The client uses the TLS protocol registered and TCP Connection established . There is a problem now, sending message KSR.uac.uac_req_send() from the kamilio server to client failed sometimes,and the error is as follows： “2023-09-04T17:22:01.515763+08:00 (3429) ERROR: tm [../../core/forward.h:292]: msg_send_buffer(): tcp_send failed 2023-09-04T17:22:01.515763+08:00 (3429) ERROR: tm [uac.c:678]: send_prepared_request_impl(): Attempt to send to precreated request failed ”. meantime, the client sending message to Kamalio server succeded, and the register status and tcp connection is normal. After 30s, obtained the $uac_req(evcode) from the event_route[uac:reply] is 408.After this error appears, all the messages sent to this client are 408. " 2023-08-29T03:48:43.738152+08:00 (3349) NOTICE: <script>: ===uac reply received, callid = 0B6ECB534AE24281AED9BBBC56B140A6`yangwang2`1693252093, code is: 408 2023-08-29T03:48:43.738825+08:00 (3349) NOTICE: <script>: ===uac reply received, callid = 0B6ECB534AE24281AED9BBBC56B140A6`yangwang2`1693252093, code is: 408 " But from other information, the TCP connection is normal. Because from Kamailio to querying this user's Location, it has not changed all day. The client sends messages is also normal. That is, the user's registration and sending messages can receive 200 OK. We use netstat commands to view the user's TCP connection information on the client and server, and the connection status is also normal.The status of the corresponding port of the TCP connection is ESTABLISHED. client: netstat -ano | findstr 49957 "172. 17. 47.125:49957 XXX.96.XXX.XXX:5061 ESTABLISHED 10516" server: "tcp 0 0 10.11.xx.xx:5061 xxx.xxx.xxx.xxx:49957 ESTABLISHED 3467/kamailio" This client is normal after logging in, and the problem that occurs after a period of use (maybe some hours), and once it occurs, it fails until the user creates a new connection and logs back in. Not all users will appear. Why is this the case? The socket connection is fine, but uac_req_send () get an ERROR? I found a problem from the log file. Is it caused by duplicate callids pushed to the same user at the same second? Looking forward to your answer, thank you very much Script information that may be required kamailio.cfg ： tm mode # ----- tm params ----- # auto-discard branches from previous serial forking leg modparam("tm", "failure_reply_mode", 3) # default retransmission timeout: 30sec modparam("tm", "fr_timer", 30000) # default invite retransmission timeout after 1xx: 120sec modparam("tm", "fr_inv_timer", 120000) modparam("tm", "auto_inv_100", 1) kamalio.lua uac_req_send() " --uac_req_send KSR.pv.sets("$uac_req(hdrs)", hdrs) KSR.pv.sets("$uac_req(method)", "MESSAGE") KSR.pv.sets("$uac_req(ruri)", "sip:" .. to .. "@" .. KAM_DOMAIN) KSR.pv.sets("$uac_req(furi)", "sip:" .. from .. "@" .. KAM_DOMAIN) KSR.pv.sets("$uac_req(turi)", "sip:" .. to .. "@" .. KAM_DOMAIN) KSR.pv.seti("$uac_req(evroute)", 1) KSR.pv.sets("$uac_req(body)", content) local ouri = (row.received ~= "" and row.received) or row.contact KSR.pv.sets("$uac_req(ouri)", ouri) local callid = rows[i].clientid .. "`" .. rows[i].username .. "`" .. os.time() KSR.pv.sets("$uac_req(callid)", callid) KSR.uac.uac_req_send()