### Description Hello, Kamailio -master version couldnt relay tcp request to other sides. it gives EAGAIN error. When i test scenario on kamailio-5.2 version , it can relay.
### Troubleshooting
#### Reproduction ``` route[TCPTEST]{ remove_hf("Route"); #loose_route(); if (!is_method("INVITE")) return;
$du="sip: mykamailio1.test:5060;transport=tcp";
if(!t_newtran()){ xlog("L_WARN"," Couldnt create new transaction \r\n"); sl_send_reply("503", "Service not available"); exit; }
if(!t_relay()){ send_reply("503", "Service not available"); exit; } exit; }
```
#### Debugging Data
#### Log Messages
<!-- Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->
``` 7(15781) DEBUG: <core> [core/tcp_main.c:1229]: tcpconn_new(): on port 5060, type 2 7(15781) DEBUG: <core> [core/tcp_main.c:1539]: tcpconn_add(): hashes: 507:2382:0, 2 7(15781) DEBUG: <core> [core/tcp_main.c:2873]: tcpconn_1st_send(): pending write on new connection 0x7fd15b5e7c98 (-1/1483 bytes written) (err: 11 - Resource temporarily unavailable) 7(15781) ERROR: <core> [core/tcp_main.c:2891]: tcpconn_1st_send(): [FD61:C23C:B3B6:0:0:0:0:5]:5060: EAGAIN and write queue full or failed for 0x7fd15b5e7c98 7(15781) DEBUG: <core> [core/tcp_main.c:3153]: tcpconn_chld_put(): destroying connection 0x7fd15b5e7c98 (2, -1) flags 0060
7(15781) ERROR: tm [../../core/forward.h:251]: msg_send_buffer(): tcp_send failed ```
#### SIP Traffic
<!-- If the issue is exposed by processing specific SIP messages, grab them with ngrep or save in a pcap file, then add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->
``` UAC: IPv6: TCP ---> Kamailio (5.2) :IPv6 :TCP ---> Client or Server IPv6:TCP ```
### Possible Solutions
<!-- If you found a solution or workaround for the issue, describe it. Ideally, provide a pull request with a fix. -->
### Additional Information
``` Kamailio master from git ```
* **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`) -->
``` NAME="CentOS Linux" VERSION="7 (Core)" 3.10.0-957.1.3.el7.x86_64 ```