

CentOS Linux release 7.7.1908 (Core)