Maybe it's not beneficial to have a receive buffer that high. If your presence server is stateful, it will send retransmissions if packets get dropped. But it will also send retransmissions if your proxy does not reply fast enough. So your proxy may be damped by retransmissions. Did you check SHM/active transactions on your proxy? Regards, Bastian On Wed, Aug 30, 2023 at 3:56 PM Ihor Olkhovskyi &lt;igorolhovskiy(a)gmail.com&gt; wrote: They are increasing, actually # ss -l -u -m src X.X.X.X/Y State       Recv-Q Send-Q                         Local Address:Port          Peer Address:Port UNCONN      25167616 0 X.X.X.X:sip                    *:* skmem:(r25167616,rb25165824,t0,tb65536,f2304,w0,o0,bl0,d514894) Le mer. 30 août 2023 à 15:04, Bastian Triller &lt;bastian.triller(a)gmail.com&gt; a écrit : Are drops increasing on that socket while it is happening? ss -l src <local_interface> -u sport 5060 -m On Tue, Aug 29, 2023 at 3:26 PM Ihor Olkhovskyi &lt;igorolhovskiy(a)gmail.com&gt; wrote: Just to add some info netstat -nlp Active Internet connections (only servers) Proto Recv-Q Send-Q Local Address     Foreign Address         State PID/Program name ... udp   25167616      0 <local_interface>:5060 0.0.0.0:* 211759/kamailio ... So I see a huge Receive Queue on UDP for Kamailio which is not clearing. Le mar. 29 août 2023 à 14:29, Ihor Olkhovskyi &lt;igorolhovskiy(a)gmail.com&gt; a écrit : Hello, I've faced a bit strange issue, but a bit of preface. I have Kamailio as a proxy (TLS/WS <-> UDP) and second Kamailio as a presence server. At some point presence server accepts around 5K PUBLISH within 1 minute and sending around the same amount of NOTIFY to proxy Kamailio. Proxy is "transforming" protocol to TLS, but at sime point I'm starting to get these type of errors tm [../../core/forward.h:292]: msg_send_buffer(): tcp_send failed tm [t_fwd.c:1588]: t_send_branch(): sending request on branch 0 failed <script>: [RELAY] Relay to <sip:X.X.X.X:51571;transport=tls> failed! tm [../../core/forward.h:292]: msg_send_buffer(): tcp_send failed tm [t_fwd.c:1588]: t_send_branch(): sending request on branch 0 failed Some of those messages are 100% valid as client can go away or so. Some are not, cause I'm sure client is alive and connected. But the problem comes later. At some moment proxy Kamailio just stops accept UDP traffic on this interface (where it also accepts all NOTIFY's), at the start of the "stopping accepting" Kamailio sends OPTIONS via DISPATCHER but not able to receive 200 OK. Over TLS on the same interface all is ok. On other (loopback) interface UDP is being processed fine, so I don't suspert some limit on open files here. Only restart of Kamailio proxy process helps in this case. I've tuned net.core.rmem_max and net.core.rmem_default to 25 Mb, so in theory buffer should not be the case. Is there some internal "interface buffer" in Kamailio that is not freed upon failure send or maybe I've missed somethig? Kamailio 5.6.4 fork=yes children=12 tcp_children=12 enable_tls=yes tcp_accept_no_cl=yes tcp_max_connections=63536 tls_max_connections=63536 tcp_accept_aliases=no tcp_async=yes tcp_connect_timeout=10 tcp_conn_wq_max=63536 tcp_crlf_ping=yes tcp_delayed_ack=yes tcp_fd_cache=yes tcp_keepalive=yes tcp_keepcnt=3 tcp_keepidle=30 tcp_keepintvl=10 tcp_linger2=30 tcp_rd_buf_size=80000 tcp_send_timeout=10 tcp_wq_blk_size=2100 tcp_wq_max=10485760 open_files_limit=63536 Sysctl # To increase the amount of memory available for socket input/output queues net.ipv4.tcp_rmem = 4096 25165824 25165824 net.core.rmem_max = 25165824 net.core.rmem_default = 25165824 net.ipv4.tcp_wmem = 4096 65536 25165824 net.core.wmem_max = 25165824 net.core.wmem_default = 65536 net.core.optmem_max = 25165824 # To limit the maximum number of requests queued to a listen socket net.core.somaxconn = 128 # Tells TCP to instead make decisions that would prefer lower latency. net.ipv4.tcp_low_latency=1 # Optional (it will increase performance) net.core.netdev_max_backlog = 1000 net.ipv4.tcp_max_syn_backlog = 128 # Flush the routing table to make changes happen instantly. net.ipv4.route.flush=1 -- Best regards, Ihor (Igor) -- Best regards, Ihor (Igor) __________________________________________________________ Kamailio - Users Mailing List - Non Commercial Discussions To unsubscribe send an email to sr-users-leave(a)lists.kamailio.org Important: keep the mailing list in the recipients, do not reply only to the sender! Edit mailing list options or unsubscribe: __________________________________________________________ Kamailio - Users Mailing List - Non Commercial Discussions To unsubscribe send an email to sr-users-leave(a)lists.kamailio.org Important: keep the mailing list in the recipients, do not reply only to the sender! Edit mailing list options or unsubscribe: -- Best regards, Ihor (Igor) __________________________________________________________ Kamailio - Users Mailing List - Non Commercial Discussions To unsubscribe send an email to sr-users-leave(a)lists.kamailio.org Important: keep the mailing list in the recipients, do not reply only to the sender! Edit mailing list options or unsubscribe: __________________________________________________________ Kamailio - Users Mailing List - Non Commercial Discussions To unsubscribe send an email tosr-users-leave(a)lists.kamailio.org Important: keep the mailing list in the recipients, do not reply only to the sender! 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